Bradycardia and heart block
Slow pulse with shock, fainting, chest pain or heart failure needs atropine now. If it fails, or the rhythm is high-risk, pace. Always check potassium and the drug chart.
A 72-year-old woman admitted with chest pain feels dizzy and nearly fainted. Pulse 34, BP 82/48. The monitor shows P waves marching through at their own rate, with no relationship to the broad QRS complexes. Potassium 4.6 mmol/L (mEq/L).
What is the rhythm, and what do you give?
- Monitor, 12-lead ECG, defibrillator pads onPads that can pace if available.
- Look for adverse featuresShock, fainting, chest pain from ischaemia, heart failure.
- Adverse features: atropine500 micrograms IV, repeat every 3–5 minutes up to 3 mg.
- Check potassium nowOn a blood gas. High potassium causes slow rhythms.
- Review the drug chartBeta-blockers, diltiazem, verapamil, digoxin, amiodarone, ivabradine.
- Atropine fails or high-risk rhythmTranscutaneous pacing or an adrenaline infusion as a bridge.
- Call for expert helpCardiology for transvenous pacing.
- Treat the causeInferior MI, high potassium, drug toxicity, hypothermia, hypothyroidism.
- Shock, fainting, chest pain or heart failure
- Mobitz II or complete heart block
- Pause over 3 seconds
- Recent asystole
- High potassium with broad QRS
- Slow pulse with high BP and falling GCS (raised intracranial pressure)
- Complete heart block
- High potassium
- Drug toxicity (beta-blocker, calcium channel blocker, digoxin)
- Inferior MI
- Raised intracranial pressure
- Organophosphate poisoning
- Low thyroid and hypothermia
| Atropine | 500 micrograms IV, repeat every 3–5 minutes to a maximum of 3 mg |
| Adrenaline infusion | 2–10 micrograms/min, titrated to response |
| Isoprenaline | 5 micrograms/min where available |
| High potassium with ECG changes | Calcium gluconate 10% 30 mL IV plus insulin-dextrose |
| High-risk rhythms | Mobitz II, complete heart block with broad QRS, pauses over 3 seconds, recent asystole |
Check every dose against a current drug reference and your hospital protocol.
On this page
Is it a problem?
A pulse below 60 is common and harmless in fit young people and during sleep. What matters is whether the patient is compromised and whether the rhythm is at risk of stopping (asystole).
- Shock (low BP, cold, confused)
- Fainting
- Ischaemic chest pain
- Heart failure
- Atropine now, then pace if needed
- Recent asystole
- Mobitz II
- Complete heart block with broad QRS
- Pauses over 3 seconds
- Expert help, prepare for pacing
- Monitor
- Find and treat the cause
- Review drugs
Reading the heart block
| Rhythm | ECG | Risk |
|---|---|---|
| First-degree block | PR longer than 200 ms, every P followed by QRS | Low |
| Mobitz I (Wenckebach) | PR gets longer each beat until a QRS is dropped, then repeats | Usually low (common in inferior MI) |
| Mobitz II | PR constant, then a QRS suddenly dropped | High: can progress to complete block |
| 2:1 block | Every second P not conducted | Cannot tell Mobitz I or II. Treat as high risk if broad QRS |
| Complete (third-degree) heart block | P waves and QRS complexes completely independent. Slow escape rhythm | High, especially with broad QRS |
Treatment
- Transcutaneous pacingPads front and back. Set rate 60–70, increase current until each spike captures a QRS. Feel the femoral pulse to confirm. Give analgesia and sedation.
- Adrenaline or isoprenaline infusionWhile pacing is arranged.
- Transvenous pacingCardiology. Definitive temporary treatment.
- Permanent pacemakerFor persistent high-grade block.
Causes to find
- Inferior MI: often causes slow rhythms and heart block, usually temporary. See Acute coronary syndrome.
- High potassium: check on a gas now.
- Drugs: beta-blockers (including eye drops), diltiazem, verapamil, digoxin, amiodarone, ivabradine, clonidine.
- Organophosphate poisoning: slow pulse with small pupils and secretions. Atropine in large doses.
- Raised intracranial pressure: slow pulse, high BP and falling GCS (Cushing's response).
- Hypothyroidism, hypothermia, sleep apnoea, vasovagal.
- Degenerative conduction disease in the elderly.
Common mistakes
- Giving atropine to a well patient with sinus bradycardia.
- Forgetting the potassium.
- Missing digoxin toxicity in an elderly patient with kidney injury.
- Relying on atropine for complete heart block with broad complexes (it often fails): prepare to pace.
- Treating the slow pulse of raised intracranial pressure with atropine instead of finding the cause.
Frequently asked questions
What is the dose of atropine for bradycardia?
500 micrograms IV, repeated every 3–5 minutes up to a total of 3 mg.
What is the difference between Mobitz I and Mobitz II?
In Mobitz I the PR interval gets longer until a beat is dropped. In Mobitz II the PR interval stays the same and beats are dropped suddenly. Mobitz II is more dangerous.
When is pacing needed for bradycardia?
When atropine fails in a compromised patient, or when there is a high risk of asystole: Mobitz II, complete heart block with broad QRS, pauses over 3 seconds, or recent asystole.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A patient has pulse 34, BP 82/48 and P waves unrelated to broad QRS complexes. What is the rhythm and first treatment?
Answer: Complete heart block: atropine. Complete heart block with adverse features: atropine, then pacing (transcutaneous, then transvenous).
- First-degree block: no treatment: P and QRS are dissociated.
- AF: digoxin: Wrong rhythm and harmful.
- Sinus bradycardia: observe: Adverse features present.
- Mobitz I: observe: Not this.
2. A well patient on bisoprolol 10 mg has pulse 48 in sinus rhythm and normal BP. What is needed?
Answer: Review the bisoprolol. Asymptomatic drug-related bradycardia needs dose review, not emergency treatment.
- Adrenaline infusion: No.
- Pacing: Not indicated.
- Atropine: Not with no adverse features.
- Isoprenaline: No.
3. A slow pulse is accompanied by peaked T waves and broad QRS complexes. What is the first drug?
Answer: Calcium gluconate. This suggests hyperkalaemia; calcium gluconate stabilises the heart, then insulin-dextrose.
- Atropine 500 micrograms: Doesn't treat the cause.
- Amiodarone infusion: Wrong.
- Adenosine: Wrong.
- Digoxin: Harmful.
4. An 80-year-old has syncope. ECG shows P waves at 80/min and regular broad QRS complexes at 35/min with no relationship between them. What is the definitive treatment?
Answer: Permanent pacemaker. Third-degree (complete) heart block: AV dissociation with a slow escape rhythm. Broad escape (ventricular) is unstable. Acute: atropine 500 µg (up to 3 mg), isoprenaline/adrenaline infusion, transcutaneous pacing; definitive treatment is a permanent pacemaker. Causes: degeneration (Lenègre/Lev), inferior MI (often transient), drugs (beta-blockers, digoxin), Lyme, infiltrative disease. Mobitz II also needs pacing.
- Repeated atropine doses: Atropine is a temporary measure.
- Beta-blocker: Beta-blockers worsen bradycardia.
- Observation on the ward: Complete heart block with syncope carries a high risk of asystole.
5. A 78-year-old man is dizzy and pale. HR 32/min, BP 78/50 mmHg. ECG shows complete heart block. What is the first drug to give?
Answer: IV atropine 500 µg. Bradycardia with adverse features (shock, syncope, ischaemia, heart failure) is treated with atropine 500 micrograms IV, repeated to a maximum of 3 mg. If ineffective, use transcutaneous pacing or an adrenaline or isoprenaline infusion, then arrange transvenous and permanent pacing. Complete heart block rarely responds well to atropine, so prepare for pacing early.
- IV adenosine 6 mg: Adenosine blocks the AV node, so it would make complete heart block worse; it is used to terminate SVT.
- IV amiodarone 300 mg: Amiodarone treats tachyarrhythmias and can further slow AV conduction; it has no role in symptomatic bradycardia.
- IV digoxin: Digoxin slows AV conduction and would deepen the block; it is used for rate control in AF.
6. A patient on digoxin starts a loop diuretic and develops nausea, yellow vision and arrhythmias. Which electrolyte change made digoxin toxicity more likely?
Answer: Hypokalaemia. Digoxin inhibits the Na⁺/K⁺ ATPase by binding where potassium normally binds. Low potassium lets more digoxin bind, increasing toxicity. Hypercalcaemia, hypomagnesaemia, renal impairment and drugs such as amiodarone and verapamil also raise the risk. Severe toxicity is treated with digoxin-specific antibody fragments.
- Hyperkalaemia: Is a feature of severe acute overdose, not a cause of increased sensitivity.
- Hyponatraemia: Not a main factor.
- Hypocalcaemia: Hypercalcaemia, not hypocalcaemia, increases toxicity.
- Hypermagnesaemia: Low magnesium, not high, increases toxicity.
7. In second-degree heart block, what would you expect on the ECG?
Answer: More P waves than QRS complexes. In second-degree block, some atrial impulses are not conducted, so some P waves are not followed by QRS complexes. In Mobitz I (Wenckebach), the PR lengthens until a beat drops; in Mobitz II, the PR is fixed and beats drop suddenly (higher risk, needs pacing).
- Prolonged PR with every P conducted: First-degree block.
- P waves and QRS fully independent: Third-degree (complete) block.
- Absent P waves throughout: Seen in atrial fibrillation.
- Wide QRS complexes with no P waves: Seen in ventricular tachycardia.
References and dose sources
- Resuscitation Council UK: adult bradycardia algorithm (2021)
- ESC 2021 guidelines on cardiac pacing
- Resus Council UK — ALS (bradycardia algorithm)
- TOXBASE
- Atropine dosing: Drugs.com: Atropine dosage (see also our atropine dose page)
- Adrenaline (epinephrine) dosing: Drugs.com: Epinephrine dosage (see also our adrenaline (epinephrine) dose page)
- Calcium gluconate dosing: Drugs.com: Calcium gluconate dosage (see also our calcium gluconate dose page)
- Glucagon dosing: Drugs.com: Glucagon dosage (see also our glucagon dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.