Thoracic Key

Voltage Abnormalities The Low-Voltage ECG Criteria Voltage is considered low in the limb leads when the average QRS amplitude is less than 5 mm and low in the precordial leads…

Abnormal T Waves The Normal T Wave The T wave represents rapid repolarization (phase 3 of the action potential). The normal T wave is asymmetric with a gradual slope followed…

Abnormal QRS Morphology Brugada Pattern and Syndrome Pathogenesis Brugada syndrome is an autosomally dominant inherited disease (with incomplete penetrance) known for causing sudden death in patients with structurally normal hearts….

AV Conduction Blocks First-Degree AV Block First-degree AV block is a delay in conduction, not a true block. First-degree block represents conduction delay somewhere between the sinus node and ventricles….

Premature Beats Premature Atrial Complex Premature atrial complexes (PACs) arise from an ectopic focus anywhere in the atria, including the coronary sinus and pulmonary veins. Most often, PACs are asymptomatic…

Bundle Branch and Fascicular Blocks Right Bundle-Branch Block 1. Septal Depolarization In the presence of a right bundle-branch block (RBBB), septal depolarization occurs in a normal left-to-right pattern (orange arrow)…

Sinus Dysfunction Sinus Arrhythmia Sinus arrhythmia occurs in young and healthy individuals and is not a form of sinus node dysfunction. Sinus arrhythmia is characterized by variation in cardiac cycle…

Concept Review Action Potential—Myocardial Cell The different phases of the action potential relate directly to the waveforms, intervals, and segments that constitute a cardiac cycle on the ECG. Each phase…