ST segment

The ST segment is the relatively flat stretch of the ECG that lies between the end of the QRS complex and the start of the T wave. It is the brief electrical lull after the ventricles have fully fired but before they begin to reset — a quiet plateau on the tracing.

Because all of the ventricular muscle is uniformly activated during this moment, a healthy ST segment normally sits level with the baseline, neither pushed up nor pulled down. That flatness is what makes it such a sensitive reference line: when part of the heart muscle is injured or starved of blood, the electrical balance is disturbed and the ST segment shifts away from the baseline.

For this reason the ST segment is one of the most closely watched parts of the ECG in suspected heart attacks and ischemia. An upward shift (elevation) and a downward shift (depression) each carry distinct meanings about the type and severity of the problem. Subtle ST changes are read carefully and compared with earlier tracings, since position alone must be interpreted with the whole clinical picture.