Respiratory energetics during exercise at high altitude

Abstract

The purpose of this study was to assess the effect of high altitude (HA) on work of breathing and external work capacity. On the basis of simultaneous records of esophageal pressure and lung volume, the mechanical power of breathing (W˙rs) was measured in four normal subjects during exercise at sea level (SL) and after a 1-mo sojourn at 5,050 m. Maximal exercise ventilation (V˙e max) and maximal W˙rs were higher at HA than at SL (mean 185 vs. 101 l/min and 129 vs. 40 cal/min, respectively), whereas maximal O2 uptake averaged 2.07 and 3.03 l/min, respectively. In three subjects, the relationship ofW˙rs to minute ventilation (V˙e) was the same at SL and HA, whereas, in one individual, W˙rs for any givenV˙e was consistently lower at HA. Assuming a mechanical efficiency (E) of 5%, the O2 cost of breathing at HA and SL should amount to 26 and 5.5% of maximal O2 uptake, whereas for E of 20% the corresponding values were 6.5 and 1.4%, respectively. Thus, at HA, W˙rs may substantially limit external work unless E is high. Although at SLV˙e max did not exceed the critical V˙e, at which any increase inV˙e is not useful in terms of body energetics even for E of 5%, at HAV˙e maxexceeded critical V˙e even for E of 20%.