decreased heart rate variability true reduced variation of beat-to-beat intervals of the heart that occurs in conjunction with the respiratory cycle https://www.ncbi.nlm.nih.gov/pubmed/26441491 reduced heart rate variability MP:0003929 decreased respiratory sinus arrhythmia MPheno.ontology Heart rate variability (HRV) is the variation of beat-to-beat intervals. A healthy heart has a large HRV, while decreased or absent variability may indicate cardiac disease. HRV also decreases with exercise-induced tachycardia. Heart rate variability (HRV) refers to the beat-to-beat alterations in heart rate. Under resting conditions, the ECG of healthy individuals exhibits periodic variation in R-R intervals. This rhythmic phenomenon, known as respiratory sinus arrhythmia (RSA), fluctuates with the phase of respiration -- cardio-acceleration during inspiration, and cardio-deceleration during expiration. RSA is predominantly mediated by respiratory gating of parasympathetic efferent activity to the heart: vagal efferent traffic to the sinus node occurs primarily in phase with expiration and is absent or attenuated during inspiration. The major reason for the interest in measuring HRV stems from its ability to predict survival after heart attack. Over half a dozen prospective studies have shown that reduced HRV predicts sudden death in patients with MI, independent of other prognostic indicators such as ejection fraction. Reduced HRV appears to be a marker of fatal ventricular arrhythmia. Moreover, a small number of studies have begun to suggest that reduced HRV may predict risk of survival even among individuals free of CHD. abnormal sinus arrhythmia