Q waves on ECG

Small - septal - Q waves in the left ventricular leads result from depolarisation of the septum from left to right. A Q wave in lead III may represent a normal finding.

Pathological Q waves occur if they are 25% or more of the height of the partner R wave and/or they are greater than 0.04 seconds in width - one small square - and greater than 2mm (two small squares) in depth (1).

Q waves are a marker of electrical silence, which, when pathological, implies full thickness death of myocardium, which happened a long time ago. They are negative because they are effectively windows through which can be seen the opposite side of the heart, in which depolarisation will be moving away from the electrode.

If a Q wave occurs in lead I then check for Q waves in AVL and the chest leads.If a Q wave occurs in lead II or AVF then check for Q waves in the other inferior leads.

As stated previously, a Q wave in lead III alone may be positional and a normal finding

• Q waves which are 25 % of the depth of the succeeding R wave, and which last for more than 20 ms may still not be pathological in lead III as long as there are no accompanying Q waves in aVF and II - these Q waves often disappear on deep inspiration

Reference:

• 1) Sahay P. E.C.G. Mystery solved.Publishing Initiatives, 1996.
• 2) Wright RA et al (1993). Prognosis in ischaemic heart disease. Medicine International, 21(10), 384-88.