Figure 2. Effect of pressure-support levels on ventilation and respiratory cycle timing during normoxia (○) and hypoxia (•).

The increase of inspiratory pressure support caused an increase in tidal volume and minute ventilation and a decrease in breathing frequency. End-tidal P_CO2 gradually decreased. Each point at pressure support levels of 0-8 cmH₂O represents the mean data for the entire group of seven subjects. The pressure-support level of 10 cmH₂O represents data from all seven subjects during normoxia but only four of seven subjects during hypoxia. During hypoxia, three of seven subjects developed apnoea at pressure support ≤ 10 cmH₂O. T_I, inspiratory time; T_E, expiratory time; V_T, tidal volume; ${\stackrel{\mathrm{.}}{V}}_{\mathrm{E}}$, minute ventilation; P_ET,CO2, end-tidal P_CO2. *P < 0.05 compared to the baseline value with zero pressure support.