Cheney's medical history

29 June 2001: Announces he will be tested for a pacemaker 5 March 2001: Hospitalised for cardiac catheterisation 22 November 2000: Undergoes operation to unclog arteries after a mild heart attack 1988: Undergoes quadruple bypass surgery 1984: Suffers heart attack 1978: Experiences his first heart attack

He said his doctors had assured him that his ability to continue as vice-president would not be affected.

He is expected to be back at work on Monday.

Mr Cheney, 60, has had four heart attacks, the most recent in November of last year. This was his third hospital visit since he took office.

With his wealth of political experience - he has been a Washington insider since he was 28 years old - he is considered among the most powerful vice-presidents in US history.

He is a key member of the administration, playing a significant role in relations with Congress, and in energy and foreign policy.

'Not resigning'

Mr Cheney said he had told President George W Bush about his hospital tests but that they had not discussed the possibility of his resigning.

He said that he had experienced no pain and did not feel anything during periods of irregular heartbeat.

Mr Bush broke the news of the operation

Describing the procedure as "good preventative medicine", he said his doctors would recommend the implant for anyone in his condition.

He called the device a "pacemaker-plus" and said it would monitor his heart rhythms and correct irregularities.

About 100,000 such devices are implanted into people with irregular heartbeats in the US each year, the vice-president said.

The battery-operated device should last five to eight years.

Able to fulfil duties

Mr Cheney's cardiologist, Dr Jonathan Reiner, said after tests in March that in his opinion, Mr Cheney was healthy enough to see out his term in office.

He was treated then after complaining of chest pains and doctors found that an artery where they had inserted a stent - or support - in November had narrowed again.

An angioplasty, in which a balloon dilates the artery, was performed.

Three sets of cardiac enzymes had shown no damage to the heart, showing that he had not had a heart attack.

In November, when the presidential election was still unresolved, Mr Cheney suffered his fourth heart attack since the age of 37. It was described as a mild attack and he returned to work shortly afterwards, having had an operation to unblock an artery.