Can I Use HRV if I'm on Beta Blockers?

There are a few things to consider when you measure HRV with beta-blockers since it alters the activity. 

The best time to measure HRV with the lowest likelihood of beta-blocker influence would be immediately upon waking in the morning, before taking any medications or consuming anything (except small amounts of water). 

If you measure at the same time each morning, the relative trends over time will likely be meaningful, even if the absolute values are somewhat skewed by the medications as compared to population norms.

For further discussion on using HRV while on medication, please check out this post: HRV Demographics, Part 3 – Health, Medication & Guidelines