What does a Q wave signify in the context of an old myocardial infarction?

A Q wave on an electrocardiogram (ECG) is indicative of myocardial necrosis, which is the death of cardiac tissue. This typically occurs as a result of a prolonged lack of blood flow, often due to a blockage in the coronary arteries, leading to an old myocardial infarction (MI). In the context of an MI, Q waves develop when myocardial tissue has died and been replaced by scar tissue.

The presence of Q waves signifies that a portion of the heart muscle has undergone irreversible damage, which is a hallmark of an old or past myocardial infarction. Clinicians often look for the presence of Q waves in conjunction with other ECG changes to determine the time frame and potentially the severity of an MI.

Other options, such as reversible ischemia, hyperventilation effects, or myocardial hypertrophy, do not accurately reflect the significance of Q waves. Reversible ischemia would not typically be associated with Q waves, as this condition implies the heart tissue is still viable and can recover, whereas Q waves indicate necrosis that has already occurred.