Cerebral CO2 vasoreactivity evaluation by transcranial Doppler ultrasound technique: a standardized methodology

Objective: In normal subjects cerebral CO2 vasoreactivity is measured during spontaneous hyperventilation, breathholding, or adding CO2 to inspiratory gases. The correlation between CO2 and cerebral blood flow may, however, be invalidated by the effects of a modified respiratory pattern on venous return, sympathovagal balance, 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 literature. We evaluated a new standardized method for overcoming these confounding factors. Design: Experimental. Participants: Twenty-one healthy volunteers. Methods: Subjects were connected through a mouthpiece to a mechanical ventilator set in the intermittent positive pressure ventilation mode. The ventilator was fed by two 40-l tanks, one of which contained 5% CO2. The inspiratory CO2 concentration was varied at fixed time intervals from 0% to 5% without modifying ventilator settings. Endtidal CO2 was measured at the mouthpiece. Mean blood velocity (Vm) and pulsatility index (PI) in the middle cerebral artery were measured by means of transcranial Doppler ultrasound. Results: The test was easily applicable and well tolerated. No hemodynamic alterations were observed during the tests. The correlation between CO2 and Vm was always linear and highly significant (R2 > 0.8, p < 0.0001). A low intersubject variability was observed. No difference was found between the two hemispheres, nor between the sexes. Conclusions: The strict standardization of the technique, avoiding hemodynamic interference, may explain the low intersubject variability. The value of this technique in ventilated neurosurgical patients is still speculative, but it might allow the collecting of valuable data together with a reduction in exposure to CO2, and hence cerebral blood flow modifications.