Determination of the Ischemic Threshold for Brain Oxygen Tension

Summary

Measuring brain tissue oxygenation is now possible due to major advances in the technical development of Clark-electrodes and fiberoptic systems. However, to make this technique clinically useful for both nurses and medical staff, the ischemic threshold for brain tissue oxygen tension (brain pO₂) must be determined. Three end points were used for determination of the critical brain pO₂ value. 1) Infarct determination after permanent middle cerebral artery occlusion in a feline model. 2) Threshold analysis using the schemic threshold for cerebral blood flow (CBF) as a “gold standard” in severely head injury patients. 3) Outcome analysis in severely head injured patients. Brain pO₂ dropped to 19 ± 6 mm Hg and 23 ± 6, 4 to 5 hours after MCA occlusion in the cat (n = 12). In severely head injured patients, a brain pO₂ ≤ 19 mm Hg was correlated with poor outcome (n = 24). The ischemic threshold for (r)CBF of 18 ml/100 g/min corresponded to a brain pO₂ of 22 mm Hg, in the same patients. By using the above mentioned end points as a reference, we found the critical value for brain pO₂ to be in between 19 and 23 mmHg. Clearly, the difference between our threshold value and the lower critical brain pO₂ level found by other groups using the Licox system, needs to be clarified in a comparison study before a uniform threshold for brain pO₂ can be determined.