Transcranial Doppler is used to estimate changes in cerebral blood flow, but the effect of hemodilution on cerebral blood flow velocity (CBFV) in anesthetized patients has not been evaluated. The aim of this study was to measure the effect of isovolemic hemodilution on CBFV and lumbar cerebrospinal fluid pressure (P(CSF)) in anesthetized patients without change in other physiological variables that may affect CBFV. Patients undergoing hemodilution were compared with a control group undergoing no hemodilution. With hemodilution, hematocrit decreased from 38% +/- 3% to 30% +/- 2%, arterial oxygen content (Cao2) decreased from 17.5 +/- 1.3 to 13.9 +/- 0.9 mL/dL, and CBFV increased from 50 +/- 10 to 58 +/- 10 cm/s. An equivalent of cerebral arterial O2 transport calculated as Cao2 x CBFV did not significantly change. Over the same time interval, there were no changes in the control group. There was no statistically significant change in P(CSF), pulsatility index, Paco2, blood pressure, heart rate, or body temperature in either group. We conclude that CBFV reflects cerebral blood flow changes after hemodilution.
Implications: Hemodilution increases cerebral blood flow but may change the cerebral artery diameter, which could confound perioperative measurement of cerebral blood flow velocity. This study found transcranial Doppler ultrasonography to accurately assess the effects of hemodilution on the cerebral circulation, but the hematocrit should be taken into account to fully understand perioperative cerebral blood flow velocity changes.