Racemic epinephrine, a nearly equimolar mixture of the two isomers (D and L) of epinephrine, is a mainstay of moderate to severe croup treatment. The majority of its activity is due to the L-isomer. The D-isomer is only about 30 percent as active as the L-isomer. The drug is quite effective when nebulized. Positive effects can be seen in as little as 10 min, with maximal effects seen in about 1 h. In most cases its pharmacologic effects are practically gone by 2 h. Nebulized epinephrine has clearly been shown to decrease airway edema, most likely by vasoconstriction of the boggy mucosal vessels. Its use does not change the natural course of disease, but it does improve ventilation. Use of racemic epinephrine decreases the number of children with croup requiring intubation, intensive care unit admission, and general admission to the hospital.
L-epinephrine, a pure isomer, is used as an advanced cardiac life support (ACLS) drug in a 1:1000 concentration. Although only recently subjected to evidence-based scrutiny,6 this form has been used exclusively in many foreign countries for years. The L-isomer is at least as effective and safe as racemic epinephrine ( Table... 1.2.9-4.).6 Although some forms contain sulfur dioxide, a known respiratory irritant, this has not been reported to cause any significant problems in this patient group. 6 Both racemic and L-epinephrine are relatively contraindicated in patients with severe left ventricular outflow obstruction (idiopathic hypertrophic subaortic stenosis, subvalvular aortic stenosis, etc.) because it may worsen the obstruction.7 When the drug is used in these patients, very close monitoring is indicated.
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