TIME-COURSE OF HYPOXIC PULMONARY VASOCONSTRICTION - A RABBIT MODEL OFREGIONAL HYPOXIA

There is disagreement in the literature about the time required for hy poxic constriction of pulmonary vessels to reach its full intensity. S ome studies suggest that only minutes are required, others that severa l 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 degre es of lung inflation: during collapse and at airway pressures of 0.3 k Pa, 0.6 kPa, and 0.9 kPa. The respective volumes (mean rt SD) of the l iquid-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 peri od from 150 to 360 min after inducing hypoxia), shunt declined at a sl ow linear rate of 2.37 x 10(-4)/min, which was independent of lung inf lation (p 0.65 analysis of variance [ANOVA]) and significantly differe nt from zero (p < 0.001). The stability of cardiac output in this anim al model, as measured sequentially try thermodilution, was confirmed i n a further 20 animals. The experiments provide evidence for a slow in tensification of blood-flow diversion at a rate that does not depend u pon the degree of lung inflation. Whether this change is a feature of hypoxic constriction itself, or some modulation of it, remains unclear .