Figure 1. Polygraph record of a single trial during normoxic (A) and hypoxic (B) breathing.

The baseline period shows spontaneous, stable breathing with continuous positive airway pressure (CPAP) of 2 cmH₂O. During normoxia, the pressure-support level was increased to 8 cmH₂O during inspiration with the end-expiratory level maintained at 2 cmH₂O (8/2 cmH₂O) and subsequently to 10/2 cmH₂O. At 10/2 cmH₂O, the reduction in P_ET,CO2 was sufficient to cause an initial hypopnoea and consequent periodic breathing. The hypopnoea threshold was determined as the mean P_ET,CO2 of the last three breaths immediately preceding the first hypopnoea. During hypoxia, arrow A indicates a hypopnoea with two successive untriggered breaths, and arrow B indicates a central apnoea with no respiratory effort.