The hemodynamic and metabolic basis for decreased aerobic capacity in the supine exercise compared with the upright exercise in coronary artery disease.

To elucidate the mechanism of reduced exercise tolerance in the supine position, 14 patients with coronary artery disease were studied by both supine ergometer exercise and upright treadmill exercise. Maximal oxygen consumption in the supine position amounted to 80% of that in the upright position (1110 +/- 453 vs 1387 +/- 470 ml/min; p less than 0.001). Maximal cardiac output was identical in both positions (12.07 +/- 4.44 vs 12.55 +/- 4.49 l/min; ns). Maximal arteriovenous oxygen difference in the supine position amounted to 83% of that in the upright position (9.22 +/- 1.92 vs 11.14 +/- 1.88 vol%; p less than 0.01). Thus, the lower maximal oxygen consumption in the supine position was not caused by the decreased cardiac output but by the impaired augmentation of arteriovenous oxygen difference. Lactate concentration at the same oxygen consumption was higher in the supine position, which means early augmentation of anaerobic metabolism. We concluded that the aerobic capacity in the supine position was significantly lower than that in the upright position in patients with coronary artery disease, and the impaired utilization of transported oxygen was considered to be one of the mechanisms of the decreased aerobic capacity in the supine position.