Sinus Rhythm: What's Normal and What's Not

Normal sinus rhythm (NSR) is another name for a normal heart rhythm. The heartbeat is controlled by electrical signals, called impulses, that arise from a structure in the heart called the sinus node. These signals are transmitted in a controlled way to the four chambers of the heart, causing them to contract and relax in sequence to pump blood in and out of the heart.

However, the sinus rhythm can sometimes become irregular, referred to as sinus arrhythmia. There are several types of arrhythmia that originate from the sinus node, some of which cause the heart to beat too slowly or too rapidly.

This article explains how the sinus rhythm regulates heartbeats. It also discusses how different arrhythmias occur, including their various causes and symptoms.

Normal Sinus Rhythm

The heart's rhythm is referred to as sinus rhythm because the electrical impulses are generated by the sinus node.

These impulses are what you see as up-and-down patterns of an electrocardiogram (ECG). With a normal sinus rhythm (NSR), the pattern will be uniform with high and low impulses falling within "normal" parameters.

The pattern of electrical impulses reflects the way that the chambers normally beat to allow blood in and out of the heart. There are two sets of heart chambers:

• Atrium: These are the upper chambers that receive blood from the body.
• Ventricles: These are the lower chambers that pump blood out of the heart and back into circulation.

The sinus node consists of a group of cells called pacemaker cells that are clustered in the right atrium of the heart. These cells produce electrical impulses that cause heart chambers to contract and relax in order so that blood can move through the heart as follows:

1. Blood enters the right atrium and passes into the right ventricle.
2. The right ventricle pumps blood to the lungs where it becomes oxygenated.
3. The oxygenated blood is brought back to the heart by the pulmonary veins and passes into the left atrium.
4. Blood passes into the left ventricle and is pumped out to the body.

A normal sinus rhythm is one in which the rate of firing is regular and neither too slow nor too quick.

Sinus Bradycardia

There are a number of circumstances in which it is normal for the sinus rhythm to be at the low end of the range of beats per minute. A slow heart rate is referred to as bradycardia.

Causes

Sinus bradycardia can occur normally during sleep, when most people experience a dip in heart rate to below 60 beats per minute. Trained athletes can have resting heart rates in the 40s even while awake.

Bradycardia may also be related to health problems that directly or indirectly affect the normal sinus rhythm. These include:

• Congenital heart disease (a heart defect present at birth)
• Heart damage caused by heart disease or heart attack
• Aging-related heart deterioration
• Obstructive sleep apnea (gaps in breathing during sleep)
• Myocarditis (heart inflammation)
• Hypothyroidism (low thyroid function)
• A complication of heart surgery
• Electrolyte imbalances, such as calcium and potassium
• Certain medications, like sedatives and opioids

Symptoms

A slow heart rhythm without symptoms is usually no cause for concern. However, when the heartbeat becomes too slow to pump enough blood throughout the body, treatment may be needed. This may include the use of a pacemaker.

When a person's heart rate is too slow while they're awake, they may have a disorder called sinus node disease or sick sinus syndrome. Symptoms include:

• Chest pain
• Severe fatigue
• Lightheadedness or dizziness
• Shortness of breath
• Confusion
• Fainting
  

Sinus Tachycardia

Sinus rhythm that is abnormally fast is referred to as sinus tachycardia. It can be normal for the heart rate to speed up during exercise to ensure there is extra blood to fuel the activity.

Causes

Sinus tachycardia also can occur in people for any number of health reasons, including:

• Fever
• Hypertension (high blood pressure)
• Hypotension (low blood pressure)
• Hyperthyroidism (overactive thyroid)
• Anemia (low red blood cells or hemoglobin)
• Electrolyte imbalances, like sodium and magnesium
• Heavy caffeine use
• Heavy alcohol use
• Certain medications, including some antidepressants and antibiotics
• Smoking
• Illegal drugs, like cocaine or methamphetamine

Some people have tachycardia for no apparent cause, referred to as inappropriate sinus tachycardia (IST). A similar condition called postural orthostatic tachycardia syndrome (POTS) occurs when a person stands upright, causing tachycardia without a drop in blood pressure.

Symptoms

Tachycardia may not cause recognizable symptoms in some people. When it does, symptoms may include:

• Rapid pulse
• A pounding heartbeat
• Palpitations (heart flutters or skipped beats)
• Chest pain
• Shortness of breath
• Dizziness or lightheadedness
• Fainting

Summary

A sinus rhythm is the beating of the heart caused by electrical impulses from the sinus node. The sinus node is a group of cells in the heart that generates these impulses, causing the heart chambers to contract and relax to move blood through the body.

A normal heartbeat is referred to as normal sinus rhythm (NSR). An abnormally slow heartbeat is called bradycardia, while an abnormally fast heartbeat is called tachycardia.

A Word From Verywell

Abnormally slow or abnormally fast heartbeats should never be ignored, especially if they occur for no known reason or are causing symptoms. The cause may end up being nothing serious, but it can also be a sign of an undiagnosed medical condition.

Speak with your healthcare provider who may refer you to a heart specialist known as a cardiologist. The cardiologist can perform an ECG and other tests used to measure the electrical activity or function of the heart muscle.

These tests can pinpoint the underlying cause of your condtion and ensure that you receive the appropriate treatment.