Cardiovascular

Bradyarrhythmias and Heart Block

Disease of the conducting tissue delays or blocks atrial impulses reaching the ventricles, leaving a slow escape rhythm whose site of origin decides how dangerous the block is.

In a nutshell

Heart block is failure of atrial impulses to reach the ventricles, from a delayed beat to full atrioventricular (AV) dissociation. If there are life-threatening features, give atropine 500 micrograms IV, repeated every 3 to 5 minutes to a maximum of 3 mg.

Classic presentation

An older adult with recurrent collapse and a pulse of 35, whose ECG shows P waves marching independently through broad QRS complexes.

Key points

First degree: every P wave conducts with a long PR interval. Mobitz I: the PR interval lengthens, then a beat drops. Mobitz II: beats drop without warning. Complete: AV dissociation.
Resuscitation Council UK (RCUK) 2025 life-threatening features are shock, syncope, myocardial ischaemia, severe heart failure, and an arrhythmia immediately after return of spontaneous circulation (ROSC).
Risk of asystole is recent asystole, Mobitz II, complete block with a broad QRS complex, or a ventricular pause over 3 seconds. Treat these even if the patient looks well.
Never give atropine after cardiac transplant: use aminophylline 100 to 200 mg by slow IV injection. Aminophylline is also the choice in spinal cord injury.
Second line after atropine: isoprenaline 5 micrograms/minute IV or adrenaline 2 to 10 micrograms/minute IV, with transcutaneous pacing alongside rather than afterwards.
Glucagon 5 to 10 mg IV over 1 to 2 minutes, then 50 to 150 micrograms/kg/hour, if a beta blocker or calcium-channel blocker may be the cause.
Mobitz II, 2:1, advanced and complete block need a permanent pacemaker even without symptoms once reversible causes are excluded. First-degree block is paced only if symptoms correlate.
NICE recommends dual-chamber pacing for symptomatic sick sinus syndrome, AV block or both, except in continuous AF or where frailty favours single-chamber ventricular pacing.
If atropine fails and no pacing equipment is to hand, fist pacing can be attempted. In apparent asystole look for P waves, because those respond to pacing.
NICE treats any degree of heart block on the ECG as a red flag after transient loss of consciousness, needing specialist cardiovascular review within 24 hours.

First-line investigation

12-lead ECG with a rhythm strip on continuous monitoring, plus potassium, calcium, magnesium, glucose, thyroid function, troponin if ischaemia is possible, and a medicines review.

Management

ABCDE, then the algorithm's two questions

ABCDE: oxygen if hypoxic, IV access, monitor ECG, blood pressure and oxygen saturation, record a 12-lead ECG. Identify and treat reversible causes such as electrolyte abnormality.²
Question one: are there life-threatening features? Shock, syncope, myocardial ischaemia, severe heart failure, or an arrhythmia immediately after return of spontaneous circulation (ROSC).²
Question two, if the answer to the first is no: is there a risk of asystole? Recent asystole, Mobitz II, complete block with a broad QRS complex, or a pause over 3 seconds.²
Yes to either question takes you to drugs and pacing. No to both means observe on continuous monitoring while the cause is treated.²

Atropine, and when not to use it

Atropine 500 micrograms IV. Reassess. If the response is unsatisfactory, repeat every 3 to 5 minutes to a maximum total of 3 mg.^2,3,4
Do not give atropine after cardiac transplant: it can cause high-degree AV block or sinus arrest. Give aminophylline 100 to 200 mg by slow IV injection.^3,2
Do not rely on atropine in high-degree AV block with a wide QRS complex. It is ineffective there and may worsen the block. Prepare pacing instead.³

When atropine fails: interim measures and pacing

Isoprenaline 5 micrograms/minute IV, titrated to response under expert supervision, or adrenaline 2 to 10 micrograms/minute IV. Stop isoprenaline if the rate reaches 130 beats per minute.^2,3,9
Glucagon 5 to 10 mg IV over 1 to 2 minutes, then 50 to 150 micrograms/kg/hour, if a beta blocker or calcium-channel blocker may be the cause.^3,10
The algorithm's other alternatives are aminophylline, dopamine, and glycopyrrolate in place of atropine. Run transcutaneous pacing alongside the drugs, and seek expert help now.²
Establish early transvenous pacing. Fist pacing can be attempted while equipment is fetched. In apparent asystole, look for P waves, because those respond to pacing.³

Permanent pacing and device choice

Mobitz II, 2:1, advanced and complete block need a permanent pacemaker even without symptoms, once a reversible cause is excluded, because they progress abruptly.¹
Mobitz I and first-degree block are paced only when symptoms correlate. ESC records weak evidence for pacing marked PR prolongation of 300 ms or more.¹
NICE recommends dual-chamber pacing for symptomatic bradycardia from sick sinus syndrome, AV block or both, and for sick sinus syndrome without AV block.^7,8
Two exceptions take single-chamber ventricular pacing: AV block with continuous AF, and frailty or comorbidity that tips the balance of risks and benefits.⁷
Leadless pacing is an option for right ventricular pacing alone. Leadless dual-chamber or right atrial pacing is limited to evidence generation or formal research.⁶

Driving, device checks and safety-net

Pacemaker implant or box change: at least one week off Group 1 (car and motorcycle) driving. Notify DVLA of an implant, but not of a box change.¹¹
Group 2 (bus and lorry): at least 6 weeks off driving after a pacemaker, and DVLA must be notified. Regular device checks are required for both groups.¹¹
A significant AV conduction defect or sinoatrial disease counts as an arrhythmia: at least 4 weeks off Group 1 driving if it caused or may cause incapacity.¹¹
Congenital complete heart block: an asymptomatic Group 1 driver may drive and need not notify. A symptomatic one must stop and notify until a pacemaker is implanted.¹¹
Unexplained syncope follows DVLA Appendix D: 6 months off Group 1 driving from an isolated episode, 12 months if recurrence risk is raised, 5 years for Group 2.¹²
Safety-net for syncope, chest pain, breathlessness, confusion or collapse. Arrange cardiology follow-up, a medicines review and the results of any ambulatory monitoring.⁵

Exam traps

Atropine can worsen high-degree block with a broad QRS complex, because that block sits below the atrioventricular (AV) node where atropine acts.
Complete heart block is AV dissociation, not simply a very long PR interval.
Mobitz II and complete block earn a pacemaker even when the patient feels perfectly well.
A drug or electrolyte trigger can unmask fixed conduction disease. Correcting it does not cancel the need for pacing.
In a transplanted heart, atropine can cause high-degree block or sinus arrest. Give aminophylline instead.
Transcutaneous pacing is painful and a bridge only. Transvenous pacing is the temporary route to establish in an unstable patient.
The 2025 algorithm added a fifth life-threatening feature: an arrhythmia appearing immediately after return of spontaneous circulation (ROSC).

Illustrations

Conduction system and levels of blockDiagram of the cardiac conduction system from the SA node through the AV node and His-Purkinje system, indicating where each degree of block typically occurs.PassFinals · original

AV dissociation and cannon 'a' wavesDiagram illustrating independent atrial and ventricular contraction in complete heart block, producing intermittent cannon 'a' waves in the JVP.PassFinals · original

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.