Respiratory heat loss in exercise-induced asthma. Measurement and clinical application

The theoretical considerations of conditioning inspired air and the application of the respiratory heat loss (RHL) formula are discussed. An on-line method for measuring RHL is described together with the apparatus for generating frigid dry and warm humid air. Exercise-induced asthma (EIA) was studied using these methods. Thirteen asthmatic and 6 normal children and adolescents participated in the study. Each subject undertook two submaximal exercise tests consisting of 6 minutes' ergometric cycling against a fixed load. One test was done while breathing cold dry air (mean temperature -22°C and 0% relative humidity) and the other while breathing warm humid air (mean temperature 36° C and 100% relative humidity). All the other exercise parameters (e.g. heart rate, minute ventilation, oxygen uptake) were carefully matched between the two tests. In the cold dry air tests with a mean RHL of 1,43 kcal/min, all asthmatic subjects developed EIA with a mean fall in forced expiratory volume in the 1st second (FEV₁) of 48% from baseline. In the warm humid air tests with negligible RHL (0,02 kcal/min) none of the asthmatics developed EIA (mean fall in FEV₁ 5%). The difference between the two tests was highly significant (P<0,001). Neither air condition caused bronchospasm in the normal subjects. A dose-response relationship was obtained between the degree of RHL and corresponding fall in FEV₁.