Table 1.
Heart rate variability (HRV) time-domain measures [15]
| Variable | Units | Description |
|---|---|---|
| Statistical measures | ||
| SDNN | ms | Standard deviation of all NN intervals |
| SDANN | ms | Standard deviation of the averages of NN intervals in all 5-minute segments of the entire recording |
| RMSSD | ms | The square root of the mean of the sum of the squares of differences between adjacent NN intervals |
| SDNN index | ms | Mean of the standard deviations of all NN intervals for all 5-minute segments of the entire recording |
| SDSD | ms | Standard deviation of differences between adjacent NN intervals |
| NN50 count | Number of pairs of adjacent NN intervals differing by more than 50 ms in the entire recording; three variants are possible counting all such NN intervals pairs or only pairs in which the first or the second interval is longer | |
| pNN50 | % | NN50 count divided by the total number of all NN intervals |
| Geometric measures | ||
| HRV 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 7.8125 ms (1/128 seconds) | |
| TINN | ms | Baseline width of the minimum square difference triangular interpolation of the highest peak of the histogram of all NN intervals |
| Differential index | ms | Difference between the widths of the histogram of differences between adjacent NN intervals measured at selected heights (e.g., at the levels of 1,000 and 10,000 samples) |
| Logarithmic index | Coefficient φ of the negative exponential curve k · e−φt, which is the best approximation of the histogram of absolute differences between adjacent NN intervals |
NN: normal-to-normal