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Concept: Purkinje fibers


According to current ESC Guidelines for the diagnosis and treatment of heart failure cardiac resynchronization therapy (CRT) is indicated in patients suffering from heart failure (HF) with reduced ejection fraction (EF) with significantly widened QRS complexes. Presence of vital myocardium proved by dobutamine stress echocardiography (DSE) is considered as a good prognostic factor for responsiveness to this treatment. Chronotropic incompetence is on the other hand known factor of unfavorable outcome in HF.

Concepts: Myocardial infarction, Cardiology, Heart failure, Ejection fraction, Echocardiography, Heart disease, QRS complex, Purkinje fibers


Atrioventricular (AV) block has been extensively studied. However, conduction inside the myocardium in patients with AV block has not been reported. In this study, we aimed to demonstrate the presence of intramyocardial block in patients with AV block. Five consecutive patients with spontaneous high-grade AV block and Torsades de pointes (TdP) were prospectively studied with standard United States Catheter Instruments (USCI) endocardial temporary catheter located at the right ventricle (RV) apex. The morphology of endocardial potentials observed in the basic QRS complexes as well as during episodes of TdP was studied. The electrogram (EGM) of the basic rhythm showed a sharp deflection of high amplitude preceded and/or followed by a smooth potential of low amplitude interpreted as far-field potentials in all patients. The sharp potential can be observed at the beginning, in the middle or at the end of the smooth potential. All these potentials were reproduced from beat to beat and were falling inside the QRS complex of the surface ECG. Therefore, these aspects are zones of electrically depressed or silent myocardium larger than the interelectrode distance of 12 mm. This situation is in agreement with recent genetic factors. In this study, we demonstrated for the first time that patients with spontaneous AV block also have trouble in ventricular activation located on the AV conduction system and inside the myocardium. It is then possible to speculate that the presence of diffuse non-conducting myocardium explains why most TdPs do not degenerate into ventricular fibrillation (VF) and generally stop spontaneously.

Concepts: Myocardial infarction, Heart, Cardiac electrophysiology, Ventricular fibrillation, Sinoatrial node, Electrocardiography, QRS complex, Purkinje fibers


Myocardial mass is a key determinant of cardiac muscle function and hypertrophy. Myocardial depolarization leading to cardiac muscle contraction is reflected by the amplitude and duration of the QRS complex on the electrocardiogram (ECG). Abnormal QRS amplitude or duration reflect changes in myocardial mass and conduction, and are associated with increased risk of heart failure and death.

Concepts: Cardiology, Heart, Muscle, Cardiac muscle, Cardiac electrophysiology, Muscular system, Electrocardiography, Purkinje fibers


Severe myocarditis associated with electrical conduction abnormalities and occasionally heart failure has been well documented following treatment with immune checkpoint blockade with an estimated incidence of less than 1%. However, the incidence, early detection, and management of less severe immune-related myocarditis are unknown since most immunotherapy trials have not included routine cardiac monitoring. Herein, we provide the first description of subclinical or smoldering myocarditis with minimal signs and symptoms following immune checkpoint blockade with a single dose of ipilimumab and nivolumab.

Concepts: Cardiology, Heart failure, Heart, Symptom, Sinoatrial node, Electrical conduction system of the heart, Purkinje fibers


Cardiac conduction abnormalities are associated with an increased risk for morbidity and mortality, and understanding factors that accelerate or delay conduction system disease could help to identify preventive and therapeutic strategies. Antifibrotic and anti-inflammatory properties of angiotensin-converting enzyme inhibitors and treatment for hyperlipidemia may reduce the risk for incident conduction system disease.

Concepts: Blood, Myocardial infarction, Heart, Cardiac arrest, Enzyme inhibitor, Sinoatrial node, Electrical conduction system of the heart, Purkinje fibers


Dysfunction of the specialized cardiac conduction system (CCS) is associated with life-threatening arrhythmias. Strategies to derive CCS cells, including rare Purkinje cells (PCs), would facilitate models for mechanistic studies and drug discovery and also provide new cellular materials for regenerative therapies. A high-throughput chemical screen using CCS:lacz and Contactin2:egfp (Cntn2:egfp) reporter embryonic stem cell (ESC) lines was used to discover a small molecule, sodium nitroprusside (SN), that efficiently promotes the generation of cardiac cells that express gene profiles and generate action potentials of PC-like cells. Imaging and mechanistic studies suggest that SN promotes the generation of PCs from cardiac progenitors initially expressing cardiac myosin heavy chain and that it does so by activating cyclic AMP signaling. These findings provide a strategy to derive scalable PCs, along with insight into the ontogeny of CCS development.

Concepts: Protein, Neuron, Stem cell, Heart, Embryonic stem cell, Cardiac pacemaker, Electrical conduction system of the heart, Purkinje fibers


Cross correlation analysis (CCA) using tissue Doppler imaging has been shown to be associated with outcome after cardiac resynchronization therapy (CRT) in patients with heart failure (HF) with wide QRS. However, its significance in patients with narrow QRS treated with CRT is unknown.

Concepts: Blood, Cardiology, Doppler echocardiography, Echocardiography, Medical ultrasound, Purkinje fibers


Duchenne muscular dystrophy (DMD) is a lethal genetic disorder caused by an absence of the dystrophin protein in bodywide muscles, including the heart. Cardiomyopathy is a leading cause of death in DMD. Exon skipping via synthetic phosphorodiamidate morpholino oligomers (PMOs) represents one of the most promising therapeutic options, yet PMOs have shown very little efficacy in cardiac muscle. To increase therapeutic potency in cardiac muscle, we tested a next-generation morpholino: arginine-rich, cell-penetrating peptide-conjugated PMOs (PPMOs) in the canine X-linked muscular dystrophy in Japan (CXMDJ) dog model of DMD. A PPMO cocktail designed to skip dystrophin exons 6 and 8 was injected intramuscularly, intracoronarily, or intravenously into CXMDJ dogs. Intravenous injections with PPMOs restored dystrophin expression in the myocardium and cardiac Purkinje fibers, as well as skeletal muscles. Vacuole degeneration of cardiac Purkinje fibers, as seen in DMD patients, was ameliorated in PPMO-treated dogs. Although symptoms and functions in skeletal muscle were not ameliorated by i.v. treatment, electrocardiogram abnormalities (increased Q-amplitude and Q/R ratio) were improved in CXMDJ dogs after intracoronary or i.v. administration. No obvious evidence of toxicity was found in blood tests throughout the monitoring period of one or four systemic treatments with the PPMO cocktail (12 mg/kg/injection). The present study reports the rescue of dystrophin expression and recovery of the conduction system in the heart of dystrophic dogs by PPMO-mediated multiexon skipping. We demonstrate that rescued dystrophin expression in the Purkinje fibers leads to the improvement/prevention of cardiac conduction abnormalities in the dystrophic heart.

Concepts: Heart, Muscle, Cardiac muscle, Muscular system, Duchenne muscular dystrophy, Electrical conduction system of the heart, Purkinje fibers, Morpholino


Identification of a possible ventriculoatrial (VA) dissociation in wide QRS complex tachycardias is one of the most reliable criteria for differentiation of tachycardia origin. The Lewis lead has been proposed for detection of atrial activity during ventricular tachycardias.

Concepts: Cardiology, Taxonomy, Supraventricular tachycardia, Ventricular fibrillation, Tachycardia, Exponential function, QRS complex, Purkinje fibers


The cardiac conduction system (CCS) consists of distinctive components that initiate and conduct the electrical impulse required for the coordinated contraction of the cardiac chambers. CCS development involves complex regulatory networks that act in stage-, tissue- and dose-dependent manners, and recent findings indicate that the activity of these networks is sensitive to common genetic variants associated with cardiac arrhythmias. Here, we review how these findings have provided novel insights into the regulatory mechanisms and transcriptional networks underlying CCS formation and function.

Concepts: Blood, Heart, Cardiac arrest, Cardiac electrophysiology, Sinoatrial node, Cardiac dysrhythmia, Electrical conduction system of the heart, Purkinje fibers