Smartwatches, phones, and headphones can all detect heart rate. What methods are used for heart rate recognition? What are their advantages and applications? Which methods have been applied to wearable devices, and how do they compare with medical applications?
Pulse oximetry
Basic measurement principle: measuring blood oxygen content and saturation is most commonly done on the finger, though toes and the earlobe are also used. These locations are the most common for blood oxygen measurement.
The principle uses red and infrared light emission. The two wavelengths must be very close and positioned so they both probe almost the same tissue volume in the finger, which helps ensure detection accuracy. This places demands on sensor technology and LEDs.
Red and infrared light operate alternately: when red light is on, infrared is off, and vice versa. Alternating operation minimizes mutual interference and helps ensure that both wavelengths sample the same body tissue.
When there is pulsation, blood flowing through capillaries delivers oxygen and then returns as reduced hemoglobin. During measurement, the detected light is attenuated by tissue structure, venous blood, and arterial blood. The arterial component has a constant baseline and a pulsatile component due to cardiac contractions, causing variations in the amount of hemoglobin that absorb light.
The heartbeat produces a pulsatile change in the transmitted or reflected light intensity. With appropriate algorithms, the pulsatile signal can be used to derive heart rate. Handling LED-driven noise is a key part of the signal processing chain.
