It is estimated that 6000 to 10,000 patients require mechanical ventilation for acute asthma in the United States each year.1 Asthma exacerbations that lead to mechanical ventilation typically progress over 1 or more days, with viral infections being the most common etiology (Table 30-1).2 Airways typically show extensive mucous plugging and eosinophilic infiltration with edema, explaining the often limited immediate response to bronchodilators.3 Approximately 20% of patients experience a more explosive onset, with attacks occurring over minutes to hours.2–4 The precise cause of “sudden asphyxial” asthma is not always apparent, but recognized triggers include aeroallergens, nonsteroidal antiinflammatory agents, airway irritants, and emotional distress.2,4 Rapid-onset attacks are characterized by profound bronchospasm with minimal mucous plugging, explaining both their sudden onset and rapid resolution.3
Table 30-1: Patterns of Near-Fatal Asthma |Favorite Table|Download (.pdf)
Table 30-1: Patterns of Near-Fatal Asthma
|Time course (onset)||One or more days||Minutes to hours|
|Triggers||Virus, unknown||Aeroallergen, nonsteroid antiinflammatory drug, airway irritant, emotional stress, unknown|
|Mechanisms of airflow obstruction||Mucous plugging and airway edema > bronchospasm||Bronchospasm (“dry airways”)|
|Response to treatment||Slow: minimal response to initial bronchodilators||Rapid: good response to initial bronchodilators|
|Prevention||Steroids early in exacerbation||Avoid triggers|
|Duration intubation||Often several days||Often < 24 hours|
Regardless of the mode of onset, life-threatening asthma is associated with a markedly increased airway resistance, pulmonary hyperinflation, and high physiologic dead space, which together lead to hypercapnia and risk of respiratory arrest.5 Hypercapnia per se is not an indication for intubation, as most asthma exacerbations with hypercapnia do respond to bronchodilator therapy and others may be successfully managed with noninvasive ventilation (Fig. 30-1).6–11 Inhalation of a helium–oxygen gas mixture (heliox) may also reduce work of breathing and might decrease the likelihood of intubation.11,12
Change in partial pressure of arterial carbon dioxide (
) and pH associated with the use of noninvasive positive-pressure ventilation by face mask in patients with severe asthma. (Used, with permission, from Meduri GU, Cook TR, Turner RE, et al. Noninvasive positive pressure ventilation in status asthmaticus. Chest.
Indications for intubation include respiratory arrest, depressed level of consciousness, or progressive fatigue and exhaustion. Rapid-sequence intubation has been advocated by some experts,13 but a high degree of confidence in being able to intubate is a prerequisite before use of a paralytic because it may be difficult to achieve effective bag-mask ventilation in patients with markedly increased airway resistance. Regardless of the technique used, intubation should be performed by the most skilled operator present. Prolonged airway manipulation and repeated failed intubations in the setting of fulminant asthma may prove catastrophic.