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 ≤ 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 ≤ 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.