Sick sinus syndrome: A condition that features symptoms including dizziness, confusion, fainting, and heart failure that is due to a problem with the sinoatrial node (SA node) of the heart, which acts as the body's natural pacemaker. If the SA node is not functioning normally, the patient usually has an abnormally slow heart rate (bradycardia), but other rhythm disturbances, including rapid heart rate (tachycardia) and an alternating fast and slow heart rate (bradycardia-tachycardia syndrome) can also occur. These disturbances can cause poor pumping by the heart, which can impair the circulation. Diagnosis is usually made via electrocardiogram (EKG). Treatment includes use of medications, such as calcium antagonists.