The continuous inhalation of oxygen-15 for assessing regional oxygen extraction in the brain of man.

A non-invasive steady-state method for studying the regional accumulation of oxygen in the brain by continuously inhaling Oxygen-15 has been investigated. Oxygen respiration by tissue results in the formation of water of metabolism which may be considered as the "exhaust product" of respiration. In turn the steady-state distribution of this product may be related to that of oxygen utilization. It has been found in monkeys that an appreciable component of the signal, recorded over the head during the inhalation of 15O2, is attributable to the local production of 15O-labelled water of metabolism. In man the distribution of radioactivity recorded over the head during 15O2 inhalation clearly relates to active cerebral tissue. Theoretically the respiration product is linearly dependent on the oxygen extraction ratio of the tissue, and at normal cerebral perfusion it is less sensitive to changes in blood flow. At low rates of perfusion a more linear dependence on flow is shown. The dual dependence on blood flow and oxygen extraction limits the interpretation of the cerebral distribution obtained with this technique. Means for obtaining more definitive measurements with this approach are discussed.