Cardiac Exercise Question?

My heart rate does not increase much doing cardio, such as bike or treadmill, due to my medications. I don't believe the Hr moves more than 10 points if that? Anyone know of a most effective way to determine the optimal cardiac rate and or machine speed and or resistance. Example on the bike I go about 70 rpm at a 4 resistance setting. I do this machine without exerting myself greatly. Any suggestions to get better results?