Near-Fatal Asthma

By Mark D. Siegel, MD, FCCP

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Management

Adjunct and Experimental Therapy

A variety of adjunct therapies have been proposed for NFA. The best studied is heliox, usually an 80/20 or 70/30 mixture of helium and oxygen, which can be delivered by face mask in nonintubated patients or through the inspiratory limb of the ventilator circuit in those who are intubated.⁴⁰ Because helium is less dense than nitrogen, airflow across narrowed airways tends to be laminar or at least less turbulent.⁴⁰ Higher oxygen concentrations cannot be used because the helium concentration becomes insufficient, making heliox less useful in those with severe hypoxia. In some patients, however, heliox may improve oxygenation, even with a lower fraction of inspired oxygen.⁴⁴

Heliox can improve lung mechanics in some patients and, in those who are not intubated, decrease the work of breathing and potentially "buy time" while waiting for corticosteroids to work.^40,45,46 Improved airway mechanics can be seen in intubated patients as well.⁴⁷ If used in this population, however, ventilators must be recalibrated to ensure accurate measurement of the tidal volume (VT) and fraction of inspired oxygen.⁴⁰ Heliox may improve the delivery of aerosolized medications as well.⁴⁸

High-dose IV magnesium may help some patients, at least in part by interfering with calcium-mediated smooth muscle contraction and decreasing acetylcholine release from parasympathetic nerve endings.^49,50 In one emergency department study, IV magnesium was associated with decreased admission rates and improved FEV₁.⁴⁹ In another report, IV magnesium decreased airway resistance in intubated patients.⁵⁰ Other studies have been unable to demonstrate benefit, however.⁴⁰ A trial of IV magnesium, 2 g over a 20-min period, may be reasonable when standard therapy fails.⁴⁰ Although generally safe, toxic levels can cause hypotension and loss of deep tendon reflexes. Caution must be exercised in patients with impaired renal function.⁴⁰

An IV form of the leukotriene-receptor antagonist montelukast may be helpful. In a recent placebo-controlled study of 191 patients with asthma refractory to albuterol, IV montelukast at 7 or 14 mg improved FEV₁ by a mean of 0.18 L at 20 min and 0.22 L at 60 min, the latter a 13.6% improvement from baseline.⁵¹ Further work is necessary before IV montelukast can be recommended for general use.

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