Seventy-seven episodes of progressive flow-limitation were analyzed in 36 patients with sleep-disordered breathing. TcCO2 and arterial oxyhaemoglobin saturation (SaO2) were measured during steady breathing and during episodes of flow-limitation with and without effort response.
After lights-off tcCO2 increased and leveled-off at plateau, when breathing stabilized. During flow-limitation tcCO2 increased at rate of 4.0 kPa/h. Flow-limitation with increasing respiratory effort associated with tcCO2 increase above the plateau (terminating at 105.2%, p < 0.001), whereas flow-limitation without effort response associated with tcCO2 increase starting below the plateau (95.8%, p < 0.001).
We conclude that the nocturnal tcCO2 plateau indicates the level above which the increasing respiratory effort is triggered as response to upper airway flow-limitation. We propose that flow-limitation below the tcCO2 plateau is an event related to stabilization of sleep and breathing.