The heart is a muscle that works like a pump. It's divided into four chambers — two atria at the top of the heart and two ventricles at the bottom. The heart beats (contracts) when an electrical impulse from the heart's "natural pacemaker" — the sinoatrial or SA node — moves through it. The normal sequence begins in the right atrium, spreads throughout the atria and to the atrioventricular (AV) node. From the AV node, the impulses (signals) travel down a group of specialized fibers (the His-Purkinje system) to all parts of the ventricles. This exact route must be followed for the heart to pump properly. As long as the electrical impulse is transmitted normally, the heart pumps and beats at a regular pace. A normal heart beats 60 to 100 times a minute.
The term arrhythmia refers to any change from the normal sequence of electrical impulses, causing abnormal heart rhythms. This can cause the heart to pump less effectively. Some arrhythmias are so brief (for example, a temporary pause or premature beat) that the overall heart rate or rhythm isn't greatly affected. But if arrhythmias last for some time, they may cause the heart rate to be too slow or too fast or the heart rhythm to be erratic.
The term tachycardia refers to a heart rate of more than 100 beats per minute. Bradycardia describes a rate of less than 60 beats per minute.
Tachycardia
A fast heart rate is called tachycardia. Tachycardias fall into two major categories. Ventricular tachycardia involves only the ventricles. Supraventricular tachycardia involves both the atria and the ventricles. The definition of "too fast" usually depends upon the person's age and physical activity.
What causes tachycardia?
Under certain conditions, the automatic firing rate of secondary pacemaker tissue may become too fast. If such an abnormal "focus" fires faster than the sinus node, it may take over control of the heart rhythm and cause tachycardia.
In another type of abnormal conduction, impulses get caught in a merry-go-round-like sequence. This process, called reentry, is a common cause of tachycardias. Regardless of their cause, tachycardias are classified by where they arise. Thus, ventricular tachycardias begin in the heart's ventricles (lower chambers). Supraventricular tachycardias arise higher in the heart — either in the upper chambers (atria) or the middle region (AV node or the very beginning portion of the His-Purkinje system.)
What are the symptoms and treatments for tachycardia (rapid heart beating)?
Rapid heart beating can produce symptoms of palpitations, rapid heart action, dizziness, lightheadedness, fainting or near fainting. Heartbeats may have either a regular or irregular rhythm. Rapid heart beating in the ventricles — called ventricular tachycardia — can be life-threatening. The most serious cardiac rhythm disturbance is ventricular fibrillation, where the lower chambers quiver and the heart can't pump any blood. Collapse and sudden death follows unless medical help is provided immediately.
If treated in time, ventricular tachycardia and ventricular fibrillation can be converted into normal rhythm with electrical shock. Rapid heart beating can be controlled with medications and by identifying or destroying the focus of rhythm disturbances. One effective way of correcting these life-threatening rhythms is by using an electronic device called an implantable cardioverter defibrillator.
Bradycardia
A heart rate that's "too slow" is called bradycardia. What's "too slow" depends upon a person's age and physical activity.
What are the symptoms and treatments for bradycardia (slow heartbeat)?
A heart rhythm that's too slow can cause fatigue, dizziness, lightheadedness, fainting or near-fainting spells. These symptoms can be easily corrected by implanting an electronic pacemaker under the skin to speed up the heart rhythm as needed.
