Overnight HRV: Comparison of PPG to ECG

Problem:  Short-duration morning HRV measurements may be susceptible to exactly when you wake, your HR, breathing rate, thinking about work etc in that moment. Different apps report widely different values of HRV.  Perhaps more importantly, morning snapshot measurements do not record HRV over a physiologically meaningful interval.  The best case would be to record HRV 24-7, but this is not feasible with current wearable technology.  However, wearables can and do report night time HRV.  At least this is measuring vagal balance over a physiologically meaningful interval, namely, the sleep recovery timeframe.

Methods:  I recorded overnight HRV using Polar H7 chest strap connected to the iOS app HRV Logger, for 12 nights over more than a month.  HRV Logger stopped recording on 2 nights, perhaps due to loss of connection with the H7 or other errors, so I have 10 nights of complete data.  The Polar H7 chest strap directly measures electrical activity (ECG) and is considered the gold standard for at-home HRV measurements.  

I also wore a Fitbit Inspire 2 each night and downloaded the data file from Fitbit.com at the end of the month.  Fitbit uses a green light PPB (photoplethysmography) sensor to measure blood flow changes at the wrist and infer heart rate and HRV.  

HRV Logger was set to 5 minute recording windows and RR-interval correction was set at 25%.  Fitbit also reports HRV in 5 minute recording windows.  However, the 5-minute interval windows are only approximately aligned as they are based on offset starting times.  HRV is reported as rMMSD.  

Results:  At this point, I am reporting preliminary data from 3 representative nights.  

Night of 9/19

Summary

	Fitbit	Polar
mean	38.5	41.48
std dev	6.75	8.44

Night of 9/21

 

Summary

	Fitbit	Polar
mean	39.82	44.32
std dev	6.94	9.23

Night of 9/28

 

Summary

	FitbitInspire2	PolarH7
mean	35.06	39.57
Std dev	5.31	7.80

Conclusion

An examination of the time course data shows that Fitbit was able to pick up the broad changes in HRV during the night.  It appears Fitbit misses spikes in HRV that the Polar strap is able to detect.  Although it is also possible that the Polar strap is biased by movement artifacts, it seems most likely that either Fitbit is not sensitive enough to detect short-duration changes in HRV, or a Fitbit error-correcting algorithm is too aggressive at removing short changes in HRV.

In addition to being less sensitive to variations in HRV (as measured by standard deviations), Fitbit also consistently records a lower average nightly HRV than the Polar strap.

The overall R squared value is only 0.38, however this statistic probably underestimates the correlation of time series data due to the fact that the 5-minute windows are not perfectly synchronized.  Even so, R squared of 0.38 is a higher correlation that I was able to achieve using the Polar H7 with 2 other apps.   

HRV "Resonant Frequency" Test

HRV is often used to assess relaxation and recovery, but it is not clear if it correlates with R&R, or somehow causes it.  I think the claim is that it causes it, because HRV is supposed to directly reflect autonomic activity, with higher values indicative of more balanced sympathetic and parasympathetic activity.  (But some recovery scores are calibrated to not let the score get too high all at once.)

But HRV is very susceptible to intentional breathing frequency.  This can be good or bad, depending on whether this manipulation is considered functional or just gaming the system.  

HR naturally slows on the exhale, and speeds up on the inhale, reflecting the dominance of parasympathetic and sympathetic systems, respectively.  By adjusting the frequency of breathing to be resonant with some natural rhythm, HRV can be increased optimally.

I tried breathing rates from 4.5 to 6.5 breaths/minute (at 0.5/bpm steps) and found 5 breaths/minute (=6 seconds in, 6 seconds out) to be optimal for increasing my HRV.  This is my personal optimized meditation breathing frequency for reducing stress.  

HRV is greatest when I take belly and chest breath deep in, then really relax completely on breath out.

Heart Disease and Blood Lipid Panels

Cholesterol is present in the diet and in the blood, and the difference bears repeating: cholesterol in the diet has very little effect on cholesterol in the blood. While cholesterol blood levels are often talked about as "less is more", we would argue that having values too low can be problematic too. For example, based on WHO data, total cholesterol between 200 and 240 is associated with lowest all-cause mortality, but that is just a simple correlation.

Based on an experimental study on a comparable "Westernized" (?) population, the Japan Lipid Intervention Trial found optimal ranges to be:

Total Cholesterol:180-260 (anything above or below has significant p>0.001 increase risk of all-cause mortality)

LDL: 80-200 (anything above or below has significant p>0.01 increase risk of all-cause mortality)

Triglycerides: N/A - no significant relationship.

HDL: anything above 50 has significant p>0.01 decrease risk of all-cause mortality.

Interestingly, LDL values are usually not measured directly, but only calculated using the Friedewald equation, which has been shown to be inaccurate, especially at low triglyceride values. If you really want to drill down on your blood lipids, your doctor should know that the standard total/LDL/HDL panel is outdated and that newer tests are more accurate.... like the NMR LipoProfile test or a VAP test.

These new tests are important, because not all LDL particles seem to cause heart disease. VLDL particles are smaller than average and are more likely to embed in arterial walls, given predisposing conditions like inflammation (C-reactive protein (CRP) is a good marker of inflammation). If your fasting triglycerides are low you probably have normal healthy LDL, but if they are high you may have much more VLDL. If you also have inflammation....watch out!