LESSON OF THE DAY: PHYSIOLOGICAL Q WAVE vs PATHOLOGICAL Q WAVE During an ECG interpretation, the electrical impulses of the heart are seen as waves deviating up or down from the isoelectric line (the flat line of the heart’s resting membrane potential). The Q wave is the first negative deflection of the QRS complex. A lot of Paramedics and other healthcare providers are taught that the presence of a Q wave indicate a prior MI (Myocardial Infarction) but this is not always the case. A PHYSIOLOGICAL Q WAVE is the first negative deflection of the QRS complex which in a normal heart, represents the Septum (the wall dividing the ventricles) depolarizing, so, Septal or Intraventricular Septal depolarization. Usually the Septum depolarizes from left to right, so when you look at the lateral limb and precordial leads (I, aVL, V5-6), you see a small Q wave which indicates Septal depolarization. Why is the Q wave negative during most normal conditions? Because the depolarization is occurring from left to right, so the impulses move away from the left lateral leads creating a negative deflection, and it’s small since it’s a short depolarization time. Occasionally, you might have a benign Q wave in an inferior lead, often lead III, which is a normal variant. Normal Septal depolarization in the Septal precordial leads (V1-2) appear as small R waves, therefore, the presence of a Q wave in those leads is abnormal. A PATHOLOGICAL Q WAVE is an abnormal Q wave that often occurs when there is transmural myocardial necrosis (death of endocardial and epicardial cardiomyocytes). In simple terms, since dead tissue can’t conduct electrical impulses, the leads of whatever site the infarction occurred in, show a pathological Q wave because the necrotic tissue (similar to a scar of the heart), can’t conduct impulses so they travel around it and it’s represented as a wider or deeper Q wave. Also, same as a reciprocal change,that lead is looking at depolarization occuring across from it. In the event of a Posterior pathological Q wave, this will be represented as a wider or taller R wave in the Septal leads. Other MIs may not produce a pathological Q wave known as Non-Q wave MI. Pathological Q waves can have either finding: - >.04s or deeper than >2mm (some say 1mm) - >1/3 the size of the R wave - Can represent posterior MI as a wide (>.04s) or taller (>2mm) R wave in Septal leads V1-2
Posted on: Mon, 24 Jun 2013 19:32:29 +0000
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