Normal sinus rhythm (NSR) is the characteristic rhythm of the healthy human heart. NSR is the rhythm that originates from the sinus node. The rate in NSR is generally regular but will vary depending on autonomic inputs into the sinus node. There can be an irregularity in the sinus rate and, when this occurs, it is termed "sinus arrhythmia". A sinus rhythm faster than the normal range is called a sinus tachycardia, while a slower rate is called a sinus bradycardia. (See "Sinus tachycardia" and "Sinus bradycardia".)
The sinoatrial (SA) node, due to its small mass, does not have a visible manifestation on the ECG. The behavior of the SA node, therefore, must be inferred from the atrial response. The upper right atrium is depolarized first, followed by the simultaneous depolarization of the remainder of the right and some of the left atrium, and finally by depolarization of the left atrial appendage. The blood supply and anatomy of the SA node along with the ECG characteristics of NSR and sinus arrhythmia will be discussed here. Abnormalities of SA nodal function are considered elsewhere. (See "Manifestations and causes of the sick sinus syndrome" and "Sinoatrial nodal pause, arrest, and exit block".)
NORMAL SINUS HEART RATE
The normal heart rate has been considered to be between 60 and 100 beats per minute, although there is some disagreement with regard to the normal rate in adults. The range (defined by 1st and 99th percentiles) is between 43 and 102 beats per minute in men and between 47 and 103 beats per minute in women (table 1) [1-3]. There is also important variability in age in young children. The normal heart rate is 110 to 150 beats per minute in infants, with gradual slowing over the first six years of life.
A variety of pharmacologic agents and physiologic conditions can result in changes to the normal sinus heart rate. These conditions are discussed in greater detail separately. (See "Sinus tachycardia", section on 'Etiology' and "Sinus bradycardia", section on 'Pathophysiologic sinus bradycardia'.)
The normal heart rate increases with exertion and decreases following the cessation of activity. The rate at which the heart rate returns to baseline following exercise can have prognostic importance, a concept which is discussed in greater detail elsewhere. (See "Exercise ECG testing to determine prognosis of coronary heart disease", section on 'Heart rate response to exercise' and "Exercise ECG testing to determine prognosis of coronary heart disease", section on 'Heart rate recovery after exercise'.)