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Your heart is a muscular organ which lies in the centre of your chest.

Your heartbeat is controlled by electrical impulses, which travel across

the heart making it contract.

The atria contract first, sending blood into the ventricles.

The ventricles then contract, sending blood to the lungs and around the body.

Nerves supplying the heart change the rate at which impulses are sent across the

heart muscle

to meet the needs of the body.

In atrial fibrillation,

the electrical impulses in the atria become disorganised,

overriding the heart's normal rate and rhythm.

This causes the atria to quiver (or 'fibrillate').

The irregular impulses can be transmitted to the ventricles,

causing the heart to pump irregularly and too fast.

In supraventricular tachycardia (or SVT), the irregular electrical impulses pass across

the ventricles

and back up into the atria in a circle, rather than travelling from the atria

to the ventricles in one direction, as they should.

This makes the heart beat faster.

In ventricular tachycardia, abnormal electrical impulses are produced in the

lower chambers of the heart.

This causes the heart to pump faster than normal.

The ventricles may not have enough time to fill up with blood properly,

so less blood is pumped around your body.

In heart block, the electrical impulses that control your heartbeat are slowed down or

blocked

as they travel through your heart.

This can result in a slow heart rate.

In ventricular fibrillation,

electrical impulses start firing from many different points in the lower

chambers of the heart, very rapidly and in an irregular rhythm.

This makes the heart quiver and unable to beat properly.

Ventricular fibrillation is fatal without prompt treatment.

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