TORONTO--Although acute illness has long been associated with sleep disturbances, few studies have evaluated the consequences of sleep deprivation in the critically ill--and fewer still have focused on patients undergoing mechanical ventilation. Addressing this problem, new research involving only mechanically ventilated subjects indicates that these patients suffer from severe sleep deprivation.[1] Moreover, sleep disturbance may hinder attempts to wean patients off the ventilator, the study authors suggest.

"We all have anecdotes about patients who were able to go off the ventilator once their sleep disturbance was controlled," Andrew B. Cooper, MD, said in an interview with PULMONARY REVIEWS. "Sleep disruption affects the load, control, and output of the ventilatory pump, and this potentially could interfere with the process of liberation from mechanical ventilation," added Dr. Cooper, who is a Staff Physician in Critical Care Medicine/Anesthesiology at Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario.

THREE STUDY GROUPS

Dr. Cooper and coworkers conducted a prospective cohort analysis of 20 patients admitted to a medical-surgical intensive care unit (ICU) at a university teaching hospital. All of the patients had mild to moderate acute lung injury and all required mechanical ventilation. Subjects underwent one 24-hour polysomnography (PSG), on average seven to 10 days after admission to the ICU. The subjects were divided into three groups according to the PSG findings: those with disrupted sleep, those with atypical sleep, and those in a coma.

According to the authors, the eight patients in the disrupted sleep group showed PSG evidence of both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep, but the times when they slept during each 24-hour period were abnormal. In seven of these patients, more than 33% of total sleep occurred during the day. Furthermore, sleep efficiency was markedly reduced both during the day and at night. "There was an increased proportion of stage 1 sleep and a reduction in the amount of REM sleep, compared with [that seen in] age-matched controls," noted the authors. In addition, the frequencies of arousals and awakenings were markedly and equally increased during the day and night periods.

ATYPICAL SLEEP AND COMA

The five patients in the atypical sleep group had electroencephalographic (EEG) features that were intermediate between those of sleep and coma and were characterized by a virtual absence of stage 2 NREM sleep. In fact, stage 2 NREM sleep could not be identified in four of the patients, and REM sleep was absent in three. The researchers observed that "another unique feature of this group was that some patients demonstrated pathologic wakefulness" (defined as the occurrence of abnormal EEG activity, such as delta waves, while patients were awake and able to obey commands). The frequency of arousals and awakenings was only slightly increased in this group.

The remaining seven patients were characterized by EEG features of coma, according to the authors. Five of the patients had class 1A coma (less than 50% delta or theta activity, with reactivity); two patients had class 1B coma (less than 50% delta or theta activity, without reactivity). On the day of the study, subjects in the atypical sleep and coma groups had higher APACHE II scores and required higher doses of sedation than did those in the disrupted sleep group.

Dr. Cooper pointed out that none of the 20 subjects had normal sleep during the study. The degree of sleep fragmentation in the disrupted sleep group was as severe as that encountered in conditions such as obstructive sleep apnea, wherein patients experience excessive daytime sleepiness and cognitive impairment. "These are factors that may further interfere with weaning the patient off the ventilator," he said. "The patient may be too cognitively impaired to carry out necessary respiratory exercises or even participate in a routine physical examination. I would caution, however, that these are preliminary observations requiring further research."

--Stanley Nelson

Reference
1. Cooper AB, Thornley KS, Young GB, et al. Sleep in critically ill patients requiring mechanical ventilation. Chest. 2000;117:809-818.

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