Q wave myocardial infarction

Introduction 

Q waves or QS complexes, in the absence of QRS confounders, are pathognomonic of a prior myocardial infarction in patients with chronic ischemic heart disease regardless of symptoms. Prior myocardial infarction is characterized by the presence of myocardial scar.

It generally takes several hours to days to develop.

ECG features 

Q waves are more likely to be diagnostic of a prior MI if there is also an inverted T wave in the same lead. The localisation of these changes depends on the localisation of MI. 

ST segment should be isoelectric, an elevation that persists more than 3 weeks after the acute event, suggests aneurysma of the myocardial wall.

Normal Q waves:
  • small Q waves may appear in most leads and do not represent any abnormality
  • deeper Q waves (>0.2 mV) may be seen in leads III and aVR as a normal variant
  • under normal circumstances, Q waves are not seen in the leads V1-V3

Pathologic Q wave is defined by:

  • any Q waves in leads V2-V3 ≥ 0.02 sec or QS complex in leads V2-V3
  • Q-wave ≥ 0.03 s and > 0.1 mV deep or QS complex in leads I, II, aVL, aVF, or V4–V6 in any two leads of a contiguous lead grouping (I, aVL,V6; V4–V6; II, III, and aVF)
  • in two contiguous leads, Q wave ≥ 0.03 sec and ≥ 0.1mV deep, or QS complex in leads I, II, aVL, aVF, or V4-V6

Simple ´´cooking book´´ for pathologic q waves in clinical practice:

  • > 40 ms (1 mm) wide
  • > 2 mm deep
  • > 25% of depth of QRS complex
  • Q waves in leads V1-3

ECG 1 Old myocardial infarction of inferior wall (Q waves in II, III, aVF, RBBB also present as a coincidental finding)

ECG 2 Old myocardial infarction of anterior wall (Q waves in V1-V3, also signs of aneurysm as a consequence of MI - ST elevations in V1-V3, LAH)

ECG 3 Subacute antero-extensive myocardial infarction (40 year old diabetic I.type with retrosternal pain >24 hours) - Q+ST elevations in V1-V4 (aneurysm), q+ ST elevations in I, aVL, V5+V6 = proximal occlusion of LAD


References

  1. Jordan M Prutkin, Ary L Goldberger, Gordon M Saperia.  ECG tutorial: Myocardial ischemia and infarction: https://www.uptodate.com/contents/ecg-tutorial-myocardial-ischemia-and-infarction
  2. Lucia PV, Anna LL, Catherine K, Tiziano M, Francesco FF (2019) Role of Q-Waves ECG in Myocardial Scar Assessment in Patients with Prior Myocardial Infarction. Med Clin Rev Vol. 5 No. 2: 4.
  3. https://litfl.com/q-wave-ecg-library/