How to Interpret an ECG - Lecture Notes

Introduction

• Presenter: Olivia from Geeky Medics
• Focus: Structured approach to ECG interpretation
• Recommendation: Watch video on how to record an ECG and subscribe for more videos.
• Structure: Covers basic principles, interpretation steps, and a case study.

Basic Principles of an ECG

• Purpose: Record electrical activity of the heart to identify pathology.
• How it works:
  • Electrodes placed on the patient record electrical activity.
  • Create graphical representation called ECG leads.
  • 12-lead ECG produces 12 graphs, but uses 10 electrodes.

Components of an ECG Cycle

• P Waves: Atrial depolarization (contraction).
• PR Interval: Time for activity to move from atria to ventricles.
• QRS Complex: Ventricular depolarization (contraction); consists of Q, R, and S waves.
• ST Segment: Isoelectric line; time between depolarization and repolarization of ventricles.
• T Wave: Ventricular repolarization.
• QT Interval: Time for ventricles to depolarize and repolarize.

ECG Interpretation Steps

Preliminary Checks

1. Confirm patient details (name, DOB).
2. Check date and time of ECG.
3. Verify ECG calibration.
4. Understand clinical context (symptoms like chest pain).

Step 1: Calculate Heart Rate

• Normal Rate: 60-100 bpm
• Methods:
  • Regular Rhythms: Count large squares in RR interval, divide 300 by this number.
  • Irregular Rhythms: Count QRS complexes in a 10-second strip, multiply by 6.

Step 2: Assess Heart Rhythm

• Types: Regular, regularly irregular, irregularly irregular.
• Assessment: Mark RR intervals, check for patterns.
• Examples: Atrial fibrillation (irregularly irregular).

Step 3: Cardiac Axis

• Definition: Direction of electrical activity.
• Normal Axis: -30° to +90°.
• Abnormal Axis:
  • Right Axis Deviation: +90° to +180°; often due to right ventricular hypertrophy.
  • Left Axis Deviation: -30° to -90°; often due to left ventricular hypertrophy or conduction abnormalities.

Step 4: Examine P Waves

• Questions: Are they present? Normal? Followed by QRS?
• Absence: Indicates atrial activity issues (flutter, fibrillation, etc.).

Step 5: PR Interval

• Normal Duration: 120-200 ms
• Prolonged PR Interval: Indicates AV block.
• Types of AV Block:
  • First Degree: Consistent prolongation, no dropped beats.
  • Second Degree Type 1 (Mobitz I): Progressive PR prolongation, eventual drop.
  • Second Degree Type 2 (Mobitz II): Consistent PR, intermittent dropped QRS.
  • Third Degree: No correlation between atria and ventricles.

Step 6: QRS Complex

• Width and Height: Normal QRS < 0.12 seconds.
• Morphology: Look for Delta waves, broad complexes.
• Bundle Branch Block:
  • Right Bundle Branch Block: RSR’ pattern in V1 (M shape), broad S wave in V6 (W shape).
  • Left Bundle Branch Block: Deep S wave in V1 (W shape), broad R wave in V6 (M shape).

Step 7: ST Segment

• Normal: Isoelectric line.
• Abnormalities: ST elevation or depression (indicates ischemia or infarction).

Step 8: T Waves

• Tall T Waves: Associated with hyperkalemia or acute MI.
• Inversion: Could be normal or indicate pathology.

Step 9: U Waves

• Appearance: After T wave; indicates bradycardia or electrolyte imbalance.

Documentation

• Include: Patient details, date/time of ECG, interpretation, impression, and plan.

Case Study Summary

• Context: Irregularly irregular rhythm, lack of P waves.
• Diagnosis: Atrial fibrillation (indicated by irregular rhythm and absent P waves).

Conclusion

• Challenge: ECG interpretation can be complex but structured approaches aid identification of pathology.
• Resources: Visit Geeky Medics website for more ECG guidance and practice.

Note: Further detailed guidance and practice can be found on the Geeky Medics website and textbook.