The trainer was dead wrong! You can't go by standard heart rate guides when you're on BP meds. Just like you figured out, they'll keep your heart rate artifically low. He's a moron, if you'll pardon my French.
Since you can't go by heart rate, there are a couple of ways to judge how hard you're working - that is, your intensity. First is Perceived Exertion. All that means is on a scale of 1 to 10 (10 being the hardest you could possibly ever go, like an all-out sprint) how hard do you feel like you're working?
Here are some descriptions from About.com
Level 1: I'm watching TV and eating bon bons
Level 2: I'm comfortable and could maintain this pace all day long
Level 3: I'm still comfortable, but am breathing a bit harder
Level 4: I'm sweating a little, but feel good and can carry on a conversation effortlessly
Level 5: I'm just above comfortable, am sweating more and can still talk easily
Level 6: I can still talk, but am slightly breathless
Level 7: I can still talk, but I don't really want to. I'm sweating like a pig
Level 8: I can grunt in response to your questions and can only keep this pace for a short time period
Level 9: I am probably going to die
Level 10: I am dead
You want to be somewhere in the 5 - 7 range most of the time, maybe occasionally going into 8.
Second is the Talk Test. The talk test method of measuring intensity is simple. If you're at a light intensity level, you should be able to sing while doing the activity. When you're at a moderate intensity level, you should be able to talk and carry on a conversation comfortably. If you can only speak in sentence fragments or not at all, that's high intensity. You'll want to be in the moderate intensity range.
You're doing fantastically well and should be very proud of yourself. Please don't listen to that idiot trainer ... and if he says anything else to you, tell him that you talked to your own personal trainer and she said you're doing it perfectly!!