Time constant of thermistors and its role in thermodilution methods

Thermolution serves for measuring either the cardiac output or the size of cardiac ventricles. In the former instance, an accurate determination of the area under the dilution curve, and in the latter, a true tracing of the thermal changes with time are of special importance. Because the size of the area is independent of the time constant, and a curve devoid of marked stroke waves is better suited for automatic integration, thermistors with time constants of 0.8 to 1.0 sec are chosen for this purpose. A reliable determination of the time constant is described. In measurements of the size of cardiac ventricles, a true course of thermal changes with time can be recorded with a thermistor possessing a very short time constant: 0.01 to 0.02 seconds. It is very difficult to make such thermistorized probes or catheters. It is easier to make thermistors with a time constant of 0.1 o 0.2 sec, well insulated, which may serve both requirements. By compensation, the time constant is reducible to 0.01 to 0.2 sec, making possible a true-shape recording of thermal changes for the measurement of the ventricular size, or the constant can be prolonged to 0.8 to 1.0 sec for instrumental integration. If both the shape and the time course of the thermal changes have to be truly recorded, then a non-insulated thermistor with a very short time constant has to be used.