Interpretation of ECGs

Introduction

• ECG/EKG interpretation is a crucial skill often underestimated.
• Each ECG component represents electrical activity in the heart corresponding to the cardiac cycle.

ECG Basics

• P wave: Atrial depolarization.
• 12 lead ECG: Uses four limb electrodes and six chest electrodes.
  • Lateral heart: Leads 1, AVL, V5, V6
  • Inferior heart: Leads 2, 3, AVF
  • Septal and anterior view: Leads V1 to V4

Preliminary Checks

• Confirm patient information, date, and calibration.
  • Calibration: Typically 25 mm/s, 10 mm/mV.

ECG Interpretation Steps

1. Heart Axis

• Definition: Overall direction of electrical activity.
• Range: -30 to +90 degrees.
• Quadrant Method:
  • Lead 1 & AVF positive: Normal axis
  • Lead 1 negative, AVF positive: Right axis deviation
  • Both negative: Extreme axis deviation
  • Lead 1 positive, AVF negative: Evaluate Lead 2
    • Lead 2 negative: Left axis deviation

2. Heart Rate

• Measurement:
  • Each small square = 40 ms.
  • Large square = 200 ms (5 small squares).
  • Calculate rate: 300 divided by number of large squares per QRS.
  • For irregular rhythms: Count QRS complexes over 10 seconds, multiply by 6.

3. Rhythm

• Regularity: Check RR intervals.
• Irregularity Types:
  • Irregularly irregular (e.g., Atrial fibrillation)
  • Regularly irregular (e.g., some heart blocks)
• P Wave and PR Interval:
  • Normal P wave positive in Lead 2, <120 ms duration, <2.5 mm amplitude.
  • PR Interval: 120-200 ms
  • Abnormalities can indicate atrial dilation or blocks.

4. QRS Complex

• Ventricular Depolarization: Narrow or widened.
  • Narrow: Supraventricular origin
  • Wide: Ventricular origin or conduction block
• Special Cases:
  • Bundle branch blocks: William-Marrow mnemonic
  • Amplitude variations can indicate pathology.

5. ST Segment & J Point

• ST Segment:
  • Represents the interval between ventricular depolarization and repolarization.
  • Elevation can indicate myocardial infarction or pericarditis.
  • Check for reciprocal changes.
• J Point:
  • Transition point between S wave and ST segment.
  • Benign early repolarization in younger individuals.

6. T Wave

• Repolarization: Normally positive, concordant with QRS.
• Abnormal T Waves:
  • Inverted T waves can indicate past ischemic events.
  • Tall T waves in hyperkalemia.
  • Biphasic T waves indicate ischemia or hypokalemia.

7. QT Interval

• Duration: From start of Q wave to end of T wave.
• Correction: Use Bizet's formula.
  • Normal: 360-440 ms (males), 360-460 ms (females).
• Consequences of Prolongation: Predisposes to arrhythmias.

8. U Waves

• Occasional post-T wave phenomenon linked to electrolytes or hypothermia.

Arrhythmia Categorization

• Tachycardia: Broad/narrow, regular/irregular.
• Bradycardia: Presence/absence of P waves, P-QRS relationship.

Final Considerations

• ECG is a snapshot; correlate with patient history and presentation.
• Consider longer monitoring for transient issues.

Resources for Further Learning

• Recommended: Life in the Fast Lane website for ECG practice and improvement.