Value of Magnetocardiographic QRST Integral Maps in the Identification of Patients at Risk of Ventricular Arrhythmias

It has been shown that regional ventricular repolarization properties can be reflected in body surface distributions of electrocardiographic QRST deflection areas (integrals). We hypothesize that these properties can be reflected also in the magnetocardiographic QRST areas and that this may be useful for predicting vulnerability to ventricular tachyarrhythmias. Magnetic field maps were obtained during sinus rhythm from 49 leads above the anterior chest in 22 healthy (asymptomatic) control subjects (group A) and in 29 patients with ventricular arrhythmias (group B). In each subject, the QRST deflection area was calculated for each lead and displayed as an integral map. The mean value of maximum was significantly larger in the control group A than in the patient group B (1,626+/-694 pTms vs. 582+/-547 pTms, P<0.0001). To quantitatively assess intragroup variability in the control group A and intergroup variability of the control and patient groups, we used the correlation coefficient r and covariance sigma. These indices showed significantly less intragroup than intergroup variation (e.g., in terms of sigma, 28.0x10(-6)+/-12.3x10(-6) vs. 3.4x10(-6)+/-12.5x10(-6), P<0.0001). Each QRST integral map was also represented as a weighted sum of 24 basis functions (eigenvectors) by means of Karhunen-Loeve transformation to calculate the contribution of the nondipolar eigenvectors (all eigenvectors beyond the third). This percentage nondipolar content of magnetocardiographic QRST integral maps was significantly higher in the patient group B than in the control group A (13.0%+/-9.1 % vs. 2.6%+/-2.0%, P<0.0001). Discriminations between control subjects and patients with ventricular arrhythmias based on magnitude of the maximum, covariance sigma, and nondipolar content were 90.2%, 90.2%, and 86.3% accurate, with a sensitivity of 89.7%, 93.1%, and 75.9%, and a specificity of 90.9%, 86.4%, and 100%. We have shown that magnitude of the maximum and indices of variability and nondipolarity of the magnetocardiographic QRST integral maps may predict arrhythmia vulnerability. This finding is in agreement with earlier studies that used body surface potential mapping and suggests that magneticfield mapping may also be a useful diagnostic tool for risk analysis.