DefinitionSick sinus syndrome is a collection of heart rhythm disorders that include:Sinus bradycardia -- slow heart rates from the natural pacemaker of the heartTachycardias -- fast heart ratesBradycardia-tachycardia -- alternating slow and fast heart rhythmsAlternative NamesBradycardia-tachycardia syndrome; Sinus node dysfunctionCauses, incidence, and risk factorsSick sinus syndrome is relatively uncommon. Sinus bradycardia occurs more often than the other types.Tachycardias that start in the upper chambers of the heart are also common forms. These include atrial fibrillation, atrial flutter/tachycardia, and supraventricular tachycardia. A period of elevated heart rates is typically followed by very slow heart rates when the tachycardia ends.Abnormal heart rhythms are often made worse by