Am. J. Respir. Crit. Care Med., Vol 155, No. 1, 01 1997, 216-221.
Time course of hypoxic pulmonary vasoconstriction: a rabbit model of regional hypoxia
NG Vejlstrup, M O'Neill, B Nagyova and KL Dorrington
University Laboratory of Physiology, Oxford, England, United Kingdom.
There is disagreement in the literature about the time required for hypoxic
constriction of pulmonary vessels to reach its full intensity. Some studies
suggest that only minutes are required, others that several hours are
needed. We examined the time course over 6 h of changes in pulmonary shunt
(as a fraction of cardiac output) following induction of unilateral hypoxia
by collapse or liquid filling of the left lung in 47 anesthetized rabbits.
The time course was examined at four degrees of lung inflation: during
collapse and at airway pressures of 0.3 kPa, 0.6 kPa, and 0.9 kPa. The
respective volumes (mean +/- SD) of the liquid-filled lung were estimated
to be 6.4 +/- 1.0, 12.8 +/- 2.5, and 15.8 +/- 1.6 ml/kg body weight (BW).
During sustained hypoxia (the period from 150 to 360 min after inducing
hypoxia), shunt declined at a slow linear rate of 2.37 x 10(-4)/min, which
was independent of lung inflation (p = 0.65 analysis of variance [ANOVA])
and significantly different from zero (p < 0.001). The stability of
cardiac output in this animal model, as measured sequentially by
thermodilution, was confirmed in a further 20 animals. The experiments
provide evidence for a slow intensification of blood-flow diversion at a
rate that does not depend upon the degree of lung inflation. Whether this
change is a feature of hypoxic constriction itself, or some modulation of
it, remains unclear.
