Basis for the heart rate variability to deep breathing test

What is the basis for the heart rate variability to deep breathing test? 

Usually, inspiration increases and expiration decreases the heart rate (sinus arrhythmia).

This phenomenon is mediated primarily by the vagus nerve. Pulmonary stretch receptors and cardiac mechanoreceptors, as well as baroreceptors, contribute to its generation.

This test is easy to perform with a commercial ECG machine. While supine with the head elevated to 30 degrees, the patient breathes deeply at six respirations per minute (5 seconds’ inspiration followed by 5 seconds’ expiration, usually for eight cycles), and minimal and maximal heart rate within each respiratory cycle is measured.

The simplest index is the heart rate variability defined as maximum heart rate minus minimum heart rate.

Another index, the E/I (excitation and inhibition) ratio, is determined by the longest R–R interval on the ECG divided by the shortest R–R interval. An abnormal test indicates parasympathetic dysfunction.

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