Disrupted passage of electrical impulse through the AVN
1st-degree: prolonged PR interval, unchanging, no missed beats
2nd-degree (Mobitz I): PR interval becomes longer and longer until a QRS is missed. Then the pattern resets
2nd-degree (Mobitz II): QRSs are regularly missed (eg 2:1 block). This is dangerous as it will progress to complete heart block
3rd-degree: Complete heart block. No impulses passed from atria to ventricles → p waves and QRSs are independent. Tissue distal to AVN then starts initiating contractions but at a much slower rate
Causes/Factors
- 1st and 2nd Mobitz I may be normal and asymptomatic in athletes or due to blockers or Digoxin.
- May also be sick sinus syndrome 3rd degree:
- Coronary Artery Disease
- Aortic Valve Calcification
- Idiopathic Fibrosis
- Digoxin toxicity
Symptoms
- Light-headedness, pre-syncope/syncope
- Chest pain (esp. with co-existent Coronary Artery Disease)
- Signs of Heart Failure
Signs
- Bradycardia (<60bpm)
- Hypotension
- Cardiac syncope - losing consciousness for seconds then coming round not knowing what happened
Diagnostic Tests
- ECG see above
Management
- Atropine administration (blocks parasympathetic action)
- Rhythm altered drugs
- Pacemaker implantation