Heart rate reduction improves myocardial ischemia in swine: role of interventricular blood flow redistribution.

Regional myocardial blood flow (MBF) distribution and function upon slowing the heart rate (HR) during ischemia were studied in anesthetized swine, a species without coronary collaterals. Perfusion of the left anterior descending artery by a pump allowed controlled production of regional ischemia. Slowing tachycardia by electrical pacing (127 to 87 beats/min) caused marked improvement of regional dysfunction [% wall thickening (WTh) from 9 to 27%] and increased subendocardial MBF [from 0.31 to 0.55 ml.min-1.g-1 (P less than 0.001)] without change of subepicardial MBF. Total left ventricular (LV) MBF increased, whereas right ventricular (RV) MBF fell by 18% (P less than 0.02). The mechanism of MBF changes during slowed HR was assessed by surgically excluding the RV and comparing findings with previous experiments with RV intact when HR was slowed from 96 to 60 beats/min. A similar improvement of regional LV function occurred (8% vs. 30% WTh) with the RV excluded, but without a change in total flow to the LV bed, whereas subendocardial MBF increased and subepicardial MBF fell, indicating transmural redistribution only. These findings show that the RV vascular bed can contribute to LV perfusion in swine during ischemia, and they document the potential for "reverse RV steal" during slowed heart rate in this setting.