Overview
This lecture covers cardiac conduction abnormalities, focusing on types and features of heart blocks, including bundle branch blocks and atrioventricular (AV) blocks.
Conduction Abnormalities Overview
- Conduction abnormalities are interruptions of electrical impulses along the heart’s conduction pathway.
- Causes include congenital defects, congestive heart failure (CHF), myocardial infarction (MI), and valve problems.
Bundle Branch Blocks & Intraventricular Conduction Delay (IVCD)
- Bundle branch blocks occur in the ventricles within the bundle branches, impeding ventricular conduction.
- Intraventricular Conduction Delay (IVCD) refers to delayed ventricular impulse conduction, detected if QRS > 0.12 seconds.
- Only a 12-lead EKG can determine whether the block is right or left bundle branch.
Atrioventricular (AV) Blocks
- AV blocks occur at the AV node and are categorized as first, second, or third degree blocks.
- First degree AV block: All impulses reach the ventricles but are delayed (PR interval > 0.20 seconds, regular rhythm).
- Second degree AV block Mobitz I (Wenckebach): PR interval progressively lengthens until a beat is dropped; irregular rhythm.
- Second degree AV block Mobitz II: Some atrial impulses fail to conduct to the ventricles; more P waves than QRS complexes; may occur with ratios like 2:1.
- Third degree (complete) heart block: No atrial impulses reach the ventricles; atria and ventricles contract independently; QRS often wide and slow (rate ~40 bpm).
Key Terms & Definitions
- Conduction Abnormality — disruption of electrical impulse travel within heart tissue.
- Bundle Branch Block — a block in the bundle branches of the ventricular conduction system.
- Intraventricular Conduction Delay (IVCD) — slowed conduction within the ventricles, seen as QRS > 0.12 sec.
- AV Block — impaired conduction at the AV node, classified into first, second (Mobitz I and II), and third degree.
- First Degree AV Block — delayed, but not blocked, conduction through AV node (PR > 0.20 sec).
- Second Degree AV Block (Mobitz I/Wenckebach) — PR interval increases before a missed beat.
- Second Degree AV Block (Mobitz II) — sudden, dropped ventricular beats without progressive PR lengthening.
- Third Degree AV Block — complete dissociation between atrial and ventricular activity.
Action Items / Next Steps
- Be able to recognize and differentiate heart blocks on EKG tracings.
- Review the process and criteria for identifying bundle branch blocks versus IVCD using a 12-lead EKG.
- Study the characteristics and key differences between first, second, and third degree AV blocks.