Relationship between local oxygen consumption and local and external cardiac work: effect of tachycardia

This study was designed to determine the extent of coupling between regional myocardial segment work and corresponding regional oxygen consumption, and to examine whether tachycardia induced changes in regional work are translated into corresponding changes in external cardiac work. In the open chest anaesthetised dog, the heart was paced at frequencies of 120-270 beats.min-1. Global and regional myocardial O2 supply, consumption, and balance were evaluated at each heart rate, and correlated with corresponding functional changes. Global cardiac function was evaluated from aortic flow, blood pressure, and left ventricular pressure. Coronary sinus flow and O2 saturation were used to calculate O2 consumption. The integrated multiple of myocardial shortening (ultrasonic dimension crystals) by corresponding force (strain gauge arch) during an averaged beat was used to express regional segment work. Regional coronary blood flow was measured with radioactive microspheres, and microspectrophotometry was used to evaluate O2 saturation in small arteries and veins. These indices were used to calculate regional myocardial oxygen consumption. NADH redox levels were recorded by surface fluorometry, and were found to increase with heart rate by up to 67%. Increasing heart rate from 120 to 180 beats.min-1 increased regional work from 3040(SEM 220) to a peak of 4290(280) mm.g-1.min-1, whereas external cardiac work did not increase [67.0(2.6) to 65.3(4.4) mm Hg.litre-1.min-1] and fell further at the highest rates. Regional oxygen consumption increased from 6.16(0.47) to 8.29(0.53) ml O2.min-1.100 g-1 and was linearly related to regional work at all heart rates (r = 0.971, p less than 0.05). External cardiac work fell by about 26% whereas global myocardial oxygen consumption increased by 49% during tachycardia. It is concluded that myocardial oxygen consumption is more closely related to regional segment work than to external work, and that tachycardia significantly raises the oxygen cost of external work of the heart.