Coronary vasodilation after a single ventricular extra-activation in the conscious dog

This study was undertaken to determine if coronary blood flow can be regulated in response to a transient increase in cardiac metabolic demand. Eight conscious dogs with experimentally produced complete heart block, a chronically implanted electromagnetic flow probe on the left circumflex coronary artery, and fluid-filled catheters for measurement of left ventricular and aortic pressures were studied. At a paced heart rate of 60 beats/min, a single ventricular extra-stimulus was introduced with a delay of 150-200 msec from the preceding R-wave. The extra-stimulus produced a ventricular extra-activation, but not a discrete mechanical extra-systole. The ensuing beats exhibited systolic potentiation, manifest by a 50 +/- 8% increase from control in maximum left ventricular dp/dt in the first potentiated beat, presumably accompanied by increased myocardial oxygen demand. In the diastole immediately following the first potentiated systole, the coronary vascular resistance index (mean aortic pressure/mean coronary flow in that diastole) fell significantly from control by 12 +/- 2%. The results indicate that a transient increase in cardiac metabolic demand is followed immediately by a compensatory coronary vasodilation that occurs within the same cardiac cycle.