UTILIZATION OF OXYGEN BY THE CONTRACTILE APPARATUS IS DISTURBED DURING REPERFUSION OF POSTISCHEMIC MYOCARDIUM

Post-ischaemic ventricular function remains depressed (=myocardial stu nning) despite nearly normal coronary blood flow during reperfusion. I n order to illuminate the causes of this phenomenon,we studied the rel ationship between ventricular function and myocardial oxygen consumpti on (MVO(2tot)) in experiments on 15 isolated rabbit hearts perfused wi th erythrocyte suspension (hct=30%). Left ventricular systolic functio n was assessed by measuring aortic flow (ml.min(-1)), peak systolic pr essure (LVP(max)), dP/dt(max), and early relaxation in terms of dP/dt( min) during control and 30 min after the onset of reperfusion, followi ng 20 min global no-flow ischaemia. The pressure-volume area was calcu lated as a measure of total mechanical energy. The external mechanical efficiency (E(ext)) was assessed from stroke work and MVO(2tot). Both contractile efficiency (E(con) = inverse slope of the MVO(2)-PVA rela tionship) and MVO(2) of the unloaded contracting heart (MVO(2uml) = ba sal MVO(2)+MVO(2) for excitation-contraction coupling) were calculated using pressure-volume area and MVO(2tot).Results: At matched heart ra re (149 +/- 30 vs 147 +/- 31 min(-1); mean +/- SD) and end-diastolic v olume (1.3 +/- 0.2 ml), the systolic variables were significantly decr eased in the stunned myocardium: aortic flow: 38 +/- 13 vs 9 +/- 11 ml .min(-1), LVP(max): 112 +/- 19 vs 74 +/- 18 mmHg, and dP/dt(max): 1475 +/- 400 vs 1075 +/- 275 mmHg.s(-1). Likewise, dP/dt(min) was signific antly impaired (-1275 +/- 250 vs -975 +/- 250). The decrease in pressu re-volume area (570 +/- 280 vs 270 +/- 200 mmHg.ml. 100 g(-1)) was not statistically significant. In contrast, both E(ext)(0.75 +/- 0.29 vs 0.18 +/- 0.26 arbitrary units) and E(con) (31 +/- 18 vs 14 +/- 7%) wer e significantly decreased whereas MVO(2tot)(40 +/- 9 vs 34 +/- 8 mu l. beat(-1).100 g(-1)) and MVO(2unl)(26 +/- 9 vs 22 +/- 6 mu l.beat(-1).1 00 g(-1)) were not. Summary: Ventricular function after. brief episode s of ischaemia is decreased whereas MVO(2tot) is maintained, ie. exter nal efficiency is decreased MVO(2) for the unloaded contraction remain ed unchanged, indicating that MVO(2) for excitation-contraction coupli ng is inappropriately high for the depressed contractile state. The de creased contractile efficiency indicates further that O-2 utilization of the contractile apparatus is disturbed during reperfusion.