Please consider adding LOINC code 80404-7 (R-R interval.standard deviation (Heart rate variability)) to the activity value set with an associated example measure. Exercise physiologists consider HRV to be a critical measure for tracking stress and planning physical activity; for example, if a patient's HRV is temporarily low due to other stressors then it may be advisable to alter the care plan to focus more on rest and recovery instead of hard exercise until the HRV is back up in the normal range. Most high-end consumer smart watches and wearable fitness trackers released in the past few years do track HRV so the data is available for many patients. At some point I think we might even want to consider adding it to the vital signs panel.
https://doi.org/10.3389%2Ffpubh.2017.00258