Determination of the Ischemic Threshold for Brain Oxygen Tension
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 f
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Determination of the Ischemic Threshold for Brain Oxygen Tension E. M. R. Doppenberg, A. Zauner, J. C. Watson, and R. Bullock Division of Neurosurgery, Medical College of Virginia. Virginia Commonwealth University, Richmond, VA, U.S.A.
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 paz) must be determined. Three end points were used for determination of the critical brain paz 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 paz 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 paz :5 19 mm Hg was correlated with poor outcome (n = 24). The ischemic threshold for (r)CBF of 18ml/l00glmin corresponded to a brain paz of 22 mmHg, in the same patients. By using the above mentioned end points as a reference, we found the critical value for brain paz to be in between 19 and 23 mmHg. Clearly. the difference between our threshold value and the lower critical brain paz level found by other groups using the Licox system, needs to be clarified in a comparison study before a uniform threshold for brain paz can be determined.
studies this is the determination of an induced infarct. In humans however, the only end points that are currently available and reliable, both quantitatively and qualitatively, are regional cerebral blood flow (rCBF) measurements and outcome. We therefore, used data obtained in our cat studies and data from our human studies in an attempt to determine "danger levels" for brain pOz'
Materials and Methods In all studies a multiparameter sensor was used. It combines a Clark electrode for measuring oxygen tension with fiberoptic sensors for carbon dioxide tension and pH measurements and a thermocouple for temperature (Paratrend 7, Diametrics Medical Inc., Saint Paul, MN, USA). (Carbon dioxide, pH and temperature data will be presented elsewhere.) To determine the critical value for brain paz, 3 objective endpoints were used: 1.
Keywords: Brain oxygen tension; Clark-electrodes; middle cerebral artery occlusion; ischemic threshold. 11.
Introduction 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 pOz) must be determined. To determine these values, true end points are needed to refer to as a "gold standard". In animal
III.
Permanent occlusion of the middle cerebral artery in the cat, from two diff
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