VO2 estimates are just that, not predictions or measurements.
Nobody here can comment on how medication or medical conditions affect fitness estimates on the Watch
You would best Contact Support.
Click here > Contact - Official Apple Support
or phone numbers
Click here > Contact Apple for support and service - Apple Support
Apple Watch should not be considered a medical device. If you have a condition requiring beta blockers then consult a doctor before working out.
You can turn off Fitness totally
Track your cardio fitness levels – Apple Support (UK)