When Apple introduced the fourth iteration of its smartwatch, the big new selling point wasn't a feature we typically associate with a watch or any sort of smart device. Instead, the company added a feature that had only recently arrived in the form of specialized consumer devices: an electrocardiograph (ECG), a device made for monitoring the heart's electrical activity. But the watch was ready before the software was, meaning an examination of the technology wasn't possible in our comprehensive review of the Apple Watch Series 4.