REDUCED ARTERIAL O-2 SATURATION DURING SUPINE EXERCISE IN HIGHLY TRAINED CYCLISTS

Performance of intense dynamic exercise in highly trained athletes is associated with a reduced arterial haemoglobin saturation for O-2 (Sao (2)) and lower arterial Po-2 (Pao(2)). We hypothesized that compared w ith upright exercise. supine exercise would be accompanied by a smalle r reduction in Sao(2) because of a lower maximal O-2 uptake (Vo(2max)) and/or a more even ventilation-perfusion distribution. Eight elite bi cyclists completed progressive cycle ergometry to exhaustion in both p ositions with concomitant determinations of ventilatory data, arterial blood gases and pH. During upright cycling Vo(2max) averaged 75+/-1.6 mt O-2 min(-1) kg(-1) (+/-SEM) and it was 10.6+/-1.7% lower during su pine cycling (P < 0.001). Also the maximal pulmonary and alveolar vent ilation were lower during supine cycling (by 15+/-2% and 21+/-3%. resp ectively; P < 0.001) which related to a 0.8+/-0.1 L lower tidal volume (P < 0.001). In all subjects and independent of work posture Pao(2) a nd Sao(2) decreased from rest to exhaustion (from 99+/-3 to 82+/-2 Tor r and 98.1+/-0.2 to 95.2+/-0.4%. respectively: P < 0.001); alveolar-ar terial Po-2 difference increased from 6+/-2 to 37+/-3 Torr in both bod y positions. At exhaustion arterial Pco(2) was lower in upright than i n supine (33.4+/-0.6 vs. 35.9+/-0.9 Torr: P < 0.01). suggesting a grea ter relative hyperventilation in upright. Arterial pH was similar in u pright and supine at rest (both 7.41+/-0.01) and at exhaustion (7.31+/ -0.01 vs. 7.32+/-0.01, respectively). We conclude that despite a lower Vo(2max) and supposedly an improved ventilation-perfusion distributio n. altering body position from upright to supine does not influence ar terial O-2 desaturation during intense exercise.