Validation of the ultra short term heart rate variability measures under changing psychophysiological state of the healthy participants at rest.

Heart rate variability (HRV) is the variability of the time intervals between successive heartbeats. The guidelines recommend at least a 5-min ECG recording to establish HRV measures reliably. However, new medical and engineering applications and dynamic environmental conditions enforce using shorter recordings (ustHRV). There are, however, no up-to-date guidelines, and many studies using ustHRV lack methodological rigor. Thus, we investigated which statistical and spectral ustHRV measures may be reliably assessed from shorter ECG lengths compared to the benchmark (5-min) recording. In addition, we investigated how changes in the psychophysiological state of the participants due to mental fatigue and drowsiness, and the difference in the sensory processing, impact the agreement of ustHRV surrogates with the reference signal. The correlation analysis, intra- and inter-group statistical tests, trend analysis, and Bland-Altman plots showed that the statistical measures and HF power from the recordings of as short as 30 s and the other frequency measures of 1 min are good surrogates of the stHRV when the participants are rested and vigilant. Longer recordings may be needed with increasing mental fatigue or drowsiness. This work lays the path to the novel applications of the ustHRV analysis in dynamic environments and factors changing humans' psychophysiological state.