Sinus Bradycardia and Sick Sinus Syndrome

In sinus bradycardia, the heart beats regularly but more slowly than usual (less than 60 beats per minute). Sick sinus syndrome is malfunction of the heart's natural pacemaker (sinus or sinoatrial node) that causes heart rhythm to vary randomly. Heart rhythm may be too slow or too fast.

A slow heart rate may be a sign of good physical condition. But it may also result when the heart's natural pacemaker (sinus node) malfunctions. If the heart's pacemaker malfunctions, other areas of the heart can produce electrical currents so that the heart continues to beat but more slowly than normal.

The heart's pacemaker is more likely to malfunction in older people. Usually, no serious problems result. However, sometimes the slow heart rate causes blood pressure to become so low that the brain does not get enough blood. Then, a person may feel dizzy or even faint.

Sinus bradycardia may be caused by a heart attack, a dangerously low body temperature (hypothermia), an underactive thyroid gland (hypothyroidism), or increased pressure within the skull (which can result from a hemorrhagic stroke). Conditions that briefly but strongly stimulate the vagus nerve (which controls heart rate) can temporarily slow the heart rate. Examples are vomiting or straining during a bowel movement or urination. Drugs given to slow the heart rate, such as beta-blockers and the calcium channel blockers verapamil and diltiazem, frequently cause sinus bradycardia.

The bradycardia-tachycardia syndrome is a type of sick sinus syndrome. In this disorder, slow heartbeats (bradycardia) alternate with fast heartbeats (tachycardia), such as atrial fibrillation and atrial flutter. Whether sick sinus syndrome causes any serious problems depends on the disorder causing it and the severity of the bradycardia or tachycardia.

The most common causes of sick sinus syndrome are coronary artery disease and scarring of the heart's electrical system.

Symptoms

Sinus bradycardia usually causes no symptoms. If the heart rate is less than 40 beats per minute, a person may feel weak and tired. If the rate is lower, the person may faint, because the brain does not get enough blood.

People with bradycardia-tachycardia syndrome may feel palpitations when the heart beats rapidly. When the heart beats slowly, they may feel dizzy. When the heart switches from beating rapidly to beating slowly, they may faint.

Diagnosis and Treatment

Doctors can often diagnose severe sinus bradycardia or sick sinus syndrome on the basis of symptoms and the results of electrocardiography (ECG). They may ask the person to wear a portable ECG (Holter) monitor to record the heart rhythm for 24 hours.

Many people with sinus bradycardia do not need treatment. If symptoms occur when straining during a bowel movement or urination, avoiding straining may prevent recurrences. If a beta-blocker or calcium channel blocker is the cause of a slow heart rate, discontinuing the drug or reducing the dose may relieve symptoms. If bradycardia causes symptoms and the cause cannot be corrected, a permanent artificial pacemaker is often implanted to speed up the heart rate. People with bradycardia-tachycardia syndrome may also need drugs to slow the rapid heart rate. After a pacemaker is implanted, a beta-blocker or a calcium channel blocker is often used to slow the heart rate and prevent tachycardia.