Sinus Node Dysfunction
Sinus node dysfunction occurs with aging, trauma, ischemia, medications, and a number of metabolic abnormalities. The sinus node may be excessively slow or exhibit sinus pauses. Patients may also exhibit “chronotropic incompetence,” meaning the sinus node is unable to speed up to an appropriate rate with exercise. Commonly, electrical disturbances of the atrial myocardium co-exist, and patients can have atrial fibrillation and/or flutter alternating with sinus bradycardia (“Tachycardia-Bradycardia Syndrome”). Atrial tachyarrhythmias may conduct quite rapidly if AV nodal function is normal or if high levels of circulating catecholamines such as adrenaline are present. Post-tachycardia sinus pauses (as seen above) may result in severe symptoms such as syncope. One interesting type of sinus node dysfunction is known as sinoatrial exit block. This disorder of impulse conduction is characterized by marked sinus arrhythmia and/or sinus pauses where the next sinus beat occurs on time. Presumably, the sinus node automaticity is normal, but the impulse is unable to exit the sinus nodal complex and reach the surrounding atrial myocardium to result in a P-wave. There are many medications that can worsen sinus node dysfunction. These include digoxin, beta-blockers, verapamil and diltiazem, clonidine, and some antiarrhythmic drugs such as propafenone and amiodarone. Eye drops containing beta-blockers may be absorbed systemically and can affect sinus node function.
In general, the indications for a permanent pacemaker in sinus node dysfunction revolve around symptoms. There must be a clear correlation between symptoms and the occurrence of bradycardia. In the absence of symptoms, permanent pacing is generally not indicated. Patients with symptomatic sinus node dysfunction should receive a dual chamber rate adaptive pacemaker to allow for normal increases in heart rate with activity. On rare occasions, patients with sinus node dysfunction and completely normal AV node function can receive an atrial pacemaker only.