Cardiac output can be used to evaluate global cardiac function. Changes in cardiac output may be used to identify a change in the hemodynamic status of a patient or to evaluate the success of a treatment and to monitoor critically ill patients over time or perioperatively.
Currently, the most widely accepted methods for the determination of cardiac output are Fick oximetry, thermodilution catheterization (TDC), and indicator dilution using dye or a radioisotope. All of these techniques are invasice and require placing a catheter.
Transthoracic electric bioimpedance (TEB), also called plethysmography or impedance cardiography (ICG), is a noninvasive measurement of cardiac output. Bioimpedance is performed by applying a small electrical current to the chest, and through electrodes placed on the neck and sides and calculating cardiac output from the impedance waveform. Changes in the impedance of the transthoracic electric current are measured electronically, processed by a computer to calculate blood flow, and displayed in real time. The computer software typically displays cardiac data collected over the preceding seconds or minutes, which allows continuous monitoring of alterations in heart rate, cardiac output, and other cardiovascular functions.
This measurment can be perfrmed in office or ER settingsm where catheter placement is impractical or not possible. The difference in time to start bioimpedance monitoring is an advantage over TDC.
A number of studies have been performed. They provide some support for the technique. However, a number of factors can interfere with the accuracy of electrical bioimpedance measurements, and absolute values of cardiac output may not be accurate. Moreover, patient selection criteria have not been clearly and questions remain regarding the appropriate role for cardiac output measurement in patient management and its impact on clinical outcomes.
Agency for Healthcare Research and Quality (AHRQ published a technology assessment on thoracic electrical bioimpedance. The assessment concluded that there was insufficient evidence for meaningful conclusions on the accuracy or clinical usefulness of electrical bioimpedance. The data provided in the available studies suggested that electrical bioimpedance measurements generally correlated similarly with measurements obtained by other testing modalities. Limitations were noted in most reported studies with a scarcity of articles reporting patient outcomes.
WH Wilson Tang,MD Measuring Impedance in Congestive Heart Failure: Current Options and Clinical Applications
Journal article,American heart, May 2009
Centers for Medicare & Medicaid Services (CMS). Technology Assessment for Thoracic Electrical Bioimpedance. Agency for Health Care Research and Quality. November 2002. Available at:http://www.cms.hhs.gov/mcd/viewtechassess.asp?where=search&tid=14&basket=ta:14:Thoracic+Electrical+Bioimpedance.
Stout CL, Van de Water JM, Thompson WM et al.. Impedance cardiography: can it replace thermodilution and the pulmonary artery catheter? Am Surg. 2006 Aug;72(8):728-32; discussion 733-4.