Bench to Bedside: Electrophysiologic and Clinical Principles of Noninvasive Hemodynamic Monitoring Using Impedance Cardiography

Abstract

The evaluation of the hemodynamic state of the severely ill patient is a common problem in emergency medicine. While conventional vital signs offer some insight into delineating the circulatory pathophysiology, it is often impossible to determine the true clinical state from an analysis of blood pressure and heart rate alone. Cardiac output measurements by thermodilution have been the criterion standard for the evaluation of hemodynamics. However, this technology is invasive, expensive, time-consuming, and impractical for most emergency department environments. Impedance cardiography (ICG) is a noninvasive method of obtaining continuous measurements of hemodynamic data such as cardiac output that requires little technical expertise. ICG technology was first developed by NASA in the 1960s and is based on the idea that the human thorax is electrically a nonhomogeneous, bulk conductor. Variation in the impedance to flow of a high-frequency, low-magnitude alternating current across the thorax results in the generation of a measured waveform from which stroke volume can be calculated by a modification of the pulse contour method. To adequately judge the possible role of this technology in the practice of emergency medicine, it is important to have a sufficient understanding of the basic scientific principles involved as well as the clinical validity and limitations of the technique.

Characterization of the spectrum of hemodynamic profiles in trauma patients with acute neurogenic shock

“…Arterial pressures measured by blood pressure cuffs are often sporadic and unreliable when compared with direct measurements using arterial catheters. In addition, impedance cardiography is a well-accepted and practical technique for determining cardiac output, but it is not a “gold standard” and can be subject to significant measurement error [9]. The advantage of using both of these noninvasive techniques in this study is that they could be readily implemented in the very early stages of the clinical shock before significant resuscitative measures were initiated.…”

“…Although all of these patients met the traditional criteria for the diagnosis of neurogenic shock (systolic blood pressure <100 mm Hg and heart rate <80 beats per minute), the goal of the study was to more extensively characterize the hemodynamics so that the mechanisms of circulatory shock could be better clarified. The hemodynamic variables collected included heart rate, systolic and diastolic blood pressure, and cardiac output that were obtained using impedance cardiography (Philips Medical ICG Monitor Model 2004; Philips Medical Systems, 3000 Minuteman Road, Andover, MA) and traditional emergency department vital sign determinations [8,9]. Additional hemodynamic variables such as systemic vascular resistance, pre-ejection period (PEP) (cardiac PEP), and left ventricular ejection time (LVET) were derived from these clinical measurements [9].…”

“…The hemodynamic variables collected included heart rate, systolic and diastolic blood pressure, and cardiac output that were obtained using impedance cardiography (Philips Medical ICG Monitor Model 2004; Philips Medical Systems, 3000 Minuteman Road, Andover, MA) and traditional emergency department vital sign determinations [8,9]. Additional hemodynamic variables such as systemic vascular resistance, pre-ejection period (PEP) (cardiac PEP), and left ventricular ejection time (LVET) were derived from these clinical measurements [9]. The reference range for the impedance measure PEP is between 0.08 and 0.15 seconds and is usually considered to be indicative of a peripheral sympathetic outflow.…”

Hemodynamic monitoring in the critically ill: an overview of current cardiac output monitoring methods

“…With this assumption, we can postulate that changes in thoracic impedance are largely dependent on three components: a baseline impedance indirectly proportional to the thoracic fluid content, tidal changes in intrathoracic blood volume caused by respiration, and small changes caused by the cardiac cycle. The latter are primarily due to changes in aortic volume, which can be used to estimate stroke volume and CO 29, 30 . However, it does have important limitations.…”

Physiologic and Clinical Principles behind Noninvasive Resuscitation Techniques and Cardiac Output Monitoring

“…This technology has been validated in over 2000 patients in multiple different settings against multiple different gold standards [85]. It is based on measuring changes in thoracic impedance of high-frequency, low-magnitude alternating currents applied across the thorax.…”