Dynamic correlation between tissue PO2 and near infrared spectroscopy

Multimodal O2 monitoring including tissue pO2 measurements and near infrared spectroscopy (NIRS) are techniques increasingly employed for monitoring patients on neurosurgical intensive care units. NIRS measures a mixed venous arterial oxygen saturation, whereas tissue pO2 evaluates the oxygen pressure in the white matter. In contrast to the tissue pO2 measurements, the NIRS at the moment has not been completely established in clinical practice. We wanted to evaluate whether both techniques are monitoring different dynamic changes. Thirteen patients were included (SAH n = 3, TBI n = 10), 12 patients were male and 1 was female. Mean age was 34 years with a range from 16-76 years. Tissue pO2 probes (Licox, GMS, Germany) were implanted in the frontal lobe showing most pathological changes on the initial CT scan. A near infrared spectroscopy sensor (Invos, Somanetics, USA) was placed simultaneously at the patient's forehead. Due to the drift of the tissue pO2 probe, only data sets were taken into further account in which a tissue pO2 value above 15 mmHg was measured. 66 data sets were analyzed by calculating the spectral coherence with multi taper methods. The coherence of two independent white noise signals were defined as an observation by chance. The significance level for correlated frequencies was 90%. In the spectral long time regime (frequency > or = 0.02), more than 80% of the data sets showed a higher percentage of correlated frequencies as compared to the observation by chance. The assumption that tissue pO2 and near infrared spectroscopy probes are measuring different dynamic changes in neurosurgical intensive care patients could not be supported by our data.