An Electrocardiogram (EKG) is a swift, non-invasive procedure that records the electrical activity of the heart muscle. Electrodes placed on the skin translate the heart’s electrical impulses into a distinctive tracing of wavy lines. A heart attack, medically known as a Myocardial Infarction (MI), is damage to the heart muscle that occurs when blood flow is blocked, leading to tissue death. Since the heart’s electrical system is intimately connected to the muscle’s health, the EKG is a primary diagnostic tool used to detect both ongoing and past heart damage from an MI.
What a Normal EKG Looks Like
A healthy EKG tracing follows a consistent, rhythmic pattern corresponding to the heart’s cycle of contraction and relaxation. The P wave represents the electrical activation of the heart’s upper chambers (atria). The QRS complex follows, signaling the electrical activation of the lower, muscular ventricles, which are responsible for pumping blood.
The ventricles then electrically reset in a process called repolarization, represented by the T wave. The ST segment is the relatively flat line connecting the end of the QRS complex to the beginning of the T wave. This segment normally sits at the baseline, indicating a period where the ventricles are contracting but no net electrical current is flowing. Disruption to the blood supply or muscle health immediately alters the shape, height, and timing of these waves.
Immediate EKG Signs of an Acute Heart Attack
When a coronary artery becomes completely blocked, cutting off blood supply to the heart muscle, time-sensitive changes appear on the EKG. The earliest signs include hyperacute T waves, which are noticeably taller, wider, and more rounded than normal. These can develop within minutes of the blockage, signaling severe muscle distress.
The most recognized sign of an acute, total blockage is ST-segment elevation. This occurs when the ST segment rises visibly above the baseline, indicating transmural ischemia—damage extending through the full thickness of the heart muscle wall. This finding defines an ST-Elevation Myocardial Infarction (STEMI), which requires immediate intervention to restore blood flow.
Alternatively, an acute event may show ST-segment depression, where the segment dips below the baseline. This change is often associated with a Non-ST-Elevation Myocardial Infarction (NSTEMI) or severe ischemia, indicating partial blood flow restriction. Both ST elevation and depression are urgent findings that reflect ongoing muscle injury and require immediate treatment.
How the EKG Reveals Past Heart Damage
The EKG provides lasting evidence of a heart attack long after the acute event has passed through scarring. When heart muscle tissue dies, it becomes electrically silent and no longer conducts the normal electrical impulse. This fundamentally changes how the EKG records ventricular activation.
The lasting evidence of this scar is the development of a pathological Q wave, which is typically wider than 0.03 seconds and deeper than 25% of the height of the subsequent R wave. This deep deflection occurs because the EKG “looks through” the dead tissue to the electrical activity on the opposite side of the heart. Pathological Q waves usually take hours to days to fully form, but once established, they often persist indefinitely, marking the territory of the previous injury.
Another sign of damage is the evolution of the T wave into a symmetric, deep T wave inversion. While T wave inversions can occur during acute ischemia, their persistence suggests the heart muscle is healing with scar tissue. The combination of pathological Q waves and persistent T wave inversions offers strong EKG evidence of a prior MI.
When EKG Findings Are Not Clear Cut
Although the EKG is a diagnostic tool, its findings are not always straightforward and require correlation with other clinical data. A significant number of heart attacks present without the characteristic ST-segment elevation, falling into the NSTEMI category. In these cases, the EKG may only show subtle ST depression, T wave inversion, or even appear normal initially.
Pre-existing heart conditions can also obscure or mimic the signs of an acute heart attack, making EKG interpretation challenging. A Left Bundle Branch Block (LBBB), for example, causes a wide QRS complex and secondary ST-T wave changes that can hide or confuse a STEMI diagnosis. In ambiguous situations, the EKG serves as one piece of a larger puzzle. A definitive diagnosis relies on combining EKG results with the patient’s symptoms and blood tests, particularly the level of cardiac troponin.