Causes

Your heart is made up of four chambers — two upper (atria) and two lower (ventricles). The rhythm of your heart is normally controlled by the sinus node, an area of specialized cells in the right upper heart chamber (atrium).

This natural pacemaker produces electrical signals that trigger each heartbeat. From the sinus node, electrical signals travel across the atria to the ventricles, causing them to contract and pump blood to your lungs and body.

If you have sick sinus syndrome, your sinus node isn't working properly, causing your heart rate to be too slow (bradycardia), too fast (tachycardia) or irregular.

Problems of the sinus node include the following:

• Sinus bradycardia. The sinus node produces an electrical charge at a slower rate than normal.
• Sinus arrest. Signals from the sinus node pause, causing skipped beats.
• Sinoatrial exit block. Signals to the upper heart chambers are slowed or blocked, causing a pause or skipped beats.
• Chronotropic incompetence. The heart rate is normal at rest, but doesn't increase with physical activity.
• Bradycardia-tachycardia syndrome. The heart rate alternates between abnormally slow and fast rhythms, usually with a long pause (asystole) between heartbeats.

What makes the sinus node misfire?

Abnormalities of the sinus node may be caused by the following:

• Age-related wear and tear of heart tissues
• Heart disease
• Inflammatory diseases affecting the heart
• Damage to the sinus node or scarring from heart surgery
• Medications to treat high blood pressure, including calcium channel blockers and beta blockers
• Drugs to treat irregular heartbeats (arrhythmias)
• Some Alzheimer's disease medications
• Neuromuscular diseases, such as muscular dystrophy
• Obstructive sleep apnea
• Rare genetic mutations