VA/Q inhomogeneity and AaDO2 in man during exercise: effect of SF6 breathing.

Pulmonary gas exchange was studied in five normal subjects both at rest and during moderate steady-state exercise on a bicycle ergometer while breathing a) room air and b) a mixture of 20.9% O2-balance sulfur hexafluoride (SF6). The alveolar-arterial oxygen pressure differences (AaDO2) widened significantly from rest to exercise. Breathing the O2-SF6 mixture reduced the AaDo2 significantly from 10.9 to 4.2 Torr at rest and from 15.5 to 10.1 Torr during exercise (P less than or equal to 0.01). There were no concurrent changes in metabolism, cardiac output, or heart rate during the SF6 breathing. Possible changes in the anatomic shunt fraction, alveolar-end-capillary equilibration, or the distribution of blood flow cannot account for these observations. We conclude that the AaDO2 increase during exercise reflects an inhomogeneity of ventilation-perfusion ratios (VA/Q) most probably arising within regions of the lung (intraregional inhomogeneity) rather than between regions (interregional inhomogeneity).

Effect of induced erythrocythemia on aerobic work capacity.

The aerobic work capacity of 11 highly trained runners was studied employing a double-blind design 1) before phlebotomy (C1), 2) following restoration of normocythemia (C2), 3) after a sham reinfusion of 50 ml of saline (sham), 4) following autologous reinfusion of approximately 900 ml of freeze-preserved blood (reinfusion), and 5) upon reestablishment of control hematologic levels after erythrocythemia (C3). There were no hematologic differences among C1, C2, sham, and C3, but following reinfusion, hemoglobin was significantly elevated (15.7-16.7 g . 100 ml-1). Maximum O2 consumption (VO2max) and running time to exhaustion were significantly increased 24 h postreinfusion (5.11-5.37 l . min-1 and 7.20-9.65 min, respectively) and 7 days postreinfusion. When sham preceded reinfusion, VO2 max and time to exhaustion were the same as control. However, 16 wk postreinfusion, despite the return to normal hematologic values, VO2max remained significantly above control levels at sham and C3. These findings indicate that there is a distinct increase in VO2max following induced erythrocythemia and suggest that oxygen transport limits maximal aerobic capacity.