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(Circulation Research. 1995;77:1201-1211.)
© 1995 American Heart Association, Inc.

Articles

Tracer Oxygen Distribution Is Barrier-Limited in the Cerebral Microcirculation

Ibrahim G. Kassissia, Carl A. Goresky, Colin P. Rose, Andreas J. Schwab, André Simard, Pierre-Michel Huet, Glen G. Bach

From the McGill University Medical Clinic in the Montreal General Hospital and the Departments of Medicine, Physiology, and Mechanical Engineering, McGill University, and the André Viallet Research Center of the Hôpital St-Luc, Montreal, Quebec, Canada.

Abstract The kinetics of tracer oxygen distribution in the brain microcirculation of the awake dog were investigated with the multiple indicator dilution technique. A bolus containing ⁵¹Cr-labeled red blood cells, previously totally desaturated and then resaturated with [¹⁸O]₂ (oxygen), ¹²⁵I-albumin, ²²Na, and [³H]water, was injected into the carotid artery, and serial anaerobic blood samples were collected from the sagittal sinus over the next 30 seconds. The outflow recovery curves were analyzed with a distributed-in-space two-barrier model for water and a one-barrier model for oxygen. The analysis provided an estimate of flow per gram brain weight as well as estimates for the tracer water and oxygen rate constants for blood-to-brain exchange and tracer oxygen parenchymal sequestration. Flow to tissue was found to vary between different animals, in concert with parallel changes in oxygen consumption. The ¹⁸O₂ outflow curves showed an early peak, coincident with and more than half the magnitude of its vascular reference curve (labeled red blood cells), whereas the [³H]water curve increased abruptly to a low-in-magnitude curve at low flow values and to a small early peak at high flow values. Analysis indicates that the transfers of both ¹⁸O₂ and [³H]water indicators from blood to brain are barrier-limited, with the former highly so because of the large red blood cell capacity for oxygen, and that the proportion of the tracer oxygen returning to the circulation from tissue is a small fraction of the total tracer emerging at the outflow.

Key Words: cerebral blood flow • blood-brain barrier • tracer oxygen • labeled water

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