Mobitz Type II (Second Degree Block)

Last updated 12/08/19

Definition

• Second degree atrioventricular block with constant PR interval and intermittent failure of ventricular conduction


P without QRS

Features

• Compared to Mobitz I (Wenckebach)
• More commonly associated with haemodynamic compromise
• Severe bradycardia
• Progression to complete heart block (CHB)
• More likely caused by structural damage
• Patients often have pre-existing left bundle branch block (LBBB) or bifascicular block
• Mobitz II manifests from intermittent failure of remaining fascicle
• Block may worsen due to
• Exercise
• Atropine
• May assist differentiating Mobitz I from Mobitz II
• Atrial pacing
• Block may improve from vagal manoeuvres
• Slows sinus rate and allows recovery of bundle of His

Causes

• Conduction system disease below level of AV node (mostly)
• Bundle of His 25%
• Bundle branches 75%
• Associated with axis deviation and wide QRS
• Acute myocardial infarction (AMI) effecting conduction system
• More common in anterior infarcts with septal involvement
• Cardiomyopathy: particularly amyloidosis, sarcoidosis
• Myocarditis
• Endocarditis with abscess formation
• Hyperkalaemia
• Hypothyroidism
• Hypervagotonia
• AV Nodal locking agents
• Digoxin
• Beta blockers
• Calcium channel blockers (non-dihydropyridine)
• Amiodarone
• Post cardiac surgery/catheter ablation
• Idiopathic progressive cardiac conduction disease with fibrosis/sclerosis
• Lenegre's disease
• Lev's disease
• +/- Genetic trait: SCN5A gene

Associated Signs & Symptoms

• Fatigue
• Dyspnoea
• Chest pain
• Presyncope or syncope (Stokes-Adams attacks)

Complications

• Bradycardia
• Cardiogenic shock
• Heart failure
• Bifascicular block
• Trifascicular block
• CHB: high risk of deterioration
• If more than 1 failed conduction in sequence
• Sudden cardiac arrest (SCA)
• Risk 35% per year

Differential Diagnosis

• First degree heart block
• All P waves are followed by QRS complex
• Mobitz type I (Wenckebach)
• Progressive prolongation of PR interval
• Cannot be differentiated if 2:1 block present
• CHB
• P waves have no relationship to QRS complex (dissociated)
• Sinoatrial exit block
• Non-conducted premature atrial complexes

Investigations

• Vitals
• HR
• Bradycardia
• Or sinus rate
• Irregular pulse
• BP: hypotension: cardiogenic shock
• ECG
• PR interval
• Unchanging PR interval
• Usually normal but may be prolonged
• QRS
• Block of 1+ P waves with failure to conduct to ventricles
• Narrow complex: block at bundle of His
• Wide QRS: block below bundle of His (75% cases)
• +/- Troponin: elevated: AMI
• +/- BNP: elevated: heart failure
• +/- Digoxin level
• U&E: hyperkalaemia
• TSH: hypothyroidism
• +/- CXR: acute pulmonary oedema (APO)
• +/- Electrophysiological study
• May identify those at risk of CHB
• Can identify level of block

Management

• Consider temporary pacing as soon as Mobitz II is identified
• Patients require admission and cardiac monitoring
• Unstable patient: cardiogenic shock
• ALS
• Apply pacing pads
• IV access
• Management of bradycardia
• Atropine 500mcg IV STAT every 3-5 minutes (max 3mg)
• May not respond
• +/- Dobutamine infusion 3-20mcg/kg/min
• Or Adrenaline infusion
• Temporary pacing
• Treat reversible causes
• As indicated by investigations
• AMI treated with revascularisation mostly resolves
• If no reversible cause identified and not resolving: permanent pace macker (PPM)
• Usually dual chamber (atrioventricular)

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References

1. Sauer W. Second degree atrioventricular block: Mobitz type II. UpToDate. 26/06/2019
2. Burns E. AV Block: 2nd degree, Mobitz II (Hay block). Life in the Fast Lane. 20/05/2019
3. Mangi M, Jones W, Napier L. Atrioventricular Block Second-Degree. StatPearls [Internet]. 06/11/2019