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The conducting system of the heart
The heart is basically a pump that maintains the circulation of blood through all the organs of the body. It has four chambers, two upper chambers called the right and left atria and two lower chambers called the right and left ventricles. The atria pass blood into the ventricles that are the main pumping chambers of the heart. The heart muscle contracts and relaxes in a rhythmic fashion, usually at a rate of 60 - 100 beats per minute at rest. Normally the heartbeat starts in the upper part of the right atrium where the natural pacemaker of the heart, called the sinus node, is located. The cells of the sinus node send out an electrical signal that first spreads through the atria causing them to contract. Normally, there is just one electrical connection between the atria and ventricles called the atrioventricular (or AV) node. The electrical impulse passes through this AV node and enters the specialised electrical conducting system of the ventricles. The impulse passes rapidly through this system, causing the ventricles to contract. The heart muscle then relaxes, ready for the next impulse from the sinus node. The purpose of this complicated system is to ensure that the heart pump works most efficiently. The atria contract a fraction of a second before the ventricles which lets them empty their blood into the ventricles before the ventricles contract.

The medical term for an abnormal rhythm of the heart is arrhythmia. If the heartbeat is too slow, it is called a bradyarrhythmia or bradycardia. If it is too fast, it is called a tachyarrhythmia or tachycardia. We consider that the normal heart rate is between 60 and 100 beats per minute but there are many exceptions. Athletic people may have heart rates less than 60 and all of us can have rates over 100 when we are exercising or under pressure etc.

What are the symptoms and treatments for slow heart beating ?
Excessive slowing of the heartbeat or bradycardia can cause fatigue, dizziness, lightheadedness, fainting or near-fainting spells. These symptoms due to slow heart beating can be corrected with an electronic pacemaker that is implanted under the skin, usually just below the left collar bone.

What are the symptoms and treatments for rapid heart beating ?
Rapid heart beating or tachycardia can produce symptoms of palpitations, dizziness, light-headedness, fainting or near fainting if the heart beats too fast to circulate blood effectively. It may be either regular or irregular in rhythm.

When rapid heart beating arises in the ventricles - called ventricular tachycardia - a life-threatening situation can arise. The most serious cardiac rhythm disturbance is called ventricular fibrillation when the electrical activity is so fast and chaotic that the heart cannot pump any blood. Collapse and sudden death follows unless cardiac massage and further medical help is provided immediately. The rhythm can be converted back to normal by an electrical shock from an external defibrillator, as seen on TV programmes such as ER. Further rapid heart beating can then be prevented by medications or in certain high risk cases by implanting an electronic device called an implantable cardioverter / defibrillator. This looks like a large pacemaker but of course it treats rapid heart beating rather than slow heart beating.

Other forms of rapid heart beating are not life-threatening in the same way as ventricular tachycardia. These are called supraventricular tachycardias during which the heart rate can increase quite suddenly to over 200 beats per minute. This usually causes distressing palpitations often with light-headedness, chest tightness, breathlessness and other symptoms. Some times these episodes are short-lived and terminate spontaneously. Other times they persist and may require drug treatment or electrical conversion in hospital. Previously, these supraventricular tachycardias required long term drug therapy but more recently curative treatment by radiofrequency ablation has been developed.

Atrial fibrillation is another form of supraventricular arrhythmia during which the normal regular rhythmic activity of the upper chambers or atria is replaced by chaotic irregular activity. This leads to an irregular and usually fast pulse rate that may cause palpitations and congestion of the lungs. It can also lead to blood clots forming within the left atrium (upper chamber of the heart) which can then break off and travel in the blood stream and block a small artery somewhere downstream. The most serious situation is when a clot lodges in an artery in the brain as this can cause a stroke. The mainstay of treatment is heart rate medication and blood-thinner (Warfarin) to minimise the risk of clots.In some patients, either drug therapy or electrical attempts at reverting them back to a regular sinus rhythm is recommended, depending on the clinical situation.

More Information
Some of the topics are discussed in more detail below. Otherwise, your family doctor or cardiologist will be able to direct you further.

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