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HRV Response to Controlled Autonomic Stimulation

HRV Response to Controlled Autonomic Stimulation

There are several techniques that allow for autonomic assessment by means of applying certain stimuli that evoke specific responses of either branch of the autonomic nervous system.

Paced Breathing Test

The reflex arc associated with breathing evokes a specific response in heart rate variability. It was first described by Genovely and Pfeifer:

Tidal volume expands the lungs, which activates stretch receptors in the lung, chest wall and heart chambers. The activated sensors stimulate the afferent nerve (vagus nerve). The central processing unit located in the brainstem (the nucleus solitarius) processes information and then decreases parasympathetic tone and/or increases sympathetic tone by sending the appropriate impulses down the efferent vagus and cervical-thoracic-sympathetic nerves, respectively. This information terminates in end-organ (the heart), which responds with an increase in heart rate. Because the reflex arc associated with the R-R variation test is reasonably well known, interpretation of test results is easier to determine than those of the reflex arcs, which are poorly described and understood (such as gastric emptying) or to reflex arcs that are more complicated.

This end-organ response is used in assessment of the autonomic function with the test method described by Wheeler and Watkins in 1973:

With this method, the subject sits quietly while his heart rate is recorded on an ECG. He is then asked to breathe deeply and regularly at a rate of six breaths per minute (5 seconds in, 5 seconds out) for one minute, while the ECG record is continued. The longest and shortest R-R intervals during each breathing cycle are measured from the ECG and converted to beats per minute.

The heart rate variability in this test may be presented in the following parameters: Expiratory/Inspiratory Index (E/I), Standard Deviation, Mean Variance of R-R, etc.

Orthostatic Test

The cardiovascular response to the act of changing a posture from supine to standing was used as an indication of the autonomic function in diabetics for a long time. It is one of several tests described by Ewing that have certain clinical value because they are simple, non-invasive, easy-to-use, reproducible and have clear difference between normal and abnormal results.

During the test, the patient rises from a supine to a standing position. Normally this causes an immediate increase in heart rate and reaches its maximum level at about 15th heartbeat after beginning to stand. It is followed by a relative bradycardia that reaches its maximum around 30th heartbeat. The phenomena can be quantified as 30:15 ratio, which is the ratio of the longest R-R interval around 30th heartbeat to the shortest interval around 15th heartbeat.

Example of an Orthostatic Test report in Heart Rhythm Scanner SE
Example of an Orthostatic Test report in Heart Rhythm Scanner SE

Bennet, Hosking and Hampton et al. h ave studied the relationship between changes in HR and blood pressure caused by standing to demonstrate its value in assessment of the autonomic effect on cardiovascular system in diabetics. They showed a complex interaction between sympathetic and parasympathetic systems and found that parasympathetic dysfunction is typically more dramatic than damage to sympathetic system. The posture test is considered as of highest value when it is performed along with other tests like paced breathing and Valsalva maneuver. The orthostatic test can be performed in Heart Rhythm Scanner Special Edition.

Valsalva Maneuver

The Valsalva maneuver was first described by Antonio M. Valsalva in 1707. In 1860 Einbrodt showed that Valsalva maneuver demonstrated “the integrity of the vagus nerves to the heart” – an acceleration of the heart rate during expiratory effort and slowing it down after it. Later Valsalva maneuver has become so well-known that wouldn’t require its explanation. The formal Valsalva maneuver definition was described by Hamilton et al. in 1943:

In practice the maneuver can be standardized by asking the seated subject to blow into a mouthpiece attached to a manometer to a pressure of 40 mm Hg for 15 seconds, while a continuous heart rate record is made with ECG. The result can then be simply calculated by measuring with a ruler the longest R-R interval after strain (representing the maximal bradycardia) and dividing it by the shortest R-R interval during strain (representing maximal tachycardia). This gives “Valsalva Ratio”.

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