Using a GPS watch has become the norm in distance running. These watches provide users with information regarding distance traveled, pace, and even maps of the route taken. Newer watches also include heart rate monitors, providing users with greater information about their fitness. The popular watch brand, Garmin, has a patented heart rate monitor [1] used in their watches, seen in Figure 1 below. 

Figure 1. Back of Garmin watch with heart rate monitor device (labeled “610”) [1].

• The device consists of an emitter, receiver, inertial sensor, and time-variant sensor. The processor determines frequency associated with the motion signal, transforms the signal from PPG into the frequency domain, identifies the cardiac component of the PPG signal, configures a time-variant filter, and calculates the time between cardiac component cycles.
• The device emits a light signal and receives an input of the light’s reflection, which eventually allows for the isolation of the cardiac component of signal.
• The cardiac component of signal allows for heart rate to be determined.
• The time between successive cycles gives insight into heart rate variability, stress, recovery time, VO2 max, and/or sleep quality.
• The device contains an interface that displays determined information to the user.

This device would be of interest to any Garmin watch user, especially those interested in heart rate during exercise. This watch, primarily used by runners, tells the user their heart rate and therefore how fast their heart is pumping blood through the body at any given time during exercise. This gives insight into the user’s fitness and exertion levels and ensures the user is in desired heart rate zones while training. Knowing how heart rate changes personally affect the user can also give insight into dehydration, stress, and needed recovery. Using this device over an extended period of time allows for users to see improvements in heart rate due to exercise.

How Does it Work?

The heart rate monitor in Garmin watches directs light from a light-emitting diode (LED) to the skin of the user. The reflection of the light is received by a photodiode, which sends a light intensity signal to the processor. The processor generates a photoplethysmogram (PPG) signal – containing cardiac, motion (determined by an inertial sensor, which senses movement of the device), and respiratory components – based on the intensity of the reflected light.

To isolate the cardiac component of the PPG signal, time-variant filters are used to remove non-cardiac components. The PPG signal can initially be filtered with a bandpass filter that only passes signals within the range of possible cardiac component frequencies. This bandwidth can be adjusted by the processor to account for lesser or greater expected cardiac frequencies based on changes in the environment. For example, if the user begins running, the processor senses rapid motion change and the bandwidth will increase since heart rate is expected to rise.

To determine which other signals to remove within the passband, the processor first identifies one or more frequencies associated with the motion signal via the inertial sensor. The processor then transforms the PPG signal into the frequency domain. Comparing the identified motion signal frequencies with the transformed PPG signal allows for the cardiac component of the signal to be determined within the frequency domain. Then, based on the identified cardiac component, the processor is able to determine filter coefficients for the cardiac component which are configured into the time-variant filter. When the PPG signal is transformed back into the time domain and filtered through this time-variant filter, the motion component is removed from the PPG signal. This results in a time domain PPG signal without the motion component, making it easier to identify the cardiac component of the PPG signal in the time domain. See Figure 2 below for a flowchart describing this filtering process.

Figure 2. Flowchart describing the process of isolating heart rate from PPG signal [1].

The device is also capable of storing memory. This allows for the device to create a model of expected cardiac component frequencies from previously determined data. Based on the model, the processor can then determine the probability of any given frequency within the PPG signal to be a frequency of the cardiac component.

Heartbeat and respiratory patterns are cyclical over a short period of time while motion data and noise can be cyclical or irregular for any length of time. Over a longer period of time, cardiac and respiratory signals can potentially have non-cyclical patterns (e.g. increasing heart rate during an exercise session). This allows for the variability in cardiac parameters to be determined. Analyzing variability in heart rate allows for estimates of parameters of stress, recovery time, VO2 max, and sleep quality.

This patent cites numerous references of inventions this device incorporates or improves upon. This device improves on a previous wrist-watch heart rate monitor (patent 2009/0048526), which was developed as an alternative to wearing a chest strap heart rate monitor. The Garmin device is different from this wrist-watch as this device does not include any inertial sensors. Therefore the Garmin device is able to better remove noise from motion [2]. Another exercise device by Samsung Electronics (patent US 7,867,142 B2) uses heart rate data to inform users about changes in their exercise speed by playing a sound. While the Garmin device does not play a sound, it uses the heart rate data to extrapolate information about stress, recovery time, VO2max, and sleep quality, which is likely to be of greater value to the user [3].

The following lists basic information regarding the Garmin heart rate monitor patent:

1. Patent title: Heart Rate Monitor With Time Varying Linear Filtering
2. Patent number: US 9,801,587 B2
3. Patent filing date: Oct. 18, 2016
4. Patent issue date: Oct. 31, 2017
5. How long it took for this patent to issue: 1 year, 13 days
6. Inventors: Paul R. MacDonald, Christopher J. Kulach
7. Assignee: Garmin Switzerland GmbH
8. U.S. classification: CPC: A61B 5/02416 (20130101); A61B 5/1112 (20130101); A61B 5/1118 (20130101); A61B 5/7285 (20130101); A61B 5/721 (20130101); A61B 5/02405 (20130101); A61B 5/02427 (20130101); A61B 5/02438 (20130101); A61B 5/0833 (20130101); A61B 5/486 (20130101); A61B 5/4815 (20130101); A61B 5/681 (20130101); A61B 5/725 (20130101); A61B 5/7278 (20130101); A61B 5/165 (20130101); A61B 2562/0219 (20130101)
9. How many claims: 29 claims

References:

[1] P. R. MacDonald and C. J. Kulach, “Heart Rate Monitor With Time Varying Linear Filtering.” U.S. Patent 9,801,587 B2, issued October 31, 2017.

[2] R. M. Aarts and M. Ouwerkerk, “Apparatus for Monitoring A Person’s Heart Rate And/Or Heart Variation; Wrist-Watch Comprising The Same.” U.S. Patent 2009/0048526 A1, issued February 19, 2009.

[3] S. K. Kim, J. S. Hwang, and K. H. Kim, “Method and Apparatus for Managing Exercise State of User.” U.S. Patent 7.867,142 B2, issued January 11, 2011.