An electrocardiogram (EKG or ECG) is one of the most common and fastest tests used to evaluate the heart. This non-invasive procedure measures the heart’s electrical activity using electrodes placed on the skin of the chest and limbs. The EKG produces a tracing that shows the rate and rhythm of the heartbeat, providing a snapshot of the heart’s electrical function at that moment. The central question is whether this electrical test can confirm the presence of a physical “blockage,” typically caused by plaque buildup in the coronary arteries. The EKG does not visualize the arteries, but it provides immediate evidence of the consequences of a severe blockage.
What an Electrocardiogram Actually Measures
The EKG’s function is to record the tiny electrical impulses that govern the heart’s contraction and relaxation cycle. These impulses originate in the heart’s natural pacemaker, the sinoatrial node, and spread through the muscle tissue to cause the chambers to pump blood. The resulting tracing displays specific waveforms—P, Q, R, S, and T—that represent the different phases of this electrical journey.
The EKG is essentially a test of the heart’s “wiring,” detecting issues with timing, strength, and rhythm. It measures electrical abnormalities like an irregular heartbeat (arrhythmia) or evidence that the heart muscle is enlarged or strained. However, an EKG cannot provide a direct image of the coronary arteries themselves. It does not detect the physical presence of plaque or narrowing within the blood vessels.
EKG Findings During an Acute Heart Attack
When a coronary artery becomes suddenly and completely blocked, it causes a heart attack (myocardial infarction), which dramatically changes the electrical signals. This complete cutoff of blood supply starves a portion of the heart muscle of oxygen, a condition known as ischemia. This acute injury to the tissue alters its electrical properties, which the EKG then records.
The EKG is a tool for immediate triage in emergency settings because it can identify characteristic patterns of injury. One of the most urgent findings is ST-segment elevation, which is indicative of a complete blockage of a major coronary artery (STEMI). This finding signals that immediate intervention is necessary to save heart muscle.
If the blockage is partial or temporary, the EKG may show other electrical changes, such as ST-segment depression or T-wave inversion. These findings suggest that the heart muscle is experiencing distress and injury, leading to a diagnosis of a non-ST-segment elevation myocardial infarction (NSTEMI) or unstable angina. The EKG detects the damage caused by the sudden lack of blood flow.
When the EKG Cannot Detect a Blockage
A major limitation of the resting EKG is its inability to detect chronic, stable blockages that are not causing active damage to the heart muscle at the time of the test. A person can have significant narrowing of their coronary arteries (coronary artery disease) and still have a completely normal EKG tracing while resting. This happens because the heart receives enough blood flow to function normally when the body is at rest.
Partial blockages often only restrict blood flow when the heart is working harder, such as during physical exertion or emotional stress. This means the resting EKG will appear normal, even if the individual experiences symptoms like chest pain (stable angina) when active. For these chronic conditions, the resting EKG is insufficient to diagnose the underlying physical narrowing of the arteries.
When a person has symptoms but a normal resting EKG, other diagnostic tools are required to confirm a blockage. A stress test involves monitoring the EKG while the patient exercises or receives medication to simulate exertion, which can expose blockages that limit blood flow only under strain. Imaging tests, such as a CT coronary angiogram or a cardiac catheterization, are the only ways to directly visualize the coronary arteries and determine the exact location and severity of the blockage.