Q Wave

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

waves of the ecg

Normal Q wave

Normal Q wave in V6

Origin of the Q 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 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

  • Myocardial infarction
  • Cardiomyopathies — Hypertrophic (HOCM), infiltrative myocardial disease
  • Rotation of the heart — Extreme clockwise or counter-clockwise rotation
  • Lead placement errors — e.g. upper limb leads placed on lower limbs

Examples

Inferior Q waves (II, III, aVF) with ST elevation due to acute MI

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

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

Lateral Q waves (V5-6) with T-wave flattening due to previous MI

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

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

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.

Related Topics

Further Reading

Author Credits

References

  • Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th Edition. Saunders Elsevier 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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