Heart Rate Variability (HRV)
Heart rate variability (HRV) refers to the beat-to-beat alterations in heart rate. Under resting conditions the heart rate of healthy individuals exhibits periodic variation. This rhythmic phenomenon, known as respiratory sinus arrhythmia (RSA), fluctuates with respiration – accelerating during inspiration, and decelerating during expiration. However, it is difficult to distinguish the contribution to this phenomenon from the vagus nerve, the sinus node, the AV node, and the afferent/efferent pathways conducting the neural impulses to and from the brain.
Although our understanding of the meaning of HRV is far from complete, it seems to be a marker of acute stress (which lowers heart rate variability) and also a marker of cumulative wear and tear. HRV has also been shown to decline with the aging process. Although resting heart rate does not change significantly with advancing age, there is a decline in HRV, which has is thought to be due to a decrease (aging) in the vagus nerve and the heart’s response to the vagus nerve effect. On the other hand, regular physical activity (which slows down the aging process) has been shown to raise HRV, presumably by increasing vagal tone. A health HRV is thus thought to be a sign of either youth or health, or both and may be a measure of the overall effect of a lifetime of abuse or the body or conversely of healthy habits, particularly exercise.