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most common ECG patterns clinical interpretation
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ECG normal sinus rhythm P wave QRS T wave
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atrial fibrillation ECG irregular rhythm absent P waves
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STEMI ST elevation myocardial infarction ECG
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left bundle branch block right bundle branch block ECG comparison
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AV heart block first degree second degree third degree ECG
Now I have plenty of material. Let me compile a comprehensive ECG teaching guide.
ECG: A Comprehensive Guide to the Most Common Patterns
1. The Normal ECG — Understanding the Baseline
Before recognizing abnormalities, you must master the normal tracing.
The Cardiac Cycle & ECG Waveforms
| Wave/Interval | What It Represents | Normal Values |
|---|---|---|
| P wave | Atrial depolarization (SA node → atria) | Duration < 0.12 s; upright in I, II, aVF |
| PR interval | AV nodal conduction delay | 0.12–0.20 s (3–5 small boxes) |
| QRS complex | Ventricular depolarization | < 0.12 s (3 small boxes) |
| ST segment | Early ventricular repolarization | Isoelectric (flat) |
| T wave | Ventricular repolarization | Upright in most leads |
| QT interval | Total ventricular depolarization + repolarization | < 0.44 s (rate-corrected QTc) |
ECG Paper Calibration
- Horizontal axis: 1 small box = 0.04 s; 1 large box = 0.20 s
- Vertical axis: 1 mm = 0.1 mV
- Rate formula: Rate = 300 ÷ (number of large boxes between R-R intervals)
Normal Sinus Rhythm (NSR) — The Template
Criteria:
- P wave before every QRS
- Regular R-R intervals
- Rate 60–100 bpm
- Normal PR interval (0.12–0.20 s)
- Normal QRS morphology with R-wave progression V1→V6
The 5-Step Interpretation Approach
- Rate — 300/large boxes between R waves
- Rhythm — regular vs. irregular; P wave present?
- Axis — normal (−30° to +90°); left or right deviation?
- Intervals — PR, QRS, QT within normal limits?
- Morphology — ST segments, T waves, Q waves, R-wave progression
2. Arrhythmias
2a. Atrial Fibrillation (AF) — Most Common Sustained Arrhythmia
ECG Hallmarks:
- Absent P waves — replaced by fine chaotic fibrillatory (f) waves
- Irregularly irregular R-R intervals (no two R-R intervals are equal)
- Narrow QRS (unless aberrant conduction)
- Ventricular rate typically 100–160 bpm if untreated (AF with rapid ventricular response)
Causes: hypertension, valvular disease, heart failure, thyrotoxicosis, alcohol (holiday heart), ischemia
2b. AV Blocks
| Type | ECG Feature | Clinical Significance |
|---|---|---|
| 1st Degree | PR > 0.20 s (>1 large box); every P conducts | Benign; no treatment usually needed |
| 2nd Degree — Mobitz I (Wenckebach) | Progressive PR lengthening → dropped QRS | Usually at AV node; benign; often inferior MI |
| 2nd Degree — Mobitz II | Fixed PR; sudden dropped QRS (no warning) | Below His bundle; can progress to complete block |
| 3rd Degree (Complete) | P waves and QRS completely dissociated; AV dissociation; slow escape rhythm | Emergency — pacemaker needed |
From Morgan and Mikhail's Clinical Anesthesiology: Mobitz II nearly always reflects infra-Hisian conduction disease and frequently progresses to complete AV block, especially after anterior MI.
3. Conduction Abnormalities — Bundle Branch Blocks
3a. Right Bundle Branch Block (RBBB)
ECG Criteria:
- QRS ≥ 0.12 s (≥ 3 small boxes)
- rSR' ("M-shape") pattern in V1
- Wide, slurred S wave in I and V6
- Discordant ST-T changes in V1–V2
Mnemonic: "MaRRoW" — in RBBB, look for the M in V1 (right), the W in V6 (left)
Clinical significance: Can be congenital or represent organic heart disease; alone with normal PR interval, rarely progresses to complete block.
3b. Left Bundle Branch Block (LBBB)
ECG Criteria:
- QRS ≥ 0.12 s
- Broad notched R without q in I, aVL, V5–V6 ("W" pattern in V1)
- rS or QS in V1
- Discordant ST-T changes throughout V1–V6
Mnemonic: "WiLLiaM" — in LBBB, W in V1, M in V6
From Goldman-Cecil Medicine and Morgan and Mikhail's: LBBB almost always indicates underlying organic heart disease and masks ischemic changes — a new LBBB in chest pain should be treated like a STEMI.
Bundle Branch Block ECG Criteria Summary (from Goldman-Cecil Medicine):
| QRS Duration | Axis | Key Morphology | |
|---|---|---|---|
| RBBB | ≥ 0.12 s | Normal | rSR' in V1; wide S in I, V6 |
| LBBB | ≥ 0.12 s | Variable | Broad notched R in I, V5–V6; rS in V1 |
| LAFB | < 0.12 s | −45° to −90° | qR in aVL; left axis deviation |
| LPFB | < 0.12 s | +90° to +180° | Right axis deviation; rS in I, aVL |
4. Ischemia & Infarction
STEMI (ST-Elevation Myocardial Infarction)
ECG criteria for STEMI:
- New ST elevation ≥ 1 mm in ≥ 2 contiguous limb leads
- Or ≥ 2 mm in ≥ 2 contiguous precordial leads
- Convex (tombstone) morphology
- Reciprocal ST depression in opposite leads
Localizing the Infarct by Leads:
| Lead Group | Territory | Culprit Artery |
|---|---|---|
| V1–V4 | Anterior wall | LAD (left anterior descending) |
| I, aVL, V5–V6 | Lateral wall | LCx (left circumflex) |
| II, III, aVF | Inferior wall | RCA (right coronary artery) |
| V1–V2 (tall R, ST depression) | Posterior wall | RCA or LCx |
NSTEMI / Ischemia
- ST depression ≥ 1 mm in ≥ 2 contiguous leads
- T-wave inversions (symmetric, deep = Wellens syndrome in anterior leads)
- No ST elevation
Pathological Q Waves (old infarction)
- Duration > 0.04 s (1 small box)
- Depth > 25% of the R wave amplitude
- Indicate prior transmural infarction
5. Other Common Patterns
Left Ventricular Hypertrophy (LVH)
- Sokolow-Lyon criteria: S in V1 + R in V5 or V6 > 35 mm
- "Strain pattern": ST depression + T-wave inversion in lateral leads (I, aVL, V5–V6)
Right Ventricular Hypertrophy (RVH)
- Right axis deviation (> +90°)
- R/S ratio > 1 in V1
- Dominant R wave in V1
Prolonged QT Interval
- QTc > 440 ms (men), > 460 ms (women)
- Risk of Torsades de Pointes (polymorphic VT)
- Causes: drugs (quinolones, antipsychotics, sotalol), hypokalemia, hypomagnesemia, congenital
Hyperkalemia — ECG Progression
| Potassium Level | ECG Change |
|---|---|
| Mild (5.5–6.5 mEq/L) | Peaked (tent-shaped) T waves |
| Moderate (6.5–7.5) | Wide QRS, flat/absent P waves |
| Severe (> 7.5) | Sine wave pattern → VF → asystole |
Pericarditis
- Diffuse ST elevation (saddle-shaped, concave upward) in virtually all leads
- PR depression (very specific sign)
- No reciprocal changes (distinguishes from STEMI)
Wolff-Parkinson-White (WPW)
- Short PR interval (< 0.12 s)
- Delta wave (slurred upstroke of QRS)
- Wide QRS
- Risk of rapid conduction via accessory pathway in AF
6. Quick Reference Summary
| Pattern | Key ECG Finding | Mnemonic/Memory Aid |
|---|---|---|
| Normal sinus | P before every QRS, rate 60–100 | — |
| Atrial fibrillation | No P waves, irregularly irregular | "Absolutely No Ps" |
| 1° AV block | PR > 200 ms | "Prolonged PR" |
| Mobitz II | Fixed PR + dropped QRS | "Dropped without warning" |
| Complete heart block | P & QRS dissociated | "Nothing gets through" |
| RBBB | rSR' in V1 (M-shape) | "MaRRoW" (M in V1) |
| LBBB | Broad R in V5–V6, rS in V1 | "WiLLiaM" (W in V1) |
| STEMI | ST elevation + reciprocal depression | Territory = culprit artery |
| Hyperkalemia | Peaked T waves → wide QRS | "PAT on the back — P waves disappear" |
| WPW | Delta wave + short PR | "Pre-excited ventricle" |
Sources: Miller's Anesthesia 10e · Medical Physiology (Boron & Boulpaep) · Morgan & Mikhail's Clinical Anesthesiology 7e · Goldman-Cecil Medicine · Tintinalli's Emergency Medicine
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