Circulatory Autonomic Testing

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Autonomic Testing, Heart Rate Variability, Blood Pressure Variability, and the Baroreflex

 

Autonomic activity primarily regulates orthostatic tolerance. Simple techniques can be used to assess certain aspects of the autonomic control of heart rate and blood pressure. 

Recently, techniques measuring the variation of heart rate around some mean or trended value have been useful in assessing the autonomic nervous system. Many believe that patterns of heart rate variation relate closely to the modulation of autonomic nervous system tone and thus reflect sympathetic and parasympathetic activity. Heart rate may be analyzed in two complementary domains: the time domain and the frequency domain. A simple example of a frequency domain parameter is the standard deviation about the mean heart rate or RR interval. Frequency domain measurements employ techniques of spectral analysis. A similar procedure may be applied to the blood pressure variation.

 

Contents:

        The quantitative Valsalva maneuver

        Circulatory findings in the Valsalva maneuver  

        Static Handgrip  

        Paced respirations

 

  1. Heart rate and blood pressure variability

  2. Coherence and baroreceptor gain

  3. Findings in POTS and CFS

  4. Return to Home Page

 

 Cardiovascular Autonomic Function

There are many potential tests of cardiovascular autonomic function. Two of the most c ommon include the quantitative Valsalva maneuver and paced or timed respirations.

 

Quantitative Valsalva Maneuver
The quantitative Valsalva maneuver is performed by blowing with an open glottis into a mouthpiece connected to the mercury column of a sphygmomanometer with an air leak. A 40-50 mmHg pressure is maintained for 15 seconds. BP recovery in phase II and cardiopressor response in phase IV are indices of vasoconstrictor and contractile integrity. Baroreceptor mediated tachycardia in phase II and bradycardia in phase IV determines if cardiovagal reflexes are intact. The tachycardia ratio (ratio of the shortest RR interval during the test to the longest RR interval before the test) and the Valsalva ratio (ratio of the longest RR interval after the maneuver divided by the shortest RR interval during the test) are typical indices. Decreases of greater than 20 mmHg during early phase II combined with absent phase IV or late phase II are indicative of blunted vasoconstrictive response.
New circulatory findings in the Valsalva maneuver  
 
Static Handgrip
Static handgrip typically uses a handgrip dynamometer as shown in the figure. Subjects first generate a maximum handgrip. thereafter a percentage of this maximum, often 30%, is sustained for a period of time or until exhaustion. 
This furnishes an afferent signal to the brain that is independent of the baroreflex (i.e. independent o blood pressure). The signal takes its origin from several mechanisms: 1) central command, a signal from higher neurological centers that initiates and sustains exercise, 2) mechanoreflexes from contracting muscle, 3) metaboreflexes which are chemoreflexes within the contracting muscle which are activated primarily during ischemia. The combination of reflexes are often called the exercise pressor reflex since they produce an increase in blood pressure and also in heart rate.  Often, after a period of static handgrip a blood pressure cuff is inflated at suprasystolic pressure on the arm used for the maneuver in order to trap the metabolites that produce the metaboreflex. Once pressure is releaased, this separates the changes due to the metaboreflex from those due to the other reflexes. 
  The exercisse pressor reflex represents one of several somatic reflexes used during autonomic testing. Another would be the cold pressor test where nociceptor afferents are activated. In all such reflexes sympathetic afferents and sometimes vagal afferents carry signals to the nucleus tractus solitarius (NTS) and through reflex and higher brain centers send signals through the rostral ventrolateral medulla where cardiovascular motor efferents are located. There are complex and as yet imperfectly understood interactions with baroreflex neurons within the RVLM such that autonomic reflexes may integrate and influence one-anothers effects.
   
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
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)
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.

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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.

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.
 
Microneurography (Muscle Sympathetic Activity, MSNA)

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 Ám, shaft diameter 200 Ám; 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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