An electrocardiogram (EKG or ECG) is a diagnostic tool that records the electrical activity of the heart. This non-invasive test uses small electrode patches on the chest, arms, and legs to detect the heart’s electrical signals. These signals are then translated into wavy lines on a monitor or paper, visually representing heart function. The EKG plays a valuable role in assessing overall heart health by showing how fast the heart is beating, its rhythm, and the timing of electrical impulses as they move through the heart.
Interpreting a Normal EKG
A typical EKG tracing displays a characteristic pattern of waves, each corresponding to a specific electrical event within the heart. The first small upward bump is the P wave, which signifies the electrical impulse spreading through the atria, causing them to contract.
Following the P wave, a larger, more complex deflection known as the QRS complex appears. This complex represents the rapid electrical activation of the ventricles. The QRS complex is usually the most prominent feature on an EKG. Finally, the T wave is a rounded wave that indicates the electrical recovery of the ventricles, preparing them for the next heartbeat. This sequence of waves reflects a healthy and coordinated electrical rhythm.
What an EKG Shows During a Heart Attack
During a heart attack, the EKG can show changes reflecting heart muscle damage from lack of blood flow. ST-segment elevation, where the segment between the S wave and the T wave is raised, is a key indicator. This elevation often signals a complete blockage of a coronary artery, a condition known as an ST-Elevation Myocardial Infarction (STEMI), where heart muscle is actively dying.
Another change is ST-segment depression, where this segment dips below the baseline. ST-segment depression typically indicates myocardial ischemia (insufficient blood flow), often due to a partial blockage, and can be seen in non-ST-Elevation Myocardial Infarction (NSTEMI). T-wave inversions, where the T wave appears upside down, can also occur during a heart attack and are another sign of ischemia. These changes arise because the affected heart muscle cells are injured and cannot conduct electrical signals normally.
What an EKG Shows After a Heart Attack
After a heart attack has occurred, the EKG can reveal lasting changes that indicate previous damage to the heart muscle. The most characteristic of these changes is the development of “pathological Q waves”. These Q waves are wider and deeper than normal Q waves. Pathological Q waves emerge because the area of heart muscle that was deprived of blood flow and died no longer conducts electrical activity.
Scar tissue replaces dead heart muscle, which is electrically inactive, creating an “electrical hole” the EKG registers as a pathological Q wave. While pathological Q waves are a strong sign of a prior myocardial infarction, they are not an early sign and generally take several hours to days to develop. Once present, these Q waves rarely disappear, serving as a permanent electrical scar of the past event, though in some cases, with early reperfusion, they may resolve. While ST-segment elevations usually resolve and return to baseline after the acute event, T-wave inversions may persist for a period or even permanently, indicating myocardial damage or ongoing ischemia.
What Happens After an EKG Reveals Changes
When an EKG shows signs of a heart attack, it serves as an important piece of information that guides further medical evaluation. The EKG is often used with other diagnostic tests for a comprehensive picture of heart health. Blood tests, particularly those measuring cardiac enzymes like troponin, are frequently performed, as these proteins are released into the bloodstream when heart muscle is damaged. Troponin levels can remain elevated for up to two weeks after a heart attack.
An echocardiogram, using sound waves to image heart structure and function, can assess damage and pumping ability. In some cases, a coronary angiogram may be performed, involving a thin tube to visualize coronary arteries and identify blockages. These diagnostic steps help healthcare providers confirm a heart attack diagnosis, determine the severity of damage, and formulate a treatment plan, including medications, angioplasty, or monitoring.