Autonomic Function Tests

Introduction

Sympathetic and parasympathetic systems control all visceral functions. Neurons of sympathetic and parasympathetic systems are affected in metabolic diseases. Therefore, autonomic dysfunctions are common in metabolic diseases such as diabetes, obesity, hypertension, hyperlipidemia, hyper and hypothyroidism, etc. India is the epicenter of diabetes, hypertension and heart diseases. Therefore, assessment of sympathetic and parasympathetic functions by conventional autonomic function tests (AFTs) is now part of routine clinical management.

Classification Of AFTs

Autonomic function tests (AFTs) are performed to confirm the clinical diagnosis of autonomic dysfunctions and to assess the degree of sympathetic and parasympathetic involvement in the process of dysfunction.

AFTs are classified as:

1. Tests for cardiovascular autonomic functions
   1. Heart rate and Blood pressure (BP) response to standing
   2. Heart rate and BP response to passive tilting
   3. Assessing baroreceptor sensitivity (BRS)
   4. Heart rate response to deep breathing
   5. Valsalva ratio
   6. BP response to hand grip
   7. Cold pressure test
   8. Nor-epinephrine spillage technique
   9. Standing to lying ratio
   10. Spectral analysis of heart rate variability (HRV)
2. Tests for sudomotor functions
   1. Sympathetic skin response
   2. Thermoregulatory sweat test (TST)
   3. Quantitative sudomotor axon reflex test (QSART)
3. Vasomotor test
   1. Laser Doppler Velocimetry for skin blood flow measurement
   2. Cold pressor test
4. Tests for pupillary functions
   1. Cocaine test
   2. Adrenaline test
5. Tests for bladder functions
   1. Test for sphincter-detrusor dyssynergia
   2. Cystometrogram
6. Other methods
   1. Muscle sympathetic nerve activity (MSNA)

Tests for Cardiovascular Autonomic Functions

Heart Rate Response to Standing

Changing the posture from supine to standing, heart rate increases immediately by about 10 to 20 beats per minute.

1. On standing, the heart rate increases until it reaches a maximum at about the 15th beat, after which it slows down to a stable state at about 30th beat.
2. The ratio of R-R intervals corresponding to the 30^th and 15th heart beat is called the 30:15 ratio.
3.  The 30:15 ratio is a measure of parasympathetic function.
4. However, relative bradycardia at 30th beat depends also on the sympathetic reactivity.
5. The normal 30:15 ratio is 1.15–1.12 at 21 to 30 years and 1.12 to 1.10 at 31 to 40 years of age.
6. This ratio decreases with age. Ratio less than 1.04 is considered abnormal.

Blood Pressure Response to Standing

The changes in blood pressure on standing are studied to assess the integrity of the sympathetic system.

1. Immediately on standing, blood pressure falls, which activates baroreceptor reflex and blood pressure returns to normal within 15 seconds.
2. When there is sustained fall in systolic pressure more than 20 mm Hg or diastolic fall more than 10 mm Hg within three minutes of standing, orthostatic hypotension is said to be present.

Assessing Baroreceptor Reflex Sensitivity (BRS)

Baroreceptor reflex sensitivity is a newer method of assessing autonomic reactivity to various stimuli, such as orthostatic challenge and injection of chemicals and drugs that change the blood pressure.

1. This is assessed by continuous blood pressure variability (BPV) measurement. Sensitivity of baroreceptors to change in dynamic component (fluctuations) of blood pressure is an important marker of sympathetic and parasympathetic systems. Thus, it is a reflection of integration of both the components.
2. BRS is expressed as ms/mm Hg.
3. BRS less than 20 ms/mm Hg reflect poor cardiovascular (CV) health, and BRS less than 15 ms/mm Hg is an indicator of increased CV risk.
4. BRS 25 ms/mm Hg or more indicates enhanced autonomic tone and improved CV health.

Norepinephrine (NE) Spillage Technique

Norepinephrine level in plasma is measured in supine position and after 5 minutes of standing. The difference in level of NE represents alteration in sympathetic-hormonal reactivity.

Heart Rate Response to Tilting

Heart rate response to head up tilt (HUT) is a useful tool in the diagnosis of autonomic dysfunctions. It is more accurate because the active change of position is avoided by passively tilting the subject on a tilt-table. Moreover, variation in time taken by individuals to stand and the manner in which they stand from supine position is avoided in this method.

1. On changing from recumbent to operate position on a tilt table to 60 to 80 degrees HUT, pooling of about 30% venous blood occurs in the peripheral vascular compartment, especially in lower limbs.
2. This decreases cardiac filling pressure and stroke volume by about 40%.
3. Heart rate rises immediately due to withdrawal of parasympathetic activity and afterward due to increased sympathetic activity.

Standing to Lying Ratio (SLR)

Heart rate (RR interval) response to lying down from standing posture is assessed by continuous recoding of ECG.

1. Following lying from standing position, increase in venous return produces reflex bradycardia.
2. Longest RR interval in standing to shortest RR interval in lying down is calculated as SLR.
3. Value of SLR below 1 is considered as abnormal.

Heart Rate Response to Deep Breathing

The variation of heart rate with respiration is known as sinus arrhythmia. Inspiration increases and expiration decreases heart rate.

1. This is primarily mediated via parasympathetic innervation of heart. Pulmonary stretch receptor, and cardiac mechanoreceptors and baroreceptors contribute to sinus arrhythmia.
2. The difference between the maximum and minimum heart rate during a deep breathing is called deep breathing difference (DBD).
3. DBD is more than 15 beats per minute in normal individual. It assesses the parasympathetic activity. DBD decreases with age.
4. It is one of the best parasympathetic reactivity test.

Normal values of DBD at different age group are:

10 to 40 years: > 18 beats per minute

41 to 50 years: > 16 beats per minute

51 to 60 years: > 12 beats per minute

61 to 70 years: > 8 beats per minute

Usually, subject is asked to inhale deeply for five seconds and then exhale for five seconds for six cycles. The ratio of shortest RR interval in inspiration to longest RR interval in expiration is calculated for each, which is called expiration-inspiration ratio (E/I ratio). The average E/I ratio of six cycles in a normal young individual is about 1:20. The E/I ratio decreases with age (Table 34.1). Normally, instead of DBD expressed in terms of beats per minute, E:I ratio is usually considered for assessing parasympathetic reactivity to deep breathing. DBD is abnormal in multisystem atrophy, progressive autonomic failure, diabetes mellitus, autonomic neuropathy and CNS depression.

Valsalva Ratio

The Valsalva ratio is a measure of parasympathetic and sympathetic function.

1. In Valsalva maneuver (named after scientist A M Valsalva, who described it), parasympathetic is the afferent and the efferent, and sympathetic is the part of the efferent pathway. Therefore, Valsalva ratio assesses more of parasympathetic (cardiovagal) than sympathetic functions.
2. The procedure is performed by closing both nostrils and then blowing into a tube connected to sphygmomanometer. By putting strain, blowing pressure is maintained at 40 mm Hg for 15 seconds.

Table 34.1: Normal values of E:I ratio.

Age (years)	E:I ratio
16 to 20	: > 1.23
21 to 25	: > 1.20
26 to 30	: > 1.18
31 to 35	: > 1.16
36 to 40	: > 1.14
41 to 45	: > 1.12
46 to 50	: > 1.11
51 to 55	: > 1.09
56 to 60	: > 1.08
61 to 65	: > 1.07
66 to 70	: > 1.06

Table 34.2: Normal values of Valsalva ratio at different age.

Age (years)	Valsalva ratio
10 to 40	: > 1.5
41 to 50	: > 1.45
51 to 60	: > 1.40
61 to 70	: > 1.35

Valsalva Maneuver

Valsalva maneuver has four phases:

Phase I: Phase I consists of the onset of strain. In this phase, there is transient increase in blood pressure that lasts for a few seconds. This occurs due to increased intrathoracic pressure and mechanical compression of the great vessels. However, heart rate does not change much.

Phase II: This is the phase of straining. In the early part of this phase, venous return decreases, which in turn decreases cardiac output and blood pressure. This change persists for 4 seconds. In the later part of this phase, blood pressure returns towards normal, which occurs due to increased peripheral resistance as a result of sympathetic vasoconstriction. However, heart rate increases steadily throughout this phase due to vagal withdrawal (in the early phase) and sympathetic activation (in the later phase).

Phase III: This phase occurs following the release of strain during which there is transient decrease in blood pressure lasting for a few seconds. This is caused by mechanical displacement of blood to pulmonary vascular bed, which was under increased intrathoracic pressure. There is little change in heart rate.

Phase IV: This is the phase that occurs with further release of strain. The blood pressure slowly increases and heart rate proportionately decreases. It occurs following 15 to 20 seconds after release of strain and lasts for about 1 minute or more. The cardiovascular changes occur due to increase in venous return, stroke volume and cardiac output.

Valsalva ratio is the ratio of minimal heart rate in phase IV to maximum heart rate in phase II as depicted in terms of RR interval.

Clinical Correlation

Valsalva ratio more than 1.45 is considered to be normal.

1. Ratio 1.2–1.45 is considered borderline, and ratio less than 1.2 is regarded abnormal.
2. The normal ratio is different at different age groups (Table 34.2).
3. Valsalva ratio is also affected by gender, posture of subject in which recording is done, expiratory pressure, duration of strain and level of yoga practice of the subject.

Changes in Valsalva ratio occur due to changes in cardiac vagal efferent and sympathetic vasomotor activity, which are stimulated by carotid sinus and aortic arch baroreceptors and other intrathoracic stretch receptors. Failure of heart rate to increase during strain suggests a sympathetic dysfunction and failure of heart rate to slow down after the strain suggests parasympathetic dysfunction. If the cardiovascular response to Valsalva maneuver is abnormal but that to cold pressure test (see blow) is normal, the lesion is supposed to be present in the baroreceptors or their afferent nerves. Such types of abnormalities occur commonly in diabetes, other neuropathies, multisystem atrophy and autonomic failure.

BP Response to Sustained Hand Grip

In hand-grip test is an isometric exercise in which the subject is asked to maintain hand grip against resistance.

1. Resistance usually offered by using a hand grip dynamometer at a 30% of maximum voluntary contraction for 5 min. BP and heart (HR) are recorded before and after the hand grip.
2. In hand-grip test, heart rate and blood pressure increase.
3. These cardiovascular responses to isometric exercise are mediated partly by central motor command and partly by mechanical changes or both, in response to contraction of the muscles that activate small fibers in the afferent limb of the reflex arch.
4. The normal response is rise in diastolic pressure more than 15 mm Hg and rise in heart rate by about 30%.
5. The blood pressure rise is due to increased sympathetic activity and heart rate rise is due to decreased parasympathetic activity.
6. The responses to hand grip test are usually not dependent on age.
7. Isometric handgrip test is one of the best sympathetic reactivity tests.

Cold Pressure Test

This test is performed by submerging the upper limb of the subject in ice cold water at 4°C for 30 to 60 sec. and BP is recorded before and after the procedure. The submersion of hand in cold water increases systolic pressure by about 20 mm Hg and diastolic pressure by 10 mm Hg.

1. The afferent limb of the reflex pathway is somatic fibers whereas the efferent pathway is the sympathetic fibers.
2. Thus, it assesses sympathetic activity.
3. Cold pressor test is one of the best sympathetic reactivity tests.

Tests for Sudomotor Functions

Sympathetic Skin Response

Sympathetic skin response (SSR) helps in studying the functions of peripheral sympathetic cholinergic (sudomotor) fibers by evaluating the changes in resistance of skin in response to electrical stimuli.

1. SSR is age dependent and is present in both hands and feet till the age of 60.
2. Composition of surface electrodes, stimulus frequency, skin temperature, and mental state of the subject affect the parameters of SSR.
3. The latency and amplitude of SSR are measured.
4. The amplitude of SSR in hand is 1.6 mV and in feet is 2.1 mV. SSR is helpful in diagnosing multisystem atrophy, progressive autonomic failure, diabetes, uremic patients and alcoholic neuropathy.

Thermoregulatory Sweat Test (TST)

Assessment of sweating response to heat also assesses sudomotor functions.

1. The subject’s body temperature is raised to by 1°C by exposing to heat of the electric heater.
2. Sweating response is studied by demarcating the area of sweating with the help of iodide starch or quinizarin powder that changes the color of the moist skin.
3. Absence of sweating in TST indicates sympathetic pre and post-ganglionic lesions.

Quantitative Sudomotor Axon Reflex Test

Quantitative sudomotor axon reflex test (QSART) is a measure of regional autonomic function by Ach-induced sweating.

1. In this test, Ach is injected intradermally and the sweat production rate is assessed.
2. Reduced or absence of sweating indicates post-ganglionic lesion of sudomotor fibers (sympathetic fibers concerned with sweating).

Tests for Pupillary Functions

Pupillary function tests assess the function of sympathetic nerve supplying iris. Two tests usually performed are: cocaine test and adrenaline test.

Cocaine Test

Dilation of pupil is observed following instillation of 4% cocaine on both eyes. Cocaine prevents reuptake of norepinephrine at adrenergic nerve endings. Therefore, pupils dilate in response to cocaine, but, Horner’s pupils do not.

Adrenaline Test

Instillation of 1:100 or 1% noradrenaline on eyes dilate Horner’s pupil more than normal pupil. This is due to the mechanism of denervation hypersensitivity of Horner’s pupil.

Tests for Bladder Function

Cystometrogram (CMG) is performed to detect autonomic dysfunctions of urinary bladder. CMG reveals decreased ability of bladder to accommodate urine. Absence of accommodation to filling indicates autonomic dysfunction. Also, contraction of bladder muscle is poor in response to the act of micturition (evacuation).

Spectral Analysis of HRV

Recently, spectral analysis of heart rate variability (HRV) has evolved as a sensitive tool for assessing integrity of sympathetic and parasympathetic functions and for determining

the sympathovagal balance. (Details are given in the next chapter)

AFTs to Assess Sympathetic and Parasympathetic Functions

A. AFTs for assessment of sympathetic functions:

1. BP response to standing/tilt
2. Cold pressor test
3. Isometric hand grip
4. Galvanic/sympathetic skin response
5. Thermoregulatory sweat test
6. Tachycardia ratio
7. Valsalva ratio
8. NE spillage test
9. LF and LFnu of HRV

B. AFTs for assessment of parasympathetic functions:

1. Resting heart rate: Basal heart rate is a good index of parasympathetic functions as heart rate in resting conditions is a measure of vagal tone. Resting HR more 75 indicates poor vagal tone and is presently considered as a CV risk.
2. 30:15 ratio
3. E:I ratio
4. Valsalva ratio
5. Bradycardia ratio
6. Baroreceptor sensitivity
7. Standing to lying ratio
8. HF and HFnu of HRV

Concept of Reactivity and Activity Tests and CAFTs

Reactivity Tests

Tests that are based on stimuli or disturbances such as change in position (standing, lying, dipping finger in cold water, hand grip against resistance, Valsalva maneuver etc.) are called reactivity tests. Accordingly, they are grouped as sympathetic and parasympathetic reactivity tests.

Activity Tests

Tests that are performed without disturbing the subject (subject at rest usually lying on couch in a comfortable room for 15 to 20 min) are called activity tests.

1. Recording of Resting HR and BP, and HRV analysis are examples.
2. Accordingly, they are grouped as sympathetic and parasympathetic activity tests.
3. Resting heart rate is parasympathetic test and resting BP is sympathetic test.

CAFTs

CAFTs refer to conventional autonomic function tests. HR and BP response to standing, HR response to deep breathing, isometric hand grip, cold pressor test and Valsalva maneuver are CAFTs.