How to read an ECG as a beginner fast and easy
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how to read ECG beginner step by step guide
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I now have everything I need to give you a thorough, well-illustrated beginner's guide to ECG reading. Here it is:
How to Read an ECG: A Beginner's Step-by-Step Guide
First, Understand What the ECG Shows
An ECG records the electrical activity of the heart from 12 different "viewpoints" (leads). Every heartbeat produces a characteristic set of waves:
Normal ECG waveform showing P, Q, R, S, and T waves - Tietz Textbook of Laboratory Medicine
What each part means:
| Wave/Interval | What it represents |
|---|---|
| P wave | Atrial depolarization (atria contract) |
| PR interval | Conduction from atria through the AV node to ventricles |
| QRS complex | Ventricular depolarization (ventricles contract) |
| ST segment | Early ventricular repolarization |
| T wave | Ventricular repolarization (ventricles "reset") |
| QT interval | Total ventricular electrical activity |
ECG waveform with key intervals labeled - Costanzo Physiology 7th Edition
The ECG Paper: Know Your Grid
Standard ECG paper runs at 25 mm/sec:
- Small box (1 mm) = 0.04 seconds (40 ms)
- Large box (5 mm) = 0.2 seconds (200 ms)
- On the vertical axis, 10 mm = 1 mV of voltage amplitude
This grid is what lets you measure everything on the ECG.
The Systematic Approach: Use RRIAMPST
A great mnemonic for beginners. Work through every ECG in this order - never skip steps:
Step 1 - Rate
Normal: 60-100 beats per minute (bpm)
Quick method for regular rhythms:
- Find an R wave on a large box line
- Count the large boxes to the next R wave
- Divide 300 by that number
| Large boxes between R waves | Heart rate |
|---|---|
| 1 | 300 bpm |
| 2 | 150 bpm |
| 3 | 100 bpm |
| 4 | 75 bpm |
| 5 | 60 bpm |
| 6 | 50 bpm |
Memory trick: 300 - 150 - 100 - 75 - 60 - 50
For irregular rhythms: count all QRS complexes across the 10-second rhythm strip, then multiply by 6.
- < 60 bpm = Bradycardia
- > 100 bpm = Tachycardia
Step 2 - Rhythm
Ask two questions:
- Is it regular? Mark R wave peaks on a piece of paper and check if spacing is equal throughout.
- Is there a P wave before every QRS, and a QRS after every P?
Normal sinus rhythm = regular, P wave before every QRS, PR interval constant.
- Regularly irregular = pattern repeats (e.g., every 3rd beat is different)
- Irregularly irregular = completely disorganized - think atrial fibrillation (no P waves, chaotic baseline)
Step 3 - Axis
The electrical axis tells you which direction the heart's electrical activity flows. A quick beginner shortcut using leads I and aVF:
| Lead I | aVF | Axis |
|---|---|---|
| Positive (upright QRS) | Positive | Normal |
| Positive | Negative | Left axis deviation |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme axis deviation |
Normal axis = -30° to +90°.
Step 4 - P Waves
Normal P wave:
- Present before every QRS
- Upright (positive) in leads I, II, aVF
- Inverted in aVR
- Duration: < 0.12 sec (< 3 small boxes)
- Amplitude: < 2.5 mm height
Absent P waves with irregular rhythm = atrial fibrillation. Peaked tall P waves = right atrial enlargement ("P pulmonale"). Broad, notched P waves = left atrial enlargement ("P mitrale").
Step 5 - PR Interval
Measured from the start of the P wave to the start of the QRS complex.
Normal: 0.12-0.20 seconds (3-5 small boxes)
- Short PR (< 0.12 sec) = pre-excitation (Wolff-Parkinson-White syndrome) or junctional rhythm
- Prolonged PR (> 0.20 sec) = 1st degree AV block
- Progressively lengthening PR until a QRS is dropped = 2nd degree AV block (Mobitz I / Wenckebach)
- Fixed prolonged PR with dropped beats = Mobitz II
- No relationship between P and QRS at all = 3rd degree (complete) heart block
Step 6 - QRS Complex
Normal QRS duration: < 0.12 sec (< 3 small boxes)
- Narrow QRS (< 0.12 sec) = normal ventricular conduction
- Wide QRS (≥ 0.12 sec) = bundle branch block, ventricular rhythm, or Wolff-Parkinson-White
Bundle branch block quick ID:
- LBBB (Left BBB): Wide QRS + "W" shape in V1 and "M" shape in V5/V6 (William)
- RBBB (Right BBB): Wide QRS + "M" shape in V1 and "W" shape in V5/V6 (MoRRoW)
Pathological Q waves: Q wave > 1 small box wide (0.04 sec) or > 25% of following R wave height = suggests old myocardial infarction in that territory.
R-wave progression in chest leads V1-V6: R wave should get progressively taller from V1 to V5. Poor R-wave progression (R stays small) = anterior MI or LBBB.
Step 7 - ST Segment
This is one of the most clinically important parts. The ST segment should be flat and at baseline (isoelectric).
ST elevation (> 1 mm in limb leads, > 2 mm in chest leads):
- Convex/tombstone ST elevation = STEMI (ST-elevation myocardial infarction) - emergency!
- Saddle-shaped ST elevation in most leads = pericarditis
- Concave ST elevation = often benign early repolarization
ST depression:
- Horizontal or downsloping = myocardial ischemia or NSTEMI
- Digoxin effect = "reverse tick" or sagging ST depression
Lead territory to infarct location:
| Leads with changes | Territory | Artery |
|---|---|---|
| II, III, aVF | Inferior | Right coronary artery (RCA) |
| V1-V4 | Anterior | LAD |
| I, aVL, V5-V6 | Lateral | Circumflex |
| V1-V2 tall R, ST depression | Posterior | RCA or circumflex |
Step 8 - T Waves
Normal T waves: upright in I, II, V3-V6; inverted in aVR (always), V1 (often normal).
- Tall peaked T waves = hyperkalemia ("peaked" or "tent-shaped") or early MI (hyperacute T waves)
- Inverted T waves = ischemia, ventricular hypertrophy, bundle branch block, pulmonary embolism (V1-V4)
- Flat T waves = hypokalemia, ischemia
Bonus - QT Interval
Measured from start of QRS to end of T wave.
Normal corrected QTc: < 0.44 sec in men, < 0.46 sec in women (corrected for rate using Bazett's formula).
- Prolonged QT = risk of Torsades de Pointes (life-threatening arrhythmia). Causes: drugs (antiarrhythmics, antibiotics, antipsychotics), hypokalemia, hypomagnesemia, congenital long QT syndromes.
Quick Summary Checklist
Every time you pick up an ECG, run through this:
✅ Rate - fast, slow, or normal?
✅ Rhythm - regular? P before every QRS?
✅ Axis - normal, left, or right deviation?
✅ P waves - present, shape, size?
✅ PR interval - 0.12-0.20 sec?
✅ QRS - narrow or wide? Q waves?
✅ ST segment - elevated or depressed?
✅ T waves - upright and normal shape?
✅ QT interval - prolonged?
Helpful Video Resource
This 13-minute video walks through all 5 steps in a clear, visual way:
Key Normal Values at a Glance
| Parameter | Normal Value |
|---|---|
| Heart rate | 60-100 bpm |
| PR interval | 0.12-0.20 sec (3-5 small boxes) |
| QRS duration | < 0.12 sec (< 3 small boxes) |
| QT interval (corrected) | < 0.44 sec (men), < 0.46 sec (women) |
| P wave duration | < 0.12 sec |
| P wave amplitude | < 2.5 mm |
Sources: Goldman-Cecil Medicine (p. 535), Harrison's Principles of Internal Medicine 22E (p. 1592), Costanzo Physiology 7th Ed (p. 149), Tietz Textbook of Laboratory Medicine 7th Ed (p. 1771-1772).
The GeekyMedics ECG guide and Life in the Fast Lane ECG library are excellent free resources for practicing on real ECG examples.
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