Sorry, just to add. I found that electronic equipment was too late in detecting the onset of problems. My brain was far quicker at detecting when something was going wrong or I was being pushed too hard. Trust your feelings, and stop. The eqiupment wont pick it up until you are in a fair amount of discomfort.
It looks from the exercise that your heart is struggling. Maybe they should have reduced your work load to begin with, especially as your blood pressure dropped like that. I had this problem in 2007, after a triple bypass. For the first two sessions I went as white as a sheet and could hardly stand up straight. Nausea and dizziness is a horrible thing after exercise.
I nearly gave up but went along for the next session and things seemed to improve. It does take time for your body to adapt, however, they should not push you beyond your limits. As soon as you feel light headed, stop. It saves a lot of discomfort and nausea.