In this post-hoc analysis assessing sleep by complete polysomnography in conscious and non-sedated critically ill patients included at different times of the ICU stay, total sleep time and deep sleep remained well-preserved in patients admitted for acute respiratory failure, whereas it was frequently replaced by atypical sleep in patients under mechanical ventilation. By contrast, total sleep time was markedly reduced after extubation. REM sleep was uncommon at any time of the ICU stay and complete disappearance of REM sleep was significantly associated with poor outcomes.
Sleep characteristics according to the timing of the ICU stay.
To our knowledge, this is the first study comparing sleep characteristics in critically ill patients at different times during their ICU stay. Sleep is deeply altered in ICUs and it is common to observe complete disappearance of the restorative sleep stages of deep sleep and REM sleep, which are sometimes replaced by abnormal sleep called atypical sleep (1–3, 5–8). However, sleep studies in ICUs have mainly explored patients under mechanical ventilation having previously received sedation. Sedation favors sleep disturbances and it has been shown that patients who had received high doses of sedation had reduced total sleep time and that they were more likely to exhibit atypical sleep (6, 14). Sleep may also directly be altered by mechanical ventilation itself via patient-ventilator asynchronies or sleep apneas, especially those induced by excessive ventilation (4, 5, 19–21). By contrast, few studies have assessed sleep in patients breathing spontaneously either early upon ICU admission or after extubation (14, 22–24). Among those studies, several have included patients treated with noninvasive ventilation and who had hypercapnic encephalopathy, which may have markedly altered sleep (22, 23). Although all patients included in the present study were conscious and free of sedation, main sleep disturbances differed according to the timing of the ICU stay and to the course of acute respiratory failure. If atypical sleep was predominant under mechanical ventilation, it may have been because sedation had been stopped shortly before sleep recording. Atypical sleep was still observed after extubation but in a lesser proportion than under mechanical ventilation, probably because it was more distant from sedation cessation. By contrast, atypical sleep was almost non-existent in patients admitted for acute respiratory failure who had never received sedation. In these patients, total sleep time and deep sleep remained preserved, which was not the case in patients breathing spontaneously after extubation, reinforcing the hypothesis that sleep disturbances are mainly favored by sedation or by sleep deprivation after prolonged ICU length of stay. Nevertheless, the only constant sleep alteration observed, whatever the timing of sleep recordings, was frequent disappearance of REM sleep. REM sleep completely disappeared in half of patients, and could be the most reliable marker to assess sleep quality at any time of the ICU stay. During REM sleep, accessory inspiratory muscles are inhibited, and as a result, the diaphragm remains the only active inspiratory muscle (25, 26). Several studies have reported a decreased ventilatory response to hypercapnia and hypoxia during REM sleep leading to hypoxemic episodes (12, 13). Consequently, REM sleep is a sleep stage that may precipitate or worsen underlying respiratory failure, and it might be hypothesized that disappearance of REM sleep is a protective mechanism to cope respiratory failure. However, disappearance of REM sleep was associated with poor clinical outcomes and might be more deleterious on brain function than protective on respiratory muscle function.
Role of sleep disturbances in ICU outcomes
Only a few studies have assessed the role of sleep on the prognosis of critically ill patients (6, 7, 22, 27).. Two of these studies showed that mechanically ventilated patients with sleep disturbances were more likely to exhibit prolonged weaning and delayed extubation (6, 7). In both studies, atypical sleep, but also pathological wakefulness, were particularly frequent and were associated with more prolonged weaning and delayed extubation (6, 7). One of these studies showed that respiratory muscle strength was not more altered in patients with atypical sleep, suggesting that sleep disturbances may be the evidence of brain dysfunction that may precipitate respiratory failure. In line with these findings, it has been shown that in healthy subjects sleep deprivation may reduce respiratory endurance by altering cortical respiratory command (10, 11). In another prospective study including 52 conscious mechanically ventilated patients, atypical sleep was for the first time shown to be associated with an increased risk of death (27). All of these studies showing potential deleterious effects of atypical sleep on patient outcomes included only mechanically ventilated patients (6, 7, 27). Atypical sleep seems mainly observed in mechanically ventilated patients and does not contribute to assessment of sleep disturbances in all critically ill patients admitted to ICUs. In our study, only one-third of patients were under mechanical ventilation at time of inclusion whereas the others were breathing spontaneously. This could explain why we did not observe any significant impact of atypical sleep on poor clinical outcomes. By contrast, we showed that disappearance of REM sleep was significantly more frequent in patients with poor clinical outcomes. Even though atypical sleep and disappearance of REM sleep could both be predictors poor clinical outcomes, complete absence of REM sleep is probably easier to detect than atypical sleep, and may be assessed not only in mechanically ventilated patients, but also in all critically ill patients admitted in ICUs. A previous study showed that low REM sleep stage was associated with poor clinical outcomes in patients admitted to ICU for acute hypercapnic respiratory failure and who were breathing spontaneously (22). Similarly, a previous study showed that low REM sleep stage was associated with poor outcomes in patients admitted to ICU for acute hypercapnic respiratory failure and who were breathing spontaneously.
Limitations
Poor outcomes included different events according to the timing of sleep exploration. However, each event is well-established as being associated with poor prognosis. Intubation occurs in around 30 to 50% in patients admitted to ICU for acute hypoxemic respiratory failure and is associated with high mortality rates (28). Reintubation occurs in around 15% of cases after planned extubation, and is associated with an increased risk of death (29–31). Similarly, prolonged weaning, defined as a duration of more than 7 days between the initial spontaneous breathing trial and extubation attempt, is well defined in the literature, and also associated with increased risk of death as compared to more simple weaning (18, 32, 33). In our study, patients in whom these events occurred had significantly higher mortality than the others, confirming that they could be considered as being associated with a poor prognosis.
This is obviously not a longitudinal analysis of sleep including several sleep explorations at different times of the ICU stay of the same patient. Even though we included three distinct populations, patients included under mechanical ventilation and those included after extubation had been admitted to ICU in more than 80% of cases for acute respiratory failure. Therefore, one may consider that sleep was explored in a similar population of patients with acute respiratory failure, and that the differences in sleep characteristics were mainly due to the timing of sleep assessment during the ICU stay.
Another major limitation is that the study was performed in a single center. Consequently, the dramatically low duration of sleep recorded in the present study may be due to an environment not conducive to sleep in that unit.