Atrioventricular Block (AV Block)

Key Points ⚡

AV block involves interruption of electrical conduction from atria to ventricles.
First-degree AV block: PR interval >0.20s consistently, no dropped beats; usually asymptomatic; causes include increased vagal tone, MI, Lyme disease, AV blocking drugs; managed by stopping AV blocking drugs.
Second-degree AV block Type 1 (Mobitz I / Wenckebach): Progressive PR prolongation until a QRS drop; usually benign; causes include vagal tone, drugs, MI; managed by stopping AV blocking drugs.
Second-degree AV block Type 2 (Mobitz II): Constant PR interval with intermittent dropped QRS complexes; always pathological; causes include MI, bundle branch disease; requires cardiac monitoring, pacing, or permanent pacemaker.
Third-degree (complete) AV block: No atrioventricular conduction; atria and ventricles beat independently; causes include congenital heart disease, ischemia, drugs, infections; requires urgent pacing and permanent pacemaker.
Symptoms in higher-grade blocks include palpitations, syncope, shortness of breath, chest pain.
Complications: progression to complete block, sudden cardiac death, symptomatic bradycardia.

AV block refers to impaired conduction from atria to ventricles causing characteristic ECG changes.
Idiopathic fibrosis is most common cause.
Investigations include ECG, labs (FBC, U&Es, TSH, troponin), echocardiogram.

Definition: Prolonged PR interval >0.20s, no dropped QRS.
Causes: Enhanced vagal tone, MI, Lyme disease, SLE, congenital, myocarditis, electrolyte disturbances, AV nodal blocking drugs (beta-blockers, CCBs, digoxin, magnesium), thyroid dysfunction.
ECG: Regular rhythm, prolonged PR, normal QRS.
Clinical: Usually asymptomatic.
Management: Stop AV blocking drugs; pacemaker if symptomatic.
Complications: Rare progression; increased atrial arrhythmias possible.

Definition: Progressive PR prolongation until a dropped QRS.
Causes: Increased vagal tone, drugs (beta-blockers, CCBs, digoxin, amiodarone), inferior MI, myocarditis, cardiac surgery.
ECG: Irregular rhythm, progressive PR lengthening, dropped QRS.
Clinical: Usually asymptomatic; possible pre-syncope/syncope.
Management: Stop AV blocking drugs; pacing if symptomatic.
Complications: Rare haemodynamic compromise.

Definition: Constant PR interval, intermittent dropped QRS complexes.
Causes: MI, idiopathic fibrosis, cardiac surgery, autoimmune diseases, hyperkalaemia, drugs, thyroid dysfunction.
ECG: Irregular rhythm (may be regularly irregular in 3:1 or 4:1 block), normal or broad QRS.
Clinical: Palpitations, pre-syncope, syncope.
Management: Cardiac monitoring, investigate cause; temporary pacing/isoprenaline if unstable; permanent pacemaker usually required.
Complications: Progression to complete heart block, asystole.

Definition: No conduction between atria and ventricles; AV dissociation.
Causes: Congenital (structural heart disease, maternal SLE), idiopathic fibrosis, ischemic heart disease, cardiomyopathies, infiltrative diseases (sarcoidosis, amyloidosis), post-cardiac surgery/ablations, drugs, infections (Lyme, Chagas), autoimmune, thyroid disease.
ECG: Variable rhythm, P waves present but unrelated to QRS, narrow or broad QRS depending on escape pacemaker site.
Clinical: Palpitations, syncope, confusion, SOB, chest pain, profound bradycardia, haemodynamic compromise.
Management: Cardiac monitoring, transcutaneous or temporary pacing, atropine may help narrow QRS escapes; permanent pacemaker insertion required.
Complications: Sudden cardiac death from ventricular arrhythmias.