BLOOD-FLOW, VOLUME, AND TRANSIT-TIME IN THE PULMONARY MICROVASCULATURE USING LASER-DOPPLER

Capillary transit time is determined by the ratio of capillary volume to flow rate. Exercise-induced hypoxemia is thought to occur because o f the short transit time of erythrocytes in capillaries. The effect of flow rate on capillary volume (recruitment vs. distension) is controv ersial. In a perfused left lower lobe preparation in canine lungs, we used laser-Doppler flowmetry (model ALF21R) to monitor changes in bloo d flow, volume, and transit time in the microvasculature near the subp leural surface. Changes in total flow, blood volume, and total transit time (t(t)) were also measured. The results showed that microvascular volume approached maximum when flow rate was at resting value (0.4 l/ min) and pressure in the pulmonary artery was > 6 mmHg relative to the level of the capillaries. In contrast, the total blood volume increas ed gradually over a wide range of flow rates. When flow increased 4.2 times (from 155 to 650 ml/min), t(t) decreased from 7.32 to 3.53 s; me anwhile, microvascular flow increased from 6.0 to 12.7 units and micro vascular transit time decreased from 3.14 to 1.81 units. The changes i n microvascular volume and transit time were essentially independent o f whether the venous pressure was higher or lower than alveolar pressu re. At very high flow (6-10 times resting value), t(t) fell gradually to similar to 1 s. Direct monitoring of transit time with the laser-Do ppler also revealed a gradual decline in microvascular transit time as flow rate increased from 2 to 10 times the normal flow. The results s uggest that the microvascular bed, including capillaries, reaches maxi mal volume when their transmural pressure exceeds 6 mmHg, whereas tota l blood volume continued to increase, presumably due to distension of larger vessels.