Some HRV resources mention that you can find the optimal breathing rate to temporarily increase HRV, and breathing for several minutes a day at this rate causes lasting increases to HRV. My setup is now good enough to test this. Below I plot the last 5 seconds of ECG data (as a sanity check to make sure the R wave peaks are identified correctly), as well as the time between beats (black), and difference in time between beats (blue) over the entire session. In retrospect, I should have plotted the absolute value of the differences. The respiratory sinus arrhythmia is very evident. During this session I tried 4-4, 5-5 and 6-6 inhale and exhale times. During a different recording sessions I tried much slower breathing and unintuitively noticed less of the arrhythmia (though this agrees with the commonly cited numbers for a good breath rate to increase HRV – I just find it unintuitive).
While ~6 breaths per minute (5s inhale, 5s exhale) is referred to as slow breathing in a lot of the literature, I find this surprisingly fast. When I’ve gone through periods where I trained slow breathing, I could get down to 1.5 breaths per minute, though with training and very relaxed.
On the electrical setup side, this is just streaming the ADC data over BLE. The PCB rests on the skin so that the PPG sensor has contact. I had an issue where the silicone conformal coating started to wear away and there was bad ECG data because the skin would intermittently contact conductive parts of the board. There was also a bit of corrosion from sweat. I cleaned the board and reapplied the same conformal coating, also adding a layer of kapton tape to improve its wear resistance. This is not a good solution, I should have used a different conformal coating, or even painted it with epoxy. There was also an issue where logging to the SD card would start failing, often after several minutes of logging. It seems that preallocating a larger file before any writing fixed that issue.
The next revision of boards are ordered and I look forward to using the improved V2 of this.
