gms | German Medical Science

Objective: The measurement of the brain tissue pO₂ is an important component of the neuromonitoring of critically ill patients with SAH or TBI. Here we compare the typical intraparenchymal pO₂ measurement with a novel opto-chemical measurement on the cortex surface. The Method is based on oxygen dependant luminescence quenching of a porphyrine dye in the sensor foil.

Methods: The measurement device (prototype, Biocam, Regensburg) was placed after a craniectomy on the cortex of intubated and ventilated wistar rats (n=6, m=355±27g). The sensor foil is excited by LED light flashes; the luminescence of the foil and also normal images of the measurement area are detected by a CCD-camera, which is coupled to the foil via a NMDA cylinder. The data is processed with PC software (IDL, Creaso). Regions of interest were defined over a cortical vein, an arteriole and the parenchyma. A Licox-probe was placed in the frontal cortex of the opposite side. Arterial pO₂ and pCO₂ were varied by changing FiO₂ and ventilation settings. Blood gas values, arterial pressure, heart rate and brain pO₂ were recorded simultaneously.

Results: The intraparenchymal (Licox) pO₂ correlated at best with the pO₂ over the cortex parenchyma (correletation coefficient =0,55; p<0,001; n=93). Correlations with the pO₂ over veins (c.c.=0,36; p<0,001) and arterioles (c.c.=0,24; p<0,05) were weaker. The correlation coefficient of the arterial pO₂ (p_aCO₂ 33-47 mmHg) with the Licox-measures was 0,37 (p<0,001), and with the cortical pO₂ it was 0,75 (arteriole, p<0,001), 0,73 (vein, p<0,001) und 0,37 (parenchyma, p<0,001). Variation of p_aCO₂ (n=54, p_aO₂ 70-130 mmHg) had no effect on the Licox measures (c.c.=-0,06, p=0,68) and small influence on the cortex pO₂ (vein: 0,04, p=0,76; arteriole: 0,30, p<0,05, parenchyma: 0,28, p<0,05).

Conclusions: The two dimensional pO₂ detection on the cortex surface allows differentiated and simultaneous measurements of the anatomical structures. The Licox measurement in the white matter correlated most of all with the pO₂ over the cortex parenchyma. The cortical measurement provides additional pO₂ values of arterioles and veins. The oxygenation of these vessels as well as probable drops in the cortex pO₂ may not be detected by the intraparenchymal pO₂ probe.