Vasomotor properties of the M2 segment of the middle cerebral artery

Vasomotor response can be tested by means of transcranial Doppler sonography. It is assumed that flow changes during hyperventilation or breath-holding are due to altered resistance caused by changes in the diameter of vessels distal to the M1 segment of the middle cerebral artery. Evaluating velocities of the M2 and M1 segments together may help elucidate the vasomotor properties of distal segments. We simultaneously evaluated the vasomotor properties of the M1 and M2 segments of the middle cerebral artery in ten healthy volunteers (mean age 21.6 +/- 0.9 years) with breath-holding and hyperventilation. The radius index (RI), defined as the ratio of the M1-M2 segment radii, was estimated after 20 s and after the maximum duration of breath-holding, as well as at the first, second, and third minute of hyperventilation, in relation to velocity ratios. RI values in the resting state were compared with the values after activation procedures. The RI estimated with basal velocities ranged from 0.72 to 0.97 (mean 0.81 +/- 0.07). The RI ranged from 0.73 to 0.97 (mean 0.82 +/- 0.07) after 20 s and 0.75 to 0.90 (mean 0.82 +/- 0.06) after the maximum duration of breath-holding. The RI was 0.72-1.02 (mean 0.85 +/- 0.11), 0.71-1.03 (mean 0.86 +/- 0.12), and 0.71-1.14 (mean 0.88 +/- 0.15) at the first, second, and third minute of hyperventilation, respectively. The differences between RI values in the resting state and during each activation procedure were not statistically significant. These results suggest that the diameter ratio of the M1-M2 segments did not change after vasomotor activation, indicating that the two segments have similar vasomotor properties.