Cerebral autoregulation dynamics in premature newborns.

BACKGROUND AND PURPOSE

Autoregulation of cerebral blood flow is easily disrupted, and loss of this normal physiological reflex may worsen the neurological outcome for patients undergoing intensive care. We studied the response of cerebral blood flow velocity to changes in mean arterial blood pressure.

METHODS

Cerebral blood flow velocity was measured with Doppler ultrasonography in one middle cerebral artery for 5-minute periods in 33 babies of gestational age < 33 weeks admitted to a neonatal intensive care unit. Two methods of evaluating autoregulation were developed. The first used linear regression analysis of blood flow velocity on blood pressure. Records were classified as showing loss of autoregulation if the regression slope was greater than a critical value. A minimum change in mean arterial blood pressure of 5 mm Hg and a critical slope of 1.5%/mm Hg were found to be adequate criteria for the classification of records by the regression method. The second method used coherent averaging, a technique similar to that used in recording evoked potentials. Spontaneous transient increases in blood pressure were automatically detected, and the instant corresponding to its maximum rate of rise was used to synchronize averages of the blood pressure and blood velocity transients. The resulting coherent averages were classified into two groups based on the morphology of the cerebral blood flow velocity average.

RESULTS

Whereas the regression method allowed the classification of only 51 of 106 records, the coherent average method classified 101 of 106 (95.3%) of the records available. For 51 records that were classified by both methods, there was agreement in 42 cases (82.3%). The coherent average of all records classified as having an active autoregulation showed cerebral blood flow velocity returning to baseline much earlier than blood pressure, suggesting that autoregulation was taking place within 1 to 2 seconds. This pattern was absent in records in which autoregulation was classified as absent.

CONCLUSIONS

Computerized coherent averaging of the cerebral blood flow velocity response to spontaneous blood pressure transients offers a promising new method for noninvasive bedside assessment of autoregulation in patients undergoing intensive care. The time course for autoregulation, when present, is in agreement with that reported in adults.