The debate tends to be between using the PQ junction which is the end of the PR segment, or the TP segment. The reference point used in determining if the ST segment is elevated or depressed has been an area of dispute. Because it is often difficult to make out the J point and/or the beginning of the T wave, the ST segment is frequently an approximation or evaluated at ST, which is the ST segment at 60 ms after the J point. For instance, diffuse J points can be seen with early repolarization, pericarditis, left ventricular hypertrophy (LVH) with strain, and acute myocardial infarctions (MI). However, it is not always easy to discern the J point because of various ST-segment morphologies. Any displacement of the ST segment above or below baseline is often measured at the J point. It marks the beginning of the ST segment. The junction (J) point is where the QRS complex and ST segment meet. In cases in which myocardial injury has occurred, if ST elevation is present, leads electrically opposite tend to show reciprocal ST depression. This affects the plateau phase of the ventricular transmembrane action potential and, hence, the ST segment. An injury current results from current flowing across a gradient of ischemic and non-ischemic zones. For example, in the presence of acute myocardial ischemia or infarction, ST-segment changes are a consequence of an injury current. Thus, any significant change in voltage gradients during the plateau phase of the action potential can result in variations in the ST segment. This near absence of voltage gradient is responsible for the normal isoelectric or practically flat ST segment on the ECG. These minimal voltage gradients are similar to those found in the period between the end of ventricular repolarization and the beginning of the next cycle of ventricular depolarization. Normally, there are slow and relatively similar transmembrane voltage changes in the ventricular myocardial cells during this phase. The ST segment corresponds to the plateau phase of the ventricular transmembrane action potential. In clinical terms, the ST segment represents the period in which the myocardium maintains contraction to expel blood from the ventricles. In other words, it corresponds to the area from the end of the QRS complex to the beginning of the T wave. The ST segment encompasses the region between the end of ventricular depolarization and beginning of ventricular repolarization on the ECG.
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