Do 1st (first) and 2nd (second) degree heart block result in a normal QRS (QRS complex) while 3rd (third) degree results in a wide QRS (QRS complex)?

QRS Complex Characteristics in Different Degrees of Heart Block

First and second-degree heart blocks typically present with normal QRS complexes, while third-degree heart block often presents with wide QRS complexes, though this depends on the anatomical location of the block.

Understanding AV Block and QRS Morphology

First-Degree AV Block

• First-degree AV block is characterized by prolonged PR interval (>200 ms) with all atrial impulses conducted to the ventricles 1
• If the QRS complex is narrow, the conduction delay is usually in the AV node 1
• If the QRS is wide, the conduction delay may be either in the AV node or in the His-Purkinje system 1
• Only a His bundle electrogram can precisely locate the site of delay when QRS is wide 1

Second-Degree AV Block

• Second-degree AV block occurs when one or more atrial stimuli are not conducted to the ventricles 1
• In Mobitz type I (Wenckebach):
  • Progressive PR interval prolongation until a P wave is not conducted 1
  • Usually associated with narrow QRS complexes 1
  • Block typically occurs at the level of the AV node 1
• In Mobitz type II:
  • PR interval remains constant before and after blocked P waves 1
  • Usually associated with wide QRS complexes 1
  • Block typically occurs in the His-Purkinje system 1

Third-Degree (Complete) AV Block

• No atrial stimuli are conducted to the ventricles 1
• Ventricles are depolarized by an escape rhythm 1
• QRS morphology depends on the site of the block:
  • If the escape rhythm originates above or within the His bundle, QRS complexes may be narrow 1
  • If the escape rhythm originates below the His bundle (more common), QRS complexes are typically wide 1

Clinical Implications and Anatomical Correlations

• The width of the QRS complex provides important clues about the anatomical location of the block 1:
  • Narrow QRS suggests AV nodal block (higher in conduction system)
  • Wide QRS suggests infra-nodal block (lower in conduction system)
• In third-degree AV block with wide QRS, the site of block is usually below the His bundle 1
• The width of native QRS complexes can predict paced QRS duration in patients requiring pacemakers for AV block 2

Important Clinical Considerations

• Type II second-degree block, especially with wide QRS, has higher risk of progressing to complete heart block 1, 3
• Permanent pacing is generally recommended for:
  • Symptomatic third-degree AV block 1
  • Type II second-degree block, particularly with wide QRS 1
• The prognosis of AV block depends on the site of block:
  • AV nodal blocks generally have better prognosis 1
  • Infra-Hisian blocks have poorer prognosis and higher risk of progression 1, 3

Diagnostic Pearls

• In cases with wide QRS and first or second-degree AV block, electrophysiological study may be required to determine the exact level of block 1
• 2:1 AV block cannot be classified as type I or II based on ECG alone since there's only one PR interval to examine before the blocked P wave 4
• Wide QRS in AV block may indicate more extensive conduction system disease and potentially worse outcomes with right ventricular pacing 2