Transcutaneous CO2 plateau as set-point for respiratory drive during upper airway flow-limitation

- Transcutaneous CO2 increases during progressive flow limitation.
- Respiratory efforts during flow limitation remain low until tcCO2 plateau is reached.
- Respiratory efforts during flow limitation increase when tcCO2 exceeds the plateau.
- TcCO2 plateau may help to identify clinically significant flow limitation.

Upper airway flow-limitation is often but not always associated with prolonged gradually increasing respiratory effort. We investigated the changes in transcutaneous carbon dioxide tension (tcCO2) during episodes of upper airway flow limitation during sleep with or without respiratory effort response.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.