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.
Some of the topics are discussed in more detail below. Otherwise, your family doctor or cardiologist will be able to direct you further.
What is atrial fibrillation ?
Atrial fibrillation, also known as 'AF' is the commonest sustained cardiac arrhythmia. Its prevalence increases with increasing years. It is estimated to affect at least 1% of the population at the age of 60 years and 5% at the age of 70 years. It occurs more frequently in those with underlying heart disease. While it is not immediately life-threatening in the same way as sustained ventricular arrhythmias, it can lead to heart failure or stroke and so it has potentially serious effects. In atrial fibrillation, 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 which may cause palpitations and congestion of the lungs. Also, because the upper chambers no longer contract and relax in the normal way, there may be stagnation of the blood which predisposes to blood clot formation. If a fragment breaks off, it will 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 initial assessment
The first step when someone developes atrial fibrillation is to look for any underlying cause or precipitating factors. The history may reveal a background of high blood pressure, heart valve disease or coronary artery disease. Alcohol plays an important role in precipitating atrial fibrillation in many patients. An overactive thyroid gland is another potential cause which must be considered. Often, a chest infection causing irritation around the heart may precipitate atrial fibrillation or certain operations, especially those involving the chest cage. In addition to a careful history and clinical exam, patients will usually have an ECG and ultrasound examination of the heart. Sometimes a coronary angiogram will be considered if there are symptoms suggesting angina or associated breathlessness.
How is atrial fibrillation treated ?
The mainstays of treatment are heart rate medication and some form of medication to minimise blood clot formation. There are a number of different medications available some of which prevent the pulse rate from going too fast and others which have a direct action on the rhythm itself. The medications which reduce the risk of blood clot formation are aspirin, clopidogrel and warfarin. As with all medications. there are potential side-effects and so the risks and benefits must be weighed up carefully before deciding what tablets are started.
One other treatment that may be required to restore the normal regular rhythm is electrical cardioversion. This is a procedure where an electrical shock is delivered through special paddles applied to the chest wall but in a controlled environment, unlike when given during a cardiac arrest like on television. Patients are put to sleep for a few minutes so that they won’t feel any pain or discomfort. Sometimes, even though regular rhythm is restored, the atrial fibrillation may come back so continued follow-up with or without medication is usually required.This is a day procedure and you will be required to fast beforehand due to the mild anaesthesia given to temporarily sedate you.
What is it?
Normally, there is just one electrical pathway connecting the upper and lower chambers (atria and ventricles, respectively) of the heart. This is the AV node. Impulses may pass in only one direction and this ensures normal, sequential contraction of the cardiac chambers. Patients with the Wolff-Parkinson-White syndrome (commonly known by its acronym, WPW syndrome) have an extra conduction pathway, called an accessory pathway, bridging the divide between the atria and ventricles. This may permit the normal electrical signal to arrive at the ventricles more quickly than usual, causing "pre-excitation" and producing characteristic changes on the electrocardiogram. The same pathway may at times function as a "short-circuit", allowing impulses to circulate rapidly down the normal AV nodal pathway and back up the accessory pathway causing one form of supraventricular tachycardia. Patients in whom this occurs will complain of rapid heart beating and may experience dizziness, chest tightness and may even faint. The most serious situation which is fortunately very rare is when patients with WPW develop atrial fibrillation (very rapid, chaotic electrical activity within the atria, see above) which may conduct very rapidly down the accessory pathway to the ventricles. This may then cause ventricular tachycardia or fibrillation and cardiac arrest. Other people with WPW never have tachycardia or other symptoms. About 80 percent of people with symptoms first experience them between the ages of 11 and 50.
How is it treated?
Once the diagnosis is made, patients are nowadays usually referred to a specialist who will try to assess the conduction properties of the accessory pathway in order to assess the risk it might pose. Patients without symptoms will not require treatment unless the pathway poses a risk to them or if they have special occupations such as airline pilot. People who have episodes of tachycardia can be treated with long term medication or can undergo radiofrequency catheter ablation (discussed in separate section) which is curative in 95 to 98% of cases. Whether an individual will be treated with medication or by an ablation procedure depends on several factors such as the severity and frequency of symptoms, risk for future arrhythmias and patient preference.