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Effect of Ventilator Mode on Sleep Quality in Critically Ill Patients
Sairam Parthasarathy and Martin J. Tobin
Division of Pulmonary and Critical Care Medicine, Edward Hines, Jr. Veterans Administrative Hospital, Hines, Illinois; and Loyola University of Chicago Stritch School of Medicine, Maywood, Illinois

To determine whether sleep quality is influenced by the mode of mechanicalventilation, we performed polysomnography on 11 critically ill patients. Because pressure support predisposes to central apneas in healthy subjects, we examined whether the presence of a backup rate on assist-control ventilation would decrease apnearelated arousals and improve sleep quality. Sleep fragmentation, measured as the number of arousals and awakenings, was greater during pressure support thanduring assist-control ventilation: 79 7 versus 54 7 events per hour (p 0.02). Central apneas occurred during pressure support in six patients; heart failure was more common in these six patients than in the five patients without apneas: 83 versus 20% (p 0.04). Among patients with central apneas, adding dead space decreased sleep fragmentation: 44 6 versus 83 12 arousals and awakenings per hour (p0.02). Changes in sleep–wakefulness state caused greater changes in breath components and end-tidal CO2 during pressure support than during assist-control ventilation. In conclusion, inspiratory assistance from pressure support causes hypocapnia, which combined with the lack of a backup rate and wakefulness drive can lead to central apneas and sleep fragmentation, especially in patients with heartfailure. Keywords: arousal; artificial respiration; critical illness; mechanical ventilator; sleep

Mechanical ventilation is used primarily to improve gas exchange and achieve respiratory muscle rest (1). To achieve this goal, it is important that a patient does not make respiratory efforts out of synchrony with the cycling of the ventilator (2, 3). Because behavioral stimuli are decreased duringsleep, respiratory muscle rest might be greater during sleep as compared with wakefulness. The operation of a ventilator, however, including its alarms, may disrupt sleep (4). Although disruptions of sleep may adversely affect critically ill patients, little information is available about the interplay between patient–ventilator synchrony and sleep. Studies of healthy volunteers and animalssuggest that sleep disruption could result in negative energy balance (5, 6), reduced host immunity (7), and decreased ventilatory responses to hypoxemia and hypercapnia (8, 9). These deleterious effects might prolong the duration of mechanical ventilation in critically ill patients. Cooper and coworkers (10) have shown that mechanically ventilated patients arouse repeatedly from sleep. In patientsfree of critical illness, arousals can result from a derangement of arterial blood gases or an increase in respiratory effort (11–13), and both are common occurrences in critically ill patients. Patient–ventilator dysynchrony or a particular ventilator mode might also lead to sleep disruption (10). Pressure support predisposes to an abnormal breathing pattern, specifically central apneas withconsequent hyperpnea (14). Meza and coworkers (14) demonstrated that healthy subjects develop central apneas during pressure support when their carbon dioxide tension (Pco2) decreases by a few torr below the apnea threshold. Whether pressure support causes apneas in critically ill patients and whether the apneas lead to disruption of sleep are unknown. Disruption of sleep secondary to central apneas,however, can cause cardiopulmonary abnormalities in ambulatory patients (8, 9, 15–20), and these effects may be magnified in critically ill patients. Unlike pressure support, assist-control ventilation delivers a fixed tidal volume on every breath, and it can be set to deliver breaths when a patient fails to make an effort. The backup rate will prevent the development of apneas, and perhaps decrease...