Diffusion Bypass of Xenon in Brain Circulation

Abstract

The diffusion bypass of 188 Xe between arteries and veins was studied in the brains of anesthetized dogs. A mixture of B1 Cr-labeled red cells and 188 Xe 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 183 Xe 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 13S Xe 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 188 Xe in the internal carotid artery and external detection of the washout is not influenced significantly by the diffusion bypass of 188 Xe in the normal flow range.KEY WORDS diffusible indicators outflow pattern vascular indicators shunting by diffusion dogs 61 Cr-labeled erythrocytes gas-tight blood sampling cerebral perfusion pressure cerebral blood flow• Exchange of gases through the walls of vessels considerably larger than capillaries must be assumed a priori, because no specialized transport mechanisms for gas exist in any vessels. Yet it is equally clear that the linear velocity of blood flow and the vessel surface area relative to blood volume favor the exchange at the capillary-venular level. This consideration has led to the classical point of view that for inert gases exchange through the walls of larger vessels can be disregarded, because complete diffusion equilibrium is established between tissue and blood in the microcirculation (1, 2). In this paper, we report observations pointing to diffusion between arteries and veins of the inert gas 13S xenon ( 138 Xe) in the circulation of the brain as a whole. This finding confirms the study that Stosseck (3) made on the local level. He showed that hydrogen gas diffused across the wall of small pial arteries and veins on the exposed brain surface. Diffusion bypass by 13S Xe also exists in skeletal muscle (4) and can probably be demonstrated in all organs. The effect of this phenomenon on flow This work was supported in part by the King Chr. Xth Foundation.Received June 5, 1972. Accepted for publication January 8, 1973. studies with freely diffusible indicators merits consideration.In the present investigation, the cerebral venous concentrations of two tracers, a vascular tracer and a freely diffusible tracer, were followed after the injection of a mixed tracer bolus into the internal carotid artery of the dog. Erythrocytes labeled with 51 chromiurn ( B1 Cr) were used as ...

Heterogeneity of cerebral capillary flow in man and its consequences for estimation of blood-brain barrier permeability.

“…A possible explanation of the increasing E might be that the test substances bypass the capillaries by diffusion from arteries to veins (shunting), as has been described for 133Xe (14). However, this seems unlikely because the E(t) curves for HOH and propranolol have almost the same slope even though the free diffusion coefficient for HOH is much higher than that for propranolol.…”

Distribution of oxygen tension on the surface of arterioles, capillaries and venules of brain cortex and in tissue in normoxia: an experimental study on rats

“…If this were the case, some microvessels with higher flow may contribute more oxygen to the venous outflow than microvessels with lower flow. However, there are no favourable conditions for O 2 diffusional shunting in the brain microcirculation [3,7]: cortical arterioles and venules do not form parallel pairs with countercurrent flows as in skeletal muscle. However, only pial arterioles and venules that cross are in immediate contact.…”

Total cerebral ischemia: application of a new model system to studies of cerebral microcirculation.

“…Diffusion bypass of 1J3 Xe between arteries and veins has been demonstrated in anesthetized dogs. 18 This could contribute to a small underestimation of cerebral blood flow, particularly in low flow states. Waltz 18 has shown that the washout curves in many experimental animal species can not always be readily resolved into a bimodal pattern, with a fast component representing grey matter flow rates and a slow component representing white matter flow rates.…”

“…The approach to microsphere use developed in this laboratory has been reported previously. 18 After the completion of each experiment, the animal was sacrificed by the intravenous injection of a saturated solution of KCL. The brain was rapidly removed and wet weight determined.…”