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(4) Heart Attack and Cardiac Disorders

What are the electrical events of the heart?

Electrical events begun by the heart’s natural (intrinsic) built-in pacemaker (the tissue that triggers the electrical impulses) control the rhythmic and continuous blood circulation (the mechanical event). That pacemaker is called the sinus node.

The sinus node is a small bundle of muscle fibers that contains numerous pacemaker cells, and it’s located in the right atrium at the junction of the superior vena cava. The sinus node constantly receives signals from nerve centers in the brain and spinal cord.

They respond to the demands by the body under different circumstances. In addition, hormone released by certain glands (in this case, the adrenal and thyroid glands) closely controls the sinus node. In simple terms, the sinus node of the heart is similar to the spark plug in an automobile. Just as the spark plug makes electric sparks to ignite the fuel and run the engine, the sinus node emits regular electrical impulses-about 60 to 100 impulses per minute in adults-to contract the heart muscle and pump blood.

These impulses are what we call our heartbeat. The sinus node will continue firing these impulses for as long as a person maintains normal heart function during his or her entire life. When the sinus node fires an electrical impulse, it spreads the right atrium in a wave-like fashion from top to bottom. This electrical event is called right atrial activation and is followed by left atrial activation in a similar manner. Both right and left atrial activation produce a P wave on an electrocardiogram.

After both atria are fully activated, the heart impulses reach the atrioventricular (AV) node, another bundle of muscles located in the upper portion of the ventricular septum. The electrical impulse then soon passes down the “bundle of His” (the common bundle) and right and left bundle branches in a network of His (Purkinje) fibers to set the ventricles in motion. This ventricular activation can actually be seen as a large wave on an electrocardiogram.

It’s termed the QRS complex. After ventricular activation, the ventricles relax; this stage is called ventricular repolarization. It produces what is known as a T wave on an electrocardiogram.

The entire electrical event started by the sinus node and ending with the ventricular repolarization stage produces the P wave, the QRS complex, and the T wave on an electrocardiogram. This entire electrical process makes up one cardiac (heart) cycle. When the electrical event of the heart is disturbed in any way by various causes, various abnormal cardiac (heart) rhythms can be produced.

The abnormal heart rhythm may be too fast, too slow, or irregular, or it may be no heartbeat at all. The term cardiac arrhythmia or cardiac dysrhythmia is used to describe various abnormal heart rhythms.

A heart attack is the most common cause of various abnormal heart rhythms. Sudden death often occurs as a result of life-threatening arrhythmias, such as ventricular fibrillation, a very rapid, chaotic, ineffective, and irregular cardiac rhythm arising from the ventricles.

The electrical events of the heart occur harmoniously and rhythmically and at the same time as the mechanical events. That is, the atrial contraction is caused by the atrial activation. It occurs immediately before theopening of the mitral and tricuspid valves. The ventricular contraction is caused by the ventricular activation, the QRS complex on the electrocardiogram mentioned earlier.

Terms:

Sinus node - natural pacemaker of the heart.

Gland - a tissue structure controlling a specific function of the human body, such as the thyroid gland.

The sinus - node of the heart is similar to the spark plug in an automobile.

Electrocardiogram (ECG or EKG) - a recording of the electrical activity of the heart.

Cardiac arrhythmia - abnormal (slow, rapid, or irregular) heart rhythm.

Ventricular fibrillation - chaotic, irregular, and ineffective heart rhythm arising from the ventricles.

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