Q Wave

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)

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Q waves in context

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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)

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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.

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Differential Diagnosis

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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.

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ECG Examples

Example 1

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

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Example 2

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

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Example 3

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

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Example 4

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

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Example 5

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

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ECG Library Basics

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Advanced Reading

Online

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

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LITFL Further Reading

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