What is ST segment depression on ECG?

As a medical professional with expertise in cardiology, I can explain the concept of ST segment depression on an ECG. The ST segment on an ECG (electrocardiogram) represents the period between the end of the P wave (which signifies atrial depolarization) and the beginning of the T wave (which signifies ventricular repolarization). Normally, the ST segment is relatively flat and isoelectric, meaning it has no significant deviation from the baseline. ST segment depression refers to a downward shift of the ST segment below the isoelectric line. This is an important finding on an ECG and is often indicative of subendocardial ischemia, which is a condition where the inner layer of the heart muscle (endocardium) is not receiving enough blood flow and oxygen. This can occur due to various reasons, including coronary artery disease. The depression can be transient and reversible if the ischemia is not severe or prolonged. However, if the ischemia is severe and leads to a heart attack (myocardial infarction), the ST segment depression may persist. In addition to ST segment depression, other ECG changes that may be seen with ischemia include T wave inversion, but typically without ST segment elevation or the appearance of Q waves, which are more characteristic of a transmural (full-thickness) infarction. It's important to note that the interpretation of ECG findings should be done by a healthcare professional, as there are many factors to consider, and other conditions can mimic the appearance of ischemia on an ECG.

The ECG sign of subendocardial ischemia is ST segment depression (A). Depression is reversible if ischemia is only transient but depression persists if ischemia is severe enough to produce infarction. T wave inversion with or without ST segment depression (B) is sometimes seen but not ST segment elevation or Q wave.