Sinus bradycardia is defined as a resting heart rate of 60 beats/minute or less. Few patients, however, actually become symptomatic until their heart rate drops to less than 50 beats/minute.
In the US: Frequency of sinus bradycardia is unknown. In cardiac patients, it has been estimated to be 3 in 5000. The morbidity and mortality associated with sinus bradycardia are related to its underlying etiology.
In patients who present with toxic exposure, the prognosis is good once the offending agent has been removed.
One of the most common causes of symptomatic sinus bradycardia is the sick sinus syndrome. Patients with sick sinus syndrome have a relatively poor prognosis, with 5-year survival rates in the range of 47-69%. It is not clear whether this high death rate is attributable to underlying heart disease, itself.
Causes of Sinus Bradycardia
Sinus bradycardia may also be caused by the sick sinus syndrome, which involves a dysfunction in the ability of the sinus node to generate or transmit an action potential, or electrical signal, to the upper portion of the heart or the atria.
Medications responsible for sick sinus syndrome include therapeutic and supratherapeutic doses of digitalis glycosides, beta-blockers, and calcium channel-blocking agents. Other drugs include class I antiarrhythmic agents and amiodarone. A broad variety of other drugs and toxins have been reported to cause bradycardia, including lithium, paclitaxel, toluene, dimethyl sulfoxide (DMSO), topical ophthalmic acetylcholine, fentanyl, alfentanil, sufentanil, reserpine, and clonidine.
Sinus bradycardia may also be seen in hypothermia, hypoglycemia, and sleep apnea.
Diphtheria, rheumatic fever and viral myocarditis can produce sinus bradycardia.
Commonly, sinus bradycardia is an incidental finding in otherwise healthy individuals, particularly in young adults or sleeping patients. Other causes of sinus bradycardia are related to increased vagal tone.
Physiologic causes of increased vagal tone include the bradycardia seen in athletes. Pathologic causes include, but are not limited to, inferior wall myocardial infarction, toxic or environmental exposure, electrolyte disorders, infection, sleep apnea, drug effects, hypoglycemia, hypothyroidism, and increased intracranial pressure.
Signs and Symptoms of Sinus Bradycardia
Sinus bradycardia is frequenty asymptomatic. When symptoms do occur, however, they may include syncope, dizziness and lightheadedness.
Chest pain and shortness of breath can occur.
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Medical Content Last Updated on 07/12/2008
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