Just take your time with them and I were you i would try and use a HR monitor to see where it goes when doing them. Ask your doctor about doing them.
Do you use any meds that control your HR?
Maybe start out on an easier move like on the smith machine and if you have been back in the game for a while now and your base is pretty good, then I wouldn't hold back as much as you think as this will only make the heart more efficient!
increase stroke volume(better cardiac output) and lower HR in general!