The Q Wave

A Q wave is any negative deflection that precedes an R wave

  • The Q wave represents the normal left-to-right depolarisation of the interventricular septum
  • Small ‘septal’ Q waves are typically seen in the left-sided leads (I, aVL, V5 and V6)

Q waves in context

Q waves in different leads

  • Small Q waves are normal in most leads
  • Deeper Q waves (>2 mm) may be seen in leads III and aVR as a normal variant
  • Under normal circumstances, Q waves are not seen in the right-sided leads (V1-3)

Pathological Q Waves

Q waves are considered pathological if:

  • > 40 ms (1 mm) wide
  • > 2 mm deep
  • > 25% of depth of QRS complex
  • Seen in leads V1-3

Pathological Q waves usually indicate current or prior myocardial infarction.

Differential Diagnosis

Loss of normal Q waves

  • The absence of small septal Q waves in leads V5-6 should be considered abnormal.
  • Absent Q waves in V5-6 is most commonly due to LBBB.

ECG Examples
Example 1
  • Inferior Q waves (II, III, aVF) with ST elevation due to acute MI

Example 2
  • Inferior Q waves (II, III, aVF) with T-wave inversion due to previous MI

Example 3
  • Lateral Q waves (I, aVL) with ST elevation due to acute MI

Example 4
  • Anterior Q waves (V1-4) with ST elevation due to acute MI

Example 5

  • Anterior Q waves (V1-4) with T-wave inversion due to recent MI

ECG Library Basics

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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner


  1. Thankyou for this Dr Burns,

    I wonder what your thoughts are on finding inferior Q waves in a patient with an indeterminate axis? Should we still expect the small inferior R waves seen in a superior axis?

  2. Thank you for this concise but informative post.

    Just a question: would that be safe to say that in example 4 and 5, there are only Q waves in Leads V1 to V4, and no R wave, no S wave, then we are onto ST elevation?

    • Yes correct. Otherwise known as poor R wave progression, we see complete loss of R waves which is common in acute infarction.

  3. Why we considered Q wave and T waves inversion as recent MI in Example 5 but in Example 2 considered as previous?

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