INTRODUCTION

Sinus bradycardia is a rhythm in which the rate of impulses arising from the sinoatrial (SA) node is lower than expected.

The normal adult heart rate, arising from the SA node, has been considered historically to range from 60 to 100 beats per minute, with sinus bradycardia being defined as a sinus rhythm with a rate below 60 beats per minute. The heart rate reflects a complex interplay between the sympathetic and parasympathetic nervous systems. It is affected by numerous factors, including age and physical conditioning (table 1) [1,2]. Sinus arrhythmia, the fluctuation in sinus rate with respiratory cycles, often accompanies sinus bradycardia. (See "Normal sinus rhythm and sinus arrhythmia".)

The etiology, clinical presentation, evaluation, and management of sinus bradycardia will be reviewed here. Primary sinus node dysfunction (ie, sinus node dysfunction) is discussed in detail separately. (See "Sinus node dysfunction: Epidemiology, etiology, and natural history" and "Sinus node dysfunction: Clinical manifestations, diagnosis, and evaluation" and "Sinus node dysfunction: Treatment".)

DEFINITION AND ECG FEATURES

Normal sinus rhythm (NSR) is the characteristic rhythm of the healthy heart. NSR is considered to be present in adults if the heart rate is between 60 and 100 beats per minute, the P wave vector on the electrocardiogram (ECG) is normal (ie, consistent with SA nodal impulse origin), and the rate is largely regular (waveform 1). The normal sinus P wave demonstrates right atrial followed by left atrial depolarization and high to low atrial activation giving rise to an upright P wave in leads I, II, and aVL, and a negative P wave in lead aVR.

By conventional definition, bradycardia indicates a heart rate less than 60 beats per minute with a normal P wave vector on the surface ECG. As such, sinus bradycardia is typically thought of as sinus rhythm occurring at a rate of less than 60 beats per minute, although one professional society has advocated a rate of less than 50 beats per minute (waveform 2) [3]. A rate less than 50 beats per minute may be a more pragmatic definition, as most patients with sinus rates in the 50s are asymptomatic. It is important to note that the rate at which a patient is labeled as having bradycardia is somewhat age dependent.