[HEART RATE VARIABILITY: THE AGE-OLD TOOL STILL REMAINS CURRENT].

INTRODUCTION Heart rate variability (HRV), the fluctuation in the time intervals between adjacent heartbeats, is generated by heart-brain interactions and dynamic autonomic nervous system processes. The study of HRV originated in the 18th century and served Einthoven in the development of the electrocardiogram (ECG), the basis for modern ECG technology. HRV reflects the regulation of autonomic balance, blood pressure, gas exchange, heart, and vascular tone. While 24-hour recording remains the gold standard for HRV assessment, short (5 minutes) and ultra-short (less than 5 minutes) HRV documentation is accurate and considered a more practical means of HRV measurement. HRV metrics include: time-domain indices (e.g. Standard Deviation of NN intervals (SDNN) - correlates with morbidity and mortality of patients after an acute coronary event); frequency-domain measurements (e.g. High Frequency - influenced by breathing and reflects parasympathetic activity); and non-linear analysis, which enables quantification of the unpredictability of a time series. While ECG is the cornerstone of chest pain triage, HRV may assist in the diagnosis of future cardiac disease, including heart failure and atrial fibrillation. In addition, HRV has been extensively studied as a means for risk stratification. Reduced SDNN following an acute coronary event, has been correlated with mortality. Lower SDNN levels were also associated with ventricular arrhythmia in pediatrics patients with acute myocarditis. Higher non-linear indices after an acute coronary event have been related to all-cause mortality.