The Measurement of Pulmonary Capillary Blood Volume and Pulmonary Membrane Diffusing Capacity in Normal Subjects; The Effects of Exercise and Position12

Abstract

The pulmonary capillary blood volume (Vc) and the true diffusing capacity of the pulmonary membrane (Dm) were computed on 23 occasions in 19 normal subjects by the method of Roughton and Forster. The average value for Vc was 65.0 ml; for DM, 98.1 ml/mm Hg/minute. In these 19 subjects Vq and DM are significantly correlated with height, weight, body surface area, vital capacity and apparent diffusing capacity for CO while breathing room air. In 4 subjects Vq proved to be stable and reproducible over a period of months while DM was quite variable. Vc and DM increased on mild exercise in 4 subjects. Vq increased while Dm did not vary significantly when 4 subjects changed from the seated to the recumbent position. The data for Vq and DM reported here probably give the correct order of magnitude of these quantities rather than their absolute values. The average Vq in our subjects is similar to that reported elsewhere; our average Dm is significantly higher. This may be due to the mathematical relation of VC to DM which makes DM highly variable in normal subjects. The hypothesis that capillaries are either shut or completely open appears to be the simplest explanation of our results both on exercise and during recumbency.