Analytical Approach To

During the past decade, dynamic data have been obtained by recording from various external monitor ing devices following administration of various ra dioactive tagged materials. From the clinical point of view, the most obvious advantage of these tech niques is the external monitoring aspect, which is both atraumatic and convenient. Conversely, this aspect has hindered a sophisticated analysis of col lected data; first because it is difficult to record from one site exclusively, and second because the data are liable to be contaminated by activity originating from other sites and occurring in the same time sequence. Since these data concern primarily the transport of material by way of the circulatory system, this process is often defined in terms of the system con cept by the so-called input-output relationship through the system or organ of interest. The data from a specific sampling, such as those obtained by use of a catheter, have been extensively analyzed by this concept (1 ), but few analytical studies based on external monitoring data have been performed. This is largely because the data of the dilution process from external recording are frequently too complex to be analyzed by straightforward means since the curves are usually defined from a composite multi modal dilution process. In order to handle these data, a compromise must therefore be made before pro ceeding too far with a refined mathematical model. The data with which we are ultimately concerned are the time sequence of spatial radioactivity changes recorded from a scintillation camera (2) . The pres ent report, however, is concerned only with the precordially monitored isotope dilution curve re corded by a single probe, the so-called radiocardio gram or RCG, analyzed in order to verify the use fulness of the concept and methodology. Several attempts at analyzing the radiocardiogram in order to get clinically useful information concerning flow and volume of the central circulatory system have been made (3—5). These attempts have thus far been of limited use because, except for cardiac output determinations, a reliable analytical method has not yet been attained. On the basis that the dynamics of the isotope dilu tion process through the central circulatory system can be approximated by a series of input-output relations representing the passage of the material through each compartment of the system, an ap proach to simulate the RCG curve was made in 1963 with the aid of an analog computer by Donato et al (6) in orderto deriverightandleftventricular vol umes and the pulmonary blood volume. Their analog model consisted of two mixing volumes each with a constant fractional rate of emptying for the left and right ventricle and a Poisson-like distribution of tran sit probabilities representing the pulmonary circula tion. This method required the introduction of the isotope by a catheter into the right atrium. Their fitting analysis by means of analog computer was somewhat tedious and complicated. Other investi gators (7) have accomplished a similar fitting analy sis with the modification of peripheral vein injection and additional analog simulation of the injection and body space. In this report, for the purpose of ascertaining the clinical usefulness of the derivation of the compre hensive information from the dynamic isotope data: (A) a simple sequential input-output model was constructed by means of an analog computer; (B) using radiocardiographic data from patient studies, a straightforward method of fitting analysis yielding volume values of each compartment of the central circulatory system was proposed; and (C) to obviate the need of an analog computer a method of graphi cal analysis has been derived.