Measuring impedance in congestive heart failure: current options and clinical applications

Abstract

Measurement of impedance is becoming increasingly available in the clinical setting as a tool for assessing hemodynamics and volume status in patients with heart failure. The 2 major categories of impedance assessment are the band electrode method and the implanted device lead method. The exact sources of the impedance signal are complex and can be influenced by physiologic effects such as blood volume, fluid, and positioning. This article provides a critical review of our current understanding and promises of impedance measurements, the techniques that have evolved, as well as the evidence and limitations regarding their clinical applications in the setting of heart failure management.

Figures

Figure 1

Band electrode impedance measurement. A, Schematic representation of the band electrode technique. B, Time derivative impedance dZ/dt plotted against time t. Point A marks the fourth heart sound of atrial contraction, point B signals the first heart sound before ventricular isovolumetric contraction and rapid ejection, point C is the maximum dZ/dt, point X is the second heart sound of the closing aortic valve, and point O marks the diastolic filling. Left ventricular ejection time (LVET) is the time between points B and X.

Figure 2

Implanted electrode impedance measurement. A, Schematic representation of the implanted electrode technique. B, Plot of the impedance Z for device-based fluid monitoring for a hypothetical episode of fluid overload. Point B is the baseline impedance in the absence of fluid overload, point C marks the steady decline in impedance with accumulating pulmonary fluid over several days or weeks given by Δt, and point D follows the restoration of baseline impedance ΔZ with applied diuretic therapy.