Normal Heart Rhythm - it's got the beat.

Your heart is a muscle which is located a little to the left of the middle of your chest, and it's about the size of your fist. It sends blood around your body to provide it with the oxygen and nutrients it needs. It also carries away waste. The right side of your heart receives blood from the body and pumps it to the lungs. The left side of the heart receives blood from the lungs and pumps it out to the body.Before each beat, your heart fills with blood. Then it contracts to squeeze the blood along.
The heart is made up of four different blood-filled chambers. There are two chambers on each side of the heart. One chamber is on the top and one chamber is on the bottom. The left and right atria are the top chambers that fill with the blood returning to the heart from the body and lungs. The two chambers on the bottom are called the ventricles. The left and right ventricles squirt out the blood to the body and lungs.
The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. The electrical system of your heart is the power source that makes this possible.
Your heartbeat is triggered by electrical impulses that travel down a special pathway through your heart. The impulse starts in a small group of cells located in the right atrium, called the SA (sinus) node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.The SA node sets the rate and rhythm of your heartbeat. Normal heart rhythm is often called normal sinus rhythm because the SA node fires regularly. This is why the SA node is called the heart’s natural pacemaker.The AV node in the centre of the heart acts like a gate that slows the electrical signal before it enters the ventricles.This delay gives the atria time to contract before the ventricles do. The signals then follow a pathway to the muscular walls of the ventricles and causes them to contract. This forces blood out of the heart to the lungs and body. The SA node fires another impulse and the cycle begins again. At rest, a normal heart beats around 50 to 99 times a minute. Exercise, emotions, fever and some medications can cause your heart to beat faster, sometimes to well over 100 beats per minute.

Abnormal Heart Rhythm - arrythmias.

The heart is an amazing organ. It beats in a steady even rhythm, about 60 to 100 times each minute (that's about 100,000 times each day! An arrhythmia (also called dysrhythmia or irregular heart rhythm) is an irregular or abnormal heartbeat. An arrhythmia can involve a change in the rhythm, producing an uneven heartbeat, or a change in the rate, causing a very slow or very fast heartbeat. Some arrhythmias change both rate and rhythm.
Arrhythmias may have many causes. The heart’s natural timekeeper—the sinus node—can malfunction and develop an abnormal electrical impulse rate. Or, because all heart tissue is capable of starting a beat, any part of the heart muscle also can interrupt the electrical rhythm or even take over as the heart’s pacemaker, setting off an abnormal heartbeat. When one of these events interrupts the heart’s normal beat, arrhythmias can occur.Other causes include coronary artery disease, changes in the heart muscle (heart failure or cardiomyopathy), valve disease, electrolyte imbalances in your blood (such as sodium or potassium), injury from a heart attack or the healing process after heart surgery.
A fast or slow heart rate does not always mean your heart rhythm is abnormal. Fast or slow heart rates are also related to anxiety, activity, medications or other normal causes. View a slideshow on arrhythmias

Types of arrythmias.

A slow heart rhythm with a rate below 60 beats per minute is called a bradycardia.
A fast heart rate of more than 100 beats per minute is called a tachycardia.
Supraventricular arrhythmias begin above the ventricles, such as in the upper chambers or atria. These are the different types of supraventricular arrhythmias:

• Premature atrial contractions (PACs)
  early,extra beats that originate in the upper chambers of the heart (atria).
• Paroxysmal supraventricular tachycardia (PSVT)
  a rapid but regular rhythm that comes from the atria. PSVT begins and ends suddenly.
• Accessory pathway tachycardia (such as Wolff-Parkinson-White syndrome)
  a fast heart rhythm due to an extra abnormal electrical pathway or connection between the atria and ventricles. The impulses  travel through the extra pathways, as well as, the usual route. This allows the impulses to travel around the heart very quickly, causing the heart to beat unusually fast.
• AV nodal re-entrant tachycardia (AVNRT)
  a fast heart rate due to having more than one pathway through the atrioventricular (AV) node.
• Atrial tachycardia
  a rapid heart rhythm that originates in the atria.
• Atrial fibrillation
  a very common irregular rhythm. Many impulses begin and spread through the atria, competing for a chance to travel through the AV node. The resulting rhythm is disorganized, rapid and irregular. Because the impulses are traveling through the atria in a disorderly fashion, there is a loss of coordinated atrial contractions.
• Atrial flutter
  an atrial arrhythmia due to one or more rapid circuits in the atrium. Atrial flutter is usually more organized and regular than atrial fibrillation.

Ventricular arrhythmias begin in the lower chambers of the heart.

• Premature ventricular contractions (PVCs)
  are early, extra beats beginning in the lower chambers of the heart (ventricles). PVCs are common. Most of the time they  cause no symptoms and require no treatment. In some people, they can be related to stress, too much caffeine or nicotine, or  exercise. But sometimes, PVCs can be caused by heart disease or an electrolyte imbalance.
• Ventricular tachycardia (V-tach)
  a rapid rhythm originating from the lower chambers of the heart. This rapid rate prevents the heart from filling adequately with  blood, and less blood is pumped through the body. This can be a more serious arrhythmia, especially in people with heart  disease, and may be associated with more symptoms.
• Ventricular fibrillation (V-fib)
  an erratic, disorganized firing of impulses from the ventricles. The ventricles quiver and cannot generate an effective contraction, making the heart unable to deliver blood to the body. This is a medical emergency that must be treated with cardiopulmonary resuscitation (CPR) and defibrillation [delivery of an energy shock to the heart muscle to restore a normal  rhythm] as soon as possible.
• Long QT
  the QT interval is the area on the electrocardiogram (ECG or EKG) that represents the time it takes for the heart muscle to contract and then recover, or for the electrical impulse to fire and then recharge. When the QT interval is longer than normal, it increases the risk for “torsades de pointes,” a life-threatening form of ventricular tachycardia.
  

The content of these pages is not intended to be a substitute for professional medical advice. Always seek the advice of your doctor or other qualified healthcare provider with any questions you may have regarding a medical condition or medical treatment.