A New Instrument for Obtaining Indicator Dilution Curves Using a Radioactive Tracer, I-125.

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 were not exactly reproducible; the continuous curve obtained was linear and needed to be replotted on semilogarithmic graph paper so that the downslope could be extrapolated to zero; the use of 1131 resulted in the patient receiving a significant radiation dose and required a detector weighing 475 lb. (216 kg.); and the ratemeter and withdrawal pump employed were standard models having many surplus components. A new instrument was, therefore, designed especially for the purpose of obtaining indicator dilution curves by this technique. The purpose of this paper is to describe the instrument and its laboratory evaluation. The use of I125 labelled albumin ('albumotope', Squibb) introduced many real advantages. (a) A lower net radiation dosage to the patient as well as to the operator (e.g. chest radiograph represents 500-2000 mrad., 50 uc I131 albumin i/v represents approximately 500 mrad., whereas 50 Mc I125 albumin i/v results in a dose of 33 mrad.). This is despite the isotope's longer half-life (ti=60 days), and is due to the emission of only 'soft' photons of 27-3 and 35 4 Kev. energies and the complete absence of beta radiation (Myers and Vanderleeden, 1960). (b) A much longer refrigerator shelf-life. (c) Z thin crystal can be used because its efficiency at higher energies is very poor, so that very little shielding is needed to reduce background interference. A spectrometer is not required. The small crystal is inexpensive and because little lead is required the unit weighs only 6 lb. (2-7 kg.).