Abstract
Recent studies have shown that left ventricular oxygen consumption (VO2) linearly correlates with systolic pressure-volume area (PVA), and VO2 is predicted by the empirical equation VO2=A·PVA+B·Emax+C. When PVA is maintained at zero in an unloaded condition, unloaded VO2 increases with increases in contractility (Ema), which indicates oxygen-wasting due to changes in contractility. To investigate the role of ejection velocity in the oxygen-wasting (Vmax) -unloaded VO2 relation in normal and hyperthyroid rabbit hearts. Thyrotoxic stress in the rabbit is known to alter the myosin isoform component from predominantly V3 (low mobility) to predominantly V1 (high mobility). Afer the establishment of retrograde coronary perfusion, the heart was isolated and a latex balloon was inserted into the left ventricular cavity. Then, the balloon was connected to a volume controller. Ejection velocity was measured using an isovelocity method under different coronary perfusions. Emax, PVA, VO2 were also measured under different coronary perfusions, The Vmax-unloaded VO2 relation and the Emax-unloaded VO2 relation was linear in normal heart. By contrast, in the hyperthyroid heart, the Emax-unloaded VO2 relasion was linear under lower coronary perfusion, but became steeper under higher coronary perfusion. When the slope of the Emax-unloaded VO2 became steeper, Vmax showed further increase. Our results suggest that increases in ejection velocity are largely responsible for the oxygen-wastiong effect in the hyperthyroid heart.
