ROLE OF RESPIRATORY HEAT-EXCHANGE IN PRODUCTION OF EXERCISE-INDUCED ASTHMA

It is possibly the total heat flux in the tracheobronchial tree during exercise that determines the degree of post-exertional obstruction in asthma; quantitative expressions that relate these 2 events were developed. This hypothesis was tested by comparing the observed responses to exercise while the subjects inhaled dry air at various temperatures ranging from subzero to 80.degree. C in a random fashion, to those that would probably occur based on calculations of respiratory heat exchange. Whether heat could be transferred from the inspired air to the mucosa so as to offset evaporative losses from the airways was determined. The observed responses fell as air temperature was increased from -11 to +37.degree. C and exactly matched theoretical predictions. Above 37.degree. C, the observed response exceeded predictions, indicating that it was not possible to provide sufficient heat per se in the air to offset the vaporization of water. When small amounts of water vapor were added to the inspirate at high temperatures, bronchospasm was virtually abolished and the response again closely matched theoretical expectations. The magnitude of exercise-induced asthma is probably directly proportional to the thermal load placed on the airways; this reaction is quantifiable in terms of respiratory heat exchange.