Introduction
The sinoatrial or sinus node is a collection of specialized pacemaker cells near the junction of the right atrium and superior vena cava that serves as the primary pacemaker for the heart. Normal sinus rhythm is defined as a heart rate of 60-100 beats/minute, though particularly fit adults may have a slower resting heart rate due to high vagal tone.
Definition
Sinus node dysfunction (SND) also called sick sinus syndrome describes the inability of the sinus node to produce a heart rate that meets the physiologic needs of a person. SND may manifest in different ways. The rate at which the cells depolarize (produce a heartbeat) may slow resulting in sinus bradycardia, an insufficient resting heart rate. There may be sinus pauses or periods of complete sinus arrest, during which there are no outputs from the sinus node; this may be associated with no heartbeat for a period of time or another part of the heart may provide a slower “escape” rhythm. Chronotropic incompetence is a common early presentation of SND wherein the resting heart rate is normal but there is an inadequate heart rate response to exertion. Finally, an impulse may be generated by the sinus node but not be propagated to the rest of the atrium; this is called sinoatrial exit block.
Epidemiology
SND is most common among the elderly, with mean age of onset in the mid-70s. Men and women are similarly affected. In addition to advanced age, hypertension (high blood pressure), sleep apnea, obesity, and other cardiac disease are all associated with SND. More than 50% of patients with SND have associated atrial arrhythmias, with atrial fibrillation being the most common; this is called tachycardia-bradycardia syndrome. Other cardiac conduction system disease also frequently accompanies SND.
Etiology
The most common cause of SND is development of fibrosis that interferes with sinus node impulse generation or conduction to the surrounding atrium. Medications and toxins may contribute to or unmask SND; cardiac medications such as beta-blockers, calcium-channel blockers, digoxin, and antiarrhythmic medications are the main culprits, but other drugs such as clonidine (typically used for control of high blood pressure) and lithium (mood stabilizer) may also impact sinus node function. SND may also be secondary to infiltrative or inflammatory disorders, ischemia (insufficient blood supply) of the sinus nodal artery, metabolic derangements, or rare genetic conditions.
Melvin Scheinman MD and Cara Pellegrini MD, San Francisco, USA
References
Circulation Cardiology Patient Page “Sick Sinus Syndrome”. http://circ.ahajournals.org/content/108/20/e143.full
Mayo Clinic. http://www.mayoclinic.org/diseases-conditions/sick-sinus-syndrome/basics/definition/con-20029161
Key words: Bradycardia, sinus pause, sinus arrest, chronotropic incompetence