The Einthoven Triangle in ECG
Fundamental Concept and Mathematical Foundation
The Einthoven triangle is a conceptual framework that represents the three standard limb leads (I, II, III) as forming an equilateral triangle around the heart, with the critical mathematical relationship that Lead II = Lead I + Lead III at any instant in the cardiac cycle (Einthoven's law). 1
Core Principles
The triangle consists of three electrode pairs: Lead I (left arm minus right arm), Lead II (left leg minus right arm), and Lead III (left leg minus left arm), where current flow toward the first electrode produces a positive deflection 1
Einthoven's law (Lead II = Lead I + Lead III) is mathematically independent of the actual geometric placement of electrodes—the equilateral triangle representation is a teaching tool, not a physical requirement 1
The three standard limb leads contain only two independent pieces of information because any one lead can be mathematically derived from the other two 1
Clinical Significance for ECG Interpretation
The triangle provides multiple vectorial perspectives of cardiac electrical activity in the frontal plane, which enhances recognition of ECG abnormalities despite the mathematical redundancy 1
Key clinical applications include:
Calculation of the electrical axis: The heart's electrical activity can be represented as a vector at the triangle's center, with its direction determined by analyzing deflections in any two leads 1, 2
Spatial morphological assessment: Redundant leads promote appreciation of spatial characteristics crucial for interpretation, particularly in evaluating ST-segment shifts during acute myocardial infarction 1
Detection of lead misplacement: Understanding the triangle's mathematical relationships allows identification of technical errors, such as the right arm-right leg transposition that produces inverted symmetry between leads I and III with near-flat lead II 3, 4
Evolution Beyond the Basic Triangle
The augmented limb leads (aVR, aVL, aVF) provide additional frontal plane perspectives but contain no new independent information—they are mathematically derived from the standard limb leads 1
The complete frontal plane system (6 leads total) still contains only two independent measured signals, with the remaining four calculated mathematically 1
The standard 12-lead ECG actually contains only 8 independent pieces of information: 2 from limb leads (from which 4 others are derived) plus 6 independent precordial leads 1
Theoretical Limitations
While Einthoven conceptualized the heart as a single dipole source with voltage explained by projection of the heart vector onto lead axes, this model is an oversimplification 1
Burger expanded this concept by treating leads as vectors with both direction and strength (length), where voltage equals the heart vector projection multiplied by lead vector strength 1, 5
Lead vector direction and strength depend on body geometry and varying tissue impedances—the anatomical lead axis differs from the electrical lead axis 1, 5
The potential at any body surface point represents instantaneous uncanceled electrical forces, with cancellation dependent on torso inhomogeneities including tissue boundaries and impedance variations 1
Critical Pitfall: Lead Misplacement
Never interpret an ECG with suspected lead misplacement—the risk of false-positive ischemia diagnosis or missed genuine ischemia is unacceptably high 3
Right arm-right leg transposition (the most common error) produces pathognomonic findings: near-flat lead II, inverted symmetry between leads I and III, and normal precordial leads 3
Always repeat the ECG with correct lead placement rather than attempting mathematical correction 3
Formal training on lead switch recognition and prevention is essential for all ECG personnel 3