Q Wave Formation

When a transmural segment of myocardium undergoes infarction, it ceases to depolarize normally and becomes essentially electrically inert. As a result, there are no forces of ventricular depolarization spreading from endocardium to epicardium and coming directly toward whichever leads are viewing the infarcted wall. Instead, the leads viewing the infarction are looking through the "window" of inactive infarcted myocardium at the forces of the opposite wall of the ventricle. These vectors in the opposite wall are also spreading from endocardium to epicardium and are therefore going away from the leads looking at the infarction. This concept of an electrical window through the infarction looking on the "back wall" of the heart, as illustrated in Figure 9.8, is a simplified but useful concept for explaining Q wave formation in the leads looking at the infarction.

In an anterior wall infarction, for example, as the anterior wall begins to die, the forces of anterior wall depolarization spreading from endocardium to epicardium gradually decline until they cease. As a result, the R wave normally seen in the anterior precordial leads gradually becomes smaller and smaller until the initial deflection finally becomes a Q. This is called loss ofR wave progression across the precordium. Figure 9.7 shows substantial loss of the normal R wave in V2-V4, but actual Q wave formation has not yet occurred. Figure 9.6 shows full Q wave formation in leads Vi-V5.

You will recall from our previous discussion of the normal ECG that, as a result of left-to-right septal depolarization, a small Q wave can be

Figure 9.8. Q wave formation in AMI. Schematic diagram of the heart illustrating how the "window" of electrically inert infarcted tissue in AMI permits the electrode viewing the infarction to look through the window at the opposite wall of the heart. Normal force vectors in the opposite wall are going away from the electrode, traveling from endocardium to epicardium, producing a negative deflection that we call a Q wave.

Figure 9.8. Q wave formation in AMI. Schematic diagram of the heart illustrating how the "window" of electrically inert infarcted tissue in AMI permits the electrode viewing the infarction to look through the window at the opposite wall of the heart. Normal force vectors in the opposite wall are going away from the electrode, traveling from endocardium to epicardium, producing a negative deflection that we call a Q wave.

Pathological
Figure 9.9. Acute inferior wall myocardial infarction showing pathologic Q wave formation and ST elevation in II, III, and aVF. A pathologic Q wave can also be seen in leads V, and V2, denoting an old anterior wall infarction,

normally seen in the inferior leads II and aVF particularly. Q waves will also be seen in lead III with LPH. With acute inferior wall infarction, however, the Q becomes deeper and wider until it reaches criteria for becoming what elec-trocardiographers call a pathologic Q wave. Most electrocardiographers define a pathologic Q as being at least 0.04 s wide (one small block) with a depth > 25% of the height of the R wave. Thus,the presence of a Q wave alone in the inferior leads is not enough to diagnose an inferior wall infarction, unless the Q wave is pathologic. Figure 9.9 shows an acute inferior wall myocardial infarction with pathologic Q wave formation along with ST elevation in leads II, III, and aVF.

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