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¹¹C-carbon dioxide fixation and equilibration in rat brain

Effects on acid-base measurements

Departments of Neurology and Radiology, University of Miami School of Medicine (Dr. Lockwood), Miami, and the Baumritter Institute of Nuclear Medicine, Mount Sinai Medical Center (Dr. Finn), Miami Beach, FL.

The positron-emitting isotope ¹¹C was used to label CO₂ for studies of metabolic fixation and equilibration after a single-breath inhalation by rats. Metabolic fixation and loss of the label via exhalation caused the metabolized fraction of the label in the brain to rise to 30.1 ± 0.7% within 30 minutes. The T₁₂ for equilibration of the label between blood and brain was 1.95 minutes. When the label was 95% equilibrated, 12% was metabolically trapped by brain, and when only 5% was trapped, the blood-brain equilibration process was only 50% complete. Labeled CO₂ thus has limited usefulness as an acid-base or metabolic tracer for positron-emission tomography.

Address correspondence and reprint requests to Dr. Lockwood, Department of Neurology (D 4-5), University of Miami School of Medicine. P.O. Box 016960, Miami, FL 33101.

This investigation was supported in part by NINCDS Positron Emission Tomography Program Project Grant NS 15639, and grants NS 14996 and NS 05820.

Presented in part at the thirty-second annual meeting of the American Academy of Neurology, New Orleans, LA, April 1980.

Accepted for publication August 18, 1981.