Ventilation-perfusion relationships during exercise-induced asthma in children.

In order to elucidate the mechanisms underlying the hypoxemia associated with exercise-induced asthma (EIA), ventilation-perfusion (VA/Q) relationships were investigated in 11 children with a history of EIA. A virtually continuous distribution of VA/Q ratios was measured by the multiple inert gas technique with the children at rest before and after an exercise test. All children displayed a unimodal distribution before the test. In 4 of the children, the exercise test did not provoke asthma, and the unimodal (VA/Q) distribution was maintained. Of the 7 children who developed asthma, 6 displayed a bimodal distribution. One mode fell within normal VA/Q regions but with increased perfusion to regions with VA/Q ratios of 0.1 to 1, which correlated well to the hypoxemia noted during asthma. The other mode fell within regions with high VA/Q ratios, and the magnitude of the mode correlated to the reduction in FEV1 and PaO2. No one had a shunt or a clearly low VA/Q mode (VA/Q less than 0.1). We hypothesize that the high VA/Q was caused by increased intrathoracic pressure impeding regional blood flow.