Page 205 - Cardiac Nursing

1
1
0:1
/09
/29
/29
/09
M
Pa
Pa
M
0:1
6 P
6 P
7-2
03.
03.
7-2
p
17
17
xd
6
6
xd
q
q
q
In
In
c.
a
ara
ara
a
LWBK340-c08_
LWB
LWB K34 0-c 08_ p pp177-203.qxd 6/29/09 10:16 PM Page 181 Aptara Inc.
K34
c.
08_
0-c
e 1
81
81
e 1
g
g
g
p
t
t
p
A
A
p
C HAPTER 8 / Sleep 181
5
most body systems. In healthy individuals, these changes are well both the intercostal and upper airway muscles, reducing the rib
tolerated; but in patients with cardiovascular disease, the sleep cage contribution to breathing and increasing upper airway resist-
state may make them vulnerable to serious, sometimes fatal, com- ance; however, the diaphragm is relatively spared from REM-
plications. 59 related paralysis. Phasic REM sleep also includes dysrhythmic
changes in the stimulation of brainstem respiratory neurons, re-
sulting in an erratic breathing pattern. 70,71
Cardiovascular Function in Sleep
The arterial blood gas changes stimulate an adaptive increase
Cardiovascular control during sleep is primarily determined by in ventilation, although the response is less effective in sleep than
variation in autonomic nervous system activity, which causes in wakefulness. In men, the ventilatory responses to both hyper-
changes in blood pressure and heart rate, the major determinants capnia and hypoxemia decrease by approximately half from wake-
of myocardial O 2 demand. 59,60 Increased parasympathetic tone fulness to NREM sleep, with a further reduction in REM sleep.
and, to a lesser extent, decreased sympathetic tone, lead to a re- Women respond similarly to hypercapnia but somewhat differ-
duction in heart rate and cardiac output in NREM sleep. Vasodi- ently to hypoxemia. Women have a lower ventilatory response
latation causes a reduction in systemic vascular resistance and a than men when awake and little change in NREM sleep, but a
5% to 15% decrease in blood pressure which can impair blood similar fall in REM sleep. 72,73
flow through stenotic coronary blood vessels and trigger myocar- Arousal from sleep is a second adaptive response to blood gas
dial ischemia or infarction. 59 In contrast, pulmonary artery pres- changes because the change stimulates ventilation and permits
sure increases slightly. 61,62 Baroreceptor gain is heightened and voluntary action to cope with the situation, such as moving a pil-
contributes to the reduction and stability of blood pressure. Brief low that interferes with breathing. Hypercapnia is a relatively ef-
surges in blood pressure and heart rate occur with K complexes, fective arousal stimulus, awakening most healthy subjects before
arousals, and large body movements. 62–64 An age-related decrease carbon dioxide tension rises 15 mm/Hg; concurrent hypoxemia
in this response has been demonstrated in middle-aged subjects in enhances the response. Hypoxemia alone; however, is a poor
comparison to younger subjects, possibly reflecting the decline in arousal stimulus; study subjects often fail to awaken despite an
65
parasympathetic function that occurs with increasing age. Sharp Sa O2 as low as 70%. 70
increases in heart rate and blood pressure occur with morning Adaptive responses that protect the airway are less effective
awakening and beginning the day’s activities 66,67 as well as after during sleep. Respiratory secretions are cleared less readily because
other periods of sleep, such as afternoon naps. 66 of diminished mucociliary clearance, 74 and the tendency to aspi-
REM sleep brings an increase in cardiovascular demands. Al- rate increases. 75 In addition, sleep suppresses the cough reflex to
though blood pressure and heart rate have average levels near irritating substances in the airways in both REM and NREM
those observed in light NREM sleep or quiet wakefulness, their sleep. 71
variability increases markedly during phasic REM sleep, with wide
erratic fluctuations. 59,68 These changes are related to bursts of in- Thermoregulation in Sleep
creased sympathetic activity and to reduced vagal input to the
heart. 59,69 Cardiac efferent vagal tone and baroreceptor regulation Body temperature is regulated at a lower set point in NREM sleep
are generally suppressed during REM sleep. 59,68 than in wakefulness. In combination with reduced motor activity,
this results in a decrease in temperature at sleep onset. 36,47 The
Respiratory Function in Sleep normal temperature-regulating mechanisms are markedly inhib-
ited during REM sleep; during this stage of sleep, body tempera-
Sleep alters breathing patterns, ventilation, and arterial blood gas ture is influenced more by the environment than the hypothala-
values. Periodic breathing, a cyclic waxing and waning of tidal vol- mus. Body temperature also has an independent circadian rhythm
ume, sometimes with brief apnea, is common at sleep onset in as- that typically peaks in the late afternoon and reaches a minimum
sociation with fluctuations between wakefulness and light sleep. in the early morning hours of sleep. 47
Breathing is remarkably regular; however, in stable stage 2 and Sleep length depends on the phase of the circadian temperature
SWS. The pattern becomes faster and more erratic during phasic rhythm at bedtime because the rising phase triggers awakening
REM sleep. 70 from sleep. A sleep period that begins when body temperature is
Minute ventilation falls, mainly because of reduced tidal vol- low—for example, going to bed at 3:00 AM—is relatively short be-
ume, with an average decrease of approximately 0.4 to 1.5 L/min cause temperature soon rises. In contrast, a sleep period that begins
70
compared with quiet wakefulness. This hypoventilation leads to when temperature is high is relatively long because temperature
small changes in arterial blood gases, including a mild hypercap- drops and a subsequent rise in temperature does not occur for some
nia (increases of 3 to 7 mm/Hg in carbon dioxide tension), de- time. 47 The body temperature rhythm shifts a little earlier (phase
creases of 0.01 to 0.06 units in pH, and a mild hypoxemia (de- advance) with aging, which may partly explain why many older
creases of 35 to 9.4 mm Hg in Pa O2 and 2% or less in Sa O2 ). 70,71 people have an earlier wake-up time than younger adults. 47,48
Data regarding REM sleep are somewhat contradictory but sug-
gest that minute ventilation, tidal volume, and respiratory fre- Cerebral Blood Flow in Sleep
quency are similar to those in NREM sleep, with similar or some-
what greater changes in blood gas values. 70,71 Other factors that Brain blood flow decreases in sleep in comparison to waking. 76
contribute to hypoventilation in NREM sleep include reduced Brain imaging studies reveal that there is a small to moderate de-
central drive to breathe caused by loss of the wakefulness stimulus crease in brain blood flow in NREM sleep in the brainstem and
and increased upper airway resistance to airflow due to reduced cortex. 77,78 In contrast, there is an marked increase in brain blood
pharyngeal muscle tone. 70,71 Tone in the intercostal muscles and flow in REM sleep in the brain stem and limbic decrease, but a
diaphragm; however, is maintained. In REM sleep, tone is lost in decrease in the frontal cortex. 79 Poor performance on tasks after