Conduction Blocks

Sinus (SA) Block

• An unhealthy Sinus (SA) Node misses one or more cycles.
• The SA Node usually resumes pacing, but the pause may evoke an “escape” response from an automaticity focus.

AV Block

• 1° AV Block: PR interval greater than .2 s.
• 2° AV Block: some P waves without QRS response
• Wenckebach - PR gradually lengthens with each series does not produce a QRS.
• Mobitz - some P waves don’t produce a QRS response. More advanced Mobitz block may produce a 3:1 (AV) pattern or even higher AV ratio.
• 3° AV Block: no P wave produces a QRS response
• P waves—SA Node origin.
• Rate of QRS determined by AV node (40-60 bpm) or ventricles (20-40 bpm)

Bundle Branch Block

• QRS greater than 0.12 sec.
• Right BBB: R and R' in V1 or V2
• Left BBB: R and R' in V5 or V6
• With Bundle Branch Block the criteria for ventricular hypertrophy are unreliable
• With Left BBB infarction is difficult to determine on EKG.