Approach to ECG Interpretation

Clinical guide

Approach to ECG Interpretation

rate → rhythm → axis → intervals → morphology → ischemia → clinical correlation

Step 1

Confirm ECG setup

  • Patient name, date, time of recording
  • Paper speed: 25 mm/s
  • Calibration: 10 mm/mV
  • Check lead placement and rule out artifacts
Standard 12-lead ECG layout
Step 2

Determine heart rate

  • Normal: 50–100 bpm
  • Regular rhythm
    • use the 300 rule: 300 / number of large squares between two R waves
  • Irregular rhythm
    • count the number of R waves in a 10-second strip and multiply by 6
  • Escape rhythms
    • atrial: 60–80 bpm
    • junctional: 40–60 bpm
    • ventricular: 20–40 bpm
Step 3

Assess rhythm

  • Regular vs. irregular vs. irregularly irregular
  • Identify the P wave before every QRS and the QRS after every P wave
  • Sinus rhythm: P upright in leads I, II, and aVF + rate 50–100 bpm
  • Irregularly irregular = think AFib
Step 4

Evaluate axis

  • Look at leads I and aVF:
    • lead I +, aVF + → Normal axis
    • lead I +, aVF − → Left axis deviation (LAD)
    • lead I −, aVF + → Right axis deviation (RAD)
    • lead I −, aVF − → Extreme axis
  • Normal axis: −30° to +90°
Step 5

Measure intervals

  • PR interval: 120–200 ms (3–5 small squares)
  • QRS duration: ≤120 ms
  • QT interval:
Step 6

Assess P wave morphology

  • Left atrial enlargement (LAE)
    • bifid P wave in lead II
    • biphasic P in V1 (terminal negative portion >1 mm)
  • Right atrial enlargement (RAE)
    • tall, peaked P waves in II and V1 (>2.5 mm)
Step 7

Examine QRS morphology

  • Bundle branch blocks (BBB)
    • LBBB: broad, notched R in V5/V6; deep S in V1
    • RBBB: rsR’ in V1; wide S in V6
  • LVH criteria
    • S in V1 + R in V5 or V6 >35 mm
  • RVH criteria
    • RAD + R/S >1 in V1
QRS morphology
LBBB vs RBBB
Hypertrophy criteria
Step 8

Review ST segment and T waves

  • Ischemia: ST depression, T wave inversion (V1–V6)
  • Infarction: ST elevation in the affected area
  • Reciprocal changes in opposite leads
  • T wave changes: hyperacute (early), inverted (later)
ST segment morphology
ST and T wave patterns
Regional ischemia patterns
T wave evolution

All images are from ecgwaves.com

Step 9

Look for pathologic Q waves

  • 1 small square wide
  • 25% of R wave height
  • Present in ≥2 contiguous leads
Step 10

Check for miscellaneous patterns

  • Electrolytes
    • hyperK: tall T waves → sine wave
    • hypoK: U waves, ST depression, long QT
    • hypoCa: long QT; hyperCa: short QT
  • Drugs: digoxin (scooped ST), antipsychotics (QT prolongation)
  • Others
    • Osborne J waves (hypothermia)
    • diffuse ST elevation + PR depression (pericarditis)
Final step

Clinical correlation

  • Summarize: rate, rhythm, axis, intervals, major abnormalities
  • Compare to previous ECGs
  • Correlate with symptoms and clinical scenario
  • For related cases, explore approach to palpitations and chest pain to connect ECG findings with real clinical decisions

References & further reading

  1. Life in the Fast Lane, ECG Library

    Huge, easy-to-navigate library with annotated ECG examples and clinical pearls.

    litfl.com/ecg-library

  2. ECG Waves, Clinical ECG Interpretation

    Detailed step-by-step explanations, beautiful visuals, and case examples.

    ecgwaves.com

  3. Geeky Medics, ECG Interpretation Guide

    Concise and OSCE-friendly; great for beginners and quick refreshers.

    geekymedics.com/how-to-read-an-ecg

  4. BMJ, ECG basics

    Evidence-based clinical approach in a structured review.

    bmj.com/content/357/bmj.j1720

  5. American Heart Association, ECG-related guidelines

    Searchable hub for official ECG-related guidance (STEMI, blocks, arrhythmias). Tip: search “ECG” or “STEMI criteria”.

    ahajournals.org

related · bradycardia · tachycardia · potassium · magnesium · calcium · palpitations

See also: MOC+ Volume 2: Cardiopulmonary, covers ACS, arrhythmias, and the ECGs that pin them down.

Last reviewed · June 2026

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