That's not quite correct. Smart watches have gotten quite good at measuring heart rate. They are sometimes a bit slow to notice drastic changes (like going from

That's not quite correct. Smart watches have gotten quite good at measuring heart rate. They are sometimes a bit slow to notice drastic changes (like going from sedentary to exercise), but they're pretty accurate when it comes to just counting beats per unit of time. They can be, however, quite sensitive. If it's not working well for you, it's likely because the sensor is wet, or dirty, too tight or too loose. The fingertip seems like a truly terrible place to measure heart rate, given that fingers are constantly moving and pretty sensitive to pressure. They're the easiest place to measure oxygen saturation (SpO2) though. Most (if not all?) smart watches have optical sensors, which register the flow of blood. You can measure the change in velocity and count the number of beats and the time between them. It doesn't tell you anything about what the heart is doing. A single lead ECG can do that reasonably well. Anything more professional (12 lead holter) quickly becomes impractical for long term use.