Ecg reading from basics
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ECG reading basics systematic approach 2024
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normal ECG waveform P wave QRS T wave labeled diagram
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I now have comprehensive material from Goldman-Cecil Medicine and Harrison's to construct a thorough ECG basics guide. Let me compile this into a well-structured answer.
ECG Reading — From Basics
1. What Is an ECG?
An electrocardiogram (ECG) records the electrical activity of the heart from the body surface. Each deflection represents electrical events in cardiac muscle — it does not directly measure mechanical contraction, but the two are tightly coupled.
2. The Cardiac Conduction System (What Generates the ECG)
The electrical impulse follows this path:
| Structure | Function |
|---|---|
| SA node | Primary pacemaker; fires at 60–100 bpm spontaneously |
| Atrial muscle | Spreads depolarization through both atria (Bachmann bundle speeds it to left atrium) |
| AV node | Physiologic delay — "gatekeeper" between atria and ventricles |
| Bundle of His | Rapid conduction from AV node downward |
| Right & Left bundle branches | Left splits into anterior and posterior fascicles |
| Purkinje fibers | Spreads impulse rapidly to ventricular muscle |
3. The ECG Waves — What Each Represents
| Wave/Segment | Represents |
|---|---|
| P wave | Atrial muscular depolarization |
| PR segment | Conduction through AV node and His-Purkinje (isoelectric) |
| PR interval | Onset of P → onset of QRS; total atrial + AV nodal + His-Purkinje conduction |
| QRS complex | Ventricular muscular depolarization |
| J point | End of QRS / start of ST segment |
| ST segment | Plateau phase of ventricular action potential (isoelectric normally) |
| T wave | Ventricular repolarization |
| QT interval | Onset of QRS → end of T wave; total ventricular electrical systole |
| U wave | Purkinje fiber repolarization (small, not always visible) |
Key concept: Atrial repolarization is buried under the QRS complex and is not visible on the surface ECG.
4. The ECG Paper and Calibration
Standard ECG paper runs at 25 mm/sec:
| Box | Time | Voltage |
|---|---|---|
| Small box (1 mm) | 0.04 sec (40 ms) | 0.1 mV |
| Large box (5 mm) | 0.2 sec (200 ms) | 0.5 mV |
| 2 large boxes (10 mm) | 0.4 sec | 1 mV (standard calibration) |
A standard ECG records 10 seconds of data across 12 simultaneous leads. Each lead group occupies 2.5 seconds.
5. Normal Intervals at a Glance
| Parameter | Normal Range |
|---|---|
| Heart rate | 60–100 bpm (some authorities: 50–90 bpm) |
| P wave duration | < 120 ms (< 3 small boxes) |
| PR interval | 90–200 ms (0.09–0.20 sec) |
| QRS duration | 75–110 ms (<3 small boxes) |
| QTc (Bazett) | Males: 390–450 ms; Females: 390–460 ms |
| QRS axis | −30° to +90° |
QTc formula (Bazett): QTc = QT ÷ √RR (RR in seconds)
6. The 12 Leads — What They View
10 electrodes produce 12 leads. The right leg electrode is a ground only.
Limb Leads (Frontal Plane)
| Lead | Comparison | View |
|---|---|---|
| I | Right arm (−) vs Left arm (+) | Lateral |
| II | Right arm (−) vs Left leg (+) | Inferior |
| III | Left arm (−) vs Left leg (+) | Inferior |
| aVR | Right arm vs (LA + LL) | Cavity — normally negative |
| aVL | Left arm vs (RA + LL) | High lateral |
| aVF | Left foot vs (RA + LA) | Inferior |
Precordial (Chest) Leads — Transverse Plane
| Lead | Position |
|---|---|
| V1 | 4th intercostal space, right sternal border |
| V2 | 4th intercostal space, left sternal border |
| V3 | Between V2 and V4 |
| V4 | 5th intercostal space, midclavicular line |
| V5 | Anterior axillary line, same level as V4 |
| V6 | Mid-axillary line, same level as V4 |
7. Heart Rate Calculation
Method 1 (regular rhythm): 300 ÷ number of large boxes between two R waves
| Large boxes between QRS | Heart Rate |
|---|---|
| 1 | 300 bpm |
| 2 | 150 bpm |
| 3 | 100 bpm |
| 4 | 75 bpm |
| 5 | 60 bpm |
| 6 | 50 bpm |
Method 2: Count beats in 10-second strip × 6
Method 3 (exact): HR = 60,000 ÷ RR interval (in ms)
8. QRS Axis
The frontal plane axis is the net direction of ventricular depolarization.
| Axis | Degrees | Significance |
|---|---|---|
| Normal | −30° to +90° | |
| Left axis deviation (LAD) | < −30° | LAHB, LVH, inferior MI |
| Right axis deviation (RAD) | > +90° | RVH, LPHB, normal in children |
| Extreme (NW) axis | < −90° or > +180° | Serious pathology |
Quick method: If QRS is predominantly positive in both I and II → normal axis.
To determine axis precisely: find the limb lead with the most isoelectric QRS — the axis is perpendicular to that lead.
9. The Systematic 14-Step Approach (Harrison's)
Harrison's recommends analyzing all 14 parameters in every ECG to avoid errors of omission:
- Standardization/calibration — confirm 1 mV = 10 mm; check paper speed
- Rhythm — sinus? Regular or irregular?
- Heart rate — fast, slow, or normal?
- PR interval — prolonged, short, or absent?
- QRS interval — narrow (<120 ms) or wide (≥120 ms)?
- QT/QTc interval — prolonged?
- QRS axis — normal, LAD, or RAD?
- P wave morphology — upright in I and II? Biphasic in V1?
- QRS voltages — high (LVH/RVH) or low?
- Precordial R-wave progression — R wave should grow V1→V6; transition at V3–V4
- Abnormal Q waves — pathological if ≥40 ms wide or ≥25% of R wave height
- ST segments — elevation or depression?
- T waves — upright in I, II, V3–V6; inverted T in aVR is normal
- U waves — prominent in hypokalemia, bradycardia; inverted U suggests ischemia
10. Key Patterns to Recognize (Basics)
Normal Sinus Rhythm
- Rate 60–100 bpm
- P wave before every QRS, QRS after every P
- PR interval 120–200 ms, consistent
- Narrow QRS (<120 ms)
- Upright P in leads I, II, aVF
P Wave Abnormalities
- P mitrale (broad, notched P, >120 ms): left atrial enlargement
- P pulmonale (tall, peaked P >2.5 mm in II): right atrial enlargement
- No P waves: atrial fibrillation (irregularly irregular QRS)
QRS Abnormalities
- Wide QRS (≥120 ms): bundle branch block, ventricular origin, paced rhythm, hyperkalemia
- RBBB pattern (V1): rSR' (M-shape) with secondary T inversion
- LBBB pattern (V6): broad notched R, absent septal q, T inversion
ST Segment
- Elevation: acute MI (STEMI), pericarditis, vasospasm — measure from J point
- Depression: ischemia, posterior MI, digoxin effect
QT Prolongation
- Risk of Torsades de Pointes (polymorphic VT)
- Causes: drugs (antiarrhythmics, antipsychotics, antibiotics), electrolyte abnormalities (↓K⁺, ↓Mg²⁺, ↓Ca²⁺), congenital channelopathies
11. Quick Reference: Regional Lead Groups
| Territory | Leads | Coronary Artery |
|---|---|---|
| Inferior | II, III, aVF | RCA (85%) |
| Lateral | I, aVL, V5, V6 | LCx |
| Anterior | V1–V4 | LAD |
| Septal | V1–V2 | LAD (septal perforators) |
| Posterior | Reciprocal ↓ST in V1–V3 | RCA or LCx |
12. Common Mistakes to Avoid
- Not checking calibration — if gain is halved, QRS looks small (pseudolowvoltage)
- Skipping leads — each lead views a different wall; check all 12
- Measuring QT in the wrong lead — use II, V5, or V6; take the longest
- Forgetting to compare with previous ECGs — subtle new changes matter
- Accepting the machine's interpretation without review — automated algorithms have significant error rates
Sources: Goldman-Cecil Medicine, 26th Ed. (GOLDMAN-CECIL MEDICINE, block 5); Harrison's Principles of Internal Medicine, 22nd Ed. (2025, block 27–28)
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