Learning Objective

After completing this section, learners will be able to differentiate first-, second, and third-degree heart blocks on ECG, describe the underlying conduction abnormalities, and recognize their characteristic ECG features.

First-Degree Heart Block

• ECG Finding: PR interval > 200 msec (long PR interval; >1 big box)
• Mechanism: Slowed conduction through the AV node
• Rate & Rhythm: Typically normal

Second-Degree Heart Block

• Every QRS is preceded by a P wave, but not every P wave is conducted
• Mechanism: Some impulses fail to pass through the AV node
• Types:

Mobitz Type I (Wenckebach)

• Progressive prolongation of the PR interval until a ventricular beat is missed
• Rhythm: unsteady (irregular)
• Cycle repeats after the dropped beat

Mobitz Type II

• PR interval is constant (does not lengthen)
• Rhythm: steady or unsteady depending on P: QRS ratio (e.g., 2:1, 3:1, 3:2)
• More likely to progress to complete heart block

Third-Degree Heart Block (Complete)

• ECG Finding: Complete dissociation of P waves and QRS complexes
• Mechanism: No impulses are transmitted through the AV node
• Rate & Rhythm:
  • Ventricular rhythm is usually slow
  • Atrial (P wave) rate differs from ventricular (QRS) rate
  • No consistent PR interval

Activity