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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Bradycardia = HR <60 bpm. Symptomatic when causing dizziness, syncope, heart failure, or shock
- First-degree AV block: prolonged PR (>200 ms), all P waves conducted — usually benign
- Second-degree: Mobitz I (Wenckebach) = progressive PR prolongation then dropped beat — usually benign. Mobitz II = sudden dropped beats without PR prolongation — HIGH RISK of progression to complete heart block
- Third-degree (complete) heart block: complete AV dissociation, P waves and QRS complexes independent — PACING REQUIRED
- Acute symptomatic bradycardia: IV atropine 500 mcg (up to 3 mg). If refractory → temporary pacing (transcutaneous or transvenous)
Overview
Bradycardia refers to a heart rate below 60 bpm. It may be physiological (athletes, sleep) or pathological. AV block is classified by the degree of impairment of conduction from atria to ventricles. First-degree is a conduction delay. Second-degree involves intermittent failure of conduction (Mobitz I/Wenckebach is at the AV node level; Mobitz II is at the infra-nodal level and has a high risk of progression to complete heart block). Third-degree (complete) heart block is complete failure of AV conduction with the ventricles driven by an escape rhythm. Causes include degenerative conduction system disease, ischaemic heart disease (inferior MI → AV node block; anterior MI → infra-nodal block), drugs (beta-blockers, CCBs, digoxin, amiodarone), hypothyroidism, and raised intracranial pressure.
Epidemiology
First-degree AV block is very common (1–3% of the general population). Complete heart block affects approximately 0.04% of the population and becomes more common with age due to progressive fibrosis of the conduction system (Lev/Lenègre disease). Bradycardia from inferior MI is usually transient (AV node blood supply). Anterior MI-related heart block carries a much worse prognosis (extensive myocardial damage). Drug-related bradycardia is a very common cause in clinical practice.
Clinical Features
Symptoms
Dizziness and lightheadedness
Syncope or presyncope (Stokes-Adams attacks: sudden LOC, pale, rapid recovery)
Fatigue and exercise intolerance
Dyspnoea (reduced cardiac output)
Chest pain (if bradycardia reduces coronary perfusion)
Heart failure symptoms
May be asymptomatic (physiological or incidental)
Signs
Bradycardia (HR <60 bpm, may be <40 bpm in complete heart block)
Hypotension
Cannon A waves in JVP (atria contracting against closed AV valves in complete heart block)
Variable intensity of S1 (changing PR interval)
Signs of heart failure
Investigations
First-line
12-lead ECG1st degree: PR >200 ms. Mobitz I: progressive PR lengthening → dropped QRS. Mobitz II: constant PR, sudden dropped QRS, may have bundle branch block. 3rd degree: complete AV dissociation, regular P-P and R-R intervals but independent, broad QRS escape
BloodsU&Es (K⁺, Mg²⁺, Ca²⁺), TFTs, digoxin level, troponin if ischaemia suspected
Second-line
Ambulatory ECG (Holter)If intermittent symptoms — capture paroxysmal bradycardia or heart block
EchocardiographyAssess LV function and structural heart disease
Specialist
Electrophysiology studyAssess conduction system function — HV interval. Rarely needed if diagnosis clear on ECG
Implantable loop recorderFor infrequent symptoms when Holter monitoring is non-diagnostic
1
Acute symptomatic bradycardia
- IV atropine 500 mcg (repeat every 3–5 min, max 3 mg)
- If atropine ineffective: transcutaneous pacing (external pads, sedation/analgesia required)
- Alternatives while awaiting pacing: isoprenaline infusion, adrenaline infusion
- Seek expert help and arrange transvenous temporary pacing if needed
2
Reversible causes — identify and treat
- Drug-related: withhold/reduce beta-blockers, CCBs, digoxin, amiodarone
- Electrolyte abnormalities: correct hyperkalaemia, hypomagnesaemia
- Inferior MI: transient AV block usually resolves — monitor, temporary pacing only if symptomatic
- Hypothyroidism: thyroid hormone replacement
3
Permanent pacemaker (PPM) — indications
- Symptomatic second-degree Mobitz II AV block
- Third-degree (complete) heart block (regardless of symptoms)
- Symptomatic sinus node disease (sick sinus syndrome)
- Post-anterior MI complete heart block (usually permanent)
- Alternating bundle branch block
- Asymptomatic Mobitz I and first-degree block do NOT require pacing
Complications
- Syncope and falls: Stokes-Adams attacks — transient complete heart block with LOC
- Heart failure: Chronic bradycardia with reduced cardiac output
- Cardiac arrest: Complete heart block → ventricular standstill (asystole)
- Pacemaker complications: Infection, lead displacement, pneumothorax (at insertion), generator malfunction
UKMLA Exam Tips
- 1Mobitz I (Wenckebach) = "longer longer longer DROP, now you have a Wenckebach" — progressive PR prolongation then dropped beat. Usually benign (AV nodal level)
- 2Mobitz II = fixed PR then sudden dropped QRS — DANGEROUS, can progress to complete heart block. Needs pacing
- 3Complete heart block: regular P-P and R-R but independent of each other. Broad QRS escape. Needs permanent pacemaker
- 4Atropine works at the AV node (vagal blockade) — effective for AV nodal block but NOT for infra-nodal (Mobitz II/CHB)
- 5Inferior MI → AV nodal block (transient, usually resolves). Anterior MI → infra-nodal block (permanent, poor prognosis, needs pacing)
- 6Drug causes: beta-blockers, digoxin, verapamil/diltiazem, amiodarone — always check medication history
practicetest your knowledge on bradycardia and heart blockApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — cardiovascular and beyond.
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