Consequences of an acute increase in P50 in anaesthetized guinea pigs

In anaesthetized guinea pigs, ventilated with ambient air, the peripheral haemodynamics and oxygen transport characteristics have been studied following a blood exchange transfusion with rat erythrocytes suspended in guinea pig plasma. Since the rat haemoglobin exhibited a lower oxygen affinity than guinea pig haemoglobin, the oxygen partial pressure at 50% of oxygen haemoglobin saturation (P50) increased from 25.2 +/- 1.1 to 37.2 +/- 0.9 mm Hg (n = 10). This increase in P50 was accompanied by a significant increase in arterial oxygen partial pressure (PaO2) and in arterio-venous difference (AVDO2). Cardiac output (Q) was decreased significantly, but oxygen consumption (VO2) remained within control values. The increase in P50 was associated with a venous oxygen partial pressure (P-VO2) which remained constant but an increase in blood lactate concentration was observed. Control exchange transfusion with fresh guinea pig blood had no effect on acid-base status, on oxygen transport, or on peripheral resistance. The sudden reduction in haemoglobin oxygen affinity induced an increase in peripheral resistance with a decrease in cardiac output, the arterial systemic pressure being maintained. These results suggested that an acute decrease in haemoglobin oxygen affinity was compensated for by a simultaneous diminution of overall tissue blood flow and reduction of capillary recruitment.