Effect of prolonged recombency on pulmonary blood volume in normal humans

We have observed a progressive decrease in pulmonary blood volume during sustained recumbency measured using two independent methods. Pulmonary capillary blood volume (Vc) was estimated by the method of Roughton and Forster (J. Appl. Physiol. 11: 290, 1957). We measured regional pulmonary blood volume (PBVR) using 99mTc-labeled erythrocytes (Gorin et al., J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 45: 225, 1978). In 21 studies in 19 normal subjects, we measured pulmonary CO diffusing capacity by the single-breath technique and calculated Vc with the subjects seated and at fixed times after lying down. After 5 min in the recumbent position, there was a mean 49% increase in Vc over the value in the seated position. With sustained recumbency Vc decreased 18.3%/h over 90 min. In 12 studies in 8 normal subjects, PBVR declined 16.7%/h with prolonged recumbency in studies lasting 60-90 min. The initial increase in Vc after subjects assumed a supine position has been well described. The subsequent fall in pulmonary blood volume to levels equal to or below that measured with the subject seated has not previously been reported.