Source: Mostly from wiki.

Heart rate variability (HRV，心率變異性)is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.

Other terms used include: "cycle length variability", "RR variability" (where R is a point corresponding to the peak of theQRS complexof theECG(心電圖)wave; and RR is the interval between successive Rs), and "heart period variability".

QRS complex (QRS波群):TheQRS complexis a name for the combination of three of the graphical deflections seen on a typicalelectrocardiogram (EKG or ECG).

In adults, it normally lasts 0.06–0.10 s; in children and during physical activity, it may be shorter.

Reduced HRV has been shown to be a predictor of mortality aftermyocardial infarctionalthough others have shown that the information in HRV relevant to acute myocardial infarction survival is fully contained in the mean heart rate.A range of other outcomes/conditions may also be associated with modified (usually lower) HRV, includingcongestive heart failure,diabetic neuropathy, depression, post-cardiac transplant, susceptibility toSIDSand poor survival inpremature babies.

Variation in the beat-to-beat interval (namely INTERBEAT INTERVAL) is a physiological phenomenon. TheSA nodereceives several different inputs and the instantaneous heart rate or RR interval and its variation are the results of these inputs.

The main inputs are thesympatheticand theparasympathetic nervous system(PSNS) andhumoral factors. Respiration gives rise to waves in heart rate mediated primarily via the PSNS, and it is thought that the lag in thebaroreceptorfeedback loop may give rise to 10 second waves in heart rate (associated withMayer wavesof blood pressure), but this remains controversial.

Factors that affect the input are thebaroreflex,thermoregulation,hormones,sleep-wake cycle, meals, physical activity, andstress.

Errors in the location of the instantaneous heart beat will result in errors in the calculation of the HRV. HRV is highly sensitive to artifact and errors in as low as even 2% of the data will result in unwanted biases in HRV calculations. To ensure accurate results therefore it is critical to manage artifact and R-R errors appropriately prior to performing any HRV analyses.

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