The validity of Transcranial Doppler cerebral blood flow velocity (TCD) as an index of cerebral blood flow hinges on the assumption that the insonated cerebral vessel diameters do not change (outside the circumstances of vasospasm). This assumption has been challenged 1 particularly when considering the sensitivity and specificity of TCD to pathological and anesthesia induced changes in CBF. Accordingly we examined changes in both TCD flow velocities and angiographically derived cerebral artery diameters, in patients undergoing elective cerebral artery aneurysmal coiling, with crossover from a vasoconstrictor to a vasodilator anesthetic.
Methods
Four patients were initially examined. Patients received a standard induction technique with propofol, paralytics and narcotics, and were stabilized on propofol infusion thereafter to control blood pressure within 20% of preanesthesia values. They were endotracheally intubated, with invasive arterial pressure monitoring, end-tidal and arterial CO2 (PaCO2) measurement. Baseline measurements of middle cerebral artery (MCA) and internal carotid artery (ICA) flow velocities were made, along with radiographic measurements of cerebral arterial diameters at A1, M1, M2 and M3 segments. The anesthetic agent was changed to 2% isoflurane (end-tidal concentration) and measurements repeated. Blood pressure and arterial CO2 was held constant with changes in ventilation and the use of phenylephrine as required.
Results
The mean change in mean arterial pressure (MAP) was 0.5% (SD 9.2) with PaCO2 of 0.25% (SD 2.1). The MCA and ICA experienced mean changes of 3% (SD 9.6) and 19.1% (SD 15.9), while the A1, M1, M2 and M3 diameters changed by 5.4% (SD 8.7), 1.7% (SD 2.4) 7.9% (SD 2.7) and 3.6% (SD 5.1) respectively.
Conclusions
Minimal changes in arterial diameter were seen accompanying similar changes in intracranial TCD flow velocities under conditions of controlled pCO2 and MAP. TCD blood flow velocity would appear to be a reasonable index of cerebral blood flow. Further data will be presented.