Magnetocardiography Study of Cardiac Anomalies

Abstract

Magnetocardiography (MCG) recordings were carried out on 40 subjects comprising of both normal and those with cardiac anomalies, referred by the collaborating hospital, with the view to compare with the routine 12 lead ECG. An indigenously built four channel Low Tc - SQUID system inside a magnetically shielded room was used for the MCG. In this paper, results on a few specific subjects are presented. In the case of a 68 years old male with ECG indicating Left Bundle Branch Block characterized by wide QRS and a barely split R peak, the MCG showed two well separated RR’ waves making a M pattern over several chest positions. A 51 years old male with normal sinus rhythm in the ECG, exhibited fragmented QRS (<120 ms) in MCG in 21 of the 36 chest positions recorded. Such very well resolved features were also obtained in MCG for subjects diagnosed with myocardial infarction on the basis of ECG. As a control study, some subjects with normal ECG exhibited regular and symmetric MCG patterns across the chest. In the MCG, several cardiac cycles were signal averaged to one cycle by appropriate time epoching. The magnetic field map at the instant of the R wave peak indicated the position of the equivalent current dipole to be in the region of the left ventricle. While the data could be fitted to one dipole in the case of the normal subject, those with fragmented QRS and other anomalies needed more than one dipole to fit the data. Limiting the bandwidth of the MCG data in representative cases by appropriate filters to the lower bandwidth employed in routine ECG did not suppress the major features, indicating that the resolved features seen in MCG are intrinsic to the technique. We conclude that MCG is inherently more sensitive and gives highly resolved information as compared to the ECG.