Increased myocardial microvascular utilisation and flow during atrial pacing

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Abstract
To determine the influence of heart rate on regional coronary flow, the coronary vascular and microvascular responses to increased heart rate were examined in anaesthetised open chest New Zealand white rabbits. Coronary blood flow was measured with labelled microspheres. Fluorescein isothiocyanate (FITC) -dextran (150 mg·kg−1) was given to mark the perfused microvessels. The FITC-dextran was given at various intervals before removal of the heart. Alkaline phosphatase stain was used to locate the total microvasculature. Increase in heart rate from 272(SD23) to 339(13) beats·min−1 increased the coronary blood flow from 198(56) to 288(82) ml·min−1·100 g−1. The subendocardial/subepicardial flow ratio was not altered by pacing. Total capillary density averaged 2508(363) mm−2 and arteriolar density was 1.74(4.1)·mm−2. There were no significant regional or treatment differences in these values. The increase in blood flow with pacing was accompanied by a uniform increase in the percentage of the microvasculature perfused in the subepicardial and the subendocardial layers of the left ventricular free wall, from 58(6)% to 75(10)% for capillaries and from 54(17)% to 81(19)% for arterioles. These results show that enhanced heart rate increases the coronary blood flow and the density of the perfused microvessels uniformly across the ventricular wall. Thus both the coronary vascular and microvascular reserves are used in the rabbit heart during tachycardia.

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