Introduction
Sinus node dysfunction (SND) is defined as an insufficient heart rate for physiologic demands. It may manifest as sinus bradycardia or slow heart rate, sinus pauses, sinus arrest with no heartbeat for a period of time or heartbeats generated by an “escape” mechanism from another part of the heart, or chronotropic incompetence, the inability of appropriate heart rate increase to match increased demand.
Presentation
Patients with SND may present with fatigue, dizziness, syncope or presyncope (fainting or feeling like about to faint). Less commonly, some experience shortness of breath with exertion or chest discomfort due to insufficient cardiac output resulting from the bradycardia. Confusion can result from insufficient blood supply to the brain, particularly in elderly patients, and disturbed sleep has been reported. More than 50% of patients with SND have associated atrial arrhythmias with atrial fibrillation being the most common. This is termed tachycardia-bradycardia syndrome, and these patients may complain of palpitations as well. Symptoms may be intermittent and insidious, with gradual worsening over time; more rarely, they may be quite severe at initial presentation. In some cases, patients with bradycardia or sinus pauses may be asymptomatic and are identified incidentally by resting electrocardiogram (ECG) or ambulatory ECG monitoring.
Diagnosis
The diagnosis of SND and ascertainment of potential need for intervention rests heavily on the association of symptoms with the observed bradycardia. Given the often nonspecific and intermittent nature of the symptoms it can sometimes be difficult to establish a symptom-rhythm correlation. A detailed history and physical is important to exclude other potential culprits, including ischemia, autonomic instability, and medications that might affect the heart rate, such as beta-blockers, calcium channel-blockers, digoxin, and antiarrhythmics. Other drugs such as clonidine (typically used for control of high blood pressure) and lithium (mood stabilizer) may also impact sinus node function. Ambulatory ECG monitoring may be helpful in identifying a temporal relationship between heart rate and symptoms. Less commonly exercise testing or, very rarely, an electrophysiology study (a minimally invasive procedure with catheters inserted into the heart to measure electrical outputs) may be performed.
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: Dizziness, fatigue, bradycardia, sinus pause, electrocardiogram, ambulatory ECG monitoring