Arrhythmias
(Abnormal Heart Rhythms · Dysrhythmia)
In this condition factsheet:
The Facts on Arrhythmias
An arrhythmia occurs when the heart's regular rhythm changes, such as speeding up or slowing down, or when it beats irregularly. 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.
Arrhythmias can be caused by either slow heartbeats (bradycardia) or fast heartbeats (tachycardia). A slow heart rate may occur due to:
- Sick sinus syndrome: This occurs when the heart's natural pacemaker breaks down, causing slower transmission of the electrical signals that contract the heart. It occurs more frequently in the elderly and may worsen with certain medications (e.g., beta blockers) that also slow down the heart rate.
- Heart block: This occurs when the electrical signal sent from the upper heart chambers (atria) to the lower heart chambers (ventricles) is interrupted. Without this signal transmission, the heart cannot contract efficiently to pump blood out into the body.
A fast heart rate can be the result of:
- 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. The atria usually contract (or beat) between 60 to 100 times per minute, but in cases of AF, the atria can contract over 400 times per minute. This usually causes the atria to contract without first allowing blood to fill the atrial chambers, and the ventricles struggle to match the atrial rate of contraction. As a result, this often causes blood to pool in the atria, which can 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. Around 200,000 Canadians are diagnosed with AF, and the lifetime risk of developing AF is around 25% for individuals over the age of 40.
- 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 of Arrhythmias
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 heartbeat 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 arrhythmias, 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 from the sinus node.
Symptoms and Complications of Arrhythmias
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.
Experiencing an occasional flutter of the heart usually doesn't mean anything on its own. 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.
