Page 252 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

156 PART 2: General Management of the Patient

TABLE 23-1 Challenges to the Study of Sleep in Critical Illness
• The typical ICU environment is hostile to sleep, consisting of exces-
sive light and noise at night, insufficient light during the day, and fre- Heterogeneous patient population
quent sleep disruption due to monitoring and patient care activities. Multiple potential sleep disrupters experienced by typical ICU patient
• In general, the sleep of critically ill patients is highly fragmented and Absence of clinically useful biomarker for sleepiness
nonconsolidated, and rapid eye movement (REM) sleep is scarce. Limited ability to perform polysomnography (limited access to equipment, sleep technologists)
Time-intensive nature of polysomnography interpretation
• Sleep and mechanical ventilation is a bidirectional interaction in Difficulty in assessing sleep in patients receiving sedation or with altered mental status
which changes in the level or pattern of mechanical ventilatory Behavioral correlates of sleep lack specificity
support may affect sleep, while changes in sleep state affect respira- Actigraphy not reliable
tion and therefore patient-ventilator interaction. EEG frequently abnormal; findings of uncertain significance
• Patients receiving mechanical ventilation are susceptible to sleep Complex subject that lies at the intersection of sleep, anesthesia, neurology, and critical
disruption if air hunger is not sated, if neural and mechanical illness, necessitating an interdisciplinary approach
inspiratory times differ, if periodic breathing is induced, and prob-
ably through other mechanisms as well.
• Sleep fragmentation from periodic breathing is most likely to occur
when pressure support ventilation is used, particularly if the patient NORMAL SLEEP: A BRIEF REVIEW
has also heart failure. Proportional assist ventilation may also induce
periodic breathing and sleep fragmentation, but appears less likely to ■ SLEEP-WAKE REGULATION
do so. Volume-cycled assist control ventilation may be the mode of Sleep and wakefulness are regulated by two basic processes: the homeo-
ventilation that is least likely to disrupt sleep in critically ill patients. static process (process S) and the circadian process (process C). The
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• Traditional sleep scoring criteria are unreliable in assessing sleep homeostatic drive to sleep increases with increasing duration of wake-
in mechanically ventilated patients receiving intravenous sedation. fulness and is dissipated by sleep. It appears that process S is mediated at
• Patients receiving continuous mechanical ventilation and continu- least in part by the neurotransmitter adenosine, the product of adenos-
ous intravenous sedation exhibit pronounced temporal disorga- ine triphosphate (ATP) metabolism, which accumulates in the arousal
nization of the sleep EEG. The extent to which sedation is itself centers of the brain with prolonged neural activity. Once a certain
restorative is not clear. threshold is passed, drowsiness and/or sleep ensue, thereby homeo-
• Sleep may play an important role in the short- and long-term statically adjusting the amount of sleep experienced by the organism. In
physical, cognitive, and psychological recovery of recent survivors contrast, the circadian process, or process C, is an endogenously driven
of critical illness. and roughly 24-hour cycle that is largely independent of process S, and
• Absent high-quality evidence to guide the clinician, efforts to which is characterized by alternating periods of high and low sleep
enhance sleep, and circadian rhythmicity in the ICU should focus propensity. The circadian timing system helps maintain wakefulness
throughout the day as the homeostatic drive to sleep increases. Similarly,
initially on relatively low-hanging fruit: for instance, administer-
ing baths during the daytime or early evening hours, and ensuring the nadir in circadian sleep propensity during the early morning hours
helps consolidate sleep. Circadian processes are also involved in the
adequate light exposure during the day.
regulation and expression of a multitude of biological processes related
to metabolism, cardiovascular regulation, cognition, and immune func-
tion, as considered later.
INTRODUCTION Synchronization of the human endogenous circadian timing system
It is still not clear exactly why we sleep, although there are many theories. to the external environment is chiefly accomplished through exposure to
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What is clear is that sleep is a ubiquitous and highly conserved evolu- light, allowing individuals to adjust their rest and activity cycles to the
tionary process in nature, with cyclic alternations of rapid eye movement light-dark cycle and to the activities of their community
every mammal studied to date, and with variations of the rest-activity ■ SLEEP ARCHITECTURE
(REM) sleep and nonrapid eye movement (NREM) sleep being found in
cycle found throughout nature. We spend roughly one-third of our time While individual requirements for sleep vary, a typical night’s sleep for
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asleep; in the words of the sleep research pioneer Allan Rechtschaffen, a healthy young adult consists of an essentially uninterrupted 8-hour
“If sleep does not serve an absolutely vital function, then it is the big- period of sleep comprised of 90-minute periods of alternating REM sleep
gest mistake the evolutionary process has ever made.” In fact, a rapidly and NREM sleep. NREM sleep is divided into three stages of sleep of
accumulating body of evidence indicates that sleep is not only necessary progressively increasing depth, from stage N1 (transitional sleep or
for maintaining alertness and performance but is highly involved in pro- drowsiness) to stage N2 to stage N3 (deep or slow wave sleep [SWS]).
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cesses as diverse as metabolism, heart disease, and cancer surveillance. 2-6 The sleep electroencephalogram (EEG) of the latter consists of ≥20%
It is therefore somewhat surprising that the sleep of critically ill delta waves and includes stages 3 and 4 in the prior classification
patients has received so little attention to date. Every year in the United scheme. REM sleep comprises approximately 20% to 25% of total sleep
States more than 5 million persons are admitted to intensive care units, time and is associated with dreaming, skeletal muscle atonia, rapid
nearly 1 million of whom will receive mechanical ventilation. All of eye movements, muscle twitches, and cardiovascular and respiratory
these patients have a biological need to sleep. If the sleep of critically instability. Healthy adult subjects who are not sleep deprived typically
ill patients matters even a little in overcoming critical illness, treatment progress from wakefulness to NREM sleep, beginning in the lighter
strategies that improve sleep quality have the potential to save thousands stages and progressing to deeper stages. The first episode of REM sleep
of lives annually while simultaneously reducing costs to society. Even if typically occurs approximately 70 to 100 minutes after sleep onset and
sleep quality were wholly irrelevant to survival in this setting—and this lasts fewer than 10 minutes. Subsequent episodes of REM sleep are
would be surprising indeed—sleep disruption is in itself distressing to progressively longer in duration and occur in alternation with bouts
patients, and therefore worthy of our efforts to ameliorate this problem. of NREM sleep at approximately 90-minute intervals, for a total of 4 to
Unfortunately, progress in understanding the sleep and circadian 5 sleep cycles per night. Thus, SWS predominates in the first part of
rhythms of critically ill patients has been slowed by a variety of logistical, the night while REM sleep predominates in the latter part of the night,
technical, and methodological challenges (Table 23-1). Fortunately, the coincident with the circadian trough in body temperature.
engagement of increasing numbers of researchers from diverse disci- In healthy young adults, SWS constitutes approximately 20% of total
plines has begun to bear fruit. sleep time. Normal aging is associated with compromised sleep that

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