How does the Apple watch track heart rate?
posted June 4th, 2024 by Cecilia Mungovan
Apple watch and all other smart watches use a technology called photoplethysmography (PPG). PPG tracks changes in volume of peripheral blood circulation using reflected light and optical sensors; blood volume changes throughout a heartbeat so light absorption changes. Basically is a way of measuring how the blood flow changes during a heartbeat, over multiple beats, or intervals.
PPG emits LED lights in a specific color and uses an optical sensor to detect changes in blood flow during different parts of the heartbeat cycle. Because you are measuring the change in blood flow, green LED is used. Blood is red and the properties of light tell us that the red blood will absorb green light and reflect red light. Other tissues will reflect less red light. The optical sensor's job is to detect the changes in light, using light sensitive photodiodes (which means they are very good at detecting tiny changes in light). Photodiodes are used to tell when the heart has just beat, meaning it has pumped blood into the body, increasing the volume peripherally (like in your wrist for example). At this moment there would be less green light, because it has been absorbed and more red light reflected. The photosensitive diodes detect this. The opposite applies when the blood is going back to the heart. This means there is less blood to detect, so higher green light signals and less red light signals. These changes in blood flow, or volume, happen in cycles with each heartbeat.
Heartbeats are electrical signals that tell the heart muscles to move in a coordinated fashion, pumping blood throughout the body. This is measured using EKG, which stands for eletro-kardio-gram (invented by a German). It uses electrodes to monitor electrical activity in the heart. The heart beats in a predictable pattern so they just measure the intervals to get the heart rate.
Unlike EKG, PPG does not directly track the heart beat but it tracks the change in blood flow that happens over the period of a heartbeat. The change in blood flow indirectly represents the heart rate. The watch tracks the cycles, and the change in blood flow that happens at even intervals. This tells us what the heart rate is, by seeing the time intervals the blood flow changes at. This can be represented over a waveform.
You are probably already familiar with PPG if you have ever been to the doctor. It has many uses in medicine. The most common one is a finger probe used to measure the amount of oxygen in your blood, or saturation. It uses the same technology to measure oxygen saturation as an Apple watch does to measure heart rate. The only difference with the finger probe is that the LED and the optical sensor are on opposite sides of the device, making them on opposite sides of the finger. This makes it more accurate because it is easier for the photodiode to detect changes in light intensity.
Smartwatches offer an easy, and depending on the model, inexpensive way to track your heart rate. Many people can benefit from it, from runners to people with heart problems. The watches use a technology that is already well known and widely used in medicine. In reality, most smartwatches don’t continuously monitor your heartrate because it takes up too much battery. During exercise though, it can track your heart rate continuously which is helpful if you mostly care about what your heart is doing during a workout. The technology can be useful for many people with a variety of needs.