The current status and future directions of impedance cardiography in ICU
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Impedance cardiography has not achieved popularity in the Intensive Care Unit (ICU) to date probably because of the limitations in technique and interpretation associated with the altered physiology of critically ill patients, and also because of interference from other machinery in the ICU. The current climate of questioning the existing technology for bedside cardiovascular assessment however spurs the need to evaluate impedance cardiography as a noninvasive alternative. Validation in noncritically ill patients is good when compared to other technologies (e.g., thermodilution, Fick, dye dilution (r greater than 0.9)). Reliability is good with a coefficient of variation in an ICU population of 8.9%, (compared to 18.6% for thermodilution). It has also shown promise in detecting the clinically significant changes of central intravascular volume. Impedance cardiography appears to be useful for measurement of stroke volume (SV) and ejection fraction (EF). From these, left ventricular end-diastolic volume (VED) can be calculated, probably a more reliable estimate of left ventricular filling than pulmonary capillary wedge pressure (PW), measured by pulmonary artery (PA) catheter. In addition, VES can be calculated and with the knowledge of left ventricular end-systolic pressure (PES) (from invasive arterial monitoring), an end-systolic pressure-volume (ES-PV) relationship can be derived. This is thought to be a measure of contractility that is independent of preload and afterload. The ultimate test in the ICU for impedance cardiography is whether clinical outcome of critically ill patients is altered by the use of this technology. Such outcome testing is essential before the true value of impedance cardiography in the management of critically ill patients can be determined.
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