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

• 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

ECG Library Basics

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
• Rawshani A. Clinical ECG Interpretation
• Mattu A. ECG’s for the Emergency Physician
• Hampton JR. The ECG In Practice, 6e