Indicator-dilution studies with "diffusible" indicators.

• The indicator-dilution principle has been applied to studies of the circulation in order to provide estimates of flows and volumes in the vascular compartment. A sine qua non of validity of these estimates is that the indicator used remain within the confines of: the vascular compartment for the entire duration of the period required to obtain appropriate samples of blood. In such a symposium as this, it may appear paradoxical to have a presentation concerned 'with the uses of indicators that do not conform to the requirement just slated. In fact, much of the information available on the flow of blood through the brain, through the liver, and through the kidney has been obtained by the use of such nonconforming indicators. The principles and technics differ to some degree from those used in the single, rapid-injection procedures. To conform to the general pattern of the symposium, no further mention will be made of these other principles and technics. Instead, an endeavor has been made to present some of the results of an extension of the indicator-dilution principles and to indicate wherein these results may have a bearing on the results obtained by the conventional technics in the particular setting of