1.8.8 ST Segment Definition

ST Segment Definition is the precise characterization of the ST segment as the portion of the electrocardiogram extending from the end of the QRS complex to the beginning of the T wave, corresponding to the plateau phase of the ventricular action potential during which the entire ventricular myocardium remains depolarized. This definition establishes the ST segment as a distinct interval representing a period of relative electrical uniformity, occurring between the completion of ventricular depolarization and the onset of repolarization.

Elements of the Definition

Position Between the QRS Complex and T Wave

The ST segment is defined by its position immediately following the QRS complex and preceding the T wave, occupying the interval during which ventricular depolarization has concluded but repolarization has not yet begun.

Correspondence to the Plateau Phase

Central to the definition is the ST segment's correspondence to the plateau phase of the cardiac action potential, a period during which calcium influx and potassium efflux are balanced, maintaining a relatively stable membrane potential across ventricular tissue.

Normally Isoelectric Baseline

The definition specifies that the ST segment normally appears as a flat, isoelectric line at the same baseline level as the segments preceding the P wave, reflecting the electrical uniformity of the fully depolarized ventricular myocardium during this period.

Distinguishing Features

Contrast with the QRS Complex and T Wave

The ST segment is distinguished from the QRS complex, which reflects active depolarization, and from the T wave, which reflects active repolarization, by representing an interval of relative electrical stability between these two dynamic phases.

Sensitivity to Alterations in Myocardial Electrical Uniformity

A defining feature of the ST segment is its heightened sensitivity to conditions that disrupt the uniform electrical state of the plateau phase, producing deviations from the normal isoelectric baseline when present.

Brief Duration Relative to Other Intervals

The definition accounts for the relatively short duration of the ST segment compared to the QT interval as a whole, reflecting its position as a single component within the broader sequence of ventricular electrical activity.

Purpose of the Definition

Establishing the Recorded Marker of the Plateau Phase

A precise definition of the ST segment establishes it as the specific electrocardiographic marker corresponding to the plateau phase of the ventricular action potential, distinct from the markers corresponding to depolarization and repolarization.

Foundation for Understanding Myocardial Electrical Stability

The definition provides the basis for understanding how the appearance of the ST segment reflects the underlying uniformity of ventricular depolarization across the myocardium during this phase.

Clarifying the ST Segment's Role Within the Electrocardiogram

By specifying that the ST segment spans from the end of the QRS complex to the beginning of the T wave, the definition delineates its relationship to these two waveforms and to the plateau phase occurring between them.