Application of impedance cardiography in critical care medicine.

In spite of good correlations between cardiac output measurements by impedance and established invasive procedures (dye- and thermo-dilution) reported by numerous authors it is doubtful uptil now whether calculations of stroke volume according to the formula of Kubicek et al. (1974) can provide absolutely reliable results. The origin of the dz/dt curve and influencing factors of impedance wave have to be cleared up prior to the total acception of impedance cardiography as a reliable method for determining non-invasive stroke volume. This is true in spite of the agreement of all authors we know, that the reproducibility of the impedance cardiography values is as good as in dye or thermo-dilution measurements. However, for patient monitoring it is sometimes more important to assess the relative changes in stroke volume than to measure its absolute value. For long-term non-invasive monitoring of myocardial contractility in critically ill patients or after pharmacological interventions impedance cardiography may be recommended. Besides systolic time intervals, such as pre-ejection time and ventricular ejection time, three more reliable parameters can be derived from the first derivate of impedance wave. Impedance plethysmography has been shown as a reliable method to diagnose deep vein thrombosis and good correlations between impedance and strain-gauge plethysmography and phlebographic findings are reported. In addition fluid volume changes in the leg, venous capacity, venous outflow and arterial inflow may be determined by impedance plethysmography in a simple way. There is no doubt that alterations in the fluid content of biological tissue may measured by impedance technique. However, correlations between changes in the transthoracic impedance and fluid content of the thorax can be quantified only in a single subject which serves as its own control. Overall standardization is not possible. The reason for interindividual differences in the thoracic impedance at a given reduction of body water are due to anatomical differences, intrapulmonary air volume and pressure, location of the electrodes, electrical conductivity of the tissue and, above all, due to the position of the body. Therefore if transthoracic impedance is determined sequentially measurements must be performed with special attention to the position of the body to get reproducible results. Rapid infusion of colloids or blood transfusion may decrease transthoracic impedance due to intravascular volume expansion even at a net fluid lost during forced furosemide-induced diuresis or extracorporal hemodialysis.(ABSTRACT TRUNCATED AT 400 WORDS)