Factors associated with respiration induced variability in cerebral blood flow velocity.

A consecutive cohort of 73 very low birthweight infants was studied to determine the presence or absence of beat to beat variability in the velocity of blood flow in the cerebral circulation and its relation with respiration. One minute epochs of information included recordings of cerebral blood flow velocity estimated with Doppler ultrasound, blood pressure, spontaneous respiratory activity, and ventilator cycling. Fourier transformation was used to resolve the frequencies present within the one minute epochs and to classify the cerebral blood flow velocity as showing the presence or absence of any respiratory associated variability. A total of 249 recordings was made on days 1, 2, 3, and 7. Forty seven infants showed respiratory variability in cerebral blood flow velocity on 97 occasions, usually during the first day of life. The infants with respiratory associated variability were of lower gestational age and when the respiratory associated variability was present they were more likely to be ventilated and receiving higher inspired oxygen; these associations were shown to be independent of gestational age. There was no significant independent association with brain injury, cerebral blood flow velocity (cm/s), or blood pressure (mm Hg). The findings suggest that artificial ventilation may entrain normal respiratory associated variability in the cerebral circulation but do not provide evidence that it is harmful.