SINGAPORE - People with a severe form of a sleep disorder called obstructive sleep apnea (OSA) were 14 times more likely to die and twice as likely to suffer from cardiovascular complications within a month of surgery, compared to those without the condition, an international study has found.

In addition, people with moderate OSA were 11 times more likely to die within a month of surgery than those without OSA, according to the study of more than 1,200 patients from five countries, including Singapore.

People with OSA have breathing that repeatedly stops and starts during their sleep due to their throat muscles intermittently relaxing and blocking their airway.

Some symptoms are loud snoring, excessive daytime sleepiness and high blood pressure.

It is more common among men than women, affecting one in four men and one in 10 women here.

"It is very well known that patients with severe and untreated OSA are more likely to die, get a heart attack or stroke in the long term, but this is the first robust study to show that even a short surgery period will make a difference," said Adjunct Associate Professor Edwin Seet from Khoo Teck Puat Hospital (KTPH).

Those with severe OSA were twice as likely to suffer from complications like heart failure, damage to the heart muscle as well as an irregular and rapid heart rate.

"Anaesthetics given to patients for surgery tend to worsen OSA because it worsens the obstruction - the tongue is more lax and tends to fall backwards," he added.

"This leads to a decrease in the oxygen level of the blood and can cause stress to the heart."

Prof Seet noted that the cardiovascular complications typically happened within a week for patients with severe OSA.

Researchers tracked patients from five places - Singapore, Malaysia, Hong Kong, Canada and New Zealand - who were 45 years and older, were undiagnosed with OSA, had one or more risk factors for cardiovascular events after surgery, and were undergoing major non-cardiac surgery like abdominal surgery, major orthopaedic or vascular surgery.

They then underwent a home sleep test during the study to determine if they had OSA, and were told of the results only one month after the operation.

The majority of patients were Chinese (50 per cent), followed by Malays and Indians (30 per cent), and Caucasians (20 per cent).

The study was published in the Journal of the American Medical Association this month (May).

Prof Seet, who heads the Department of Anaesthesia at KTPH, advises patients to get screened for OSA before going under the knife and to inform their anaesthetist if they already have the condition.

At KTPH, a risk-assessment tool for OSA called Stop-Bang, which can also be found online, is a mandatory procedure for all surgical patients.

In addition, doctors can use a shorter acting anaesthetic, give less opioids and other drugs which do not stop their breathing as much, he said.

Doctors should also monitor patients very closely after surgery.

Another option is to undergo positive airway pressure therapy, which opens up the airway, but the jury is still out on the procedure's effectiveness.

Researchers will next look at possible interventions to avoid complications in susceptible OSA patients.

The study will involve around 200 patients from the same five countries, and researchers plan to begin at the end of the year.