Atrioventricular (AV) Block
- Atrioventricular (AV) block occurs due to delayed conduction of an impulse between the atria or ventricles, due to intrinsic or extrinsic causes.
Causes of Atrioventricular (AV) Block
- Idiopathic degeneration
- Myocardial infarction
- Congenital - congenital heart disease, neonatal lupus
- Infiltrative disease - sarcoidosis, amyloidosis, haemochromatosis
- Cardiac procedures - valvular surgery, correction of congenital heart disease, catheter ablation
- Systemic lupus erythematosus
- Physiologic - sleep, athletes
- Autonomic - carotid sinus massage, carotid sinus hypersensitivity
- Drugs - beta blockers, calcium channel blockers, digoxin, adenosine, amiodarone
- Physiologic and autonomic causes of AV block tend to result in first degree or Mobitz I heart block, while pathologic causes tend to result in Mobitz II, advanced second degree or complete heart block.
1st Degree AV Block
- First degree AV block indicates delayed conduction between the atria and the ventricles.
- Prolonged PR interval >200ms (5mm).
2nd Degree AV Block: Type I Mobitz
- Also known as Wenckebach AV block.
- Type I second degree AV block occurs when there is progressively delayed AV transmission eventually resulting in a missed QRS complex.
- Initial normal PR interval with progressive prolongation of PR interval followed by a dropped QRS complex.
2nd Degree AV Block: Type II Mobitz
- Type II second degree AV block occurs when there is an intermittently missed QRS complex due to failure of conduction through the AV node.
- Constant PR interval with intermittently dropped QRS complexes.
Advanced 2nd Degree Heart Block
- In advanced or ‘high-grade' second degree heart block, multiple beats are blocked and it is difficult to distinguish between Mobitz type I and II.
- Two or more consecutive P waves without QRS complexes
- May be in a 3:1, 4:1 or higher pattern
3rd Degree AV Block
- Complete atrioventricular block represents complete failure of conduction between the atria and ventricles.
- Complete dissociation of P waves and QRS complexes.