A79-07 Assessment of Circuit Rebreathing During Continuous Positive Airway Pressure (CPAP) Therapy

Abstract Rationale Obstructive sleep apnea (OSA) is a highly prevalent disorder with major health consequences. The 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, where exhaled carbon dioxide (CO2) and hypoxic gas is re-inspired due to inadequate exhaust flow through the CPAP circuit. The current study sought to carefully quantify levels of rebreathing under various exhaust flow conditions. Methods In a study of 13 participants with moderate-to-severe OSA, we performed wakefulness testing while patients breathed 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, noting that 13-23 L/min captures the typical range of exhaust flow with commercially-available CPAP masks at 5 cmH2O. 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 (including perception of lack of cool/fresh air, dyspnea, claustrophobia, 0-10 points). 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, estimate95%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. Conclusions Our study suggests that rebreathing can occur at exhaust flow levels observed in commercially-available CPAP circuits, and may impact patient comfort. This abstract is funded by: SleepRes