Sudden high pulse during and after exercise

Dear Ask The Doctor: I am a 48 year old male. I am having a problem with my pulse increasing dramatically during exercise. My resting heart rate is around 60. Up until 6 months ago I could run for 25 minutes at a 12 minute mile. Over the past 6 months, my exercise capacity has diminished dramatically. My maximum is now 1.5 miles at a 15 minute mile. Over this recent period I've begun to have tachycardia incidents. Sometimes I'm running on the treadmill for as little as 10 minutes and my pulse will rise from 120 to 130 very quickly into the 170's. Then it can stay up there for as long as 15 minutes before stabilizing. I've also had it shoot up while resting. I'm wearing a king of hearts monitor for the second time. I recorded what I described above while exercising last week, but the doctor who read the report said my high pulse was normal for exercising. I know this is not normal as my exercise ability has diminished over 6 months, and my tachycardia incidents have increased. I do have a bicuspid aortic valve, but I have that regularly checked. Any ideas what can be causing this? I'm quite depressed as I'm now afraid to exercise. Thank you.

Dear Peter: Although a bicuspid aortic valve (BAV) usually causes no difficulties, it may cause decrease in the capacity to exercise as the patient grows older. This may be due to calcification, stenosis or regurgitation. It would be best to talk to your doctor about your limitations for further assessment of the condition.