How to Read an ECG | ECG Interpretation | EKG

Introduction

• Purpose: Understanding how to read an ECG through a systematic approach.

Normal ECG

• Reference: Normal 12-lead ECG demonstrating sinus rhythm.

Initial Steps

• Confirm Patient Details: Name, date of birth, date/time of ECG.
• Check Calibration: Standard is 25mm/s and 10mm/1mV.

Heart Rate

Normal Adult Heart Rate

• Normal: 60-100 bpm
• Tachycardia: > 100 bpm
• Bradycardia: < 60 bpm

Calculating Heart Rate

• Regular Rhythm: Count large squares in R-R interval, divide 300 by this number.
  • Example: 4 large squares = 75 bpm
• Irregular Rhythm: Count complexes on 10-sec rhythm strip, multiply by 6.
  • Example: 10 complexes = 60 bpm

Heart Rhythm

• Can be regular or irregular.
• Irregular Types: Regularly irregular or irregularly irregular.
• Atrioventricular Block: Mark P and R waves to check PR interval changes or QRS omissions.

Cardiac Axis

• Description: Direction of electrical spread, normal: 11 to 5 o'clock direction.
• Axis Determination: Examine leads I, II, III.

Normal Axis

• Lead II: Most positive deflection.

Deviations

• Right Axis Deviation: Lead III positive, Lead I negative. Linked to right ventricular hypertrophy.
• Left Axis Deviation: Lead I positive, Leads II/III negative. Linked to conduction abnormalities.

P Waves

• Questions: Presence, QRS following, normal appearance, atrial activity.
• Hints: Absence and irregular rhythm may indicate atrial fibrillation.

PR Interval

• Normal Range: 120-200 ms (3-5 small squares).

Prolonged PR Interval

• Indication: Atrioventricular (AV) block.
• Types of AV Block:
  • First-degree: Fixed prolonged PR.
  • Second-degree Type 1 (Mobitz I): Progressive PR prolongation, QRS drop.
  • Second-degree Type 2 (Mobitz II): Consistent PR, intermittent QRS drop.
  • Third-degree (Complete Block): No atria-ventricles communication.

Shortened PR Interval

• Causes: Closer P wave origin to AV node or accessory pathway.

QRS Complex

Characteristics

• Width: Narrow (<0.12s) or Broad (>0.12s).
• Height: Small or Tall (ventricular hypertrophy).
• Morphology: Delta waves, Q-waves, R and S wave progression.

Specifics

• Bundle Branch Block: LBBB (deep S wave in V1, M-shaped R in V6) and RBBB (RSR in V1, broad S in V6).
• Pathological Q-wave: >25% R wave, >2mm height or >40ms width.

ST Segment

• Should be isoelectric.

Abnormalities

• Elevation: >1mm in limb leads or >2mm in chest leads, indicating myocardial infarction.
• Depression: >0.5mm in 2 contiguous leads, indicating ischaemia.

T Waves

Characteristics

• Tall T Waves: >5mm limb, >10mm chest, associated with hyperkalaemia or acute STEMI.
• Inverted T Waves: Normal in V1, Lead III; otherwise indicates multiple conditions.
• Biphasic and Flattened T Waves: Indicate ischaemia or electrolyte imbalance.

U Waves

• Uncommon: Seen in electrolyte imbalances, hypothermia, antiarrhythmic therapy.

Documentation

• Importance: Record interpretation in patient notes.

Additional Resources

• ECG Case Collection with 85 cases.
• Guides on understanding, recording, and documenting ECGs.

Contributor

• Dr Ben Marrow, Cardiology Registrar