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Cardiac Arrhythmias: Reentry, Automaticity, and Triggered Activity

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Action Potential Initiation: Threshold, All-or-None, and Depolarization Cardiac Anatomy and the Electrical Conduction System +1 more→→Atrial Fibrillation: Atrial Remodeling, Substrate Formation, and Arrhythmia Progression

Core Idea

Arrhythmias arise from three mechanisms: reentry (circular conduction block and recovery), abnormal automaticity (ectopic pacing), and triggered activity (afterdepolarizations). Reentry requires unidirectional block and slow conduction, often in scarred tissue; afterdepolarizations occur from calcium overload or hypokalemia.

Explainer

From your cardiac conduction prerequisite, you know that a normal heartbeat starts at the SA node, propagates through the AV node and His-Purkinje system in a coordinated wave, and then the entire system resets before the next beat. An arrhythmia is any disruption of this orderly sequence. The three underlying mechanisms — reentry, abnormal automaticity, and triggered activity — each represent a fundamentally different kind of failure, and distinguishing them matters because they respond to different treatments.

Reentry is the most clinically important mechanism and requires the most careful conceptual work. Imagine the electrical wave reaching a fork in the road where two pathways connect the same two points. Normally the wave travels down both paths, they collide at the far end, and the circuit extinguishes — it cannot circle back because both ends are refractory. Now suppose one pathway has been damaged by scar tissue from a myocardial infarction. The wave travels normally down the fast pathway, reaches the far end, and tries to enter the damaged slow pathway retrogradely. If the slow pathway has recovered by the time the retrograde wave reaches it, the wave travels backward through it and re-excites the tissue it already passed — creating a circus movement that sustains itself indefinitely, producing a rapid, regular tachycardia. The two required conditions are unidirectional block (the damaged pathway cannot conduct antegrade) and slow conduction (enough time for the fast pathway to recover before the returning wave arrives). Ablating the slow pathway interrupts the circuit, which is why catheter ablation is curative for many reentrant arrhythmias.

Abnormal automaticity arises when cells that are not supposed to pace begin spontaneously depolarizing. Your action potential prerequisite established that phase 4 spontaneous depolarization is normally unique to SA and AV nodal cells. In pathological states — hypokalemia, ischemia, digitalis toxicity, or catecholamine excess — other cells can acquire this property. A focus in the atrium, ventricle, or AV junction fires at its own rate, competing with or overriding the SA node. Unlike reentry, this mechanism requires no circuit — it is simply an ectopic pacemaker. Rate is typically 60–100 bpm for junctional automaticity, 20–40 bpm for ventricular escape rhythms, and highly variable for accelerated idioventricular rhythms seen after reperfusion.

Triggered activity is conceptually distinct from both. It arises from afterdepolarizations: membrane potential oscillations that follow an action potential rather than arising independently. Early afterdepolarizations (EADs) occur during phase 2 or 3 when channels re-open prematurely from a prolonged action potential duration — the mechanism behind torsades de pointes in the setting of hypokalemia or QT-prolonging drugs. Delayed afterdepolarizations (DADs) occur during phase 4 from intracellular calcium overload: the sarcoplasmic reticulum misfires after the action potential, releasing calcium spontaneously, which the Na⁺/Ca²⁺ exchanger extrudes in exchange for inward sodium current — a transient inward current that can depolarize the membrane to threshold. If either type reaches threshold, it triggers a new action potential, which can trigger another, producing a run of tachycardia. The defining feature of triggered activity is that it requires a preceding beat to initiate — it cannot start from rest, unlike automaticity — which is why it is more common at fast rates (EADs) or short-long-short sequences (DADs).

Practice Questions 5 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → One-to-One Correspondence → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Making 10 as an Addition Strategy → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals in Polar Coordinates → Double Integrals in Polar Coordinates → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → The Measurement Problem → Interpretations of Quantum Mechanics → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Enzyme Structure and Function → Transcription: DNA to RNA → RNA Types and Structure → RNA Structure and Intramolecular Base Pairing → RNA Processing and Splicing → Translation: RNA to Protein → Ribosomes: Protein Synthesis Machines → Translation: Initiation and Elongation → Post-Translational Modifications → Proteasomal Degradation and Ubiquitin-Mediated Marking → Cell Cycle Regulation and Checkpoints → Cell Cycle Checkpoints: Ensuring Genome Integrity → Cell Cycle Checkpoints and Cancer Prevention → Mitotic Spindle Checkpoint and Chromosome Segregation → Kinetochore Structure and Function → Mitochondria: Structure and Function → Cellular Respiration Overview → Glycolysis → Pyruvate Oxidation → The Krebs Cycle (Citric Acid Cycle) → Electron Transport Chain → ATP Synthesis and Oxidative Phosphorylation → ATP Hydrolysis and Cellular Free Energy → The Na+/K+-ATPase: Maintaining Ion Gradients → Resting Membrane Potential → Ligand-Gated Ion Channels → Voltage-Gated Sodium Channels → Action Potential Phases → Cardiac Electrophysiology and Action Potentials → Cardiac Anatomy and the Electrical Conduction System → Cardiac Arrhythmias: Reentry, Automaticity, and Triggered Activity

Longest path: 234 steps · 1239 total prerequisite topics

Prerequisites (3)

Cardiac Anatomy and the Electrical Conduction Systemhard Action Potential Initiation: Threshold, All-or-None, and Depolarizationhard Cardiac Arrhythmogenesis Mechanismssoft

Leads To (1)

Atrial Fibrillation: Atrial Remodeling, Substrate Formation, and Arrhythmia Progressionhard