ECG Topic - The Q Wave

Last updated 1/28/26

Introduction

Q waves are among the most misunderstood findings on the ECG. While classically associated with myocardial infarction, not all Q waves represent scar, and not all infarctions produce Q waves. The key is distinguishing normal depolarization vectors from true loss of electrically active myocardium.

Electrophysiology

The first portion of the QRS complex reflects initial ventricular depolarization, which normally begins in the interventricular septum and proceeds left-to-right. When this initial vector is directed away from a lead, a small negative deflection — a Q wave — is recorded.

• Benign Q waves reflect normal septal activation.
• Pathologic Q waves reflect absence of viable myocardium in a region, allowing unopposed electrical forces from the opposite wall to dominate.

Thus, Q waves do not directly represent “scar,” but rather electrical silence in the territory that would normally generate early depolarization forces.

Benign (Physiologic) Q Waves

1. Septal Q Waves (Leads I, aVL, +/- V5–V6)

• Small, narrow (<30 ms), shallow
• Represent normal left-to-right septal depolarization
• Common and normal

ECG: Septal Q waves

2. Isolated QS Complex in V1

• A QS pattern in V1 alone is frequently normal
• Due to lead position and septal orientation
• Becomes concerning if QS extends into V2 or V3 → suggests anteroseptal infarction

ECG: QS in V1

3. Isolated Q Wave in Lead III

• Often positional
• Can vary with respiration, especially in patients with a horizontal heart (obesity, pregnancy, ascites)
• Must be present in II and aVF as well to localize an inferior infarct

ECG: Q in lead III

4. Pseudo-infarct Q Waves

Seen in conditions that alter early ventricular activation:

• WPW (delta wave masks initial R wave)
• LBBB or paced rhythm
• Hypertrophic cardiomyopathy
• Severe RVH or extreme axis deviation

ECG: Type B WPW

ECG: LBBB

Respiratory and Positional Variation

Changes in diaphragmatic position can rotate the heart and alter the apparent depth of inferior Q waves with breathing. This produces cyclical Q wave variation and minor axis shifts. This is benign and should not be confused with evolving infarction or electrical alternans.

Images adapted to Tomas Garcia’s 12 Lead ECG - Art of Interpretation

ECG: Q wave variation in lead III

Q wave respiratory variation causes gradual, cyclical changes in QRS amplitude or axis with breathing, whereas electrical alternans shows beat-to-beat, abrupt swings in QRS morphology and axis, classically seen with large pericardial effusion.

ECG: Electrical Alternans for comparison

What Makes a Q Wave Pathologic?

A Q wave is considered pathologic when it reflects myocardial infarction rather than normal activation.

A Q wave is abnormal if it is:

• Wide: ≥ 30-40 ms (≥ 1 small box) ← most specific
• Deep: ≥ 25–33% of the ensuing R wave
• Territorial: Present in ≥ 2 contiguous leads
• Unexplained: Not due to conduction disease or pre-excitation

Images adapted to Tomas Garcia’s 12 Lead ECG - Art of Interpretation

ECG: Pathologic Qs in the inferior and lateral leads

Source: ECG Wave Maven

ECG: Pathologic Qs in the inferior leads

ECG: Pathologic Qs in the anterior leads

ECG: Pathologic Qs in the anterior and lateral leads

Posterior MI (Q-Wave Equivalent)

Posterior infarction does not produce Q waves on the standard 12-lead. Instead, it creates:

• Tall, wide R waves in V1–V2 - This represents the mirror /reciprocal image of posterior Q waves

ECG: Posterior MI

ECG: Posterior MI

Key Pitfalls & Takeaways

• A single Q wave in lead III is usually positional, not infarction. Lead III is highly sensitive to axis and respiratory shifts
• A QS in V1 alone is often normal; extension into V2–V3 is abnormal.
• Consider WPW, LVH, RVH, and bundle branch block before labeling Q waves as infarct.
• Width matters more than depth (≥30 ms is most specific), and territory matters more than a single lead.
• Q waves reflect loss of early depolarization forces—not simply “old scar”—and do not exclude acute coronary occlusion.
• Interpret Q waves in clinical context, with ST–T changes and prior ECGs.

Sources

• Brady WJ, Harrigan RA. Critical Decisions in Emergency and Acute Care Electrocardiography. 2nd ed. Wiley-Blackwell.
• Burns E, Buttner R. Pathological Q Waves. LITFL ECG Library.
• Davila E. The ECG. 2025.
• Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Lippincott Williams & Wilkins.
• Mattu A, Brady WJ. ECGs for the Emergency Physician. Vols 1–2. BMJ Books.
• Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th ed. Elsevier.

This post is for education and not medical advice.