BACKGROUND AND PURPOSE: The use of flow velocity changes in the middle cerebral artery (MCA) measured by Doppler techniques as an index of corresponding cerebral blood flow (CBF) changes is based on the assumption that the insonated arterial diameter remains stable. The postulate of unchanging vessel calibers during CBF changes, however, is still under debate. We performed simultaneous measurements of arterial and venous blood flow velocities by transcranial Doppler ultrasound during various stages of hypercapnia to analyze diameter changes in the insonated vessels by comparing differences in the vasomotor reactivity.
METHODS: Simultaneous Doppler recordings of 1 MCA and of a contralateral venous vessel thought to represent the sphenoparietal sinus (SPS) were carried out with a pair of 2-MHz range-gated transducers in 16 young healthy subjects during variations of end-tidal PaCO2.
RESULTS: During hypercapnia the mean blood flow velocity of the MCA rose from 62. 5+/-10.2 to a maximum of 99+/-12.2 cm/s (vasomotor reactivity of 60. 1+/-17.3%). The corresponding values in the SPS were significantly higher (P<0.001), revealing a rise from 17.8+/-5.7 to 34.9+/-14.3 cm/s (vasomotor reactivity of 91.4+/-25.9%). Exponential and linear regression analyses revealed an identical high correlation (r2=0.97 and 0.98 for the MCA and SPS, respectively). Slopes were 0.034+/-0. 01 on the arterial and 0.048+/-0.01 on the venous side. The CO2 reactivity (percentage per mm Hg, EtCO2) was found to be 4.5+/-1%/mm Hg in the MCA and 6.8+/-1.5%/mm Hg in the SPS. This difference indicates a vasodilation of the MCA in comparison to the venous vessel.
CONCLUSIONS: We have demonstrated a different reaction pattern between intracranial venous and arterial vessels related to end-tidal CO2. Relating the flow velocities to the square of the vessel diameter and assuming a global rise of CBF and not extensible sinus walls, our results indicate that the MCA undergoes a vasodilation of 9.5+/-7% in maximal hypercapnia.