In the latest story of an Apple Watch saving its user’s life, a Canadian firefighter’s life was saved when his watch alerted him to a serious heart condition. Firefighter Travis Chalmers, 44, suddenly felt “hot” in his chest and a sharp headache while playing road hockey with his son. He initially dismissed these symptoms as the flu or allergies, but Chalmers soon discovered that his heart rate was abnormally high, which was a warning from his Apple Watch that ultimately led him to seek medical help.

According to a report by Canada’s Global News, Chalmers’ Apple Watch initially indicated atrial fibrillation, a condition in which the heartbeat speeds up, increasing the risk of stroke and heart failure. Despite attempts to rest, his heart rate remained irregular. Seeing the constant alerts from the watch, Chalmers decided to go to the hospital.

“About half an hour later, I was lying down with my daughter and my heart rate was still racing out of my chest — when I explained atrial fibrillation and told them the symptoms, I was rushed to the hospital,” she said. “That’s when they told me I was probably having a heart attack.”

Upon arriving and describing his symptoms, he was immediately assessed and told he was possibly having a heart attack. Subsequent tests by doctors confirmed the diagnosis, revealing a completely blocked artery.

Chalmers, who has no significant history of heart disease in his family and lives an active lifestyle, was taken aback by the diagnosis. He then thanked his Apple Watch for detecting the irregularities that prompted him to seek help, saying the watch proved to be a second pair of eyes in taking care of his health. “I’m very fortunate that the watch gave me a second pair of eyes.”

Apple designed the irregular heart rhythm notification feature on Apple Watch (Series 1 or later) with watchOS 6 to alert users if it detects an irregular heart rhythm that may be a sign of atrial fibrillation (AFib). The feature monitors the user’s heart rhythm in the background over a period of time using the watch’s optical heart sensor. Monitoring occurs at regular intervals while the user is at rest. If the sensor detects an irregular heart rhythm that indicates AFib, it performs additional checks to confirm the findings.

For the Apple Watch to issue a notification, it must identify an irregular heart rhythm on five separate occasions over a period of at least 65 minutes. Once this criterion is met, the watch sends a notification to alert the user of the detected irregular heart rhythm. This notification prompts the user to seek further medical evaluation, potentially leading to early identification of AFib and allowing for timely medical intervention.

In addition to the background monitoring feature, Apple Watch also includes an ECG app that provides a more detailed analysis of heart rhythm. Apple also validated the ECG app’s accuracy through a clinical trial that included nearly 600 participants. During the trial, the ECG app’s results were compared to a traditional 12-lead ECG interpreted by a cardiologist. The study showed that the ECG app had a sensitivity of 98.3 percent in correctly identifying AFib and a specificity of 99.6 percent in correctly identifying sinus rhythm in classified recordings.

These features enable the Apple Watch to help users effectively monitor their heart health. While Apple notes that the Apple Watch cannot alert for heart attacks, it can help users identify potential cases of AFib, and allow users to take proactive steps in managing their heart health.