Q waves or QS complexes, in the absence of QRS confounders, are pathognomonic of a prior myocardial infarction in patients with chronic ischemic heart disease regardless of symptoms. Prior myocardial infarction is characterized by the presence of myocardial scar.
It generally takes several hours to days to develop.
Q waves are more likely to be diagnostic of a prior MI if there is also an inverted T wave in the same lead. The localisation of these changes depends on the localisation of MI.
ST segment should be isoelectric, an elevation that persists more than 3 weeks after the acute event, suggests aneurysma of the myocardial wall.
Simple ´´cooking book´´ for pathologic q waves in clinical practice:
ECG 1 Old myocardial infarction of inferior wall (Q waves in II, III, aVF, RBBB also present as a coincidental finding)
ECG 2 Old myocardial infarction of anterior wall (Q waves in V1-V3, also signs of aneurysm as a consequence of MI - ST elevations in V1-V3, LAH)
ECG 3 Subacute antero-extensive myocardial infarction (40 year old diabetic I.type with retrosternal pain >24 hours) - Q+ST elevations in V1-V4 (aneurysm), q+ ST elevations in I, aVL, V5+V6 = proximal occlusion of LAD