Power spectral analysis of heart rate and arterial blood pressure oscillation in brain-dead patients.

To clarify the mechanism of the slow arterial blood pressure oscillation seen in brain-dead patients, we investigated the frequency of fluctuations in arterial blood pressure and heart rate using power spectral analysis. The electrocardiogram, arterial blood pressure and respiration were recorded simultaneously from 9 brain-dead patients and 8 vegetative patients. Power spectral analysis of these data revealed a very slow fluctuation (0.002-0.01 Hz) in arterial blood pressure in brain-dead patients, the frequency of which was equal to that of the low-frequency spectrum of heart rate, indicating vasomotor sympathetic activity. Neuropathological examinations of the medulla and spinal cords of 4 autopsied brain-dead patients revealed that the spinal cord, ventral and dorsal nerve roots, and the nucleus intermediolateralis of the lateral horn below the level of C3/4 were virtually intact. These findings suggest strongly that the slow oscillation of arterial blood pressure in brain-dead patients originates from the vasomotor tone controlled by spinal sympathetic nerves.