How to Interpret an ECG

Introduction

• Presenter: Olivia from Geeky Medics
• Objective: Guide to ECG interpretation
• Pre-requisite: Watch the video on how to record an ECG

Basic Principles of an ECG

• Purpose: Record electrical activity of the heart to identify and locate pathology
• Method: Electrodes placed on the patient create a graphical representation (ECG leads)
• 12-lead ECG: Produces 12 separate graphs but uses 10 electrodes

Components of an ECG Cycle

• P waves: Atrial depolarization (contraction), precedes each QRS complex
• PR interval: From start of P wave to start of Q wave, time for electrical activity to move from atria to ventricles
• QRS complex: Ventricular depolarization (contraction)
• ST segment: Isoelectric line, time between ventricular depolarization and repolarization
• T-wave: Ventricular repolarization
• QT interval: From start of QRS complex to end of T-wave, duration for ventricles to depolarize and repolarize

Pre-interpretation Checks

• Confirm patient’s name and date of birth
• Check date and time of ECG
• Verify ECG calibration
• Understand clinical context (symptoms, reason for ECG)

Structured ECG Interpretation

Heart Rate Calculation

• Normal: 60-100 beats per minute
• Tachycardia: >100 bpm
• Bradycardia: <60 bpm

Methods

1. RR Interval (Regular):
   • Count large squares within RR interval
   • Formula: 300 / number of large squares
   • Example: 7 large squares → 300 / 7 = 48 bpm
2. Standard ECG Strip (Irregular):
   • Count QRS complexes in 10-second strip
   • Multiply by 6
   • Example: 11 QRS complexes → 11 * 6 = 66 bpm

Rhythm Assessment

• Regularity: Regular or irregular
• Pattern: Regularly irregular vs. irregularly irregular

Cardiac Axis

• Normal Axis: -30° to +90°, positive deflection in lead I and II
• Right Axis Deviation: +90° to +180°, positive in lead III, AVR negative
  • Causes: Right ventricular hypertrophy, tall individuals
• Left Axis Deviation: -30° to -90°, positive in lead I, negative in lead II and III
  • Causes: Left ventricular hypertrophy, conduction abnormalities

P Waves

• Checklist: Presence, followed by QRS, normal appearance
• Absence Indicators: Sawtooth for flutter, chaotic baseline for fibrillation, flatline for no activity

PR Interval

• Normal: 120-200 ms (3-5 small squares)
• Prolongation: Suggests AV block

Types of AV Blocks

1. First Degree: PR interval >200 ms, every P wave followed by QRS, often asymptomatic
2. Second Degree Type 1 (Mobitz I/Wenckebach): Progressive PR prolongation & dropped QRS, usually benign
3. Second Degree Type 2 (Mobitz II): Consistent PR interval with intermittent dropped QRS, always pathological
4. Third Degree (Complete Block): No communication between atria and ventricles, independent function

Pathological Conditions

• Wolf Parkinson White Syndrome: Accessory pathway creating Delta wave, short PR, wide QRS
  • Risk: Tachyarrhythmias

QRS Complex

• Width: Normal <0.12 seconds, broad if abnormal depolarization
• Height: Small (<5 mm limb, <10 mm chest), tall (hypertrophy)
• Morphology: Delta wave, bundle branch block patterns (William-MarRRoW mnemonic)

Bundle Branch Blocks

1. Right Bundle Branch Block (RBBB): Broad QRS, rsR’ pattern in V1 (M shape), broad S in V6 (W shape)
2. Left Bundle Branch Block (LBBB): Deep S in V1 (W shape), broad R in V6 (M shape)

ST Segment

• Normal: Isoelectric line
• Elevation: >1 mm in limb leads, >2 mm in chest leads, suggests myocardial infarction
• Depression: Indicates myocardial ischemia
• Benign Variant: High takeoff

T Waves

• Tall: Hyperkalemia, hyperacute STEMI
• Inversion: Normal variants or pathological (myocardial ischemia, myocarditis)
• Normal Variants: Isolated inversion in AVR, V1, V3

U Waves

• Characteristics: >0.5 mm after T-wave, seen in electrolyte imbalances, hypothermia

Documentation

• Patient details, date and time, interpretation, impression, and plan

Case Study Example

• Details: Check patient details, clinical context, calibration
• Heart Rate: 102 bpm (17 QRS * 6)
• Rhythm: Irregularly irregular, no clear patterns (Atrial fibrillation)
• Cardiac Axis: Normal (positive in lead I and II)
• PR Interval: Not calculable (no P waves)
• QRS Complex: Normal width and height
• ST Segment and T-waves: Normal (no elevation or depression, T-waves normal)
• Diagnosis: Atrial Fibrillation