Correlations between brain tissue oxygen tension, carbon dioxide tension, pH, and cerebral blood flow--a better way of monitoring the severely injured brain?

BACKGROUND

The ideal method for monitoring the acutely injured brain would measure substrate delivery and brain function continuously, quantitatively, and sensitively. We have tested the hypothesis that brain PO2, pCO2, and pH, which can now be measured continuously using a single sensor, are valid indicators of regional cerebral blood flow (CBF) and oxidative metabolism, by measuring its product, brain pCO2.

METHODS

Twenty-five patients (Glasgow Coma Score < or = 8) were studied. A Clark electrode, combined with a fiber optic system (Paratrend 7, Biomedical Sensors, Malvern, PA) was used to measure intraparenchymal brain PO2, pCO2, and pH. Data were averaged over a 1-h period before and after CBF studies. Regional CBF was measured around the probe, using stable xenon computed tomography. Regression analyses and Spearman Rank tests were used for data analysis.

RESULTS

Regional CBF and mean brain PO2 were strongly correlated (r=0.74, p=0.0001). CBF values < 18 mL/100 g/min were all accompanied by brain PO2 < or = 26 mm Hg. The four patients with a brain PO2 < 18 mm Hg died. Brain pCO2 and pH, however, were not correlated with CBF (r=0.36, p=0.24 and r=0.30, p=0.43, respectively).

CONCLUSIONS

Until recently, substrate supply to the severely injured brain could only be intermittently estimated by measuring CBF. The excellent intra-regional correlation between CBF and brain pO2, suggests that this method does allow continuous monitoring of true substrate delivery, and offers the prospect that measures to increase O2 delivery (e.g., increasing CBF, CPP, perfluorocarbons etc.) can be reliably tested by brain PO2 monitoring.