SINGAPORE - For more than 30 years, Mr Ong Chee Fatt, 57, would keep fit by running 15 rounds around his flat every evening without fail, a distance of about 3km or 4km.

That routine came to an abrupt end in 2010, when he was 49.

"I felt like my fitness had suddenly dropped, even though there was no reason for it as I had been running consistently," said Mr Ong, who was speaking during a media briefing on Thursday (Dec 6) at the National Heart Centre Singapore (NHCS).

"Suddenly, I would feel breathless. It got so bad that I would have to stop after just half a round," he added.

Although he did not know it at the time, Mr Ong had a condition known as chronic thromboembolic pulmonary hypertension (CTEPH), which affects about one in 30,000 people in Singapore.

In November 2016, he became one of the first few patients to undergo treatment for the condition as part of a programme that was being developed at NHCS.

The programme is now fully operational and accepting new patients.

Patients with the condition have high blood pressure in their lungs caused by clotting in the blood vessels.

Mr Ong, a senior lecturer who teaches civil engineering at Singapore Polytechnic, was recommended surgery called pulmonary endarterectomy.

Under this procedure, his body temperature was gradually lowered to 20 deg C to slow down his metabolism.

At this temperature, his heart stopped pumping and a machine took over his blood circulation.

The machine then completely drained his body of blood. Surgeons had 20 minutes to operate on his lungs to attempt to remove the clots.

Without cooling the body, irreversible brain damage would start to set in and other organs would begin to fail in just three minutes.

After the clot was removed, the blood was pumped back into Mr Ong's body and he was returned to a normal temperature.

The procedure was repeated then to remove clots in the other side of his lungs, with the whole operation lasting about eight hours.

Following his recovery, Mr Ong went back to his old fitness routine.

"Before the surgery, I would even have problems walking up the slope to get to this clinic," said Mr Ong, who is married with three children.

However, not all patients with his condition are suitable for the surgery.

Mrs Linda Ang, 40, similarly experienced breathlessness when climbing stairs.

She was recommended another surgery being offered under the programme known as balloon pulmonary angioplasty as she has a petite frame and a depression in her chest.

This surgery is more suitable for clots in smaller blood vessels, which surgeons might not be able to reach directly, said Associate Professor Lim Soo Teik, who is a senior consultant and deputy medical director at the department of cardiology in NHCS.

Under this procedure, a catheter was passed through a blood vessel in Mrs Ang's leg to her lungs.

A tiny balloon device was then used to physically push the clot to the side of the blood vessels.

Mrs Ang, a former sales manager, underwent the procedure four times between May and November last year.

The two procedures being offered under the programme were not previously readily available in Singapore.

So far, a total of 22 people have undergone the procedures.

"Due to the non-specific nature of the symptoms and a lack of awareness of the condition, many cases go undiagnosed," said Prof Lim.

"It is important that we raise awareness of this condition so that more can be diagnosed and treated early," he added.