Abstract
The conventional impedance cardiogram is a record of pulsatile changes in the electrical impedance of the chest with each heartbeat. The signal seems intuitively related to cardiac stroke volume. However doubts persist about the validity of stroke volume measurements based on electrical impedance. This paper presents a new electrical axis for impedance cardiography that is perpendicular to the conventional head-to-foot axis in an anterior-posterior direction. Dual chest and back electrodes are concentric, permitting tetrapolar technique. A relatively simple analytical model is developed, and this model is validated in a three-dimensional finite element model of current flow through the human chest. Three-dimensional simulations show predictable relationships between the fractional increase in anterior-posterior chest impedance and the ventricular ejection fraction (cardiac stroke volume/ventricular end-diastolic volume). Ejection fraction can be computed accurately with a roughly 30-fold increase in signal level compared to the conventional impedance cardiogram. Breathing causes only modest changes in the signal. When the axis of current flow is optimized, one can interpret the impedance changes during the cardiac cycle with greater confidence as noninvasive, beat-by-beat indicators of ventricular ejection fraction in a wide variety of clinical settings.
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Notes
The change in heart volume is the integral of the difference between emptying rate and filling rate of the heart during systole. If only the cardiac ventricles are within the volume sampled by impedance recording then the measured ejection fraction is the same as the true ventricular ejection fraction. If the cardiac atria are also sampled and we assume temporarily that the rate of venous return is nearly constant, then the amount of atrial filling during ventricular systole is the same as the mean flow, multiplied by the ejection time. In this case the sensed or effective ejection fraction is diminished by a factor of \( \left( {1 - t_{e} /T} \right) \), where t e = systolic ejection time, T = cycle time (1/cardiac frequency). In the remaining discussion ejection fraction and stroke volume are taken to mean the effective or net ejection fraction, including any sensed venous return to the atria. In a future computer based system times t e and T could be, if needed, obtained by waveform analysis of the front-to-back impedance cardiogram itself.
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Babbs, C.F. Anterior-Posterior Impedance Cardiography: A New Approach to Accurate, Non-Invasive Monitoring of Cardiac Function. Cardiovasc Eng 10, 52–65 (2010). https://doi.org/10.1007/s10558-010-9094-z
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DOI: https://doi.org/10.1007/s10558-010-9094-z