The most common pattern of respiratory disturbances was PVA, detected in 55% of patients studied, occurring mainly in stages 1 and 2 of NREM sleep, and in four cases, lasting > 40% of TST. Although in acute care settings, PVA may contribute to failure of NPPV,13,21 PVA was not, in this study, associated with significant decreases in Spo2 or increases in TcPco2 when compared with patients without PVA (Table 4). However, PVA was associated with more fragmented sleep and a decrease in slow wave sleep and REM sleep. Two thirds of PVA episodes were associated with microarousals in stage 1 and 2 of NREM sleep, often followed by resynchronization of patient and ventilator. PVA-associated arousals occurred much less frequently in slow wave sleep or REM sleep, probably because of a higher arousal threshold during these sleep stages. PVA may result from defective inspiratory triggering (causing delayed pressurization or unrewarded inspiratory efforts) or either delayed or premature cycling.,11–12 In patients without increased intrinsic positive end-expiratory pressure, major leaks are probably the most important contributors to these events. Mouth leaks were underestimated by the methods used in our polysomnography evaluation and were not quantified. The relevance of detecting PVA is, in the present study, mainly related to its deleterious impact on sleep structure, and its theoretical negative impact on WOB and relief of respiratory muscles. Indeed, Fanfulla et al22 showed how two different ventilator settings, which did not induce significantly different diurnal patterns of breathing, blood gas levels, or respiratory mechanics, could affect quite differently nocturnal patient/ventilator synchrony, nocturnal blood gas levels, and sleep quality (sleep efficiency, amount of REM sleep, number of arousals). The authors suggest that inappropriate settings of home ventilators leading either to ineffective inspiratory efforts or central apnea may be more easily detected by sleep studies than through daytime assessment. In fact, the later study22and the study by Collard et al23 both suggest that for patients receiving long-term mechanical ventilation, sleep studies are necessary for determining appropriate ventilator settings thus minimizing PVA, and improving ABG levels and sleep architecture. Furthermore, detection of PVA may orient the clinician toward the following: (1) mouth leaks, and (2) defective inspiratory triggering, or cycling, which are adjustable parameters in many recent bilevel pressure respirators.