VENTILATION-PERFUSION INEQUALITY IN ASYMPTOMATIC ASTHMA

Abstract

Ventilation-perfusion (.ovrhdot.VA/.ovrhdot.Q) inequality was measured by a multiple inert gas elimination method in 6 asymptomatic patients with asthma and in a 7th patient during a severe asthmatic episode. Measurements were made before and at 5 min intervals after administration of aerosolized isoproterenol. All patients had some residual airway obstruction as measured during forced expirations. All except 1 patient had clearly bimodal distributions of .ovrhdot.VA/.ovrhdot.Q ratios during all phases of the study, as confirmed by an extensive distribution exploration compatible with each set of inert gas data. One mode lay within range of .ovrhdot.VA/.ovrhdot.Q but the other, containing 19.8% of the cardiac output on the average, was centered on a .ovrhdot.VA/.ovrhdot.Q ratio of only 0.07. There was essentially no shunt. Five minutes after administration of isoproterenol, the blood flow to the low .ovrhdot.VA/.ovrhdot.Q mode approximately doubled, accounting for the observed decrease in arterial PO2 [partial pressure of O2]. Breathing 100% O2 had little effect on the distribution. The presence of a bimodal distribution of .ovrhdot.VA/.ovrhdot.Q ratios without shunt suggests that collateral ventilation may be an important mechanism determining the distribution of .ovrhdot.VA/.ovrhdot.Q ratios and preventing the development of shunts. In some asymptomatic asthmatic patients, as many as 12 of the lung units may lie behind completely closed airways and may have very low but finite .ovrhdot.VA/.ovrhdot.Q ratios as a result of collateral ventilation.