[Variability of heart rate as a diagnostic criterion for determining brain death]

Determination of brain death and proof of its irreversibility are necessary when intensive care applied to patients with cerebral areflexia is to be discontinued. In almost all countries a 12-hour observation time with consecutive clinical confirmation of brain death is sufficient to meet the legal needs. If it is possible to get organs for transplantation purposes, a reduction of the observation period is required. Permissible and suitable technical methods for this purpose are reported on. Heart rate variability as a clinical parameter has not yet been sufficiently investigated. Its validity for diagnosing brain death is examined. The beat-to-beat heart interval was measured in 46 patients 68 times and dispersion as well as the coefficient of variability were calculated. In 25 cases the time of investigation was shortly after clinical deterioration and in 27 cases after brain death had been confirmed by a neurologist. Furthermore, 14 and 7 preliminary investigations, respectively, were considered comparatively. It was shown that the significant decrease in heart rate variability was a prognostically bad symptom. Moreover, a fast Fourier transformation was carried out. The most important frequency ranges, marked A, B and C, and their respective percentage were compared. In the course of intensive therapy a remarkable decrease in the B percentage and a simultaneous increase in the C percentage are indicators for bad prognosis.