ST segment morphology is very important for many diagnoses, including myocardial ischemia, ST elevation myocardial infarction and pericarditis. First, remember that the ST segment begins with the J point and ends with the beginning of the T wave. In the frontal leads, the normal ST segment is usually isoelectric. In the horizontal leads the normal ST segment commonly shows elevation, more in the right or mid-precordial leads than the left leads. The ST elevation is more common and higher with younger age and in males more than females. ST segment elevation when normal is concave upward. ST elevation or depression of 1 mm or more can reflect disease states, such as ischemia or infarction. The extent of ST depression or elevation should be compared relative to the “isoelectric line,” which is the interval from the end of the T wave to the beginning of the P wave (the T-P interval). If it is difficult to identify, such as with sinus tachycardia where the P wave may merge with the T wave, then use the PR segment as a baseline to compare with the ST segment.