Tracer Studies of Peripheral Circulation

Studying the blood flow of the various tissues by tracers (also called indicators) one assumes steady state conditions for the system. It means, that the relevant parameters of the system are taken to be constant, the flow, oxygen uptake, etc. Systemic steady state is elementary for deriving the basic equations. It must persist during the time needed for making the measurement. This does not mean, however, that some degree of variability of the systemic parameter in question cannot be allowed. Consider for example the determination of cardiac output by injection of below-body-temperature saline in the right side of the heart with downstream temperature measurement in the pulmonary artery (“heat” clearance). In this case the flow rate of the blood in the system measured oscillates enormously. Yet, because the duration of the period of oscillation is fairly short relative to the duration of the heat clearance curve itself, one can get a good estimate of the average blood flow. In contrast to the systemic parameters, the tracer parameters, as for example the blood concentration, often vary as a function of time, i.e. for the tracer the steady state condition does not (in many cases) pertain.