At Danderyd Hospital in Stockholm, 206 patients took part in a quiet revolution in heart care: instead of waiting for procedure day to discover their rhythm had already healed, they simply held their smartphones to their fingertips twice a day, and let the camera do the work. The results were striking—so striking that they challenge how hospitals prepare patients for a common procedure that costs time, money, and staff resources to perform.
Atrial fibrillation, the most common cardiac arrhythmia in adults, causes the heart to beat irregularly and often too fast. When medication can't control the symptoms, doctors use electrical cardioversion—a controlled electrical impulse delivered while the patient is under brief general anesthesia—to restore normal rhythm. It's an established, effective treatment, but it requires skilled staff and careful preparation. Here's the problem hospitals face: many patients spontaneously return to normal rhythm on their own before the procedure date, without knowing it. When this goes undetected until the morning of surgery, the entire procedure gets canceled at the last minute, and all those allocated resources—the operating room, the anesthesiologist, the surgical team—sit unused.
Researchers at Karolinska Institutet and Danderyd Hospital tested whether daily home monitoring using a smartphone camera could prevent these costly last-minute cancellations. The technology, called CORAI, uses photoplethysmography, or PPG—the phone's camera measures tiny changes in blood flow at the fingertip, and from those pulse waves, the software calculates heart rhythm with high accuracy. Between 2022 and 2025, patients scheduled for cardioversion were randomly assigned to either monitor themselves at home or receive standard care with no advance monitoring.
The numbers tell the story. In the monitored group, only 4.8% of cardioversions were canceled on the same day. In the control group, 23.2% were canceled—nearly one in four. But the most telling comparison: when counting only cancellations caused by spontaneous return to normal rhythm, the monitored group had just 1.0% of cancellations versus 18.2% in the control group. That's a 94.7% relative risk reduction—nearly eliminating the problem that hospitals have struggled with for years.
What's equally revealing is what happened in the control group when researchers reviewed their records. Many of those patients had actually recorded normal heart rhythm before their scheduled procedure, but only three bothered to contact their health care provider about canceling. As Jonatan Fernstad, the physician-engineer at Karolinska Institutet who developed the technology, notes, objective heart rhythm monitoring gives doctors information patients wouldn't think to report on their own.
The accessibility angle matters too. Among participants, 99% owned a smartphone despite a median age of 70 years—a finding that suggests smartphone-based heart diagnostics could democratize rhythm assessment more broadly. That's significant because untreated atrial fibrillation increases the risk of stroke and heart failure, conditions that demand early detection.
The research points toward a future where a smartphone isn't just for calls and emails, but a reliable health partner that extends medical monitoring beyond hospital walls and clinic appointments. Johan Engdahl, professor of cardiology at Karolinska Institutet, is already planning the next phase: studying how effectively the method detects previously unknown atrial fibrillation compared with current health care screening methods. What began as a solution to one problem—canceled procedures—may become something larger entirely.