Respiratory sinus arrhythmia (RSA) is a naturally occurring variation in heart rate that occurs during a breathing cycle. RSA is also a measure of parasympathetic nervous system activity - which denotes "rest and digest" behaviors.
Vagal tone cannot be directly measured. Instead, other biological processes are measured that represent the functionality of vagal tone. An increase in vagal tone both slows the heart and makes heart rate more variable (i.e. there is more beat-to-beat change between heart beats). During the process of respiratory sinus arrhythmia (RSA), inhalation temporarily suppresses vagal activity, causing an immediate increase in heart rate. Exhalation then decreases heart rate and causes vagal activity to resume. Thus, while we do not explicitly measure vagal tone, we do measure changes in heart rate which are a result of respiratory sinus arrhythmia. This is done by measuring periodic changes in the heart rate during a resting state of cardiovascular activity, a process known as heart rate variability (HRV). Currently, there are about 40 published methods of quantifying HRV. However, the vast majority of studies use only a few of the available measurements. Most common is high-frequency HRV, a measurement of the amount of heart rate variability there is between typical breathing cycles (approximately between 2.5 and 6.7 seconds).
On an electrocardiogram ECG, RSA is seen as subtle changes in the R-R interval (time between two of the distinctive, large, upward "R" spikes on an electrocardiogram) synchronized with respiration. The R-R interval on an ECG is shortened during inspiration and prolonged during expiration. Meditation and relaxed breathing techniques can temporarily alter RSA.
Read more about Respiratory Sinus Arrhythmia: Age Differences
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