February 21, 2017

Atrial Fibrillation (AF, AFib)

Atrial fibrillation definition and facts

  • Atrial fibrillation (also referred to as AFib or AF) is the most common type of abnormal heart rhythm.
  • Atrial fibrillation is caused by abnormal electrical discharges (signals) that generate chaotically throughout the upper chambers of the heart (atria).
  • It reduces the ability of the atria to pump blood into the ventricles, and usually causes the heart to beat too rapidly.
  • One-half million new cases of are diagnosed every year in the U.S., and billions of dollars are spent annually on its diagnosis and treatment.
  • Symptoms of the condition include:
  • Some people have no signs or symptoms.
  • Complications of atrial fibrillation include heart failure and stroke.
  • The diagnosis of atrial fibrillation can be made by physical examination, electrocardiogram, Holter monitor, or patient-activated event recorder.
  • Treatment of for the condition is directed toward controlling underlying causes, slowing the heart rate and/or converting the heart to normal rhythm, and stroke prevention using blood-thinning medications.
  • Medications are commonly used in the longer term to control or prevent recurrence of AF, but medications may not be effective and may have intolerable side effects.
  • Electrical cardioversion is successful in over 95% of individuals with AF, but 75% of them have a recurrence of the disease within 1 to 2 years.
  • Some doctors may leave a patient in AF for the long term provided the rate is under control, blood flow is adequate, and blood is adequately thinned with medications.
  • Non-medication treatments of atrial fibrillation include pacemakers, AV node ablation, atrial defibrillators, and the Maze procedure.
  • Pulmonary vein isolation shows promise for the treatment of the disease and has a high rate of success; however, longer-term experience is necessary.
  • The prognosis for a person with this condition depends upon the cause and extent of this type of heart disease.

What is atrial fibrillation?

Atrial fibrillation is an abnormal and irregular heart rhythm in which the electrical signals are generated chaotically throughout the upper atria (chambers) of the heart. Many people with this condition have no signs or symptoms (asymptomatic). It is the most common supraventricular tachycardia.

What is the normal function of the heart, and how does its electrical system work?

What is the normal function of the heart?

The heart has four chambers (The upper two chambers are the atria, and the lower two chambers are the ventricles.); 1) the right atrium (upper right); 2) the left atrium (upper left); 3) the right ventricle (lower right); and 4) the left ventricle (lower left). Blood returning to the heart from the body contain low levels of oxygen and high levels of carbon dioxide. This blood flows into the right atrium and then down into the adjacent right ventricle. After the right ventricle fills, contraction of the right atrium pumps additional blood into the right ventricle. The right ventricle then contracts and pumps the blood to the lungs where the blood takes up oxygen and gives off carbon dioxide. The blood then flows from the lungs to the left atrium, and then down into the adjacent left ventricle. Contraction of the left atrium pumps additional blood into the left ventricle. The left ventricle then contracts and pumps the blood to the aortic artery and then to rest of the body via the vascular system. The heartbeat (pulse) that we feel is caused by the contraction of the ventricles. The ventricles must deliver enough blood to the body for the body to function normally. The amount of blood that is pumped depends on several factors. The most important factor is the rate of contraction of the heart (the heart rate). As the rate increases, more blood is pumped. The heart pumps more blood with each beat when the atria contract and fill the ventricles with additional blood just before the ventricles contract.

How does the electrical system of the heart work?

With each beat of the heart, an electrical discharge (current) passes through the electrical system of the heart. This discharge causes the muscle of the atria and ventricles to contract and pump blood.

The electrical system of the heart consists of the SA node (sinoatrial node), the AV node (atrioventricular node) and special tissues in the atria and the ventricles that conduct the current.

  • The SA node is the heart's electrical pacemaker. It is a small patch of cells located in the wall of the right atrium; the frequency with which the SA node discharges determines the rate at which the heart beats. The current passes from the SA node, through the special tissues of the atria and into the AV node.
  • The AV node serves as an relay station between the atria and the ventricles. Signals from the atria must pass through the AV node to reach the ventricles.

The electrical discharges from the SA node cause the atria to contract and pump blood into the ventricles. The same discharges then pass through the AV node to reach the ventricles, traveling through the special tissues of the ventricles and causing the ventricles to contract. In a normal heart, the rate of atrial contraction is the same as the rate of ventricular contraction.

At rest, the frequency of the discharges originating from the SA node is low, and the heart beats at the lower range of normal (60 to 80 beats/minute). During exercise or excitement, the frequency of discharges from the SA node increases, increasing the rate in individuals that are in good clinical health.

What causes atrial fibrillation?

  • The cause of atrial fibrillation is an abnormal heart rhythm. During this rhythm, electrical discharges are not generated solely by the SA node. Instead, discharges come from other parts of the atria. These abnormal discharges are rapid and irregular and may exceed 350 discharges per minute. The rapid and irregular discharges cause ineffective contractions of the atria. In fact, the atria quiver rather than beat as a unit. This reduces the ability of the atria to pump blood into the ventricles.
  • The rapid and irregular discharges from the atria then pass through the AV node and into the ventricles, causing the ventricles to contract irregularly and (usually) rapidly. The contractions of the ventricles may average 150/minute, much slower than the rate in the atria. (The ventricles are unable to contract at 350/minute.) Even at an average rate of 150/minute, the ventricles may not have enough time to fill maximally with blood before the next contraction, particularly without the normal contraction of the atria. Thus, AFib decreases the amount of blood pumped by the ventricles because of their rapid rate of contraction and the absence of normal atrial contractions.
Reviewed on 1/30/2017
Atrial Fibrillation Quiz