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Contents:
The
quantitative Valsalva maneuver
New circulatory
findings in the Valsalva maneuver
Static Handgrip
Paced respirations
Heart rate
and blood pressure variability
Coherence and
baroreceptor gain
Findings
in POTS and CFS
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Cardiovascular Autonomic Function
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| Timed
rhythmic breathing is performed in the supine position.
Equal inspirations and expirations at rates of 6 breaths
per minute timed by a metronome are used. Cardiovagal
inhibition and activation are tested. The difference
between maximum and minimum heart rate for each cycle can
be obtained and averaged over the sampling period as an
index of the vagally mediated respiratory arrhythmia. |
Variability
Techniques
| Heart
rate (upper panels) and blood pressure (lower
panels) variability and corresponding frequency
spectra |
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As shown in the figure there are two prominent peaks
in the heart rate variability spectrum, at about 0.1 Hz (the
low frequency peak) and 0.25 Hz (the high frequency peak).
Blood pressure has a similar low frequency peak but much
attenuated high frequency power. The high frequency peak
is generally regarded as related to the respiratory sinus
arrhythmia and therefore represents the modulation of
vagal tone, while the low frequency component is thought
to have its origins in variations in the blood pressure (so-called
Mayer waves) related to sympathetic tone. Sympathetic
blood pressure modulation is transduced into changes in
heart rate through the actions of the vagal arm of the
sinoaortic baroreflex. Thus the low frequency component
relates both to parasympathetic and sympathetic factors. |
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| Coherence (top panel) and baroreceptor
gain (bottom panel) |
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From the relation between variation in
blood pressure and variation in heart rate we can compute
a relation that reflects the action of the baroreflex
based on transfer function analysis. Such a relation
yields the change in heart rate with change in blood
pressure, or equivalently, the vagal arm of the
baroreflex as shown on the left. The "goodness of
fit" of such a relation is reflected by the
coherence between heart rate and pressure signals. |
Findings in POTS
and CFS
Loss of heart rate variability, increased blood pressure
variability , and impaired baroreflex appear to be characteristic
of moderate to severe POTS and are seen also in CFS patients with
POTS.
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These findings are consistent of a state
of vagal withdrawal as may be seen in dehydration and
related states and can be reliably reproduced through
classic vagolytic experiments with agents such as
atropine. Results also suggest a state of relative
sympathetic activation. Resting sympathetic activation
has been observed by investigators in their patients with
chronic orthostatic intolerance many of whom fit criteria
for POTS. |
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| Microneurography
(Muscle Sympathetic Activity, MSNA) |
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Microneurography (MSNA) is a real-time measure of sympathetic nerve
activity. Multiunit recordings of efferent postganglionic MSNA will be
obtained with a tungsten microelectrode into a muscle fascicle of the
peroneal nerve, posterior to the fibular head. A unipolar tungsten
electrode (uninsulated tip diameter 1 to 5 µm, shaft diameter 200 µm;
Frederick Haer and Co.) is inserted into the muscle nerve fascicles of
the peroneal nerve at the fibular head for multi-U recordings. Nerve
activity will be amplified with a total gain of 100 000, band pass
filtered (0.7 to 2 kHz), and integrated (Biomedical Engineering
Department; University of Iowa, Iowa City). A low impedance reference
electrode will be inserted a few centimeters away. After acquiring a
stable recording site, resting MSNA will be recorded. The integrated
MSNA appear as upright “bursts”. Bursts identified by inspection of the
mean voltage neurogram will be expressed as burst frequency (bursts per
min) and burst incidence (bursts per 100 heart beats). Details of the
nerve recording technique and criteria for MSNA have been reported
previously. Criteria for adequate MSNA recording will include: (1) pulse
synchrony; (2) facilitation during Valsalva straining and suppression
during the hypertensive overshoot after release; (3) increases in
response to breath-holding; and (4) insensitivity to startle (ie, loud
noise).
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Circulatory Autonomic Testing
