SDNN
SDNN (Standard Deviation of NN intervals) is a time-domain measure of heart rate variability (HRV). It captures the overall spread of your normal heartbeat intervals. Per HRV Task Force standards, SDNN is mainly reported over 24-hour recordings. Over a full day, it reflects both your sympathetic and parasympathetic nerve input, plus circadian rhythms and slower low-frequency swings. A short 5-minute SDNN is heavily shaped by your breathing and vagal activity. (For pure vagal tone, RMSSD and HF power are more specific.) A low 24-hour SDNN is tied to higher cardiovascular and all-cause death risk, especially after a heart attack.
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Sources
- Malik M, Bigger JT, Camm AJ, et al.. (1996). Heart rate variability: Standards of measurement, physiological interpretation, and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. *European Heart Journal*doi:10.1093/oxfordjournals.eurheartj.a014868
- Tsuji H, Venditti FJ Jr, Manders ES, et al.. (1994). Reduced heart rate variability and mortality risk in an elderly cohort. The Framingham Heart Study. *Circulation*doi:10.1161/01.CIR.90.2.878