Scorpion Sting

Background

Scorpion stings are a major public health problem in many underdeveloped tropical countries. For every person killed by a poisonous snake, 10 are killed by a poisonous scorpion. In Mexico, 1000 deaths from scorpion stings occur per year. In the United States, only 4 deaths in 11 years have occurred as a result of scorpion stings. Furthermore, scorpions can be found outside their normal range of distribution, ie, when they accidentally crawl into luggage, boxes, containers, or shoes and are unwittingly transported home via human travelers.

A scorpion has a flattened elongated body and can easily hide in cracks. It has 4 pairs of legs, a pair of claws, and a segmented tail that has a poisonous spike at the end. Scorpions vary in size from 1-20 cm in length.

Out of 1500 scorpion species, 50 are dangerous to humans. Scorpion stings cause a wide range of conditions, from severe local skin reactions to neurologic, respiratory, and cardiovascular collapse.

Almost all of these lethal scorpions, except the Hemiscorpius species, belong to the scorpion family called the Buthidae. The Buthidae family is characterized by a triangular-shaped sternum, as opposed to the pentagonal-shaped sternum found in the other 5 scorpion families. In addition to the triangular-shaped sternum, poisonous scorpions also tend to have weak-looking pincers, thin bodies, and thick tails, as opposed to the strong heavy pincers, thick bodies, and thin tails seen in nonlethal scorpions. The lethal members of the Buthidae family include the genera of Buthus, Parabuthus, Mesobuthus, Tityus, Leiurus, Androctonus, and Centruroides. These lethal scorpions are found generally in the given distribution:

• Buthus - Mediterranean area
• Parabuthus - Western and Southern Africa
• Mesobuthus - Asia
• Tityus - Central and South America, Caribbean
• Leiurus - Northern Africa and Middle East
• Androctonus - Northern Africa to Southeast Asia
• Centruroides - Southwest USA, Mexico, Central America

However, these scorpions may be found outside their habitat range of distribution when inadvertently transported with luggage and cargo.

In general, scorpions are not aggressive. They do not hunt for prey; they wait for it. Scorpions are nocturnal creatures; they hunt during the night and hide in crevices and burrows during the day to avoid the light. Thus, accidental human stinging occurs when scorpions are touched while in their hiding places, with most of the stings occurring on the hands and feet.

Pathophysiology

Scorpions use their pincers to grasp their prey; then, they arch their tail over their body to drive their stinger into the prey to inject their venom, sometimes more than once. The scorpion can voluntarily regulate how much venom to inject with each sting. The striated muscles in the stinger allow regulation of the amount of venom ejected, which is usually 0.1-0.6 mg. If the entire supply of venom is used, several days must elapse before the supply is replenished. Furthermore, scorpions with large venom sacs, such as the Parabuthus species, can even squirt their venom.

The venom glands are located on the tail lateral to the tip of the stinger and are composed of 2 types of tall columnar cells. One type produces the toxins, while the other produces mucus. The potency of the venom varies with the species, with some producing only a mild flu and others producing death within an hour. Generally, the venom is distributed rapidly into the tissue if it is deposited into a venous structure. Venom deposited via the intravenous route can cause symptoms only 4-7 minutes after the injection, with a peak tissue concentration in 30 minutes and an overall toxin elimination half-life of 4.2-13.4 hours through the urine. The more rapidly the venom enters the bloodstream, the higher the venom concentration in the blood and the more rapid the onset of systemic symptoms.

Scorpion venom is a water-soluble, antigenic, heterogenous mixture, as demonstrated on electrophoresis studies. This heterogeneity accounts for the variable patient reactions to the scorpion sting. However, the closer the phylogenetic relationship between the scorpions, the more similar the immunological properties. Furthermore, the various constituents of the venom may act directly or indirectly and individually or synergistically to manifest their effects. In addition, differences in the amino acid sequence of each toxin account for their differences in the function and immunology. Thus, any modifications of the amino acid sequence result in modification of the function and immunology of the toxin.

The venom is composed of varying concentrations of neurotoxin, cardiotoxin, nephrotoxin, hemolytic toxin, phosphodiesterases, phospholipases, hyaluronidases, glycosaminoglycans, histamine, serotonin, tryptophan, and cytokine releasers. The most potent toxin is the neurotoxin, of which 2 classes exist. Both of these classes are heat-stable, have low molecular weight, and are responsible for causing cell impairment in nerves, muscles, and the heart by altering ion channel permeability.

The long-chain polypeptide neurotoxin causes stabilization of voltage-dependent sodium channels in the open position, leading to continuous, prolonged, repetitive firing of the somatic, sympathetic, and parasympathetic neurons. This repetitive firing results in autonomic and neuromuscular overexcitation symptoms, and it prevents normal nerve impulse transmissions. Furthermore, it results in release of excessive neurotransmitters such as epinephrine, norepinephrine, acetylcholine, glutamate, and aspartate. Meanwhile, the short polypeptide neurotoxin blocks the potassium channels.

The binding of these neurotoxins to the host is reversible, but different neurotoxins have different affinities. The stability of the neurotoxin is due to the 4 disulfide bridges that fold the neurotoxin into a very compact 3-dimensional structure, thus making it resistant to pH and temperature changes. However, reagents that can break the disulfide bridges can inactivate this toxin by causing it to unfold. Also, the antigenicity of this toxin is dependent on the length and number of exposed regions that are sticking out of the 3-dimensional structure.

Frequency

United States

A total of 13,000 stings have been reported, with the majority being from the nonlethal scorpions. Only 1 of 30 scorpion species found in the United States is dangerous to humans. This lethal scorpion species is the straw-colored Centruroides. Less than 1% of stings from Centruroides are lethal to adults; however, 25% of children younger than 5 years who are stung die if not treated. The epidemiological features of a patient who has been envenomed show a disposition for rural areas (73%), with most of the stings occurring in the summer months between 6:00 pm and 12:00 am (49%) and a second peak from 6:00 am to 12:00 pm (30%). Both of these peaks coincide maximum human activity with maximum scorpion activity. Furthermore, the larger the scorpion population, the larger the incidence rate. Because the offending scorpion is recovered for identification in only 30% of the cases, local knowledge of the type of scorpion populating the area is useful.

International

Scorpion stings occur in temperate and tropical regions, especially between the latitudes of 50°N and 50°S of the equator. Furthermore, stings predominantly occur during the summer and evening times. In addition, the majority of patients are stung outside their home.

A recent 5-year surveillance study in Saudi Arabia found 6465 scorpion sting cases with a mean patient age of 23 years, a male-to-female ratio of 1.9, and a higher incidence of stings in the months of May-October.¹

Mortality/Morbidity

The underreporting of scorpion stings is frequent because most envenomations occur in desert and jungle areas that do not have large medical facilities. Furthermore, reporting is not required.

Most deaths occur during the first 24 hours after the sting and are secondary to respiratory or cardiovascular failure.

Children and elderly persons are at the greatest risk for morbidity and mortality. A smaller child, a lower body weight, and a larger ratio of venom to body weight lead to a more severe reaction. A mortality rate of 20% is reported in untreated babies, 10% in untreated school-aged children, and 1% in untreated adults.

Furthermore, patients in rural areas tend to fare worse than patients in urban areas because of the delay in getting medical help due to a longer travel time to medical centers. Fortunately, better public education, improved control of the scorpion population, increased supportive therapies, and more technologically advanced intensive care units have combined to produce a substantial decrease in mortality from these envenomations.

Race

No racial predilection exists. Any differences in individual reactions to the scorpion sting are a reflection of that individual's genetic composition rather than race.

Sex

Females are more susceptible than males to the same amount of scorpion venom because of their lower body weight.

Age

While adults are stung more often than children, children are more likely to develop a more rapid progression and increased severity of symptoms because of their lower body weight. Furthermore, elderly persons are more susceptible to stings because of their decreased physiologic reserves and increased debilitation.

Treatment

Medical Care

Because the clinical manifestations and severity of the symptoms vary among patients, individualize management of scorpion stings. Furthermore, frequent patient monitoring allows earlier recognition of the life-threatening problems of scorpion envenomation. Treatment generally consists of moving the patient away from the scorpion and stabilizing the patient's airway and vital signs, followed by administration of antivenin and institution of symptomatic and local treatment.

• Local treatment is discussed as follows:
• • A negative-pressure extraction device (ie, the extractor) may be useful, although the benefit is unproven. The extractor creates a negative pressure of 1 atm. Apply it to the sting site after incision. Oral extraction is contraindicated.
  • Use ice bags to reduce pain and to slow the absorption of venom via vasoconstriction. This is most effective during the first 2 hours following the sting.
  • Immobilize the affected part in a functional position below the level of the heart to delay venom absorption.
  • Calm the patient to lower the heart rate and blood pressure, thus limiting the spread of the venom.
  • For medical delay secondary to remoteness, consider applying a lymphatic-venous compression wrap 1 inch proximal to the sting site to reduce superficial venous and lymphatic flow of the venom but not to stop the arterial flow. Only remove this wrap when the provider is ready to administer systemic support. The drawback of this wrap is that it may intensify the local effects of the venom.
  • Apply a topical or local anesthetic agent to the wound to decrease paresthesia; this tends to be more effective than opiates.
  • Administer local wound care and topical antibiotic to the wound.
  • Administer tetanus prophylaxis.
  • Administer systemic antibiotics if signs of secondary infection occur.
  • Administer muscle relaxants for severe muscle spasms (ie, benzodiazepines.)
• Systemic treatment is instituted by directing supportive care toward the organ specifically affected by the venom.
  • Establish airway, breathing, and circulation (ie, ABCs) to provide adequate airway, ventilation, and perfusion.
  • Monitor vital signs (eg, pulse oximetry; heart rate, blood pressure, and respiratory rate monitor).
  • Use invasive monitoring for patients who are unstable and hemodynamic.
  • Administer oxygen.
  • Administer intravenous fluids to help prevent hypovolemia from vomiting, diarrhea, sweating, hypersalivation, and insensible water loss from a tropical environment.
  • Perform intubation and institute mechanical ventilation with end-tidal carbon dioxide monitoring for patients in respiratory distress.
  • For hyperdynamic cardiovascular changes, administration of a combination of beta-blockers with sympathetic alpha-blockers is most effective in reversing this venom-induced effect. Avoid using beta-blockers alone because this leads to an unopposed alpha-adrenergic effect. Also, nitrates can be used for hypertension and myocardial ischemia.
  • For hypodynamic cardiac changes, a titrated monitored fluid infusion with afterload reduction helps reduce mortality. A diuretic may be used for pulmonary edema in the absence of hypovolemia, but an afterload reducer, such as prazosin, nifedipine, nitroprusside, hydralazine, or angiotensin-converting enzyme inhibitors, is better. Inotropic medications, such as digitalis, have little effect, while dopamine aggravates the myocardial damage through catecholaminelike actions. Dobutamine seems to be a better choice for the inotropic effect. Finally, a pressor such as norepinephrine can be used as a last resort to correct hypotension refractory to fluid therapy.
  • Administer atropine to counter venom-induced parasympathomimetic effects.
• Insulin administration in scorpion envenomation animal experiments has helped the vital organs to use metabolic substrates more efficiently, thus preventing venom-induced multiorgan failure, especially cardiopulmonary failure. Unfortunately, no human studies have been conducted.
• Administer barbiturates and/or a benzodiazepine continuous infusion for severe excessive motor activity.
• The use of steroids to decrease shock and edema is of unproven benefit.
• Antivenin is the treatment of choice after supportive care is established. The quantity to be used is determined by the clinical severity of patients and by their evolution over time. Unfortunately, predicting the evolution of symptoms and, thus, the amount of antivenin that is needed in the future, is difficult.
• • The antivenin significantly decreases the level of circulating unbound venom within an hour. The persistence of symptoms after the administration of antivenin is due to the inability of the antivenin to neutralize scorpion toxins already bound to their target receptors.
  • Time guidelines for the disappearance of symptoms after antivenin administration are as follows:
    • Centruroides antivenin: Severe neurologic symptoms reverse in 15-30 min. Mild-to-moderate neurologic symptoms reverse in 45-90 min.
    • Non-Centruroides antivenin: In the first hour, local pain abates. In 6-12 hours, agitation, sweating, and hyperglycemia abate. In 6-24 hours, cardiorespiratory symptoms abate.
  • While an anaphylaxis reaction to the antivenin is possible, the patient is at lower risk for this than with other antivenins for other poisonous envenomations because of the huge release of catecholamines induced by the scorpion venom. However, the larger the dose of antivenin, the greater the chance for serum sickness.
• A vaccine preparation was tried in experimental animals but was not pursued because of the need to prepare different antigens according to different geographical areas and to different species of scorpions living in the same area.
• In some cases, be aware that meperidine and morphine may potentiate the venom. Also, the concurrent use of barbiturates and narcotics may add to the respiratory depression in patients who have been envenomated.

Activity

• Rest and immobilization of the sting site is recommended to prevent rapid absorption of the venom into the circulation.

Medication

The goals of pharmacotherapy are to reduce morbidity, to prevent complications, and to neutralize the toxin.

Drug Category: Antivenins

Scorpion toxins are not good antigens because of small size and poor immunogenicity. They do not induce antibodies that cross-react against toxins of other scorpion species unless a 95% amino acid sequence homology exists between the 2 toxins. Thus, no universal antivenin is available. Instead, 22 types of scorpion antivenin exist.

Furthermore, the neurotoxin component of the scorpion venom tends to be the least immunogenic, resulting in the low efficiency for neurological complications. It usually is prepared from horses because they yield larger quantities. Sheep, goat, or bovine antivenin may be prepared if patient sensitivity to horse serum occurs.

A recent idea was to mix a batch of different scorpion antivenin together to create a universal antivenin, but this exposes the patient to unnecessary antivenin from scorpion species not from the patient's region.

Perform a skin test prior to administering the antivenin. First, dilute 0.1 mL of antivenin in a 1:10 ratio with isotonic sodium chloride solution. Second, administer 0.2 mL intradermally. A positive test result is if a wheal develops within 10 minutes. The skin test has a sensitivity of 96% and a specificity of 68%.

The best result occurs when antivenin is administered as early as possible (preferably within the first 2 h after the sting) and with adequate quantities to neutralize the venom (usually 50-100 times the LD50 amount). A decrease in curative effects occurs with longer sting-serotherapy delay and administration of insufficient amounts of antivenin.

Drug Name	USA-APL Centruroides scorpion antivenin
Description	Used to neutralize toxins from scorpions. Produced in Arizona (for use in Arizona only). Not approved by FDA. Use remains controversial, but many physicians recommend it in grade III and IV envenomations. Shown to produce rapid resolution of systemic symptoms but does not affect pain or paresthesias. Results in resolution of symptoms within min to 2 h after administration. Antivenin treatment is based on venom burden, not patient's size. The smaller the victim, the more important it is to administer the full dose because of the venom dose-dependent severity.
Adult Dose	Grade I and II: None Grade III and IV: 1 vial (5 mL) in 50 mL saline IV over 30 min; if severe symptoms still persist after 1 h, repeat once prn
Pediatric Dose	Administer as in adults
Contraindications	Documented hypersensitivity; may administer in severe envenomation, despite hypersensitivity
Interactions	None reported
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Due to presence of horse serum, agents for emergency treatment of anaphylaxis should be available; premedicate with antihistamines or steroids

Drug Category: Benzodiazepines

By increasing the action of GABA (inhibitory neurotransmitter), counteract scorpion-induced excessive motor activity and nervous system excitation.

Drug Name	Lorazepam (Ativan)
Description	Sedative hypnotic with short onset of effects and relatively long half-life. By increasing action of GABA, which is a major inhibitory neurotransmitter in the brain, may depress all levels of CNS, including limbic and reticular formation.
Adult Dose	1-4 mg IV over 2-5 min; may repeat dose in 10-15 min prn
Pediatric Dose	0.05 mg/kg IV over 2-5 min; may repeat dose in 10-15 min prn
Contraindications	Documented hypersensitivity; preexisting CNS depression, hypotension, and narrow-angle glaucoma
Interactions	Toxicity in CNS increases when used concurrently with alcohol, phenothiazines, barbiturates, and MAOIs
Pregnancy	D - Unsafe in pregnancy
Precautions	Caution in renal or hepatic impairment, myasthenia gravis, organic brain syndrome, Parkinson disease, hypotension, and respiratory depression

Drug Name	Midazolam (Versed)
Description	Short-acting benzodiazepine that can be administered in continuous infusion for severe nervous system excitation.
Adult Dose	0.1 mg/kg IV bolus then 0.1 mg/kg/h; titrate dose upward q5min until symptoms controlled
Pediatric Dose	Administer as in adults
Contraindications	Documented hypersensitivity; preexisting hypotension, narrow-angle glaucoma, and sensitivity to propylene glycol (the diluent)
Interactions	Sedative effects may be antagonized by theophyllines; narcotics and erythromycin may accentuate sedative effects because of decreased clearance
Pregnancy	D - Unsafe in pregnancy
Precautions	Caution in congestive heart failure, pulmonary disease, renal impairment, and hepatic failure; may require intubation and pressor support

Drug Category: Barbiturates

Used to counteract scorpion-induced hyperactivity.

Drug Name	Pentobarbital (Nembutal)
Description	Short-acting barbiturate with sedative and anticonvulsant properties used to produce barbiturate coma for severe CNS hyperexcitation. Requires patient intubation prior to use.
Adult Dose	12 mg/kg IV bolus, then 5 mg/kg/h; titrate to symptom abatement or EEG inactivity
Pediatric Dose	Administer as in adults
Contraindications	Documented hypersensitivity; liver failure; porphyria
Interactions	Concomitant use with alcohol may produce additive CNS effects and death; chloramphenicol may inhibit metabolism; may enhance chloramphenicol metabolism; MAOIs may enhance sedative effects of barbiturates; valproic acid appears to decrease barbiturate metabolism, increasing toxicity; barbiturates can decrease effects of anticoagulants (patients may require dosage adjustments if barbiturates are added to or withdrawn from regimen); decreased contraceptive effect may occur due to induction of microsomal enzymes (alternate form of birth control is suggested); barbiturates may decrease corticosteroid and digitoxin effects through induction of hepatic microsomal enzymes that increase metabolism; barbiturates decrease theophylline levels and may decrease effects; may decrease verapamil bioavailability
Pregnancy	D - Unsafe in pregnancy
Precautions	Patient may become tolerant to hypnotic effects; caution in patients with hypovolemic shock, respiratory dysfunction, hypotension, renal dysfunction, congestive heart failure, previous addiction to sedative hypnotics, and congestive heart failure

Drug Category: Local anesthetics

Tend to be more effective than opiates to control paresthesia and pain at the sting site.

Drug Name	Bupivacaine (Marcaine)
Description	May reduce pain by slowing nerve impulse propagation and reducing action potential, which, in turn, prevents initiation and conduction of nerve impulses.
Adult Dose	1.25 mg/kg/dose intralesionally until pain subsides; not to exceed 3-4 mg/kg
Pediatric Dose	Administer as in adults
Contraindications	Documented hypersensitivity; septicemia, spinal deformities, severe hypertension, and existing neurologic disease
Interactions	May enhance effects of CNS depressants; coadministration may increase toxicity of MAOIs, TCAs, beta-blockers, vasopressors, and phenothiazines
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Test a dose and monitor for CNS toxicity, cardiovascular toxicity, and signs of unintended intrathecal administration; caution with inflammation or sepsis in region of proposed injection; monitor patient's state of consciousness after each injection; caution in hypertension, cerebral vascular insufficiency, peripheral vascular disease or heart block, hypoxia, hypovolemia, and arteriosclerotic heart disease

Drug Category: Adrenergic blocking agents and vasodilators

Used to counteract the scorpion-induced adrenergic cardiovascular effect.

Drug Name	Labetalol (Normodyne, Trandate)
Description	Blocks beta1-adrenergic, alpha-adrenergic, and beta2-adrenergic receptor sites, decreasing blood pressure.
Adult Dose	20 mg IV then 40 mg IV repeated q10-15min until BP controlled or until the maximum accumulative dose of 300 mg is reached
Pediatric Dose	Not established Suggested: 0.1 mg/kg IV; repeat q15-20min as last resort
Contraindications	Documented hypersensitivity; cardiogenic shock, pulmonary edema, bradycardia, atrioventricular block, uncompensated congestive heart failure, reactive airway disease, and severe bradycardia
Interactions	Decreases effect of diuretics and increases toxicity of methotrexate, lithium, and salicylates; may diminish reflex tachycardia resulting from nitroglycerin use without interfering with hypotensive effects; cimetidine may increase blood levels; glutethimide may decrease effects by inducing microsomal enzymes
Pregnancy	C - Fetal risk revealed in studies in animals but not established or not studied in humans; may use if benefits outweigh risk to fetus
Precautions	Caution in impaired hepatic function; discontinue therapy if signs of liver dysfunction occur; in elderly patients, a lower response rate and higher incidence of toxicity may be observed; caution with concomitant beta-blockers; beware of continued hypertension despite decreasing heart rate due to insufficient alpha blockade

Drug Name	Prazosin (Minipress)
Description	Counteracts scorpion-induced adrenergic cardiovascular effects. May improve pulmonary edema through vasodilatory effects.
Adult Dose	1 mg PO bid/tid; not to exceed 5 mg/dose
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity
Interactions	Acute postural hypotensive reaction from beta-blockers may worsen; indomethacin may decrease antihypertensive activity; verapamil may increase serum levels and may increase patient's sensitivity to prazosin-induced postural hypotension; may decrease antihypertensive effects of clonidine
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Caution in renal insufficiency and hypotension

Drug Name	Hydralazine (Apresoline)
Description	Decreases systemic resistance through direct vasodilation of arterioles
Adult Dose	10-20 mg IV q4-6h
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity
Interactions	MAOIs and beta-blockers may increase toxicity
Pregnancy	B - Usually safe but benefits must outweigh the risks.
Precautions	May cause hydralazine-induced tachycardia, SLE-type syndrome, and peripheral neuritis

Drug Category: Anticholinergics

Used to counteract scorpion-induced cholinergic symptoms.

Drug Name	Atropine (Atropair)
Description	Used to increase heart rate through vagolytic effects, causing an increase in cardiac output. Also treats bronchorrhea associated with scorpion envenomations.
Adult Dose	0.5 mg IV q15min until desired effect (Note: for vagolytic cardiac effects, there is a 3-mg limit)
Pediatric Dose	0.01 mg/kg IV q15min until desired effect (Note: For cardiac vagolytic effects, there is a 3-mg limit)
Contraindications	Documented hypersensitivity; thyrotoxicosis, narrow-angle glaucoma, and tachycardia
Interactions	Coadministration with other anticholinergics has additive effects; pharmacologic effects of atenolol and digoxin may increase; antipsychotic effects of phenothiazines may decrease; TCAs with anticholinergic activity may increase effects
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Avoid in Down syndrome and/or children with brain damage to prevent hyperreactive response; also avoid in patients with coronary heart disease, tachycardia, congestive heart failure, cardiac arrhythmias, and hypertension; caution in patients with peritonitis, ulcerative colitis, hepatic disease, and hiatal hernia with reflux esophagitis; in patients with prostatic hypertrophy, prostatism may cause dysuria and may require catheterization; monitor patients for anticholinergic effects (eg, hyperthermia, dilated pupils, dry mucous membrane, tachycardia)

Drug Category: Vasopressors/inotropics

Used to combat hypotension refractory to IV fluid therapy.

Drug Name	Norepinephrine (Levophed)
Description	Indicated for persistent hypotension not responsive to judicious fluid loading and sodium bicarbonate.
Adult Dose	0.05-0.15 mcg/kg/min IV infusion; titrate to effect
Pediatric Dose	0.1-1 mcg/kg/min IV infusion; titrate to effect
Contraindications	Documented hypersensitivity
Interactions	Chlorpromazine enhances pressor response by blocking reflex bradycardia caused by norepinephrine
Pregnancy	D - Unsafe in pregnancy
Precautions	Administer into a large vein because extravasation may cause severe tissue necrosis; caution in occlusive vascular disease

Drug Name	Dobutamine (Dobutrex)
Description	Sympathomimetic amine with stronger beta than alpha effects. Increases inotropic state with afterload reduction.
Adult Dose	5-20 mcg/kg/min IV continuous infusion, titrate to desired response; not to exceed 40 mcg/kg/min
Pediatric Dose	Administer as in adults
Contraindications	Documented hypersensitivity
Interactions	Beta-blockers antagonize effects
Pregnancy	B - Usually safe but benefits must outweigh the risks.
Precautions	Higher dosages may cause increase in heart rate and exacerbate hypotension

Drug Name	Milrinone (Primacor)
Description	Positive inotropic agent and vasodilator with little chronotropic activity.
Adult Dose	50 mcg/kg loading dose IV over 10 min, followed by 0.375-0.75 mcg/kg/min continuous IV infusion
Pediatric Dose	Administer as in adults because has been used in the pediatric ICUs, although safety and efficacy not well established
Contraindications	Documented hypersensitivity
Interactions	May precipitate if infused in the same IV line as furosemide
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Slow or stop infusion in patients showing excessive decreases in blood pressure

Respiratory Failure

Background: Respiratory failure is a syndrome in which the respiratory system fails in one or both of its gas exchange functions: oxygenation and carbon dioxide elimination. In practice, respiratory failure is defined as a PaO₂ value of less than 60 mm Hg while breathing air or a PaCO₂ of more than 50 mm Hg. Furthermore, respiratory failure may be acute or chronic. While acute respiratory failure is characterized by life-threatening derangements in arterial blood gases and acid-base status, the manifestations of chronic respiratory failure are less dramatic and may not be as readily apparent.

Classification of respiratory failure

Respiratory failure may be classified as hypoxemic or hypercapnic and may be either acute or chronic.

Hypoxemic respiratory failure (type I) is characterized by a PaO₂ of less than 60 mm Hg with a normal or low PaCO₂. This is the most common form of respiratory failure, and it can be associated with virtually all acute diseases of the lung, which generally involve fluid filling or collapse of alveolar units. Some examples of type I respiratory failure are cardiogenic or noncardiogenic pulmonary edema, pneumonia, and pulmonary hemorrhage.

Hypercapnic respiratory failure (type II) is characterized by a PaCO₂ of more than 50 mm Hg. Hypoxemia is common in patients with hypercapnic respiratory failure who are breathing room air. The pH depends on the level of bicarbonate, which, in turn, is dependent on the duration of hypercapnia. Common etiologies include drug overdose, neuromuscular disease, chest wall abnormalities, and severe airway disorders (eg, asthma, chronic obstructive pulmonary disease [COPD]).

Distinctions between acute and chronic respiratory failure

Acute hypercapnic respiratory failure develops over minutes to hours; therefore, pH is less than 7.3. Chronic respiratory failure develops over several days or longer, allowing time for renal compensation and an increase in bicarbonate concentration. Therefore, the pH usually is only slightly decreased.

The distinction between acute and chronic hypoxemic respiratory failure cannot readily be made on the basis of arterial blood gases. The clinical markers of chronic hypoxemia, such as polycythemia or cor pulmonale, suggest a long-standing disorder.

Pathophysiology: Respiratory failure can arise from an abnormality in any of the components of the respiratory system, including the airways, alveoli, CNS, peripheral nervous system, respiratory muscles, and chest wall. Patients who have hypoperfusion secondary to cardiogenic, hypovolemic, or septic shock often present with respiratory failure.

Hypoxemic respiratory failure: The pathophysiologic mechanisms that account for the hypoxemia observed in a wide variety of diseases are ventilation-perfusion (V/Q) mismatch and shunt. These 2 mechanisms lead to widening of the alveolar-arterial oxygen difference, which normally is less than 15 mm Hg. With V/Q mismatch, the areas of low ventilation relative to perfusion (low V/Q units) contribute to hypoxemia. An intrapulmonary or intracardiac shunt causes mixed venous (deoxygenated) blood to bypass ventilated alveoli and results in venous admixture. The distinction between V/Q mismatch and shunt can be made by assessing the response to oxygen supplementation or calculating the shunt fraction following inhalation of 100% oxygen. In most patients with hypoxemic respiratory failure, these 2 mechanisms coexist.

Hypercapnic respiratory failure: At a constant rate of carbon dioxide production, PaCO₂ is determined by the level of alveolar ventilation (Va), where VCO₂ is ventilation of carbon dioxide and K is a constant value (0.863).

(Va = K x VCO₂)/PaCO₂

A decrease in alveolar ventilation can result from a reduction in overall (minute) ventilation or an increase in the proportion of dead space ventilation. A reduction in minute ventilation is observed primarily in the setting of neuromuscular disorders and CNS depression. In pure hypercapnic respiratory failure, the hypoxemia is easily corrected with oxygen therapy.

Ventilatory capacity versus demand

Ventilatory capacity is the maximal spontaneous ventilation that can be maintained without development of respiratory muscle fatigue. Ventilatory demand is the spontaneous minute ventilation that results in a stable PaCO₂. Normally, ventilatory capacity greatly exceeds ventilatory demand. Respiratory failure may result from either a reduction in ventilatory capacity or an increase in ventilatory demand (or both). Ventilatory capacity can be decreased by a disease process involving any of the functional components of the respiratory system and its controller. Ventilatory demand is augmented by an increase in minute ventilation and/or an increase in the work of breathing.

Pathophysiologic mechanisms in acute respiratory failure

The act of respiration engages 3 processes: (1) transfer of oxygen across the alveolus, (2) transport of oxygen to the tissues, and (3) removal of carbon dioxide from blood into the alveolus and then into the environment. Respiratory failure may occur from malfunctioning of any of these processes. In order to understand the pathophysiologic basis of acute respiratory failure, an understanding of pulmonary gas exchange is essential.

Physiology of gas exchange

Respiration primarily occurs at the alveolar capillary units of the lungs, where exchange of oxygen and carbon dioxide between alveolar gas and blood takes place. Following diffusion into the blood, the oxygen molecules reversibly bind to the hemoglobin. Each molecule of hemoglobin contains 4 sites for combination with molecular oxygen, 1 g of hemoglobin combines with a maximum of 1.36 mL of oxygen. The quantity of oxygen combined with hemoglobin depends on the level of blood PaO₂. This relationship, expressed as the oxygen hemoglobin dissociation curve, is not linear, but has a sigmoid-shaped curve with a steep slope between a PaO₂ of 10 and 50 mm Hg and a flat portion above a PaO₂ of 70 mm Hg. The carbon dioxide is transported in 3 main forms: (1) in simple solution, (2) as bicarbonate, and (3) combined with protein of hemoglobin as a carbamino compound.

During ideal gas exchange, blood flow and ventilation would perfectly match each other, resulting in no alveolar-arterial PO₂ difference. However, even in normal lungs, not all alveoli are ventilated and perfused perfectly. For a given perfusion, some alveoli are underventilated while others are overventilated. Similarly, for known alveolar ventilation, some units are underperfused while others are overperfused. The optimally ventilated alveoli that are not perfused well are called high V/Q units (acting like dead space), and alveoli that are optimally perfused but not adequately ventilated are called low V/Q units (acting like a shunt).

Alveolar ventilation

At steady state, the rate of carbon dioxide production by the tissues is constant and equals the rate of carbon dioxide elimination by the lung. This relationship is expressed as PaCO₂ = VCO₂ x 0.862/Va. This relationship signifies whether the alveolar ventilation is adequate for metabolic needs of the body.

The efficiency of lungs at carrying out of respiration can be further evaluated by measuring alveolar-to-arterial PaO₂ difference. This difference is calculated by the following equation:

PaO₂ = FIO₂ x (PB – PH₂O) – PaCO₂/R

For the above equation, PaO₂ = alveolar PO₂, FIO₂ = fractional concentration of oxygen in inspired gas, PB = barometric pressure, PH₂O = water vapor pressure at 37°C, PaCO₂ = alveolar PCO₂, assumed to be equal to arterial PCO₂, and R = respiratory exchange ratio. R depends on oxygen consumption and carbon dioxide production. At rest, VCO₂/VO₂ is approximately 0.8.

Even normal lungs have some degree of V/Q mismatching and a small quantity of right-to-left shunt, alveolar PO₂ is slightly higher than arterial PO₂. However, an increase in alveolar-to-arterial PO₂ above 15-20 mm Hg indicates pulmonary disease as the cause of hypoxemia.

Pathophysiologic causes of acute respiratory failure

Hypoventilation, V/Q mismatch, and shunt are the most common pathophysiologic causes of acute respiratory failure. These are described in the following paragraphs.

Hypoventilation is an uncommon cause of respiratory failure and usually occurs from depression of the CNS from drugs or neuromuscular diseases affecting respiratory muscles. Hypoventilation is characterized by hypercapnia and hypoxemia. The relationship between PaCO₂ and alveolar ventilation is hyperbolic. As ventilation decreases below 4-6 L/min, PaCO₂ rises precipitously. Hypoventilation can be differentiated from other causes of hypoxemia by the presence of a normal alveolar-arterial PO₂ gradient.

V/Q mismatch is the most common cause of hypoxemia. V/Q units may vary from low to high ratios in the presence of a disease process. The low V/Q units contribute to hypoxemia and hypercapnia in contrast to high V/Q units, which waste ventilation but do not affect gas exchange unless quite severe. The low V/Q ratio may occur either from a decrease in ventilation secondary to airway or interstitial lung disease or from overperfusion in the presence of normal ventilation. The overperfusion may occur in case of pulmonary embolism, where the blood is diverted to normally ventilated units from regions of lungs that have blood flow obstruction secondary to embolism. Administration of 100% oxygen eliminates all of the low V/Q units, thus leading to correction of hypoxemia. As hypoxemia increases the minute ventilation by chemoreceptor stimulation, the PaCO₂ level generally is not affected.

Shunt is defined as the persistence of hypoxemia despite 100% oxygen inhalation. The deoxygenated blood (mixed venous blood) bypasses the ventilated alveoli and mixes with oxygenated blood that has flowed through the ventilated alveoli, consequently leading to a reduction in arterial blood content. The shunt is calculated by the following equation:

QS/QT = (CCO₂ – CaO₂)/CCO₂ – CVO₂)

QS/QT is the shunt fraction, CCO₂ (capillary oxygen content) is calculated from ideal alveolar PO₂, CaO₂ (arterial oxygen content) is derived from PaO₂ using the oxygen dissociation curve, and CVO₂ (mixed venous oxygen content) can be assumed or measured by drawing mixed venous blood from pulmonary arterial catheter.

Anatomical shunt exists in normal lungs because of the bronchial and thebesian circulations, accounting for 2-3% of shunt. A normal right-to-left shunt may occur from atrial septal defect, ventricular septal defect, patent ductus arteriosus, or arteriovenous malformation in the lung. Shunt as a cause of hypoxemia is observed primarily in pneumonia, atelectasis, and severe pulmonary edema of either cardiac or noncardiac origin. Hypercapnia generally does not develop unless the shunt is excessive (>60%). When compared to V/Q mismatch, hypoxemia produced by shunt is difficult to correct by oxygen administration.

Frequency:

• In the US: Respiratory failure is a syndrome rather than a single disease process, and the overall frequency of respiratory failure is not well known. The estimates for individual diseases mentioned here can be found in the appropriate article.

Mortality/Morbidity: The mortality rate associated with respiratory failure varies according to the etiology. For acute respiratory distress syndrome, the mortality rate is approximately 50% in most studies. Acute exacerbation of COPD carries a mortality rate of approximately 30%. The mortality rates for other causative disease processes have not been well described.

Treatment
Medical Care: Hypoxemia is the major immediate threat to organ function. Therefore, the first objective in the management of respiratory failure is to reverse and/or prevent tissue hypoxia. Hypercapnia unaccompanied by hypoxemia generally is well tolerated and probably is not a threat to organ function unless accompanied by severe acidosis. Many experts believe that hypercapnia should be tolerated until the arterial blood pH falls below 7.2. Appropriate management of the underlying disease obviously is an important component in the management of respiratory failure.

A patient with acute respiratory failure generally should be admitted to a respiratory care or intensive care unit. Most patients with chronic respiratory failure can be treated at home with oxygen supplementation and/or ventilatory assist devices along with therapy for their underlying disease.

• Airway management
  • Assurance of an adequate airway is vital in a patient with acute respiratory distress.
  • The most common indication for endotracheal intubation (ETT) is respiratory failure.
  • ETT serves as an interface between the patient and the ventilator.
  • Another indication for ETT is airway protection in patients with altered mental status.
• Correction of hypoxemia
  • After securing an airway, attention must turn to correcting the underlying hypoxemia, the most life-threatening facet of acute respiratory failure.
  • The goal is to assure adequate oxygen delivery to tissues, generally achieved with a PaO₂ of 60 mm Hg or an arterial oxygen saturation (SaO₂) of greater than 90%.
  • Supplemental oxygen is administered via nasal prongs or face mask; however, in patients with severe hypoxemia, intubation and mechanical ventilation often are required.
• Coexistent hypercapnia and respiratory acidosis may need to be addressed. This is done by correcting the underlying cause or providing ventilatory assistance.

• Mechanical ventilation is used for 2 essential reasons: (1) to increase PaO₂ and (2) to lower PaCO₂. Mechanical ventilation also rests the respiratory muscles and is an appropriate therapy for respiratory muscle fatigue.

• Ventilator management
  • The use of mechanical ventilation during the polio epidemics of the 1950s was the impetus that led to the development of the discipline of critical care medicine.
  • Prior to the mid 1950s, negative-pressure ventilation with the use of iron lungs was the predominant method of ventilatory support.
  • Currently, virtually all mechanical ventilatory support for acute respiratory failure is provided by positive-pressure ventilation. Nevertheless, negative-pressure ventilation still is used occasionally in patients with chronic respiratory failure.
  • Over the years, mechanical ventilators have evolved from simple pressure-cycled machines to sophisticated microprocessor-controlled systems. A brief review of mechanical ventilation is presented as follows.
• Overview of mechanical ventilation
  • Positive-pressure versus negative-pressure ventilation: In order for air to enter the lungs, a pressure gradient must exist between the airway and alveoli. This can be accomplished either by raising pressure at the airway (positive-pressure ventilation) or by lowering pressure at the level of the alveolus (negative-pressure ventilation). The iron lung or tank ventilator is the most common type of negative-pressure ventilator used in the past. These ventilators work by creating subatmospheric pressure around the chest, thereby lowering pleural and alveolar pressure, and thus facilitating flow of air into the patient's lungs. These ventilators are bulky, poorly tolerated, and are not suitable for use in modern critical care units. Positive-pressure ventilation can be achieved by an endotracheal or tracheostomy tube or noninvasively through a nasal mask or face mask.
  • Controlled versus patient-initiated (ie, assisted): Ventilatory assistance can be controlled (AC) or patient-initiated. In controlled modes of ventilation, the ventilator delivers assistance independent of the patient's own spontaneous inspiratory efforts. In contrast, during patient-initiated modes of ventilation, the ventilator delivers assistance in response to the patient's own inspiratory efforts. The patient's inspiratory efforts can be sensed either by pressure or flow-triggering mechanisms Triggering mechanism).
  • Pressure-targeted versus volume-targeted: During positive-pressure ventilation, either pressure or volume may be set as the independent variable. In volume-targeted (or volume preset) ventilation, tidal volume is the independent variable set by the physician and/or respiratory therapist, and airway pressure is the dependent variable. In volume-targeted ventilation, airway pressure is a function of the set tidal volume and inspiratory flow rate, the patient's respiratory mechanics (compliance and resistance), and the patient's respiratory muscle activity. In pressure-targeted (or pressure preset) ventilation, airway pressure is the independent variable and tidal volume is the dependent variable. The tidal volume during pressure-targeted ventilation is a complex function of inspiratory time, the patient's respiratory mechanics, and the patient's own respiratory muscle activity.

• Interface between patient and ventilator (mask vs endotracheal intubation)
  • Mechanical ventilation requires an interface between the patient and the ventilator. In the past, this invariably occurred through an endotracheal or tracheostomy tube, but in recent years, an increasing trend has occurred towards noninvasive ventilation, which can be accomplished by the use of either a full face mask (covering both the nose and mouth) or a nasal mask Noninvasive ventilatory support).
  • Care of an endotracheal tube includes correct placement of the tube, maintenance of proper cuff pressure, and suctioning to maintain a patent airway.
  • Following intubation, the position of the tube in the airway (rather than esophagus) should be confirmed by auscultation of the chest and, ideally, by a carbon dioxide detector. As a general rule, the endotracheal tube should be inserted to an average depth of 23 cm in men and 21 cm in women (measured at the incisor). Confirming proper placement of the endotracheal tube with a chest radiograph is recommended.
  • The tube should be secured to prevent accidental extubation or migration into the mainstem bronchus, and the endotracheal tube cuff pressure should be monitored periodically. The pressure in the cuff generally should not exceed 25 mm Hg.
  • Endotracheal suctioning can be accomplished by either open-circuit or closed-circuit suction catheters. Routine suctioning is not recommended because suctioning may be associated with a variety of complications, including desaturation, arrhythmias, bronchospasm, severe coughing, and introduction of secretions into the lower respiratory tract.
• Specific modes of ventilatory support
  • Pressure support ventilation (PSV): PSV can be categorized as patient-initiated, pressure-targeted ventilation. With PSV, ventilatory assistance occurs only in response to the patient's spontaneous inspiratory efforts. With each inspiratory effort, the ventilator raises airway pressure by a preset amount. When the inspiratory flow rate decays to a minimal level or to a percentage of initial inspiratory flow (eg, 25% of peak flow), inspiration is terminated. During PSV, the patients are free to choose their own respiratory rate; inspiratory time, inspiratory flow rate, and tidal volume are determined, in part, by the patient's respiratory efforts. This mode of ventilation should not be used in patients with unstable ventilatory drive, and care must be exercised when the patient's respiratory mechanics are changing because of bronchospasm, secretions, or varying levels of auto–positive end-expiratory pressure (auto-PEEP).
  • Intermittent mandatory ventilation (IMV): IMV is a mode whereby mandatory breaths are delivered at a set frequency, tidal volume, and inspiratory flow rate. However, the patient can breathe spontaneously between the machine-delivered breaths. Most modern ventilators have synchronized IMV (SIMV), whereby the ventilator attempts to deliver the mandatory breaths in synchrony with the patient's own inspiratory efforts. In essence, the ventilator allows the patient an opportunity to breathe. If the patient makes an inspiratory effort during a window of time determined by the IMV rate, the ventilator delivers a mandatory breath in response to the patient's inspiratory effort. However, if no inspiratory effort is detected by the ventilator, a time-triggered breath is delivered. Compared to IMV, SIMV may improve patient comfort and may limit dynamic hyperinflation, which may occur when a preset breath is delivered immediately after the patient's spontaneous inspiratory effort (ie, prior to exhalation).
  • Assist-control ventilation: In assist-control ventilation, patients receive a fixed tidal volume and inspiratory flow rate with each inspiratory effort, regardless of their respiratory rate. However, a “back-up rate" is selected that guarantees that the patient receives a minimum number of breaths per minute. If the patient's respiratory rate falls below the back-up rate, the ventilator delivers the number of breaths necessary to reach the back-up rate; such breaths are delivered independently of any inspiratory effort by the patient.
  • Volume-control: In this mode, respiratory rate, tidal volume, and inspiratory flow rate (or inspiratory time) are fixed. This mode is used most often in heavily sedated or paralyzed patients.
  • Pressure-control: In contrast to volume control, in pressure-control mode, airway pressure is raised by a set amount at a fixed number of times per minute. The physician or respiratory therapist also sets the inspiratory-to-expiratory (I:E) ratio or the inspiratory time. This mode is used most often in heavily sedated or paralyzed patients.
  • Pressure-control inverse-ratio ventilation (PCIRV): PCIRV is a variation of simple pressure-control ventilation. In this mode, inspiration is set to be longer than expiration. The I:E ratio should rarely, if ever, exceed 3:1.
• Triggering mechanism: pressure versus flow triggering
  • In patient-initiated (assisted) ventilation, the ventilator must sense the patient's inspiratory effort in order to deliver assistance. Ventilator triggering may be based on either a pressure or a flow change.
  • With pressure-triggering, the ventilator is set to detect a certain change in pressure. The ventilator is triggered whenever airway pressure drops by the set amount. For example, in a patient on no positive end-expiratory pressure (PEEP) with a trigger sensitivity set at 1 cm water, a breath is triggered whenever airway pressure falls below -1 cm water. In a patient on 5-cm water PEEP with the same trigger sensitivity, a breath is triggered whenever airway pressure falls below +4 cm water.
  • In flow-triggering, a continuous flow of gas is sent through the ventilator circuit. In some ventilators, this continuous flow rate may be set by the physician or respiratory therapist, whereas in other ventilators, the continuous flow rate is fixed. A flow sensitivity is selected, and the ventilator senses the patient's inspiratory efforts by detecting a change in flow. When the patient makes an inspiratory effort, some of the gas that was previously flowing continuously through the circuit is diverted to the patient. The ventilator senses the decrease in flow returning through the circuit, and a breath is triggered. One problem with flow-triggering is that autotriggering sometimes results from leaks in the ventilator circuit.

• Positive end-expiratory pressure
  • By maintaining airway (and hence alveolar) pressure greater than zero, PEEP may recruit atelectatic alveoli and prevent their collapse during the succeeding expiration. PEEP also shifts lung water from the alveoli into the perivascular interstitial space and helps with recruitment of alveoli. However, it does not decrease the total amount of extravascular lung water.
  • In patients with disorders such as ARDS or acute lung injury, PEEP is applied to recruit atelectatic alveoli, thereby improving oxygenation and allowing a reduction in FiO₂ to nontoxic levels (FiO₂ <0.6).>
  • In an ARDS Network trial, higher PEEP produced better oxygenation and lung compliance but no benefit to survival, time on ventilator, or nonpulmonary organ dysfunction. While sufficient PEEP is essential in ventilator management of patients with ARDS, this level varies from patient to patient. Ideal PEEP helps to achieve adequate oxygenation and decrease the requirement for high fractions of inspiratory oxygen without causing any of the harmful effects of PEEP. Current evidence does not support routine application of high PEEP strategy in people with ALI or ARDS.
  • PEEP causes an increase in intrathoracic pressure, which may decrease venous return and cardiac output, particularly in patients with hypovolemia.
• Inspiratory flow
  • In volume-targeted ventilation, inspiratory flow is a variable that is set by the physician or respiratory therapist. The inspiratory flow rate is selected on the basis of a number of factors, including the patient's inspiratory drive and the underlying disease.
  • Two flow patterns are used commonly: (1) a constant-flow (ie, square-wave) pattern and (2) a decelerating-flow pattern. With a constant-flow pattern, inspiratory flow is held constant throughout the breath, whereas with a decelerating-flow pattern, flow rises quickly to a maximal value and then decreases progressively throughout the breath.
  • In pressure-targeted ventilation, the inspiratory flow rate is a dependent variable that varies as a function of the preset pressure and the patient's own inspiratory effort. Because airway pressure is held constant while alveolar pressure rises during inspiration, the pressure difference between airway and alveoli decreases, leading to a decelerating pattern of inspiratory flow.
• Determinants of ventilator-associated lung injury
  • Mechanical ventilation is associated with a variety of insults to the lung.
  • In the past, physicians focused on barotrauma, including pneumothorax, pneumomediastinum, and subcutaneous and pulmonary interstitial emphysema. The manifestations of barotrauma likely occur because of excessive alveolar wall stress. Excessive airway pressure by itself does not appear to cause barotrauma. In critically ill patients, the manifestations of barotrauma can be subtle. For example, the earliest sign of pneumothorax in supine patients may be the deep-sulcus sign or a collection of air anteriorly along cardiophrenic angle.
  • More recently, lung damage indistinguishable from ARDS has been recognized to possibly be caused by certain patterns of ventilatory support. Early experiments in animals showed that mechanical ventilation employing high peak airway pressures and high tidal volume led to the formation of pulmonary edema. The mechanism was thought to be due to direct parenchymal injury and altered microvascular permeability secondary to high peak alveolar pressures. Recently, other investigators have shown that excessive tidal volumes resulting in alveolar overdistension are the most important factor in ventilator-associated lung injury.
  • A strategy of using low tidal volumes in patients with ARDS who are on mechanical ventilation has led to a reduced incidence of barotrauma and improved survival rates in recently published clinical trials.
• Mechanical ventilation in specific diseases

• General guidelines
  • The mode of ventilation should be suited to the needs of the patient. Following the initiation of mechanical ventilation, the ventilator settings should be adjusted based on the patient's lung mechanics, underlying disease process, gas exchange, and response to mechanical ventilation.
  • SIMV and assist-control ventilation often are used for the initiation of mechanical ventilation.
  • In patients with intact respiratory drive and mild-to-moderate respiratory failure, PSV may be a good initial choice.
• Supplemental oxygen
  • The lowest FiO₂ that produces an SaO₂ greater than 90% and a PaO₂ greater than 60 mm Hg generally is recommended.
  • The prolonged use of FiO₂ less than 0.6 is unlikely to cause pulmonary oxygen toxicity.
• Acute respiratory distress syndrome
  • The primary objective is to accomplish adequate gas exchange while avoiding excessive inspired oxygen concentrations and alveolar overdistension.
  • The traditional ventilatory strategy of delivering high tidal volumes leads to high end-inspiratory alveolar pressures (ie, plateau pressure).
  • Many investigators currently believe that repeated cycles of opening and collapsing of inflamed and atelectatic alveoli are detrimental to the lung. Failure to maintain a certain minimum alveolar volume may further accentuate the lung damage. Furthermore, transalveolar pressure (reflected by plateau pressure) exceeding 25-30 cm water is considered to be an important risk factor for stretch injury to the lungs.
  • Patients with ARDS should be targeted to receive a tidal volume of 6 mL/kg. Importantly, remember that the set tidal volume should be based on ideal rather than actual body weight. If the plateau pressure remains excessive (>30 cm water), further reductions in tidal volume may be necessary.

    ARDSNet, a prospective randomized clinical trial, demonstrated a striking reduction in hospital mortality in patient with ARDS who were ventilated with 6 ml/kg predicted body weight compared with 12 ml/kg. Patients who received the lower tidal volume strategy also had more ventilator-free and organ failure-free days. In the lower tidal volume group, the target tidal volume was 6 mL/kg of predicted body weight. This strategy may lead to respiratory acidosis, which requires either high respiratory rates and or sodium bicarbonate infusion.

  • Application of PEEP sufficient to raise the tidal volume above the lower inflection point (Pflex) on the pressure-volume curve may minimize alveolar wall stress and improve oxygenation. A pressure-volume curve can be constructed for an individual patient by measuring plateau pressures at different lung volumes. Pflex is the point at which the slope of the curve changes, indicating that the lung is operating at the most compliant part of the curve.
  • A lung-protective strategy where the PaCO₂ is allowed to rise (permissive hypercapnia) may reduce barotrauma and enhance survival.
  • In some patients with ARDS, the prone position may lead to significant improvements in oxygenation; whether this translates to improved outcome is unknown.
• Obstructive airway diseases
  • In patients with COPD or asthma, institution of mechanical ventilation may worsen dynamic hyperinflation (auto-PEEP or intrinsic PEEP [PEEPi]). The dangers of auto-PEEP include a reduction in cardiac output and hypotension (because of decreased venous return) and barotrauma.
  • The goals of mechanical ventilation in obstructive airway diseases are to unload the respiratory muscles, achieve adequate oxygenation, and minimize the development of dynamic hyperinflation and its associated adverse consequences.
  • Following the initiation of mechanical ventilation, patients with status asthmaticus frequently develop severe dynamic hyperinflation, which often is associated with adverse hemodynamic effects. The development of dynamic hyperinflation can be minimized by delivering the lowest possible minute ventilation in the least possible time. Therefore, the initial ventilatory strategy should involve the delivery of relatively low tidal volumes (eg, 8-10 mL/kg) and lower respiratory rates (eg, 8-12 breaths per min) with a high inspiratory flow rate.
  • In the absence of hypoxia, hypercapnia generally is well tolerated in most patients. Even marked levels of hypercapnia are preferable to attempts to normalize the PCO₂, which could lead to dangerous levels of hyperinflation.
  • Patients often require large amounts of sedation and occasionally paralysis until the bronchoconstriction and airway inflammation have improved.
  • If a decision is made to measure trapped-gas volume (VEI), as recommended by some investigators, an attempt should be made to keep it below 20 mL/kg. The routine measurement of VEI is not recommended because measurement of plateau pressure and auto-PEEP provide similar information and are much easier to perform.
  • Patients with COPD have expiratory flow limitation and are prone to the development of dynamic hyperinflation. Here again, the goal of mechanical ventilation is to unload the respiratory muscles while minimizing the degree of hyperinflation. The use of extrinsic PEEP may be considered in spontaneously breathing patients in order to reduce the work of breathing and to facilitate triggering of the ventilator. Care must be exercised to avoid causing further hyperinflation, and the set level of PEEP should always be less than the level of auto-PEEP.
• Facilitating patient-ventilator synchrony
  • During mechanical ventilation, many patients sometimes experience asynchrony between their own spontaneous respiratory efforts and the pattern of ventilation imposed by the ventilator. This can occur with both controlled and patient-initiated modes of ventilation.
  • In order to achieve synchrony, the ventilator must not only sense and respond quickly to the onset of the patient's inspiratory efforts, it also must terminate the inspiratory phase when the patient's “respiratory clock" switches to expiration. Asynchronous interactions, commonly referred to as “fighting the ventilator," may occur when ventilator breaths and patient efforts are out of phase. This may lead to excessive work of breathing, increased respiratory muscle oxygen consumption, and decreased patient comfort.
  • Patient-ventilator asynchrony should be minimized, and a variety of ways is available to achieve this. Modern ventilators are equipped with significantly better valve characteristics compared to older-generation ventilators. Flow-triggering (with a continuous flow rate) appears to be more sensitive and more responsive to patient's spontaneous inspiratory efforts.
  • Patient-ventilator asynchrony often occurs in the presence of auto-PEEP. Auto-PEEP creates an inspiratory threshold load and thereby decreases the effective trigger sensitivity. This may be partially offset by the application of external PEEP.
  • Sometimes, additional sedation may be necessary to achieve adequate patient-ventilator synchrony.
• Noninvasive ventilatory support
  • The application of ventilatory support through a nasal or full face mask in lieu of ETT is being used increasingly for patients with acute or chronic respiratory failure.
  • Noninvasive ventilation should be considered in patients with mild-to-moderate acute respiratory failure. The patient should have an intact airway, airway-protective reflexes, and be alert enough to follow commands.
  • In clinical trials, noninvasive positive-pressure ventilation (NPPV) has proven beneficial in acute exacerbations of COPD and asthma, decompensated CHF with mild-to-moderate pulmonary edema, and pulmonary edema from hypervolemia. Reports conflict regarding its efficacy in acute hypoxemia due to other causes (eg, pneumonia). A variety of methods and systems are available for delivering noninvasive ventilatory support.
  • The benefits of NPPV depend on the underlying cause of respiratory failure. In acute exacerbations of obstructive lung disease, NPPV decreases PaCO₂ by unloading the respiratory muscles and supplementing alveolar ventilation. The results of several clinical trials support the use of NPPV in this setting.
  • In a large randomized trial (Brochard 1995) comparing NPPV with a standard ICU approach, the use of NPPV was shown to reduce complications, duration of ICU stay, and mortality. In patients in whom NPPV failed, mortality rates were similar to the intubated group (25% vs 30%).
  • Plant and colleagues recently published the largest prospective randomized study comparing NPPV to standard treatment in patients with COPD exacerbation. NPPV was administered on the ward; the nurses were trained for 8 hours in the preceding 3 months. Treatment failed in significantly more patients in the control group (27% vs 15%), and in-hospital mortality rates were significantly reduced by NPPV (20% to 10%).
  • In addition, 3 Italian cohort studies with historical or matched control groups have suggested that long-term outcome of patients treated with NPPV is better than that of patients treated with medical therapy and/or endotracheal intubation.
  • In acute hypoxemic respiratory failure, NPPV also helps maintain an adequate PaO₂ until the patient improves.
  • In cardiogenic pulmonary edema, NPPV improves oxygenation, reduces work of breathing, and may increase cardiac output.
  • When applied continuously to patients with chronic ventilatory failure, NPPV provides sufficient oxygenation and/or carbon dioxide elimination to sustain life by reversing or preventing atelectasis and/or resting the respiratory muscles.
  • Patients with obesity-hypoventilation syndrome benefit from NPPV by reversal of the alveolar hypoventilation and upper airway obstruction.
  • Most studies have used NPPV as an intermittent rather than continuous mode of support. Most trials have used inspiratory pressures of 12-20 cm water; expiratory pressures of 0-6 cm water; and excluded patients with hemodynamic instability, uncontrolled arrhythmia, or a high risk of aspiration.
• Weaning from mechanical ventilation
  • Weaning or liberation from mechanical ventilation is initiated when the underlying process that necessitated ventilatory support has improved. In some patients, such as those recovering from uncomplicated major surgery or a toxic ingestion, withdrawal of ventilator support may be done without weaning. In patients who required more prolonged respiratory therapy, the process of liberating the patient from ventilatory support may take much longer.
  • A patient who has stable underlying respiratory status, adequate oxygenation (eg, PaO₂/FiO₂ >200 on PEEP <10>
  • Over the years, many criteria have been used to predict success in weaning, including a minute ventilation of less than 10 L/min, maximal inspiratory pressure more than -25 cm water, vital capacity more than 10 mL/kg, absence of dyspnea, absence of paradoxical respiratory muscle activity, and agitation or tachycardia during the weaning trial. However, recent studies suggest that the rapid-shallow breathing index, ie, the patient's tidal volume (in liters) divided by the respiratory rate (breaths per min) during a period of spontaneous breathing, may be a better predictor of successful extubation. In a recent study, a daily trial of spontaneous breathing in patients with a rapid-shallow breathing index of less than 105 resulted in a shorter duration of mechanical ventilation. A spontaneous breathing trial of only 30 minutes appears adequate to identify patients in whom successful extubation is likely.
  • In patients who are not yet ready to be liberated from the ventilator, one should focus on the cause of ventilator dependency, such as excessive secretions, inadequate respiratory drive, impaired cardiac function, and ventilatory muscle weakness, rather than the type of ventilator or the mode of assistance.
  • The weaning protocol could be designed with assist-control ventilation, with gradually increasing time spent in trials of spontaneous breathing or by gradually reducing the level of PSV.
  • SIMV appears to result in less rapid weaning than PSV or trials of spontaneous breathing.
  • Patient-ventilator desynchrony is an important component in a carefully designed weaning protocol.
  • Attention must be directed towards patient comfort, avoidance of fatigue, adequate nutrition, and prevention and treatment of medical complications during the weaning period.
  • Ventilator monitoring: Peak inspiratory and plateau pressures should be assessed frequently. Attempts should be made to limit the plateau pressure to less than 25 cm water. Expiratory volume is checked initially and periodically (continuously if ventilator-capable) to assure that the set tidal volume is delivered. In patients with severe airflow obstruction, auto-PEEP (PEEPi) should be monitored on a regular basis.
• Monitoring of patients with acute respiratory failure
  • A patient with respiratory failure requires repeated assessments, which may range from bedside observations to the use of invasive monitoring.
  • These patients should be admitted to a facility where close observation can be provided. Most patients who require mechanical ventilation are critically ill; therefore, constant monitoring in a critical care setting is a must.
  • Cardiac monitoring, blood pressure, pulse oximetry, SaO₂, and capnometry are recommended. An arterial blood gas determination should be obtained 15-20 minutes after the institution of mechanical ventilation. The pulse oximetry readings direct efforts to reduce FiO₂ to a value less than 0.6, and the PaCO₂ guides adjustments of minute ventilation.
• Treatment of underlying cause
  • After the patient's hypoxemia is corrected and the ventilatory and hemodynamic status have stabilized, every attempt should be made to identify and correct the underlying pathophysiologic process that led to respiratory failure in the first place.
  • The specific treatment depends on the etiology of respiratory failure.
  • In , a brief discussion of medications used to treat common causes of respiratory failure, such as cardiogenic pulmonary edema, chronic obstructive pulmonary disease, and asthma, is provided.
  • The reader is recommended to review the article specific to the disease for the workup and management of the various disorders, all of which progress by different means but ultimately converge on a final common pathway of respiratory failure.

Consultations:

• Consultation with a pulmonary specialist and an intensivist often are required.

• Patients with acute respiratory failure or exacerbations of chronic respiratory failure need to be admitted to the intensive care unit for ventilatory support.

Activity:

Patients generally are prescribed bed rest during early phases of respiratory failure management. However, ambulation as soon possible helps ventilate atelectatic areas of the lung.

Medication
The pharmacotherapy of cardiogenic pulmonary edema and acute exacerbations of COPD is discussed here. The goals of therapy in cardiogenic pulmonary edema are to achieve a pulmonary capillary wedge pressure of 15-18 mm Hg and a cardiac index greater than 2.2 L/min/m², while maintaining adequate blood pressure and organ perfusion. These goals may need to be modified for some patients. Diuretics, nitrates, analgesics, and inotropics are used in the treatment of acute pulmonary edema.
Drug Category: Diuretics -- First-line therapy generally includes a loop diuretic such as furosemide, which inhibits sodium chloride reabsorption in the ascending loop of Henle.

Drug Name	Furosemide (Lasix) -- Administer loop diuretics IV because this allows for both superior potency and a higher peak concentration despite increased incidence of adverse effects, particularly ototoxicity.
Adult Dose	10-20 mg IV for patients symptomatic with CHF not already using diuretics 40-80 mg IV for patients already using diuretics 80-120 mg IV for patients whose symptoms are refractory to initial dose after 1 h of administration or who have significant renal insufficiency Higher doses and more rapid redosing may be appropriate for patients in severe distress
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, hepatic coma, anuria, state of severe electrolyte depletion
Interactions	Metformin decreases concentrations; conversely, furosemide interferes with the hypoglycemic effect of antidiabetic agents; also antagonizes muscle-relaxing effect of tubocurarine Auditory toxicity appears to be increased with concurrent use of aminoglycoside and furosemide; hearing loss of varying degrees may occur Anticoagulant activity of warfarin may be enhanced when taken concurrently Increased plasma lithium levels and toxicity are possible when taken concurrently
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Monitor for electrolyte imbalance; caution with coadministration of nephrotoxic drugs

Drug Name	Metolazone (Mykrox, Zaroxolyn) -- Has been used as adjunctive therapy in patients initially refractory to furosemide. Has been demonstrated to be synergistic with loop diuretics in treating refractory patients and causes a greater loss of potassium. Potent loop diuretic that sometimes is used in combination with Lasix for more aggressive diuresis. Also used in patients with a degree of renal dysfunction for initiating diuresis.
Adult Dose	5-10 mg PO before redosing with furosemide
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, hepatic coma, encephalopathy, anuria
Interactions	Thiazides may decrease effect of anticoagulants, sulfonylureas, and gout medications; anticholinergics and amphotericin B may increase toxicity of thiazides; effects of thiazides may decrease when used concurrently with bile acid sequestrants, NSAIDs, and methenamine When coadministered, thiazides increase toxicity of anesthetics, diazoxide, digitoxin, lithium, loop diuretics, antineoplastics, allopurinol, calcium salts, vitamin D, and nondepolarizing muscle relaxants
Pregnancy	D - Unsafe in pregnancy
Precautions	Exercise caution with hepatic and renal disease, diabetes mellitus, gout, and systemic lupus erythematosus

Drug Category: Nitrates -- These agents reduce myocardial oxygen demand by lowering preload and afterload. In severely hypertensive patients, nitroprusside causes more arterial dilatation than nitroglycerin. Nevertheless, due to the possibility of thiocyanate toxicity and the coronary steal phenomenon associated with nitroprusside, IV nitroglycerin may be the initial therapy of choice for afterload reduction.

Drug Name	Nitroglycerin (Nitro-Bid, Nitrol) -- SL nitroglycerin and Nitrospray are particularly useful in the patient who presents with acute pulmonary edema with a systolic blood pressure of at least 100 mm Hg. Similar to SL, onset of Nitrospray is 1-3 min, with a half-life of 5 min. Administration of Nitrospray may be easier, and it can be stored for as long as 4 y. One study demonstrated significant and rapid hemodynamic improvement in 20 patients with pulmonary edema who were given Nitrospray. Topical nitrate therapy is reasonable in a patient presenting with class I-II CHF. However, in patients with more severe signs of heart failure or pulmonary edema, IV nitroglycerin is preferred because it is easier to monitor hemodynamics and absorption, particularly in patients with diaphoresis. Oral nitrates, due to delayed absorption, play little role in the management of acute pulmonary edema.
Adult Dose	Nitrospray: 1 puff (0.4 mg) equivalent to a single 1/150 SL; may repeat q3-5min as hemodynamics permit, not to exceed 1.2 mg Ointment: Apply 1-2 inches of nitropaste to chest wall Injection: Start at 20 mcg/min IV and titrate to effect in 5- to 10-mcg increments q3-5min
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, severe anemia, shock, postural hypotension, head trauma, closed-angle glaucoma, cerebral hemorrhage
Interactions	Aspirin may increase nitrate serum concentrations; marked symptomatic orthostatic hypotension may occur when coadministered with calcium channel blockers, adjustment in dose of either agent may be necessary
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Caution in coronary artery disease and low systolic blood pressure

Drug Name	Nitroprusside sodium (Nitropress) -- Produces vasodilation of venous and arterial circulation. At higher dosages, may exacerbate myocardial ischemia by increasing heart rate. Easily titratable.
Adult Dose	10-15 mcg/min IV; titrate to effective dose range of 30-50 mcg/min and a systolic blood pressure of at least 90 mm Hg
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, subaortic stenosis, optic atrophy, tobacco amblyopia, idiopathic hypertrophic, atrial fibrillation or flutter
Interactions	Patients receiving other hypertensive therapy may be more sensitive to sodium nitroprusside
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Exercise caution with increased intracranial pressure, hepatic failure, severe renal impairment, and hypothyroidism. In renal or hepatic insufficiency, levels may increase and can cause cyanide toxicity Has potent effects on blood pressure (use only in those patients with mean arterial pressures >70 mm Hg)

Drug Category: Analgesics -- Morphine IV is an excellent adjunct in the management of acute pulmonary edema. In addition to being both an anxiolytic and an analgesic, its most important effect is venodilation, which reduces preload. Also causes arterial dilatation, which reduces systemic vascular resistance and may increase cardiac output.

Drug Name	Morphine sulfate (Duramorph, Astramorph, MS Contin) -- DOC for narcotic analgesia due to reliable and predictable effects, safety profile, and ease of reversibility with naloxone. Morphine sulfate administered IV may be dosed in a number of ways and commonly is titrated until desired effect is obtained.
Adult Dose	2-5 mg and repeated q10-15min IV unless respiratory rate is <20>
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, hypotension, potentially compromised airway with uncertain rapid airway control, respiratory depression, nausea, emesis, constipation, urinary retention
Interactions	Phenothiazine may antagonize analgesic effects of opiate agonists; tricyclic antidepressants, MAOIs, altered mental status, and other CNS depressants may potentiate adverse effects of morphine when used concurrently
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Exercise caution with atrial flutter and other supraventricular tachycardias; morphine has vagolytic action and may increase the ventricular response rate; due to addictive nature, abuse also is a possibility, although this is not a significant concern in a critically ill patient

Drug Category: Inotropics -- Principal inotropic agents include dopamine, dobutamine, inamrinone (formerly amrinone), milrinone, dopexamine, and digoxin. In patients with hypotension presenting with CHF, dopamine and dobutamine usually are employed. Inamrinone and milrinone inhibit phosphodiesterase, resulting in an increase of intracellular cyclic AMP and alteration in calcium transport. As a result, they increase cardiac contractility and reduce vascular tone by vasodilatation.

Drug Name	Dopamine (Intropin) -- Stimulates both adrenergic and dopaminergic receptors. Hemodynamic effects depend on the dose. Lower doses stimulate mainly dopaminergic receptors that produce renal and mesenteric vasodilation. Cardiac stimulation and renal vasodilation are produced by higher doses. Positive inotropic agent at 2-10 mcg/kg/min that can lead to tachycardia, ischemia, and dysrhythmias. Doses >10 mcg/kg/min cause vasoconstriction, which increases afterload.
Adult Dose	5 mcg/kg/min IV and increase at increments of 5 mcg/kg/min IV to dose of 20 mcg/kg/min
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, pheochromocytoma, ventricular fibrillation
Interactions	Phenytoin, alpha- and beta-adrenergic blockers, general anesthesia, and MAOIs increase and prolong effects, thus, lower dosage
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Closely monitor urine flow, cardiac output, pulmonary wedge pressure, and blood pressure during infusion; prior to infusion, correct hypovolemia with either whole blood or plasma, as indicated; monitoring of central venous pressure or left ventricular filling pressure may be helpful in detecting and treating hypovolemia

Drug Name	Norepinephrine (Levophed) -- Used in protracted hypotension following adequate fluid replacement. Stimulates beta1- and alpha-adrenergic receptors, which in turn increases cardiac muscle contractility and heart rate, as well as vasoconstriction. As a result, increases systemic blood pressure and cardiac output. Adjust and maintain infusion to stabilize blood pressure (eg, 80-100 mm Hg systolic) sufficiently to perfuse vital organs.
Adult Dose	0.05-2 mcg/kg/min IV titrated according to hemodynamic response not to exceed 10 mcg/kg/min
Pediatric Dose	0.05-0.1 mcg/kg/min IV titrated according to hemodynamic response; not to exceed 1-2 mcg/kg/min
Contraindications	Documented hypersensitivity; peripheral or mesenteric vascular thrombosis because ischemia may be increased and the area of the infarct extended
Interactions	Atropine sulfate may enhance the pressor response of norepinephrine by blocking the reflex bradycardia caused by norepinephrine; effects increase when administered concurrently with tricyclic antidepressants, MAOIs, antihistamines, guanethidine, methyldopa, and ergot alkaloids
Pregnancy	D - Unsafe in pregnancy
Precautions	Correct hypovolemia before administering norepinephrine; extravasation may cause severe tissue necrosis; therefore, administer into large vein; use with caution in occlusive vascular disease

Drug Name	Dobutamine (Dobutrex) -- Produces vasodilation and increases inotropic state. At higher dosages, may cause increased heart rate, thus exacerbating myocardial ischemia. Strong inotropic agent with minimal chronotropic effect and no vasoconstriction.
Adult Dose	2.5 mcg/kg/min IV initially; generally therapeutic in the range of 10-40 mcg/kg/min
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, idiopathic hypertrophic subaortic stenosis, atrial fibrillation or flutter
Interactions	Beta-adrenergic blockers antagonize effects of nitroprusside; general anesthetics may increase toxicity
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Following an MI, use dobutamine with caution; correct hypovolemia before using

Drug Category: Bronchodilators -- These agents are an important component of treatment in respiratory failure caused by obstructive lung disease. These agents act to decrease muscle tone in both small and large airways in the lungs. This category includes beta-adrenergics, methylxanthines, and anticholinergics.

Drug Name	Terbutaline (Brethaire, Bricanyl) -- Acts directly on beta2-receptors to relax bronchial smooth muscle, relieving bronchospasm and reducing airway resistance.
Adult Dose	0.25 mg (0.25 cc of 1-mg/mL concentration) SC; not to exceed 0.5 mg SC q4h
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity, tachycardia resulting from cardiac arrhythmias
Interactions	Concomitant use with beta-blockers may inhibit bronchodilatory, cardiac, and vasodilatory effects of beta-agonists; coadministration of MAOIs with beta-sympathomimetics may result in severe hypertension, headache, and hyperpyrexia, which may result in a hypertensive crisis MAOIs also may potentiate activity of beta-adrenergic agonists on vascular system Coadministration of oxytocic drugs (eg, ergonovine with terbutaline) may result in severe hypotension
Pregnancy	B - Usually safe but benefits must outweigh the risks.
Precautions	Caution in coronary disease; through intracellular shifts, may decrease serum potassium levels, which can produce adverse cardiovascular effects; however, decrease usually is transient and may not require supplementation

Drug Name	Albuterol (Proventil) -- Beta-agonist useful in the treatment of bronchospasm. Selectively stimulate beta2-adrenergic receptors of the lungs. Bronchodilation results from relaxation of bronchial smooth muscle, which relieves bronchospasm and reduces airway resistance.
Adult Dose	5 mg/mL of solution for nebulization, usually mixed as 0.5-1 cc with 2.5 cc of water and nebulized prn in acute setting
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity to albuterol, adrenergic amines, or related products
Interactions	Beta-adrenergic blockers antagonize effects; inhaled ipratropium may increase duration of bronchodilation induced by albuterol; cardiovascular effects may increase when coadministered with MAOIs, inhaled anesthetics, tricyclic antidepressants, and sympathomimetic agents
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Caution in hyperthyroidism, diabetes mellitus, or cardiovascular disorders

Drug Name	Theophylline (Theo-Dur, Slo-bid, Theo-24) -- Has a number of physiological effects, including increases in collateral ventilation, respiratory muscle function, mucociliary clearance, and central respiratory drive. Partially acts by inhibiting phosphodiesterase, elevating cellular cyclic AMP levels, or antagonizing adenosine receptors in the bronchi, resulting in relaxation of smooth muscle. However, clinical efficacy is controversial, especially in the acute setting.
Adult Dose	Target concentration: 10 mcg/mL Dosing = (target concentration - current level) x 0.5 (ideal body weight); alternatively, 1 mg/kg results in approximately 2 mcg/mL increase in serum levels
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity to theophylline, xanthines, or related products; uncontrolled arrhythmias; hyperthyroidism
Interactions	Aminoglutethimide, barbiturates, carbamazepine, ketoconazole, loop diuretics, charcoal, hydantoins, phenobarbital, phenytoin, rifampin, isoniazid, and sympathomimetics may decrease effects; effects may be increased by coadministration with allopurinol, beta-blockers, ciprofloxacin, corticosteroids, disulfiram, quinolones, thyroid hormones, ephedrine, carbamazepine, cimetidine, erythromycin, macrolides, propranolol, and interferon
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Caution in tachyarrhythmias, hyperthyroidism, and patients with compromised cardiac function; do not inject IV solution faster than 25 mg/mm; patients diagnosed with pulmonary edema or liver dysfunction are at greater risk of toxicity because of reduced drug clearance

Drug Name	Ipratropium bromide (Atrovent) -- Anticholinergic medication that appears to inhibit vagally mediated reflexes by antagonizing action of acetylcholine, specifically with the muscarinic receptor on bronchial smooth muscle. Vagal tone can be significantly increased in COPD; therefore, this can have a profound effect. Dose can be combined with a beta-agonist because ipratropium may require 20 min to begin having an effect.
Adult Dose	0.5 mg/nebulizer treatment
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity
Interactions	Albuterol and ipratropium together are more efficacious than either one alone Drugs with anticholinergic properties (eg, dronabinol) may increase toxicity
Pregnancy	B - Usually safe but benefits must outweigh the risks.
Precautions	Not indicated for initial treatment of acute episodes of bronchospasm; caution in narrow-angle glaucoma, prostatic hypertrophy, and bladder neck obstruction

Drug Category: Corticosteroids -- Have been shown to be effective in accelerating recovery from acute COPD exacerbations and are an important anti-inflammatory therapy in asthma. While they may not make a clinical difference in the ED, they have some effect 6-8 h into therapy; therefore, early dosing is critical.

Drug Name	Methylprednisolone (Solu-Medrol, Depo-Medrol) -- Usually given IV in ED for initiation of corticosteroid therapy, although PO should theoretically be equally efficacious.
Adult Dose	The optimal dosage is uncertain 125 mg IV q6h often administered for the first 24-48 h of therapy
Pediatric Dose	Not established
Contraindications	Documented hypersensitivity; viral, fungal, or tubercular skin infections
Interactions	Clearance may decrease when coadministered with estrogens; when coadministered with digoxin, may increase digitalis toxicity secondary to hypokalemia; phenobarbital, phenytoin, and rifampin also may increase metabolism of glucocorticoids; therefore, consider increasing maintenance dose; monitor patients for hypokalemia with concurrent use of diuretics
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Hyperglycemia, edema, osteonecrosis, peptic ulcer disease, hypokalemia, osteoporosis, euphoria, psychosis, growth suppression, myopathy, and infections are possible complications of glucocorticoid use