Asynchronous transmural perfusion during coronary reactive hyperaemia

Dynamic variation in the transmural distribution of myocardial blood flow was studied in 36 anaesthetised open-chest dogs during the course of the reactive hyperaemia following a 90 s occlusion of the left anterior descending coronary artery. Myocardial blood flow was estimated from tissue uptake of radioactive microspheres, 9 to 10 micrometers in diameter. Endo/epi flow ratios indicated asynchronous reactive hyperaemic responses across the free wall of the left ventricle; flow was preferentially to subepicardium during the initial (pre-peak) phase, whereas flow was preferentially to subendocardium during the prolonged recovery phase. The mean of endo/epi flow ratios obtained during the peak plateau of coronary artery reactive hyperaemia indicated essentially uniform transmural distribution. However, further analysis indicated that this was result of ratios less than 1 early in the peak plateau and ratios greater than 1 late in the peak plateau. Reversal of the endo/epi flow ratios due to regionally asynchronous maximum flows is responsible for the prolonged plateau at peak flow observed in measurements of coronary artery reactive hyperaemia. Transmural asynchrony of peak reactive hyperaemic flow may also account for underestimates in vasodilator capability of ischaemia based on measurements of either coronary artery or myocardial blood flow.