The Q Wave
A Q wave is any negative deflection that precedes an R 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 (HCM), 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 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
Related Topics
LITFL Further Reading
- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest
Advanced Reading
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
- Wagner GS. Marriott’s Practical Electrocardiography 12e
- Chan TC. ECG in Emergency Medicine and Acute Care
- Dubin D. Rapid Interpretation of EKG’s
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e
ECG LIBRARY
Electrocardiogram
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