Cerebral CO2 reactivity in severe head injury. A transcranial Doppler study

Abstract

Cerebral circulation is profoundly affected by changes in PaCO2. CO2 manipulation plays a basic role in the management of intracranial hypertension; CO2 reactivity (CO2R) defines the changes in CBF in response to changes in PaCO2. Transcranial Doppler has allowed exploring its effects "on line". We conducted a prospective clinical trial, with the objective of studying CO2R in severe head injury patients. Sixteen severe traumatic brain injury patients, mechanically ventilated, were included. Monitoring of MAP, ICP, CPP, SjO2, ETCO2, and cerebral blood flow velocity (CBFV) was performed. Taking into account basal cerebral hemodynamic pattern, minute ventilation was modified to attain a negative ("A") or positive ("B") deltaPCO2. CO2R was calculated as: CO2R = % deltaCBFV/deltaETCO2 in mmHg (normal value 3.7 +/- 1%/mmHg). CO2R was compared with deltaICP/ deltaPCO2 in each patient. Three patients were excluded because the change in ETCO2 was too low (deltaETCO2 < 3 mmHg). The median value of CO2R in the total group of 13 patients was 3.38. In "A" the values tended to be lower than in "B". There were four low CO2R values in "A" and none in "B". There was no significant correlation between CO2R and deltaICP/deltaPCO2. The different "A" and "B" behavior might be due to dissimilar mechanisms involved in the basis of vasodilatation and vasoconstriction. Changes in ventilation must be performed with caution, avoiding sudden increases in CO2 that may increase ICP. The absence of correlation between CO2R and deltaICP/deltaPCO2 is explained, at least partially, by different cranio-cerebral compliance in each patient. Therefore, induced blood volume changes are not directly transmitted to ICP, but their effects depend on the shape of the pressure-volume curve and the position on the curve in which each situation is working.