Color Doppler ultrasound of changes in small vessel diameter and cerebral blood flow during acute anemia in the newborn lamb.

RATIONALE AND BACKGROUND: The relationship between changes in small vessel diameter as determined by arteriography and those estimated by color Doppler ultrasound were tested. These techniques were used to examine the effects of acute isovolemic anemia on cerebral blood flow and small vessel diameter in the newborn lamb. MATERIALS AND METHODS: Validation of Doppler measurements was obtained in two lambs. Color Doppler diameter estimates of the anterior tibial artery (1-2 mm) were compared with simultaneous arteriographic measurements during vasodilatation and vasoconstriction effected by intra-arterial infusions of papaverine and norepinephrine, respectively. Acute anemia was produced in five newborn lambs by serial isovolemic exchange transfusions with adult sheep plasma. Doppler estimates of intracranial vessel diameter and mean blood flow velocity were correlated with simultaneous cerebral blood flow (CBF) determinations corrected for cerebral oxygen consumption (CBF/CMRO2). RESULTS: In the anterior tibial artery, color Doppler measurements of vessel diameter correlated well with arteriographic measurements (r2 = .77, P = .001), but overestimated the absolute vessel diameter by 10% to 27%. Hemodilution resulted in significant increases in CBF/CMRO2 (r2 = .61, P = .001), and estimated diameter of small cerebral arteries (r2 = .93, P = .001). Percent change in mean blood flow velocity correlated well with changes in CBF/CMRO2, but underestimated true changes in blood flow by approximately 40% (r2 = .93, P = .0001). Conversely, changes in Doppler volume flow overestimated true change in flow by approximately 50% (r2 = .84, P = .0001). CONCLUSIONS: Under certain physiological conditions, small cerebral arteries may make an important contribution to the overall regulation of CBF. Doppler estimates of relative flow and vessel diameter correlate well with true changes in CBF and small vessel diameter.