Brain tissue oxygen monitoring for assessment of autoregulation: preliminary results suggest a new hypothesis.

Brain tissue oxygen monitoring (P(ti)O2 (Neurotrend, Codman, Germany) was employed in addition to standard intracranial pressure (ICP) and cerebral perfusion pressure (CPP) monitoring in seven patients with severe neuronal damage of heterogeneous etiology. The correlation between P(ti)O2 changes and CPP fluctuations during periods of 30 minutes were analyzed, when CPP was above 70 mmHg and lower than 100 mmHg. A new ratio, the CPP-oxygen-reactivity (COR) index was calculated as COR=delta p(ti)O2 %/delta CPP%. The patient COR values were compared to those found in the brain of six noninjured anesthetized piglets. The analysis was performed to determine the significance of synchronous fluctuations of CPP and P(ti)O2, when CPP is above the lower threshold of autoregulation. The correlation between CPP variations and p(ti)O2 variations was found to be strong (R(mean)) = 0.74 +/- 0.17) in the patients and was weak in the uninjured animals (R(mean)) =0.38 +/- 0.43). The COR (mean) was 2.05 +/- 0.57 in patients and 0.78 +/- 0.6 in the animals. In the injured brain of our patients, we observed an unexpectedly close correlation between P(ti)O2 and CPP variations when CPP levels were within a therapeutically targeted range (70 to 100 mmHg). In a porcine model, we could not find this relationship in the noninjured brain. We speculate that an increased COR might be indicative for an impaired local pressure autoregulation. The preliminary data suggest that COR values above "1" might be pathologic. However, the reported sample sizes are too small to provide sufficient statistical power to justify inferential statistical analyses. As such, results are presented with descriptive statistics only, and should be regarded as a hypothesis.