Because little is known about spontaneous changes in cerebral blood flow in neonates, a newly developed online Doppler technique was used to insonate continuously the middle cerebral arteries of a group of sick (n = 20) and full term healthy (n = 16) newborn infants for a period of one minute. A total of 290 recordings of epochs each lasting one minute were analysed, and pronounced regular, cyclical variations were seen in the velocity traces of these infants. The cycles occurred 1.5-5 times/minute and were present for at least one epoch in all 20 of the sick infants and in 15 of the 16 healthy mature neonates. Simultaneous recordings of the systemic blood pressure in the sick infants rarely showed the same cyclical variations. The cyclical variation is different from the beat to beat variability seen in the waveforms previously described, and is an additional factor to account for the wide variation in 'normal' velocity recordings obtained when Doppler ultrasound is measured over a short period of time.
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