Myocardial oxygen extraction and oxygen-hemoglobin equilibrium curve during moderate exercise.

Conditions of oxygen extraction by the myocardium have been studied in 12 subjects (44 +/- 9 years old) with pure mitral stenosis without clinical, metabolic or electrical sign of coronary insufficiency. Oxygen-hemoglobin equilibrium curves (OHEC) have been determined on arterial, mixed venous and coronary sinus blood, at rest and during a moderate (60 W, 10 min) exercise performed on a bicycle ergometer in supine position. Physiological values, at rest and during exercise, of the following functional parameters of the OHEC were determined in the in-vivo conditions of pH and PCO2 : P50, nHill, DS max = maximal value of OHEC slope (DS = delta SO2/delta PO2), PDSmax, SDS max. The concentration of plasma electrolytes capable to modify one of these parameters was controlled in each blood sample. In coronary sinus blood, P50 rises from 27.5 +/- 1.7 to 28.9 +/- 1.6 Torr during exercise (p less than 0.01). At rest, Hill's n in myocardial venous blood (2.67 +/- 0.09) is significantly higher than in arterial blood (2.61 +/- 0.08, p less than 0.01). A decrease in DS max (2.67 +/- 0.20 to 2.53 +/- 0.12%. Torr -1; p less than 0.01) and an increase in PDSmax (20.9 +/- 1.9 to 22.6 +/- 2.1 Torr, p less than 0.01) are observed. At rest, the myocardial "venous point" (PCSO2, SCSO2) is not significantly distinct from the maximal slope point (PDSmax, SDS max). After 1 min of exercise, a small gap appears which becomes significant at the end of the exercise (PDS max-PCSO2 = 0.2 Torr at rest, 2.7 Torr after exercise; SDS max-ScsO2 = 4.1% at rest, 7.9% after exercise). DS value at the myocardial venous point is only 2% at rest and 6% after exercise, lower than its maximal value. The gap between venous point and DS max point could constitute an error signal in the regulation loop of the coronary circulation. The existence of a physiological receptor, sensitive to instant variations in myocardial tissue PO2 and able to maintain venous point near DS max point could be considered.