Many things can lead to, or cause, an arrhythmia, including:
- A heart attack that's occurring right now
- Scarring of heart tissue from a prior heart attack
- Changes to your heart's structure, such as from cardiomyopathy
- Blocked arteries in your heart (coronary artery disease)
- High blood pressure
- Overactive thyroid gland (hyperthyroidism)
- Drinking too much alcohol or caffeine
- Drug abuse
- Dietary supplements and herbal treatments
- Electrical shock
- Air pollution
What's a normal heartbeat?
When your heart beats, the electrical impulses that cause it to contract follow a precise pathway through your heart. Any interruption in these impulses can cause an arrhythmia.
Your heart is divided into four chambers. The chambers on each half of your heart form two adjoining pumps, with an upper chamber (atrium) and a lower chamber (ventricle).
During a heartbeat, the atria contract and fill the relaxed ventricles with blood. This contraction starts when the sinus node — a small group of cells in your right atrium — sends an electrical impulse causing your right and left atria to contract.
The impulse then travels to the center of your heart, to the atrioventricular node that lies on the pathway between your atria and your ventricles. From here, the impulse exits the atrioventricular node and travels through your ventricles.
In a healthy heart, this process usually goes smoothly, resulting in a normal resting heart rate of 60 to 100 beats a minute. Conditioned athletes at rest commonly have a heart rate less than 60 beats a minute because their hearts are so efficient.
Types of arrhythmias
Doctors classify arrhythmias not only by where they originate (atria or ventricles) but also by the speed of heart rate they cause:
- Tachycardia (tak-ih-KAHR-dee-uh). This refers to a fast heartbeat — a resting heart rate greater than 100 beats a minute.
- Bradycardia (brad-e-KAHR-dee-uh). This refers to a slow heartbeat — a resting heart rate less than 60 beats a minute.
Not all tachycardias or bradycardias mean you have heart disease. For example, during exercise it's normal to develop tachycardia as the heart speeds up to provide your tissues with more oxygen-rich blood.
Tachycardias in the atria
Tachycardias originating in the atria include:
- Atrial fibrillation. This fast and chaotic beating of the atrial chambers is a common arrhythmia. It often affects older people. Your risk of developing atrial fibrillation increases with age, mostly due to wear and tear on your heart, especially if you've had high blood pressure or other heart problems. During atrial fibrillation, the electrical signal that causes your heart to beat becomes uncoordinated. The atria beat so rapidly — as fast as 300 or more beats a minute — that instead of producing a single forceful contraction, they quiver (fibrillate). Atrial fibrillation can be dangerous, for over time it can cause more-serious conditions, such as stroke.
- Atrial flutter. Atrial flutter is similar to atrial fibrillation. Both can occur, coming and going in an alternating fashion. The heartbeats in atrial flutter are more-organized and more-rhythmic electrical impulses than in atrial fibrillation.
- Supraventricular tachycardia (SVT). SVT is a broad term that includes many forms of arrhythmia originating above the ventricles (supraventricular). SVTs usually cause a burst of rapid heartbeats that begins and ends suddenly and can last from seconds to hours. These bursts often start when the electrical impulse from a heartbeat begins to circle repeatedly through an extra pathway. Although SVT is generally not life-threatening in an otherwise normal heart, symptoms from the racing heart may feel quite uncomfortable.
- Wolff-Parkinson-White syndrome. One cause of SVT is known as Wolff-Parkinson-White syndrome. People with this condition have an extra electrical pathway between the atria and the ventricles. This pathway may allow electrical signals to pass between the atria and the ventricles without passing through the atrioventricular node, leading to short circuits and rapid heartbeats.
Tachycardias in the ventricles
Tachycardias occurring in the ventricles include:
- Ventricular tachycardia (VT). This fast, regular beating of the heart is caused by abnormal electrical impulses that start in the ventricles. Often these are due to a problem with the electrical impulse traveling around a scar from a previous heart attack. VT can cause the ventricles to contract more than 200 beats a minute.
Most VT episodes occur in people with some form of heart-related problem, such as scars or damage within the ventricle muscle from coronary artery disease or a heart attack. Sometimes VT can last for 30 seconds or less (unsustained), and it might not cause any symptoms, although it causes inefficient heartbeats. Still, an unsustained VT may put you at risk of more-serious ventricular arrhythmias, such as longer lasting (sustained) VT. An episode of sustained VT is a medical emergency. Without prompt medical treatment, sustained ventricular tachycardia often worsens into ventricular fibrillation.
- Ventricular fibrillation. In ventricular fibrillation, rapid, chaotic electrical impulses cause your ventricles to quiver uselessly instead of pumping blood. Without an effective heartbeat, your blood pressure plummets, cutting off blood supply to your vital organs — including your brain. Most people lose consciousness within seconds and require immediate medical assistance, including chest compressions, defibrillation and cardiopulmonary resuscitation (CPR). Your chances of survival are better if chest compressions are delivered until your heart can be shocked back into a normal rhythm with a device called a defibrillator. Without CPR or defibrillation, death results in minutes. Most cases of ventricular fibrillation are linked to some form of heart disease. Ventricular fibrillation is frequently triggered by a heart attack.
- Long QT syndrome. Long QT syndrome (LQTS) is a heart disorder that carries an increased risk of fast, chaotic heartbeats. The rapid heartbeats, caused by changes in the electrical system of your heart, may lead to fainting, which can be life-threatening. In some cases, your heart's rhythm may be so erratic that it can cause sudden death.
You can be born with a genetic mutation that puts you at risk of long QT syndrome. In addition, more than 50 medications, many of them common, may cause long QT syndrome. Some medical conditions, such as congenital heart defects, also may cause long QT syndrome.
Bradycardia — a slow heartbeat
Although a heart rate below 60 beats a minute while at rest is considered bradycardia, a low resting heart rate doesn't always signal a problem. If you're physically fit, you may have an efficient heart capable of pumping an adequate supply of blood with fewer than 60 beats a minute at rest. However, if you have a slow heart rate and your heart isn't pumping enough blood, you may have one of several bradycardias, including:
- Sick sinus. If your sinus node, which is responsible for setting the pace of your heart, isn't sending impulses properly, your heart rate may be too slow, or it may speed up and slow down intermittently. Sick sinus can also be caused by scarring near the sinus node that's slowing, disrupting or blocking the travel of impulses.
- Conduction block. A block of your heart's electrical pathways can occur in or near the atrioventricular node, which lies on the pathway between your atria and your ventricles. A block can also occur along other pathways to each ventricle. Depending on the location and type of block, the impulses between the upper and lower halves of your heart may be slowed or blocked. If the signal is completely blocked, certain cells in the atrioventricular node or ventricles can make a steady, although usually slower, heartbeat. Some blocks may cause no signs or symptoms, and others may cause skipped beats or bradycardia.
Although it often feels like a skipped heartbeat, a premature heartbeat is actually an extra beat. Even though you may feel an occasional premature beat, it seldom means you have a more serious problem. Still, a premature beat can trigger a longer lasting arrhythmia — especially in people with heart disease. Premature heartbeats are commonly caused by stimulants, such as caffeine from coffee, tea and soft drinks; over-the-counter cold remedies containing pseudoephedrine; and some asthma medications.
Feb. 27, 2013
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