Table 2

Time Domain HRV Parameters

Time-domain indices
Description

Mean RR interval
The average NN interval per hour; a single 24 hour average will be reported.
SDNN
Standard deviation of the NN interval every hour; a single 24-hour average will be reported.
SDNNi
Standard deviation of the NN interval in every 5 minutes; a single 24-hour average will be reported.
RMSSD
The root mean square of successive differences of adjacent NN intervals; a single 24-hour average will be reported.
sNN50
Number of pairs of adjacent NN intervals differing by >50 ms; a single 24-hour value for the increases, decreases and total will be reported.
Triangular Index
Total number of all NN intervals divided by the height of the histogram of all NN intervals measured on a discrete scale with bins of 1/128s; a single 24-hour average will be reported.
sNN6%
Number of pairs of adjacent NN intervals differing by >6%; a single 24-hour value for the increases, decreases and total will be reported.
VPC > 10%/hour
The number of hourly segments with > 10% SVE and VPCs will be reported and the absolute number of SVE and VPCs will also be reported.

Pater et al. Current Controlled Trials in Cardiovascular Medicine 2003 4:2   doi:10.1186/1468-6708-4-2