How Your Heart Beats

The electrical heart conduction pathway must be followed to ensure the heart pumps properly.

The heartbeat starts in the right atrium when a special group of cells (the sinus node or "pacemaker" of the heart) sends an electrical signal.

The signal spreads throughout the atria and to the atrioventricular (A-V) node.

The A-V node connects to a group of fibers in the ventricles that conduct the electric signal.

The impulse travels down these specialized fibers to all parts of the ventricles.

Atrial Fibrillation

What is an arrhythmia?

Arrhythmias (or dysrhythmias) are abnormal rhythms of the heart which cause the heart to pump less effectively.

Normally, as the electrical impulse moves through the heart, the heart contracts - about 60 to 100 times a minute. Each contraction represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract.

Under some conditions almost all heart tissue is capable of starting a heartbeat, or becoming the pacemaker. An arrhythmia occurs when:

  • the heart's natural pacemaker develops an abnormal rate or rhythm.
  • the normal conduction pathway is interrupted.
  • another part of the heart takes over as pacemaker.

What is an electrocardiogram (ECG)?

The electrical activity of the heart is measured by an electrocardiogram (ECG or EKG). By placing electrodes at specific locations on the body (chest, arms, and legs), a graphic representation, or tracing, of the electrical activity can be obtained. Changes in an ECG from the normal tracing can indicate arrhythmias, as well as other heart-related conditions.

How does the physician know what an ECG means?

Almost everyone knows what a basic ECG tracing looks like. But what does it mean?

Illustration of  a basic EKG tracing
  • The first little upward notch of the ECG tracing is called the "P wave." The P wave indicates that the atria (the two upper chambers of the heart) are electrically stimulated to pump blood to the ventricles.
  • The next part of the tracing is a short downward section connected to a tall upward section. This spike-like section is called the "QRS complex." This part indicates that the ventricles (the two lower chambers of the heart) are electrically stimulated ("depolarization") to pump out blood.
  • The next short flat segment is called the "ST segment." The ST segment indicates the amount of time from the end of the electrical signal for the contraction of the ventricles to the beginning of the "T wave".
  • The next upward curve is the T wave. The T wave indicates the electrical recovery period ("repolarization") of the ventricles.

When your physician studies your ECG, he/she looks at the size and length of each part of the ECG. Variations in size and length of the different parts of the tracing may be significant. The tracing for each lead of a 12-lead ECG will look different, but will have the same basic components as described above. Each lead of the 12-lead is "looking" at a specific part of the heart, so variations in a lead may indicate a problem with the part of the heart associated with the lead.

What is atrial fibrillation?

Atrial fibrillation is a type of arrhythmia. With atrial fibrillation, the electrical signals in the atria (the two small chambers of the heart) are fired in a very fast and uncontrolled manner. The atria quiver instead of contract. The electrical signals then arrive in the ventricles in an irregular fashion. When atria do not contract effectively, the blood may pool and/or clot. If a blood clot becomes lodged in an artery in the brain, a stroke (brain attack) may occur. About 15 percent of strokes occur in persons with atrial fibrillation. Aspirin, warfarin, and other cardiac medications may be used to treat atrial fibrillation.

How is atrial fibrillation treated?

According to the latest recommendations from the American College of Physicians and the American Academy of Family Physicians, for most patients with atrial fibrillation, slowing heart rate with beta-blocker medication such as atenolol or metoprolol, or calcium-channel blockers such as diltiazem or verapamil, is the most appropriate treatment. Patients with atrial fibrillation should receive blood thinners (warfarin) to prevent stroke unless they have a condition that would make anticoagulation dangerous (such as alcoholism with frequent falls).

Patients who choose conversion to normal heart rhythm instead of rate control because of symptoms can select electrical or medical cardioversion. For patients who choose cardioversion, outcomes are similar whether patients have immediate cardioversion following a special test (transesophageal echocardiogram) to make sure no blood clots exist in the atria or if they delay cardioversion until blood thinners take effect. Most patients take medications to maintain normal rhythm after cardioversion.

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