High exercise HR, Low BP, PVCs

My wife is 38 years old, not overweight, doesn't smoke, drinks occasionally, and has experienced heart palpitations since she was in her early 20s. She has always had a fairly high heart rate during moderate exercise (eg. 185 bpm when running 3 miles at 10 min/mile). She exercises typically 4-5x per week. Her heart rate goes to 150 and higher very soon after she starts. When she is lying down her heart rate is in the low 50's to mid 60's. When she stands, it goes into the mid 70's, and when walking around, it goes into the mid-80s, up to 100. Her blood pressure is low-normal. Historically it as been ~ (100-110)/(65-72).

The palpitations have been increasing, and she went to see a cardiologist. She had an in-office EKG, Echo, and was given a 30-day monitor to record the palpitations. The in-office EKG was normal, the Echo was normal, except for mild regurgitation of the aortic valve. The cardiologist said the monitor shows PVCs (skipped beats), and has a wide HR range, including high HR during exercise, and she was concerned with just one episode when her heart rate went up to 140bmp after climbing the stairs. The cardiologist perscribed a beta-blocker to control the heart-rate, but my wife and I are concerned about the beta blocker and her fairly low BP.

I'm concerned about the cluster of high exercise HR, low BP and palpitations. The palpitations do diminish if she takes a break from running. I'm wondering what causes her type of physiology, could it be a structural defect that would only be apparent in an exercising echo? She had her blood checked recently and she is not aneimic. Could she just be built with a heart with a low-stroke volume? Her aorta is also on the upper side of normal in diameter, but it has been like that for years. I fear her exercise tolerance is diminishing over time, and there is some deteriorating condition. What should we look for? Or is this reasonably within normal physiology?

[Jerry_NJ]

I can't help much but can say from experience when I started taking a high dose of BB my BP went too low.  Not a danger in itself, but did make me light headed when I got up from sitting, never passed out, but close a couple of times.  However, over a few months my body seems to have adjusted to the BB and the BP is back to normal, no more dizziness.

An echocardiogram does give a measure of the ejection efficiency and measures the volume of all chambers as well as check the operation of all valves.  This should provided the data to answer some of your questions about exercise cardio capacity.  

A HR of 185 under stress may be ok for a person 38, in good physical condition.  I'm using 220 - age as the maximum to stay below.