An Electrocardiogram (ECG or EKG) is a simple, non-invasive diagnostic tool that records the electrical activity of the heart. This test translates the heart’s complex electrical impulses into a wave-like tracing. Each wave and segment corresponds to a specific phase of the heart’s cycle. The ST segment is a particularly important section that medical professionals closely examine for signs of cardiac health or distress.
What the ST Segment Represents on the ECG
The ST segment is the relatively flat line on the ECG tracing that appears immediately after the QRS complex and just before the T wave. The QRS complex represents the electrical signal that causes the ventricles to contract, while the T wave signifies their electrical recovery. The ST segment marks the brief period between the end of ventricular contraction and the beginning of repolarization.
The exact point where the QRS complex ends and the ST segment begins is known as the J-point, which is the location from which any deviation is measured. Physiologically, the ST segment represents the plateau phase of the ventricular action potential, meaning the heart muscle cells are completely depolarized and are holding a steady electrical charge. Because all the ventricular cells are electrically stable, the ST segment typically appears isoelectric, or flat, along the baseline. A deviation from this flat baseline indicates a disruption in the normal electrical stability of the heart muscle.
ST Elevation and Acute Myocardial Infarction
ST elevation, a significant upward deviation of the ST segment, indicates a severe and time-sensitive medical event. This finding is the hallmark of an ST-Elevation Myocardial Infarction (STEMI), the most dangerous form of heart attack. The elevation occurs because a coronary artery, which supplies oxygen-rich blood to the heart muscle, has become completely or nearly completely blocked, usually by a blood clot. This blockage causes transmural ischemia, meaning the lack of blood flow affects the entire thickness of the heart muscle wall.
Injured heart muscle cells cannot maintain their normal electrical potential during the plateau phase, creating an “injury current” picked up by the ECG electrodes. This current causes the ST segment to shift upward from the baseline in the leads facing the damaged area. Significant ST elevation is defined as a measurement of at least one millimeter above the baseline in two or more contiguous leads. In chest leads V2 and V3, thresholds are often higher and sex-specific (two millimeters for men over 40 or 1.5 millimeters for women). Recognizing this pattern demands immediate medical intervention, such as catheterization, because heart muscle tissue is dying rapidly.
Understanding ST Depression and Other Abnormalities
ST depression occurs when the ST segment drops below the isoelectric baseline, signaling a problem with the heart’s blood supply. This downward shift is often associated with subendocardial ischemia, where the blood flow restriction is partial and affects only the inner layers of the heart muscle wall. This condition is frequently seen in non-ST-Elevation Myocardial Infarction (NSTEMI) or unstable angina, which require urgent treatment. A horizontal or downsloping ST depression of 0.5 millimeters or more at the J-point in two or more contiguous leads is considered a concerning sign of ischemia.
ST depression can also be caused by factors unrelated to a blood flow blockage, complicating its interpretation. For example, certain heart medications, such as digitalis, can cause a characteristic scooping or downward sloping of the ST segment. Very fast heart rates, known as tachycardias, can also temporarily cause ST depression due to the heart muscle needing more oxygen than the blood flow can supply. Non-ischemic conditions like pericarditis (inflammation of the sac surrounding the heart) often result in widespread ST elevation. Furthermore, a common, benign variant known as early repolarization can mimic mild ST elevation in young, healthy individuals, where the J-point is elevated but the rest of the segment is normal.