AKT · Cardiovascular/Arrhythmias & conduction

Bradyarrhythmias & AV block

Sinus node dysfunction or AV conduction block → slow ventricular rate

Overview

Pathologically slow rhythms from sinus node disease or atrioventricular conduction block. First-degree (long PR), Mobitz I (Wenckebach — progressive PR lengthening then dropped beat), Mobitz II (intermittent dropped QRS, high block risk) and third-degree (complete AV dissociation). Mobitz II and complete block are dangerous and usually need pacing.

Recognise

Fatigue, dizziness, syncope (Stokes-Adams attacks), breathlessness; bradycardia on examination
1st degree: PR >200 ms, every P conducts. Mobitz I: progressive PR prolongation then a dropped beat. Mobitz II: constant PR with sudden dropped QRS
Complete (3rd-degree) block: P waves and QRS independent (AV dissociation), slow escape rhythm

Red flags

Adverse features (shock, syncope, myocardial ischaemia, heart failure) or risk of asystole (Mobitz II, complete block, recent asystole, ventricular pause >3 s) → urgent treatment/pacing
Complete heart block after anterior MI → temporary then permanent pacing

Differentials & how to tell them apart

Sinus bradycardia (physiological/drug)normal P-QRS relationship; athletic/drug-induced — often benign

Vasovagal syncopeclear trigger, prodrome, rapid recovery; no fixed conduction block

Hypothyroidism / hyperkalaemia / drug effectreversible cause on bloods/drug history

Complete (third-degree) AV block — P waves and QRS dissociated (ECG)
Gregory Marcus, MD / CC BY 3.0 — Wikimedia Commons

Investigations

12-lead ECG and rhythm strip (classify the block); ambulatory monitoring for intermittent symptoms; electrolytes, TFTs, digoxin level, troponin if ischaemic; review rate-limiting drugs.

Management

Atropine for symptomatic bradycardia → pacing (temporary then permanent) for high-risk block

1Identify adverse features/asystole risk and reversible causes (drugs, ischaemia, electrolytes, hypothyroidism). Symptomatic → atropine 500 mcg IV, repeated up to 3 mg.Gate: If no response, or there is Mobitz II/complete block/recent asystole/pause >3 s → transcutaneous pacing or isoprenaline/adrenaline as a bridge to transvenous pacing — atropine alone is insufficient.
2Definitive treatment of irreversible Mobitz II, complete heart block or symptomatic sinus node disease = permanent pacemaker.

Atropine 500 mcg IV (repeat to 3 mg) for symptomatic bradycardia — first-line for adverse features; review/stop rate-limiting drugs (beta-blocker, CCB, digoxin)

Transcutaneous pacing / isoprenaline / adrenaline infusion — if no response to atropine or high risk of asystole — bridge to transvenous pacing

Permanent pacemaker — Mobitz II, complete heart block, symptomatic sinus node disease, or symptomatic irreversible bradycardia

Key points

Mobitz I (Wenckebach) with progressive PR lengthening is usually benign; Mobitz II and complete heart block (AV dissociation, slow escape) are high-risk → atropine first, but they need pacing. Always check rate-limiting drugs and electrolytes.

Monitor & prognosis

Telemetry, pacing thresholds, device checks; resolve reversible causes.

Excellent once paced; complete block carries syncope/sudden-death risk untreated.

Source: Resuscitation Council UK bradycardia algorithm; NICE