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Pathogenesis and diagnosis of Q waves on the electrocardiogram

INTRODUCTION

By definition, a Q wave on the electrocardiogram (ECG) is an initially negative deflection of the QRS complex. Thus, a Q wave indicates that the net direction of early ventricular depolarization forces is oriented away from (by more than 90º) the positive axis of the lead in question. Although prominent Q waves are a characteristic finding in myocardial infarction (MI), they can also be seen in a number of noninfarct settings. Failure to appreciate the other causes of Q waves can lead to important diagnostic errors. (See "Basic principles of electrocardiographic interpretation".)

The presence of a Q wave does not indicate any specific electrophysiological mechanism. To the contrary, Q waves can be related to one or more of the following four factors (table 1) [1,2]:

  • Physiologic and positional effects
  • Myocardial injury or replacement
  • Ventricular enlargement
  • Altered ventricular conduction

Clinicians should be aware of three principles with respect to Q waves: 1) not all Q waves are pathologic; 2) not all pathologic Q waves are due to myocardial infarction caused by fixed coronary artery occlusion; and 3) there is no firm consensus on the criteria for the diagnosis of pathologic Q waves [3].

A broader discussion of the electrocardiogram in MI is found elsewhere. (See "Electrocardiogram in the diagnosis of myocardial ischemia and infarction".)

             

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Literature review current through: Apr 2013. | This topic last updated: Apr 1, 2013.
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References
Top
  1. Goldberger, AL. Clinical Electrocardiography: A Simplified Approach, 7th ed, Mosby, St. Louis 2006.
  2. Mirvis, DM, Goldberger, AL. Electrocardiography. In: Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 9th ed, Bonow, RO, Mann, DL, Zipes, DP, Libby, P (Eds), W.B. Saunders Company, Philadelphia 2011.
  3. Delewi R, Ijff G, van de Hoef TP, et al. Pathological Q waves in myocardial infarction in patients treated by primary PCI. JACC Cardiovasc Imaging 2013; 6:324.
  4. Pirwitz MJ, Lange RA, Landau C, et al. Utility of the 12-lead electrocardiogram in identifying underlying coronary artery disease in patients with depressed left ventricular systolic function. Am J Cardiol 1996; 77:1289.