Multiparameter brain tissue monitoring - Correlation between parameters and identification of CPP thresholds

Continuous monitoring of brain interstitial gas concentrations allows direct regional evaluation of the pathophysiology of cerebral tissues. We have incorporated the Paratrend 7 (P7) multiparameter sensor into our established multimodal monitoring of head injured patients, to investigate the relationship between brain and arterial pO(2), pCO(2), and pH, as well as defining thresholds for cerebral perfusion pressure (CPP). A P7 sensor was inserted into the brain tissue of 40 adult head injured patients via a modified Camino bolt or triple lumen bolt. A second sensor was placed in the femoral artery for continuous monitoring of blood gases. Data signals from 19 monitored parameters were collected onto computer at the bedside for up to 14 days. No complications were seen. For individual patients the changes in brain tissue parameters showed large variations over 24 hours and the relationship between parameters varied considerably both between patients and during the period of monitoring any one individual. Changes related to periods of arterial desaturation, cerebral hypoperfusion and therapeutic manoeuvres could be seen. Good correlation was seen between brain pCO(2) and arterial pCO(2) (r=0.58). Poor correlation was seen between CPP and brain pO(2), and between brain pO(2) and ICP. However, by grouping values for intracranial pressure (ICP) and CPP, thresholds for brain tissue pO, were identified in 16 patients where CPP fell below 60 mmHg. No patients where CPP was always >60 mmHg showed a significant threshold for a drop in brain pO(2) (n=16). In conclusion, the P7 shows potential as a monitor of regional brain oxygenation and for detection of potentially damaging secondary insults. The results must be interpreted whilst considering catheter position, autoregulation and systemic arterial changes for each individual.