The prognostic assessment of patients with severe brain damage is based on clinical judgement using coma scales and electrophysiological examinations, in particular electroencephalography and multimodal evoked potentials. The suitability of the parameter heart rate variability (HRV) and the increase of its suitability by Fast-Fourier-Analysis for such an assessment is discussed. It was demonstrated that the spectral analysis is able to exhibit changes in the different spectral fields in dependence on the degree of severity of the cerebral damage. The comparability with other investigations such as electroencephalography and computer tomography is discussed. Drug influences, the influence of controlled ventilation and the dependence of results on the time of investigation are considered.