The ECG Blog with Reid

Second-degree type I AV block, also known as Mobitz I or Wenckebach, is typically characterized by a progressive prolongation of the PR interval followed by a dropped beat. While often evaluated for underlying conduction disease, it is not always pathological—especially when it occurs exclusively du...

Fusion beats are one of the hallmark findings that support the diagnosis of ventricular tachycardia (VT)—particularly in the case of monomorphic VT. They represent a unique moment in cardiac electrophysiology where two wavefronts—one ventricular and one supraventricular—collide, creating a hybrid QR...

When confronted with a wide QRS complex that appears unexpectedly on an ECG, one of the key clinical challenges is to determine whether it's due to a premature ventricular contraction (PVC) or a premature atrial contraction (PAC) with aberrant conduction, also known as the Ashman phenomenon. Though ...

Bundle branch blocks (BBBs) occur when there's a delay or interruption in the electrical conduction through either the right or left bundle branch of the His-Purkinje system. These conduction abnormalities affect the timing and sequence of ventricular depolarization, leading to characteristic change...

Electrical alternans is a fascinating ECG phenomenon typically associated with large pericardial effusion—but it is not pathognomonic for it. This beat-to-beat variation in the amplitude or axis of the QRS complex (and sometimes P or T waves) can also be seen in supraventricular tachyarrhythmias, es...

Left ventricular hypertrophy (LVH) is most often recognized on ECG by increased QRS voltage—tall R waves in left-sided leads and deep S waves in right-sided leads. However, in some cases, LVH is also associated with a widened QRS complex, raising the question: Why would a structural enlargement of t...

Atrial fibrillation (AF) is marked by chaotic atrial activity that can drive the ventricles at dangerously high rates. While the atria may be firing at 400–600 impulses per minute, the AV node acts as a gatekeeper—filtering which signals reach the ventricles. Calcium channel blockers (CCBs), particu...

AV Nodal Reentry Tachycardia (AVNRT) is one of the most common types of paroxysmal supraventricular tachycardia (PSVT), frequently encountered in the emergency department and electrophysiology lab. It arises not from an ectopic focus, but from a reentrant circuit within or around the AV node, exploi...

Complete heart block—or third-degree AV block—is a feared complication of ST segment elevation myocardial infarction (STEMI), particularly when the infarct involves the inferior or posterior walls of the heart. This pattern is not random. It’s grounded in the anatomical relationship between the righ...

Looking to master all things cardiology? Let's walk through my top study resources for those looking to up their cardiac framework in 2025. Whether it's cardiac electrophysiology, ECG interpretation, anti-arrhythmic mastery, or other cardiac or vasoactive pharmacology, here are my picks!

Disclosu...

Pathological Q waves are among the most recognizable signs of a prior myocardial infarction (MI) on the electrocardiogram. Yet their presence is not always straightforward. To understand their significance, we need to explore the electrophysiological mechanisms and cardiac anatomy that give rise to ...

Atrial fibrillation (AF) is classically described as an irregularly irregular rhythm, defined by chaotic atrial activity and a ventricular response that lacks any predictable pattern. However, a unique exception occurs in patients who have ventricular pacing, especially those with complete AV block ...