Heart Rate, as a smart-watch feature, means measuring your pulse from the wrist, and on most watches it works through PPG, or photoplethysmography. Green LEDs on the caseback shine light into the skin, and a photodetector reads how that light reflects as blood volume rises and falls with each beat; hemoglobin absorbs green wavelengths strongly, which is why the lights look green. This optical method enables 24/7 monitoring, resting heart rate, and HRV estimates. ECG is a separate technology that reads the heart's electrical signal, usually as a single on-demand recording through a second electrode. The first mass-market example was the Apple Watch Series 4 in September 2018, which shipped an FDA-cleared, over-the-counter ECG plus an AFib-detection algorithm.
The current examples are easy to point to. Apple Watch Series 10 (fall 2024) pairs optical Heart Rate with the ECG app, which runs on Series 4 and later, with Apple Watch pricing starting around $399. On the Garmin side, the latest optical unit is the Elevate Gen 5 sensor, found on the Venu 3 and the Fenix 8 series; the Fenix 8 runs roughly $999.99 to $1199.99 depending on case and size. For tighter budgets, the Garmin Forerunner 165 (around $250) and Fitbit Charge 6 (around $159) both offer 24/7 tracking and high/low heart rate alerts. The reference standard remains the Polar H10 chest strap.
When you shop, separate everyday tracking from serious training. For resting heart rate, sleep, and general wellness, optical PPG is plenty: studies put the Apple Watch within 5bpm about 98% of the time while seated. The trade-off shows up under hard, interval-style effort, where wrist optical sensors lag and accuracy drops. If you do sprints, HIIT, or fast cycling, the safest path is pairing a chest strap like the Polar H10 to your watch over ANT+ or Bluetooth, then letting the watch handle data and display.
It helps to know the sub-types. There are cheap bands that do optical Heart Rate only; there are watches that add ECG and AFib notifications, such as the Apple Watch, Samsung Galaxy Watch, and Google Pixel Watch; and there are sports watches like the Garmin Fenix that pair the sensor with multi-day battery life. AFib notification is not a diagnosis, but the high sensitivity Apple cites for its ECG app makes it a useful early-warning flag. If medical monitoring matters most, choose an ECG model; if performance does, look for the Elevate Gen 5 or solid chest-strap support.