The sick sinus syndrome is characterized by intrinsic inad-equacy of sinus node pacemaking and/or conduction failure be-tween the sinus node and the rest of the atrium. In the brady-cardia-tachycardia syndrome, recurrent supraventricular tach-yarrhythmias alternate with sinus bradycardia and/or subatrial bradyarrhythmias. Conduction disturbances are common in the atria, AV node, bundle branches, and ventricles, but ventric-ular ectopic activity is rare.

Symptoms in sick sinus syndrome may be intermittent, varied, and difficult to correlate with ECG changes. Syncope, dizziness, and palpitations are common, probably because these symptoms are used for case finding and diagnosis. Con-gestive heart failure or angina can be aggravated. Cerebral thromboembolism is common in the bradycardia-tachycardia syndrome.

Management Persistent, symptomatic sinus bradycardia is an indication for pacemaker therapy. Digitalis can be used to control the atrial tachyarrhythmis and, contrary to expecta-tion, usually does not aggravate coexistent bradyarrhythmias. After pacemaker implantation, drugs with class I antiarrhyth-mic action can be used to control tachyarrhythmias. Symptoms can be improved with pacemaker therapy in the bradycardia-tachycardia syndrome, but cerebral thromboembolism contin-ues. The prognosis of effectively treated sick sinus syndrome is determined by associated heart disease. Treatment of atrial fib-rillation in the sick sinus syndrome can cause severe bradycar-dia. A temporary ventricular pacemaker should be used when attempting to convert atrial fibrillation with slow ventricular rate to sinus rhythm.

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