Journal of Applied Physiology, Vol 76, Issue 6 2643-2650, Copyright © 1994 by American Physiological Society
Blood flow, volume, and transit time in the pulmonary microvasculature using laser-Doppler
T. S. Hakim, E. Gilbert and E. M. Camporesi
Department of Surgery, State University of New York Health Science Center, Syracuse 13210.
Capillary transit time is determined by the ratio of capillary volume to
flow rate. Exercise-induced hypoxemia is thought to occur because of the
short transit time of erythrocytes in capillaries. The effect of flow rate
on capillary volume (recruitment vs. distension) is controversial. In a
perfused left lower lobe preparation in canine lungs, we used laser-Doppler
flowmetry (model ALF21R) to monitor changes in blood flow, volume, and
transit time in the microvasculature near the subpleural surface. Changes
in total flow, blood volume, and total transit time (tt) 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 increased gradually over a wide range
of flow rates. When flow increased 4.2 times (from 155 to 650 ml/min), tt
decreased from 7.32 to 3.53 s; meanwhile, microvascular flow increased from
6.0 to 12.7 units and microvascular transit time decreased from 3.14 to
1.81 units. The changes in microvascular volume and transit time were
essentially independent of whether the venous pressure was higher or lower
than alveolar pressure. At very high flow (6-10 times resting value), tt
fell gradually to approximately 1 s. Direct monitoring of transit time with
the laser-Doppler also revealed a gradual decline in microvascular transit
time as flow rate increased from 2 to 10 times the normal flow. (ABSTRACT
TRUNCATED AT 250 WORDS)
