Istituto di Matematica Applicata e Tecnologie Informatiche     
Taccardi B., Veronese S., Colli Franzone P., Guerri L. Multiple components in the unipolar electrogram: a simulation study in a three-dimensional model of ventricular myocardium. Preprint ercim.cnr.ian//1998-1086, 1998.
For many decades the interpretation of unipolar electrograms (EGs) and electrocardiograms (ECGs) was based on simple models of the heart as a current generator, e.g. the uniform dipole layer and, more recently, the 'oblique dipole layer'. However, a number of recent and old experimental data are inconsistent with the predictions of these models. To address this problem, we implemented a numerical model simulating the spread of excitation through a parallepipedal myocardial slab, with a view to identifying the factors that affect the shape, amplitude and polarity of unipolar electrograms generated by the spreading wave front. The numerical model represents a portion of the left ventricular wall as a parallelepipedal slab ($6.5 times 6.5 times 1$ cm), the myocardial tissue is represented as an anisotropic bidomain with epi-endocardial rotation of fiber direction and unequal anisotropy ratio. Following point stimulation, excitation times in the entire volume are computed by using an eikonal formulation. Potential distributions are computed by assigning a fixed shape to the action potential profile. Electrograms at multiple sites in the volume are computed from the time-varying potential distributions. The simulations show that the unipolar QRS waveforms are the sum of a 'field' component, representing the effect of an approaching or receding wave front on the potential recorded by a unipolar electrode, and a previously unrecognized 'reference' component which reflects the drift, during the spread of excitation, of the reference potential, which moves from near the positive to near the negative extreme of the potential distribution during the spread of excitation. The drift of the reference potential explains the inconsistencies between the predictions of the models and the actual shapes of the EGs. The drift modifies the slopes of EG wave forms during excitation and recovery and can be expected to affect the assessment of excitation and recovery times and QRS and ST-T areas. Removing the drift re-establishes consistency between potential distributions and electrographic wave forms.
Subject Electrograms, Bidomain Model, Eikonal Equation, QRS, Reference Potential, Cardiac Potential Maps

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