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

Abstract

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.

Fig. 1.

Schematic drawing of left ventricular (LV) pressure, aortic (Ao) pressure. and coronary blood flow (CBF) delineating the various segments of phasic CBF used for data analysis. See methodsfor terms as defined in the text.

Fig. 2.

Representative recording of left ventricular pressure (LVP), CBF, and aortic blood flow (ABF) at rest and during graded treadmill exercise. Aortic pressure (AoP) is presented at rest to illustrate the position of the dicrotic notch in relation to aortic flow. Vertical lines represent the borders of systole (S) and diastole (D). mph, miles/h.

Fig. 3.

Total antegrade and retrograde coronary blood flow during systole and diastole expressed as mean flow rates (A) and as a percent of net flow across the cardiac cycle (B) at rest (R) and during graded treadmill exercise (2–5 mph). Total retrograde flow values include early- and late-systolic flow reversals. Values are means ± SE. *P < 0.05 vs. all other speeds. **P < 0.05 vs. at rest and 2 mph. †P < 0.05 vs. at rest and 2 and 3 mph.

Fig. 4.

Early (E)- and late (L)-systolic retrograde coronary flows expressed as mean flow rates (A) and as a percent of net flow across the cardiac cycle (B) at rest (R) and during graded treadmill exercise (2–5 mph). Values are means ± SE. *P < 0.05 vs. at rest. **P < 0.05 vs. zero. †P < 0.05 vs. 5 mph., ‡P < 0.05 vs. 3 mph.

Fig. 5.

Representative results of aortic wave-intensity analysis in systole at rest (A) and within 20 s after exercise at 5 mph (B). Net, forward-going and backward-going wave intensities (WI) are shown in relation to CBF. Wave intensity units: W/m²; CBF units: ml/min. Vertical lines represent aortic valve opening (AO) and closure (AC). Black bars in upper right of each panel equal 5 ms. C: peak wave intensities in late systole prior to aortic valve closure at rest and immediately following exercise at 5 mph. D: peak wave intensity plotted against maximum rate of fall in LV pressure (dP/dt_min). Data were fitted to a first-order linear regression equation. Results for individual animals (C and D) along with group means ± SE (C) are presented. *P < 0.05 vs. at rest.