1: Tony Blair's condition, supraventricular tachycardia, is a non-life threatening heart rhythm disturbance caused by rapid electrical activity in the upper heart (the atria)
2: The same condition can affect the lower heart (the ventricles) as ventricular tachycardia - usually a sign of underlying heart disease
Prime Minister Tony Blair underwent a heart procedure in hospital on Friday to correct a continuing "flutter".
He was treated in October last year for supraventricular tachycardia (SVT), but the problem has returned.
BBC News Online explains the condition, looks at what may have caused it and what treatments are given.
What is supraventricular tachycardia?
SVT is a disturbance of the heart rhythm caused by rapid electrical activity in the upper parts of the heart - the atria.
The heart goes from beating at a normal resting pulse of around 70 beats per minute to anywhere between 140 and 240 beats per minute.
Patients usually experience this surge as palpitations - a fluttering of the heart - but they may also feel dizzy or faint.
Experts say the condition is uncomfortable but not harmful.
What treatment did Mr Blair receive on Friday?
Mr Blair underwent a procedure called catheter ablation to restore his heart's natural rhythm.
The procedure does not involve a surgical incision and will be carried out under local anaesthetic and sedation.
His cardiologist, Dr Punit Ramrakha, who treated the prime minister last year, said catheter ablation has been developed in recent years and restores the heart's rhythm.
A catheter - a long, thin wire - is introduced through the skin into large veins and moved inwards under x-ray control until the tip lies inside the heart chambers.
Electrical sensors at the tip allow the cardiologist to perform the procedure to locate the exact site of the short circuit.
The catheter then delivers pulses of energy which destroy - ablate - the short circuit.
What treatment did the Prime Minister receive last year?
Once SVT is diagnosed, the aim is to return the heart back to its normal rhythm.
This was done by using a treatment called cardio version where, under anaesthetic, a mild electrical charge is carefully applied to switch the heart beat back to normal.
Patients are then kept in hospital for observation for a few hours to check the procedure has been successful.
Is the condition caused by stress?
It is possible that SVT can be triggered by stress - although there are other potential triggers, such as being rundown because of a cold or flu or drinking too much caffeine.
However, the underlying cause is likely to be a slight congenital abnormality of the electrical activity of a person's heart.
This can lie dormant and not affect people for decades.
Professor Sir Charles George, medical director of the British Heart Foundation, told BBC News Online: "People tend to associate these rhythm disorders with periods of anxiety.
"But in most people, there's no obvious immediate cause."
Although people can suffer repeated attacks of SVT, many only have one episode in their lives.
How common is SVT?
Experts say it is a fairly common heart rhythm disorder.
Thousands of people in the UK will have suffered SVT.
Is there any follow-up treatment?
Patients may simply be advised to avoid potential triggers such as caffeine, alcohol and some over-the-counter cold remedies.
Others may be fitted with devices where electrodes are fitted to the chest for between 24 and 48 hours to monitor the heart's rhythm to see if they experience further episodes of SVT.
In the longer term, it is a matter of waiting to see if a patient does suffer another attack.
If they do, they can be given an electrophysiological test to see exactly where the problem lies.
Patients with persistent palpitations may be given anti-arrhythmic drugs to control the condition.
Will the Prime Minister's illness have any effect on his ability to do his job?
The recovery time is short and Mr Blair will be able to return to work and undertake full activity within three to four days.
Mr Blair says he is determined to lead his party in a third term.