Cardiac Conduction System Animation | The Heart's Electrical Wiring

This comprehensive section covers cardiac arrhythmias including atrial fibrillation, preexcitation syndromes, and ventricular arrhythmias. Atrial fibrillation risk factors include aging, hypertension, diabetes, thyrotoxicosis, heart failure, coronary artery disease, valvular disease, COPD, sleep apnea, cardiac surgery, alcohol, and increased vagal tone. Pathophysiology involves premature depolarization from pulmonary veins leading to structural and electrical remodeling. Consequences include loss of atrial contribution to ventricular filling, blood stasis, and thrombus formation. Classification includes paroxysmal, persistent, and permanent types with symptom severity grading. Preexcitation syndromes involve accessory pathways bypassing normal AV conduction, with Wolff-Parkinson-White syndrome featuring delta wave on ECG. Brugada syndrome is hereditary with sodium channel mutations and characteristic ST elevation. Ventricular arrhythmias arise from automaticity abnormalities or reentry currents, ranging from premature beats to ventricular fibrillation.

Cardiac arrhythmia refers to any abnormality in heart rate, which can be classified into sinus arrhythmias (originating from the SA node, including tachycardia, bradycardia, and sinus arrhythmia), atrial arrhythmias (including atrial extrasystoles, atrial tachycardia, and atrial fibrillation), and ventricular arrhythmias (including ventricular extrasystoles, ventricular tachycardia, and ventricular fibrillation); the underlying mechanisms involve abnormalities in the cardiac conduction system, including ectopic foci, re-entry circuits, and conduction blocks that disrupt the normal electrical impulse propagation through the heart.

Las arritmias cardíacas se generan principalmente por alteraciones en el automatismo (trastornos del automatismo normal y anormal, y actividad desencadenada) o por alteraciones en la conducción (bloqueos y reentrada). El automatismo normal alterado ocurre cuando el nodo sinusal no funciona correctamente, causando taquicardia sinusal o bradicardia. El automatismo anormal ocurre cuando células de respuesta rápida adquieren propiedades de automatismo, generando focos ectópicos que producen extrasístoles o taquicardias. La actividad desencadenada se divide en potenciales tardíos (debidos a aumento de calcio intracelular, causados por frecuencia cardíaca elevada, isquemia, digital y catecolaminas, asociados a taquicardia ventricular polimórfica y taquicardia bidireccional) y potenciales precoces (asociados a prolongación del QT y taquicardia ventricular polimórfica tipo torsades de pointes). Los bloqueos son alteraciones en la conducción del impulso, mientras que la reentrada requiere un bloqueo unidireccional, un trayecto de conducción lento, una masa crítica y un desencadenante, y se clasifica en anatómica (calcificaciones, isquemias, vías accesorias) o funcional (alteraciones en la conducción del tejido).

When atrial myocytes become irritable and depolarize prematurely, they can serve as abnormal depolarization hubs, leading to atrial fibrillation characterized by random irregular heart rates. Heart block occurs when signal transmission between atria and ventricles is impaired. First-degree block shows prolonged PR interval on ECG. Second-degree block involves intermittent signal transmission. Third-degree (complete) block represents complete failure of signal transmission from atria to ventricles.

Cardiac arrhythmias arise from four primary mechanisms: abnormal pacemaker function, pacemaker shift, conduction system block, or ectopic impulse generation. The SA node normally generates 60-100 bpm impulses, but physiological variations like fever (10 bpm/°F increase), dehydration, anemia, and athletic training alter heart rate. Pathological causes include ischemia, inflammation, infections, and medications. Heart block occurs because the AV bundle is the sole pathway connecting atria and ventricles; conditions causing block include ischemia, compression, inflammation, vagal stimulation, degeneration, or drugs. First-degree block shows prolonged PR interval (>0.2 sec) with preserved conduction. Second-degree block involves intermittent non-conducted beats (Mobitz Type I shows progressive PR prolongation; Type II shows fixed ratios). Third-degree (complete) block creates complete dissociation with independent escape pacemakers. Stokes-Adams syndrome involves intermittent complete block causing syncope due to overdrive suppression. Premature contractions occur when abnormal foci generate impulses before the SA node's scheduled beat, showing characteristic ECG features depending on origin. Long QT syndrome involves prolonged QT interval from delayed repolarization, increasing susceptibility to torsades de pointes and sudden death.

Antiarrhythmic drugs exert their effects by targeting specific phases of the cardiac action potential. Class 1 drugs (1a, 1b, 1c) block fast sodium channels during Phase 0 in ventricular myocytes, slowing depolarization. Class 3 drugs block potassium channels during Phase 3, prolonging action potential duration by preventing potassium efflux. Class 2 drugs (beta blockers) affect funny current and calcium channels in SA/AV nodes, reducing automaticity. Class 4 drugs (calcium channel blockers) block L-type calcium channels during the plateau phase (Phase 2), reducing calcium influx. The effective refractory period—the time when cells cannot regenerate action potentials—is critical for drug efficacy and safety, as drugs can alter this period to prevent reentry circuits that cause arrhythmias.

The heart contains different cardiac cells with distinct action potentials. Atrial muscle cells require stimulation to depolarize, with sodium influx followed by potassium efflux and calcium entry during the plateau phase. The SA node has automaticity due to calcium influx during phase 4. The AV node has a longer plateau phase that prolongs action potential duration. Beta-blockers reduce sodium channel activation by blocking beta receptors, prolonging phase 4 slope. Calcium channel blockers directly block calcium channels, reducing calcium influx. Adenosine increases potassium efflux, hyperpolarizing cells and delaying the next action potential, particularly effective for AV nodal reentrant tachycardia.

Antiarrhythmic drugs suppress abnormal cardiac rhythms by modifying action potentials through various mechanisms. Class 1 agents block sodium channels: 1A provides moderate blockade increasing action potential duration (quinidine, procainamide, disopyramide); 1B provides mild blockade decreasing duration (lidocaine IV, mexiletine oral); 1C provides marked blockade without duration change (flecainide, propafenone). These agents decrease phase 4 depolarization and increase threshold potential. Class 2 agents are beta blockers that counteract catecholamines, reducing heart rate, contractility, and automaticity while prolonging AV node repolarization. Class 3 agents block potassium channels (specifically DRK channels), prolonging phase 3 repolarization. Class 4 agents block L-type calcium channels, primarily affecting SA and AV nodes to decrease heart rate and terminate re-entrant rhythms.

Antiarrhythmic medications treat abnormal heart rhythms by targeting the electrical conduction system of the heart, which consists of the SA node (primary pacemaker), Bachmann's bundle, AV node (which delays signals), bundle of His, bundle branches, and Purkinje fibers that coordinate synchronized contraction of cardiac myocytes. The heart relies on two distinct action potentials: the pacemaker action potential (found in SA/AV/Purkinje cells) features a continuous depolarization (phase 4) due to funny currents that slowly allow sodium to leak in, reaching threshold at -40mV and causing calcium influx; while the myocyte action potential (in contractile cells) has a resting phase at -90mV, followed by rapid depolarization (phase 0), early repolarization (phase 1), plateau phase (phase 2) where calcium influx triggers contraction via sarcoplasmic reticulum release, and final repolarization (phase 3). These medications are classified into five classes (Class I further divided into Ia, Ib, and Ic) based on their mechanisms of action, with each class affecting different phases of the action potential to restore normal cardiac rhythm.

Class 4 antiarrhythmics (calcium channel blockers) decrease the slope of phases 0, 3, and 4 of the cardiac action potential. They also prolong repolarization via the AV node. This is remembered using the fact that calcium has seven letters, and 0+3+4 equals seven.

Implantable devices are cornerstone therapies for bradyarrhythmias and life-threatening tachyarrhythmias. Pacemakers (since 1958) sense intrinsic cardiac activity and deliver pacing stimuli when rates fall below programmed thresholds. They're implanted percutaneously with leads in the right atrium and/or ventricle, or as leadless devices (capsule implanted directly in the right ventricle). Indications include symptomatic sinus node dysfunction and AV conduction disease. Implantable cardioverter defibrillators (ICDs) detect and treat life-threatening ventricular arrhythmias (VT/VF) through anti-tachycardia pacing (ATP) or high-energy shocks. Indications include primary prevention (high-risk patients without prior events) and secondary prevention (survivors of cardiac arrest). Subcutaneous ICDs avoid intravenous leads, reducing infection risk while providing defibrillation capability.

Implantable devices include pacemakers (stimulating heart rhythm) and ICDs (delivering shocks or ATP to terminate fast rhythms). ICD technology evolved from abdominal epicardial patches (1985) to pectoral devices (1995) to modern transvenous systems. Modern ICDs offer programmable therapy, triple-chamber capability, and extended longevity. Clinical trials established ICD importance in secondary prevention for syncope, cardiac arrests, and VT. The SCD-HeFT trial demonstrated primary prevention effectiveness. ICD leads placed via subclavian vein or right atrium provide sensing, pacing, and defibrillation capabilities. This evolution reflects increasing sophistication in life-saving rhythm management.

Pacemakers are electronic devices with pulse generators and leads placed in heart chambers. They remain inactive when the heart beats adequately but deliver electrical stimuli when heart rate drops below programmed thresholds. Implantable cardioverter defibrillators (ICDs) evolved from large abdominal devices requiring sternotomy to small chest implants placed through collarbone veins. ICDs can detect and terminate dangerous rhythms like ventricular fibrillation through electrical shocks. Survival from cardiac arrest depends critically on time to defibrillation—survival drops dramatically after 10-15 minutes without intervention. With rapid response systems, survival rates can reach 40%, compared to only 3-16% overall.

Pacemakers are implanted computers with batteries that monitor heart rate and deliver regular beats when electrical conduction fails. Modern leadless pacemakers eliminate wires, reducing infection risk. Ventricular tachycardia appears as broad, fast, disorganized rhythm where the heart pumps too fast to fill, potentially causing death. Defibrillation interrupts this rhythm. Implantable cardioverter-defibrillators (ICDs) are larger devices that automatically detect and defibrillate dangerous rhythms. ICD implantation criteria include prior cardiac arrest, family history of sudden death, syncope episodes, heart muscle thickening exceeding 3 cm, apical aneurysm, or ejection fraction below 50%. ICDs carry risks including surgical complications, lead failure, infection, and inappropriate shocks causing anxiety.

Pacemakers and ICDs are implantable cardiac devices that help maintain heart rhythm; pacemakers regulate slow heart rates (bradycardia, typically below 50 bpm) by delivering gentle electrical impulses to ensure regular beating, while ICDs monitor heart rhythm and deliver stronger electrical shocks to restore normal rhythm when life-threatening arrhythmias occur, preventing sudden cardiac death.