An electrocardiogram (EKG) is a straightforward, non-invasive test that records the heart’s electrical activity. By placing electrodes on the skin, the EKG captures the electrical signals that regulate the heart’s rhythm and pumping action. While frequently used to diagnose current heart issues, the EKG also serves as a historical record of cardiac health. A past heart attack leaves a permanent electrical signature on the heart muscle, which can sometimes be identified years later, even if the individual was unaware of the event.
How Scar Tissue Affects Electrical Signals
A heart attack (myocardial infarction) occurs when blood flow to heart muscle is severely reduced or blocked, causing cells to die. This dead tissue, called an infarct, does not regenerate into healthy muscle. Instead, it is gradually replaced by dense, non-contracting scar tissue in a process called fibrosis.
This fibrous scar tissue is electrically inert, meaning it cannot conduct the electrical impulses needed for the heart to beat. The scar creates an “electrical hole” or dead zone in the heart wall. When the electrical signal travels through the chambers, it must detour around this non-conducting tissue.
This rerouting of the electrical current permanently alters the EKG tracing. The primary, long-term sign of a past heart attack is the presence of a pathological Q wave. A Q wave is normally the first downward deflection of the QRS complex, representing the electrical activation of the ventricles.
A pathological Q wave is abnormally deep and wide, typically lasting longer than 0.04 seconds or being more than 25% of the following R wave’s height. This pattern indicates the electrical signal is moving away from the lead because the underlying tissue is electrically silent. The location of these abnormal Q waves on the 12-lead EKG corresponds to the damaged area, localizing the previous infarction.
Differentiating Recent vs. Past Heart Events
The EKG distinguishes a recent (acute) heart attack from a chronic, past event by recognizing transient and permanent markers. An acute heart attack is characterized by dynamic changes evolving over hours to days. These temporary signs include ST-segment elevation or depression and T-wave inversions, reflecting immediate injury and lack of oxygen to the heart muscle.
ST-segment elevation, where the segment between the S and T waves rises above the baseline, indicates an ongoing, full-thickness heart attack. T-wave inversions, where the wave flips upside down, often follow ST changes as the heart muscle suffers from ischemia. These acute markers generally resolve as the heart attack stabilizes, either disappearing or stabilizing into chronic patterns.
The pathological Q wave, however, represents the permanent consequence of the event, signifying that a portion of the heart muscle has died and scarred. While the acute ST and T-wave changes fade over days or weeks, the Q wave persists indefinitely, serving as a lasting record of myocardial tissue death. Therefore, a finding of pathological Q waves without the accompanying acute ST-segment changes suggests that the heart attack occurred in the past, allowing clinicians to determine the timing of the injury.
When the EKG Cannot Tell the Full Story
While the pathological Q wave is a reliable marker, the EKG is not a perfect diagnostic tool and has limitations. The test may fail to show evidence of a past event if the damage was very small (a microinfarction). If the scarred tissue is minimal, it may not be large enough to significantly alter electrical conduction, resulting in an “electrically silent” infarct.
Some heart attacks occur in areas difficult for the standard 12-lead EKG to monitor effectively, such as the posterior wall of the left ventricle. In these cases, electrical changes are masked or require specialized lead placement to be visible. Pre-existing heart conditions can also complicate the EKG reading.
Conditions like Left Bundle Branch Block (LBBB) or certain types of left ventricular hypertrophy can produce patterns that mimic the signs of a previous heart attack, potentially leading to a false-positive reading.
Alternative Diagnostic Tools
When the EKG is inconclusive or negative despite suspicion of past damage, additional diagnostic tools are necessary. For instance, cardiac enzymes like troponin confirm a recent event, as they elevate in the blood for several weeks after the injury.
An echocardiogram visualizes the heart’s physical structure and movement, revealing areas of the wall that are thinned or moving abnormally due to scar tissue. Cardiac magnetic resonance imaging (MRI) is considered the most definitive technique, using contrast to directly visualize and quantify the extent of the scar tissue. While the EKG is a valuable initial screening test, confirmation from these advanced imaging and laboratory tests is often required for a complete picture of a patient’s cardiac history.