A QS complex in which the QRS is entirely negative can also occur in lead V1 as a normal variant and rarely in leads V1 and V2. Depending upon the electrical axis, more prominent Q waves (as part of QS or QR-type complexes) can also appear in the limb leads: in aVL with a vertical axis and in leads III and aVF with a horizontal axis. As a result, small (<0.04 s in duration) "septal" Q waves typically occur in the lateral precordial leads (which have a left-right spatial orientation) and in one or more of the limb leads (except aVR). These early septal depolarization forces are oriented anteriorly and to the right. PHYSIOLOGIC AND POSITIONAL FACTORS - Physiologic activation of the ventricles begins at the left side of the interventricular septum. (See "Electrocardiogram in the diagnosis of myocardial ischemia and infarction".) Ī broader discussion of the ECG in MI is provided elsewhere. In contrast, Q-wave regression was associated with significantly lower risk of major adverse cardiac events. In one large prospective study (CIRCUS) including 780 patients with anterior ST-elevation MI treated with a percutaneous coronary intervention, persistent Q waves after reperfusion were associated with a fourfold increase in the risk of heart failure or death compared to non-Q-wave MI. Furthermore, pathologic Q waves due to infarction may regress or disappear entirely following the event. Accordingly, Q waves should always be interpreted in the clinical context (eg, acute MI presentation, asymptomatic presentation). To the contrary, Q waves can be related to one or more of the following four factors ( table 1) :Ĭlinicians should be aware of three principles with respect to Q waves: not all Q waves are pathologic, not all pathologic Q waves are due to MI caused by fixed coronary artery occlusion, and there is no consensus on the precise criteria for the diagnosis of pathologic Q waves with respect to their width, extent, and location. The presence of a Q wave does not indicate any specific electrophysiological mechanism. (See "Basic principles of electrocardiographic interpretation".) Failure to appreciate the other causes of Q waves can lead to important diagnostic errors. Although prominent Q waves are a characteristic finding in myocardial infarction (MI), they can also be seen in a number of noninfarct settings. Technically, a Q wave indicates that the net direction of early ventricular depolarization (QRS) electrical forces projects toward the negative pole of the lead axis in question. INTRODUCTION - By definition, a Q wave on the electrocardiogram (ECG) is an initially negative deflection of the QRS complex.
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