What is the most common ECG change seen in a patient with myocardial ischemia?

In a patient experiencing myocardial ischemia, the most common ECG change observed is depressed ST segments. This alteration occurs due to the subendocardial ischemia where the affected myocardium does not receive adequate blood flow, thus impacting the repolarization phase of the cardiac cycle.

During ischemia, the surrounding tissues can still generate electrical activity, but the impaired blood flow leads to a delay in the return of the electrical potential to its baseline after depolarization. This manifests as a downward displacement of the ST segment from the baseline on the ECG, often seen during episodes of angina or in early stages of myocardial infarction.

Other changes, such as elevated ST segments typically indicate a more severe form of damage like acute myocardial infarction (specifically STEMI) rather than ischemia. Prolonged QT intervals could signal other issues like electrolyte imbalances or drug effects but are not specifically indicative of myocardial ischemia. Inverted T waves can occur later in the course but are not as common in initial ischemic episodes compared to ST segment depression. Thus, depressed ST segments are the hallmark of ischemia, reflecting the compromised blood flow and its effects on cardiac repolarization.