Normal ST segment

ECG: Isoelectric ST segment, with no abnormality and minimal physiological elevation in V1-V2 (< 0.5 mm);

Comments: The ST segment corresponds to the portion of the electrical conduction which occurs between the end of the QRS-complex, which reflects the end of ventricular depolarization, and the beginning of the T-wave, which corresponds to the beginning of the repolarization. The transition between the QRS-complex and the ST segment is generally well-defined, angular and clearly identifiable, the point of junction is called the J point. Conversely, the transition to the T-wave is more gradual. A systematic analysis of the ECG includes the study of the position and morphology of the ST segment but not its duration. Indeed, since the onset of the T-wave is often difficult to determine, it does not allow for an accurate measurement. The ST segment typically lasts 80 to 120 ms even though there is no firmly established standard.

•  Position of the ST segment: the position of the ST segment corresponds to that of the J point or in the case of inaccuracy of the latter, to that of the most horizontal part of the segment; this segment is normally more or less isoelectric and on the same line as the ensuing TP segment, i.e. the J point  is at the same height as the baseline (potential 0); there may be a slight deviation from the baseline if the different ventricular myocardial layers (endocardium versus epicardium) show a slight difference in potential during this phase; a possible elevation or depression is assessed relative to the TP segment and is measured in mm;
• ST segment morphology: this segment is usually horizontal even if an ascending oblique delay is physiological in young subjects; the ST-T-wave complex is normally smooth, rounded with no pronounced angle;
• Physiological depression: a slight drop in J-point and of the initial portion of the ST segment below the isoelectric line is frequently observed in frontal leads and left precordial leads; a low amplitude depression (< 1 mm) can be physiological if the ST segment is not in a downward slope;
• Physiological elevation: the normality criterion is that the ST segment must not deviate by more than 1 mm above the isoelectric line; there is, however, a variant of normal, which is the early repolarization observed preferentially in young patients, with an elevation often maximal in the mid-precordial leads, concave upwards, often connected to a slurring or notching of the ascending branch of the R wave and followed by a tall and symmetrical T-wave;

Take-home message: The ST segment is generally isoelectric and horizontal; a deviation of less than 1 mm above or below the baseline may be physiological.