Extracorporeal sensing techniques

Many physiologic variables have been measured in the extracorporeal circulation by experimental systems but only a few systems have reached technical maturity for everyday application. Variables relating to cardiovascular function, which today can be measured in the extracorporeal system, are pressure, temperature, and measures of blood composition such as hematocrit, hemoglobin, and total protein concentration. While the measurement of blood composition and temperature is well established, and while the use of extracorporeal pressure information awaits further analysis for robust application, recent interest focused on continuous measurement of plasma sodium concentration which is believed to be of major importance for optimal blood treatment. However, problems with a simple, reliable, and continuous measurement of plasma sodium for everyday use have not yet been resolved. As can be seen from the growing interest in isothermic or isonatremic treatment modes which turns away from constant and profiled treatment modes without feedback control, the treatment goal is now to provide stable conditions within the patient so as to minimize interference with intrinsic physiological control mechanisms. This, however, requires valid and reliable measurement of the specific patient variables of interest.