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beta activity. 40 The two oscillations were coupled with a
phase shift of several minutes, with cardiac changes pre-
ceding the EEG changes. Although the mechanisms
underlying this coupling is not known, it has been hypoth-
esized that there might be a central generator synchroniz-
ing the oscillatory process in autonomic and sleep
functions, where cardiovascular function may anticipate
sleep-stage changes. 39
Autonomic Responses Associated with
Arousal from Sleep and from Periodic
Leg Movements
AROUSALS
Electrocortical arousal from sleep (i.e., EEG desynchroni-
zation with appearance of low-voltage, high-frequency
EEG) either spontaneous, provoked by an exogenous
stimuli, or in the context of sleep-disordered breathing, is
associated with sympathetic neural surges, leading to tran-
sient increases in HR, BP, and MSNA. 41-43 The typical
cardiac response is biphasic with tachycardia lasting 4 to 5
seconds followed by bradycardia, with HR increasing
prior to cortical arousals. Using Time-Variant analysis it
appears that the surge in sympathoexcitation as repre-
sented by LF components of RR variability and BP vari-
ability remains substantially elevated above baseline long
after the HR, BP, and MSNA return to baseline values. 18
This can be particularly relevant in conditions character-
ized by frequent arousals across the night, conceivably
leading to a sustained sympathetic influence on the cardio-
vascular system.
Auditory stimuli during sleep may result in autonomic
and respiratory modifications even in the absence of overt
EEG activation (“autonomic arousal”), or in association
with an EEG pattern different from conventional arousal,
such as K-complexes or bursts of delta waves not followed
by EEG desynchronization (“subcortical arousal”). 42 , 43
These observations imply that there is a range of partial
arousal responses implicating autonomic responses with
EEG manifestations different from classical arousals and
even without any EEG response. The different EEG pat-
terns and the associated cardiac response indicate a hier-
archical spectrum of increasing strength from the weaker
high-amplitude delta burst to a stronger low-voltage alpha
rhythm 43 ( Fig. 20-3 ).
AWAKE
STAGE 4
SNA
SNA
125
125
0
0
REM
STAGE 2
SNA
K
SNA
125
125
0
0
STAGE 3
T 10 sec
SNA
125
0
Figure 20-2 Recordings of sympathetic nerve activity (SNA),
and mean blood pressure (BP) in a single subject while awake
and while in stages 2, 3, 4 and REM sleep. SNA and BP gradually
decrease with the deepening of NREM sleep. Heart rate, BP and
BP variability increase during REM sleep, together with a pro-
found increase in the frequency and amplitude in SNA. Reprinted
from Somers VK, Dyken ME, Mark AL, et al. Sympathetic-nerve
activity during sleep in normal subjects. N Engl J Med
1993;328:303-307. (K, K complexes; T, muscle twitches.)
sensitivity appears also to be increased during NREM
sleep compared to wakefulness 37 ; however, the response is
variable. Namely, compared with wakefulness, baroreflex
gain is heightened in response to BP increments rather
than decrements during NREM sleep. This mechanism
probably serves to ensure the maintenance of stable low
BP and HR during NREM sleep.
In contrast, REM sleep is a state of autonomic instabil-
ity, dominated by remarkable fluctuations between para-
sympathetic and sympathetic influences, which produce
sudden and abrupt changes in heart rate and blood pres-
sure. 38 The average HR and BP are higher during REM
compared to NREM sleep, as is sympathetic neural vaso-
motor drive. 25 The cardiovascular excitation of REM sleep
is also reflected by a significant increase of the low fre-
quency components, and a shift of the LF to HF ratio
toward sympathetic predominance. 8
PERIODIC LEG MOVEMENTS DURING SLEEP
Periodic leg movements (PLMs) are described as a repeti-
tive rhythmic extension of the big toe and dorsiflexion of
the ankle, with occasional flexion at the knee and hip.
Periodic leg movements can occur during wakefulness
(PLMW) as well as during sleep (PLMS). Periodic leg
movements during sleep recur frequently in several sleep
disorders (such as restless leg syndrome, narcolepsy, REM
sleep behavior disorder, and sleep apnea) and in patients
with congestive heart failure 44 but are also a frequent
finding in healthy, asymptomatic subjects especially with
advancing age. 45 In the context of sleep apnea, PLMS
may coexist with (and are often difficult to distinguish
from) respiratory-related leg movements, which are part
of the arousal response at the end of airway obstruction
(in obstructive sleep apnea) or at the peak of ventilation
(in central sleep apnea). Approximately 30% of PLMS
RR Variability and EEG Coupling
Studies assessing the overnight relationship between RR
variability and EEG profiles showed the dynamic of RR
variability is closely related to the dynamic of EEG reflect-
ing the depth of sleep. The presence of an ultradian 80- to
120-minute rhythm in the normalized LF, with high levels
during REM sleep and low levels during slow-wave sleep
(SWS) was recently described. 39 These oscillations were
strikingly coupled in a “mirror-image” to the overnight
oscillations in delta wave activity, which reflect sleep deep-
ening and lightening. Similarly, it was reported that nor-
malized HF components of RR variability were coherent
with all EEG spectral bands, with a maximum gain (ratio
HF amplitude/EEG amplitude was higher) for delta activ-
ity and minimum (i.e., the same ratio was lower) with
 
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