Impedance cardiography

Impedance cardiography (ICG) is a non invasive technology measuring total electrical conductivity of the thorax and its changes in time to process continuously a number of cardiodynamic parameters, such as stroke volume (SV), heart rate (HR), cardiac output (CO), ventricular ejection time (VET), pre ejection period and used to detect the impedance changes caused by a high frequency, low magnitude current flowing through the thorax between additional two pairs of electrodes located outside of the measured segment. The sensing electrodes also detect the ECG signal, which is used as a timing clock of the system.

Impedance cardiography (ICG), also referred to as electrical impedance plethysmography (EIP) or Thoracic Electrical Bioimpedance (TEB) has been researched since the 1940s. NASA helped develop the technology in the 1960s. The use of impedance cardiography in psychophysiological research was pioneered by the publication of an article by Miller and Horvath in 1978. Subsequently, the recommendations of Miller and Horvath were confirmed by a standards group in 1990. A comprehensive list of references is available at ICG Publications. With ICG, the placement of four dual disposable sensors on the neck and chest are used to transmit and detect electrical and impedance changes in the thorax, which are used to measure and calculate cardiodynamic parameters.

The primary function of the cardiovascular system is transport of oxygen: blood is the vehicle, oxygen is the cargo. The task of the healthy cardiovascular system is to provide adequate perfusion to all organs and to maintain a dynamic equilibrium between oxygen demand and oxygen delivery. In a healthy person, the cardiovascular system always increases blood flow in response to increased oxygen demand. In a hemodynamically compromised person, when the system is unable to satisfy increased oxygen demand, the blood flow to organs lower on the oxygen delivery priority list is reduced and these organs may, eventually, fail. Digestive disorders, male impotence, tiredness, sleepwalking, environmental temperature intolerance, are classic examples of a low flow state, resulting in reduced blood flow.

Clinical measurement of cardiac output has been available since the 1970s. However, this blood flow measurement is highly invasive, utilizing a flow directed, thermodilution catheter (also known as the Swan Ganz catheter), which represents significant risks to the patient. In addition, this technique is costly (several hundred dollars per procedure) and requires a skilled physician and a sterile environment for catheter insertion. As a result, it has been used only in very narrow strata (less than 2%) of critically ill and high risk patients in whom the knowledge of blood flow and oxygen transport outweighed the risks of the method. In the United States, it is estimated that at least two million pulmonary artery catheter monitoring procedures are performed annually, most often in peri operative cardiac and vascular surgical patients, decompensated heart failure, multi organ failure, and trauma.

In theory, a noninvasive way to monitor hemodynamics would provide exceptional clinical value because data similar to invasive hemodynamic monitoring methods could be obtained with much lower cost and no risk. While noninvasive hemodynamic monitoring can be used in patients who previously required an invasive procedure, the largest impact can be made in patients and care environments where invasive hemodynamic monitoring was neither possible nor worth the risk or cost. Because of its safety and low cost, the applicability of vital hemodynamic measurements could be extended to significantly more patients, including outpatients with chronic diseases. ICG has even been used in extreme conditions such as outer space and a Mt. Everest expedition. Heart failure, hypertension, pacemaker, and dyspnea patients are four conditions in which outpatient noninvasive hemodynamic monitoring can play an important role in the assessment, diagnosis, prognosis, and treatment. Some studies have shown ICG cardiac output is accurate, while other studies have shown it is inaccurate. Use of ICG has been shown to improve blood pressure control in resistant hypertension when used by both specialists and general practitioners. ICG has also been shown to predict worsening status in heart failure.