Sick sinus syndrome (SSS) is a disorder of the conducting tissue that controls the heart rate and rhythm. The sinoatrial (SA) node is located in the right atrium (upper chamber of the heart) and initiates an electrical impulse that spreads through specialized tissue of the heart and triggers the heart to beat. Dogs with SSS develop changes in the SA node and other conducting tissue that results in irregular heart rhythms including bradycardia (slow heart rates), pauses, or sinus arrest. Another variation, referred to as brady-tachy syndrome, involves periods of bradycardia followed by tachycardia (fast heart rates). The cause of SSS is unknown but likely involves degeneration of the conduction tissue. It likely has a genetic component since certain breeds, such as the miniature schnauzer, are predisposed to SSS. Other breeds that are commonly affected include the West Highland white terriers, cockers spaniels, dachshunds, and pugs.

Clinical signs of SSS vary depending on the frequency and severity of the arrhythmias. Brief periods of tachycardia or bradycardia may not result in any noticeable changes. However, sustained periods of bradycardia or pauses often result in signs of syncope (collapse), muscle weakness, or exercise intolerance. Work-up typically includes blood pressure measurement, electrocardiography (ECG), radiographs (x-rays), echocardiogram (ultrasound of the heart) and 24-hour Holter monitor evaluation. An atropine challenge may also be performed to test how well the sinus node is responding. In some cases, a cardiac event monitor is needed for a more definitive diagnosis.

Treatment varies on the frequency of the arrhythmias and associated clinical signs. Dogs that are asymptomatic do not usually require treatment; however, SSS can progress over time and may warrant treatment at a later date. Since symptoms are usually associated with bradycardia, treatment is aimed at preventing slow heart rates or pauses. Although medical therapy is an option it is often ineffective. Therefore, an artificial pacema┬Čker is usually recommended to prevent the heart rate from dropping to low in patients that are weak or collapsing from SSS.