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160 PART 2: General Management of the Patient

Parthasarathy and Tobin were among the first to conclusively dem- pressure support ventilation is used, particularly if the patients also have
onstrate that ventilator mode impacts sleep quality in the ICU setting. congestive heart failure. Similar problems with sleep fragmentation may
In a detailed study of 11 patients receiving mechanical ventilation sleep occur when assist-control ventilation and proportional assist ventilation
fragmentation was found to be severe with frequent arousals. Pressure are used, but these modes are probably more forgiving in this respect.
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support adjusted to achieve a tidal volume equivalent to that achieved
during assist-control ventilation (8 mL/kg) caused greater sleep frag- ■ NONINVASIVE VENTILATION
mentation than did assist-control ventilation, and six subjects—five Noninvasive ventilation has been used for years in the management
of whom had heart failure—exhibited central apneas. The addition of of patients with chest wall disease and/or neuromuscular weakness,
dead space to the ventilator circuit decreased sleep fragmentation in the and has been associated not only with improvements in dyspnea and
pressure support mode. Subsequently, a randomized crossover study health-related quality of life, but also with improved sleep quality (see
of patients nearing extubation after treatment for acute-on-chronic Chapter 44). In contrast, little is known about the effects of noninvasive
respiratory failure compared the effect on sleep quality of assist-control ventilation on sleep quality in the acutely ill patient. In one study of
ventilation and pressure support ventilation with a low, fixed (6 cm H O) patients being treated in a medical ICU with noninvasive ventilation
2
level of inspiratory pressure. The investigators noted improvements in for >48 hours, patients who ultimately failed noninvasive ventilation
38
sleep quality, including increased SWS, with the assist-control setting. exhibited worse sleep quality than those who did not, manifest as the
In contrast, Cabello et al found no differences in sleep quality between more frequent occurrence of an abnormal electroencephalographic
assist-control ventilation, clinically adjusted pressure support ven- pattern, reduced REM sleep, and a reduced percentage of total sleep time
tilation, and automatically adjusted pressure support ventilation in 15 occurring at night. A particular strength of this study was the inclusion
44
conscious, nonsedated, mechanically ventilated patients. Differences of the behavioral state in the analysis of the EEG; however, the reproduc-
39
in results between these studies may reflect differences in the subjects’ ibility of the method used by the investigators has not been examined.
severity of illness, residual effects of sedation, differing degrees of prior Of course, even if the results are valid, whether the “sleep” abnormalities
sleep deprivation and/or circadian-determined sleep propensity, or witnessed by the investigators actually led to harm or were themselves an
subtle differences in ventilator settings. Common to all three studies was epiphenomenon that served only to mark worsening illness is unknown.
the scarcity of REM sleep.
Proportional assist ventilation is designed to adjust the level of support ■ SEDATION AND SLEEP
to instantaneous flow and volume and has been shown in many stud- 45
ies to improve synchrony between the patient and the ventilator (see The relationship between sedation and sleep is extraordinarily complex.
Chapter 50). Conceivably, such a strategy could improve sleep quality Sedatives and narcotics commonly used in the ICU may, in fact, pro-
as well. In fact, one study conducted by Bosma et al showed propor- mote sleep in some situations by alleviating pain, dyspnea, and anxiety.
tional assist ventilation to be superior to pressure support ventilation On the other hand, even relatively small doses of these medications may
in this regard. This advantage was not reproduced in a study that impair sleep quality: benzodiazepines can cause in increase in stage
40
compared pressure support ventilation with proportional assist ventila- N2 sleep while reducing slow wave sleep and REM sleep, while opiates
tion with load-adjustable gain factors (so-called PAV+). This study powerfully suppress REM sleep. While the overall effect of such doses
41
was performed in a somewhat different patient population than that of on the EEG is relatively minor, high doses of sedatives and narcotics
Bosma et al, however. Notably, both modes were able to induce periodic are capable of inducing significant EEG suppression, leading even to an
breathing during sleep in both sedated and nonsedated patients. This isoelectric state. It appears likely that, somewhere in the middle between
study confirms the importance of individualized titration of ventila- these two extremes, exist most acutely ill patients receiving continuous
tor support to patient effort. It also highlights the primitive nature of intravenous sedation.
current forms of mechanical ventilatory support, in that even those When considering the relationship between sleep and sedation it is
systems that employ some form of feedback control—like propor- worth remembering that, while sedation is similar to sleep in many ways,
tional assist ventilation—do not effectively navigate changes in sleep there are also many differences between the two (Table 23-4). Normal
state. However, in a study comparing a more sophisticated method of sleep is a naturally occurring state that is reversible with external stimuli.
mechanical ventilation—neurally adjusted ventilator assist (NAVA)— It is highly oscillatory and characterized by approximately 90-minute
against pressure support ventilation, Delisle et al demonstrated that alternating episodes of REM sleep and NREM sleep, by the presence of
pressure support resulted in frequent central apneas (a mean of 10 per
hour) as opposed to none during NAVA. In the same controlled study of TABLE 23-4 Relationship Between Sleep and Sedation
critically ill nonsedated patients, NAVA resulted in better sleep—more
REM sleep, less sleep fragmentation, and fewer ineffective efforts—than Similarities
that achieved during pressure support. 42 Overlapping neurophysiologic pathways
It should be mentioned that understanding the influence of mechani- Muscle hypotonia
cal ventilation on sleep is made more challenging in the ICU by the Temperature dysregulation
frequent occurrence of abnormalities of circadian timing. Absent mark- Disconjugate eye movements (REM)
ers of circadian timing—for instance, melatonin secretion patterns—it is Altered sensorium and mentation
impossible to know whether patients enrolled in these studies exhibited Respiratory depression
similar degrees of sleep propensity at the time they were studied, or Differences
whether some patients were studied during their biological day while Sleep is spontaneous; sedation is not
others were studied during their biological night. Sleep is circadian; sedation is not
Sleep and mechanical ventilation is thus a bidirectional interaction in Sleep is an essential biologic function; sedation is not
which mechanical ventilation may affect sleep, while sleep affects res- Sleep is completely reversible with external stimuli; sedation is not
piration and therefore patient-ventilator synchrony. In particular, the Sleep is associated with decreased release of norepinephrine from locus coeruleus;
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variable neural respiratory pattern seen in healthy subjects during REM norepinephrine release continues during sedation
sleep may be processed rather crudely by currently available ventilatory Sleep is associated with cyclic progression of EEG stages; sedation variably alters
modes, promoting patient-ventilator dyssynchrony and abruptly termi- normal sleep architecture
nating efforts by the patient to enter this stage. Taken together, the avail- EEG, electroencephalogram; REM, rapid eye movement.
able evidence suggests that sleep fragmentation, chiefly from periodic Reproduced with permission from Weinhouse GL, Schwab RJ. Sleep in the critically ill patient. SLEEP.
breathing, is likely to be most problematic in critically ill patients when May 2006;29(5):707-716.

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