Dr. Roach: How can an EKG show evidence of an old heart attack?
Dear Dr. Roach: When I went to my new doctor, whom I like very much, 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. After thinking about it, 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, but I managed to drop my friend off at her house and get myself home. I could have passed out and killed us both. What a scary thought!
Could you explain how EKGs show past history? I always thought it was a determination of your present condition.
Dear S.B.: The electrocardiogram looks at the electrical impulses in the heart. It is a critical tool for evaluating rhythm disturbances. Despite its limitations, 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 (called ischemia, the cause of angina). These are different from the EKG changes of injury, when the heart cells are dying — which is what happens with a myocardial infarction, or heart attack. 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. So, the EKG can show both your present condition and your past, but it isn’t terribly good at determining how long ago the damage might have occurred.
I say “consistent with” because the EKG is not perfect. In a recent study, about 5% of routine EKGs showed changes that the computer algorithm read as an inferior myocardial infarction — “inferior” being from the lower part of the heart. Comparing the EKG with more sophisticated testing of the heart, it was found that about half of the people actually had had a heart attack, while the other half had not. A skilled cardiologist was better than the computer at determining old heart attack from a false positive test, but even in the ideal situation, the EKG is not a perfect test for diagnosing old heart attack.
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. It’s far better to be reassured everything is OK than take the chance of having a catastrophic event, especially when driving. The second is that the EKG isn’t perfect, and your doctor may want to do further testing to examine whether you are at risk for a heart attack now. 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 to prevent another event.
Expert groups, such as the U.S. Preventive Services Task Force, recommend against EKGs for people at low risk, as it isn’t clear that the benefits outweigh the harms.
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