Arrhythmias affect approximately half of patients with Costello syndrome (CS, OMIM # 218040), with non-reentrant atrial tachycardia being the most common. This case describes an infant with Costello syndrome carrying the pathogenic HRAS c.34G>A (p.G12S) variant who developed early-onset, drug-refractory multifocal atrial tachycardia (MAT). Despite multiple antiarrhythmic therapies, rhythm control remained inadequate until trametinib was initiated, resulting in rapid resolution of MAT and allowing stepwise discontinuation of adjunct antiarrhythmics. Although MAT did not recur after the MEK inhibitor was stopped, the patient subsequently developed hypertrophic cardiomyopathy only after trametinib was discontinued, suggesting a potential disease-modifying cardioprotective effect during active treatment. This case supports MEK inhibition as a targeted antiarrhythmic strategy in Costello syndrome, with benefits that may extend beyond acute rhythm stabilization to include prevention of cardiac hypertrophy. While spontaneous resolution of MAT can occur in Costello syndrome, the rapid and sustained response in this patient strengthens the hypothesis that trametinib exerts a direct antiarrhythmic effect.

Monensin is an antibiotic ionophore used as feed additive in production animals, however, overdose with monensin is associated with poisoning in livestock. This study describes a mortality outbreak in sheep associated with ingestion of a feed supplement containing elevated concentrations of monensin. The objectives were characterize pathological lesions from monensin toxicosis, correlate these findings with serum creatine kinase myocardial band (CKMB) to estimate the extent of myocardial injury, as well as with electrocardiogram to evaluate the progression and severity of cardiac damage. A farm with 49 Texel sheep provided 275 Kg of supplement containing 1.102 mg/kg of monensin undiluted, freely available in the trough for seven days (40 Kg/day). From the 5th day onward animals presented sternal recumbency, submandibular edema, dyspnea and tachycardia followed by death in 15 animals. At necropsy, the main lesions were globular heart with eccentric dilatation of cardiac chambers, pale myocardium, lungs with edema and congestion, hydrothorax, hydropericardium, ascites, liver with enhanced lobular pattern and pale skeletal musculature. Histologically, in six sheep that died within the first days necrotic myocarditis multifocal to coalescing, mild to moderate with discrete degree of fibrosis, and in nine sheep that died later fibrosis became pronounced. Mapping of cardiac lesions revealed predominance of lesions in left and right ventricles, atria, and interventricular septum, with degree varying between 2 and 3. In apex, right and left papillary muscles lesions ranged from grades 0-2. There was evidence of discrete multifocal necrotic myositis, pulmonary edema and congestion, necrotic hepatitis and centrilobular congestion. In the animals which the CKMB enzyme was measured, the mean value was 1008.88 u/L (104-2480 u/L). Electrocardiogram in four animals showed sinus tachycardia, atrial fibrillation and decreased wave P amplitude. Monensin poisoning caused predominant damage to heart with right and left congestive heart failure, which continued to be present in the surviving animals.

Paroxysmal supraventricular tachycardias (PSVTs) affect approximately 36 out of 100,000 individuals annually in the United States, with women being twice as likely as men to develop the condition. The risk of PSVT increases significantly with age, with older individuals being 5 times more likely to experience the arrhythmia compared to younger people. PSVT accounts for 10% to 15% of cases in adults undergoing catheter ablation, while in children, it accounts for 10% to 20% of cases.  Focal atrial tachycardia (AT) is a supraventricular arrhythmia with rapid heart rates originating from ectopic atrial foci independent of the sinus or atrioventricular (AV) nodes, and it is a common mechanism of PSVT. Presenting as either unifocal or multifocal, AT has distinct pathophysiological mechanisms. Unifocal AT arises from enhanced automaticity, triggered activity, or micro-reentry mechanisms, resulting in a single, consistent P-wave morphology with an atrial rate of 150 to 250 bpm. Typically benign and paroxysmal, unifocal AT is often nonsustained. However, incessant tachycardia may cause tachycardia-induced cardiomyopathy. On the other hand, multifocal atrial tachycardia (MAT) involves three or more distinct P-wave morphologies and an irregular atrial rate exceeding 100 bpm. Enhanced automaticity is the primary mechanism, with triggered activity also playing a role. MAT is frequently associated with underlying conditions such as pulmonary disease, metabolic imbalances, or drug effects.  Electrophysiology (EP) studies and catheter ablation are crucial for diagnosing and managing AT. Successful elimination of unifocal AT is frequently achieved through catheter ablation with low complication rates, while the management of MAT focuses on treating underlying causes. Advances in mapping technologies and ablation techniques have greatly enhanced outcomes for patients undergoing these procedures, making ablation a cornerstone in focal AT treatment. This review explores the mechanisms, diagnostic strategies, and therapeutic approaches, emphasizing the essential role of EP studies and ablation in contemporary cardiac care.

The atrioventricular node (AV) measures approximately 1 by 3 by 5  millimeters and sits within an anatomic region bordered anteriorly by septal tricuspid valve annulus, superiorly by the tendon of Todaro and posteriorly by the coronary sinus ostium. This anatomic region is also commonly referred to as the triangle of Koch. The AV node is a subendocardial structure located in the inferior-posterior right atrium. The blood supply to the AV node is from the AV nodal branch of the right coronary artery  (90%) or the left circumflex artery (10%) depending on right or left dominant blood supply to the heart. The first septal perforator of the left anterior descending artery also supplies blood to the AV node.[1][2][3][4][5] Paroxysmal supraventricular tachycardia (PSVT) accounts for intermittent episodes of supraventricular tachycardia with sudden onset and termination. PSVT is part of the narrow QRS complex tachycardias with a regular ventricular response in contrast to multifocal atrial tachycardia, atrial fibrillation, and atrial flutter. SVTs are classified based on the origin of the rhythm and whether the rhythm is regular or irregular.[6][7][8][9][10][11] Atrial in Origin and Regular Rhythm : Sinus tachycardia. Inappropriate sinus tachycardia. Sinoatrial nodal reentrant tachycardia. Atrial flutter. Atrial fibrillation. Atrial in Origin and Irregular Rhythm : Multifocal atrial tachycardia. Atrial flutter with variable block. Atrial fibrillation. AV Node in Origin and Regular Rhythm : Junctional tachycardia  Atrioventricular nodal reentrant tachycardia. Atrioventricular reentrant tachycardia. AV Node in Origin and Irregular Rhythm None. SVT is known to occur in individuals of all ages, but treatment is often difficult. The clinical presentation of SVT is variable- ranging from asymptomatic to severe palpitation. Electrophysiologic studies are usually necessary to determine the pathophysiology of impulse formation and pathways of conduction.

In a familial cohort with 8 heterozygous carriers of a rare pathogenic SCN5A mutation (c.2482C>T), 4 female mutation carriers manifested with fetal ventricular tachycardia and 2:1 atrioventricular block. One presented with multifocal ectopic premature Purkinje-related complexes-like phenotype and atrial fibrillation later in life. These novel findings inform the need for robust fetal monitoring of mutation carriers.

Although a high-resolution three-dimensional mapping system has made it possible to treat complicated atrial tachyarrhythmia (AT), there remain cases that are difficult to diagnose and treat. However, when multiple different ATs alternately appear, mapping and diagnosis of those are more difficult. Parallel mapping module is well known as a good option to simultaneously map two or more different ATs when they alternately appear. When performing parallel mapping of two different ATs, one bipolar signal of the reference catheter is used as a timing reference and a cycle length filter is used for differentiating AT1, AT2, and others, including sinus rhythm, fusion beats, or catheter-induced premature atrial complex. Therefore, it has some limitations for differentiating multifocal ATs. We present a case wherein multifocal ATs were successfully eliminated by combining parallel mapping module and dual-chamber intra-cardiac pattern matching technique that we have previously reported.

While the majority of individuals with Costello syndrome share characteristic findings affecting multiple organ systems, the phenotypic spectrum is wide, ranging from a milder or attenuated phenotype to a severe phenotype with early lethal complications. Costello syndrome is typically characterized by failure to thrive in infancy as a result of severe postnatal feeding difficulties; short stature; developmental delay or intellectual disability; coarse facial features (full lips, large mouth, full nasal tip); curly or sparse, fine hair; loose, soft skin with deep palmar and plantar creases; papillomata of the face and perianal region; diffuse hypotonia and joint laxity with ulnar deviation of the wrists and fingers; tight Achilles tendons; and cardiac involvement including: cardiac hypertrophy (usually typical hypertrophic cardiomyopathy), congenital heart defect (usually valvar pulmonic stenosis), and arrhythmia (usually supraventricular tachycardia, especially chaotic atrial rhythm/multifocal atrial tachycardia or ectopic atrial tachycardia). Relative or absolute macrocephaly is typical, and postnatal cerebellar overgrowth can result in the development of a Chiari I malformation with associated anomalies including hydrocephalus or syringomyelia. Individuals with Costello syndrome have an approximately 15% lifetime risk for malignant tumors including rhabdomyosarcoma and neuroblastoma in young children and transitional cell carcinoma of the bladder in adolescents and young adults.

Electrocardiogram (ECG) interpretation software mistakes can lead to incorrect diagnoses and inappropriate treatments. Occasionally, however, repetitive and consistent computer errors may hide important clues for correct diagnoses that otherwise could have been missed. We present a collection of a few common and clinically important such peculiarities, and provide tools on how to prove or disprove the suspected diagnosis. In addition to the illustrations in print, an online supplement (OS) shows more examples of the discussed phenomena. In each ECG, the original computer interpretations were enlarged for legibility.