Respiratory energetics during exercise at high altitude

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 br eathing ((W) over dot rs) was measured in four normal subjects during exerc ise at sea level (SL) and after a 1-mo sojourn at 5,050 m. Maximal exercise ventilation ((V) over dot E-max) and maximal Wrs were higher at HA than at SL (mean 185 vs. 101 1/min and 129 vs. 40 cal/min, respectively), whereas maximal O-2 uptake averaged 2.07 and 3.03 1/min, respectively. In three sub jects, the relationship of Wrs to minute ventilation ((V) over dot E) was t he same at SL and HA, whereas, in one individual, (W) over dot rs for any g iven (V) over dot E was consistently lower at HA. Assuming a mechanical eff iciency (E) of 5%, the O-2 cost of breathing at HA and SL should amount to 26 and 5.5% of maximal O-2 uptake, whereas for E of 20% the corresponding v alues were 6.5 and 1.4%, respectively. Thus, at HA, (W) over dot rs may sub stantially limit external work unless E is high. Although at SL (V) over do t E-max did not exceed the critical (V) over dot E, at which any increase i n (V) over dot E is not useful in terms of body energetics even for E of 5% , at HA (V) over dot E-max exceeded critical (V) over dot E even for E of 2 0%.