If you are being treated for obstructive sleep apnoea (OSA) at Singapore General Hospital​ (SGH), don’t be surprised if the sleep disorders doctor refers you to a cardiologist at the National Heart Centre Singapor​​e​ (NHCS) too. This is because people who suffer from this common sleep disorder, in particular the severe form, are more likely to have calcium buildup in their arteries, a study by SGH and NHCS has found.

How does calcium buildup lead to heart disease?

Calcium deposits or calcifications are a sign of heart disease as the deposits narrow and harden the arteries, reducing the flow of oxygen-rich blood to the heart. Over time, this can cause problems like chest pain and heart attack. “No matter how little, once calcium starts to build up, it means the process of atherosclerosis​ (artery hardening and narrowing) has started,” said Dr Tan Swee Yaw​, Senior​ Consultant, Department of Cardiology​, National Heart Centre Singapore​, and principal investigator of the study.

For the study, patients with severe OSA had to undergo a special computed tomography (CT) scan of the heart to find out their coronary artery calcium score, or the amount of calcium in the walls of the arteries. “The calcium score tells you indirectly how much atherosclerosis has occurred,” said Dr Tan. The risk of a heart attack is 0.1 per cent for someone with a zero score, but 10 times more than normal for someone with a score of more than 100, and 25 times more than normal for a score of more than 400, he added.

In the study, which enrolled 150 mostly male participants in their 50s, nearly half were found to have atherosclerosis while 8.7 per cent had a calcium score of more than 400. The participants had been newly diagnosed with OSA. The link between OSA and heart disease has been known for some time, but it is still not fully understood why OSA should lead to an increased risk of atherosclerosis. Using the calcium score may help explain the link, said co-investigator Dr Toh Song Tar. “It may be because oxygen desaturation (that occurs during the sleep cycle of OSA patients) causes inflammation and damage to the blood vessels, which then becomes the trigger for lipids and calcium to be deposited in the arteries,” said Dr Toh Song Tar​, Senior Consultant, Department of Otolaryngology​, and Director, Sleep Disorders Unit, Singapore General Hospital​ (SGH).

What causes obstructive sleep apnoea (OSA)?

OSA occurs when something – the tongue or fatty tissues – partly or completely blocks the airways during sleep, reducing the flow of oxygen to the brain. Th​e brain then sends a signal to rouse the person to reopen his airways and breathe. People with severe OSA can get more than 30 such episodes a night, said Dr Toh.

The study was presented at a few international conferences, including the ACC (American College of Cardiology) conference in April in Chicago. It is now being readied for submission for peer review. Started in 2012, the study was completed in 2015. What began as a casual chat over coffee has now become routine practice: since 2015, patients with severe OSA seen at SGH will be referred to an NHCS cardiologist for testing for heart disease.

“We are trying to prevent cardiac disease from happening. If a patient comes to us with severe sleep apnoea – and we now know there’s a high risk of coronary disease happening – we hope that by referring him to the heart centre, cardiologists will do some intervention, be it medication or other treatments, to stop heart disease from developing or worsening,” said Dr Toh.

Added Dr Tan: “We used to be firefighters, but now we want to be fire preventers. I’d rather detect and see who is at risk, start the appropriate medication, and prevent that heart attack from happening in the first place.” Some 1,000 patients are diagnosed with OSA in SGH annually, including 300-500 with moderate to severe forms of the disease. About 15 per cent of Singapore’s population has OSA.

See next page to learn about the link between glaucoma ​and sleep apnoea​.

​Ref: O17