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

Quantification of Beat-To-Beat Variability of Action Potential Durations in Langendorff-Perfused Mouse Hearts

Background: Beat-to-beat variability in action potential duration (APD) is an intrinsic property of cardiac tissue and is altered in pro-arrhythmic states. However, it has never been examined in mice. Methods: Left atrial or ventricular monophasic action potentials (MAPs) were recorded from Langendorff-perfused mouse hearts during regular 8 Hz pacing. Time-domain, frequency-domain and non-linear analyses were used to quantify APD variability. Results: Mean atrial APD (90% repolarization) was 23.5 ± 6.3 ms and standard deviation (SD) was 0.9 ± 0.5 ms (n = 6 hearts). Coefficient of variation (CoV) was 4.0 ± 1.9% and root mean square (RMS) of successive differences in APDs was 0.3 ± 0.2 ms. The peaks for low- and high-frequency were 0.7 ± 0.5 and 2.7 ± 0.9 Hz, respectively, with percentage powers of 39.0 ± 20.5 and 59.3 ± 22.9%. Poincaré plots of APDn+1 against APDn revealed ellipsoid shapes. The ratio of the SD along the line-of-identity (SD2) to the SD perpendicular to the line-of-identity (SD1) was 8.28 ± 4.78. Approximate and sample entropy were 0.57 ± 0.12 and 0.57 ± 0.15, respectively. Detrended fluctuation analysis revealed short- and long-term fluctuation slopes of 1.80 ± 0.15 and 0.85 ± 0.29, respectively. When compared to atrial APDs, ventricular APDs were longer (ANOVA, P < 0.05), showed lower mean SD and CoV but similar RMS of successive differences in APDs and showed lower SD2 (P < 0.05). No difference in the remaining parameters was observed. Conclusion: Beat-to-beat variability in APD is observed in mouse hearts during regular pacing. Atrial MAPs showed greater degree of variability than ventricular MAPs. Non-linear techniques offer further insights on short-term and long-term variability and signal complexity.

In vivo human sock-mapping validation of a simple model that explains unipolar electrogram morphology in relation to conduction-repolarization dynamics

INTRODUCTION

The unipolar electrogram (UEG) provides local measures of cardiac activation and repolarization and is an important translational link between patient and laboratory. A simple theoretical model of the UEG was previously proposed and tested in silico.

METHOD AND RESULTS

The aim of this study was to use epicardial sock-mapping data to validate the simple model's predictions of unipolar electrogram morphology in the in vivo human heart. The simple model conceptualizes the UEG as the difference between a local cardiac action potential and a position-independent component representing remote activity, which is defined as the average of all action potentials. UEGs were recorded in 18 patients using a multielectrode sock containing 240 electrodes and activation (AT) and repolarization time (RT) were measured using standard definitions. For each cardiac site, a simulated local action potential was generated by adjusting a stylized action potential to fit AT and RT measured in vivo. The correlation coefficient (cc) measuring the morphological similarity between 13,637 recorded and simulated UEGs was cc  =  0.89 (0.72-0.95), median (Q₁ -Q₃ ), for the entire UEG, cc  =  0.90 (0.76-0.95) for QRS complexes, and cc  =  0.83 (0.58-0.92) for T-waves. QRS and T-wave areas from recorded and simulated UEGs showed cc> 0.89 and cc> 0.84, respectively, indicating good agreement between voltage isochrones maps. Simulated UEGs accurately reproduced the interaction between AT and QRS morphology and between RT and T-wave morphology observed in vivo.

CONCLUSIONS

Human in vivo whole heart data support the validity of the simple model, which provides a framework for improving the understanding of the UEG and its clinical utility.

Analytical description of the slope of the APD-restitution curve to assess the interacting contribution of conduction and repolarization dynamics

The restitution of the action potential duration (APDR) is a mechanism whereby cardiac excitation and relaxation adapt to changes in heart rate. Several studies, mainly carried out in-vitro and in-silico, have demonstrated that a steep APDR curve is associated with increased vulnerability to fatal arrhythmias. However, the mechanisms that link the steepness of the APDR curve to arrhythmogenesis remain undetermined. Although APDR is known to interact with conduction dynamics, few studies have focused on these interactions. In this paper, an analytical expression of the slope of the APDR is derived. This expression explicitly describes the dependency of the slope of the APDR curve on the activation time and/or conduction velocity changes. The study of this expression shows that conduction dynamics are among the main determinants of the slope of the APDR curve. A small absolute increment in the steepness of the activation time restitution slope can cause the steepness of the APDR slope to dramatically increase. Theoretically, the APDR slope quickly diverges to infinity when the increase in activation time matches the decrease in the pacing interval. High density epicardial mapping performed in a patient undergoing open heart surgery, shows excellent agreement between measures of the slope of the APDR curve and its analytical prediction (linear correlation > 0.95). The in-vivo recordings suggest that activation time restitution is the main determinant of the slope of the APDR curve.

On how 2∶1 conduction block can induce T-wave alternans in the unipolar intracavitary electrogram: Modelling in-vivo human recordings from an ischemic heart

Repolarization alternans is a marker of increased vulnerability to fatal arrhythmias. At the tissue level, in unipolar electrograms (UEGs) recorded on the myocardium, repolarization alternans is often measured as an alternating change of the T-wave, so called T-wave alternans (TWA). During ischemia, UEG-TWA is used as a marker of cardiac instability and is considered as a key parameter to assess pharmacological strategies. However, during ischemia it is not clear whether UEG-TWA is a sign of repolarization alternans which may promote 2:1 conduction block, or whether it is induced by ongoing regional 2:1 conduction block. In this study, we first show in-vivo human data recorded during an ischemic event that suggest that 2:1 conduction block induces UEG-TWA beyond the region of 2:1 conduction block. We then develop an analytical forward model of the UEG by coupling an analytical description of the cardiac action potential with a theoretical expression of the UEG, where each UEG is the combination of a local and a remote component and noise. With this model, we were able to generate signals that closely resemble UEGs recorded in-vivo, with a maximum correlation ρ > 0.94. Finally, we interrogate the model and demonstrate that whenever 2:1 conduction block is present, UEG-TWA arises as a consequence of alternating imbalance of both the local and remote components of the UEG. The statistical significance of UEG-TWA depends on the interactions between local and remote dynamics and noise.We conclude that in an ischemic model, UEG-TWA is likely to be a sign of 2:1 conduction block, either proximal or distal from the recording site.

Accuracy of measurements derived from intracardiac unipolar electrograms: A simulation study

The ventricular action potential duration (APD) is a fundamental determinant of cardiac electrical stability and can be estimated by measuring the activation recovery interval (ARI) from the unipolar electrogram (UEG), which represents the electrical activity of the heart at the tissue level. Under experimental conditions, automatic estimation of ARIs is challenging due to non-related interferences and low signal-to-noise ratios (SNRs). In this simulation study, we investigated how the reliability of ARI estimates is affected by noise and artefacts in the UEG. Real-like electrograms were generated using a 257-node whole heart model to synthesize 20 real-like UEGs exhibiting constant and dynamic ARI patterns. Controlled degrees of noise and contamination (ectopic beats) were added to obtain a range of signal qualities. The generated recordings were automatically analyzed using a proposed standard method to estimate the ARI. The performance was compared with two improvements of the standard method including a narrow search window and a correlation filter. The results show that the robustness of automatic ARI analysis was dramatically improved by using the proposed improvement methods. For typical recordings with a SNR of 10dB and filtered with often used cutoff frequency of 30Hz to measure repolarization, the average mean absolute error of the estimations was reduced from 16.2ms (range:12.2-29.0ms) for the standard method to 11.6ms (range:6.0-13.4ms) for the improved method. The standard deviation was reduced from 38.2ms (range:26.8- 58.5ms) to 14.6ms (range:7.6-16.9ms). Detection of cyclical variation of ARI was also improved by using the improvement strategy: for 0.2Hz ARI oscillations with an amplitude of 5ms, the highest average detection rate increased from 41% for the standard method to 100% using the improved method for recordings with a SNR of 10dB.

Comparative evaluation of methodologies for T-wave alternans mapping in electrograms

Electrograms (EGM) recorded from the surface of the myocardium are becoming more and more accessible. T-wave alternans (TWA) is associated with increased vulnerability to ventricular tachycardia/fibrillation and it occurs before the onset of ventricular arrhythmias. Thus, accurate methodologies for time-varying alternans estimation/detection in EGM are needed. In this paper, we perform a simulation study based on epicardial EGM recorded in vivo in humans to compare the accuracy of four methodologies: the spectral method (SM), modified moving average method, laplacian likelihood ratio method (LLR), and a novel method based on time-frequency distributions. A variety of effects are considered, which include the presence of wide band noise, respiration, and impulse artifacts. We found that 1) EGM-TWA can be detected accurately when the standard deviation of wide-band noise is equal or smaller than ten times the magnitude of EGM-TWA. 2) Respiration can be critical for EGM-TWA analysis, even at typical respiratory rates. 3) Impulse noise strongly reduces the accuracy of all methods, except LLR. 4) If depolarization time is used as a fiducial point, the localization of the T-wave is not critical for the accuracy of EGM-TWA detection. 5) According to this study, all methodologies provided accurate EGM-TWA detection/quantification in ideal conditions, while LLR was the most robust, providing better detection-rates in noisy conditions. Application on epicardial mapping of the in vivo human heart shows that EGM-TWA has heterogeneous spatio-temporal distribution.