Sinus bradycardia is a rhythm in which fewer than the normal number of impulses arise from the sinoatrial (SA) node. The normal heart rate 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, a study in 500 normal subjects using an electrocardiographically recorded heart rate found the mean afternoon heart rate for men and women to be about 70 beats per minute with two standard deviation limits of 46 and 93 beats per minute in men and 51 and 95 beats per minute for women; there was no significant age-related effect (table 1) [1,2]. Sinus arrhythmia often accompanies sinus bradycardia, but the P waves have a normal morphology unless atrial disease is present. A PR interval of up to 0.21 is normal in sinus bradycardia. (See "Normal sinus rhythm and sinus arrhythmia".)
The causes, manifestations and management of sinus bradycardia, both in the normal heart and secondary to other conditions, are reviewed here. Primary sinus node dysfunction (sick sinus syndrome) is discussed in detail separately. (See "Manifestations and causes of the sick sinus syndrome" and "Treatment of the sick sinus syndrome".)
SINUS BRADYCARDIA IN THE NORMAL HEART
Sinus bradycardia occurs in normal children and adults, particularly during sleep when rates may transiently drop as low as of 30 beats per minute and pauses of up to 2 seconds are not uncommon [3-6]. It may also be seen in the absence of heart disease in the following settings:
●At rest, in 25 to 35 percent of asymptomatic individuals under 25 years of age .
●In well-conditioned athletes [8,9].