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What is the Sinoatrial Node?

The sinoatrial node (SA node) is a bundle of nerve cells that function as a natural pacemaker for the heart. The SA node creates and sends electrical signals throughout the heart, causing the heart muscle to contract and pump blood through the body. The sinoatrial node is also called the sinus node.

Location

The heart has four chambers; the top two are called atria (singular: atrium), and the bottom two are called ventricles. The sinoatrial node is located in the center of the heart on the right atrial wall. It is near the opening of the superior vena cava, which is one of the primary veins of the heart.

Understanding the cardiac conduction cycle

The heart has two nodes: the sinoatrial node and the atrioventricular node (AV node). Most nervous tissue only relays electrical signals throughout the body. The SA and AV nodes are different. The SA node is longer than the AV node and looks like a flattened banana. The shorter AV node has a half-oval shape. However, both specialized nodes can contract like muscle tissue and produce or relay signals like nerve tissue.

Beginning in the SA node and atria

The signal starts in the sinoatrial node. The tissue contracts rhythmically, creating and releasing electrical impulses. This depolarization and repolarization process is responsible for setting the pulse rate. Healthy adults should have a resting pulse rate of 60-80 beats per minute.

When the SA node fires, the electrical impulse causes the atria to contract. The right atrium draws in deoxygenated blood from the rest of the body via the heart’s primary veins: the superior vena cava and inferior vena cava. The left atrium receives oxygen-rich blood from the lungs via the pulmonary vein.

Following the path to the AV node

If the sinoatrial node is a pacemaker, the atrioventricular node is a pace regulator. The same signal which makes the atria contract travels downward to the AV node, near the center of the heart. The AV node sits on the right side of the wall between the left and right atria, close to the bottom of the right atrium. The atrioventricular node acts as a gate, delaying the signal for a split second before passing it on to the ventricles. The delay gives the atria time to contract and empty their contents into the ventricles.

Relaying impulses to the ventricles

The delayed electrical signal travels into a bundle of nerve fibers called the atrioventricular bundle. The bundle splits into two bundles, which carry the impulses to the right and left ventricles. The bundles transmit the signal to specialized fibers called Purkinje fibers, which are spread throughout the ventricles. The signal tells the ventricles to contract, and blood flows to the rest of the body. The right ventricle is responsible for returning blood to the lungs, while the left ventricle supplies the rest of the body.

The autonomous nervous system

The entire process is governed by the autonomous nervous system, which controls automatic functions like breathing, digestion, and blood pressure. This nervous system and the cardiac conduction cycle are so connected that responses between the two only take a few seconds. When you ride a bike or swim, the autonomous nervous system tells the SA node you need more oxygen. The node responds by increasing your heart rate, which increases blood flow throughout the body.

Understanding sinoatrial node dysfunction (SND)

When the SA node does not work correctly, other cells in the heart try to mimic its function to keep the heart beating. This results in a signal overload for the heart, preventing the heart’s atria from contracting completely. The oxygenated and deoxygenated blood are not fully exchanged, which decreases the available oxygen level in the blood.

SND may go by other names, including sinoatrial node disease. When the cause of the sinoatrial node dysfunction cannot be determined, medical professionals may use the term “sick sinus syndrome” (SSS).

Types of SND

Sinoatrial node dysfunction is classified according to the affect it has on the heart. The following disorders are all categorized as SND or SSS.

  • Bradycardia – slow heartbeat, under 60 beats per minute
  • Tachycardia – resting heartbeat is too fast
  • Bradycardia-tachycardia syndrome – the heart randomly beats too slowly or quickly
  • Sinoatrial block – the electrical signal from the sinus node cannot reach the atria
  • Sinus arrest or pause – the SA node temporarily quits working or skips a beat or two; also called atrial fibrillation

Symptoms of sinoatrial node dysfunction

People with SND may not experience any symptoms. However, it is more likely that the symptoms are subtle enough that the patient may not recognize the severity of their condition. For example, a sluggish pulse can cause fatigue and weakness. However, a person experiencing this symptom could assume they simply need more sleep.

A person with SND may experience more than one of the following symptoms, but not necessarily all of them.

  • Fainting or feeling lightheaded
  • Dizziness
  • Fatigue
  • Chest pain
  • Slow pulse
  • Abnormal heartbeat
  • Trouble breathing
  • Mental confusion or memory problems
  • Restless sleep

People with SND have a higher risk of a heart attack, especially if they have a family history of heart disease. See your current medical provider immediately if you experience upper body pain, especially in the chest or arms, profuse sweating, trouble breathing, panic or confusion, dizziness, blackouts, vomiting, or abdominal pain.

Assessing sinus node dysfunction risk factors

Elderly people have a higher risk of developing SND. The primary cause of sinus node dysfunction in children and young adults are heart defects that are present at birth. Thyroid disease, sleep apnea, and coronary artery disease can also cause SND. Other factors in developing SND include scar tissue in the heart from disease or previous surgeries, calcium channel blockers or beta blockers use to treat high blood pressure, or a weakened heart muscle due to aging.

Diagnosing and Treating SND

Doctors may use a few different tests to diagnose sinus node dysfunction. The simplest of these is an electrocardiogram (ECG). In this test, electrical impulses on the skin are measured with electrodes. If the symptoms are subtle or random, the doctor may ask the patient to wear a Holter monitor. This portable ECG measures heart activity over a period of 24-48 hours. If necessary, the physician may order an echocardiogram. This ultrasonic image test can provide more information on blood flow in the heart.

Less often, the doctor may order a transesophageal echocardiogram (TEE). An ultrasound device is inserted into the patient’s throat and threaded through the esophagus to get a clear picture of the heart’s size. A TEE also provides specific information regarding the heart’s strength and any damaged areas.

Treatment options

For patients with bradycardia, a pacemaker is the best option. The patient is put to sleep while a thin metal box with electrodes is inserted into the heart. The pacemaker takes over for the damaged SA node, regulating the patient’s heartbeat.

When patients have tachycardia, the physician may prescribe antiarrhythmic drugs. These medications help keep the heart from beating too fast. Patients with bradycardia-tachycardia syndrome may receive both a pacemaker and antiarrhythmic medications.

Diet and exercise are both important parts of recovery and prevention. Your doctor may ask you to limit or refrain from certain foods, especially high-fat or fried foods, caffeine, and alcohol. He or she may also recommend moderate aerobic activity on a daily basis, such as swimming, walking, or jogging. Like any muscle, the heart weakens over time if it is not used regularly. Following a heart-healthy diet and staying active will help your heart function normally.

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