. Author manuscript; available in PMC: 2011 Mar 16.

Published in final edited form as: Am Heart J. 2008 Dec 16;157(3):402–411. doi: 10.1016/j.ahj.2008.10.016

Table I.

Factors affecting impedance measurements

Factor	Method affected (ICG or fluid)	Summary of mechanism
Blood volume¹⁰,¹¹	ICG and fluid	Causes approximately 60% of the total impedance change during the cardiac cycle. For fluid status, hypervolemia is associated with fluid overload.
Aortic volume change¹⁰,¹¹	ICG	Aortic expansion is attributed to approximately 30% of the impedance change during ventricular ejection.
Blood velocity¹⁰,¹¹	ICG	Approximately 10% of the impedance signal. Shear stress from blood flow across vessel walls affects the blood resistivity, which can be significant for post-CABG surgery.
Valvular regurgitation¹²–¹⁴	ICG	Affects the flow of electrical current through the aorta, which can give widely varying CO intrapatient readings.
Sensor placement¹⁵,¹⁶	ICG and fluid	Conflicting results for the accuracy of whole-body impedance measurements with electrodes placed at the extremities. Pacer leads at different positions may have different sensitivity to fluid overload.
Algorithm²,¹⁷	ICG and fluid	Early ICG algorithms are inaccurate, later versions have better correlation to catheterization. Fluid monitor affected by impedance threshold causing interpatient variation.
Atrial fibrillation¹⁸,¹⁹	ICG	Greater deviation N15% from TD for CO measurements from decreased impedance. Unknown if any correlation exists between atrial fibrillation and fluid overload episodes.
Body dimensions²⁰	ICG	Extreme dimensions show poor correlation with TD and Fick principle for CO. Excessive fat influences total resistivity and sensitivity of the impedance signal.
Body posture²¹	ICG and fluid	Changes in posture can shift fluid distribution, which causes deviations in impedance over time.

ICG, Impedance cardiography; CABG, coronary artery bypass graft; CO, cardiac output.