Airway cooling and nocturnal asthma.

It has long been recognized that asthma often worsens at night. The nature of nocturnal asthma is unclear. Previously we have demonstrated that acute asthma attacks can be induced by exposing the body to cold, and that such attacks can be prevented by breathing warm, humidified air (WHA). Since body temperature begins to decrease late at night and continues through the early morning hours, we suspected that part if not all of nocturnal asthma may be triggered by the same mechanism as asthma induced by body cooling. To test this hypothesis, eight asthmatic patients were studied on four occasions. The subjects breathed either ambient room air (RA; 23 degrees C, 20 percent relative humidity) or WHA (37 degrees C, 100 percent relative humidity) for eight hours during the night with or without taking their regular evening medication. Each night, oral temperature and pulmonary function were measured at 9:45 PM (baseline measurement) and again at 6:15 AM (morning measurement). Results showed that when RA was used, morning pulmonary function and oral temperature decreased significantly from baseline. Using WHA in conjunction with regular evening medication completely prevented both nocturnal asthma and changes in oral temperature. Using WHA without regular evening medication did not prevent but substantially reduced bronchoconstriction compared with RA. These results suggest that airway cooling, which occurs as a consequence of body cooling, plays a significant role in triggering nocturnal asthma.