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Arrythmia

The heart has four chambers - two atria and two ventricles. The walls of these chambers are mainly made of special heart muscle. The chambers have to squeeze in a orderly fashion for the heart to pump blood correctly with each heartbeat.

The sinoatrial node (SA node) in the right atrium is a tiny in-built clock. It fires off an electrical impulse at regular intervals (about 60-80 per minute when you rest and faster when you exercise; this controls your heart rate). Each impulse spreads across both atria. This causes them to contract and pump blood through one-way valves (mitral valve and tricuspid valve) into the ventricles.

The electrical impulse now gets to the atrioventricular node (AV node) at the lower right atrium. A thin bundle of conducting fibers called the atrioventricular bundle (AV bundle) acts like 'wires' and carries the impulse from the AV node to the ventricles (the major pumping chambers of the heart).The AV bundle splits into two - a right and a left branch. These then split into many tiny fibers (the Purkinje system) which conduct the electrical impulse throughout the ventricles.

  • The artery going from the right ventricle (pulmonary artery) takes blood to the lungs.
  • The artery going from the left ventricle (aorta) takes blood to the rest of the body.
  • The heart then rests for a short time (diastole). Blood coming back to the heart from the large veins fills the atria during diastole.
  • The veins coming into the left atria bring blood from the lungs (oxygen loaded).
  • The veins coming into the right atria bring blood from the body (oxygen depleted).
Sinus tachycardia

This means a normal heartbeat which is faster than usual (more than 100 beats a minute). This is usual when you exercise. It may also occur if you: are anemic, have a fever, have an overactive thyroid gland, or are frightened or stressed.

Sinus bradycardia

This means a normal heartbeat which is slower than usual (fewer than 60 beats per minute). Many people who are fit have a heart rate between 50 and 60 beats per minute - sometimes slower. It may also occur if you have an underactive thyroid gland.

Ectopic beats

These are extra heartbeats which occur out of sync to the normal regular rate. They are very common and are usually harmless. Caffeine in tea or coffee, and alcohol may cause you to have more ectopic beats than usual. Some people with some types of heart disease have frequent ectopic beats.

An arrhythmia is an abnormal rate and/or rhythm of the heartbeat. There are various types, but all are due to some problem with the electrical conducting system of the heart. Some arrhythmias are more serious than others.

Supraventricular tachycardia (SVT)

In this arrhythmia, the heartbeat is not controlled by the SA node (the normal timer of the heart). Another part of the heart overrides this timer with faster regular impulses. The source or trigger of the impulse in an SVT is somewhere above (supra) the ventricles. The heart then contracts (beats) faster than normal, usually between 140 and 240 beats per minute. The heartbeat is regular. In most cases an episode (paroxysm) of SVT occurs now and again.

Atrial fibrillation

In atrial fibrillation (AF) many random electrical impulses 'fire off' from different parts of the atria. The atria then fibrillate (quiver like a mass of jelly). This means they cannot effectively contract. Only some of these impulses pass through to the ventricles in a haphazard way. So, the ventricles contract between 160 and 180 beats per minute, but in an irregular way with varying force.

AF is common in older people, but it also affects some younger people.

Ventricular tachycardia

This is an uncommon arrhythmia. In this condition the ventricles beat faster than normal (between 120 and 200 beats per minute). This very high rate means that the heart cannot effectively relax in between beats and starve for oxygen rich blood itself. The rate in the atria is normal. So, there is a trigger of electrical impulses somewhere in the ventricles which overrides the normal impulses coming down from the atria. If uncontrolled this can soon deteriorate into an unstable rhythm and the heart may arrest (stop beating altogether).

Ventricular fibrillation

In ventricular fibrillation (VF) many random electrical impulses 'fire off' from different parts of the ventricles. The ventricles then fibrillate. This means they only partially contract, which is not enough to push blood out of the heart. This is life-threatening and a common cause of cardiac arrest. It is fatal unless corrected within a few minutes. It is a complication of various heart disorders, most commonly after a large heart attack (myocardial infarction).

Heart block

This is where the electrical impulses are partially or fully blocked between the atria and the ventricles. The SA node in the right atrium 'fires' at the normal rate, but the rate at which the ventricles contract depends on how many impulses get through to the ventricles.

First-degree heart block means there is a slight delay in each impulse going from the atria to the ventricles. But, each impulse does get through and the heart rate is normal.

Second-degree heart block means that some impulses from the atria are not conducted through to the ventricles. The rate that the ventricles contract can then be slow.

Third-degree, or complete, heart block means that no impulses are conducted through. The ventricles then contract at their own in-built rate of about 20-40 beats per minute. So, you have a very slow pulse.

Sick sinus syndrome

This condition is where the SA node (the heart's natural pacemaker) becomes damaged. The heart then tends to beat slowly or miss a few beats. But, in some cases, the heart alternates between beating slow for a while, and then fast for a while.

Many arrhythmias occur as a complication of a heart condition.

  • Ischemic heart disease (which causes angina and heart attacks). This reduces the blood supply to parts of the heart which may include parts of the conducting system.
  • Heart valve diseases can cause the heart muscle to enlarge which can trigger abnormal electrical activity. For example, AF is a common complication of mitral valve disease.
  • High blood pressure can put strain on the heart and cause arrhythmias.
  • Age-related degeneration (ageing) around the conducting fibers is one cause of complete heart block.
  • Cardiomyopathy (a disorder of the heart muscle) can sometimes cause arrhythmias.
  • Some arrhythmias are due to abnormalities in the electrical pathways, which are present from birth.
  • Some congenital heart defects are associated with certain arrhythmias.
  • Inflammation of the heart and other less common heart disorders are other possible causes.
Non-heart causes

Certain medicines and excess thyroid hormone can sometimes trigger an arrhythmia.

What are the symptoms of arrhythmias?
  • Palpitations (an abnormal awareness of your heartbeat).
  • An abnormally fast, slow, or irregular pulse.
  • Dizziness or feeling faint.
  • Shortness of breath.
  • Chest pains which sometimes develop.
Diagnosis:
  • Electrocardiogram (ECG)
  • The ECG test is painless and harmless. Small metal electrodes are stuck on to your arms, legs and chest. Wires from the electrodes are connected to the ECG machine. The electrical impulses in your heart can be detected by the ECG machine which records them on to a paper or computer.

    Different arrhythmias cause different ECG patterns, so this test can often clarify the type of arrhythmia.

  • Ambulatory ECG (Holter)
  • If you have an intermittent arrhythmia, this may not be detected by a standard ECG done at one particular time. You may then be advised to have an ambulatory ECG. This test records the electrical activity of your heart when you are walking about (ambulatory) and doing your normal activities. Wires from electrodes placed on your chest are connected to a small lightweight recorder. The recorder is attached to a belt which you wear round your waist. The electrical activity is usually recorded for 24-48 hours.

  • Exercise ECG
  • Sometimes an ECG is taken whilst you exercise on a treadmill or bike to try to provoke symptoms which may be an intermittent arrhythmia.

  • Electrophysiological study
  • A very sophisticated test done in a cardiac cath lab wherein a electrical stimulus sensitive catheter (thin wire) is threaded up into your heart where it studies the electrical activity map of the various chambers.

What are the treatments for arrhythmias?
  • Medication
  • Various medicines can interfere with the electrical impulses in your heart. They are often used to prevent intermittent arrhythmias, or to control the heart rate in AF. The type of medicine to be used will be decided by your cardiologist.

  • Catheter ablation (destruction) treatment
  • This is an option for some cases of SVT, ventricular tachycardia and AF. A catheter (small wire) is passed via a large vein in your leg into the chambers of your heart. It is guided by special X-ray techniques. The tip of the catheter can destroy a tiny section of heart tissue that is the source or trigger of abnormal electrical impulses.

  • Cardioversion
  • This may be an option for some types of tachycardia - for example, in some cases of AF which have recently developed, and some cases of ventricular tachycardia. Whilst under anesthesia, an electrical shock over the heart is given. This may revert the abnormal rhythm back to normal.

  • Artificial pacemakers
  • These are used in cases of complete heart block and in certain other situations. An artificial pacemaker is a small device which is inserted just under the skin on the upper chest. Wires from the pacemaker are passed through veins into the heart chambers. The pacemaker can then stimulate the heart to maintain a regular normal heartbeat.

  • Implantable cardioverter defibrillators (ICDs)
  • They are small devices which are similar to pacemakers and are inserted under the skin in the upper chest. Wires are passed through a vein to the heart. The device monitors the heartbeat. If it detects a change to an abnormal rhythm, the device can send a short electrical shock to the heart to stop the abnormal rhythm. It is cardioversion done directly to the heart, rather than doing it through the chest.


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