The pulsatile nature of the pulmonary capillary blood flow.

The nature of the pulmonary capillary blood flow is fundamental for the understanding both of pulmonary hemodynamics and of respiratory gas exchange. Direct examinations of the surface of the lung in the cat, rabbit, dog and rat (1-5) have indicated that pulmonary capillary blood flow is basically non-pulsatile. On the other hand, experimental studies by Lee and DuBois (6), using the body plethysmograph in man, have suggested that pulmonary capillary blood flow is consistently pulsatile. Their experimental evidence consisted of a rhythmic distortion of the record, synchronous with the heart beat, obtained during breathholding with open glottis, after a single inspiration of nitrous oxide. However, since the first experiences with the pneumotachograph, it has been recognized that cardiovascular pulsations produce rhythmic changes of pressure in the conducting airways of the lungs. These rhythmic changes in intrapulmonary pressure are detected by the pneumotachograph through the changes in the rate of air flow which they produce. This phenomenon, illustrated in the pneumotachographic record of Figure 1, was recognized by Lilly (7) and designated by him as "cardiac air pulse." Subsequently, Bartlett, Brubach and Specht (8) found that rhythmic changes in the pneumotachographic record could be correlated with the pulsations observed in the plethysmographic pressure record obtained during breathholding after an inhalation of ambient air.