Q wave

Definition

The first negative (downward) deflection of the QRS complex, before any R wave. Small septal q waves, produced as the interventricular septum depolarizes from left to right, are normal in the lateral leads (usually I, aVL, V5 and V6); the Fourth Universal Definition of MI describes normal septal Q waves as shorter than 30 ms and less than a quarter of the R-wave amplitude, and a small Q in lead III or aVL can also be normal depending on the QRS axis. By that definition (in the absence of left ventricular hypertrophy or left bundle branch block), a pathological Q wave is a Q at least 30 ms wide and at least 1 mm deep, or a QS complex, in leads I, II, aVL, aVF or V4 to V6, in two leads of a contiguous group (I and aVL; II, III and aVF; V1 to V6), or any Q wider than 20 ms, or a QS complex, in V2 and V3. Pathological Q waves are associated with prior myocardial infarction.

Clinical noteA QS complex in lead V1 is normal. Q waves or QS complexes can also occur without infarction, for example with pre-excitation, cardiomyopathy, left bundle branch block or left ventricular hypertrophy.