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

Objective: In normal subjects cerebral CO2 vasoreactivity is measured durin g spontaneous hyperventilation, breathholding, or adding CO2 to inspiratory gases. The correlation between CO2 and cerebral blood flow may, however, b e invalidated by the effects of a modified respiratory pattern on venous re turn, sympathovagal balance, and cathecolamine release. Moreover, the durat ion of the test, usually not considered, may play an important role. This m ay 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 venti lator set in the intermittent positive pressure ventilation mode. The venti lator was fed by two 40-1 tanks, one of which contained 5 % CO2. The inspir atory CO2 concentration was varied at fixed time intervals from 0 % to 5 % without modifying ventilator settings. End-tidal CO2 was measured at the mo uthpiece. Mean blood velocity (V-m) and pulsatility index (PI) in the middl e 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 V-m was always linear and highly significant (R-2 > 0.8, p < 0.0001). A lo w intersubject variability was observed. No difference was found between th e two hemispheres, nor between the sexes. Conclusions: The strict standardization of the technique, avoiding hemodyna mic interference, may explain the low intersubject variability. The value o f this technique in ventilated neurosurgical patients is still speculative, but it might allow the collecting of valuable data together with a reducti on in exposure to CO2, and hence cerebral blood flow modifications.