Pathologic Q wave

Definition

An abnormal initial negative QRS deflection that usually indicates established myocardial necrosis; it appears when viable myocardium beneath the recording electrode has been lost. By the fourth universal definition of MI (in the absence of left ventricular hypertrophy and left bundle branch block), a Q wave is pathologic when any Q wave in leads V2 to V3 is wider than 0.02 s (20 ms) or there is a QS complex in V2 and V3, or when a Q wave 0.03 s (30 ms) or wider and 0.1 mV (1 mm) or deeper, or a QS complex, appears in leads I, II, aVL, aVF or V4 to V6 in any two contiguous leads of a lead group (I, aVL; II, III, aVF; V1 to V6). A normal septal q wave, by contrast, is small and brief (under 0.03 s, 30 ms, and under one quarter of the R-wave amplitude) and is seen in leads such as I, aVL, aVF and V4 to V6. Q waves or QS complexes can also occur without infarction, for example with pre-excitation, cardiomyopathy, left bundle branch block, left anterior fascicular block, or left or right ventricular hypertrophy.

Clinical noteDo not confuse a pathologic Q wave with the small normal septal q wave, with a QS complex in V1 (which is normal), or with a small Q wave in lead III (under 0.03 s and under one quarter of the R wave), which is normal when the frontal QRS axis lies between -30 and 0 degrees. The fourth universal definition uses the one-quarter-of-R measure only to describe these normal q waves, not as a criterion for a pathologic Q wave.