sudden quick heartbeat, sudden slow down

  : I'm 23 years old, was a smoker for 6 years (quit only 4 days ago), and was on phentermine monotherapy off and on for about 2 years(ending last October).  I've been having this problem since this past November, but now it is really starting to scare me and occupy my thoughts.  Basically, with any small physical exertion (i.e., standing up, brushing teeth, walking short distances, etc.)my heart rate goes from around 85 to 110 instantly. It usually returns to normal within minutes, if not seconds, after I've stopped whatever I was doing and there is no shortness of breath. I'm afraid to start exercizing again because I don't know what is going on.  Also, I can usually feel my heartbeat throughout my lung area, but I don't know if this is just due to increased sensitivity in general.  
  : Is this sudden increase then decrease in heart rate something that is normal? Did I give myself a death sentence by taking phentermine, and should I go see a doctor for this? If so, what types of tests would I have to do in order to find out the cause? Basically, any thoughts or suggestions you could give me would be greatly appreciated, as this has become quite upsetting.  If you need to ask further questions please do.
  : Thank you,
  : Jennifer
  

[Cleveland Clinic, MD]

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Dear  Debbie,
Thank you for your question. There are many causes of tachycardia  (fast heart beat) and they can be divided roughly into sinus (originating from the sinus node or heart's natural pacemaker) and non-sinus tachycardias.  Nonsinus tachycardias are either  supraventricular (coming from the upper chambers of the heart) or ventricular (coming from the lower chambers of the heart).  Supraventricular tachycardias include:  paroxysmal supraventricular tachycardia, atrial flutter, atrial fibrillation and AV nodal tachycardia. Ventricular tachycardias are more serious in nature and are due to a rapid depolarization of  the ventricles.  
Sinus tachycardia is defined as a heart rate of greater than 100 beats per minute originating from the sinus node.  Sinus tachycardia is classified as either appropriate or inappropriate.  There are many causes of appropriate sinus tachycardia such as exercise, anxiety, panic attacks, dehydration, deconditioning, volume loss due to bleeding or other loss of body fluids, hyperthyroidism, electrolyte abnormalities and many other conditions.
Inappropriate sinus tachycardia can only be diagnosed when all causes of appropriate sinus tachycardia have been ruled out.  It is not clear what causes inappropriate sinus tachycardia but possible etiologies are an increase in the rate at which the sinus node depolarizes and an increased sensitivity to adrenaline.  Once the diagnosis has been made by ruling out all of the potential causes of appropriate sinus tachycardia  there are several treatment options.  If the symptoms are not overly concerning no treatment needs to be done.  There is no increase in morbidity or mortality in persons with this condition and they can expect to have a normal life-span.  For persons in whom the symptoms are unbearable medications such as beta blockers or calcium channel blockers can be used, usually with good results.  In the rare person unable to tolerate medical treatment catheter ablation (burning) of the sinus node with insertion of a pacemaker or surgical removal of the sinus node have been used in the past.  Newer techniques are being developed using catheter ablation to modify and not destroy the sinus node thus avoiding the need for a pacemaker.   This procedure is still in it's infancy and should only be undertaken at a major medical center after consultation with an electrophysiologist.
Yes, you probably have not done yourself any favors by taking the phenteramine and this may be playing a part of the problem.  However, I do not think this is a "death sentence".  I think this is most likely one of the above conditions and your doctor should be able to help you work through it.
Information provided in the heart forum is for general purposes only.  Only your physician can provide specific diagnoses and therapies.  Please feel free to write back with additional questions.
If you would like to make an appointment at the Cleveland Clinic Heart Center, please call 1-800-CCF-CARE or inquire online by using the Heart Center website at www.ccf.org/heartcenter.  The Heart Center website contains a directory of the cardiology staff that can be used to select the physician best suited to address your cardiac problem.