Histamine-induced asthma in children: effects on the ventilation-perfusion relationship.

Asthma was provoked by histamine inhalation in five children in order to study the hypoxaemia that might ensue and the underlying ventilation-perfusion (VA/Q) mismatching. The distribution of the VA/Q ratios was measured by a multiple inert gas technique before the provocation, during the asthmatic attack and after salbutamol inhalation. All children displayed a unimodal distribution of ventilation and perfusion under baseline conditions. During asthma they all developed a bimodal distribution, one mode lying within normal VA/Q regions but with increased perfusion to regions with VA/Q ratios of 0.1-1, which correlated with the observed hypoxaemia; the other mode was centered on a VA/Q ratio of approximately 10 and the magnitude of this mode correlated with FEV1 in percent of the predicted value. Salbutamol improved the VA/Q distribution and restored the blood gases to normal. We hypothesize that histamine-induced asthma causes a state of hyperinflation which compromises regional ventilation and blood flow, resulting in a VA/Q mismatching with one normal and one high VA/Q mode, and hypoxaemia.