Short hypobaric hypoxia and breathing pattern: effect of positive end expiratory pressure.

The ventilatory effects of a 5-cm H2O positive end expiratory pressure (PEEP) and its influence on the breathing pattern during short hypoxic exposure both at rest and during physical exercise were studied. There were 22 healthy subjects who were submitted to normoxia and to 4-h of hypoxia in a hypobaric chamber (4500 m, PB = 589 hPa) both at rest and during an 8-min cycle ergometer exercise (100 W) without and with a 5 cm H2O PEEP. The results show that hypoxia compared with normoxia induces increases in tidal volume (VT) (+28.5%, p < 0.05 at rest; and +19.4%, p < 0.01 at 100 W) and in respiratory frequency (f) at 100 W (p < 0.05), and significant decreases in inspiratory (tI) (p < 0.05 at rest and at 100 W), and expiratory (tE) durations (p < 0.05 at 100 W). However, the breathing pattern expressed as duty cycle (tI/tt) is unchanged, whereas an increased mean inspiratory flow (VT/tI) is observed (p < 0.01 at rest and at 100 W). This study also demonstrates that PEEP during a 4-h hypobaric hypoxia significantly increases VT (+22.2% p < 0.01 at rest, +8.9% p < 0.05 at 100 W), tI, and tE at rest (p < 0.05), but not during exercise and tends to decrease f (p = 0.06 at rest and at 100 W). However, PEEP does not alter the breathing pattern in hypoxia since VT/tI and tI/tt are unchanged. Heart rate and arterial O2 saturation are also unaffected by PEEP. In conclusion, this study shows that a 4-h hypoxia modifies ventilatory parameters and mean inspiratory flow (VT/tI) at rest and during exercise (100 W), whereas a 5-cm H2O PEEP does not alter the breathing pattern despite changes in ventilatory parameters are observed.