Blood flow velocity in the middle cerebral artery during carotid cross-clamping: loss of regulatory response to carbon dioxide partial pressure. A transcranial Doppler intraoperative study.

The CO2 reactivity of blood flow velocity in the Middle Cerebral Artery (MCA) was explored by Doppler ultrasound, in nine patients, at the time of common carotid artery cross-clamping during vascular surgery, in order to detect if a regulatory response apply during operative occlusion of common carotid artery. Transcranial Doppler was used to monitor ipsilateral MCA blood velocity during carotid surgery. MCA velocity, arterial blood pressure and pCO2 (arterial or end-tidal) were measured, during carotid cross clamping, before and after an increase in pCO2 was induced by changing respiratory rate and volumes. No relevant changes in MCA flow velocities were detected in patients with stable arterial blood pressure. In those patients showing an increase in blood pressure during the surgical procedure the MCA velocity increased accordingly. It is suggested that the cross-clamping of common carotid artery is a critical condition in which no regulatory response can be elicited: blood flow velocity tends to parallel the perfusion pressure and also CO2 reactivity is lost.