Am. J. Respir. Crit. Care Med., Vol 152, No. 6, 12 1995, 2014-2020.
Distribution of pulmonary capillary transit times
RA Klocke, HJ Schunemann and BJ Grant
Department of Medicine, State University of New York at Buffalo, USA.
The length of time that blood remains in the pulmonary capillary is an
important variable in gas exchange. We have investigated the distribution
of capillary transit times in isolated rabbit lungs perfused with a
bicarbonate-free buffer. The time course of gas exchange was monitored by
enclosing the lungs in a plethysmograph. A bolus of buffer containing
dissolved acetylene was injected into the perfusion system. Exchange of
this inert gas occurred as soon as the bolus reached the capillary bed,
thereby describing the input function into the bed. A separate bolus
injection of bicarbonate solution resulted in production and excretion of
CO2 as long as the bolus remained in the capillary bed. The rate of CO2
production was adjusted by partial inhibition of endothelial carbonic
anhydrase. The distribution of capillary transit times was computed from a
model of CO2 production in the capillary bed and the observed rates and
volumes of acetylene and carbon dioxide excretion. The recovered
distributions indicate that there is a fairly wide distribution of
capillary transit times (relative dispersion, 0.45) around the mean value
of 1.71 s (+/- 0.53 [SD]). Only 10% of capillary transit times are less
than one half of mean transit time. It is likely that gas exchange reaches
equilibrium in the capillary bed except possibly during strenuous exercise
or exposure to high altitude or in disease.
