Transconjunctival Oxygen Tension Measurements During Carotid Endarterectomy-Reply

In Reply.—Dr Gibson describes her findings that ipsilateral Pcjo₂ decreased in all patients (N=18) undergoing temporary carotid artery occlusion during elective carotid endarterectomy but that no correlation was found between Pcjo₂ and rCBF, as measured by xenon Xe 133 and 16-channel EEGs. Indeed, we would not expect Pcjo₂ to be a measure of flow or to be directly correlated to EEG or cerebral blood flow changes because Pcjo₂ measures tissue oxygen tension, using the conventional Clark polarographic oxygen electrode in a manner comparable with the blood gas analyzer's measurement of oxygen tension. We found that Pcjo₂, like transcutaneous partial pressure, may reflect tissue oxygen tension and, therefore, track tissue perfusion; both of these measurements may be limited by flow, but that does not mean that they must be correlated with flow. For example, tissue oxygenation may decrease when normal or marginal flow decreases or