BACKGROUND: Nasal continuous positive airway pressure (NCPAP) is widely used as a treatment for obstructive sleep apnoea. However, to date there are no randomised controlled trials of this therapy against a well-matched control. We undertook a randomised prospective parallel trial of therapeutic NCPAP for obstructive sleep apnoea compared with a control group on subtherapeutic NCPAP. METHODS: Men with obstructive sleep apnoea, defined as an Epworth sleepiness score of 10 or more and 10 or more dips per h of more than 4% SaO2 caused by obstructive sleep apnoea on overnight sleep study, were randomly assigned therapeutic NCPAP or subtherapeutic NCPAP (about 1 cmH2O) for 1 month. Primary outcomes were subjective sleepiness (Epworth sleepiness score), objective sleepiness (maintenance of wakefulness test), and SF-36 questionnaire measurements of self-reported functioning and wellbeing. FINDINGS: 107 men entered the study: 53 received subtherapeutic NCPAP and 54 therapeutic NCPAP. Use of NCPAP by the two treatment groups was similar: 5.4 h (therapeutic) and 4.6 h (subtherapeutic) per night. Subtherapeutic NCPAP did not alter the overnight number of SaO2 dips per h compared with baseline, and thus acted as a control. Therapeutic NCPAP was superior to subtherapeutic NCPAP in all primary outcome measures. The Epworth score was decreased from a median of 15.5 to 7.0 on therapeutic NCPAP, and from 15.0 to 13.0 on subtherapeutic NCPAP (between treatments, p < 0.0001). Mean maintenance of wakefulness time increased from 22.5 to 32.9 min on therapeutic NCPAP and, not significantly, from 20.0 to 23.5 min on therapeutic NCPAP on subtherapeutic NCPAP (between treatments p < 0.005). Effect sizes for SF-36 measures of energy and vitality were 1.68 (therapeutic) and 0.97 (subtherapeutic) NCPAP (between treatments p < 0.0001). For mental summary score, the corresponding values were 1.02 and 0.4 (between treatments p = 0.002). INTERPRETATION: Therapeutic NCPAP reduces excessive daytime sleepiness and improves self-reported health status compared with a subtherapeutic control. Compared with controls, the effects of therapeutic NCPAP are large and confirm previous uncontrolled clinical observations and the results of controlled trials that used an oral placebo.

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