The surgery was performed because MM Lee had been experiencing symptoms of cardiac arrhythmia - an irregular heart rhythm - which showed up with his atrial flutter in mid-September, the PMO said in a statement.

A pacemaker is often implanted in the chest and attached to the heart of a person with a slow or irregular heartbeat.

When it detects that the heart rate has fallen below a certain limit, it will generate an electrical impulse to correct this.

Atrial flutter, an abnormal heart rhythm, is not uncommon among people of MM Lee's age. He is 85.

The statement said that he is recovering from the surgery, and is expected to be discharged from the hospital today.

The Minister Mentor will also go ahead with a scheduled trip to Hong Kong from today till Wednesday to have a dialogue session with former United States president Bill Clinton at the Clinton Global Initiative (CGI) Asia Meeting.

'His doctors have agreed to and will accompany him on his trip,' the statement said.

The CGI is a four-year-old non-partisan effort that brings together global leaders to devise and implement innovative solutions to some of the world's most pressing issues such as poverty and climate change. The Hong Kong meeting will focus on issues important to Asia.

According to the programme for the CGI meeting, MM Lee's dialogue is scheduled for tomorrow afternoon.

MM Lee and Mr Clinton 'will engage in a dialogue on the challenges and priorities for Asia in education, energy and climate change, and public health', the programme said.

The Hong Kong meeting is the CGI's first outside of the United States.

Other speakers include United Nations Secretary-General Ban Ki Moon, Philippine President Gloria Arroyo, actor and philanthropist Jet Li, and World Toilet Organisation founder, Singaporean Jack Sim.

This will not the first time that MM Lee has kept his appointments despite his health.

Even when he was warded for his atrial flutter on Sept 13, he kept his date with the Global UBS Philanthropy Forum that night, addressing 200 participants at the Arts House via video link from the Singapore General Hospital.

In his five-minute speech, he stressed the importance of successful individuals giving back to society, to ensure that a widening income gap did not lead to society losing cohesiveness.

Mr Lee, who has undergone surgery twice to clear a blocked heart artery, is known to exercise regularly.

In an e-mail interview with The Straits Times in September this year, he said he cycled and swam daily and ate moderately to keep in shape.

He underwent two heart operations in 1996, following the discovery of the blocked vessel. In the second operation, he had a stent inserted.

A stent is a steel scaffolding that is placed inside an artery to hold it open.

zakirh@sph.com.sg