I'm not a doctor, so take what I'm saying with a grain of salt, and ask your wife's own doctor to read the EKG and tell her what he really thinks.    If you aren't satisfied that he's giving you an honest answer, or he won't run another EKG to confirm the results of the first, go see another doctor and get a second opinion.

The reason he asked about her potassium is because ST segment depression on an EKG can often be due to electrolyte imbalances, or in her case low serum potassium (also called hypokalemia).   That is a possible cause of the ST depression in her EKG.  You should ask the doctor to confirm her potassium level by running a CBC (complete metabolic panel) which is a simple blood test.  That should confirm whether or not her potassium is low, and whether taking additional potassium supplements might help.

Unfortunately, the image you loaded of the EKG was too small to read the text or to be able to count the boxes (that's how you tell whether and how far the ST segment is below the QRS or not), so you'll need to upload a larger image if possible.

There are several other causes of ST segment depression, including ischemia (a blockage somewhere), hypothermia, hypokalemia (low levels of serum potassium), tachycardia (abnormal heart rhythm, in the case of tachycardia, a very quickly beating heart), subendocardial infarct (muscle damage to the subendocardium, i.e., a heart attack), a reciprocal ST elevation (which is just what it sounds like), ventricular hypertrophy (enlargment of the ventricle, often a precursor to heart failure, but not always, as well trained athletes also often have enlarged hearts), bundle branch block, or the use of medications like digitalis (sometimes used to treat patients with congestive heart failure or to treat high  blood pressure).   Those are the most probable causes of ST segment depression.

Since you say your wife is healthy and has never had any sypmtoms of heart disease (and by that I'm assuming you mean she's never had shortness of breath, angina, or an MI that she knows about), there are a few causes listed above which can immediately be ruled out.  Hypothermia (having the body subjected to extreme cold) is probably one which doesn't apply here.  Likewise, while you don't mention her BP (i.e., whether she has high blood pressure or normal, untreated BP),  if she's not taking meds for CHF (congestive heart failure) or to treat BP (e.g., diuretics which cause a loss of potassium) those causes can also be ruled out.  

On the other hand, if she's taking "loop" diuretics, she is likely losing a lot of potassium in her urine.  If so, you might ask her doctor to either switch her to a potassium-sparing diuretic (e.g., dyrenium, spironolactone, or Inspra, which is a new drug which doesn't have the side-effects of spironolactone).

While you also say she has no heart disease symptoms, other than the obvious signs of a heart attack, many people have infarcts that they aren't even aware of, especially women.   But rather than worry about her condition, the best thing to do is take a pro-active approach:   She should speak with her doctor, and have him run a couple of tests to find out the cause of the ST segment depression, and if he doesn't know or can't discern the reason, ask him or her for a referral to a cardiologist, i.e. a specialist. 

Good luck.