What specific change on an ECG indicates myocardial injury?

The elevation of the ST segment on an ECG is a critical indicator of myocardial injury, often associated with conditions such as acute myocardial infarction. When there is damage to the heart muscle, the repolarization phase of the cardiac cycle is altered due to the ischemic environment, resulting in an upward displacement of the ST segment from the baseline. This elevation reflects a serious heart condition that requires immediate medical attention.

In contrast, a flattened T wave typically suggests ischemia but does not necessarily indicate active injury to the myocardium. A wide QRS complex might indicate a delay in conduction, such as in bundle branch blocks or ventricular hypertrophy, but it does not specifically reflect myocardial injury. Likewise, a shortened PR interval often suggests an abnormality in the conduction system but is not associated with direct myocardial injury. Hence, the elevated ST segment is the most specific and critical sign of myocardial injury on an ECG.