Indicators and detectors for circulatory dilution studies and their application to organ or region blood-flow determination.

• Increasing application of the indicator-dilution technic to the study of various segments of the circulation has focused attention on the different forms of this teehnic that have been devised" as well as on requirements of the ideal indicator and of the ideal detecting-recording system to be used in such studies. This paper will be largely confined to a discussion of indicators and detectors used primarily in dilution studies in the central circulation employing the classic forms of the indicator-dilution technic and to the application of these indicators to the measurement, of regional and organ blood flow. Indicators used in central circulatory dilution studies may be conveniently divided into two groups, the liquid and the gaseous indicators, the former being inserted directly into the vascular system, the latter entering the vascular system either via diffusion in the lungs or by injection of solutions saturated with the gas. Gaseous indicators being freely diffusible, strictly speaking, are not in the province of this paper but will be discussed for reasons detailed later. To be useful as an indicator a substance must have some chemical or physical property that allows for its detection and preferably also for its quantitation in whole blood or plasma. Thus some substances, notably dyes (even if colorless), are useful because they absorb light of a particular spectral region, some because they emit light, that is, fluoresce, and some because they emit ionizing radiation. Among the earliest indicators to be used in circulatory dilution studies were strong electrolyte solutions, which change the