Second Degree Atrioventricular Block

Second degree AV block is intermittent failure of atrial impulses to conduct to the ventricles.

Mobitz type I (Wenckebach): progressive PR prolongation until a dropped beat; block usually within the AV node. Often benign, seen with high vagal tone (athletes, sleep), inferior infarction, drugs (beta blockers, calcium channel blockers, digoxin), myocarditis.

Mobitz type II: constant PR interval with sudden non conducted P wave; block infranodal (His Purkinje), usually wide QRS; may progress to complete heart block. Causes: anteroseptal infarction, degenerative disease, cardiomyopathy, sarcoid, post cardiac surgery.

2:1 block: cannot be classified from the surface ECG; wide QRS favours infranodal block.

High grade (advanced) block: two or more consecutive non conducted P waves with some conducted beats.

Prognosis: type I benign if asymptomatic; type II carries high risk of progression and syncope or sudden death.

• Often asymptomatic (type I), incidental ECG finding.
• Fatigue, lightheadedness, presyncope, syncope, dyspnoea, chest pain, bradycardia, irregular pulse with dropped beats.
• Type II: Stokes Adams attacks, heart failure.

• ECG:
  • Type I: group beating, progressively shorter RR intervals, longest PR before the dropped beat, narrow QRS usually.
  • Type II: fixed PR before and after a dropped QRS, typically with bundle branch block.
• Ambulatory monitoring (Holter, loop recorder).
• Exercise test: Wenckebach improves with exercise (nodal), infranodal block worsens.
• Atropine response: improvement indicates nodal block, worsening suggests infranodal.
• Evaluate reversible causes: drugs, electrolytes, Lyme disease, thyroid, ischaemia, sleep apnoea.
• Echocardiography and cardiac MRI for structural disease; electrophysiology study (HV interval over 70 ms, infra Hisian block) in uncertain cases.

• Type I asymptomatic: observation, no pacing. Remove reversible causes (reduce dose or stop nodal blockers). Athletes with block at rest that resolves on exercise need no treatment.
• Symptomatic type I: atropine 0.5 mg IV acutely; permanent pacing if persistent and causing symptoms.
• Type II, 2:1 infranodal and high grade block: permanent pacemaker (dual chamber) even if asymptomatic. Temporary transcutaneous or transvenous pacing as bridge, especially in acute anterior infarction.
• Acute inferior infarction with type I block: observe; atropine only if symptoms.
• Lyme carditis: ceftriaxone 2 g IV daily or doxycycline 100 mg twice daily for 14 to 21 days; temporary pacing if needed.
• Conduction system pacing or biventricular pacing for patients with reduced ejection fraction (anticipated high pacing burden).
• Avoid AV nodal blockers in untreated type II block.
• Follow up with device interrogation and ECG.