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The diffusion bypass of 133Xe between arteries and veins was studied in the brains of anesthetized dogs. A mixture of 51Cr-labeled red cells and 133Xe was injected as a bolus into the internal carotid artery. Gas-tight venous samples were taken every 0.4–0.8 seconds from the superior sagittal sinus. An excess of 133Xe in the early venous blood samples over that expected from blood-tissue equilibration considerations was demonstrated in all the experiments. The excess was most marked in the first samples, and it was enhanced by lowering cerebral blood flow by decreasing the cerebral perfusion pressure to 20–30 mm Hg. We concluded that diffusion bypass of the tissue by 133Xe is the most likely explanation for these experimental findings. The fraction of the bolus which appeared to bypass the tissue amounted to about 2–3% at the normal perfusion pressure (i.e., normal blood flow) and about 10% at a reduced cerebral perfusion pressure of 20–30 mm Hg (i.e., about half of normal blood flow). We concluded that measurement of cerebral blood flow by a bolus injection of 133Xe in the internal carotid artery and external detection of the washout is not influenced significantly by the diffusion bypass of 133Xe in the normal flow range.

Diffusion Bypass of Xenon in Brain Circulation