O/sub 2/ exchange between blood and brain tissues studied with /sup 18/O/sub 2/ indicator-dilution technique

A technique has been developed to record /sup 18/O/sub 2/ dilution curves of an organ in vivo by use of /sup 51/Cr-labeled erythrocytes as a reference tracer. The technique employs anaerobic sampling of venous outflow following an intraarterial injection of tracer-laden blood and off-line determination of (/sup 18/O/sub 2/) and (/sup 51/Cr) profiles in the venous outflow. O/sub 2/ and reference indicator-dilution curves of cerebral circulation were recorded in eight experiments with six halothane-anesthetized dogs. Autologous blood labeled with the tracers was injected into a carotid artery, and brain venous outflow was sampled from the sagittal sinus. The total net extraction of O/sub 2/ tracer was equal to the extraction of elemental O/sub 2/. Instantaneous extraction of /sup 18/O/sub 2/ along the outflow curve fell linearly with time, from an initial value of 0.6-0.7 to very small or even negative values toward the end of a pulse. This indicates that O/sub 2/ undergoes a flow-limited distribution. In all experiments, the mean transit time of unmetabolized /sup 18/O/sub 2/ was longer than the mean transit time of the Cr tracer. An index of the tissue O/sub 2/ dilution space, hence the mean tissue PO/sub 2/, is calculated from this data withmore » the use of a modified central volume principle. This estimate of mean tissue PO/sub 2/ increases as a linear function of sagittal sinus PO/sub 2/ with a slope of 0.97. The method may provide an index of the critical PO/sub 2/ of venous blood, the PO/sub 2/ below which O/sub 2/ diffusion from blood to tissue may limit its rate of metabolic uptake.« less