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Bronchodilators relax constricted airway smooth muscle in vitro. Because of this property, bronchodilators reverse airway obstruction, prevent bronchoconstriction and provide protection from constrictor stimuli.1 In this chapter, bronchodilators employed in mechanically ventilated patients are discussed with a special emphasis on inhalation therapy.
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Ventilator-dependent patients with acute exacerbations of chronic obstructive pulmonary disease (COPD) and acute severe asthma routinely receive bronchodilators to relieve bronchoconstriction. By reducing airway resistance, bronchodilators reduce the pressure required to ventilate the lung. This reduction in pressure may protect the lung against injury and enhance patient comfort. A general population of ventilated patients in a medical intensive care unit (ICU)2,3 and patients with acute respiratory distress syndrome4,5 showed improvement in expiratory airflow and airway resistance after bronchodilators. Infants with bronchopulmonary dysplasia, and children with asthma, or bronchiolitis also receive bronchodilators on a routine basis.6–10 In ventilated patients with COPD, elevated airway resistance and intrinsic positive end-expiratory pressure are major causes for weaning failure.11 In these patients, bronchodilators may facilitate weaning.12 Therapy with bronchodilators is, therefore, routinely and commonly employed for many indications in ventilator-dependent patients.13
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β-adrenergic agonists, anticholinergic drugs, and methylxanthines are the three major classes of bronchodilators employed in the ICU. β-adrenergic agonists and anticholinergics are usually administered by the inhaled route, whereas methylxanthines can only be administered enterally or parenterally. Although they are not bronchodilators in the classic sense, corticosteroids, both inhaled and systemic, are commonly employed in acutely ill patients to reduce airway inflammation and increase airway caliber.14–16
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The pharmacology of the β-agonists was extensively reviewed in the second edition of this textbook,17 and other excellent reviews are available.18
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Route of Administration of β2-Adrenergic Agonists
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β-agonists have been given by oral, subcutaneous, intravenous, and inhaled routes. Table 63-1 lists doses for individual drugs. Inhaled therapy is preferred because the drug is delivered directly to its site of action in the airways, a smaller quantity of drug produces an effect comparable to that observed with systemic administration, onset of effect is rapid, and systemic absorption of the drug is limited, thus minimizing side effects. The oral approach has been all but abandoned, and there appears to be no advantage to the intravenous route even in severe asthma with hypercapnia.19 A meta-analysis found no evidence of benefit for the intravenous use of β-agonists in patients who are refractory to inhaled β-agonists.20
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