A NEW INSTRUMENT FOR OBTAINING INDICATOR DILUTION CURVES USING A RADIOACTIVE TRACER, 1125 BY

The use of indicator dilution curves has now become established as a routine procedure in studies of the circulation. Dyes are the commonly used indicators and the time concentration curve is obtained by intermittent sampling or by a continuous technique with an indwelling needle at a suitable sampling site. More recently radio-iodinated human serum albumin (1125HSA) has been successfully used as the indicator in determining cardiac output. Because of the slow response of earlier nuclear instruments, counting of individual blood samples, each collected over a one-minute period, was the method of choice. The technique was described by Fields (Fields and Seed, 1961). Rapid advances in the design of nuclear equipment have made available detectors of high sensitivity and ratemeters of such rapid response and precision that accurate continuous recording is possible. An apparatus employing this method was described for measuring blood flow in the femoral artery (Hobbs and Edwards, 1962). By simply reversing the technique, i.e. injecting the indicator into the femoral vein and sampling from the femoral artery, apparently reliable cardiac output indicator dilution curves were obtained. Although this method gave seemingly reliable results the apparatus was bulky and had several disadvantages. The injection was made by hand so that the volume and duration of injection wer