Chapter 28: Disorders of Cardiac Conduction and Rhythm

The conduction system, spearheaded by the sinoatrial (SA) node (the heart's primary pacemaker), directs electrical impulses through internodal pathways to the atrioventricular (AV) node, the bundle of His, and the specialized Purkinje fibers to coordinate atrial and ventricular activity. Electrical function relies on action potentials, which are sequential changes in membrane potential driven by the movement of ions (Na+, K+, Ca++) and divided into five distinct phases, from rapid depolarization (Phase 0) to the resting potential or spontaneous firing (Phase 4). Cells are categorized by their firing patterns: fast response (myocardium and Purkinje fibers) or slow response (SA and AV nodes), and the refractory periods ensure the necessary alternating contraction and relaxation cycles. Electrical activity is clinically assessed using an Electrocardiogram (ECG), which visualizes key events like atrial depolarization (P wave) and ventricular depolarization (QRS complex). Arrhythmias and conduction defects arise from alterations in four inherent properties—automaticity, excitability, conductivity, or refractoriness—and can range from benign to fatal. Common rhythm disturbances include sinus node arrhythmias (e.g., sinus bradycardia or tachycardia), atrial arrhythmias (e.g., atrial flutter or the highly disorganized atrial fibrillation [AF]), and serious ventricular arrhythmias (e.g., premature ventricular contractions [PVCs], ventricular tachycardia [VT], and the critical ventricular fibrillation [VF]). A major mechanism for tachyarrhythmias is reentry, which involves abnormal circulation of the electrical impulse. Conduction defects are often seen as AV heart blocks, categorized as first-, second-, or third-degree, representing delays or failures in impulse transmission from atria to ventricles. The text also discusses inherited arrhythmogenic disorders (channelopathies) like Long QT Syndrome (LQTS) and Brugada syndrome. Diagnosis is achieved through various methods, including Holter monitoring, exercise stress testing, and electrophysiologic studies (EPS). Treatment strategies involve adjusting underlying causes, using antiarrhythmic medications (classified based on ion channel effects), and employing electrical interventions such as pacemakers, cardioversion, defibrillation, and ablation therapy (e.g., radiofrequency or cryoablation).