Table 2: Mortality Risk Associated with Various Heart Rate Variability Measurements Using Resting ECG.

Study	Number of Patients	Monitoring Method	HRV Parameters	Conclusions
Zutphen study Dekker et al. 1997[2]	878 (middle-aged cohort) 885 (elderly cohort)	Resting ECG, 15–30 sec	SDNN	SDNN <20 ms associated with increased risk of CHD (RR 2.1, 95 % CI [1.1–4.1]) and all-cause mortality (RR 2.1, 95 % CI [1.4–3.0])
Rotterdam study De Bruyne et al. 1999[3]	5,272	Resting ECG, 10 sec	SDNN	SDNN in lowest and highest quartiles had increased risk of cardiac mortality; HR 1.8 (95 % CI [1.0–2.3]) and 2.3 (95 % CI [1.3–4.0]) respectively
ARIC study Dekker et al. 2000[4]	900	Resting ECG, 2 min	SDNN, rMSSD, SDSD, pNN50	Increased risk of all-cause mortality for patients in the lowest tertile of all parameters (RR 1.47–1.91)
Yoo et al. 2011[5]	85	Resting ECG	SDNN, rMSSD, VLF, LF, HF, LF/HF, TP	SDNN (28ms versus 36ms, p=0.037), rMSSD (28ms versus 29ms, p=0.007), and lnHF (4.7ms2 versus 5.5ms2, p=0.008) are depressed in patients with FRS >10 %
La Rovere et al. 2003[6]	444	Resting ECG, 8 min	SDNN, LF and HF	Increased risk of sudden death with reduced LFP (RR 2.8, 95 % CI [1.2–6.8], p=0.02)

FRS = Framingham risk score; HF = high frequency power; HRV = heart rate variability; LF = low frequency power; LF/HF = low frequency to high frequency power ratio; ln HF = natural log of the high-frequency measurement; pNN50 = percentage of RR intervals that differ by 50ms; rMSSD = root mean square of the differences in successive R-R intervals; SDNN = standard deviation of NN intervals; SDSD = standard deviation of absolute differences between successive intervals; TP = total power; VLF = very low frequency power.