Key Facts

Second degree heart block describes intermittent failure of conduction from the atria to the ventricles. It is classified into Mobitz type I (Wenckebach) and Mobitz type II, which have significantly different clinical implications and management.

Epidemiology

• Mobitz type I: relatively common, especially in young/athletic individuals and during sleep
• Mobitz type II: less common, typically occurs in older patients with structural heart disease
• Mobitz II accounts for ~20-30% of second-degree blocks

Aetiology

Mobitz Type I:

• Increased vagal tone (athletes, sleep)
• Drugs: beta-blockers, CCBs, digoxin
• Inferior MI (transient AV nodal ischaemia)
• Myocarditis, rheumatic fever

Mobitz Type II:

• Degenerative conduction system disease (Lenegre/Lev disease)
• Anterior MI (septal branch of LAD supplies His-Purkinje)
• Post-cardiac surgery
• Infiltrative: sarcoidosis, amyloidosis
• Aortic valve calcification extending to conduction system

Pathophysiology

Mobitz I (Wenckebach):

• Block occurs at the level of the AV node
• Incremental delay in AV conduction until an impulse fails entirely
• QRS is usually narrow (unless pre-existing bundle branch block)

Mobitz II:

• Block occurs below the AV node (His bundle or bundle branches)
• Intermittent complete failure of infra-nodal conduction
• QRS is often wide (reflecting bilateral bundle branch disease)
• Higher risk of sudden progression to complete heart block

Mobitz Type I (Wenckebach)

• Often asymptomatic
• May cause mild dizziness or palpitations
• Irregularly irregular pulse (grouped beating pattern)

Mobitz Type II

• May be asymptomatic or present with presyncope, syncope, fatigue, dyspnoea
• Regular pulse with 'dropped' beats
• May present with sudden complete heart block and Stokes-Adams attacks

ECG Features

Mobitz I: Progressive PR prolongation, shortening RR intervals, dropped QRS, then cycle repeats (Wenckebach periodicity) Mobitz II: Fixed PR interval, sudden non-conducted P wave, often with wide QRS 2:1 block: Every other P wave conducted — needs clinical context to differentiate

Red Flags

• Syncope or presyncope (suggests haemodynamic compromise)
• Wide QRS with second-degree block (likely infra-nodal)
• New Mobitz II in context of anterior MI
• Rapid progression of PR prolongation or increasing dropped beats
• Associated symptoms of cardiac failure

Bedside

• 12-lead ECG: identify type I vs II pattern, QRS width
• Continuous cardiac monitoring: capture intermittent block
• Observations: HR, BP, symptoms

Bloods

• U&Es: potassium, magnesium
• Digoxin level: if applicable
• Troponin: if ischaemia suspected
• TFTs: hypothyroidism
• Lyme serology: if risk factors present

Imaging

• Echocardiography: structural heart disease, LV function
• Cardiac MRI: if sarcoidosis or myocarditis suspected

Special Tests

• Holter monitor: quantify frequency and duration of block, correlate with symptoms
• Exercise testing: Mobitz I that resolves with exercise is nodal (benign); Mobitz II that worsens is infra-nodal
• Electrophysiology study: rarely needed; to localise level of block in uncertain cases

Non-pharmacological

• Mobitz I: usually observation and reassurance; review medications
• Mobitz II: admit for cardiac monitoring
• Avoid AV nodal blocking drugs in both types if symptomatic

Pharmacological

• Atropine 500 mcg IV (up to 3mg): for acute symptomatic bradycardia; more effective in nodal (type I) block
• Atropine may be ineffective or worsen infra-nodal (type II) block
• Isoprenaline infusion: temporising measure for symptomatic Mobitz II awaiting pacing

Surgical/Interventional

• Temporary transvenous pacing: for symptomatic Mobitz II or haemodynamic instability, as a bridge to permanent pacing
• Permanent pacemaker: indicated for:
  • Mobitz type II (even if asymptomatic) — class I indication
  • Symptomatic Mobitz type I not responsive to drug withdrawal
  • 2:1 block with symptoms
  • Second-degree block with broad QRS

Referral Criteria

• Mobitz II: urgent cardiology referral for pacemaker assessment
• Symptomatic Mobitz I: cardiology referral
• Post-MI conduction disturbance: specialist monitoring

• Mobitz I: excellent prognosis in isolation; low risk of progression to complete heart block (~2-5% over years)
• Mobitz II: significant risk of progression to complete heart block (~35-50% without pacing)
• Post-inferior MI Mobitz I: usually transient, resolves within days-weeks
• Post-anterior MI Mobitz II: poor prognosis, indicates extensive myocardial damage
• Permanent pacemaker for Mobitz II normalises life expectancy
• 2:1 block prognosis depends on the level of block (nodal vs infra-nodal)

Comparison: Mobitz I vs Mobitz II