Mitral Annular Disjunction in Idiopathic Ventricular Fibrillation Patients: Just a Bystander or a Potential Cause?

AIMS

Previously, we demonstrated that inferolateral mitral annular disjunction (MAD) is more prevalent in patients with idiopathic ventricular fibrillation (IVF) than in healthy controls. In the present study, we advanced the insights into the prevalence and ventricular arrhythmogenicity by inferolateral MAD in an even larger IVF cohort.

METHODS AND RESULTS

This retrospective multicentre study included 185 IVF patients (median age 39 [27, 52] years, 40% female). Cardiac magnetic resonance images were analysed for mitral valve and annular abnormalities and late gadolinium enhancement. Clinical characteristics were compared between patients with and without MAD. MAD in any of the 4 locations was present in 112 (61%) IVF patients and inferolateral MAD was identified in 24 (13%) IVF patients. Mitral valve prolapse (MVP) was found in 13 (7%) IVF patients. MVP was more prevalent in patients with inferolateral MAD compared with patients without inferolateral MAD(42% vs. 2%, p < 0.001). Proarrhythmic characteristics in terms of a high burden of premature ventricular complexes (PVC) and non-sustained ventricular tachycardia (VT) were more prevalent in patients with inferolateral MAD compared to patients without inferolateral MAD (67% vs. 23%, p < 0.001 and 63% vs 41%, p = 0.046, respectively). Appropriate implantable cardioverter defibrillator therapy during follow-up was comparable for IVF patients with or without inferolateral MAD (13% vs. 18%, p = 0.579).

CONCLUSION

A high prevalence of inferolateral MAD and MVP is a consistent finding in this large IVF cohort. The presence of inferolateral MAD is associated with a higher PVC burden and non-sustained VTs. Further research is needed to explain this potential interplay.

An emerging role for DPP6: reciprocal regulation of INa-Ito and implications for arrhythmogenesis

Background

Since the association of a chromosomal risk haplotype harboring dipeptidyl peptidase-like protein-6 (DPP6) to familial idiopathic ventricular fibrillation (iVF), a growing number of DPP6 missense variants has been reported in patients with ventricular tachyarrhythmias. The mechanisms underlying DPP6 mediated-arrhythmogenesis are not yet fully elucidated. DPP6 is a subunit of the transient outward potassium (Ito) channel complex in Purkinje cells (PC) and ventricular myocytes (VM).

Purpose

Since other Ito-channel subunits (Navβ1, KChIP2, KCNE4 and DPP10) are also known to antagonize INa, we examined whether DPP6 could play a broader role in the inter-regulation of Kv4.3 and Nav1.5 channels. We identified two novel DPP6 variants (p.Arg274His and p.His213Tyr), each segregating in families with QT/QU prolongation. DPP6 p.Arg274His carriers suffered from iVF, ectopic beats from the conduction system, and mitral valve prolapse. Other DPP6 variants (p.Ala751Val identified in this study; p.Gln526His and DPP6-T p.His332Arg published) are associated with Brugada syndrome (BrS). We hypothesized that DPP6 has opposing effects on INa and Ito displaying a reciprocal regulation of these currents.

Methods and results

First, we determined the effect of the DPP6 variants on INa and Ito in transfected CHO cells. Ito density was significantly reduced only when PC subunits were co-expressed with the DPP6 p.Arg274His or p.His213Tyr variants. Indeed, DPP6 modulates Nav1.5 channels in CHO cells by reducing INa Peak and INa Late, whereas DPP6 mutants p.Arg274His or p.His213Tyr resulted in an increase of both components compared to WT. Co-immunoprecipitation experiments in human endocardium confirmed an interaction between DPP6 and Nav1.5 channels. Computing of mutant DPP6-driven Ito-INa changes in a published human PC model led to significant prolongation of the action potential duration, mainly caused by increased INa Late.

On the other hand, the DPP6 p.Gln526His and p.Ala751Val variants, linked to BrS, led to a decreased INa Peak compared to the WT, while there was a tendency towards increased Ito density in both PC and VM molecular setups.

DPP6 (p.Arg274His and p.Ala751Val) transfection experiments in hiPSC cardiomyocytes, expressing endogenous INa and Ito, confirmed the reciprocal results obtained in CHO cells.

Conclusions

DPP6 regulates INa and Ito in a reciprocal manner. The cardiac phenotype of DPP6 variants could encompass a spectrum between two opposite poles: 1) QT/QU prolongation by DPP6 variants causing loss of Ito and gain of INa, like p.Arg274His and p.His213Tyr versus 2) BrS by DPP6 variants leading to gain of Ito and loss of INa, like p.Gln526His and p.Ala751Val.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): ESC Personal research grant, obtained in 2019

Local areas of earlier repolarization cause epicardial repolarization heterogeneity in patients with apparently idiopathic ventricular fibrillation

Background

Sudden cardiac arrest is often due to ventricular fibrillation (VF). In 5–10% of cases, no cause can be identified despite extensive cardiac examination, hence the designation idiopathic VF. Early repolarization with down sloping ST segments has been previously identified in patients with idiopathic VF. Early repolarization may increase repolarization heterogeneity with steep local repolarization time gradients, and thus form a substrate for idiopathic VF.

Purpose

To study the presence of local earlier repolarization and increased repolarization dispersion in idiopathic VF patients with noninvasive electrocardiographic imaging (ECGI).

Methods

A validated, non-commercial, potential-based formulation of ECGI was performed in 17 patients with idiopathic VF and 10 controls with no structural or electrical abnormalities. The ECGI measurement consisted of a body surface potential map with 184–256 electrodes in combination with a CT scan to obtain the torso and heart geometries. ECGI provided local epicardial repolarization times (RT) and RT isochrones. We determined the 1st (RT1%) and 99th percentile (RT99%) of RTs, the total epicardial RT dispersion (ERD: RT99%-RT1%), and the mean RT. Heart-rate corrected QT (QTc), TpTe intervals, and presence of the ER pattern were determined from the 12-lead ECG. All metrics were normalized to the body-surface Q.

Results

QTc and TpTe did not differ between the two groups (P=0.40 and P=0.83, respectively, Figure 1, panel A). One (10%) control subject and three (17.6%) idiopathic VF patients showed an ER pattern on the 12-lead ECG, with a down sloping ST segment only in 2/4 of the latter. With ECGI, the mean RT was similar between the groups (P=0.31), but the ERD was significantly increased in patients with idiopathic VF (P=0.01, figure 1, panel B). Moreover, RT1% was significantly lower in idiopathic VF patients in comparison to the controls (P=0.002), whereas the RT99% did not differ significantly (P=0.40). Subgroup analysis between ER positive and negative patients did not yield significantly different RT results.

Conclusion

Noninvasive ECGI, in contrast to the 12-lead ECG, revealed a wider range of epicardial RTs in patients with idiopathic VF, implying increased repolarization heterogeneity. This heterogeneity is caused by areas of earlier repolarization. Our data indicate the value of noninvasively diagnosing these repolarization abnormalities, and suggest promising potential value of the 1st percentile of RT to identify idiopathic VF patients with true early repolarization.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): Dutch Heart Foundation Figure 1

Noninvasive unipolar electrogram T-wave upslope is an accurate marker of local refractoriness in explanted hearts with drug-induced repolarization dispersion

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): French National Research Agency Matthijs Cluitmans is supported by a Veni grant from the Netherlands Organization for Scientific Research

Background

The relationship between T-wave morphology in local unipolar electrograms (UEGs) as mapped with noninvasive electrocardiographic imaging (ECGI) and local repolarization time (RT) has not been validated in pronounced RT dispersion.

Purpose

To study the time of upslope of the T-wave (Tup) in epicardial UEGs mapped with ECGI as a marker of time of local refractoriness (trefr, a surrogate for RT) in intact hearts with RT dispersion.

Methods

Six pig hearts were Langendorff-perfused with selective perfusion of the LAD artery and submersed in a torso-shaped tank containing 256 electrodes on the torso surface (panel A). RT was prolonged in the non-LAD regions by infusing dofetilide (‘Dof’) and shortened in the LAD region using pinacidil (‘Pin’). Tup was determined in both invasive UEGs (recorded with epicardial electrodes) and in noninvasive UEGs (reconstructed with ECGI). Programmed stimulation was used to determine trefr, defined as the shortest coupling interval with capture. Both metrics were determined relative to the common pacing spike.

Results

In all six hearts, selective dofetilide and pinacidil infusion resulted in delayed trefr and Tup in the non-LAD region, and shortened invasive trefr and noninvasive Tup in the LAD region, respectively (panel B and C). Over all 59 observations, Tup showed high agreement with trefr (values close to line of identity, panel D) with strong correlation (r = 0.91). This finding was independent of T-wave polarity (positive, negative or biphasic).

Conclusion

The moment of steepest upslope of the T-wave in a noninvasively reconstructed UEG accurately reflects end of local refractoriness in intact hearts, in case of pronounced RT dispersion. Under these circumstances, when local RT is defined by moment of re-excitability (allowing to link it to conduction block and re-entry), ECGI T-wave upslope can be taken as a truthful marker for local RT. Abstract Figure. Results

P933Novel missense variant in DPP6 in familial ventricular fibrillation

Funding Acknowledgements

This work was supported by an ESC Research Grant to A.R., and by The Netherlands CardioVascular Research Initiative (CVON PREDICT2), Den Haag, The N

Background

The DPP6 gene, encoding dipeptidyl aminopeptidase-like protein-6, has been associated with familial ventricular fibrillation (VF) and early repolarization. The DPP6 protein is part of the macromolecular Ito-channel complex and functionally expressed in ventricular myocytes (VM) and Purkinje cells (PC). Here we report a novel missense variant in DPP6 identified in a family with sudden cardiac death/arrest due to VF.

Purpose

To examine the consequences of the novel DPP6 variant c.821G &gt; A, p.(R274H) for cardiac Ito and their potential contribution to a VF substrate.

Methods: Clinical

Co-segregation of DPP6-R274H with major adverse cardiac events in the family members was investigated. Ambulatory ECGs, echocardiograms, exercise tests and ICD recordings were systematically analyzed.

Cellular

Chinese hamster ovary cells were transiently transfected with Ito α and β subunits in order to mimic the VM (Kv4.3 + KChIP2) or the PC phenotype (Kv4.3 + NCS1). Additionally, DPP6 wild type (WT) or variant was transfected. Ito was assessed by patch clamping.

Results: Clinical

DPP6-R274H, affecting a conserved region in the extracellular protein domain segregated with an autosomal dominant pattern of inheritance. The female index patient experienced first VF at 17 years, and subsequent ICD therapy, after her brother had died suddenly at 13 years. Both carriers were also known with mitral-valve prolapse (MVP), but otherwise without structural heart disease. A third sibling without arrhythmia, but with mild MVP, did not carry the DPP6 variant. No other pathogenic mutations were found by next generation sequencing in the index patient. ECGs of the index patient revealed dynamic early repolarization, U waves with varying amplitudes and pause-dependent prominence, and &gt;8000/24 h superimposed premature ventricular complexes (PVCs) and (non)sustained VT. PVCs (QRS duration 120 ms) displayed both a left posterior (80%) and anterior (20%) fascicular origin. PVC burden was higher during daytime and exercise, with pause-dependent occurrence.

Cellular

In the PC model DPP6-R274H markedly decreased Ito density (at +30 mV: 291 pA/pF vs 995 pA/pF in WT; p &lt; 0.001), whereas it did not in the VM model. In VM, but not in PC, Ito inactivation was slowed by the variant (Tau at 0 mV: 91 ms vs 49 ms in WT; p &lt; 0.001).

Conclusions

The novel DPP6 variant R274H has differential effects on Ito in cellular models of VM versus PC. Our main finding of a loss-of-function in the PC model combined with a slowed Ito inactivation in the VM phenotype may exaggerate electrical heterogeneity, especially at the PC-VM junction. Ito loss in PC would prolong action-potential duration and promote early afterdepolarizations, consistent with the dynamic patterns of T-U-dispersion and (fascicular) ectopy in the index case. Mechano-electric influences by the MVP further promote arrhythmogenesis.

Novel use of repolarization parameters in electrocardiographic imaging to uncover arrhythmogenic substrate

BACKGROUND

Measuring repolarization characteristics is challenging and has been reserved for experienced physicians. In electrocardiographic imaging (ECGI), activation-recovery interval (ARI) is used as a measure of local cardiac repolarization duration. We hypothesized that repolarization characteristics, such as local electrogram morphology and local and global dispersion of repolarization timing and duration could be of significance in ECGI.

OBJECTIVE

To further explore their potential in arrhythmic risk stratification we investigated the use of novel repolarization parameters in ECGI.

MATERIALS AND METHODS

We developed and compared methods for T-peak and T-end detection in reconstructed potentials. All methods were validated on annotated reconstructed electrograms (EGMs). Characteristics of the reconstructed EGMs and epicardial substrate maps in IVF patients were analyzed by using data recorded during sinus rhythm. The ECGI data were analyzed for EGM morphology, conduction, and repolarization.

RESULTS

We acquired ECGI data from 8 subjects for this study. In all patients we evaluated four repolarization parameters: Repolarization time, T-wave area, Tpeak-Tend interval, and T-wave alternans. Most prominent findings were steep repolarization time gradients in regions with flat EGMs. These regions were also characterized by low T-wave area and large differences in Tpeak-Tend interval.

CONCLUSIONS

Measuring novel repolarization parameters in reconstructed electrograms acquired with ECGI is feasible, can be done in a fully automated manner and may provide additional information on underlying arrhythmogenic substrate for risk stratification. Further studies are needed to investigate their potential use and clinical application.

Life-long tailoring of diagnosis and management of patients with idiopathic ventricular fibrillation-future perspectives in research

The diagnosis and management of idiopathic ventricular fibrillation is challenging, as it requires extensive diagnostic testing and offers few curative options due to unknown underlying disease. The resulting population is a heterogeneous group of patients with a largely unknown natural history. Structural patient characterisation, follow-up and innovations in diagnostic testing can improve our understanding of the disease mechanisms of idiopathic ventricular fibrillation, detect underlying disease during follow-up and aid in therapeutic management. Recently, initiatives have been launched in the Netherlands to investigate the role of high-resolution non-invasive electrocardiographic imaging and genetic and familial screening in idiopathic ventricular fibrillation.

The role of the SCN5A-encoded channelopathy in irritable bowel syndrome and other gastrointestinal disorders

BACKGROUND

Gastrointestinal functional and motility disorders, like irritable bowel syndrome (IBS), have a high prevalence in the Western population and cause significant morbidity and loss of quality of life leading to considerable costs for health care. A decade ago, it has been demonstrated that interstitial cells of Cajal and intestinal smooth muscle cells, cells important for gastrointestinal motility, express the sodium channel alpha subunit Nav 1.5. In the heart, aberrant variants in this sodium channel, encoded by SCN5A, are linked to inherited arrhythmia syndromes, like the long-QT syndrome type 3 and Brugada syndrome. Mounting data show a possible contribution of SCN5A mutants to gastrointestinal functional and motility disorders. Two percent of IBS patients harbor SCN5A mutations with electrophysiological evidence of loss- and gain-of-function. In addition, gastrointestinal symptoms are more prevalent in cardiac SCN5A-mutation positive patients.

PURPOSE

This review firstly describes the Nav 1.5 channel and its physiological role in ventricular cardiomyocytes and gastrointestinal cells, then we focus on the involvement of mutant Nav 1.5 in gastrointestinal functional and motility disorders. Future research might uncover novel mutation-specific treatment strategies for SCN5A-encoded gastrointestinal channelopathies.

Founder mutations in the Netherlands: familial idiopathic ventricular fibrillation and DPP6

In this part of a series on founder mutations in the Netherlands, we review familial idiopathic ventricular fibrillation linked to the DPP6 gene. Familial idiopathic ventricular fibrillation determines an intriguing subset of the inheritable arrhythmia syndromes as there is no recognisable phenotype during cardiological investigation other than ventricular arrhythmias highly associated with sudden cardiac death. Until recently, it was impossible to identify presymptomatic family members at risk for fatal events. We uncovered several genealogically linked families affected by numerous sudden cardiac deaths over the past centuries, attributed to familial idiopathic ventricular fibrillation. Notably, ventricular fibrillation in these families was provoked by very short coupled monomorphic extrasystoles. We were able to associate their phenotype of lethal arrhythmic events with a haplotype harbouring the DPP6 gene. While this gene has not earlier been related to cardiac arrhythmias, we are now able, for the first time, to identify and to offer timely treatment to presymptomatic family members at risk for future fatal events solely by genetic analysis. Therefore, when there is a familial history of unexplained sudden cardiac deaths, a link to the DPP6 gene may be explored as it may enable risk evaluation of the remaining family members. In addition, when closely coupled extrasystoles initiate ventricular fibrillation in the absence of other identifiable causes, a link to the DPP6 gene should be suspected.

End–diastolic myofiber stress and ejection strain increase with ventricular volume overload

BACKGROUND

Myocardial stress and strain are considered primary mechanical stimuli for hypertrophic remodeling. Their values and significance in the intact beating heart during chronic overload remain poorly characterized.

METHODS AND RESULTS

Left-ventricular (LV) dimensions (echocardiography) and pressure (invasive) were simultaneously recorded in anesthetized dogs at sinus rhythm (SR), acute and 1, 2, 6, 12 weeks of atrioventricular block (AVB), leading to structural, electrical and contractile remodeling. Mechanical load of the myocardium was quantified as myofiber stress (sigma(f)), being force along myofiber orientation per cross-sectional area, and natural myofiber strain (e(f)), being change in natural logarithm of myofiber length (l) divided by its reference length: e(f) = ln(l/l(ref)). Time courses of sigma(f) and e(f) were calculated from LV pressure and dimensions, using a validated mathematical model of cardiac mechanics. End-diastolic sigmaf increased from 2.0 +/- 0.1 kPa at SR to 3.4 +/- 0.3 kPa at acute AVB, remaining elevated for > 6 weeks. Systolic sigma(f) was not affected by AVB. Ejection strain rose instantly upon AVB, reaching a maximum at 2 weeks: 0.24 +/- 0.02 vs. 0.10 +/- 0.01 at SR. The increase of myofiber stroke work (sigma(f)-e(f) loop area) from 3.1 +/- 0.3 at SR to 6.0 +/- 0.5 kJ/m(3)/beat at 1 week AVB was attributed mainly to an increase of strain during ejection. Stroke work and ejection strain remained elevated up to 12 weeks. The rate of LV-mass increase was maximal (2.2 +/- 0.4 g/day) at 1 week AVB.

CONCLUSIONS

Serial mechanical phenotyping is feasible in the intact anesthetized dog with chronic ventricular overload. Our new approach yields values of mechanical load that are comparable to those found in isolated myocardium by others. In chronic AVB, both end-diastolic myofiber stress and ejection strain are increased. Early increases of both parameters coincide with peak hypertrophic growth, suggesting their important role for mechanotransduction. Peak systolic sigmaf is likely not important for hypertrophy in this model, since it does not change throughout the experiment.

Giant negative T waves after treatment of severe hyperkalaemia

We present the case of a 79-year-old female with severe hyperkalaemia and severe prerenal insufficiency due to dehydration and nephrotoxic medications, including spironolactone. The ECG showed AV nodal rhythm and tented T waves. After treatment with fluids, insulin, polystyrene sulphonate and sodium bicarbonate, the serum potassium level and kidney function normalised. Several days later, she developed QT prolongation with giant negative T waves without signs of ischaemia. In this report, we review the effect of hyperkalaemia on cardiac ion channel function and the associated changes on the ECG. In addition, the causes and mechanisms of giant negative T waves are discussed.