Cardiac signal extraction in patients with Implantable Cardioverter Defibrillators

An epoch based methodology to denoise magnetocardiogram (MCG) signals and its application to measurements on subjects with implanted devices

Magnetocardiograms (MCG) provide clinically useful diagnostic information in a variety of cardiac dysfunctions. Low frequency baseline drifts and high frequency noise are inevitably present in routine MCG even for those measured inside magnetically shielded rooms. These interferences sometimes exceed subtle cardiac features in MCG recorded on subjects with implanted devices like cardiac pacemakers; this makes interpretation of cardiac magnetic fields difficult. The present study proposes a correlation-based beat-by-beat approach and principal component analysis to eliminate drifts and high frequency noise respectively; the approach is suitable for denoising both single and multi-channel MCG data. The methodology is critically evaluated on simulated noisy measurements using a 37 channel MCG system, when objects such as implantable permanent pacemaker and stainless-steel wire are sequentially kept externally on the chests of five healthy subjects. By characterizing the noise introduced by each of these objects, the deterioration in the quality of MCG and its subsequent restoration by using the proposed method is assessed. The performance of the proposed method is also compared with other conventional denoising techniques namely, bandpass filters, wavelets and ensemble empirical mode decomposition. The proposed method not only exhibits least distortion, but also preserves the beat-by-beat dynamics of cardiac time series. The method has also been illustrated on actual MCG measurements on two subjects with implanted pacemaker which highlight the ability of the proposed method for denoising MCG in general and during extremely noisy measurement situations. Keywords: Magnetocardiography (MCG), pacemaker, baseline correction, PCA.

Prognostic value of electromagnetic QRS fragmentation in survivors of sustained ventricular tachycardia or ventricular fibrillation compared with healthy controls

BACKGROUND

Magnet field imaging (MFI) is a noninvasive method to determine cardiac electromagnetic activity.

AIM OF THE STUDY

This study aims to compare the electromagnetic QRS fragmentation index (eQFI) in survivors of sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) to healthy individuals.

METHODS

Twenty-five consecutive patients with documented sustained VT or VF who underwent a MFI investigation between December 2009 and October 2011 were compared with 25 age- and sex-matched healthy individuals.

RESULTS

Patients with documented VT or VF showed a trend to higher eQFI values compared with the control group (p = 0.06). This increase was mainly driven by VT/VF patients with ischemic cardiomyopathy (CMP) which was markedly elevated compared with the healthy controls (1.48 vs. 1.07; p = 0.01). In patients with nonischemic CMP or acute coronary syndrome, eQFI was not different from the healthy group.

CONCLUSIONS

Electromagnetic QRS fragmentation is increased in VT/VF patients with ischemic CMP but not in patients with ventricular arrhythmias of other origin. Further investigations in prospective cohorts should evaluate the prognostic value of electromagnetic QRS fragmentation in patients with ischemic heart disease to predict the occurrence of VT/VF and to guide therapy.