Q wave notation in this module takes three forms: 

1. “Q wave” denotes an ECG tracing where Q > R.
2. “QS wave” or “QS complex” denotes an ECG tracing where the QRS complex is entirely negative.
3.  “q wave” denotes an ECG tracing where Q < R.

A normal q wave is smaller than the R wave and represents depolarization in the interventricular septum. In most leads, a normal q wave lasts up to 0.015 seconds. It may exceed 0.020–0.025 seconds in the presence of normal variants of the Q wave morphology in limb lead III. It may also appear as a QS wave in precordial leads V₁ and V₂.  

Most leads do not show q waves in the normal ECG because the vectors in the interventricular septum are conducted in a positive manner. It may also be manifested in the first 0.015 seconds of the R wave.

Lead aVR is located at −150° in the frontal plane. Since aVR detects impulses from the interventricular septum traveling away (negative) from it, pathologic q wave patterns such as Qr, QS, or even an rS or rSr’ pattern may be present as a normal variant in lead aVR.

A pathologic Q wave has a duration ≥ 0.04 seconds.