Predictive value of unshielded magnetocardiographic mapping to differentiate atrial fibrillation patients from healthy subjects

Abstract

Background: P-wave duration, its dispersion and signal-averaged ECG, are currently used markers of vulnerability to atrial fibrillation (AF). However, since tangential atrial currents are better detectable at the body surface as magnetic than electric signals, we investigated the accuracy of magnetocardiographic mapping (MCG), recorded in unshielded clinical environments, as predictor of AF occurrence.

Methods: MCG recordings, in sinus rhythm (SR), of 71 AF patients and 75 controls were retrospectively analyzed. Beside electric and magnetic P-wave and PR interval duration, two MCG P-wave subintervals, defined P-dep and P-rep, were measured, basing on the point of inversion of atrial magnetic field (MF). Eight parameters were calculated from inverse solution with "Effective Magnetic Dipole (EMD) model" and 5 from "MF Extrema" analysis. Discriminant analysis (DA) was used to assess MCG predictive accuracy to differentiate AF patients from controls.

Results: All but one (P-rep) intervals were significantly longer in AF patients. At univariate analysis, three EMD parameters differed significantly: in AF patients, the dipole-angle-elevation angular speed was lower during P-dep (p < 0.05) and higher during P-rep (p < 0.001) intervals. The space-trajectory during P-rep and the angle-dynamics during P-dep were higher (p < 0.05), whereas ratio-dynamics P-dep was lower (p < 0.01), in AF. At DA, with a combination of MCG and clinical parameters, 81.5% accuracy in differentiating AF patients from controls was achieved. At Cox-regression, the angle-dynamics P-dep was an independent predictor of AF recurrences (p = 0.037).

Conclusions: Quantitative analysis of atrial MF dynamics in SR and the solution of the inverse problem provide new sensitive markers of vulnerability to AF.

Keywords: atrial fibrillation; discriminant analysis; inverse solution; magnetocardiography; surface cardiac mapping.

Figure 1

Schematic representation of atrial MCG measurements (PWD: P wave duration; MFD: magnetic field distribution; P‐dep and P‐rep: as explained in the text)

Figure 2

(a) Time‐variant dynamics of the MF distribution during the whole P‐wave. Typical inversion of the MF polarity is evident during the P‐wave descending limb (thick dashed line). (b) After subtraction of atrial repolarization MF, residual MF distribution consistent with the underlying atrial depolarization is unmasked. (c) Inverse EMD localization within the 3D heart model shows a trajectory consistent with atrial depolarization from the right (* PW onset) to the left atrium