The Facts
An arrhythmia occurs when the heart's regular rhythm changes - it may speed up or slow down, and it may beat irregularly. In North America, about 5 million people get arrhythmias, most over the age of 50. Some also have heart disease, but many don't. In the vast majority of cases, the arrhythmia by itself isn't life-threatening, but may predispose someone to a number of problems such as a stroke. There are many different types of arrhythmias and their significance and consequences are varied.
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.
Some common arrhythmias include:
- Premature beats, the most common arrhythmia, is basically harmless. Every once in a while, the heart's electrical signals fire early. The heart beats twice quickly, then pauses and returns to normal. The premature beat may come from the ventricle or the atrium.
- Atrial fibrillation (AF) is where disordered signals are fired off in rapid succession, causing fibrillation, an uncoordinated quivering of the muscle. 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. Atrial fibrillation is potentially dangerous, however, because blood can pool in the atrium predisposing 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, affecting almost 1% of the population. It is more common in elderly people.
- Ventricular fibrillation is the most dangerous 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 basic 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 ventricles are hindered; this is often termed 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 fire abnormally. High blood pressure and an overactive thyroid gland also increase the chances of arrhythmias. Alcohol can cause atrial arrhythmias as well. 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 often at fewer than 40 beats a minute instead of the usual 60 to 100 with the sinus node.
