THE MEASUREMENT OF CEREBRAL BLOOD FLOW BY EXTERNAL ISOTOPE COUNTING*

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Abstract
The initial dis-tribution of 4-iodol31antipyrine is used to estimate the total cephalic fraction[long dash]brain, head, and neck[long dash]of the cardiac output. Since there is no readily exchangeable cerebral Rb86 reservoir, and since there is a large, rapidly exchangeable, noncerebral cephalic pool, the stable cephalic content of Rb86Cl corrected for recirculation is used to estimate the noncerebral cephalic fraction[long dash]just head and neck[long dash]of the cardiac output. The difference between the total cephalic flow fraction and the noncerebral cephalic flow fraction represents the total cerebral blood flow fraction, which when multiplied by cardiac output, determines the total cerebral blood flow; The effects of voluntary hyperventilation and breathing 5% CO2 in air upon cardiac output, cerebral blood flow, and cerebral vascular resistance were evaluated with this technique. Voluntary hyperventilation increased cardiac output, reduced cerebral blood flow, and doubled cerebrovascular resistance. Five per cent CO2 minimally increased cardiac output, increased cerebral blood flow, and significantly reduced cerebrovascular resistance. These results agree with those reported by the nitrous oxide method, but have the advantage of estimating total cerebral blood flow as a fraction of the cardiac output.