Heart rate does not increase with intense workout

[calex]

I am a 54 year old women with high blood pressure (managed), overweight, and recently had multi level fusion/Disc L1 - S1. Over the last 3 months I have been working out with weights and riding exercise bike, gradually increasing the time and weight loads. My concern is that during a 40 min bike workout with a mid range of 91rpm 12.5 k workout that I cannot increase my heart rate beyond 89. I try to push the rpm and still no response. This is not true in all workouts as 1 in 4 workouts my heart rate during cardio workout of bike will be 137 - 155. I'm not sure if this is cause for concern. My normal routine is 5- 7 min of bike, 40 min weight/ball (core)and then finish with 40min on bike. In the beginning I thought the readings on the bike were not functioning properly and changed bikes but it didn't make a difference.
What am I doing wrong?

[anxiousmomtobe?]

Are you working with a trainer?  Mine recommends 30 minutes cardio, 10 minutes stretching than 20 minutes with weights.  The weight training will increase with my fitness level.

[calex]

Initially that was my routine. I have private session with Pilates instructor weekly so try and incorporate some core with my weight training.
My concern still remains my heart rate during cardio workout. This morning my rate was 130 - 151, but 2 days ago I couldn't get the rate to rise higher than 89 during a 40 min session on the bike.

[anxiousmomtobe?]

Have you seen a doctor, maybe you need a stress test.

[mmfd]

You mentioned that your hypertension was managed.  That usually means that you are taking some medication for it.  Many of them can affect your heart rate, esp. its response to exercise.  Are you on a beta blocker for your bp?  It is reasonable and common to have your heart rate not go above 89 on a beta blocker.   Depending on the time of day you take your med and when you exercise, your heart rate will vary.  If you aren't on any meds then I have no clue, but it is much better for your heart rate to not go up really high with exercise, than to have it go too high and take too long to recover!