ST depression is a significant ECG finding associated with ischemia, particularly during periods of increased myocardial oxygen demand, such as exercise or stress. This change illustrates a lack of adequate blood flow and oxygen to the heart muscle due to narrowed or blocked coronary arteries.
When there is ischemia, the affected myocardial cells may be unable to generate enough electrical activity, leading to a downward shift in the ST segment on the ECG. This depression often indicates that the heart is not receiving enough oxygen for its workload, reflecting an ongoing ischemic process.
ST elevation, while significant, typically indicates myocardial infarction rather than ischemia itself. Q waves can appear after a heart attack, showing old myocardial damage but are not directly indicative of acute ischemia. A normal sinus rhythm reflects a healthy heart rhythm with no immediate signs of ischemia or cardiac distress. Thus, the presence of ST depression is the most direct indicator of ischemia on an ECG.