Distribution of pulmonary capillary erythrocyte transit times.
Presson, Robert G., Jr, Jacquelyn A. Graham, Christopher C. Hanger,
Patricia S. Godbey, Sarah A. Gebb, Richard A. Sidner, Robb W. Glenny,
and Wiltz W. Wagner, Jr.
Departments of Anesthesia, Physiology/Biophysics, Pediatrics, and
Surgery, Indiana University Medical School, Indianapolis, IN 46202,
and Department of Medicine, University of Washington, Seattle, WA
98195
APStracts 2:0191A, 1995.
In theory, red blood cells can pass through the pulmonary capillaries
too rapidly to be completely saturated with oxygen during exercise.
This idea has not been directly tested because the transit times of
the fastest red cells are unknown. We report the first measurements
of the entire transit time distribution for red blood cells crossing
single subpleural capillary networks of canine lung using in vivo
fluorescence video microscopy and compare those times with the
distribution of plasma transit times in the same capillary networks.
On average, plasma took 1.4 times longer than red cells to pass
through the capillary bed. Decreased transit times with increased
cardiac output were mitigated both by capillary recruitment and a
narrowing of the transit time distribution. This design feature of
the pulmonary capillary bed kept the shortest times from falling
below the theoretical minimum time for complete oxygenation. As red
blood cells cross pulmonary capillaries, hemoglobin becomes saturated
with oxygen. This process requires approximately 0.25 s for
completion (19). Normal red cell capillary transit times (> 1 s) are
believed to decrease to < 0.25 s during strenuous exercise and result
in hemoglobin desaturation in man at sea level (20) and in race
horses (21). This mechanism remains hypothetical because direct
measurements of the entire distribution of pulmonary capillary red
cell transit times have been technically impossible, especially
during high cardiac output. Further, these measurements are
confounded by the complex morphology of the pulmonary
microcirculation which causes transit times to be widely dispersed
about a mean (9). Direct estimates of the distribution of transit
times have been made by measuring the time required for a fluorescent
dye bolus to cross the pulmonary capillaries (4,15,23). These plasma
transit times however are likely to differ from erythrocyte transit
times because red cells travel through capillaries more rapidly than
plasma (1). There is also evidence that some pulmonary capillaries
may be only partially open at times, admitting plasma flow but not
erythrocytes (11). In that case, the path lengths and resistances for
plasma would differ from those for erythrocytes resulting in a
difference in transit times. For these reasons, direct measurements
of red cell transit times are necessary to determine how closely red
cell transit times approach the theoretical limit for complete
saturation. To make these determinations we measured the entire
transit time distribution of fluorescently-labeled erythrocytes
crossing the pulmonary capillaries at rest and during increased
cardiac output and compared them to the transit time distribution of
fluorescently-labeled plasma during increased cardiac output.
Received 6 September 1994; accepted in final form 13 March 1995.
APS Manuscript Number A925-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 9 May 1995.