What You Should Know About Normal Sinus Rhythm

Normal sinus rhythm (NSR) is another name for the normal heart rhythm. The heartbeat is controlled by regular electric signals (also called an electrical impulses) that spontaneously arise in a structure called the sinus node. These signals then spread across the heart starting at the atria and then the ventricles.

This orderly spread of the electrical impulse ensures the distinct portions of the heart contract in an orderly, sequential way: First the atria beat, ejecting blood into the ventricles and then the ventricles contract; the right ventricle ejects blood into the lungs and the left ventricle ejts blood to the rest of the body.

A normal sinus rhythm chart
Ed Reschke / Getty Images

The heart's rhythm is referred to as “sinus rhythm” because the electrical impulse is generated in the sinus node. A normal sinus rhythm is one in which the rate of firing is not too fast nor too slow.

Normal sinus rhythm is generally defined as between 60 beats and 99 beats per minute.

Sinus Bradycardia

There are a number of circumstances in which it's normal for the sinus rhythm to be at the low end of the range of beats per minute or at the high end. A slow heart rate is called bradycardia. Sinus bradycardia occurs during sleep, for example, when most people experience a dip in heart rate to well below 60 beats per minute. Trained athletes can have resting heart rates in the 40s even while awake.

However, when a person's heart rate is too low while they're awake, they may have a disorder called sinus node disease, or sick sinus syndrome, which can cause them to get tired easily and experience other symptoms such as lightheadedness and fainting.Sinus node disease often must be treated with a pacemaker.

Sinus Tachycardia

Sinus rhythm that is at the high end of the normal range is referred to as sinus tachycardia. It can be normal for the heart rate to speed up during exercise, as this allows it to pump extra blood throughout the body to fuel physical exertion.

Sinus tachycardia also can occur under circumstances that aren't normal, such as during periods of extreme stress or when someone has a fever. An abnormally fast heart rate also can be a symptom of hyperthyroidism or other medical problems that require a higher cardiac output (such as anemia)

A form of sinus tachycardia called sinus re-entrant tachycardia is a rare type that comes and goes suddenly and is caused by extra electrical pathways within the sinus node. It is sometimes treated with ablation.

Some people have sinus tachycardia without any apparent underlying cause, a condition called inappropriate sinus tachycardia (IST). A similar condition is seen called postural orthostatic tachycardia syndrome (POTS), in which sinus tachycardia—and a drop in blood pressure—occurs when someone is standing upright. Both IST and POTS often produce significant palpitations, lightheadedness, and other symptoms and can be challenging diagnoses.

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