Determination of this HRV is often performed to assess patients with cardiovascular disease.
Spectral analysis provides an assessment of the vagal modulation of the R-R interval.
Nonspectral or time domain parameters involve computing indexes that are not directly related to specific cycle lengths. This method offers a simple means of defining patients with decreased variability in the mean and standard deviations of R-R intervals. The height of the histogram equals the total number of intervals found in the modal bin. These 2 analytical techniques are complementary in that they are different mathematical analyses of the same phenomenon. HRV increases with increased periods of observation, and it is important to distinguish ranges on the basis of duration of recording.