Left ventricular mechanoenergetics during asynchronous left atrial-to-aortic bypass. Effects of pumping rate on cardiac workload and myocardial oxygen consumption.

ABSTRACT

The purpose of this study was to analyze left ventricular energetics during asynchronous, pulsatile left atrial to aortic bypass in the failing heart with the use of the pressure-volume relationship. In 12 anesthetized Holstein calves (body weight 94 +/- 7 kg), 10 microns microspheres (3.3 x 10(7) +/- 1.1 x 10(7)/100 gm left ventricular weight) were injected into the left main coronary artery to induce heart failure. Baseline left ventricular end-systolic elastance significantly decreased from 7.9 +/- 0.7 to 5.5 +/- 0.4 mm Hg/ml 100 gm left ventricular weight. Left ventricular pressure was measured with a micromanometer, and ultrasonic dimension transducers measured left ventricular orthogonal diameters. Ellipsoidal geometry was used to calculate simultaneous left ventricular volume. End-systolic elastance, pressure-volume area, external work, potential energy, and myocardial oxygen consumption were analyzed during steady-state contractions. After pre-pulsatile left atrial to aortic bypass measurements were taken, the measurements were repeated during asynchronous pulsatile left atrial to aortic bypass at the maximal pumping rate (69 +/- 13 beats/min) termed 100%, and then 80%, 60%, and 40% of the maximal pumping rate in the full to empty mode. With increases in pumping rate, pressure-volume area and external work proportionally decreased, whereas potential energy remained unchanged except for 100% of maximal pumping rate. Pressure-volume area correlated linearly with myocardial oxygen consumption during asynchronous pulsatile left atrial to aortic bypass (r = 0.971). As a result, pumping rate correlated linearly with conservation of myocardial oxygen consumption (r = 0.998). In conclusion, decreased pressure-volume area accounts for the reduction in myocardial oxygen consumption during asynchronous pulsatile left atrial to aortic bypass. Conservation of myocardial oxygen consumption is mainly attributed to the reduction of external work.