STsegment

The ST segment is the portion of the electrocardiogram (ECG) between the end of the QRS complex and the beginning of the T wave. It corresponds to the plateau phase (phase 2) of the ventricular action potential. In a healthy heart, the ST segment is typically isoelectric, lying on the baseline.

Normally the ST segment is level with the baseline in most leads. Small deviations can occur due to electrode position, respiration, or normal anatomic variation. The appearance can be horizontal, upsloping, or downsloping depending on the lead and individual factors.

Abnormal ST segments are of clinical interest. ST-segment elevation can indicate acute myocardial injury, most notably ST-elevation myocardial infarction (STEMI), when seen in contiguous leads corresponding to a region of the heart. ST-segment depression or T-wave inversion can indicate ischemia, subendocardial infarction, or nontransmural infarction. Diffuse ST elevation with PR segment depression is classically associated with acute pericarditis. Other conditions that can alter the ST segment include left ventricular hypertrophy, bundle-branch blocks, electrolyte disturbances, and drug effects such as digoxin.

Measurement and interpretation: The ST segment is measured from the end of the QRS complex, at the J point, to the start of the T wave. Clinically significant ST elevation is often defined as about 1 mm (0.1 mV) or more in limb leads or 2 mm (0.2 mV) or more in precordial leads, though thresholds vary with age, sex, and lead. ST depression is typically considered significant at 0.5 mm or more in two or more contiguous leads.

In practice, ST-segment changes are interpreted alongside symptoms, clinical history, and other ECG findings to distinguish ischemic from nonischemic causes.