- 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.
- 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
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 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
- 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.