A complete, detailed reference for understanding the 12-lead ECG — from fundamental electrophysiology to advanced pathological pattern recognition.
Electrophysiology & Recording Principles
An Electrocardiogram (ECG/EKG) is a non-invasive diagnostic tool that records the electrical activity of the heart over time using electrodes placed on the skin surface. It captures the summation of all cardiac action potentials as they propagate through the myocardium.
Einthoven's Law: Lead II = Lead I + Lead III. The bipolar limb leads form an equilateral triangle around the heart, providing frontal plane vectors.
Interactive ECG Complex Analysis
Click on any wave component below to see its details
Represents: Atrial depolarization
Duration: <0.12s (3 small squares)
Amplitude: <2.5mm in Lead II
Morphology: Upright in I, II, aVF; inverted in aVR
Best seen: Lead II and V1
Represents: Ventricular depolarization
Duration: 0.06–0.10s (normal)
Q wave: First negative deflection before R
R wave: First positive deflection
S wave: Negative deflection after R
Represents: Ventricular repolarization
Direction: Usually concordant with QRS
Amplitude: <5mm limb leads, <10mm precordial
Morphology: Asymmetric (slow rise, rapid descent)
Inverted normally in: aVR, V1 (sometimes V2-V3)
| Parameter | Normal Value | Represents | Abnormality Significance |
|---|---|---|---|
| PR Interval | 0.12–0.20s | Atrial depolarization + AV delay | >0.20s = 1° AV block; <0.12s = pre-excitation (WPW) |
| QRS Duration | 0.06–0.10s | Ventricular depolarization | >0.12s = BBB, ventricular rhythm, hyperkalemia |
| ST Segment | Isoelectric | Early ventricular repolarization | Elevation = STEMI/pericarditis; Depression = ischemia/digitalis |
| QT Interval | 0.36–0.44s | Total ventricular electrical activity | Prolonged QTc >0.46s = risk of Torsades de Pointes |
| RR Interval | 0.6–1.0s | Ventricular rate (300/# large boxes) | Irregular = AFib; Regular short = SVT/VT |
Electrode Placement & Lead Views
RA (−) → LA (+). Views the lateral wall. Measures horizontal electrical axis.
RA (−) → LL (+). Views the inferior wall. Best lead for rhythm analysis — P waves most prominent here.
LA (−) → LL (+). Views the inferior wall. Complements Lead II for inferior MI diagnosis.
Right arm perspective. Normally all negative (inverted P, negative QRS, inverted T). Upright QRS in aVR = critical finding (sodium channel blockade, dextrocardia, lead misplacement).
Left arm perspective. Views the high lateral wall. ST elevation here with Lead I = high lateral MI (diagonal branch / LCx).
Left foot perspective. Views the inferior wall directly. Key lead for inferior MI (RCA territory).
Augmented leads are unipolar — they measure voltage at a single electrode relative to a modified Wilson's Central Terminal. They are "augmented" by 50% to produce adequate deflections. Together with bipolar leads, they complete the hexaxial reference system in the frontal plane.
Axis Determination Quick Method:
Look at Lead I and aVF:
• Both positive = Normal axis (0° to +90°)
• Lead I negative, aVF positive = Right axis deviation
• Lead I positive, aVF negative = Left axis deviation
• Both negative = Extreme axis deviation
Normal R-wave progression shows increasing R-wave amplitude from V1 to V5, with the transition zone (R=S) normally at V3-V4.
Poor R-wave Progression (PRWP):
Anterior MI, LBBB, LVH, COPD, cardiomyopathy, or lead misplacement.
Leads: II, III, aVF
Artery: Right Coronary Artery (RCA) — 80% of cases; Left Circumflex (LCx) — 20%
Reciprocal changes in: I, aVL
Leads: V1, V2, V3, V4
Artery: Left Anterior Descending (LAD)
Reciprocal changes in: II, III, aVF
Leads: I, aVL, V5, V6
Artery: Left Circumflex (LCx) or diagonal branches of LAD
Reciprocal changes in: V1, III
Leads: V1, V2
Artery: LAD (septal perforators)
No direct leads (use V7–V9)
Artery: RCA or LCx (posterior descending)
Mirror changes: tall R and ST depression in V1–V3
Right-sided leads: V3R, V4R
Artery: Proximal RCA
Always check V4R in inferior STEMI!
Step-by-Step Approach
Methods to calculate heart rate:
300 Method (Regular)
HR = 300 ÷ (# large boxes between R-R)
Example: 4 large boxes = 75 bpm
1500 Method (Precise)
HR = 1500 ÷ (# small boxes between R-R)
6-Second Method (Irregular)
Count QRS in 30 large boxes × 10
Normal: 60–100 bpm • Bradycardia: <60 bpm • Tachycardia: >100 bpm
Criteria for Normal Sinus Rhythm:
Irregularly irregular = AFib • Regularly irregular = 2° AV block, PACs/PVCs in pattern
Quick Axis Determination (Lead I + aVF):
Causes of LAD: LAFB, inferior MI, LVH. Causes of RAD: LPFB, RVH, PE, lateral MI.
P Wave Assessment
Shape, size, and consistency. Look for P-mitrale (LA enlargement) or P-pulmonale (RA enlargement). Absent P waves suggest AFib or junctional rhythm.
QRS Morphology
Duration (<0.12s normal), height, pathological Q waves, bundle branch block patterns (rsR' in V1 = RBBB; broad monophasic R in V5-V6 = LBBB).
ST Elevation Causes
STEMI (convex up), Pericarditis (concave up, diffuse, PR depression), Benign early repolarization, LV aneurysm, Brugada syndrome, Prinzmetal angina.
ST Depression Causes
Subendocardial ischemia, Digitalis effect (reverse tick), LVH strain pattern, Reciprocal changes in STEMI, Hypokalemia.
T Wave Abnormalities
Hyperacute T waves: Tall, broad, symmetric = earliest STEMI sign. Deep T inversions: Wellens' (V2-V3) = critical LAD. Peaked T waves: Hyperkalemia. Flattened T waves: Hypokalemia, ischemia.
Bazett's Formula
QTc = QT ÷ √RR
Normal QTc: <440ms (♂) / <460ms (♀). Prolonged QTc increases risk of Torsades de Pointes — check medications (antiarrhythmics, antibiotics, antipsychotics) and electrolytes (↓K⁺, ↓Mg²⁺, ↓Ca²⁺).
Critical Diagnoses & Recognition
QRS >0.12s. rsR' pattern ("M-shaped") in V1-V2. Wide S wave in I, V5-V6. Remember: "MaRRoW" — M in V1 (R'), W in V6 (S).
QRS >0.12s. Broad monophasic R in I, V5-V6. Deep rS in V1. Remember: "WiLLiaM" — W in V1, M in V6. New LBBB + chest pain = cath lab!
Interactive ECG Quiz