0506 Circuit Rebreathing During Continuous Positive Airway Pressure (CPAP) Therapy

Abstract Introduction Obstructive sleep apnea (OSA) first-line therapy, continuous positive airway pressure (CPAP), is highly efficacious but limited by adherence. We propose that an underrecognized factor influencing adherence is circuit rebreathing. The current study sought to carefully quantify levels of rebreathing under various exhaust flow conditions. Methods 13 participants with moderate-to-severe OSA performed wakefulness testing while breathing on a modified CPAP circuit that allowed independent manipulation of exhaust flow. Exhaust flow levels of 8, 13, 18, 23, and 35 L/min were studied. Patients breathed on the circuit at 5 cmH2O for 1-2 min for each exhaust flow level, repeated in duplicate. We also assess effects of lowered exhaust flow on the global breathing discomfort scale. Mean inspired CO2 and inspired O2 levels were carefully estimated using delay-corrected gas sampling. Effects of circuit exhaust on inspired CO2 and inspired O2 were assessed using mixed model analysis. Impact of circuit rebreathing was also examined during sleep (5-10 min periods). Results In the five participants studied to date (4M:1F; age 61±6 y; BMI 29±4 kg/m2), inspired CO2 increased progressively with reduced exhaust flow (continuous model: inspired CO2 [mmHg] ~ 1/exhaust, β = 19.6 [14.6, 24.6], estimate[95%CI]; P< 0.0001). Compared to exhaust at 35 L/min, progressive exhaust reductions to 23, 18, 13, and 8 L/min led to inspired CO2 increases by +0.4 [−0.5, 1.2], +0.35 [−0.5, 1.2], +0.7 [−0.3, 1.7], and +2.1 [1.3, 2.9] mmHg respectively. Likewise, we found that inspired O2 reduced significantly with reduced exhaust flow (continuous model β = −25.1 [−31.0, −19.1]; P< 0.0001). Compared to exhaust at 35 L/min, exhaust reductions to 23, 18, 13, and 8 L/min led to inspired O2 changes by −0.7 [−1.8, 0.4], −0.6 [−1.7, 0.4], −1.3 [−2.5, −0.03], and −2.7 [−3.7, −1.6] mmHg respectively. With reductions to 23, 18, 13, and 8 L/min, the global breathing discomfort scale changed by −0.2, 0, +0.4, and +1.4 respectively. Similar effects appeared present during sleep. Conclusion Our study suggests that rebreathing can occur at exhaust flow levels observed in commercially-available CPAP circuits, and may impact patient comfort. Support (if any)