The ST segment starts at the J point (the junction between the end of the QRS and the beginning of the ST segment) and ends at the beginning of the T wave.  The normal ST segment is isoelectric; is essentially level with the PR and TP (baseline) segments, and has a duration of 0.080 to 0.120 secs.  The shape and direction of the ST segment are the important indicators to measure along with T-wave changes when trying to determine myocardial ischemia/infarction.  Significant ST depression in the Ambulatory ECG (A-ECG) is determined when the ST segment is ≥ 2mm below the baseline.   When viewing a 12-lead A-ECG, reciprocal changes in the ST segments and T-waves among the leads should be noted.