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Compliance to continuous positive airway pressure (CPAP) in patients with OSA is similar when using pressures derived from manual titration compared to auto-titration. However, auto-titration pressures are usually 2-5 cmH2O higher than manually titrated pressures, indicating complicated CPAP titration could be simplified by selecting a highest tolerable pressure for longterm treatment. We examine the effects of CPAP on respiratory load and breathing discomfort in healthy volunteers and in CPAP naïve patients with obstructive sleep apnea (OSA) to determine the threshold pressure of breathing difficulty. We further observed the effect of a fixed CPAP threshold of 10 cmH2O on treatment efficacy in patients with OSA. Expiratory reserve volume, diaphragm EMG, expiratory muscle EMG (recorded from the transversus abdominis) were measured in 12 healthy subjects at different CPAP levels …