Q Waves

Q waves are the first deflection of the QRS complex, and are the representation of septal depolarisation within the heart.

They are usually absent from most leads of the ECG, but small Q waves are normal in the leads that observe the heart from the left;

• I
• II
• aVL
• V₅
• V₆

Q waves are sometimes seen in lead III, but disappear on deep inspiration.

Q waves in leads other than the above can be considered pathological, particularly if they are:

• >2mV / small squares deep

or

• >25% of associated R wave

and/ or

• >0.04s

Wide or deep Q waves may indicate:

• Myocardial infarction
• LVH
• Bundle branch block

Abnormal Q waves in lead III can be a sign of pulmonary embolism.