ST depression

ST depression is when the ST segment of the ECG sits below its normal baseline, dipping downward rather than staying level. It most often signals that some heart muscle is not getting enough blood for the work being asked of it — a sign of ischemia rather than completed full-thickness injury.

Unlike the full-thickness damage behind ST elevation, ST depression typically reflects partial-thickness or demand-related shortfall: the inner layers of the heart wall are the first to suffer when supply cannot keep up. This is why ST depression often appears during exertion or stress and may ease with rest, and why it is a key finding watched for during an exercise stress test.

ST depression can also appear as a mirror image, or reciprocal change, in leads opposite a region of ST elevation, and it can result from other causes such as certain medicines or chamber strain. As with all ST changes, it is interpreted together with the symptoms, the leads involved, and the rest of the workup rather than in isolation.