The evolution of low frequency variability in cerebral blood flow velocity (CBFV) was examined in 30 consecutive admissions of very low birthweight babies. Measurement of CBFV was made on the first day of life and at weekly intervals until discharge. Altogether 133 recordings were subjected to visual classification and described as showing presence or absence of variability at a frequency between 1 and 5/minute. Amplitude of variability was expressed as the largest variation in peak systolic velocity as a percentage of the maximum systolic change. Slow variability was usually obvious and was sometimes present for only part of the minute recorded. There was a significant trend for decreased incidence of slow variability in relation to both postconceptional and postnatal ages. Amplitude of slow variability was also damped with increasing age. Slow variability appeared to be unrelated to the type of sedation, severity of illness, or intracranial pathology; it may be a normal phenomenon in which damping occurs as the autonomic nervous system matures.
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