Analysis of CO2 vasomotor reactivity and vessel diameter changes by simultaneous venous and arterial Doppler recordings

Background and Purpose-The use of flow velocity changes in the middle cereb ral artery (MCA) measured by Doppler techniques as an index of correspondin g cerebral blood flow (CBF) changes is based on the assumption that the ins onated 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 now velocities by tr anscranial Doppler ultrasound during various stages of hypercapnia to analy ze diameter changes in the insonated vessels by comparing differences in th e vasomotor reactivity. Methods-Simultaneous Doppler recordings of 1 MCA and of a contralateral ven ous vessel thought to represent the sphenoparietal sinus (SPS) were carried out with a pair of 2-MHz range-gated transducers in 16 young healthy subje cts during variations of end-tidal PaCO2. Results-During hypercapnia the mean blood flow velocity of the MCA rose fro m 62.5 +/- 10.2 to a maximum of 99 +/- 12.2 cm/s (vasomotor reactivity of 6 0.1 +/- 17.3%). The corresponding values in the SPS were significantly high er (P<0.001), revealing a rise from 17.8 +/- 5.7 to 34.9 +/- 14.3 cm/s (vas omotor reactivity of 91.4 +/- 25.9%). Exponential and linear regression ana lyses revealed an identical high correlation (r(2) = 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 CO, 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 compari son to the venous vessel. Conclusions-We have demonstrated a different reaction pattern between intra cranial 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 M CA undergoes a vasodilation of 9.5 +/- 7% in maximal hypercapnia.