Diaphragm fatigue during exercise at high altitude: the role of hypoxia and workload

The effect of high altitude (HA) on exercise-induced diaphragm fatigue in n ormal subjects nas examined. Eight normal subjects completed an incremental exercise test at sea level ( SL) and at 3,325 m. Before (baseline), during, and after exercise (recovery ), maximal transdiaphragm pressure (Pdi,sniff), breathing pattern, and diap hragmatic effort (PTPdi) were measured. Arterialized blood lactate was meas ured at baseline and during recovery. At maximal exercise (WRmax) Pdi,snif(f) fell to 72% and 61'%, of baseline a t SL and HA respectively, recovering to baseline in 60 min at SL, and >60 m in at HA, At the 5th min of recovery, circulating lactate was six-fold and seven-fold baseline at SL and HA, respectively. The time course of circulat ing lactate recovery mas as for Pdi,sniff. At WRmax PTPdi was 80.74+9.87 kP a.s(-1) at SL and 64.13+/-8.21 kPa.s(-1) at HA. HA WRmax compared to isowor k rate, SL data showed a lower Pdi,sniff (8.90+/-0.68 versus 11.24+/-0.59 k Pa) and higher minute ventilation (117+/-11 ver sus 91+/-13 L.min(-1)), PTP di being equal. To conclude, in normal subjects hypoxia-related effects, and not an increas e in diaphragm work, hastens exercise-induced diaphragm fatigue and delays its recovery at high altitude compared to sea level.