The effect of hypoxia on capillary flow direction in the isolated perfused rat heart.

To characterize regional capillary flow direction in the isolated perfused heart subjected to acute hypoxia.A technique incorporating sequential infusion of differently coloured 10 microns microspheres into hearts of adult male Sprague-Dawley rats was employed in two series of experiments. In series A, a control group (n = 10) received three infusions of coloured microspheres (each distinctly coloured) in sequence during normoxic perfusion. A 'hypoxic' group (n = 12) received the initial two infusions under identical conditions, with the third infusion under conditions of experimental hypoxia. Hearts in series B (n = 5) received two differently coloured infusions, the initial under normoxic conditions and the second during hypoxia. Subsequent histological examination of 40 microns sections obtained from midmyocardial (MID) and subendocardial (SUB-E) regions of the heart revealed flow vectors based on the sequence of colours trapped within neighbouring capillaries.Series A-A prevalence in concurrent flow was observed among neighbouring capillaries, with significantly lower percentages in SUB-E than in MID (P < 0.01). A higher percentage of countercurrent flow was revealed subsequent to the hypoxic intervention than during the earlier normoxic period, as well as in comparison with normally perfused controls (P < 0.01). Flow vector observations from series B depicted lower proportions of concurrent flow compared with the two-colour normoxic interval in 'hypoxic' hearts from series A (P < 0.05).Reversals in capillary flow direction, suggested by the reorientation of final injected microspheres relative to the positions of earlier injected microspheres, were interpreted as a response to sustain myocyte oxygenation due to severe reductions in arterial oxygen tension.