An electrocardiogram, commonly known as an EKG or ECG, is a non-invasive test that records the electrical signals of your heart. It involves placing small electrodes on your chest and sometimes your limbs, which detect the electrical activity generated by your heart as it beats. These electrical impulses are translated into wave patterns, providing insights into your heart’s rate, rhythm, and the timing of its electrical signals. This test helps healthcare professionals evaluate heart health and diagnose various heart conditions.
How an EKG Can Indicate a Past Heart Attack
An EKG can reveal signs of a previous heart attack by detecting disruptions in the heart’s electrical patterns caused by damaged tissue. One definitive sign is the presence of pathological Q waves. These are wider than 0.04 seconds or deeper than 2 millimeters, and appear in specific leads.
Pathological Q waves develop because heart muscle that died during a heart attack becomes electrically silent. The EKG records electrical activity from the surrounding healthy tissue, creating these abnormal deflections. Q waves persist indefinitely once formed.
Persistent T-wave inversions in certain leads also suggest an old heart attack, though they are less specific than Q waves. T-wave inversions indicate altered ventricular repolarization, the process of the heart’s lower chambers electrically resetting. While acute ST-segment elevation characterizes a current heart attack, some chronic ST-segment changes, particularly persistent elevation with deep Q waves, can indicate an old myocardial infarction.
When an EKG Might Not Show a Past Heart Attack
Despite its utility, an EKG has limitations in detecting all previous heart attacks. Small heart attacks or those that do not involve the full thickness of the heart muscle may not leave permanent, detectable EKG changes. Electrical patterns can normalize over time, making it difficult to diagnose a past event solely based on EKG findings.
Some heart attacks are “silent,” meaning they occur without noticeable symptoms, and these may not always produce clear or lasting EKG evidence. The standard 12-lead EKG may not detect damage in certain areas of the heart due to electrode placement. Other heart conditions can also cause EKG changes that mimic those of a past heart attack, potentially leading to false positives. An EKG is often used as one tool among several for a comprehensive diagnosis.
Other Methods for Diagnosing a Previous Heart Attack
When an EKG is inconclusive or additional information is needed, healthcare professionals employ other diagnostic tools to assess a previous heart attack. An echocardiogram uses sound waves to create real-time images of the heart’s structure and function. This test can visualize areas of weakened or scarred heart muscle resulting from a past heart attack, as well as assess chamber size, wall thickness, and blood flow.
Cardiac MRI (Magnetic Resonance Imaging) offers highly detailed images of the heart muscle, making it effective at detecting scar tissue from old infarcts. It can identify small areas of dead heart tissue that other imaging techniques may miss. During a cardiac MRI, a contrast medium can be injected to highlight scarred areas, providing information on the extent and thickness of the scarring.
Blood tests for cardiac biomarkers, such as troponin, are primarily used to diagnose an acute heart attack. Troponin levels rise within a few hours of heart damage and can remain elevated for up to two weeks, but they return to normal after this period. While they indicate recent heart muscle injury, they are not used for diagnosing heart attacks that occurred in the distant past.
Stress testing, which can include exercise EKGs, stress echocardiograms, or nuclear stress tests, helps identify areas of the heart that do not receive sufficient blood flow during exertion. These tests can reveal underlying coronary artery disease that may have led to a past heart attack by showing how the heart responds under stress. A nuclear stress test, for instance, uses a radioactive tracer to show blood flow to the heart at rest and during exercise, highlighting areas of poor blood flow or damage.