Indicator dilution curves recorded downstream following single injections of indicators are widely used for measurement of cardiac output. For injections of a constant amount of the indicator the area under the primary time-concentration curve is inversely proportional to volume flow. The shape of the indicator dilution curve is influenced by such technical factors as the dead space of the sampling system, the speed of withdrawal of blood, and the dynamic frequency response of the concentration measuring instrument. Ideally such a curve describes what will happen to a population of blood cells or molecules of blood plasma, starting simultaneously from a point in the bloodstream, in terms of their times of arrival at a sampling point downstream. This depends on variations in path length and in velocity along parallel paths, attachment of indicator to cells or plasma, intravascular turbulence and mixing, and the velocity profile in vessels. The curve forms a statistical frequency distribution of transit times of individual particles, from which can be calculated the shortest and longest transit times, the mean transit time, the degree of dispersion of transit times, the volume flow in the circulation. and the volume of blood in the system between the injection and sampling points.
There are practical restrictions on the use of indicator dilution curve analysis. The indicator must be nontoxic, haemodynamically inert, rapidly miscible, and capable of accurate and continuous quantitation in the sampled blood stream. The injection must be rapid and complete. All the injected dye must travel directly to the common mixing chamber. After passage through the mixing chamber the dyed blood must not be further diluted by admixture of collateral blood flow. The injection and sampling sites must be separated by a sufficient volume to permit satisfactory mixing of the indicator, and this is generally held to preclude study of flow from one chamber of the heart to an adjoining one.
Assuming these conditions are met, we may record from a single sampling point (e.g., the brachial artery) dilution curves from one or more injection sites in the left or right heart. Alternatively, following a single right-sided injection, curves may