An Open-Circuit Helium Method for Measuring Functional Residual Capacity and Defective Intrapulmonary Gas Mixing1

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Abstract
A convenient open-circuit method using helium is described for measuring the functional residual capacity of the lungs. After an initial period of equilibration on a He-O2 mixture, the subject begins to breathe O2, and the expired gas is collected for measurement of He. Continuous analysis of the He concentration in the expired gas allows calculation of the volume and ventilation rate of slowly ventilated lung spaces in patients with defective intrapulmonary gas mixing. Eight of 16 normal subjects had unequal intrapulmonary gas mixing by this method. In emphysematous subjects there were very large lung spaces with very low ventilation rates. The findings suggest that the conventional open-circuit N method may significantly underestimate the functional residual volume in patients with severely impaired intrapulmonary mixing.