How can an EKG show evidence?

Dear Dr. Roach: When I went to my new doctor, I found that they do a lot of things in their office for which I would normally be sent elsewhere. The nurse drew four vials of blood and gave me an EKG.

The EKG showed I had a heart attack sometime in the past. I decided it was in 2010. My friend and I were driving home and I thought I had heartburn. I had terrible pain in my chest.

Could you explain how EKGs show past history? — S.B.

The electrocardiogram looks at the electrical impulses in the heart. It is a critical tool for evaluating rhythm disturbances. It is a useful tool for looking at problems with blood flow to the heart. The EKG can show changes consistent with poor blood flow to a specific area of the heart. These are different from the EKG changes of injury, when the heart cells are dying. Over time, the injured cells die completely and form a scar. The scar may have yet another type of EKG finding. The EKG you had with your new doctor was likely this last one, consistent with a scar.

I say “consistent with” because the EKG is not perfect. Even in the ideal situation, the EKG is not a perfect test for diagnosing old heart attacks.

I can’t say whether your 2010 episode was a heart attack or if it really was heartburn, but there are at least two lessons. The first is that unexpected chest discomfort should prompt an evaluation. The second is that the EKG isn’t perfect, and your doctor may want to do further testing. The more risk factors you have for heart disease, the more likely the EKG was correct. If you are at higher risk, you should be on treatment, including diet, other lifestyle choices and usually medication.

Expert groups recommend against EKGs for people at low risk, as it isn’t clear that the benefits outweigh the harms.