Pathologic Q Waves

Pathologic Q waves are a sign of usually prior MI. They may change over time and actually begin during an acute MI. Small q waves are typical in some leads  (III, aVR, V1 and left-sided leads) without any pathology due to septal depolarization. To be pathologic (a sign of ischemia/infarction), they should be wide. The definition is at least 0.04ms (or 1 small box) wide. Sometimes, this is also defined as a depth of >25% of the R wave amplitude, but the definition using width is more specific and should be used preferably. Although usually a sign of prior MI, some studies show that up to 25% of anterior OMIs will have Q wave formation develop within the first hour. 

Examples:

Further Reading:

https://litfl.com/q-wave-ecg-library/

http://hqmeded-ecg.blogspot.com/search/label/q-waves?max-results=20