Interpretation of Electrocardiogram (ECG/EKG)

Overview

• ECG interpretation is a critical but often overlooked skill in medicine.
• Represents electrical activity in the heart, corresponding to different cardiac cycle points.

Components of ECG

• P wave: Atrial depolarization.
• Leads: 12-lead ECG
  • Four limb and six chest electrodes.
  • Different leads offer different heart views:
    • Leads 1, AVL, V5, V6: Lateral heart.
    • Leads 2, 3, AVF: Inferior heart.
    • Leads V1-V4: Septal and anterior heart.

Initial Steps in ECG Interpretation

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

Axis Interpretation

• Overall direction of electrical activity.
  • Typically between -30 and 90 degrees.
• Quadrant Method:
  • Lead 1 and AVF:
    • Both positive: Normal.
    • Lead 1 negative, AVF positive: Right axis deviation.
    • Both negative: Extreme axis deviation.
    • Lead 1 positive, AVF negative: Check Lead 2 for left axis deviation.

Analyzing Rate and Rhythm

• Rate:
  • Each small square: 40 ms, large squares: 200 ms.
  • 300 divided by large squares between QRS gives ventricular rate.
  • Normal rate: 60-100 bpm. Below 60: Bradycardia, Above 100: Tachycardia.
• Rhythm:
  • Regular intervals between beats.
  • RR Interval: Regular vs irregular rhythm.
  • Irregularly irregular: Atrial fibrillation.
  • Regularly irregular: Some second-degree heart blocks.

P Wave and PR Interval

• P Wave:
  • Positive in Lead 2, duration < 120 ms, amplitude < 2.5 mm.
  • Abnormalities: Increased duration/amplitude, absence (e.g., atrial fibrillation).
• PR Interval:
  • Time from atrial to ventricular depolarization.
  • Normal: 120-200 ms.
  • Prolongation: First degree AV block.
  • Shortening: Accessory pathway (e.g., Wolff-Parkinson-White).

QRS Complex

• Represents ventricular depolarization.
• Morphology:
  • Narrow: Supraventricular origin.
  • Wide: Ventricular origin or conduction block.
• Voltage:
  • Large amplitude: Left ventricular hypertrophy.
  • Alternating amplitude: Pericardial effusion.
• Q Waves:
  • Pathological if > 25% of QRS and > 40 ms.

ST Segment and T Waves

• ST Segment:
  • Interval between ventricular depolarization and repolarization.
  • Elevation: ST elevation myocardial infarction (STEMI), pericarditis.
  • Depression: Ischemia.
• T Waves:
  • Represent ventricular repolarization.
  • Inversions indicate possible ischemia.
  • Tall T waves: Hypokalemia.

QT Interval and U Waves

• QT Interval:
  • From start of Q wave to end of T wave.
  • Normal: 360-440 ms (males), <460 ms (females).
  • Importance: Prolongation can lead to arrhythmias.
• U Waves:
  • Can appear after T waves due to electrolyte imbalances or hypothermia.

ECG as a Diagnostic Tool

• Categorizes rhythm: Tachycardia (broad/narrow complex), Bradycardia.
• A snapshot of the heart's activity; must correlate with patient history.
• Longer monitoring (e.g., Holter monitor) may be needed for arrhythmias not caught during ECG.

Further Reading

• Recommended: Life in the Fast Lane website for comprehensive ECG resources.