Impedance cardiography.  The impact of new technology. 
The 1990s have witnessed major advances in impedance cardiography technology.  Problems existed with the methods used to calculate cardiac output.  Excessive lung fluid, as often found in critically ill patients, may also invalidate measurements.  The signal processing and measurement techniques used in older systems were deficient.  The newer systems, of which there are at least six, incorporate novel and improved signal processing techniques.  They also offer analog visual displays, personal computer interfacing, sophisticated analytical software and haemodynamic patient management systems.  Evaluation of these systems is difficult because no true 'gold standard' method of cardiac output measurement exists.  When compared with thermodilution techniques, limits of agreement of +/- 20-30% seem acceptable.  These limits can be achieved in normal subjects but not in critically ill patients.  Validation data are available for only half of the new systems.  Until recently, the main application for impedance cardiography has been research but improved accuracy should lead to increased clinical usage.