Heart rate variability (HRV) can be assessed through a time frequency analysis, the smoothed pseudo Wigner-Ville transformation (SPWVT). Such an analysis has allowed the calculation of ICF, a frequential index, extremely sensitive to the modulation of the sympatho-vagal balance. The use of ICF has been applied in the present study to two clinical situations for which a role of this balance was probable, brain death and atrial fibrillation (AF). The use of the time frequency analysis immediately identified the timing of brain death. No difference could be found in the analysis of the 30 min preceding an atrial fibrillation episode. It is suggested that a time frequency analysis should be used in clinical situations in which transitory fluctuations of the sympatho-vagal balance are expected and crucial to the well being of the patients.