Wearable medical technology promises a new, and better, way to manage personal health. Whether it’s Fitbits counting steps and calories burned, blood glucose monitors aiding insulin dosing for diabetic patients, or Bluetooth earpieces offering round-the-clock heart rate and body temperature tracking, wearable devices sell the promise of the coldly clinical made portably intimate. Intermittent EKG monitoring, like that available in the latest Apple Watch, might seem like a small technological leap, putting what was once the sole purview of hospitals and doctor’s offices neatly around a consumer’s wrist.*

But EKG monitoring is a little different from other, more discrete medical information. Unlike devices that measure more cleanly numerical metrics—step counts or target heart rates or blood glucose levels—a wearable EKG display doesn’t give the user an easy sense of hitting targets or falling short. Reading an EKG tracing is nuanced and interpretive, more art than math. A Fitbit gives you a number. An EKG paints a picture.

The 12-lead EKG, the gold standard of the diagnostic, measures the flow of current from 12 points on the patient’s body, offering a 360-degree view of the heart’s electrical activity. Its tracing reports the patient’s heart rate, rhythm, and regularity. Because the various parts of the heart produce different shapes of electrical activity owing to their size and muscularity, the EKG can also detect which chambers are beating at what time, and whether these chambers are correctly synced up and beating effectively. The larger a muscle is, the stronger its electrical impulses, so the size of an EKG wave can also indicate whether parts of the heart muscle are enlarged or dangerously thickened.

The most urgent diagnostic use of the device determines the presence and location of cardiac damage due to decreased blood flow. Areas of the heart getting less oxygen will show changes in their electrical conduction, and the 12-lead EKG provides real-time information: not just indicating whether a patient is having a heart attack, but also which coronary vessels are most likely blocked. The 12-lead EKG can also detect the location of scarring left behind by prior, sometimes silent heart attacks. An EKG tracing will clearly show an area of dead heart muscle no longer conducting electrical signals. Dead meat don’t beat, as cardiologists put it.

Still, there’s plenty that a snapshot EKG can’t do, including diagnosing intermittent problems with rhythm or changes that only occur with certain activities. An EKG doesn’t capture the shape or function of the heart’s valves, nor can it diagnose precarious plaques in the coronary arteries that could signal heart attacks waiting to happen.

The fewer number of leads an EKG has, the less information it can give you. A one-lead EKG, such as the kind that appears in the latest iteration of the Apple Watch, gives just a single vantage point. For the diagnosis of some cardiac abnormalities, that might be akin to only solving one side of a Rubik’s Cube. A watch endowed with this kind of EKG feature likely won’t have a large public-health impact, despite promotional materials from Apple touting the “momentous achievement” of a wearable “that can provide critical data for doctors and peace of mind for you.” Apple’s not alone, either. Another smartwatch-EKG offering, from Withings, promises “the opportunity to take an ECG anytime and anywhere.”