Spectral analysis of heart rate variability (HRV) provided an estimate of the sympathetic and parasympathetic influences on heart rate without drugs or other invasive procedures. However, there are many situations where heart rate changed rapidly over time and the control of those changes was 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. As a result the assessment of the autonomic nervous system during rapid changes in heart rate was made.

There were three advantages to calculating vagal tone and sympatho-vagal balance by using timefrequency analysis. First, vagal tone and sympatho-vagal balance were described as functions of time. Hence, a better understanding was attained about autonomic control of rapidly changing signals. Second, the rate of increase or decrease of vagal tone and sympatho-vagal balance was derived. These rates were used to determine the integrity of the autonomic system. Third, a mental component affecting vagal tone was suggested. It is anticipated that the effects of anxiety on vagal tone can be revealed using time-frequency analysis.