ST Segment

ST Segment represents the flat, isoelectric section of the electrocardiogram (ECG) tracing that begins at the end of the QRS complex and extends to the start of the T wave. It corresponds to the period between ventricular depolarization and repolarization, during which the ventricles remain in a state of electrical neutrality. This segment reflects the phase of early ventricular repolarization when the ventricular myocardium is uniformly depolarized.

Definition and Location

The ST Segment starts at the J point, which is the junction between the end of the QRS complex and the beginning of the ST segment. It ends at the onset of the T wave. On the ECG waveform, it is typically a flat, horizontal line at the baseline level, representing no net electrical activity because the ventricular muscle cells are in a plateau phase of their action potentials.

Electrophysiological Basis

During the ST Segment, ventricular myocytes are in phase 2 of their action potentials, also known as the plateau phase. This phase is characterized by a balance between inward calcium (Ca²⁺) currents and outward potassium (K⁺) currents, maintaining a stable membrane potential. This electrical stability results in no net change in voltage, which manifests as an isoelectric line on the ECG.

Clinical Significance

Normal ST Segment

A normal ST Segment is usually level with the baseline (the PR segment level), indicating normal ventricular repolarization. Slight variations in elevation or depression (usually less than 1 mm) can be normal depending on the lead and patient characteristics.

Abnormal ST Segment Changes

• ST Segment Elevation: Elevation of the ST Segment above the baseline indicates myocardial injury or acute ischemia, commonly seen in conditions such as acute myocardial infarction (heart attack). It reflects current of injury caused by damaged myocardial cells altering the electrical potential of the ventricular wall.

• ST Segment Depression: Depression below the baseline suggests subendocardial ischemia, electrolyte imbalances, or digitalis effect. It indicates delayed or abnormal repolarization in the ventricular myocardium.

• ST Segment Morphology Changes: Abnormal shapes or slopes of the ST Segment, such as upsloping, downsloping, or horizontal depression, provide diagnostic clues to various cardiac pathologies.

Measurement and Interpretation

The ST Segment is assessed by its elevation or depression in millimeters relative to the isoelectric baseline, usually measured 60 to 80 milliseconds after the J point. The degree, location (specific leads involved), and morphology of ST Segment deviations help determine the underlying cardiac condition.

Summary of ECG Waveform Components Related to ST Segment

Pathophysiological Context

The ST Segment abnormalities are critical markers of myocardial oxygen supply and demand imbalance. In ischemic heart disease, the ionic shifts and cellular injury alter the normal plateau phase, causing shifts in the electrical vector that appear as ST deviations on the ECG. Prompt recognition of these changes enables early diagnosis and treatment of potentially life-threatening cardiac events.

Summary

The ST Segment is an essential component of the ECG waveform, reflecting the phase of electrical neutrality during early ventricular repolarization. Precise analysis of its level and shape provides vital information on cardiac health, especially regarding ischemia, injury, and electrolyte disturbances. Understanding the ST Segment’s electrophysiological basis is fundamental in interpreting ECGs and guiding clinical decision-making.