The BioActive sensor was tested against clinical measurements, and it was up to 98% accurate
Most smartwatches and activity bands use heart rate variability (HRV), or the interval between heartbeats. A higher HRV is good while a lower HRV is usually an indication of fatigue and stress.
The Samsung Galaxy Watch 4 comes in two models with Bluetooth and LTE versions.
5 smartwatches that offer an SpO2, or blood oxygen saturation, sensor; a couple of them also offer ECG monitoring.