Figure 1: Heart rate variability. Note: The heart rate variability refers to the beat-to-beat variability in heart rate. It can be assessed in both the time and the frequency domain. The most common parameters for evaluation of the heart rate variability include the root mean square of successive differences in the time domain, very low frequency, low frequency, and high frequency in the frequency domain. See Table 1 for a more detailed description of the parameters used in the evaluation of the sympatheci and parasympathetic nervous system.