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Electrocardiogram and Cardiac Rhythm

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ecg arrhythmia cardiac-conduction

Core Idea

The electrocardiogram (ECG) records the electrical activity of the heart on the body surface, translating the heart's electrical signals into a standardized waveform of P waves, QRS complexes, and T waves. Deviations from the normal ECG pattern reveal arrhythmias, conduction blocks, myocardial infarction, and electrolyte abnormalities.

Explainer

You already understand that cardiac muscle cells generate action potentials and that these electrical signals propagate through the heart in a coordinated sequence starting from the SA node. The electrocardiogram (ECG or EKG) is the clinical tool that lets us observe this electrical activity from outside the body, using electrodes placed on the skin. It does not measure the action potential of any single cell — instead, it detects the sum of all electrical activity across millions of cardiac cells at each instant, projected onto different recording axes called leads. The standard 12-lead ECG provides twelve different "viewing angles" of the same electrical events, giving a comprehensive picture of how depolarization and repolarization sweep through the heart.

The normal ECG tracing has three main deflections that map directly onto the conduction sequence you already know. The P wave represents atrial depolarization — the electrical wave spreading from the SA node across both atria. The QRS complex represents ventricular depolarization — the rapid, powerful wave traveling through the bundle of His, bundle branches, and Purkinje fibers to activate the thick ventricular muscle. The QRS is much larger than the P wave because the ventricular muscle mass is far greater. The T wave represents ventricular repolarization — the recovery phase as ventricular cells return to their resting membrane potential. Atrial repolarization also occurs, but it is hidden within the larger QRS complex and is not normally visible.

The intervals between these waves carry critical diagnostic information. The PR interval (from the start of the P wave to the start of the QRS) reflects conduction time through the atria and AV node — normally 120–200 ms. A prolonged PR interval indicates a conduction delay at the AV node. The QRS duration (normally < 120 ms) reflects how quickly the ventricles depolarize; a widened QRS suggests a bundle branch block or abnormal conduction pathway. The QT interval (from QRS onset to the end of the T wave) represents the total time for ventricular depolarization and repolarization; prolongation increases the risk of dangerous arrhythmias.

The power of the ECG lies in pattern recognition. A missing P wave before a QRS complex suggests the impulse did not originate from the SA node — possibly an ectopic ventricular beat or atrial fibrillation. An irregularly irregular rhythm with no identifiable P waves is the hallmark of atrial fibrillation, where the atria quiver chaotically instead of contracting in an organized manner. ST segment elevation (the flat segment between the QRS and T wave being pushed upward) is a classic sign of acute myocardial infarction — injured ventricular muscle creates abnormal current flow that distorts this baseline. Each deviation from the normal pattern tells a specific physiological story, making the ECG one of the most information-dense diagnostic tools in medicine.

Practice Questions 5 questions

Prerequisite Chain

Counting to 10 → Counting to 20 → Understanding Zero → The Number Zero → Counting to Five → One-to-One Correspondence → Combining Small Groups Within 5 → Addition Within 10 → Addition Within 20 → Two-Digit Addition Without Regrouping → Two-Digit Addition with Regrouping → Addition Within 100 → Repeated Addition as Multiplication → 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 → Two-Digit by One-Digit Division → Division with Remainders → Remainders and Quotients in Division → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Reading and Writing Decimals → Comparing and Ordering Decimals → Adding and Subtracting Decimals → Multiplying Decimals → Dividing Decimals → Dividing Fractions → Mixed Number Arithmetic → Order of Operations → Integer Order of Operations → Variable Expressions → Combining Like Terms → One-Step Equations → 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 → 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 over Rectangular Regions → 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 → Green's Theorem → Surface Integrals and Flux of Vector Fields → Surface Integrals and Flux of Vector Fields → Divergence Theorem: Flux and Outflow → Divergence Theorem → Electric Flux → 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 → The Electromagnetic Spectrum → Blackbody Radiation and Planck's Law → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → Heisenberg Uncertainty Principle → Wavefunction and the Born Rule → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Scattering Theory → Introduction to Scattering Theory → Partial Wave Analysis in Scattering → Spin Angular Momentum → Electron Spin and Intrinsic Magnetic Moment → Stern-Gerlach Experiment: Spin Quantization and Measurement → Electron Diffraction and Matter Wave Properties → Davisson-Germer Experiment: Crystal Diffraction of Electrons → Electron Diffraction and Matter Wave Interference → Wavefunctions and Probability Density Interpretation → Quantum Superposition and Linear Combinations of States → Quantum Operators and Observables → Canonical Commutation Relations and Uncertainty → Heisenberg Uncertainty Principle and Measurement Limits → Time-Independent Schrödinger Equation and Eigenvalues → Hydrogen Atom in Quantum Mechanics → Spectral Lines and Energy Transitions → Selection Rules for Atomic Transitions → LS and jj Coupling Schemes in Multi-Electron Atoms → Pauli Exclusion Principle and Antisymmetric Wavefunctions → Electron Configuration and the Aufbau Principle → The Periodic Table and Atomic Electronic Structure → The Periodic Table → Electron Configuration → Periodic Trends → Ionization Energy → Ionic Bonding → Lewis Structures → Resonance Structures and Delocalized Electrons → Resonance and Formal Charge → 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 → Organic Reaction Mechanisms and Arrow Pushing → 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 → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Amino Acid Classification and Biochemical Properties → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Ion Channels and Selective Permeability Mechanisms → Cardiac Electrophysiology and Action Potentials → Electrocardiogram and Cardiac Rhythm

Longest path: 186 steps · 822 total prerequisite topics

Prerequisites (2)

Cardiac Electrophysiology and Action Potentialshard Cardiac Pacemaker Activity and the Sinoatrial Nodesoft

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