Compliance with nasal CPAP therapy for obstructive sleep apnoea: how much is enough?
Authors: WT McNicholas
Journal: European Respiratory Journal (1997)

Nasal continuous positive airway pressure (NCPAP) is a highly effective therapy for obstructive sleep apnoea (OSA), and can completely abolish apnoeas when effective pressure levels are used during sleep [1]. However, the device is cumbersome, and several studies have demonstrated only moderate compliance, with reports indicating 3‐6 h use on an average nightly basis [2‐4]. This observation indicates that patients spend considerable parts of the night without effective therapy, and raises the question of what happens to the patients' OSA during the sleep period without effective therapy. Since the severe end of the OSA spectrum is typically characterized by apnoea and hypopnoea frequencies in excess of 50 events·h-1, and associated with episodic profound hypoxaemia, even a few hours without effective therapy could be associated with several hundred apnoeas and/or hypopnoeas, with consequent episodic severe hypoxaemia. There is considerable evidence that OSA carries a substantial morbidity and mortality, particularly from cardiovascular complications [5‐8]. Whilst there is evidence that NCPAP reduces mortality [9], the literature is limited on the long-term outcome of OSA, both treated and untreated. Major concern persists about the possible adverse health effects of leaving OSA patients without effective therapy for up to half of each night. However, several studies have demonstrated that, in clinical practice, NCPAP dramatically reduces the daytime sleepiness which is a typical feature of OSA, both in patients and their bed partners [10, 11]. Clear-cut objective benefits in terms of reduced daytime sleepiness and improved cognitive function have been demonstrated from average nightly treatment durations of only 3 h [12]. These findings indicate that either the limited period of normal sleep while on NCPAP is sufficient to produce these benefits, or that there is a benefit from NCPAP that persists into the latter part of the night, even though the device might not be in use. The paper by HERS et al. [13] in this issue of the Journal provides important information on what happens to OSA in that part of the night without effective NCPAP therapy, and demonstrates a clinically important residual benefit which persists in the latter part of the night after NCPAP withdrawal. The frequency of apnoeas and hypopnoeas was almost halved, and the severity of oxygen desaturations greatly reduced, when compared with the latter part of a previous study night without NCPAP therapy. Furthermore, movement arousals were fewer, despite the fact that sleep stages were similar between the two study periods of therapy. These findings provide reassurance to clinicians who may be concerned about the possible adverse health effects of limited compliance with NCPAP in many of their OSA patients. Nevertheless, the authors are careful to draw attention to other reports that have failed to demonstrate a significant residual benefit after acute NCPAP withdrawal. Most of these reports, however, have examined the effects of withdrawal for at least one full night. The mechanisms of this residual effect are not clear, since NCPAP appears to act as a simple positive pressure splint to counteract the upper airway collapse caused by negative suction pressure during inspiration [14], and one might, therefore, expect OSA to return to its previous severity immediately following withdrawal of therapy. Recent evidence from our department suggests that reflex mechanisms may also be involved in the action of NCPAP [15], although, again, this finding would not necessarily suggest a residual beneficial effect after NCPAP withdrawal. It is recognized that the duration of apnoeas increases