Effect of isocapnic hypoxia on variational activity of breathing

In the presence of either hypocapnia or sleep, hypoxia has been shown to in duce periodic breathing and increase the total variational activity of brea th components. It is not known whether hypoxia induces alterations in breat hing variability during wakefulness and in the absence of hypocapnia. To ad dress this issue, we studied nonobtrusively 14 healthy awake subjects befor e and during the delivery of a hypoxic gas mixture via a plastic hood; the subjects' oxygen saturation decreased from 98 to 79% and end-tidal carbon d ioxide tension was kept constant. Compared with air, isocapnic hypoxia incr eased the gross variability of minute ventilation ((V) over dot l), tidal v olume (VT), inspiratory time (Tl), and expiratory time (TE) (all p < 0.004) . Isocapnic hypoxia decreased the autocorrelation coefficient at a lag of o ne breath for TE (p < 0.008) and (V) over dot l (p = 0.07), the number of c onsecutive breath lags having significant autocorrelation coefficients for TE (p = 0.03), and the cycle time of oscillations in (V) over dot l (p = 0. 03). When partitioned, the increase in total variational activity during Is ocapnic hypoxia was found to result from increases in the random fractions of Vl, VT, Tl, and TE (all p < 0.05), and the oscillatory fractions of (V) over dot l, VT, and TE (all p < 0.03). In conclusion, hypoxia induced hidde n oscillations in VI, VT, and TE despite wakefulness and an isocapnic state , suggesting that neural responses may have a more important role in the ge nesis of hypoxia-induced oscillations than previously reported.