An artificial intelligence model reading data from an Apple Watch tracked drops in patients' cardiopulmonary fitness that preceded unplanned hospital visits and urgent treatment by roughly a week. The finding points to a passive, everyday way to catch heart failure decline before symptoms force a crisis.
The Ted Rogers Understanding Exacerbations of Heart Failure study, or TRUE-HF, was published in Nature Medicine in March 2026 and featured on the journal's cover. Researchers at Toronto's University Health Network trained the model on 154 patients and validated it on another 63.
Each 10 percent drop in the model's estimate of peak oxygen uptake, a measure of the most oxygen the body uses during exertion, was linked to a more than threefold higher risk of an unplanned medical event. A modified version of the model extended the warning window to about 21 days.
"The findings of this study are a potential game-changer because they allow us to identify signals that would tell us a patient was in trouble before they ended up coming to the emergency room," cardiologist Heather Ross told Medical Xpress.
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Warning signs people brush off
Heart failure affects roughly 6.7 million adults in the United States, according to the American Heart Association's 2026 statistics update. Prevalence rises sharply with age, reaching 8 to 9 percent in people over 65.
The condition often creeps in quietly. Early symptoms overlap with what many people put down to normal aging or being out of shape, and they can come and go.
The American Heart Association describes shortness of breath, fatigue, and swelling in the feet, ankles, legs or abdomen as the most common signs. A nagging or dry cough can appear when fluid builds up in the lungs.
Unexplained weight gain of more than two to three pounds in a day, or five pounds in a week, may also signal worsening heart failure, the AHA notes. That kind of jump reflects fluid retention rather than fat.
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The Cleveland Clinic adds waking up short of breath at night, chest pain, a need to urinate while resting at night, and a bloated or hard stomach. Symptoms can range from mild to severe.
When to see a doctor
Any one of these signs on its own can have a benign explanation. Several together, especially in someone with high blood pressure, diabetes or a prior heart attack, warrant a doctor's evaluation.
The American Heart Association advises reporting one or more of these symptoms to a health care professional even without a prior heart diagnosis. Sudden shortness of breath, chest pain or fainting is a reason to seek emergency care.
Wearables are not a replacement for that judgment. But the Toronto team argues that data from a device already on millions of wrists could help clinical teams step in earlier.
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"We're not only measuring how fast someone walks down a hallway in the hospital while their clinical team is standing behind them and encouraging them," Chris McIntosh, senior scientist at UHN and co-senior author, told Medical Xpress. "We're seeing what happens to their heart rate when they're walking at the mall, on the street or at home."
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