Limitations to the Efficiency of Pulmonary Gas Exchange During Exercise in Man

Abstract

The human lung must function over a wide range of metabolic demands and environmental conditions. It is not rare for oxygen consumption ($\rm\dot{v}$O₂) to vary from 3-5 ml · kg · min⁻¹ at rest to as much as 70 ml · kg · min⁻¹ during exercise only moments later, or for inspired PO₂ (PIO₂) to range from 150 Torr (sea level) to 80 Torr (equivalent altitude 4500 meters) or less over a period of hours, to days. The ability to function in these different circumstances comes with a small price: although the lung is remarkably efficient at rest at sea level, it becomes less so at higher $\rm\dot{v}$O₂ (Asmussen and Nielsen, 1960), particularly at high altitude. For example, in healthy resting subjects the ideal alveolar-arterial PO₂ difference (A-aDO₂) is normally only 5-10 Torr, but it may increase to 25 Torr or more during neavy exercise (Dempsey et al., 1984). While this increased gradient has relatively little effect on arterial O₂ content at sea level, it can lead to substantial additional arterial desaturation at altitude, where subjects are operating on the steep descending slope of the oxyhemoglobin dissociation curve.