Sinus Bradycardia

Sinus bradycardia is a regular rhythm originating in the sinus node at a rate below 60 beats per minute in adults. It is normal in athletes and during sleep and is often asymptomatic, but may reflect drug effect, sinus node dysfunction, raised intracranial pressure or metabolic disease.

• Physiological: fit young adults and endurance athletes (rates in the 40s are possible), sleep, high vagal tone.
• Drugs: beta blockers, rate limiting calcium channel blockers (verapamil, diltiazem), digoxin, amiodarone, ivabradine, donepezil and other cholinesterase inhibitors, clonidine, opioids, lithium, ophthalmic timolol.
• Pathological: hypothyroidism, hypothermia, hypokalaemia or hyperkalaemia, raised intracranial pressure (Cushing response), obstructive sleep apnoea, inferior myocardial infarction, sick sinus syndrome, anorexia nervosa, severe hypoglycaemia, cholestatic jaundice, infections (typhoid, Lyme disease, Chagas disease).
• Prognosis depends on cause and symptoms.

• Often asymptomatic.
• Fatigue, exercise intolerance, light headedness, presyncope or syncope, dyspnoea, chest pain, confusion in older adults.
• Palpitations from compensatory beats or pauses.
• Examination: slow regular pulse, check blood pressure, temperature, thyroid signs (dry skin, slow reflexes), signs of heart failure, cannon A waves if heart block.
• Red flags: syncope, hypotension, chest pain, heart failure, rate under 40, pauses of 3 seconds or more, new bradycardia after inferior myocardial infarction, headache with vomiting and papilloedema (raised intracranial pressure).

• 12-lead ECG: normal P waves preceding each QRS at a rate under 60 with normal PR interval; look for sinus arrest, sinoatrial block or atrioventricular block.
• Medication review.
• Bloods: thyroid function, urea and electrolytes, calcium, magnesium, glucose, full blood count, troponin if chest pain, cortisol where indicated.
• 24 to 72 hour ambulatory ECG or event recorder for symptomatic patients or intermittent bradycardia. Exercise test to document chronotropic response.
• Echocardiography if structural heart disease suspected.
• Differentials: second or third degree atrioventricular block, junctional rhythm, blocked atrial ectopics, sick sinus syndrome, athlete's heart.

Asymptomatic

• No treatment. Reassure athletes and those with normal exercise tolerance. Avoid unnecessary drugs that slow the rate.

Treat the cause

• Withdraw or reduce causative drug (for example reduce bisoprolol, verapamil, diltiazem or digoxin dosage).
• Hypothyroidism: levothyroxine 25 to 50 micrograms orally once daily in older adults or cardiac disease, titrated by 12.5 to 25 micrograms every 4 to 6 weeks; 1.6 micrograms per kg in healthy younger adults.
• Correct electrolytes and hypothermia. Treat sleep apnoea with continuous positive airway pressure.

Symptomatic or unstable bradycardia (emergency care)

• Atropine 500 micrograms intravenously, repeated every 3 to 5 minutes to a maximum of 3 mg. Further options include isoprenaline, adrenaline infusion or transcutaneous pacing, followed by cardiology review and pacemaker consideration.
• Do not give atropine to a heart transplant recipient (ineffective).

Persistent symptomatic bradycardia (specialist)

• Permanent pacing for sick sinus syndrome with documented symptom correlation, or for high grade atrioventricular block.
• Theophylline or aminophylline as a bridge in selected cases; specialist advice.

Special groups

• Athletes: asymptomatic rates in the 40s are normal; refer if syncope, exertional symptoms or ECG abnormalities.
• Older adults: bradycardia with syncope is often sinus node disease; low threshold for ambulatory monitoring.
• Pregnancy: unusual; look for cause.

Follow up and referral

• Urgent referral for syncope, pauses of 3 seconds or more, haemodynamic compromise or high grade block.
• Routine referral for symptomatic bradycardia persisting after drug withdrawal.