Middle cerebral artery flow velocity correlates with common carotid arteryvolume flow rate after CO2 inhalation

Cerebral vasoreactivity can be studied with transcranial Doppler (TCD) by m onitoring CO2-induced middle cerebral artery (MCA) velocity changes. Expect ed MCA mean velocity (Vm) changes due to changes in end-expiratory CO2 (EE- CO2) are established, but reactivity of common carotid artery (CCA) volume flow rate (VFR) has not been extensively reported. The authors assess the r elationship between MCA Vm, CCA VFR, and EE-CO2. Ten normal individuals wit hout cerebrovascular disease and with CCA diameters of more than 3.0 mm wer e studied. CCA VFR was obtained by Color Velocity Imaging Quantification an d ipsilateral MCA Vm by standard TCD methods. Each side was studied before, during, and after inhalation of 5% CO2. EE-CO2, blood pressure, and pulse rate were monitored. Four women and 6 men with mean age of 36 years were in cluded. Significant correlations between MCA Vm and EE-CO2, CCA VFR and EE- CO2, and MCA Vm and CCA VFR were found. MCA Vm and CCA VFR increased 5.2% a nd 4.3% per mm Hg increase in EE-CO2, respectively. MCA Vm increased 0.3 cm /s for each ml/min increase in CCA VFR. In normal individuals, there is a d irect correlation between MCA Vm, CCA VFR, and EE-CO2. Measurement of CCA V FR changes during CO2 inhalation may be an alternative method to estimate c erebral vasoreactivity when the MCA velocity cannot be obtained because of inadequate acoustic temporal windows.