This is a topic for discussion respiratory rate, a type of data that is becoming popular due to both it’s proposed value for COVID-19 infection tracking and it’s increased availability from wearables like the Apple Watch, Oura Ring, and others.
Please contribute to this discussion if you have questions or answers about how exactly respiratory rate is measured by these different systems.
Hey Gary, here is some basic stuff about how the ring RR interval/calculates respiration rate: oura collects continuous ppg data during the night, so you are getting every QRS curve that each heart beat makes. It can then temporally isolate the r peaks and calculate the distance between them. It does this and then saves out the data in the form of HRV (RMSSD of the RR interval) and HR (number of QRS complexes per minute). Only on a research-only version of the ring (not available for public sale) is all of that ppg data saved out each night and available for anlaysis – as one might imagine, saving all that data kills the battery life fast. Respiration rate can be garnered from looking at frequency modulation of RR interval (because the RR interval is modulated at the frequency you are breathing!). I’m not sure what kind of frequency analysis is used to garner this information though, but it is read off from the Gen2 ring as an average every 5 minutes. I know they are working on improving their methods, but am not sure on more details.
Great answer and a great way to start this topic. PPG data is the “photoplethysmogram” data that is generated by the light on the inside of the ring, if I understand it correctly. That is, the light shines onto the skin and the intensity(?) and/or wavelength (?) of the light that comes back to the sensor can detect the pulse. The pulse can be used to generate HRV, and HRV can be used to determine the number of breaths per minute.
Almost: As @azure mentioned above, the basic measure is RR intervals, i.e. the exact amount of time between each heartbeat. From a sequence of RR intervals you can calculate the pulse (bpm), various HRV metrics, or try to infer how fast someone is breathing.
One limitation of PPG is that it has trouble capturing accurate-enough RR intervals when there is movement. You can use an ECG device (like a chest strap) instead, but even then a high heart rate can throw off the breathing rate calculations because the variability of RR intervals goes way down as heart rate goes up.
Devices like the Hexoskin use sensors that measure how your chest (and belly) expand and contract to track actual breathing rate (and volume), so that’s another option.