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Autonomic Nervous System Monitoring of Patients with Excess Parasympathetic Responses
psychological states
19
can cause autonomic imbalance and associated
Figure 1: Valsalva Maneuver—Sympathetic Challenge
P dysfunction. Severe acute conditions can precipitate PE, including
trauma,
20
injury, infection, surgery, cancer, and myocardial infarction (MI).
160
Preliminary evidence suggests that multiple pregnancies can cause PE in
140
women, and severe or chronic exposure to chemicals, cold, and allergens
can also affect PE. Stress, excess caffeine, nicotine, and other chemical
21,22
120
and environmental exposures can also affect autonomic balance. PE may
100
also be genetically mediated, as evidenced by colic in infants.
80
Establishing PE may help clarify a diagnosis when patients demonstrate
60
54 Time in seconds 161
multiple seemingly contradictory symptoms (e.g. hypertension with
depression) and provide a more integrated approach to therapy.
12 3 456 7 8
Disease can cause autonomic imbalance; for example, pain causes S
Respiratory response HR response
excess that can lead to early hypertension. Disease can also be caused
by autonomic imbalances. For example, dizziness on standing up
A recording of normal subject data during the Valsalva challenge of the clinical exam (see
‘Methods’). Instantaneous heart rate (HR) changes (in beats per minute; purple) and respiratory
(orthostasis or syncope) is caused by S abnormalities. Therefore, it
activity (RA: in volts; blue) are plotted against time. At (1) the short Valsalva is initiated by
seems reasonable to hypothesize that every autonomic imbalance is a
inhalation. At (2) the sudden increase in intrathoracic pressure stimulates the thoracic
baroreceptors, causing a momentary parasympathetic (Vagal) response, followed by a drop in
separate and distinct dysfunction. A single agent can often address both
HR. Due to decreased venous return to the heart, sympathetic activity is stimulated and there is
the primary disease and autonomic disorders. PE can also be treated
a gradual increase in HR starting at (3). Upon release of Valsalva during exhalation (4), there is
an overshoot of blood pressure (BP) resulting from the sudden rush of blood back to the heart.
concurrently. Once the PE is corrected, the patient is more stable and
This overshoot is compounded by the residual sympathetic activity (causing peripheral
the primary disease(s) can be treated more aggressively.
vasoconstriction) exaggerating the blood rush into the thorax and opposing venous return into
the extremities. The deep inhalation (5) causes parasympathetic inhibition and a subsequent rise
in HR. After the release of the Valsalva, HR and RA finish returning to normal (6). Normal sinus
Measures of P and S activity are critical to understanding the true nature
(HR) rhythm returns (7), synchronized with RA, and continues until the end of the recording (8).
of autonomic dysfunction and its clinical implications. Simultaneous,
independent documentation of P and S activity has provided more increase, the commonly perceived net effect is that postural change is
insight into many commonly observed clinical conditions. Such an S challenge. It should be noted that the predominant S response to
measures have identified failures in the reactive push–pull dynamics Valsalva is a beta-adrenergic response, and the predominant S
within the ANS. PE appears to be the primary autonomic disorder and the response to postural change is an alpha-adrenergic response. In this
(reactionary) S abnormalities appear to be secondary. This article will way, it is possible to detect both an SE (in response to Valsalva) as well
discuss longitudinal studies showing PE, its correction, and outcomes. as an S insufficiency or S withdrawal (in response to postural change).
Background Normal Autonomic Nervous System
PE presents in response to an S challenge. For example, any type of Responses to a Valsalva Maneuver
P increase in response to head-up postural change (standing) is Figure 1 depicts the instantaneous HR (purple) and respiratory (blue)
considered abnormal. Normally, P activity decreases first, potentiating responses of a normal subject during a short Valsalva. During a short
the S increase that follows to perpetuate the expected vasoconstriction Valsalva, there is a sudden increase in intrathoracic pressure. This
required to counter orthostasis. Stress responses such as short Valsalva mechanical pressure increase is falsely interpreted by the baroreceptors
maneuvers (<15 seconds) are expected to cause a decrease in average at the heart as a sudden increase in BP due to an increase in cardiac
P activity. An increase in P activity to either of these S challenges output. In actuality, the Valsalva lowers cardiac output by shunting blood
appears to force a higher S response than typical for that patient and away from the heart. The Valsalva is initiated by inhalation (see Figure 1,
the condition. These relative S excesses (SE) are often experienced as #1, respiratory response in blue), causing P (vagal) inhibition, immediately
increases in blood pressure (BP) or heart rate (HR), and as such may be followed by an increase in HR (see Figure 1, #1, HR response in purple)
treated as a primary SE. However, patient responses under these from the cardiovagal inhibition as stretch receptors in the lungs are
autonomic conditions are unexpected. By definition, PE is a dynamic unloaded. The sudden increase in intrathoracic pressure stimulates the
autonomic imbalance and not a resting imbalance. Understanding the thoracic baroreceptors, which causes a momentary P response, followed
physiology of some commonly used clinical assessment challenges can by a drop in HR (see Figure 1, #2, HR response). As blood is shunted away
help to elucidate PE and its clinical implications. The short Valsalva from the thorax, two reflexes, the baroreceptor reflex and the
maneuver is normally a significant S challenge and should have little or venoarteriolar axon reflex, stimulate S activity. Due to decreased venous
no net P effect. Valsalva simulates normal stresses that occur daily, return to the heart and decreased cardiac output, aortic pressure
such as exercise, bowel movements, and domestic, environmental, and reduces and the aortic baroreceptors are unloaded. In addition, there is
workplace-related stress. Clinically, Valsalva can help differentiate no opposing peripheral vasoconstriction and no pre-existing S activity
normalized hypertensives from those who are only normotensive at rest upon initiation of the Valsalva. Therefore, as blood is shunted freely into
and still at risk for stroke or myocardial infarction (MI) due to SE from the peripheral vessels, the venoarteriolar axon reflex is initiated as
stress. The head-up postural change challenge is a challenge to both transmural pressure within the blood vessels begins to exceed 25mmHg.
ANS branches, and a test of coordination between the two. As the P As a result, S activity is stimulated and there is a gradual increase in HR
activity is expected to decrease and the S activity is expected to (see Figure 1, #3, HR response).
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