Introduction Cerebrovascular reactivity to CO 2 is commonly investi- gated by measuring changes in mean blood velocity (V m ) in middle cerebral artery by means of transcranial Doppler ultrasound (TCD). Hypocapnia is obtained by voluntary hyperventilation, while hypercapnia is in- duced adding CO 2 to inspiratory gases or by breath holding. TCD values are obtained after a steady state is reached. Surprisingly, no standardization of the method M. Cigada S. Marzorati S. Tredici G. Iapichino Cerebral CO 2 vasoreactivity evaluation by transcranial Doppler ultrasound technique: a standardized methodology Received: 26 May 1999 Final revision received: 14 February 2000 Accepted: 16 March 2000 M. Cigada ( ) ) ´ S. Marzorati ´ S. Tredici ´ G. Iapichino Neurosurgical Intensive Care, Department of Anesthesia and Intensive Care, IRCCS Ospedale Maggiore di Milano, Italy e-mail: marcocigada@yahoo.it Tel.: + 39-02-81 84 44 76 Fax: + 39-02-81 84 45 43 Mailing address: M. Cigada Reparto di Rianimazione, Ospedale San Paolo, Via A. Di Rudinì, 8, 20 142 Milano, Italy Abstract Objective: In normal sub- jects cerebral CO 2 vasoreactivity is measured during spontaneous hy- perventilation, breathholding, or adding CO 2 to inspiratory gases. The correlation between CO 2 and cerebral blood flow may, however, be invalidated by the effects of a modified respiratory pattern on ve- nous return, sympathovagal bal- ance, and cathecolamine release. Moreover, the duration of the test, usually not considered, may play an important role. This may justify the scattering of values found in litera- ture. We evaluated a new standard- ized method for overcoming these confounding factors. Design: Experimental. Participants: Twenty-one healthy volunteers. Methods: Subjects were connected through a mouthpiece to a mechani- cal ventilator set in the intermittent positive pressure ventilation mode. The ventilator was fed by two 40-l tanks, one of which contained 5 % CO 2 . The inspiratory CO 2 concen- tration was varied at fixed time in- tervals from 0 % to 5 % without modifying ventilator settings. End- tidal CO 2 was measured at the mouthpiece. Mean blood velocity (V m ) and pulsatility index (PI) in the middle cerebral artery were mea- sured by means of transcranial Doppler ultrasound. Results: The test was easily applica- ble and well tolerated. No hemody- namic alterations were observed during the tests. The correlation between CO 2 and V m was always linear and highly significant (R 2 > 0.8, p < 0.0001). A low inter- subject variability was observed. No difference was found between the two hemispheres, nor between the sexes. Conclusions: The strict standardiza- tion of the technique, avoiding hemodynamic interference, may ex- plain the low intersubject variability. The value of this technique in venti- lated neurosurgical patients is still speculative, but it might allow the collecting of valuable data together with a reduction in exposure to CO 2 , and hence cerebral blood flow mod- ifications. Key words Cerebral CO 2 vasoreactivity ´ Transcranial Doppler ´ Normal subjects Intensive Care Med (2000) 26: 729±732 Ó Springer-Verlag 2000 ORIGINAL