An electrocardiogram (EKG or ECG) is a common, non-invasive diagnostic test that measures the electrical activity of the heart. The EKG machine records these signals as a wave pattern, allowing healthcare professionals to assess the heart’s rate and rhythm. An EKG can often provide evidence of a past heart attack, known medically as a myocardial infarction (MI), by detecting permanent changes left behind in the heart muscle.
The EKG’s Primary Role in Heart Monitoring
The EKG monitors the timing and strength of electrical impulses as they travel through the heart’s chambers. The resulting graph displays characteristic wave shapes, traditionally labeled P, QRS, and T, which correspond to specific electrical events. The P wave represents the electrical activation of the heart’s upper chambers, the atria, while the QRS complex shows the depolarization of the lower chambers, the ventricles. The T wave signifies the electrical recovery of the ventricles after contraction. By analyzing these waves, doctors can identify irregularities in the heart’s rhythm or rate. Changes in this electrical flow can signal various cardiac issues, including previous muscle damage.
Identifying Permanent Electrical Changes from a Past Heart Attack
A heart attack occurs when blood flow to a section of the heart muscle is blocked, causing that tissue to die and form a scar. This scar tissue, unlike healthy muscle, is electrically inert and cannot conduct the electrical impulses generated by the heart. The dead tissue forces the electrical signal to detour around the damaged area.
This rerouting of the electrical current is what the EKG records as a permanent alteration in the QRS complex, most notably the appearance of pathological Q waves. A Q wave is the first downward deflection of the QRS complex, and small, narrow Q waves are often normal. However, a pathological Q wave is typically wider and deeper, indicating the presence of non-viable, infarcted tissue.
The presence of these pathological Q waves acts as a persistent electrical footprint of the injury. Because the electrical signal moves away from the dead tissue, the EKG leads positioned over the scarred area record this deep negative deflection. The criteria for a pathological Q wave often include specific duration and depth measurements in connected leads.
Why an EKG Might Not Reveal Prior Damage
Despite its utility, the EKG is not a perfect marker for a past heart attack and has limitations. Studies have shown that an EKG may only correctly identify a previous heart attack in less than half of cases when compared to more advanced imaging. One common reason for a negative EKG result is a “silent” heart attack, where the event was too small or occurred without noticeable symptoms, leading to minimal scarring.
Furthermore, not all heart attacks cause the characteristic pathological Q wave; these are often referred to as non-Q-wave MIs. This type of injury involves damage that does not extend through the full thickness of the heart wall, meaning enough healthy muscle remains to conduct the electrical impulse normally. The location of the damage also plays a role, as scars on certain parts of the heart are less likely to be detected by standard EKG lead placement. The age of the heart attack can also influence EKG readability, as some patients experience a diminution of the Q-wave area over time.
Confirmatory Tests Beyond the EKG
Because the EKG has low sensitivity for detecting prior damage, it is often used as a screening tool, followed by more sophisticated tests for confirmation.
Echocardiogram
An echocardiogram, or “echo,” uses high-frequency sound waves to create moving images of the heart’s structure and function. This test can visually identify areas of the heart muscle that have abnormal wall motion, which is a sign of scar tissue from a past heart attack.
Cardiac MRI
Cardiac magnetic resonance imaging (MRI) is considered a highly accurate method for visualizing heart damage. The cardiac MRI uses magnets and radio waves to produce detailed pictures and, with the use of contrast agents, can clearly distinguish between healthy muscle and scarred tissue. This ability to directly visualize the scar makes it a powerful tool for confirming a prior MI.
Stress Tests
Stress tests assess the heart’s response to increased demand, such as exercise. Stress echocardiography or nuclear stress tests can reveal areas of the heart that receive poor blood flow under exertion, indicating blockages or existing damage that may not be apparent at rest. These advanced imaging techniques provide structural and functional evidence that complements the electrical information provided by the EKG.