Cardiac Electrophysiology

Cardiac Electrophysiology is the branch of cardiology that studies the electrical properties and activities of the heart. It involves understanding the mechanisms of electrical impulse generation, propagation, and regulation within cardiac tissues, which are fundamental to maintaining normal heart rhythm and contractile function. This discipline integrates cellular, molecular, and systemic aspects of cardiac electrical activity to diagnose, treat, and manage cardiac arrhythmias and conduction disorders.

Fundamentals of Cardiac Electrophysiology

Electrical Activity of the Heart

The heart's electrical system controls the timing and sequence of cardiac muscle contractions. Specialized pacemaker cells generate spontaneous electrical impulses that propagate through the conduction system, coordinating contraction and relaxation phases to maintain efficient blood circulation.

Cardiac Electrophysiological Anatomy

The cardiac conduction system includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers. Each component has distinct electrophysiological properties facilitating impulse initiation, delay, and rapid conduction to ensure synchronized ventricular contraction.

Cardiac Membrane Biophysics

Cardiac myocytes possess excitable membranes characterized by a transmembrane potential difference. The membranes contain ion channels and transporters that regulate ionic fluxes, generating and shaping action potentials that underlie cardiac excitability and conduction.

Cardiac Ion Channels and Ionic Currents

Ion Channels Overview

The cardiac action potential depends on various ion channels, including voltage-gated sodium (Na⁺), calcium (Ca²⁺), and potassium (K⁺) channels. These channels mediate inward and outward ionic currents that determine the phases of the cardiac action potential.

Ionic Currents and Their Functions

• INa (Fast Sodium Current): Responsible for rapid depolarization (phase 0) of atrial and ventricular myocytes.
• ICa,L (L-type Calcium Current): Contributes to the plateau phase (phase 2) and triggers calcium-induced calcium release for contraction.
• IK (Potassium Currents): Multiple potassium currents mediate repolarization phases (phases 1 and 3), restoring resting membrane potential.

Cardiac Action Potentials

Phases of the Action Potential

The cardiac action potential consists of five phases:

• Phase 0: Rapid depolarization due to Na⁺ influx.
• Phase 1: Initial repolarization caused by transient outward K⁺ current.
• Phase 2: Plateau phase resulting from balance between inward Ca²⁺ and outward K⁺ currents.
• Phase 3: Final repolarization due to dominant K⁺ efflux.
• Phase 4: Resting membrane potential maintained by ionic gradients and background currents.

Variations in Different Cardiac Cells

Pacemaker cells (SA and AV nodes) have a slower upstroke velocity and lack a stable resting potential, exhibiting spontaneous diastolic depolarization that initiates rhythmic firing. Ventricular and atrial myocytes have a rapid upstroke and stable resting potentials.

Calcium Handling and Excitation-Contraction Coupling

Role of Calcium in Cardiac Electrophysiology

Calcium ions link electrical excitation to mechanical contraction. The influx of Ca²⁺ during the action potential triggers release of stored calcium from the sarcoplasmic reticulum, initiating myofilament contraction.

Calcium Cycling

Calcium is tightly regulated through channels, pumps, and exchangers such as the ryanodine receptor, SERCA pump, and the sodium-calcium exchanger (NCX), ensuring proper relaxation and readiness for subsequent contractions.

Pacemaking and Automaticity

Mechanisms of Automaticity

Pacemaker cells generate spontaneous action potentials via gradual diastolic depolarization, primarily mediated by the "funny current" (If), T-type and L-type calcium currents, and reduced potassium conductance.

Hierarchy of Pacemakers

The SA node acts as the primary pacemaker with the highest intrinsic rate, followed by the AV node and Purkinje fibers as subsidiary pacemakers that can initiate impulses if the SA node fails.

Cardiac Conduction and Electrical Propagation

Impulse Conduction Pathways

Electrical impulses propagate from the SA node through atrial myocardium to the AV node, where conduction slows to allow ventricular filling. Subsequent rapid conduction via the His-Purkinje system ensures synchronized ventricular activation.

Conduction Velocity Determinants

Conduction velocity depends on cell-to-cell coupling via gap junctions, the amplitude and velocity of the action potentials, and the properties of the myocardial tissue.

Refractoriness, Restitution, and Rate Dependence

Refractory Periods

The cardiac refractory period prevents premature re-excitation, divided into absolute refractory period (ARP) where no new action potential can be initiated, and relative refractory period (RRP) where a stronger stimulus can provoke a response.

Restitution and Rate Adaptation

Action potential duration and refractoriness shorten with increasing heart rate, a relationship described by electrical restitution, which is essential for adaptation to physiological demands but can contribute to arrhythmogenesis if abnormal.

Electrical Heterogeneity and Dispersion

Variability Among Cardiac Regions

Different regions of the heart exhibit distinct electrophysiological properties, contributing to heterogeneous repolarization and conduction patterns.

Role in Arrhythmogenesis

Electrical heterogeneity creates dispersion of refractoriness, which can facilitate reentrant circuits, a common mechanism underlying many arrhythmias.

Modulation of Cardiac Electrophysiology

Autonomic Nervous System Effects

Sympathetic stimulation enhances heart rate and conduction velocity via β-adrenergic receptors, increasing calcium currents. Parasympathetic stimulation decreases heart rate and conduction via muscarinic receptors, increasing potassium currents.

Pharmacological Modulation

Antiarrhythmic drugs target specific ion channels or receptors to modify action potential characteristics and conduction, aiming to restore or maintain normal rhythm.

Cardiac Electrical Remodeling

Structural and Functional Changes

Chronic cardiac diseases induce remodeling of ion channel expression and gap junctions, altering electrophysiological properties and increasing susceptibility to arrhythmias.

Impact on Therapy

Understanding remodeling processes is critical for developing effective arrhythmia treatments and predicting therapeutic outcomes.

Mechanisms of Arrhythmogenesis

Abnormal Impulse Formation

Includes enhanced automaticity, triggered activity due to early or delayed afterdepolarizations.

Abnormal Impulse Conduction

Conduction block and reentry mechanisms create circuits of continuous excitation, underlying many tachyarrhythmias.

Atrial Electrophysiology

Unique Properties

Atrial myocardium exhibits distinct action potential durations, ion channel expression, and conduction properties compared to ventricles.

Clinical Relevance

Atrial fibrillation, the most common sustained arrhythmia, arises from atrial electrophysiological abnormalities.

Atrioventricular and His-Purkinje Electrophysiology

AV Node

Slows conduction to coordinate atrial and ventricular contraction, with slow upstroke action potentials mediated largely by calcium currents.

His-Purkinje System

Rapid conduction network with fast sodium channel-dependent action potentials, ensuring synchronized ventricular activation.

Ventricular Electrophysiology

Electrophysiological Characteristics

Ventricular myocytes have long action potentials with prominent plateau phases critical for contraction duration.

Vulnerability to Arrhythmias

Ventricular arrhythmias, including ventricular tachycardia and fibrillation, often arise from electrical disturbances in ventricular tissue.

Cardiac Electrophysiology Genetics and Genomics

Genetic Influences

Mutations in ion channel genes can cause inherited arrhythmia syndromes such as Long QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia.

Personalized Medicine

Genomic insights guide diagnosis and tailored treatment strategies for patients with inherited electrophysiological disorders.

Electrocardiography

Surface ECG

Non-invasive recording of the heart’s electrical activity, providing information on rhythm, conduction, and ischemic changes.

Interpretation

Analysis includes P wave, QRS complex, T wave morphology, intervals, and segments to diagnose arrhythmias and conduction abnormalities.

Intracardiac Electrograms

Invasive Recording

Catheter-based recordings of electrical activity within cardiac chambers, essential for precise localization of arrhythmogenic foci and conduction pathways.

Clinical Use

Guides catheter ablation and device therapy for arrhythmias.

Electrical Stimulation and Pacing Electrophysiology

Cardiac Pacing

Artificial initiation of cardiac impulses using implanted devices to treat bradyarrhythmias and heart block.

Electrophysiological Testing

Programmed stimulation protocols assess conduction properties and arrhythmia inducibility.

Electrophysiological Mapping

Techniques

3D mapping systems create electrical activation maps of cardiac chambers, aiding in identification of arrhythmia substrates.

Applications

Essential in complex ablation procedures for atrial and ventricular arrhythmias.

Experimental Cardiac Electrophysiology

Model Systems

Use of isolated cells, tissues, and whole hearts to study electrophysiological mechanisms under controlled conditions.

Technological Advances

Optical mapping, patch-clamp techniques, and molecular biology tools enhance understanding of cardiac electrophysiology.

Computational Cardiac Electrophysiology

Mathematical Modeling

Simulations of ionic currents, action potentials, and tissue conduction provide insights into normal and pathological cardiac electrical behavior.

Clinical Translation

Models support arrhythmia mechanism elucidation, drug development, and personalized therapy prediction.

Cardiac electrophysiology integrates multidisciplinary knowledge to elucidate the heart’s electrical function, diagnose and treat arrhythmias, and improve cardiovascular health through advanced diagnostic and therapeutic technologies.