REGIONAL ASYNCHRONY OF SEGMENTAL CONTRACTION MAY EXPLAIN THE OXYGEN-CONSUMPTION PARADOX IN STUNNED MYOCARDIUM

Despite apparently depressed function, stunned myocardium maintains ox ygen consumption and has the capacity to increase contractility with i notropic stimulation. We hypothesized that during stunning, O-2 demand is maintained because regional segment work is performed, but is asyn chronous with global left ventricular contraction, and that inotropic stimulation would restore regional work and synchrony. Thirteen open-c hest anesthetized dogs were subjected to three left anterior descendin g (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 myoca rdial work was calculated as the integrated product of instantaneous f orce and shortening, during two periods: 1) over the entire cardiac cy cle (Positive Work), and 2) limited to the systolic portion of the car diac cycle (Systolic Work). Regional myocardial O-2 consumption (MVO(2 )) was calculated from regional blood flow (radiolabeled microspheres) and O-2 saturation data (microspectrophotometry). Occrusion of the LA D produced a delay in onset of segment shortening in the ischemic regi on, but not in regional force development. A time delay of 67-81 ms pe rsisted through the three stages of occlusions and reperfusions. Systo lic regional work was depressed to a greater extent (924 +/- 182 to 14 9 +/- 118 gmm*min(-1)) than total positive regional work (1437 +/- 33 7 to 857 +/- 174 gmm*min(-1)). Regional subepicardial MVO(2) in the s tunned region was not different than in the control region (7.3 +/- 1. 5 vs. 6.9 +/- 1.4 ml O-2min(-1)*100 g(-1)). Local infusion of isoprot erenol reversed the delay in regional shortening from 73 +/- 7 to 21 /- 8 ms, thereby augmenting systolic work (298%) more than positive wo rk (60%), without a significant increase in MVO(2) (7.3 +/- 1.5 to 10. 5 +/- 3.2 ml O-2min(-1)*100 g(-1)). It is concluded that myocardial s tunning decreases regional systolic work due to regional mechanical as ynchrony, while MVO(3), is used supported total positive work which wa s not significantly reduced. Isoproterenol restores regional work by r estoring synchrony, without greatly affecting regional MVO(2).