Measurement of Local Cerebral Blood Flow in the Unanesthetized Rat Using a Hydrogen Clearance Method

The hydrogen clearance method for measuring local tissue blood flow was adapted for use in the brain of the unanesthetized rat by means of a stainless steel plate screw mounted in the skull as the reference electrode instead of a calomel electrode. Blood flow was measured at 3 sites in the brains of 5 three-month-old rats over a 6-week period. Blood flow in the frontal cortex and the cerebellum for all the animals averaged 79 ± 22 (SD) and 81 ± 25 ml/min/100 g tissue, respectively. The other site, supposedly the hippocampus, exhibited a two-component clearance curve in most cases. The initial rapid component was of questionable validity; the second slow component averaged 110 ± 23 ml/min/100 g. Autopsy revealed that the hippocampal electrode was actually positioned in more than one type of tissue or proximal to an interspace. Hypoxia (Po 2 = 50 mm Hg) and hypercapnia (produced by inhalation of 10% CO 2 ) increased the clearance rates two- to four-fold. The rate of local blood flow and its variability were affected by the state of the animal; anesthesia (sodium pentobarbital) decreased the rate of blood flow as anticipated, and markedly reduced the variation in rate seen in the conscious state.