An Electrocardiogram (EKG or ECG) is a quick, non-invasive diagnostic test that records the heart’s electrical activity using electrodes placed on the skin. This simple test is often the first step when a health concern like chest pain or shortness of breath suggests a heart problem. The EKG provides an immediate picture of the heart’s rhythm and electrical timing, allowing medical professionals to assess its current function. While the EKG cannot visualize the inside of an artery, it is highly effective at detecting the consequences of a severe blockage, such as the lack of oxygen supply that causes heart muscle injury.
What an EKG Measures
The heart generates electrical impulses that coordinate the contraction of its muscle chambers. The EKG translates these impulses into a characteristic wave pattern composed of distinct waves and segments representing different phases of the heartbeat cycle. The P wave represents the electrical activation of the atria, the heart’s upper chambers, as they contract.
Following the P wave is the QRS complex, a larger spike that signifies the rapid electrical activation of the ventricles, the powerful lower pumping chambers. The T wave reflects the electrical recovery, or repolarization, of the ventricles as they relax. Between the QRS complex and the T wave lies the ST segment, which normally appears as a flat line and represents the period when the ventricles are contracted but electrically stable.
The Critical Distinction: Damage Versus Blockage
A “blockage” refers to an anatomical narrowing of a coronary artery, often caused by plaque buildup in Coronary Artery Disease (CAD). The EKG is not an imaging device; it cannot visualize the physical artery or the plaque itself. Instead, the EKG tests the heart’s electrical function.
When a blockage becomes severe, it restricts blood flow, causing a shortage of oxygen in the heart muscle tissue. This oxygen deprivation, called ischemia, immediately alters the electrical stability of the affected muscle cells. The EKG detects this electrical disturbance, which is the functional consequence of the blockage. Therefore, the EKG detects injury to the heart muscle, serving as an indirect sign of a severe underlying blockage.
Interpreting EKG Signals of Ischemia and Injury
Changes in the EKG tracing result directly from the heart muscle’s electrical instability when starved of oxygen. When blood flow is reduced but the damage is reversible (ischemia), the EKG may show a flattened or inverted T wave, indicating abnormal ventricular repolarization. Another sign of ischemia is ST segment depression, where the ST segment drops below the baseline.
If the blockage is complete and causes acute, irreversible damage—a heart attack or myocardial infarction—the EKG shows a more dramatic change. The most recognizable sign of an acute heart attack is ST segment elevation, where the ST segment is lifted above the baseline. This finding, often called a STEMI, indicates a complete obstruction and is a medical emergency. In cases of older damage, the EKG may also show pathological Q waves, which are permanent electrical scars marking past heart muscle death.
Comprehensive Diagnosis Beyond the EKG
While a finding like ST segment elevation is highly suggestive of a severe blockage, a normal EKG does not completely rule out Coronary Artery Disease. Blockages can exist without causing EKG changes, especially if the patient is resting or if the obstruction is not yet severe enough to cause a large area of damage. This is why the EKG is often one component of a broader diagnostic process to definitively assess for coronary blockages.
Blood Tests
Blood tests measuring cardiac biomarkers, such as troponin, are used to detect proteins released into the bloodstream when heart muscle cells are damaged.
Stress Testing
If the EKG is normal but a blockage is still suspected, a stress test may be performed to monitor the EKG while the heart is working harder. This can reveal blockages that only become problematic under physical strain.
Coronary Angiography
The definitive anatomical assessment, considered the gold standard for visualizing blockages, is cardiac catheterization, or coronary angiography. This invasive procedure involves injecting a contrast dye directly into the arteries to produce real-time X-ray images, allowing medical professionals to see the exact location and severity of any arterial narrowing.