Slow heart rate/energy

  When my sister was going through medical school, and would practice physicals on me, she would often comment on how slow my heart seemed to beat.  A few months ago, in an oral surgeons office, the nurse commented on my rate again, (45BPM), a few weeks after that if measured at 40BPM on a blood pressure machine at the gym.  I brought it up to my doctor during a visit for something else and he didn't seemed alarmed, but did order and EKG.  After reading the results he said they looked fine, but the rate was unusually low, 43, so he was going to send me for a cardiology consultation.  That appointment is coming up mext week.  I am experiencing lightheadedness.  I am in fairly good shape, running 30 min a day for about 2 months now after taking a break from running for about a year.
  My question is, can a slow heart rate be related to energy at all?  I slepp 10-12 hours a night and still have to sit down and often nap after being awake for 5-6 hours.  I rarely have the energy to do the things I want to do, just getting done what needs to be done.  Several years ago I mentioned this to a doctor and he quickly just told me that it's probably depression, which I know it is not.  Depression runs in my family and I'm very well aware as to what that looks like, as well as the need to combat it in my own life.  Depression is more like laying on the couch with no will to do anything, but I have plenty of will, just not much energy.
  Also, I was anorexic for about 8 months back in 1993, and have since bounced back to a healthy weight (5'8" - 143 lbs.) and healthy eating habits.  But could there have been any lone-term affects to my heart from that?

[Cleveland Clinic, MD]

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Dear Karen,
Thank you for your question.  There are many different causes of bradycardia (slow heart rate).  By definition bradycardia is present when the heart rate is less than 50.  Causes may be broadly divided into physiologic and non-physiologic causes.  
Physiologic causes are due to normal adaptations of the heart.  The most common example of this type of bradycardia is in the athlete or extremely fit individual.  The heart has become so powerful in its pumping capability that very few beats per minute are needed to provide blood supply to the rest of the body.  The heart rate may be as low as the 20's to 30's in some athletes.  This type of bradycardia does not cause symptoms and the heart increases to normal rates with exercise.
Non-physiologic bradycardia is usually due to disease of the conduction system of the heart.  There are several areas in the heart that can be affected: the sinus node (natural pacemaker) and  AV node are common sites of conduction disease. The exact location can usually be determined with an EKG but sometimes a special study called an electrophysiology study is required.   The incidence of this type of bradycardia increases with age.  If this type of bradycardia is symptomatic (i.e. lightheadedness, fainting) treatment is usually recommended.  Treatment is generally with a pacemaker device.
Your cardiologist should be able to determine which type of bradycardia you have and make appropriate recommendations from there.  Good luck.
I hope you find this information useful.  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.