Amplitude of Dominant T-Wave Alternans assessment on ECGs obtained from a biophysical model

A Potential-Based Inverse Spectral Method to Noninvasively Localize Discordant Distributions of Alternans on the Heart From the ECG

T-wave alternans (TWA), defined as the beat-to-beat alternation in amplitude of the T-waves, has been shown to be linked to ventricular fibrillation (VF). However, current TWA tests have high sensitivity but low specificity in determining who is at risk. To overcome this limitation, it might be helpful to determine the spatial distribution of any regions on the heart that alternate in opposite phase. Understanding these spatial distributions in relation to the regular activation of the heart could help explain the mechanism for the genesis of VF and thus disambiguate the low specificity of TWA.

GOAL

Image the spatial distribution of TWA on the heart surface from ECG measurements.

METHODS

We introduced the inverse spectral method (ISM), a tailored inverse (or ElectroCardioGraphic Imaging) solution designed specifically to noninvasively image cases of TWA on the heart.

RESULTS

We evaluate the ISM on its capacity to reliably detect the spatial distributions of TWA compared against a standard TWA detection method applied directly to the electrograms on the heart surface. We report on results from both a series of synthetic simulations of TWA generated using the ECGSIM software and a set of continuous epicardial surface voltage recordings from a canine experiment. ISM detected TWA distributions that matched the phase of the true underlying out-of-phase regions over and of the heart surface, respectively.

CONCLUSION

Our results suggest that ISM is capable of reliably detecting the different regions present in a TWA distribution across a wide variety of TWA locations on the heart in simulation and in the face of transients and nonidealities in the canine recordings.

Detection of transient, regional cardiac repolarization alternans by time-frequency analysis of synthetic electrograms

Repolarization alternans (RA), originating at the cellular level, is thought to produce an arrhythmogenic substrate, and surface ECG T-wave alternans (TWA) is a marker of risk for sudden cardiac death. In this paper we study RA in the unipolar electrograms (EGM), which represent the electrical activity of the heart at the tissue level. We first describe a simple analytical model to study how RA, simulated as alternating variations of action potential duration, affects EGM-TWA, and then we propose a novel methodology based on time-frequency analysis to detect EGM-TWA which occurs intermittently in few consecutive beats. In a simulation study, we used a 257-node whole heart model to reproduce several patterns of RA. RA involved specific subsets of adjacent nodes (11, 65 and 257), exhibited different amplitudes (0.25, 0.5 and 1 ms) and lasted for 40 consecutive beats of a 80-beat-long test sequence. Results show a relationship between the spatial distribution of RA and EGM-TWA: the smaller the region where RA occurs, the higher the extent of EGM-TWA. With the proposed methodology, we localized those portions of myocardium which exhibited EGM-TWA with an accuracy higher than 90%.