The ScanWatch collects data on heart rate using photoplethysmography (PPG) and calculates a measurement based on dedicated algorithms. PPG uses light absorbed by the blood and skin to measure the variation in blood flow to the veins, which can be used to calculate the heart rate. That is why it may be difficult to obtain an accurate measurement if the watch is not worn optimally.

To obtain the most accurate results please follow these guidelines:

• The ScanWatch should be worn tightly enough that the sensor on the back of the watch face touches your skin.
• Wear the watch 1 to 2 finger width away from your wrist bone.
• Use the wristband Withings provides with your watch.
• Make sure that your watch stays in place during exercise. If it moves around at all, tighten your band one notch.
• Make sure your band is secure, but not too tight. It should be comfortable and allow proper circulation.
• Make sure the watch is clean, so the green LEDs are not obstructed by dust or dirt.

Additional factors that can cause the heart rate to be measured inaccurately:

Skin perfusion

Skin perfusion, or how much blood is flowing through the skin, varies among individuals and according to the environment. In certain circumstances, the watch may not be able to provide an accurate heart rate measurement.

Skin and blood properties

The technology used by the watch is based on the light absorbed by your skin and blood. Any permanent or temporary changes to your skin, such as tattoos, sweat, or even hair on the wrist may affect the measurements.

Environment

Environmental factors may affect the ability of the watch to take an accurate measurement. For example, water may obstruct or modify the light dispersion, and cold weather may impact skin perfusion.

Motion

The watch needs to stay securely in place to provide a heart rate measurement. Activities that cause force to the wrist, such as boxing or volleyball, may not allow the watch to provide an accurate heart rate measurement.

What is the proactive heart scanning feature?

Since AFib symptoms are irregular or may not even be known to users, ScanWatch offers a new early-warning detection capability. Through its embedded PPG sensor, the device has the ability to continuously monitor heart rate, which allows it to alert users to a potential issue even if they don’t feel palpitations. When ScanWatch detects an irregular heartbeat through its heart rate sensor, it will prompt the user, via the watch display, to take an ECG reading.

When does ScanWatch measure your heart rate?

When you use the Workout Mode, ScanWatch measures your heart rate continuously during the workout. In addition, ScanWatch measures your heart rate throughout the day when you’re motionless, and periodically when you’re walking. Since ScanWatch takes these background readings based on your activity, the time between these measurements will vary. 

How does ScanWatch measure your heart rate?

The optical heart sensor in ScanWatch uses what is known as photoplethysmography (PPG). This technology is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. ScanWatch uses green LED paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights thousands of times per minute, ScanWatch can calculate the number of times the heart beats each minute — your heart rate. The optical heart sensor supports a range of 30–210 beats per minute. In addition, the optical heart sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.

ScanWatch uses green LED lights to measure your heart rate during workouts.

ScanWatch also measures your heart rate when manually recording an electrocardiogram (ECG). It has built-in electrodes in the bezel (the upper ring) and the back, which can measure the electrical signals across your heart when used with the ECG recording. When you place your finger on the bezel, it creates a closed circuit between your heart and both arms, capturing the electrical impulses across your chest. 

To use the electrical heart sensor to measure your heart rate, select the ECG mode on the watch and place your finger on the bezel.

Important:

• ScanWatch cannot detect heart attacks. If you ever experience chest pain, pressure, tightness, or what you think is a heart attack, call emergency services immediately.  
• ScanWatch does not constantly look for AFib. This means ScanWatch cannot detect all instances of AFib, and people with AFib may not get a notification.
• If you’re not feeling well, you should talk to your doctor even if you don’t get a notification. Symptoms such as a rapid, pounding, or fluttering heartbeat, dizziness, or fainting, can indicate a serious condition.  
• Do not change your medication without talking to your doctor.
• In some instances, the notification may indicate the presence of an irregular heart rhythm other than AFib.

Notes:

• ScanWatch uses green LED lights to take measurements. It is possible for you to notice it if your wristband is worn loose.
• Each time a heart rate measurement is launched (by you, automatically or during a workout session) LED lights are on, in general for 30 seconds.
• If the sensor remains constantly on, even when no measurement is launched nor scheduled, resetting the watch by pressing and holding the button for 20 seconds should resolve the problem.