ECG
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I now have solid textbook content to build the full guide. Here is a comprehensive, structured guide to basic ECG interpretation.
ECG Basic Interpretation - A Systematic Guide
A good ECG read always follows a fixed order so nothing gets missed. Harrison's (22e, 2025) lists 14 parameters to check on every ECG; in practice these collapse into the steps below.
1. Technical Quality Check (Step 0)
Before interpreting anything, verify:
- Calibration: Standard is 10 mm/mV (1 mV = 10 mm tall). If non-standard, all voltage measurements are off.
- Paper speed: Standard is 25 mm/s.
- Lead placement: Limb lead reversal is a common artifact that mimics axis deviation or dextrocardia.
- Artifact: Muscle tremor, poor contact, baseline wander.
Errors of omission are the most common mistakes in ECG interpretation. A systematic approach is essential. - Harrison's Principles of Internal Medicine, 22e
2. The ECG Grid
| Box | Time | Voltage |
|---|---|---|
| Small box | 0.04 s (40 ms) | 1 mm |
| Large box (5 small) | 0.20 s (200 ms) | 5 mm |
| 5 large boxes | 1.0 s | - |
| 1 strip (standard) | 10 s | - |
3. Rate
Method 1 - "300 rule" (regular rhythms):
Count the number of large boxes between two consecutive R waves, then divide 300 by that number.
| Large boxes between R-R | Rate (bpm) |
|---|---|
| 1 | 300 |
| 2 | 150 |
| 3 | 100 |
| 4 | 75 |
| 5 | 60 |
| 6 | 50 |
Method 2 - "QRS count" (irregular rhythms):
Count the number of QRS complexes in the 10-second strip and multiply by 6.
- Normal: 60-100 bpm
- Bradycardia: < 60 bpm
- Tachycardia: > 100 bpm
Common causes of bradycardia: medications (beta-blockers, digoxin), hypothyroidism, ischemia, vagal tone, electrolyte disturbance, sinoatrial node degeneration.
4. Rhythm
Ask three questions:
- Is it regular? - Are R-R intervals equal?
- Is there a P before every QRS? - Confirms sinus rhythm.
- Is the P wave normal? - Upright in I and II, inverted in aVR.
A normal sinus rhythm requires:
- Rate 60-100 bpm
- Regular R-R intervals
- Upright P in leads I and II, inverted in aVR
- Constant and normal PR interval (0.12-0.20 s)
- Every P followed by a QRS
5. Axis
The axis represents the net vector of ventricular depolarization. Assess leads I and aVF (perpendicular views at 0° and +90°).
| Lead I | Lead aVF | Axis |
|---|---|---|
| Positive | Positive | Normal (-30° to +90°) |
| Positive | Negative | Left axis deviation (LAD) |
| Negative | Positive | Right axis deviation (RAD) |
| Negative | Negative | Extreme/"northwest" axis |
- Normal axis: -30° to +90°
- LAD (-30° to -90°): left ventricular hypertrophy, left anterior fascicular block, inferior MI, LBBB
- RAD (+90° to +180°): right ventricular hypertrophy, left posterior fascicular block, RBBB, pulmonary embolism, lateral MI
Axis deviation is most commonly caused by ventricular hypertrophy shifting the position of the heart in the chest. - Kaplan & Sadock's
6. Intervals
PR Interval
- Measured from start of P to start of QRS
- Normal: 0.12-0.20 s (3-5 small boxes)
- Short PR (< 0.12 s): pre-excitation (WPW), junctional rhythm, LGL syndrome
- Long PR (> 0.20 s): first-degree AV block
QRS Duration
- Normal: < 0.12 s (< 3 small boxes)
- Wide QRS (≥ 0.12 s): bundle branch block, ventricular rhythm, hyperkalemia, sodium channel blocker toxicity (TCAs)
QT/QTc Interval
- Measured from start of Q to end of T wave
- Varies with heart rate; corrected using Bazett formula: QTc = QT / √(R-R interval in seconds)
- Normal QTc: < 440 ms in males, < 450-460 ms in females
- Prolonged QTc > 500 ms: high risk for Torsades de Pointes (TdP)
Common QT-prolonging drugs (from Kaplan & Sadock's):
| Drug class | Examples |
|---|---|
| Antiarrhythmics | Amiodarone, sotalol, procainamide, quinidine |
| Antibiotics | Azithromycin, erythromycin, clarithromycin, moxifloxacin |
| Antifungals | Fluconazole, ketoconazole |
| Antipsychotics | Haloperidol, quetiapine, ziprasidone |
| Antidepressants | Citalopram (most notable among SSRIs) |
| Others | Methadone, chloroquine, cocaine |
7. The 12 Leads - What They "See"
| Leads | View |
|---|---|
| II, III, aVF | Inferior wall (RCA territory) |
| I, aVL, V5, V6 | Lateral wall (LCx territory) |
| V1, V2 | Septal / anterior |
| V3, V4 | Anterior wall (LAD territory) |
| V1 (reciprocal) | Posterior wall (look for tall R in V1) |
| aVR | Cavity (inverted view of all walls) |
8. Waveform Analysis
P Wave
- Represents atrial depolarization
- Normal: upright in I, II; inverted in aVR; < 2.5 mm tall and < 120 ms wide
- Peaked P in II (P pulmonale): right atrial enlargement
- Notched/bifid P in II (P mitrale): left atrial enlargement
QRS Complex
- Represents ventricular depolarization
- Q waves: normal small septal Q waves in I, aVL, V5-V6; pathological if > 40 ms wide or > 25% of R height (= prior infarction)
- R-wave progression: R wave should grow from V1 to V5 (poor progression = anterior MI or LBBB)
- Voltage: Low voltage (< 5 mm all limb leads or < 10 mm all precordial leads) = effusion, obesity, hypothyroidism, infiltrative disease
ST Segment
- Represents early ventricular repolarization
- Should be isoelectric (flat)
- ST elevation: STEMI, pericarditis (diffuse), Brugada, LV aneurysm, early repolarization
- ST depression: NSTEMI/ischemia (reciprocal or primary), digoxin effect ("reverse tick"), hypokalemia
Pericarditis shows diffuse ST elevation with PR depression in 4 stages: (1) diffuse ST elevation + PR depression, (2) normalization, (3) T-wave inversion, (4) normalization. - Roberts & Hedges' Clinical Procedures
T Wave
- Represents ventricular repolarization
- Normally upright in I, II, V3-V6; inverted in aVR
- Peaked/tall T: hyperkalemia, early ischemia ("hyperacute T")
- Inverted T: ischemia, PE (V1-V4), hypertrophy strain pattern, CNS events
- Flattened T: hypokalemia, drugs
U Wave
- Small deflection after T wave, best seen in V2-V3
- Prominent U: hypokalemia, bradycardia, drugs
- Inverted U: ischemia, hypertension
9. Harrison's 14-Parameter Checklist (Complete)
Harrison's (22e) recommends checking these in every ECG:
- Standardization / calibration / technical features
- Rhythm
- Heart rate
- PR interval / AV conduction
- QRS interval
- QT / QTc interval
- Mean QRS electrical axis
- P waves
- QRS voltages
- Precordial R-wave progression
- Abnormal Q waves
- ST segments
- T waves
- U waves
"Many mistakes in ECG interpretation are errors of omission." - Harrison's Principles of Internal Medicine, 22e
10. Quick Pattern Recognition Summary
| Pattern | Key ECG Finding |
|---|---|
| STEMI | ST elevation ≥ 1 mm in ≥ 2 contiguous leads |
| LBBB | Wide QRS, notched R in I/V6, rS in V1 |
| RBBB | Wide QRS, RSR' ("rabbit ears") in V1, wide S in I/V6 |
| WPW | Short PR, delta wave, wide QRS |
| AF | Absent P waves, irregularly irregular rhythm |
| VT | Wide QRS tachycardia, AV dissociation |
| Complete heart block | P waves and QRS completely independent, slow rate |
| Hyperkalemia | Peaked T → wide QRS → sine wave |
| PE | Sinus tach, S1Q3T3, RBBB, RV strain |
Sources:
- Harrison's Principles of Internal Medicine, 22e (2025) - Clinical Interpretation of the ECG
- Kaplan & Sadock's Comprehensive Textbook of Psychiatry - Electrophysiology and ECG Interpretation
- Roberts and Hedges' Clinical Procedures in Emergency Medicine - Electrocardiography
Want me to go deeper on any specific section - arrhythmia recognition, bundle branch blocks, axis determination, ischemia patterns, or any other topic?
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