lack of a chronotropic effect of a beta blocker

Posted By lyn on July 01, 1998 at 11:59:44:







My wife is a 47 year old postmenopausal female on estrogen replacment
therapy and in excellent physical condition.   She has a very stressful
and demanding job.   During a routine checkup a year ago she found that
she was hypertensive (~160/100) and her resting heart rate was ranging from
90-105 bpm.  A stress echo was performed and it showed normal heart function
and wall thickness with no evidence of valvular malfunction.   Blood and
urine analysis ruled out problems such as an adrenal tumor.  She was started
intially on HCTZ and  then atenolol, neither of which she tolerated well.
  She was then switched to cardizem CD.  This controlled her BP initially
and then it began creeping up.  Her physician increased the dose but
control was only temporary.  She was then switched to losartan (50 mg),
once daily.  Initially this controlled her bp and decreased her resting
heart rate to the low 80s.  When she did aerobics her heart rate would get
up to the 150 range.  When her bp started creeping up her physician
increased the dose to twice daily.  Two weeks ago she went to see her
physican who found her bp was again high and prescribed Ziac
(2.5mg bisoprolol/6.25 mg HCTZ) added to her losartan regimen.  This
controlled her bp nicely and decreased her resting heart rate to the 60-70
bpm range.  She was then told that she could resume her aerobics class but
that she should expect that her  heart rate would show only a minimal
increase with exercise.  When she exercised she found her heart rate was
up to 200.  What could explain such a finding?  Could she be getting such
an adrenalin rush from exercise that it just overloads the beta
adrenergic receptors so that beta blockade is no longer effective?  What
kind
of cardiac structural or endocrine pathology could explain such findings?