What is a primary indicator of myocardial ischemia on an ECG?

The presence of ST-segment deviations on an ECG is a primary indicator of myocardial ischemia. Ischemia occurs when the heart muscle does not receive enough oxygen-rich blood, commonly due to coronary artery blockage. When this happens, the electrical activity of the heart is affected, leading to characteristic changes in the ECG.

During ischemia, the ST segment may become elevated or depressed, reflecting the altered repolarization of the myocardial cells due to the lack of adequate blood flow. Elevated ST segments can indicate acute myocardial injury or infarction, while depressed ST segments can suggest less severe ischemia. Both types of ST-segment changes serve as crucial diagnostic markers in assessing the heart's condition.

Other choices, while they may indicate various cardiac conditions, do not serve as primary indicators of myocardial ischemia. Ectopic beats are irregular heartbeats that can occur in many contexts, QRS duration pertains to the conduction time through the ventricles but does not specifically indicate ischemia, and heart rate variability is more related to autonomic nervous system function than to specific ischemic events. Thus, ST-segment deviations are a key focus for detecting myocardial ischemia on an ECG.