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8 October 2023

As personal heart monitors, smart watches are not so smart

The popularity of wearables is muddying health data and fuelling an epidemic of scans and anxiety.

By Phil Whitaker

We were debriefing after morning surgery. Sam, my registrar, clicked open the next set of notes. “OK, so this was a 65-year-old guy, went into AF on holiday. The hospital sent him back to us to sort out. I started him on edoxaban and gave him a warning card.”

It seemed entirely reasonable. Atrial fibrillation (AF) is the commonest heart rhythm disturbance in the UK, becoming more prevalent with advancing age. Many cases are “silent”, causing no symptoms, but when AF provokes a rapid pulse rate, people can feel breathless or light-headed, or can develop chest pain.

[See also: Why we should be worried by the new Covid wave]

The main worry, though, arises from the cessation of coordinated contractions in the atria, the upper chambers of the heart. The stagnation of blood flow allows clots to form. And if pieces break off into the circulation they lodge and cause blockage, usually in the arteries supplying the brain. Around one in five strokes are thought to be caused by AF.

Edoxaban, the medication Sam had prescribed, is a blood thinner. It can reduce stroke risk by 80 per cent. However, interfering with clot formation can also cause life-threatening haemorrhage – hence the alert card Sam had issued, which all anticoagulated patients should carry. We use risk scores to help decide whether an individual is more likely to benefit than be harmed. In Sam’s patient, his age and sex put the balance in favour of treatment.

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“He was in sinus rhythm, though, was he?” I nodded towards the screen, where Sam’s notes stated the pulse had been normal and regular.

“Yes.” He didn’t sound unduly worried; some patients have paroxysmal AF, going in and out of altered rhythm at different times.

“How about at the hospital?”

“Must have.” Sam scrolled the cursor and opened the letter. “No. He was in sinus rhythm there, too.”

“So how do we know he’d been in AF?”

“It was his Apple Watch,” Sam said. “It alarmed during the night. He phoned 111, who told him to go straight to hospital.”

Apple’s early attempts to get its smart watches to detect AF were wildly inaccurate. Since 2018, though, the introduction of single-lead ECG recording capability has greatly reduced false positives, though they do still occur. Where Apple led, others have followed: most smart watches now have similar functionality. I get several patients a year coming in with news of AF from their wrist monitors. According to the British Heart Foundation, the number of people diagnosed with AF has risen from 1 million a decade ago to 1.5 million today. Some of that huge rise will be due to population ageing, but the greater part will reflect a much higher rate of detection.

It might seem self-evidently a good thing: think of all those strokes being prevented. But here’s where it gets murky. The evidence on which we base treatment recommendations – including the risk calculators that help strike the balance between benefit and harm – comes from studies conducted on patients whose AF came to light due to symptoms. Treating a patient whose smart watch has incidentally picked up a burst of silent AF may not be the same at all. A Danish study in 2021, which used implantable ECG recorders more sensitive than smart watches, trebled the number of AF cases and anticoagulant prescribing in the monitoring group. You’d expect this to have prevented a lot of strokes, but the rates were the same as in the control group. Screening for and treating “silent” AF may do no good at all.

The term “Vomit syndrome” – victim of medical imaging technology – was coined to describe the scenario where CT and MRI scans make unexpected findings of uncertain significance. This happens up to 25 per cent of the time. Patients are then sucked into a vortex of repeat scans – and sometimes invasive procedures such as biopsies – and must live with the nagging anxiety that there might be something serious going on. The increasing popularity of “wearables” like smart watches, which are constantly measuring our biological parameters, is fuelling a fresh epidemic of Vomit, this time centred on the heart.

Sam spoke to his patient again. Having had the uncertainties explained, he opted to discontinue the blood thinners. He said he would let us know if his watch raised the alarm again. If it does, though, I’m still not sure we would know what we ought to do.

“What Is a Doctor?” by Phil Whitaker is published by Canongate

[See also: How to save the NHS]

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This article appears in the 11 Oct 2023 issue of the New Statesman, War Without Limits