The interplay of sex-specific electrophysiology and repolarization heterogeneity governs ventricular arrhythmia sustainability in women

Background

The role of sex-specific electrophysiology in the development of lethal ventricular arrhythmia is poorly understood. Since female ventricles have a longer action potential duration (APD) than male ventricles, we hypothesized that female hearts are more vulnerable to sustained ventricular arrhythmia in the presence of known arrhythmogenic substrates.

Purpose

To determine if prolonged APD in females leads to sustained ventricular arrhythmia in the presence of left ventricular (LV) hypertrophy, diffuse fibrosis, and repolarization heterogeneity.

Methods

Magnetic resonance imaging (MRI) data were used to develop computational biventricular models of two aortic stenosis patients (male, 54 years; female, 64 years) before (LV hypertrophy) and 3 months after (regressed LV hypertrophy) aortic valve replacement (AVR). These patients were chosen since they exhibit reduced LV mass, less fibrosis, and no ventricular arrhythmias after AVR. The models were assigned sex-specific cellular electrophysiology and an apicobasal repolarization gradient derived from human data. Simulations of apical pacing with a cycle length (CL) of 750 ms were performed while APD at 90% repolarization (APD90) and repolarization time (activation time + APD90) were calculated. Reentrant arrhythmia was induced by rapidly pacing the LV with a 2 mm wide apicobasal line electrode at CL≤265 ms. The simulations were repeated in the presence of diffuse myocardial fibrosis derived from the MRI patient data, and repeated again with an apicobasal repolarization gradient reduced by ≈35%. Ventricular mass was calculated as the product of model volume and myocardial density.

Results

In hypertrophic ventricles with default apicobasal repolarization gradient, rapid pacing led to reentry that lasted longer in the female (>25 s) than in the male (1.0 s) ventricles. The duration of reentry increased in the presence of fibrosis in the male ventricles (male: 6.6 s; female >25 s) and decreased in the female ventricles when the apicobasal repolarization gradient was reduced (Figure). Sustained reentry (>25 s) was not observed in the ventricles with regressed hypertrophy, regardless of the presence of fibrosis or repolarization gradient. Average APD90 was longer in the female ventricles both before (male: 267±35 ms; female: 318±34 ms) and after (male: 272±35 ms; female: 328±38 ms) AVR. The female heart had lower myocardial mass than the male heart both before (280 vs. 406 g) and after AVR (227 vs. 310 g).

Conclusion

Longer APD and steeper apicobasal repolarization gradients in hypertrophic female ventricles resulted in sustained arrhythmia, despite their lower mass when compared to males. The absence of sustained reentry in the post-AVR models indicates a potential electrophysiological benefit of AVR and encourages future clinical studies to quantify sex-specific differences in spatial repolarization heterogeneity.

Funding Acknowledgement

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): European Union's Horizon 2020 research and innovation program under the ERA-NET co-fund action No. 680969 with ANR (ERA-CVD SICVALVES) and Agence Nationale de la Recherche

Mechanisms linking electrical alternans and clinical ventricular arrhythmia in human heart failure

BACKGROUND

Mechanisms of ventricular tachycardia (VT) and ventricular fibrillation (VF) in patients with heart failure (HF) are undefined.

OBJECTIVE

The purpose of this study was to elucidate VT/VF mechanisms in HF by using a computational-clinical approach.

METHODS

In 53 patients with HF and 18 control patients, we established the relationship between low-amplitude action potential voltage alternans (APV-ALT) during ventricular pacing at near-resting heart rates and VT/VF on long-term follow-up. Mechanisms underlying the transition of APV-ALT to VT/VF, which cannot be ascertained in patients, were dissected with multiscale human ventricular models based on human electrophysiological and magnetic resonance imaging data (control and HF).

RESULTS

For patients with APV-ALT k-score >1.7, complex action potential duration (APD) oscillations (≥2.3% of mean APD), rather than APD alternans, most accurately predicted VT/VF during long-term follow-up (+82%; -90% predictive values). In the failing human ventricular models, abnormal sarcoplasmic reticulum (SR) calcium handling caused APV-ALT (>1 mV) during pacing with a cycle length of 550 ms, which transitioned into large magnitude (>100 ms) discordant repolarization time alternans (RT-ALT) at faster rates. This initiated VT/VF (cycle length <400 ms) by steepening apicobasal repolarization (189 ms/mm) until unidirectional conduction block and reentry. Complex APD oscillations resulted from nonstationary discordant RT-ALT. Restoring SR calcium to control levels was antiarrhythmic by terminating electrical alternans.

CONCLUSION

APV-ALT and complex APD oscillations at near-resting heart rates in patients with HF are linked to arrhythmogenic discordant RT-ALT. This may enable novel physiologically tailored, bioengineered indices to improve VT/VF risk stratification, where SR calcium handling and spatial apicobasal repolarization are potential therapeutic targets.

A novel rule-based algorithm for assigning myocardial fiber orientation to computational heart models

Electrical waves traveling throughout the myocardium elicit muscle contractions responsible for pumping blood throughout the body. The shape and direction of these waves depend on the spatial arrangement of ventricular myocytes, termed fiber orientation. In computational studies simulating electrical wave propagation or mechanical contraction in the heart, accurately representing fiber orientation is critical so that model predictions corroborate with experimental data. Typically, fiber orientation is assigned to heart models based on Diffusion Tensor Imaging (DTI) data, yet few alternative methodologies exist if DTI data is noisy or absent. Here we present a novel Laplace-Dirichlet Rule-Based (LDRB) algorithm to perform this task with speed, precision, and high usability. We demonstrate the application of the LDRB algorithm in an image-based computational model of the canine ventricles. Simulations of electrical activation in this model are compared to those in the same geometrical model but with DTI-derived fiber orientation. The results demonstrate that activation patterns from simulations with LDRB and DTI-derived fiber orientations are nearly indistinguishable, with relative differences ≤6%, absolute mean differences in activation times ≤3.15 ms, and positive correlations ≥0.99. These results convincingly show that the LDRB algorithm is a robust alternative to DTI for assigning fiber orientation to computational heart models.

A novel rule-based algorithm for assigning myocardial fiber orientation to computational heart models

Electrical waves traveling throughout the myocardium elicit muscle contractions responsible for pumping blood throughout the body. The shape and direction of these waves depend on the spatial arrangement of ventricular myocytes, termed fiber orientation. In computational studies simulating electrical wave propagation or mechanical contraction in the heart, accurately representing fiber orientation is critical so that model predictions corroborate with experimental data. Typically, fiber orientation is assigned to heart models based on Diffusion Tensor Imaging (DTI) data, yet few alternative methodologies exist if DTI data is noisy or absent. Here we present a novel Laplace-Dirichlet Rule-Based (LDRB) algorithm to perform this task with speed, precision, and high usability. We demonstrate the application of the LDRB algorithm in an image-based computational model of the canine ventricles. Simulations of electrical activation in this model are compared to those in the same geometrical model but with DTI-derived fiber orientation. The results demonstrate that activation patterns from simulations with LDRB and DTI-derived fiber orientations are nearly indistinguishable, with relative differences ≤6%, absolute mean differences in activation times ≤3.15 ms, and positive correlations ≥0.99. These results convincingly show that the LDRB algorithm is a robust alternative to DTI for assigning fiber orientation to computational heart models.