Autonomic nervous system evaluation using instantaneous frequency [ECG analysis]

Spectral analysis of heart rate variability (HRV) provides an estimate of sympathetic and parasympathetic influences on heart rate without drugs or other invasive procedures. However, there are many situations where heart rate changes rapidly over time and the control of those changes is of considerable interest. Time-frequency analysis was utilized to expand the concept of spectral analysis of HRV to describe changes in vagal tone and sympatho-vagal balance as a function of time. Examining the Wigner distribution (WD) and instantaneous frequency (IF) of respiration, it was found that under certain conditions, such as exercise, the upper limit of the frequency contents of respiration exceeded the designated upper 0.4 Hz limit. As a result, new techniques were proposed to compensate and reflect these changes in the measure of vagal activity.