How to Read an ECG — Beginner Guide

The electrocardiogram — ECG or EKG — is one of the most powerful diagnostic tools in medicine. It can diagnose a heart attack within seconds, reveal life-threatening arrhythmias, and guide critical treatment decisions. Yet for most medical students, reading an ECG feels overwhelming at first. This guide breaks it down into a simple, systematic approach anyone can follow.

What Is an ECG and Why Does It Matter?

An ECG records the electrical activity of the heart from multiple angles using electrodes placed on the skin. Each electrode detects tiny electrical signals produced as the heart muscle contracts and relaxes. These signals are plotted against time to produce the waveform pattern you see on the paper or screen.

A standard 12-lead ECG gives you 12 different views of the heart simultaneously — similar to taking photographs of a building from 12 different angles. This allows you to localise problems precisely, whether it is an anterior MI, a lateral STEMI, or a posterior infarct.

Reading an ECG systematically is a core clinical skill. Whether you are an FY1 doctor covering the wards at night, a GP managing a patient with palpitations, or a medical student preparing for finals, having a reliable method prevents missed diagnoses.

The ECG Paper: Understanding the Grid

Before interpreting any waveform, you must understand what the squares mean. ECG paper runs at a standard speed of 25 mm per second.

• Small square = 1 mm = 0.04 seconds horizontally, 0.1 mV vertically
• Large square = 5 mm = 0.2 seconds horizontally, 0.5 mV vertically
• One second = 5 large squares
• Standard calibration = 10 mm = 1 mV (check the calibration box at the start of the trace)

Always check the calibration marker first. If the calibration is set to half standard (5 mm = 1 mV), all your voltage measurements will be doubled in reality. Missing this is a classic beginner mistake.

The Systematic Approach: 8 Steps to Read Any ECG

The most important thing about ECG interpretation is consistency. Use the same method every time — even when the answer seems obvious from a quick glance. Missed findings are almost always the result of skipping steps.

Step 1 — Rate

The normal heart rate is 60–100 beats per minute (bpm). Below 60 is bradycardia. Above 100 is tachycardia.

The 300 method (regular rhythms): Divide 300 by the number of large squares between two consecutive R waves. For example, if there are 4 large squares between R waves, the rate is 300 ÷ 4 = 75 bpm.

The 6-second method (irregular rhythms): Count the number of QRS complexes in a 30-large-square strip (6 seconds) and multiply by 10. This is less precise but reliable for atrial fibrillation.

Step 2 — Rhythm

Is the rhythm regular or irregular? Use a piece of paper or callipers to mark off several RR intervals and compare them. A regular rhythm has consistent, equal spacing. An irregular rhythm varies.

Common irregular rhythms include:

• Atrial fibrillation — irregularly irregular, no P waves
• Atrial flutter with variable block — saw-tooth flutter waves at 300 bpm
• Second-degree AV block (Mobitz I) — progressively lengthening PR then dropped beat
• Ectopic beats — occasional early beats disrupting the pattern

Step 3 — Axis

The cardiac axis describes the overall direction of electrical activity through the ventricles. Normal axis is between -30° and +90°.

Quick method using leads I and aVF:

• Both positive → Normal axis
• Lead I positive, aVF negative → Left axis deviation (LAD)
• Lead I negative, aVF positive → Right axis deviation (RAD)
• Both negative → Extreme axis deviation (rare)

Left axis deviation is seen in left bundle branch block, inferior MI, and left anterior hemiblock. Right axis deviation is seen in right ventricular hypertrophy, pulmonary embolism, and left posterior hemiblock.

Step 4 — P Wave

The P wave represents atrial depolarisation — the electrical signal spreading across the atria before they contract. A normal P wave is:

• Upright in leads I, II, aVF
• Inverted in aVR (normal)
• Duration less than 0.12 seconds (3 small squares)
• Amplitude less than 2.5 mm in limb leads

What to look for:

• Absent P waves → Atrial fibrillation or sinoatrial block
• Inverted P waves in II → Junctional rhythm (origin near AV node)
• Bifid P waves (P mitrale) → Left atrial enlargement
• Peaked P waves (P pulmonale) → Right atrial enlargement

Step 5 — PR Interval

The PR interval is measured from the start of the P wave to the start of the QRS complex. It represents the time taken for the impulse to travel from the SA node through the AV node and His-Purkinje system.

Normal PR interval: 0.12–0.20 seconds (3–5 small squares)

• Short PR (<0.12s) → Pre-excitation (Wolff-Parkinson-White), junctional rhythm
• Long PR (>0.20s) → First-degree AV block (benign in isolation)
• Progressively lengthening PR then dropped QRS → Mobitz I (Wenckebach)
• Fixed long PR with occasional dropped QRS → Mobitz II (more sinister)
• No relationship between P and QRS → Complete (third-degree) heart block

Step 6 — QRS Complex

The QRS complex represents ventricular depolarisation — the electrical activation of both ventricles. It is the most important part of the ECG for diagnosing serious pathology.

Normal QRS duration: less than 0.12 seconds (3 small squares)

A wide QRS (>0.12s) means the ventricles are depolarising abnormally. This occurs in:

• Left bundle branch block (LBBB)
• Right bundle branch block (RBBB)
• Ventricular tachycardia
• Pre-excitation (WPW)
• Hyperkalaemia
• Sodium channel blocker toxicity (tricyclic antidepressants)

Pathological Q waves are wider than 0.04s (one small square) or deeper than 25% of the following R wave. They indicate previous myocardial infarction and are found in the territory of the infarct.

R wave progression should increase from V1 to V5. Poor R wave progression (dominant S waves persisting into V4–V5) suggests anterior MI or left ventricular hypertrophy.

Step 7 — ST Segment

The ST segment represents the period between ventricular depolarisation and repolarisation. This is where you look for myocardial ischaemia and infarction.

The ST segment is measured relative to the TP segment (isoelectric baseline). Normal ST segments sit on or very close to the baseline.

ST elevation (>1 mm in limb leads, >2 mm in chest leads):

• STEMI — convex (tombstone) elevation in a coronary territory
• Pericarditis — saddle-shaped elevation in multiple leads without territory
• Left ventricular aneurysm — persistent elevation after old MI
• Benign early repolarisation — seen in young men, concave elevation, notching

ST depression (>0.5–1 mm):

• NSTEMI or unstable angina — horizontal or downsloping depression
• Digoxin effect — reverse tick (Salvador Dalí moustache) pattern
• Reciprocal changes in STEMI
• Left ventricular hypertrophy with strain

Step 8 — T Wave and QT Interval

The T wave represents ventricular repolarisation. Normal T waves are:

• Upright in leads I, II, V4–V6
• Inverted in aVR (normal)
• Variable in III, aVL, V1–V3

T wave abnormalities:

• Tall peaked T waves → Early hyperkalaemia, hyperacute STEMI
• T wave inversion → Ischaemia, PE (V1–V4), RBBB, LVH strain
• Flattened T waves → Hypokalaemia, hypothyroidism

The QT interval is measured from the start of the QRS to the end of the T wave. It must be corrected for heart rate using the Bazett formula to give the QTc.

Normal QTc: less than 440 ms in men, less than 460 ms in women

A prolonged QT interval increases the risk of Torsades de Pointes — a potentially fatal arrhythmia. Common causes include hypokalaemia, hypomagnesaemia, hypocalcaemia, and numerous medications including antipsychotics, antihistamines, and antiarrhythmics.

Localising an MI: Which Leads, Which Artery?

One of the most important applications of ECG reading is localising myocardial infarction. The 12 leads view the heart from different angles, allowing you to identify which coronary artery is occluded.

Bundle Branch Blocks: LBBB vs RBBB

Bundle branch blocks occur when conduction down one of the main branches of the His-Purkinje system is delayed or blocked. The result is a wide QRS complex (>0.12s) with characteristic patterns.

Right Bundle Branch Block (RBBB) — RSR’ pattern (rabbit ears) in V1, wide S wave in V5–V6, lead I. RBBB can be a normal variant, but new RBBB in the context of chest pain suggests pulmonary embolism or anterior MI.

Left Bundle Branch Block (LBBB) — Broad notched R waves in V5–V6, I, aVL. Deep S in V1. LBBB makes it impossible to interpret the ST segments reliably for ischaemia. New LBBB with chest pain should be treated as a STEMI equivalent and triggers the same pathway.

A simple memory aid: WiLLiaM MaRRoW

• W pattern in V1 and M pattern in V6 → LBBB
• M pattern in V1 and W pattern in V6 → RBBB

Common ECG Findings You Must Know

Beyond MI and blocks, several other ECG patterns appear frequently on the wards and in exams:

Atrial fibrillation: No P waves, irregularly irregular rhythm, fibrillatory baseline. The most common sustained cardiac arrhythmia — affects 1 in 4 people over their lifetime.

Atrial flutter: Saw-tooth flutter waves at 300 bpm (best seen in II, III, aVF). Usually conducted with 2:1 block giving a ventricular rate of 150 bpm. If a tachycardia is exactly 150 bpm, always think flutter first.

Supraventricular tachycardia (SVT): Narrow complex tachycardia at 150–250 bpm with no visible P waves (buried in QRS). Regular rhythm. Responds to vagal manoeuvres or adenosine.

Ventricular tachycardia (VT): Wide complex tachycardia. Any wide complex tachycardia should be treated as VT until proven otherwise — this is the safe approach. VT is a medical emergency.

Hyperkalaemia: Classic progression — tall peaked T waves → PR prolongation → wide QRS → sine wave pattern → cardiac arrest. Check electrolytes whenever you see unexplained wide QRS.

A Practical ECG Reading Template

Use this checklist every time you read an ECG. Writing it out in your notes demonstrates systematic thinking and protects you medicolegally.

1. Patient details and date — always confirm you have the right patient and ECG
2. Rate — bpm, bradycardia or tachycardia?
3. Rhythm — regular, irregular, regularly irregular?
4. Axis — normal, LAD, RAD?
5. P waves — present, morphology, relationship to QRS?
6. PR interval — normal, short, long, variable?
7. QRS — duration, morphology, Q waves, R progression?
8. ST segment — elevation, depression, territory?
9. T waves — normal, inverted, peaked?
10. QTc — normal or prolonged?
11. Impression — one sentence summary with clinical correlation

Tips to Get Better at Reading ECGs Fast

Like all clinical skills, ECG reading improves dramatically with practice. Here are the most effective ways to build confidence quickly:

Read ECGs every day. Even five minutes on a free ECG app or website builds pattern recognition over time. The brain learns to recognise abnormalities faster when exposed to normal ECGs repeatedly.

Always read the ECG before looking at the report. The computer interpretation is often wrong or incomplete. Form your own opinion first, then compare — this is how you learn.

Present ECGs out loud. Saying “rate 78, regular rhythm, normal axis, normal P waves, PR 160ms, narrow QRS, no ST changes, normal T waves — normal sinus rhythm” trains you to be systematic even under pressure.

Correlate with the clinical picture. An ECG never exists in isolation. A patient with chest pain, diaphoresis and new ST elevation is an emergency. The same ST elevation in an asymptomatic 22-year-old athlete is almost certainly benign early repolarisation.

Frequently Asked Questions

What is the difference between ECG and EKG?
Nothing — they are exactly the same test. ECG comes from the English term electrocardiogram. EKG comes from the German elektrokardiogramm. Both are used interchangeably worldwide.

How long does it take to learn to read ECGs?
With systematic practice, most medical students can read basic ECGs confidently within 4–6 weeks. Complex arrhythmia interpretation takes longer but develops naturally with clinical experience.

What is a normal ECG?
A normal ECG shows sinus rhythm (P wave before every QRS, regular rate 60–100 bpm), normal axis, narrow QRS, no ST changes, and normal T waves. However, many normal variants exist — always interpret in clinical context.

Can a normal ECG rule out a heart attack?
No. Up to 20% of MIs present with a normal initial ECG. Serial ECGs and troponin measurements are essential in any patient with suspected acute coronary syndrome. A normal ECG never rules out MI.

Conclusion

Reading an ECG is a skill, not a talent. With a systematic approach — rate, rhythm, axis, P wave, PR interval, QRS, ST segment, T wave, QTc — you can work through any ECG confidently and safely. The key is consistency: use the same method every single time, even when the trace looks normal at first glance.

Practice daily, correlate with clinical findings, and never skip steps under pressure. The ECG that looks “probably fine” at 3 am is the one that catches you out. Be systematic. Be safe.

References:
1. Hampton JR. The ECG Made Easy. 9th ed. Elsevier; 2019.
2. Thaler MS. The Only EKG Book You’ll Ever Need. 9th ed. Wolters Kluwer; 2018.
3. Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th ed. Saunders; 2008.
4. ESC Guidelines for the management of acute coronary syndromes. Eur Heart J. 2023.