Clinical Applications of Heart Rate Variability in the Triage and Assessment of Traumatically Injured Patients

Table 1

Definitions of time-domain metrics of heart rate variability.

Variable	Abbreviation	Description
Standard deviation of normal-normal RR intervals (NN)	SDNN	Measures the standard deviation of RRI for an entire measurement [3]
Standard deviation of average NN interval	SDANN	Measures the standard deviation of RRIs for short segments of ECG (usually 5 minutes) [3]
Root mean squared successive difference	RMSSD	Calculation of the square root of the mean squared differences in successive RRIs [3]
Proportion of successive NN intervals >50 ms	pNN50	Number of the number of interval differences of successive NN intervals >50 ms divided by total number of NN intervals [3]
Integer heart rate variability/heart rate volatility	HRVi	Calculation of the standard deviation of the integer heart rate for 5-minute increments [11, 15, 19–21]
Cardiac volatility-related dysfunction/cardiac uncoupling	CVRD	Percentage of time per 24-hour period that HRVi falls within a critically low range (0–0.5 or 0.3–0.6 bpm) [6, 12, 22]