Use of external counting technics in studies of the circulation.

• A number of new technics involving external detection of radioactivity in the blood stream or in tissues have been proposed as means for determining the rate, volume and pathway of blood flow in the intact subject. By virtue of their dependence on indirect, external measurement of indicator (isotope) activity in the body as opposed to direct measurement, these technics have been categorized as "external counting technics." Since the critical departure actually lies in the method of indicator detection, the preferable term would perhaps be "external detection technics." In either case, they are concerned with measurement of indicator dilution within the blood stream or measurement of indicator-transfer rates between blood and tissue. When preeordial external counting is performed, the dilution curve so obtained is often referred to as a "radiocardiogram.' Although these methods represent innovations, adequate isotope detection, counting and recording equipment becoming available only after "World War II, methods to date have been described for measurement of total blood flow (cardiac output) and regional blood flow in muscle, heart, brain, liver and kidney, for determination of circulation time and cardiac volume, for detection of intracardiae and extraeardiac shunt, and for organ visualization. The rapidity of these developments suggests that a variety of new external counting methods and modifications of earlier ones must be anticipated in the near future. It would appear that many of these technics will have an area of useful clinical application. Whether any will ever be widely used in clinical study remains problematical.