Tracer Oxygen Distribution Is Barrier-Limited in the Cerebral Microcirculation

Abstract

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 51 Cr-labeled red blood cells, previously totally desaturated and then resaturated with [ 18 O] 2 (oxygen), 125 I-albumin, 22 Na, and [ 3 H]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 18 O 2 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 [ 3 H]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 18 O 2 and [ 3 H]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.