The Apple Watch Series 4 will include a more advanced heart-monitoring technology called electrocardiogram. This feature has received clearance from the US Food and Drug Administration, meaning it can be used as a medical device — a move that is part of Apple’s increasing push to brand the Watch as more than a fitness device.

The Apple Watch Series 4 is the first major design overhaul of the Watch since its debut more than three years ago. The existing Apple Watch already had a way to measure heart rate, of course. Like most wearables, it monitors heart rate using green LED lights embedded in the device. The lights reflect on the skin to detect the pulse and changes in blood volume; this is turned into the heart rate number.

This method is convenient but indirect. Watches are convenient, but the wrist isn’t always the best place for taking accurate measurements this way because there are so many layers of tissue, and studies suggest this method isn’t very accurate.

ekg is far more accurate than guessing based on pulse

The electrocardiogram, or EKG (or ECG as Apple calls it), is much better, and there have long been rumors about Apple developing this feature. Though this is a new feature in the Watch itself, last year the FDA approved the AliveCor KardiaBand, a watch accessory that essentially does the same thing.

On the Series 4, users put their finger on the digital crown. The Watch passes a current across the chest to track electrical signals in the heart, directly, and it’s far more accurate than interpreting based on pulse. The process is meant to take about 30 seconds and the user will receive a heart rhythm classification. Normal rhythms will be classified as “sinus” rhythm, and the Watch will also classify irregular rhythms, such as atrial fibrillation. The information will be stored in the Apple Health app.

EKG is a serious feature and the next step for Apple as a health company. It can be used to more accurately diagnose or monitor heart disease. Heart disease is the most common cause of death around the world, according to the World Health Organization, so of course companies want to work on a problem that could have a huge impact and be a real moneymaker.

Apple is no exception. Last November, Apple announced that it would partner with researchers from Stanford University to run the Apple Heart Study to investigate atrial fibrillation (AFib). (Stanford will collect data from Apple until January 2019.) Third-party apps already work with Apple to see if their technology can detect common heart problems like irregular patterns. FDA clearance for the EKG feature on the Watch will pave the way for medical clearance of even more Apple features.

The new Series 4 Apple Watch begins shipping on September 21st, but the EKG app and Irregular rhythm notifications won’t be available until later this year.