Quantitative analysis of exercise-induced enhancement of early- and late-systolic retrograde coronary blood flow

Coronary blood flow (CBF) is reduced and transiently reversed during systole via cardiac contraction. Cardiac contractility, coronary tone, and arterial pressure each influence systolic CBF (CBF(SYS)), particularly by modulating the retrograde component of CBF(SYS). The effect of concurrent changes in these factors on CBF(SYS) during dynamic exercise has not been examined. Using chronically instrumented swine, we hypothesized that dynamic exercise enhances retrograde CBF(SYS). Phasic CBF was examined at rest and during treadmill exercise [2-5 miles/h (mph)]. Absolute values of mean CBF over the cardiac cycle (CBF(CYCLE)) as well as mean CBF in diastole (CBF(DIAS)) and mean CBF(SYS) were increased by exercise, while relative CBF(DIAS) and CBF(SYS) expressed as percentage of mean CBF(CYCLE) were principally unchanged. Early retrograde CBF(SYS) was present at rest and increased in magnitude (-33 +/- 4 ml/min) and as a percent of CBF(CYCLE) (-0.6 +/- 0.1%) at 5 mph. This reversal was transient, comprising 3.7 +/- 0.3% of cardiac cycle duration at 5 mph. Our results also reveal that moderately intense exercise (>3 mph) induced a second CBF reversal in late systole before aortic valve closure. At 5 mph, late retrograde CBF(SYS) amounted to -53 +/- 11 ml/min (-3.1 +/- 0.7% of CBF(CYCLE)) while occupying 11.1 +/- 0.3% of cardiac cycle duration. Wave-intensity analysis revealed that the second flow reversal coincided with an enhanced aortic forward-going decompression wave (vs. rest). Therefore, our data demonstrate a predictable increase in early-systolic CBF reversal during exercise and additionally that exercise induces a late-systolic CBF reversal related to the hemodynamic effects of left ventricular relaxation that is not predictable using current models of phasic CBF.