Individual and group mean data comparing NREM PetCO2, PetCO2–apnea threshold (AT), ΔPetCO2–AT, and controller gain gain before (mean pre–continuous positive airway pressure [CPAP] value, closed hexagon) and after (mean post-CPAP value, open hexagon) use of CPAP. There were significant differences in the ΔPetCO2–AT (*P < 0.001) and controller gain (+P < 0.016).