Q Wave

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

Normal Q wave

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

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

• P wave
• T wave
• R wave

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

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.