An electrocardiogram (EKG) can provide evidence of a past heart attack by detecting abnormal electrical patterns in the heart. These patterns suggest that a portion of the heart muscle may have been damaged due to a lack of oxygen. While a valuable diagnostic tool, an EKG is not always definitive for confirming a past heart attack. It serves as an initial assessment, guiding further investigation into cardiac health.
The EKG’s Role in Detecting Past Heart Attacks
An EKG measures the electrical activity of the heart, and changes can indicate prior damage. The most characteristic EKG sign of a past heart attack, also known as a myocardial infarction, is the presence of pathological Q waves. These waves signify electrically inactive heart tissue, occurring when heart muscle dies due to prolonged lack of blood flow. Pathological Q waves are typically wider and deeper than normal Q waves, often defined as being 0.03 seconds or more in width and at least 1 mm in depth.
Other EKG changes, though less specific, can also suggest a past heart attack, including persistent ST-segment abnormalities or T-wave inversions. Persistent ST-segment elevation can sometimes point to a ventricular aneurysm, a complication of a large past heart attack. T-wave inversions can persist after an infarction, though other conditions may also cause them. The location of these EKG changes on a 12-lead EKG helps pinpoint the previously affected heart region.
Limitations of EKG for Past Heart Attacks
An EKG has limitations in definitively detecting all past heart attacks. Small or non-transmural heart attacks, where damage does not extend through the full thickness of the heart muscle, may not always produce clear EKG changes. The electrical alterations might be too subtle or localized to be consistently captured. Additionally, some “electrically silent” areas of the heart may not manifest damage prominently on an EKG.
The timing since the heart attack also plays a role. While pathological Q waves generally persist, other EKG changes can normalize over time, making older events harder to identify. Various other cardiac conditions, such as left bundle branch block or pericarditis, can also mimic EKG patterns seen after a heart attack, potentially leading to misinterpretation.
Confirmatory Tests for Heart Attack History
Given EKG limitations, other diagnostic tools are often used to confirm a heart attack history. An echocardiogram uses sound waves to create heart images, assessing function and identifying damaged or scarred heart muscle. This non-invasive test provides visual evidence of structural changes.
Cardiac Magnetic Resonance Imaging (MRI) offers highly detailed heart images, effective at identifying scarring or damage from past heart attacks. MRI can differentiate between acute and chronic myocardial infarction and quantify damaged tissue. Elevated cardiac troponins, proteins released when heart muscle is damaged, can indicate a recent heart attack. However, troponin levels typically return to normal within weeks, so they are not a direct test for distant past events. These advanced imaging techniques provide a clearer picture when an EKG is inconclusive.