Videomicroscopy provides accurate in vivo assessment of pulmonary microvascular reactivity in rabbits

When defining the mechanism of hypoxic pulmonary vasoconstriction (HPV), in vestigators have employed ex vivo preparations because of the belief that a ccurate, quantitative assessment of pulmonary microvessels could not be obt ained in vivo. We hypothesize that accurate, quantitative assessment of pul monary microvascular reactivity can be performed using a simple, in vivo pr eparation. Our aim was to provide this quantitative assessment in a defined animal model, and to confirm that the chosen preparation could discriminat e changes in microvascular reactivity as influenced by endogenous mediators . New Zealand rabbits were instrumented for in vivo microscopy and direct m easurement of subpleural arterioles, Rabbits were first randomized to eithe r control (n = 7) or endotoxin (n = 5), infusion of Escherichia coli lipopo lysaccharide (200 Fg/kg), All rabbits were then exposed to a repeated proto col of normoxia (21% O-2) for 20 min and then hypoxia (15% O-2) for 10 min over 2 h. The changes in arteriole diameter were measured at the end of eac h interval. Normal pulmonary arterioles repeatedly constrict 15 +/- 3.5% du ring hypoxia, Altering endogenous vasoactive mediators, as with infusion of endotoxin, caused a loss of hypoxia-induced vasoconstriction. The results of our study validate this experimental preparation for the reliable quanti fication of pulmonary microvascular reactivity and investigation of hypoxic pulmonary vasoconstriction under both normal and pathologic conditions.