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