Sinus Bradycardia

What sinus bradycardia means on an ECG

"Sinus" means the heartbeat starts in the sinoatrial (SA) node, the normal pacemaker in the right atrium. On an ECG:

• each beat has a normal P wave followed by a QRS complex
• the rhythm is regular
• the rate is below 60 beats per minute

If the rhythm is slow but does not start in the sinus node, or some beats are blocked, it is a different condition, such as heart block or a junctional rhythm.

For the general symptom of a slow pulse and how to check it, see decreased heart rate.

When sinus bradycardia is normal

• Athletes: endurance training increases vagal tone and the heart's stroke volume, so resting rates of 40 to 60, and sometimes lower, are common
• Sleep: rates often fall into the 40s or even lower in healthy people
• Young adults: commonly have resting rates in the 50s
• Vagal stimulation: straining, vomiting or pain can slow the heart briefly

The key question is whether the heart speeds up appropriately with exercise and whether there are symptoms.

Causes of problematic sinus bradycardia

Sinus bradycardia vs sick sinus syndrome

Sinus node dysfunction, also called sick sinus syndrome, is an age-related wear of the sinus node. It can cause:

• persistent sinus bradycardia
• pauses or sinus arrest
• failure to speed up with exercise (chronotropic incompetence)
• alternating slow and fast rhythms (tachy-brady syndrome), often with atrial fibrillation

It usually affects older adults and is a common reason for pacemaker implantation.

Symptoms

Many people have none. When the rate is too slow for the body's needs:

• dizziness or lightheadedness
• fainting or near-fainting
• fatigue and weakness
• shortness of breath, especially with exertion
• reduced exercise tolerance
• confusion or memory problems in older adults
• chest discomfort

Diagnosis

• 12-lead ECG confirms sinus rhythm and rate
• Holter or patch monitor for 24 hours to 2 weeks, capturing pauses and rate variation, and correlating symptoms
• Exercise stress test to check that the rate rises appropriately
• Blood tests: thyroid function, electrolytes, and drug levels such as digoxin
• Echocardiogram to check heart structure
• Sleep study if sleep apnea is suspected

Treatment

• No symptoms: usually no treatment; regular check-ups if rates are very low
• Medicine adjustment: reduce or switch rate-slowing medicines (never stop them suddenly without advice)
• Treat the cause: thyroid hormone, CPAP for sleep apnea, correcting potassium
• Pacemaker: recommended when symptoms are clearly linked to bradycardia from sinus node dysfunction and no reversible cause is found, or when needed medicines cause symptomatic slowing
• Emergency treatment: atropine and temporary pacing for severe symptomatic bradycardia

Outlook

Sinus bradycardia without symptoms, especially in athletes and young people, has an excellent outlook. When symptoms are present, treatment of the cause or a pacemaker relieves them in most people.

Frequently asked questions

What does sinus bradycardia mean on an ECG?

It means the heart's rhythm starts normally in the sinus node and every beat looks normal, but the rate is below 60 beats per minute. It is a very common finding and is often normal, especially in fit or young people.

Is sinus bradycardia dangerous?

Usually not. It is often a sign of a healthy, well-conditioned heart. It needs attention if it causes dizziness, fainting, fatigue or breathlessness, or if it is caused by medicines, thyroid problems or sinus node disease.

What causes sinus bradycardia?

Normal causes include fitness, sleep and youth. Other causes include beta-blockers and other heart medicines, an underactive thyroid, sleep apnea, sinus node disease, heart attack, high potassium and hypothermia.

Do athletes have sinus bradycardia?

Yes. Endurance athletes often have resting heart rates in the 40s or 50s because training strengthens the heart and increases vagal tone. If they have no symptoms and the rate rises normally with exercise, it is considered healthy.

When does sinus bradycardia need a pacemaker?

A pacemaker is recommended when sinus bradycardia or pauses from sinus node dysfunction clearly cause symptoms such as fainting, dizziness or fatigue and no reversible cause, such as a medicine or thyroid problem, is found.