- Munther K Homoud, MD
Munther K Homoud, MD
- Associate Professor of Medicine
- Tufts University School of Medicine
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. However, the "normal" heart rate is, in part, the result of the complex interplay between the sympathetic and parasympathetic nervous systems. It is affected by numerous factors and varies in part with age and physical conditioning (table 1) [1,2]. Sinus arrhythmia, changes in the sinus rate as a result of respiration 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, sick sinus syndrome) is discussed in detail separately. (See "Sick sinus syndrome: Epidemiology, etiology, and natural history" and "Sick sinus syndrome: Clinical manifestations, diagnosis, and evaluation" and "Sick sinus syndrome: Treatment".)
DEFINITION AND ECG FEATURES
Normal sinus rhythm (NSR) is the characteristic rhythm of the healthy human heart. NSR is considered to be present in adults if the heart rate is between 60 and 100 beats per minute, the P waves on the electrocardiogram (ECG) are normal (unless there is concurrent atrial disease), and the rate does not vary significantly (waveform 1). The normal sinus P wave demonstrates right followed by left atrial depolarization giving rise to an upright P wave in leads I, II and aVL, and a negative P wave in lead aVR. The duration generally ranges between 80 and 110 milliseconds, and the amplitude is <0.25 millivolts. (See "ECG tutorial: Basic principles of ECG analysis", section on 'P wave'.)
By conventional definition, a bradycardia requires the heart rate to be less than 60 beats per minute with normal-appearing P waves on 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) . The rate at which a patient is labelled as having bradycardia is somewhat age dependent as heart rate decreases during early childhood but tends to increase normally with age.
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