The ST segment is a component of an electrocardiogram (ECG) that provides insight into the heart’s electrical activity. Its appearance on an ECG tracing is important for assessing heart health, as changes can indicate various cardiac conditions. Accurate interpretation of the ST segment is fundamental for cardiovascular diagnostics.
Understanding the Electrocardiogram
An electrocardiogram (ECG or EKG) is a non-invasive test that measures the electrical signals of the heart. These signals represent the heart’s rhythmic contractions and relaxations, which are necessary for pumping blood effectively throughout the body. An ECG records these electrical impulses as a waveform with distinct peaks and valleys.
The typical heartbeat waveform on an ECG is composed of several characteristic waves, each corresponding to a specific electrical event in the heart. The P wave signifies the electrical activation of the atria, the heart’s upper chambers, as they contract. Following the P wave, the QRS complex represents the rapid electrical activation of the ventricles, the heart’s lower pumping chambers. The T wave then indicates the electrical recovery, or repolarization, of the ventricles. The ST segment is located between the end of the QRS complex and the beginning of the T wave, representing a period where the ventricles are fully activated but have not yet begun to recover electrically.
The Normal ST Segment
A normal ST segment on an ECG tracing appears as a flat, isoelectric line. It lies at or very close to the ECG baseline, often established by the PR segment (the flat line between the P wave and QRS complex). The “J point” marks the junction where the QRS complex ends and the ST segment begins.
Physiologically, the normal appearance of the ST segment signifies the period between ventricular depolarization (contraction) and repolarization (relaxation). During this phase, the heart muscle cells in the ventricles are uniformly activated and are in a plateau phase, meaning there are minimal changes in electrical voltage across the heart. This near absence of electrical potential differences is why the normal ST segment is observed as a flat line on the ECG. A smooth transition from the ST segment into the T wave is also a characteristic of a normal tracing.
Abnormal ST Segment Findings
Deviations from the normal flat ST segment can indicate underlying heart issues. The two primary types of abnormal ST segment findings are ST elevation and ST depression. These terms describe whether the ST segment appears above or below the established baseline.
ST elevation occurs when the ST segment is displaced upwards from the baseline. Visually, this means the line after the QRS complex rises noticeably above the baseline. The J point, where the QRS complex meets the ST segment, is also elevated in these cases. This upward shift can vary in magnitude and morphology, sometimes appearing convex or concave.
Conversely, ST depression is observed when the ST segment falls below the baseline. On an ECG, this appears as a noticeable downward shift of the line after the QRS complex, dipping below the baseline. Similar to elevation, ST depression can present with different shapes, such as horizontal, downsloping, or upsloping. The extent of the depression is measured from the J point relative to the baseline.
Clinical Implications of ST Segment Deviations
Changes in the ST segment are indicators of various cardiac conditions, particularly those involving reduced blood flow to the heart muscle. The most significant cause of ST segment abnormality is myocardial ischemia or infarction. However, these changes are not always indicative of disease, as some normal variations can also present with mild ST segment deviations.
ST elevation is a sign of an acute myocardial infarction (heart attack). This type, an ST-elevation myocardial infarction (STEMI), results from a complete blockage of a coronary artery, leading to severe heart muscle damage. The elevated ST segment indicates that the entire thickness of the heart muscle wall is affected by the lack of blood flow, a condition known as transmural ischemia. Prompt recognition of ST elevation is important because immediate medical intervention, such as restoring blood flow, can minimize damage and improve outcomes. Other causes of ST elevation include pericarditis (inflammation of the heart sac) or benign early repolarization, a normal variant.
ST depression indicates myocardial ischemia, a condition where the heart muscle receives insufficient blood flow and oxygen, often without a complete blockage. This can result from a partial blockage of a coronary artery or an imbalance between the heart’s oxygen supply and demand. The appearance of ST depression, particularly if horizontal or downsloping, is considered more specific for myocardial ischemia. ST depression can also be a reciprocal change, appearing in leads opposite to an area of ST elevation in a heart attack.
Other causes include hypokalemia (low potassium levels), certain medications like digitalis, or left bundle branch block. The presence of ST segment deviations on an ECG requires professional medical assessment to determine the underlying cause and appropriate course of action.