Disorders of Sinus Function

Sinus node dysfunction is a common entity with significant clinical implications. Establishing a diagnosis may, on occasion, tax the skills of the clinician. Many causes have been cited, but no single factor appears to be established. Immunologic abnormalities may play a part in the etiologic process. Clinical invasive electrophysiology studies may be used to establish a diagnosis. In general, medical therapy must be integrated. Controversy exists regarding the best method of permanent pacing. Treatment may need to be individualized to the type of arrhythmia noted. Long-term prognosis is a large factor in choice of therapy, related to the underlying disease. Prevention of atrial fibrillation may occur with dual-chamber pacing; however, anticoagulation appears essential in this patient subgroup. The 5-year mortality rate in these patients is high and does not appear to be significantly improved with artificial pacing. Mortality is prominently influenced by the coexistence of cardiovascular and valvular heart disease. Patients who die do not differ substantially from survivors with regard to type of sinus dysfunction, occurrence of tachyarrhythmia, or distal conduction abnormalities. The survival rate in patients with sick sinus syndrome and congestive heart failure is significantly lower, and the incidence of embolic events remains high in patients with permanent pacing and the sick sinus syndrome. Thus, it has been proposed that all patients exhibiting the bradycardia-tachycardia syndrome be fully anticoagulated. The incidence of atrial fibrillation is significantly lower in patients with atrial demand pacing (22.3% versus 3.9%) than in patients with ventricular pacing and is accompanied by a decreased incidence of systemic embolization (13% versus 1.6%). Reports comparing survival with use of dual-chamber pacing versus ventricular pacing are encouraging in patients with congestive heart failure. At present, the natural history of the disease is unknown; furthermore, clinical risk factors for the development of symptoms have not been defined, and no electrophysiologic measure of sinus node function has been demonstrated to have reliable predictive value. Therefore, common practice has been to withhold pacemaker therapy in the asymptomatic patient.