Regional Asynchrony of Segmental Contraction May Explain the "oxygen Consumption Paradox" in Stunned Myocardium

Despite apparently depressed function, stunned myocardium maintains oxygen consumption and has the capacity to increase contractility with inotropic stimulation. We hypothesized that during stunning, O2 demand is maintained because regional segment work is performed, but is asynchronous with global left ventricular contraction, and that inotropic stimulation would restore regional work and synchrony. Thirteen open-chest anesthetized dogs were subjected to three left anterior descending (LAD) coronary artery occlusions (10 min) and reperfusions (15 min) to produce regional myocardial stunning. Segment shortening and force development were measured independently and simultaneously in the LAD (experimental) region and circumflex (control) regions. Regional myocardial work was calculated as the integrated product of instantaneous force and shortening, during two periods: 1) over the entire cardiac cycle (Positive Work), and 2) limited to the systolic portion of the cardiac cycle (Systolic Work). Regional myocardial O2 consumption (MVO2) was calculated from regional blood flow (radiolabeled microspheres) and O2 saturation data (microspectrophotometry). Occlusion of the LAD produced a delay in onset of segment shortening in the ischemic region, but not in regional force development. A time delay of 67-81 ms persisted through the three stages of occlusions and reperfusions. Systolic regional work was depressed to a greater extent (924 +/- 182 to 149 +/- 118 g*mm*min-1) than total positive regional work (1437 +/- 337 to 857 +/- 174 g*mm*min-1). Regional subepicardial MVO2 in the stunned region was not different than in the control region (7.3 +/- 1.5 vs. 6.9 +/- 1.4 ml O2*min-1*100 g-1). Local infusion of isoproterenol reversed the delay in regional shortening from 73 +/- 7 to 21 +/- 8 ms, thereby augmenting systolic work (298%) more than positive work (60%), without a significant increase in MVO2 (7.3 +/- 1.5 to 10.5 +/- 3.2 ml O2*min-1*100 g-1). It is concluded that myocardial stunning decreases regional systolic work due to regional mechanical asynchrony, while MVO2 is used supported total positive work which was not significantly reduced. Isoproterenol restores regional work by restoring synchrony, without greatly affecting regional MVO2.