On Man

Abstract

Although it is obvious that the blood flow through the pulmonary artery is pulsatile, it is not so certain that it remains so within the pulmonary capillaries. On the contrary, the direct Fick method for measuring cardiac output expresses mean flow per unit time, and calculation of pulmonary arteriolar resistance tacitly assumes the existence of a non-pulsatile blood flow. Further-more, it is not known whether the rate of gas dif- fusion from alveolus to capillary varies with the phase of the cardiac cycle. The present communication describes a method for measuring instantaneous blood flow in the pulmonary capillary bed in man. A continuous recording of pressure within an airtight body plethysmograph is made before and after the in-halation of nitrous oxide. As the gas is absorbed by the blood entering the lung capillaries, the pressure within the body plethysmograph falls. This fall in pressure is proportional to the pulmonary capillary blood flow, and was found to vary according to the phase of the cardiac cycle.