Effects of increases in the inspired oxygen fraction on brain surface oxygen pressure fields in pig and man.

In six patients undergoing neurosurgical operation, brain surface oxygen pressure was studied during an increase of the inspired oxygen fraction (FiO2). The eight-channel oxygen surface electrode (MDO-electrode) was placed directly on the brain cortex. FiO2 was increased to four levels, from baseline level 0.21 to 0.3, 0.5, 0.7 and 1.0, respectively. During these four stages and FiO2 0.21, brain surface oxygen pressure (PtO2) was measured. The physiological variables such as blood pressure, PaCO2, pH and temperature were stable throughout the study. The results are presented as mean values +/- s.d. and a PtO2 histogram for each FiO2-level. Already at an FiO2 of 0.3 (at a PaO2 of 16.3 +/- 3.4 kPa) scattered histograms were seen in five of six patients. A scattered histogram indicates disturbed microcirculation. At the FiO2 levels of 0.5, 0.7 and 1.0, all histograms were scattered. The PtO2 values did not increase proportionally to PaO2 at FiO2 levels 0.3, 0.5 or 0.7. But at FiO2 1.0 four patients had normal mean PtO2 values and two patients very high mean PtO2 values. It is possible that the four patients with normal PtO2 values succeeded in regulating the cerebral microcirculation as a response to the high FiO2 leading to a high PaO2 (60.1 +/- 6.4 kPa). The same study was initially done on six pigs in which the regional cerebral blood flow (rCBF) was also measured. MDO-electrode measurements at different FiO2-levels gave the same results as in the patients. rCBF decreased when FiO2 was increased.