The effects of changes in heart rate and aortic systolic pressure on left ventricular myocardial oxygen consumption in lambs

To determine whether changes in heart rate and aortic systolic pressure contribute equally to the determination of left ventricular myocardial oxygen consumption, we independently varied heart rate and pressure and compared the resultant oxygen consumption for similar rate-pressure products. In 6 young lambs which underwent atrioventricular node ablation, we varied heart rate by ventricular pacing at 250 beats/min, 300 beats/min, and 120 beats/min while aortic pressure remained stable and varied aortic systolic pressure by infusion of phenylephrine (to 132 +/- 15 mm Hg and 155 +/- 14 mm Hg) and by infusion of sodium nitroprusside (to 79 +/- 6 mm Hg) while heart rate was maintained stable at 200 beats/min. The 3 levels of change in aortic systolic pressure were chosen so that the ratepressure product during the pressure changes matched the rate-pressure product during the heart rate changes. We found that left ventricular myocardial oxygen consumption was the same at all 3 levels of the rate-pressure product whether heart rate was changed and pressure remained stable or pressure was changed and heart rate remained stable. Also, the correlation between oxygen consumption and the rate-pressure product was similar for both heart rate and pressure changes. During nitroprusside infusion at a fixed heart rate, oxygen extraction was significantly lower than during pacing at a heart rate of 120 beats/min when the rate-pressure product was comparable because of the direct vasodilatory effects of nitroprusside. We conclude that heart rate and aortic systolic pressure contribute equally to left ventricular myocardial oxygen consumption at the same rate-pressure product, even though there may be differences in myocardial blood flow and oxygen extraction.