The Electrocardiogram (ECG) is a non-invasive diagnostic tool that records the electrical activity generated by the heart. This graphical representation allows medical professionals to assess the heart’s rhythm and overall function. Myocardial ischemia is a condition where blood flow to the heart muscle is restricted, causing an insufficient supply of oxygen. When heart muscle cells lack oxygen, their normal electrical function is compromised, and the ECG captures these significant changes, making it a rapid diagnostic method.
Electrical Disruption in Ischemic Tissue
A lack of oxygen causes heart muscle cells to become electrically unstable, altering how they transmit signals. Oxygen deprivation affects the movement of ions across cell membranes, leading to a change in the resting electrical potential of the affected tissue. This difference in electrical charge between normal and oxygen-starved tissue generates an “injury current.” The ECG detects this current as a deviation from the normal electrical baseline. This injury current shifts the baseline of the tracing, causing the ST segment—the segment between the QRS complex and the T wave—to appear elevated or depressed. This ST segment shift is the direct electrophysiological manifestation of acute oxygen deprivation.
Indicators of Acute Myocardial Injury
The most immediate signs of active ischemia on an ECG involve changes to the ST segment, which represents the phase where the ventricles are fully depolarized. Depending on the depth of the muscle wall affected, the ST segment will either elevate or depress.
ST-Segment Elevation
ST-segment elevation indicates a severe, complete blockage of a coronary artery, known as transmural ischemia. This injury spans the entire thickness of the heart wall. The total lack of blood flow causes a large injury current that pulls the ST segment upward above the normal baseline. Detecting ST elevation is a time-sensitive finding, as it signifies a major heart attack requiring immediate intervention to restore blood flow.
ST-Segment Depression
ST-segment depression indicates that the blood flow restriction is partial, affecting only the inner layer of the heart muscle (subendocardium). The subendocardium is the most vulnerable to oxygen shortage. The resulting injury current is directed differently than in a full blockage, causing the ST segment to drop below the baseline. ST depression signals an acute cardiac event requiring urgent medical attention. Ischemic ST depression is characterized by a horizontal or downsloping appearance, which helps distinguish it from non-ischemic causes.
Markers of Resolved Ischemia or Infarction
As the acute phase of an ischemic event passes, the ECG tracing evolves, reflecting either muscle recovery or permanent damage.
T-Wave Inversion
T-wave inversion often appears as ischemia is resolving or in a subacute phase. The T wave represents the electrical repolarization (relaxation) of the heart muscle, and ischemia disrupts this process. When the T wave points downward, it suggests the sequence of repolarization has been altered. T-wave inversion is less specific for an acute, full-thickness blockage than ST-segment changes.
Pathological Q Waves
A more permanent indicator of past damage is the development of a pathological Q wave. A Q wave is the first downward deflection of the QRS complex, representing the initial phase of electrical activation. Pathological Q waves signify an area of the heart muscle that has died and turned into electrically inactive scar tissue (myocardial infarction). These non-functioning zones no longer generate a normal electrical signal, causing the ECG to record a deep and wide Q wave. A Q wave is considered pathological if it is wider than 0.04 seconds or deeper than 25% of the following R wave’s height. These permanent Q waves serve as an electrical “footprint” of a previous heart attack.
When the ECG Does Not Show Ischemia
Despite its utility, the ECG is not infallible, and its findings must be interpreted alongside a patient’s symptoms and blood tests. Some individuals experience silent ischemia, where the heart muscle is deprived of oxygen but they feel no noticeable symptoms. In these cases, ischemic changes on the ECG may be transient or subtle.
Pre-existing heart conditions can also obscure or mimic the signs of ischemia, leading to a non-diagnostic ECG. For example, a left bundle branch block alters the heart’s electrical pathways, creating ST-segment changes similar to those caused by a heart attack. Patients with implanted pacemakers or ventricular hypertrophy may also have abnormal baseline ECGs, making it difficult to identify new ischemic events.