The normal beating of the heart is controlled by electrical signals sent from a particular segment of heart muscle tissue called the sinus node. This natural pacemaker is located near the top of the right atrium. The heart is divided into four chambers: two atria on top, and two ventricles underneath. The job of the atria is to fill the ventricles with blood, which then do the heavy work of pumping it through the rest of the body.
In a normal heartbeat, an electrical pulse travels down the muscle tissue, activating the ventricles a split second after the atria. In arrhythmias, there's a problem with this signal. There are many different kinds of arrhythmias, but those that affect the ventricles are generally more serious than arrhythmias of the atria.
- atrial fibrillation (AF): This involves disordered signals that are fired off in rapid succession, causing fibrillation, which is an uncoordinated quivering of the muscle wall of the atria. This has often been described as looking like a "bag of worms." The atria stop pumping blood effectively, yet enough blood still reaches the ventricles to allow the heart to function. AF is potentially dangerous, however, because blood can pool in the atrium and lead to clot formation. If one of these clots travels to the brain, it causes a stroke. AF is the most common form of harmful arrhythmia. It affects approximately 2% of people younger than age 65 and about 9% of people aged 65 years or older
- ventricular fibrillation (V-fib or VF): This is the most dangerous form of arrhythmia. The ventricles twitch but don't pump blood. If the twitching does not stop on its own or by a shock from a defibrillator, it is always fatal.
Causes
There are two common ways you can develop arrhythmias:
- problems with initiating the electrical signal: either the sinus node fires abnormally, or there is a competing impulse elsewhere in the heart
- problems with the conduction of the electrical impulse: connections from the atria to the ventricles are hindered (this is often called a heart block)
People with heart disease are particularly likely to develop arrhythmias, since damage to the heart can stop the beat signal from reaching the ventricles or cause certain areas of the heart to fire abnormally.
High blood pressure and an overactive thyroid gland also increase the chances of arrhythmias. Alcohol can also cause atrial and ventricular arrhythmias. Certain medications such as decongestants as well as many prescription medications can make the heart susceptible to arrhythmias and must be used with caution in people with heart disease.
There are also inherited and congenital (present since birth) types of arrhythmia, often resulting in a weak or late signal getting to the ventricles. The ventricles can emit their own signal, but this is often fewer than 40 beats a minute instead of the usual 60 to 100 with the sinus node.
Symptoms and Complications
When the heart beats faster than normal, it's called tachycardia. Symptoms include chest discomfort, palpitations, lightheadedness, dizziness, and sometimes fainting. When it beats slower than normal, it's called bradycardia. Bradycardia can cause fatigue, lightheadedness, dizziness, and fainting as it tends to produce low blood pressure.
Everyone knows what it feels like to experience an occasional flutter of the heart, and usually it doesn't mean anything. But if you get chest pains, feel faint, or you notice your pulse to be irregular or either very rapid or very slow over a prolonged period, it's time to see a doctor.
Making the Diagnosis
The stethoscope is still a valuable instrument when it comes to detecting arrhythmias, but there are modern tests that can pinpoint the problem.
The electrocardiogram (ECG) prints a graph of the heart's electrical activity using small electrodes taped to the chest. The pattern on these graphs reveals the type of arrhythmia. Since the arrhythmia might not occur at the hospital, there are portable ECGs that you can bring home. Some are constantly turned on over a specified period of time (called a Holter monitor), while others are turned on when you feel an arrhythmia (called an event monitor or loop recorder). Some of these devices can download the heart signal data over a phone line or a cell phone. Certain arrhythmias may be associated with exercise, therefore you may be asked to walk on a treadmill or ride a stationary bicycle while hooked up to an ECG machine.
Electrophysiologic study (EPS) is a more elaborate test. Thin tubes are inserted into a blood vessel in the leg and guided up to the heart. They hold electrodes that can find the muscle tissue that may be causing abnormal electrical activity.
Treatment and Prevention
In some cases, arrhythmias are due to bad habits like drinking too much alcohol. Breaking the habit can cure the problem. Reducing stress, avoiding caffeine, improving your diet, and increasing the amount of exercise you do in a week can also reduce the frequency of experiencing arrhythmias. You should talk to your doctor first before beginning such changes. In other people, the arrhythmias are a symptom of heart disease, and they won't go away unless this underlying problem is addressed. Fortunately, many people can benefit from modern medical interventions, especially in the area of surgery and the insertion of special electrical devices.
There are several medications that can slow down a rapid heartbeat (known as "rate control"). Beta-blockers (e.g., atenolol, metoprolol)* are medications that are very useful for rate control in people with a variety of heart conditions. Calcium-channel blockers (e.g., diltiazem, verapamil) may also be used for rate control. Often, these medications can be used for more than one purpose (i.e., heart rate control, high blood pressure, heart protection after a heart attack). Another medication, digoxin, is derived from a substance called digitalis that has been used for arrhythmias for over 200 years.
Other medications called antiarrhythmics can convert an abnormal rhythm back to normal, and prevent it from recurring (known as "rhythm control"). Examples of antiarrhythmics include amiodarone, propafenone, and sotalol. Patients with atrial fibrillation (AF) are usually given anticoagulants such as warfarin, apixaban, or rivaroxaban. These medications thin the blood to keep blood clots from forming and causing strokes. Recent research has shown that rate control and adequate anticoagulation are very important (perhaps even more important than rhythm control) for people with certain kinds of arrhythmias (e.g., AF).
In some cases, AF is converted back to a normal rhythm by a treatment called cardioversion, in which a doctor gives the patient a sedative followed by a small electrical shock that "resets" the natural pacemaker in the heart.
Many arrhythmias can be cured outright with radiofrequency ablation. The same tubes used in EPS (see "Making the Diagnosis") are inserted into the heart to send radio waves directly onto electrical pathways carrying inappropriate signals. These waves ablate (destroy) the abnormal tissue, preventing it from triggering an abnormal heart rhythm.
Artificial pacemakers can take over the job of generating the electrical signals. Once limited to treating slow heart rates, the latest pacemakers can control fast heart rates, too. They can run up to 15 years on one battery. Some save power by switching off when the heartbeat is normal. Most units are placed under the skin, requiring only minor surgery.
Defibrillators are devices that restart arrested hearts with jolts of electricity to the chest. Defibrillators are effective at stopping ventricular fibrillation, but since ventricular fibrillation can be fatal in less than 4 minutes, they must be used early. Most defibrillators are external devices, but now there are automatic implantable cardioverters/defibrillators (AICD). These devices can be as small as pacemakers. They can detect a dangerous fibrillation and jolt the heart back to normal before any harm is done. As one doctor puts it, "it's like having an emergency room implanted in your chest."
