Microheterogeneity of myocardial blood flow in rabbit hearts during normoxic and hypoxic states

The goal of this study was to evaluate microheterogeneity of myocardial blood flow and its dependence on arterial O2 tension (PaO2). We measured within-layer distribution of regional blood flows in the left ventricles of anesthetized rabbits in both normoxic and hypoxic states with myocardial region sizes in the range of 0.01-1.0 mm2. A novel method of digital radiography combined with the technique of 3H-labeled desmethylimipramine deposition enabled us to visualize and accurately quantitate regional blood flow at such high levels of resolution. To analyze myocardial blood flow patterns, we computed the coefficient of variation (CV) and the correlation between adjacent regional flows (CA). The CA values were larger in the hypoxic state (PaO2 = 26 +/- 5 mmHg) than in the normoxic state (PaO2 = 97 +/- 20 mmHg) at all levels of resolution (P < 0.001). In the normoxic state, there was a transmural difference in CA (P < 0.001); CA increased with depth of the left ventricle (from subepicardium to subendocardium). However, the relation between CA and the depth of the left ventricle was not statistically significant in the hypoxic state. The CV values were smaller in the hypoxic state than in the normoxic state at all levels of resolution (P < 0.001). When the degree of resolution was reduced from 0.01 to 1.0 mm2, CV decreased by 75% in the normoxic and by 69% in the hypoxic state. Thus we conclude that 1) the decrease in PaO2 increases similarity of blood flows in nearby regions and decreases myocardial blood flow heterogeneity, and 2) similarity of regional blood flows increases with depth of the left ventricle in the normoxic state, but this transmural difference disappears in the hypoxic state.