An Electrocardiogram (EKG or ECG) is a simple, non-invasive test that records the heart’s electrical activity using electrodes placed on the skin. A heart attack, medically termed a Myocardial Infarction (MI), involves irreversible damage and death of heart muscle tissue due to a lack of blood flow. An EKG can detect a heart attack long after the event has occurred. While acute electrical signs fade quickly, the physical scarring creates a permanent electrical change visible on the tracing.
The Electrical Signature of Past Damage
Permanent damage to the heart muscle alters the normal pathway of electrical conduction, creating a distinct and lasting “electrical signature” that the EKG records. This signature is primarily characterized by the presence of pathological Q waves, which are the definitive markers of prior myocardial cell death. The dead muscle tissue is replaced by a fibrous scar that is electrically inert, meaning it cannot conduct the electrical signal.
The electrically silent scar tissue acts as a “hole,” causing the electrical signal of the surrounding healthy muscle to move away from the EKG lead. The EKG records this movement as a significant negative deflection, known as the pathological Q wave. These waves are distinguished from normal, small Q waves by their size, typically being \(\geq 0.04\) seconds in duration and \(\geq 25\%\) of the height of the subsequent R wave. The location of these permanent Q waves also allows clinicians to determine the anatomical area of the heart that was damaged.
Distinguishing Recent Events from Old Injuries
Healthcare providers differentiate between a recent, acute event and an old injury by observing the dynamic nature of the EKG’s changes over time. In the immediate aftermath of a heart attack, the EKG shows transient changes that reflect ongoing injury and ischemia. The earliest signs of acute injury often include ST-segment elevation, where the tracing rises above the baseline, sometimes preceded by tall, symmetrical hyperacute T waves.
As the event evolves over hours to days, the ST-segment elevation typically resolves, and the T-waves begin to invert, reflecting the phase of muscle ischemia. Only after this acute phase, often days or weeks later, do the permanent pathological Q waves fully develop, marking the completion of the myocardial scarring process. The presence of stable, deep Q waves without accompanying acute ST-segment or dynamic T-wave changes points toward an infarction that happened in the past. Comparing a current EKG to previous tracings is an effective method for determining the age of the damage.
When the EKG Requires Confirmation
The EKG is a powerful tool for detecting past heart attacks, but it has limitations. Not all myocardial infarctions produce lasting Q waves; smaller areas of damage may be “electrically silent.” Furthermore, certain pre-existing heart conditions, such as severe left ventricular hypertrophy or a bundle branch block, can produce EKG patterns that mimic a prior MI.
For this reason, a complete diagnosis often relies on a combination of electrical and physical evidence. Cardiac biomarkers, such as Troponin, are measured in the blood to confirm a recent event because these proteins are released when heart muscle cells die. Since these biomarkers clear from the bloodstream within days or weeks, they cannot be used to confirm an old injury.
Imaging modalities provide physical visualization of the damage, complementing the EKG’s electrical data. An Echocardiogram uses sound waves to assess heart function and can reveal wall-motion abnormalities. Cardiac Magnetic Resonance Imaging (cMRI) is considered the gold standard for quantifying scar tissue, as it uses a contrast agent to highlight non-viable muscle, providing anatomical proof of a past infarction.