Refractory period after hyperventilaton-induced asthma

Nine young asthmatc subjects undertook isocapnic hyperventilation while breathing air under different conditions. Each subject undertook 2 pairs of tests. Pair A consisted of 2 hyperventilation challenges performed while breathing cold (2.8 ± 1.4° C) dry (2.3 ± 0.05 mg H₂O/L) air. Pair B consisted of an initial warm (38.0 ± 0.9°C) saturated air challenge followed by a cold dry challenge. Tests were closely matched in terms of ventilation and respiratory heat loss in the cold dry tests. The subjects were rendered refractory by the first cold dry hyperventilation challenge, the fall in forced expiratory volume in one second (FEV₁) after hyperventilation in the first test (ΔFEV₁ = 39 ± 5%) being significantly greater than that after the second challenge of Pair A (ΔFEV₁ = 21 ± 5%, p < 0.005). In test Pair B, the warm humid hyperventilation challenge neither caused significant asthma (ΔFEV₁, 6 ± 3) nor rendered the subjects refractory to the subsequent cold dry test (ΔFEV₁, 38 ± 4%). Because in a previous study it was shown that exercise while breathing warm humid air could induce a refractory period without itself causing asthma, we conclude that hyperventilation-induced asthma is not the same as exercise-induced asthma in most subjects.