Changes in oxygen-haemoglobin affinity and myocardial function during extracorporeal perfusion in immature pigs

Abstract

An alteration in oxygen-haemoglobin affinity, as indicated by the partial pressure of oxygen at 50% haemoglobin saturation (P50), may theoretically affect oxygen delivery to the myocardium and, thus, alter myocardial tissue PO2 and possibly myocardial function. We studied the effect of changes in P50 on heart function, metabolism, and coronary sinus PO2 in 10 anaesthetized immature domestic pigs. To facilitate exchange transfusions and control of preload (left ventricular end diastolic pressure, LVEDP), afterload (mean arterial pressure), cardiac output, and heart rate, the animals were placed on right heart bypass. Stroke work (SW), myocardial oxygen consumption (MVO 2), coronary blood flow (CBF), arterial-coronary sinus O2 content difference (CaO2 -CcsO2), and P50 were measured with a control blood prime and after an exchange transfusion with blood having either a low (Group I) or high (Group II) affinity for oxygen. Results (Mean ± SEM, Control versus Test): P50 (mmHg) at 37°C and pH 7.4 increased in Group 1 (35.5 ± 0.7 versus 40.1 ± 1.2, p <0.01) and decreased in Group II (34.7 ± 1.3 versus 26.3 ± 0.5, P <0.001). At 8mmHg LVEDP, SW (gram·metres, g.m) remained constant in Group I (12.2 ± 1.1 versus 11.7 ± 0.8 g·m), but decreased (p <0.05) in Group II (11.7 ± 0.9 versus 8.6 ± 0.61 g.m). In addition, Starling curves relating SW to LVEDP between 4 and 18mmHg were uniformly depressed in Group II but not Group I. No significant change in MVO2 occurred in either group. Group II demonstrated decreased CaO2- CcsO2 (9.3 ± 0.8 versus 6.0 ± 0.3ml O2/dl, p <0.05) and increased CBF (121.6 ± 16.5 versus 188.9 ± 16.9ml/min/1 00gm, p <0.01). PcsO2 increased significantly in Group I (26.6 ± 4.2 versus 36.1 ± 2.7, P <0.02) but not Group II. In conclusion, a decrease in P50 appeared to decrease myocardial function despite a lack of change in PcsO2 or MVO2. A decrease in P50 also led to an increase in CBF. An increase in P50 did not alter function, but did increase coronary sinus and, presumably, tissue PO2.