Bradycardia

Bradycardia is a heart rate under 60 beats/minute at rest in adults. It is physiological in athletes and during sleep and often asymptomatic. It becomes pathological when it causes symptoms (fatigue, dizziness, presyncope, syncope, breathlessness, confusion, chest pain, heart failure) or haemodynamic compromise, usually when the rate is under 40 or there are pauses over 3 seconds while awake.

Clinical relevance. Symptomatic bradycardia can produce asystole and sudden death. Reversible causes (drugs, hypothyroidism, hyperkalaemia, hypothermia, raised intracranial pressure, increased vagal tone, inferior myocardial infarction) are common and must be sought first. High degree atrioventricular block and sick sinus syndrome need permanent pacing.

Classification by site.

• Sinus node: sinus bradycardia, sinus pause or arrest, sinoatrial exit block, tachycardia bradycardia syndrome (sick sinus syndrome).
• Atrioventricular node and conduction: first degree block (PR over 200 ms), second degree Mobitz type I (Wenckebach; progressive PR lengthening) and type II (fixed PR with dropped QRS), third degree (complete) heart block.
• Atrial fibrillation with slow ventricular response.

Classification by cause.

• Physiological: athletic conditioning, sleep, young healthy, vagal tone.
• Intrinsic: degenerative fibrosis (age), ischaemia, cardiomyopathy, infiltrative disease (amyloid, sarcoid, haemochromatosis), myocarditis, endocarditis with abscess, rheumatic fever, Lyme carditis, Chagas disease, congenital heart block, post surgery or ablation, collagen vascular disease, tumours.
• Extrinsic: drugs, hypothyroidism, hyperkalaemia, hypothermia, increased intracranial pressure (Cushing reflex), obstructive sleep apnoea, vagal reflexes, hypoxia, neurally mediated syncope, anorexia nervosa, jaundice, typhoid fever.

History.

• Symptoms: fatigue, exercise intolerance, dizziness, presyncope, syncope (with injury), breathlessness, chest pain, palpitations, confusion, falls.
• Timing: relation to exertion, sleep, vomiting, micturition, defaecation, cough, swallowing, neck movement.
• Cardiac: heart disease, myocardial infarction, rheumatic fever, endocarditis, cardiac surgery, family history of pacemaker or sudden death.
• Systemic: hypothyroid symptoms (cold intolerance, weight gain), eating disorder, sleep apnoea, headache, vomiting and visual disturbance (raised intracranial pressure), fever, tick bite (Lyme disease), travel (Chagas disease, typhoid).
• Drugs: beta blockers (including eye drops), verapamil, diltiazem, digoxin, amiodarone, sotalol, flecainide, clonidine, donepezil and other cholinesterase inhibitors, lithium, opioids, dexmedetomidine, ivabradine, propofol, phenylephrine, antipsychotics, ondansetron, organophosphates.
• Athletic training level.

Examination.

• Pulse rate, rhythm (regularly slow, regularly irregular, irregular), volume, blood pressure, temperature, respiratory rate, oxygen saturation, glucose.
• Cannon A waves in the jugular venous pulse and variable intensity first heart sound (complete heart block); large volume pulse; heart failure signs.
• Hypothyroidism (dry skin, slow reflexes, goitre), hypothermia, signs of raised intracranial pressure (papilloedema, hypertension, irregular respiration), anorexia, jaundice.
• Murmurs, signs of endocarditis, rheumatic heart disease.
• Response to exercise (chronotropic response) and atropine.

Red flags (adverse features).

• Systolic blood pressure under 90 mmHg, shock, syncope, myocardial ischaemia, heart failure.
• Heart rate under 40, pauses over 3 seconds, Mobitz II or third degree block, broad QRS escape rhythm.
• Risk of asystole: recent asystole, Mobitz II block, complete heart block with broad complex escape, ventricular pause over 3 seconds.
• Acute inferior or anterior myocardial infarction.
• Drug overdose (beta blocker, calcium channel blocker, digoxin).
• Raised intracranial pressure.
• Symptomatic bradycardia with exertional intolerance.

Physiological and benign. Athlete's heart (resting rate 30 to 50, with first degree block and Wenckebach at night), sleep, normal variant, vasovagal reflex.

Sinus node disease. Sick sinus syndrome (idiopathic fibrosis), sinus node ischaemia, post cardiac surgery, atrial fibrillation with slow response, amyloidosis, sarcoidosis, myocarditis, rheumatic carditis, congenital.

Atrioventricular conduction disease. Degenerative conduction disease (Lenègre, Lev), inferior myocardial infarction (transient), anterior infarction (high risk), aortic valve calcification, endocarditis with root abscess, Lyme disease, Chagas disease, rheumatic fever, sarcoidosis, amyloidosis, haemochromatosis. Other causes are myocarditis, cardiac tumours, congenital complete heart block (maternal anti Ro antibodies), post surgical or transcatheter aortic valve replacement, muscular dystrophies (myotonic dystrophy, Emery-Dreifuss), neuromuscular disease.

Drugs and toxins. Beta blockers, non dihydropyridine calcium channel blockers, digoxin, amiodarone, sotalol, flecainide, clonidine, cholinesterase inhibitors, lithium, opioids, organophosphates, carbamates, plant toxins (oleander, foxglove, aconite), cardiac glycoside poisoning, sedatives, anaesthetic agents.

Metabolic and systemic. Hypothyroidism, hyperkalaemia, hypothermia, hypoglycaemia, hypoxia, severe acidosis, sepsis (late), raised intracranial pressure, anorexia nervosa, jaundice, typhoid fever, brucellosis, legionellosis (relative bradycardia), obstructive sleep apnoea.

Neurally mediated. Vasovagal syncope, carotid sinus hypersensitivity, situational syncope, glossopharyngeal neuralgia.

1. ABCDE assessment and 12 lead ECG with monitoring. Determine the rhythm and degree of block; look for P waves, PR interval, QRS width, QTc, ischaemic changes, hyperkalaemic changes, Osborn waves (hypothermia).
2. Blood tests: urea, creatinine, electrolytes (potassium, magnesium, calcium), glucose, thyroid stimulating hormone, troponin, full blood count, C reactive protein, digoxin level, drug screen, cortisol, blood cultures if fever or suspected endocarditis, lactate, and arterial gas.
3. Chest radiograph; echocardiography for structural disease, ventricular function, valve disease, and infiltrative disease; cardiac MRI for sarcoid or amyloid.
4. Ambulatory monitoring: Holter (24 to 72 hours), event recorder, implantable loop recorder for infrequent symptoms. Correlation of symptoms with bradycardia is critical (symptom rhythm correlation).
5. Exercise testing for chronotropic incompetence (failure to reach 80% of age predicted maximum heart rate). Atropine test (1 to 2 mg intravenously) to differentiate vagally mediated bradycardia from intrinsic disease. Carotid sinus massage for suspected hypersensitivity. Tilt table testing for neurally mediated syncope.
6. Electrophysiology study for unexplained syncope with bundle branch block.
7. Sleep study for suspected obstructive sleep apnoea. Serology for Lyme disease and Chagas disease when exposure suggests them. Antinuclear antibodies, anti Ro antibodies.
8. Imaging of the brain for raised intracranial pressure.

Limits. Wenckebach at night in athletes is normal. Pauses up to 2 seconds in sleep are normal. A normal Holter does not exclude intermittent block if symptoms do not occur.

Stable, asymptomatic. Identify the cause, monitor, and review drugs. No treatment for athletes and healthy individuals, and for first degree and Mobitz type I block without symptoms.

Acute symptomatic or unstable bradycardia (adverse features).

1. Airway, oxygen if hypoxic (saturation under 94%), intravenous access, monitor.
2. Atropine 500 micrograms intravenously, repeat every 3 to 5 minutes to a maximum of 3 mg. Ineffective in heart transplant recipients, and in Mobitz II and complete block with wide complex escape.
3. If atropine ineffective or high risk of asystole: transcutaneous pacing at 80 to 100 beats/minute starting at 50 to 70 mA and increasing until capture; analgesia and sedation; transvenous pacing to follow.
4. Alternatives while arranging pacing: adrenaline infusion 2 to 10 micrograms/minute, isoprenaline 5 micrograms/minute, dopamine 5 to 20 micrograms/kg/minute. Also glucagon 5 to 10 mg intravenous bolus then infusion 1 to 5 mg/hour for beta blocker toxicity. Also aminophylline 100 to 200 mg slow intravenous injection (for inferior infarction and spinal cord injury).
5. Transvenous temporary pacing wire (right ventricular apex), then permanent pacemaker.

Cause directed treatment.

• Drug toxicity: stop the drug. Beta blocker overdose: glucagon 5 mg intravenously (up to 10 mg) then 2 to 5 mg/hour, high dose insulin euglycaemia therapy (insulin 1 unit/kg bolus then 1 to 10 units/kg/hour with glucose and potassium), calcium chloride 10% 10 to 20 mL, lipid emulsion, and pacing.
• Calcium channel blocker overdose: calcium chloride 10% 10 to 20 mL (or calcium gluconate 30 to 60 mL), high dose insulin, vasopressors.
• Digoxin toxicity: digoxin-specific antibody fragments (80 mg for each 1 mg of digoxin ingested, or 400 to 800 mg empirically), correct potassium, avoid calcium, atropine for bradycardia.
• Organophosphate poisoning: atropine 2 mg intravenously, doubling every 5 minutes until secretions dry; pralidoxime 30 mg/kg over 30 minutes then 8 mg/kg/hour.
• Hyperkalaemia: calcium gluconate 10% 30 mL over 10 minutes, insulin 10 units with glucose 25 g, salbutamol 10 to 20 mg nebulised, sodium bicarbonate if acidotic, dialysis.
• Hypothyroidism: levothyroxine 1.6 micrograms/kg daily (start 25 micrograms in older adults or ischaemic disease); myxoedema coma: levothyroxine 200 to 400 micrograms intravenously then 1.6 micrograms/kg daily with hydrocortisone 100 mg every 8 hours.
• Hypothermia: rewarming; avoid unnecessary shocks; atropine ineffective.
• Raised intracranial pressure: neurosurgical emergency.
• Myocardial infarction: inferior bradycardia usually settles with atropine, reperfusion and time; anterior infarction with new block needs pacing.
• Lyme carditis: ceftriaxone 2 g intravenously daily for 14 to 21 days (or doxycycline 100 mg twice daily), temporary pacing as needed.
• Endocarditis with abscess: surgical referral.
• Obstructive sleep apnoea: continuous positive airway pressure.
• Vagal reflexes: reassurance and avoidance of triggers.
• Anorexia nervosa: nutritional rehabilitation with electrolyte monitoring.

Permanent pacing. Indications:

• Symptomatic sinus node dysfunction with documented correlation.
• Symptomatic chronotropic incompetence.
• Mobitz II or third degree block (irrespective of symptoms).
• Alternating bundle branch block.
• Bifascicular block with syncope.
• Prolonged asystole (over 3 seconds) with symptoms.
• Cardioinhibitory carotid sinus hypersensitivity.
• Tachycardia bradycardia syndrome needing rate controlling drugs.

Dual chamber pacing for atrioventricular block. Complications: infection, lead failure, pneumothorax, tamponade, pacemaker syndrome. Pacemaker follow up and device checks every 6 to 12 months. Avoid strong magnetic fields; MRI conditional devices.

Monitoring. Continuous ECG, blood pressure, urine output and neurological status until stable; repeat electrolytes; check temporary pacing threshold daily.

Referral and escalation. Cardiology urgently for high degree block or symptomatic sinus node disease. Electrophysiology for pacing. Critical care for shock. Poison centre for toxic ingestion. Endocrinology for severe hypothyroidism. Neurosurgery for raised intracranial pressure.

Admission. Symptomatic bradycardia, heart rate under 40, Mobitz II, third degree block, pauses over 3 seconds, adverse features, drug toxicity, acute myocardial infarction, syncope with bundle branch block, and hyperkalaemia.

Safety netting. Return for fainting, near fainting, chest pain, breathlessness, or heart rate under 40 with symptoms. For pacemaker patients: wound problems, fever, dizziness, or hiccups.

Special groups. Athletes: asymptomatic bradycardia is normal, but investigate syncope or heart rate under 30 and detraining may reverse. Children: normal heart rates differ by age (neonates under 100 abnormal); congenital heart block with maternal antibodies. Pregnancy: congenital heart block carries risk; pacing is feasible. Older adults: sick sinus syndrome and conduction disease; review drugs, including eye drops. Endemic regions: Chagas disease, rheumatic carditis, typhoid fever, myocarditis, and organophosphate poisoning.