Bradycardia and Sinus Node Dysfunction: an Internal Medicine Approach

Definitions

Sinus node dysfunction may include sinus bradycardia, sinus pauses, sinus exit block, sinus arrest, tachy-brady syndrome, persistent atrial standstill and chronotropic incompetence.

Symptoms, when present, are usually related to cerebral hypoperfusion. These include dizziness, presyncope, and syncope. Other symptoms include congestive heart failure, fatigue, decrease exercise capacity, recurrent falls, confusion, memory loss, and palpitations or stroke (if paroxysmal atrial fibrillation is part of the picture). When symptomatic, sinus node dysfunction is called sick sinus syndrome.

It’s safe to say that more than 90% of all cases of subnormal sinus node function are caused by physicians…. The sick sinus syndrome is rare. In our busy county hospital, where many of our patients are sick and old, I might see only one genuine sick sinus syndrome in a year.

(Phibbs, 2006)

Culprit medications are, of course, the beta blockers, calcium channel blockers, digitalis, and clonidine. Less obvious medicinal causes include the antipsychotics, opioids and lithium. Ciguatera fish poisoning is also a cause of bradycardia.

Physical examination

1. Stop offending agents (including, if applicable, skin examination for clonidine or opioid patches, external decontamination of organophosphates)
2. Pulse oximeter or arterial blood gas (to evaluate for hypoxemia)
3. Temperature, rectal if appropriate (to evaluate for hypothermia)

Workup

1. Serum electrolyte (especially potassium to evaluate for hyperkalemia)
2. Electrocardiogram (to evaluate for acute ischemia, particularly inferior wall MI; also evidenced on ECGs are heart blocks, pericarditis, hypothyroidism, hypothermia, intracranial hypertension, cardiac glycoside toxicity, and infiltrative disease)
3. Serial troponins if appropriate
4. TSH (to evaluate for hypothyroidism)
5. Sepsis workup if appropriate
6. Liver chemistry studies (to evaluate for obstructive jaundice)
7. Lyme titers if appropriate
8. Anti-Ro antibodies (associated with congenital third degree (complete) atrioventricular block in patients with neonatal lupus)
9. Cardiac imaging if appropriate (to evaluate for infiltrative diseases)
10. Funduscopic examination or brain imaging as appropriate (to evaluate for intracranial hypertension)
11. Digitalis or lithium levels if appropriate
12. Sleep study (if obstructive sleep apnea is suspected)

References

• Brendan P. Phibbs MD, Advanced ECG: Boards and Beyond (2006).
• Hurst’s The Heart (2010)
• Mattu, Amal, MD, Emergency ECG Videos, Episode No. 107