Background: Many deaths from attacks of asthma may be preventable. However, the difficulty in preventing fatal attacks is that not all the pathophysiologic risk factors have been identified.
Methods: To examine whether dyspnea and chemosensitivity to hypoxia and hypercapnia are factors in fatal asthma attacks, we studied 11 patients with asthma who had had near-fatal attacks, 11 patients with asthma who had not had near-fatal attacks, and 16 normal subjects. Their respiratory responses to hypoxia and hypercapnia, determined by the standard rebreathing technique while the patients were in remission, were assessed in terms of the slopes of ventilation and airway occlusion pressure as a function of the percentage of arterial oxygen saturation and end-tidal carbon dioxide tension, respectively. The perception of dyspnea was scored on the Borg scale during breathing through inspiratory resistances ranging from 0 to 30.9 cm of water per liter per second.
Results: The mean (+/- SD) hypoxic ventilatory response (0.14 +/- 0.12 liter per minute per percent of arterial oxygen saturation) and airway occlusion pressure (0.05 +/- 0.05 cm of water per percent of arterial oxygen saturation) were significantly lower in the patients with near-fatal asthma than in the normal subjects (0.60 +/- 0.35, P < 0.001, and 0.16 +/- 0.08, P < 0.001, respectively) and the patients with asthma who had not had near-fatal attacks (0.46 +/- 0.29, P = 0.003, and 0.15 +/- 0.09, P = 0.004). The Borg score was also significantly lower in the patients with near-fatal asthma than in the normal subjects, and their lower hypoxic response was coupled with a blunted perception of dyspnea.
Conclusions: Reduced chemosensitivity to hypoxia and blunted perception of dyspnea may predispose patients to fatal asthma attacks.