Work

Real-ECG extraction and stroke volume from MR-Compatible 12-lead ECGs; testing during stress, in PVC and in AF patients

Year: 2011

Type: article

Abstract: Due to the Magneto-Hydro-Dynamic (MHD) effect, blood flow within the MRI’s magnetic field (B 0 ) produces a large voltage during the S-T cardiac segment [ 1 ]. The peak MHD voltage (V MHD ) can be comparable, in higher-field MRIs, to the R-wave amplitude of the real Electrocardiogram (ECG real ), so that V MHD reduces ECG-gating reliability and prevents ischemia-monitoring during cardiac imaging/interventions. We hypothesized that (1) separation of ECG real and V MHD from 12-lead ECGs acquired within a 1.5T MRI could be achieved, using adaptive filtering, based on a set of ECG calibration measurements, and (2) a non-invasive beat-to-beat stroke-volume estimation could be achieved from time-integrated systolic V MHD . Fig. 1 shows 3 sets of 20-sec breath-held ECGs measured at positions (i), (ii) and (iii), utilizing an MRI-compatible Cardiolab-IT digital ECG-recording system [ 2 ]. The adaptive filtering procedure was tested in 5 healthy subjects, and 2 patients with Premature Ventricle Contractions (PVCs) and Atrial Fibrillation (AF). Validation was based on comparing the filter-derived ECG real with ECGs measured periodically outside the MRI. The data processing block diagram (Fig. 2 ) includes training of adaptive Least-Mean-Square filters with ECG real input (i), application of the trained filters to ECGs acquired in (ii) and (iii), which separates the V MHD from ECG real . ECGs measured at 3 positions; outside the field (i) and at isocenter with head-in (ii) and feet-in (iii). Adaptive filtering diagram used for intermittent PVC patients, with beats separated and then processed independently at abnormal/normal beat filters. PVC patient’s results (Fig. 3 ): (a) unprocessed surface-lead V6, (b) extracted ECG real , and (c) V MHD . In (b) S-T segment voltage is restored, and the R-wave dominates for gating. Aortic-flow vortices (c) generate oscillating-polarity V MHD , with V MHD peaking during S-T segment. Cardiac beat-to-beat stroke volume (d) was estimated from time-integrated systolic V MHD . PVC beats produce substantially lower stroke volume than during sinus-rhythm. AF patient results (Fig. 4 ): (c) Irregular V MHD and (d) irregular stroke volume are due to ventricular-filling differences at varying heart rates (100-140bpm). Athlete subject results (Fig. 5 ): Filter tracking of rapid heart-rate changes from 44bpm to 87bpm is shown during a treadmill stress test performed inside the MRI. V MHD (b) and stroke volume (c) increase with heart rate, suggesting that the cardiac output matches higher demand. A stroke-volume comparison of all subjects (Fig 6 ), derived from time-integrated systolic V MHD s, demonstrates the measurement’s sensitivity to pathology. Results from a PVC patient (Ejection Fraction 20-25%, mitral regurgitation. Results from AF patient (100-150 bpm) Results from athlete subject during treadmill stress test (44-87 bpm) Stroke-volume comparison (cardiac cycles n=20 per subject). Athlete (+24%), PVC (-54%) and AF (-23%), relative to average of volunteers. The filtering extracts ECG real from measured 12-lead ECG, preserving ECG real for ischemia monitoring and MRI gating. Stroke volume can be non-invasively derived from the time-integrated systolic V MHD .

Source: Journal of Cardiovascular Magnetic Resonance

DOI: 10.1186/1532-429x-13-s1-p6