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First negative deflection of the QRS complex, measured in duration (ms) from the begining to the end when it returns to baseline
If the QRS deflection is all negative, the deflection is a Q wave, but the complex is called a "QS" complex
Normal Q waves
Small (<30 ms), common finding in most leads (except aVR, V1-V3)
Abnormal Q waves
Any Q wave in leads V1-V3
Q waves > 40ms (1 small box) in leads I, II, aVL, aVF, V4-V6
Q wave MI - abnormal Q waves in at least 2 contiguous leads and > 1 mm in depth
Q waves in MI:
Q waves may be indicative of lower acuity of onset of MI
However, it is important to note that QR waves are present in about half of anterior MIs within the first hour of symptom onset, representing ischemia of the conduction system rather than completed infarction
QR waves alone should not be used as a reason to withhold emergent reperfusion therapy
Patients with pathologic Q waves already present within the first hour have a larger final infarct size but benefit equally from thrombolytics
Pathologic Q waves appear to be an independent marker of a worse outcomes (death, shock, and CHF)
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