Nitrous oxide (3-10 per cent) is administered through the breathing circuit such that the patient's lungs are ventilated with a mixture of oxygen, nitrogen, and nitrous oxide throughout the period of measurement (usually 10-15 min, although a longer period may be required if cerebral blood flow is low). Diffusion equilibrium must be achieved by the end of the period of measurement such that the final arterial and venous concentrations are within 10 per cent of each other. Timed paired samples of arterial and jugular venous blood are withdrawn during the period of nitrous oxide administration. These can be aspirated intermittently (usually six pairs in 10 min) or continuously to give 'integrated' arterial and cerebral venous concentrations. The nitrous oxide concentrations in the blood are determined either spectrophotometrically or chemically.

The basic Kety-Schmidt technique has a number of limitations. First, it is invasive; it is necessary to cannulate an artery (any artery) and the vein which drains the organ being studied. In the case of the brain the appropriate sampling site is the jugular bulb. This introduces the further problem of what is, and what is not, the true venous drainage from the brain itself. The third limitation is that the measurement of flow obtained is the mean of the flows through all the tissues draining into the vein from which the blood samples have been withdrawn; regional variations in flow cannot be detected. Finally, nitrous oxide is not totally inert; in higher concentrations it can alter cerebral metabolism and hence influence cerebral blood flow.

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