As you read the ECG, you must decide which Q waves indicate pathology, and which Qs are normal. For example in lead I, a Q less than 1/4 of the R height, and less than one box wide, is considered normal.
Normal Qs indicate activation of the intraventricular septum. They will therefore appear in the leads that look left the rightward electrical activity across the septum will cause a negative deflection in these leads.
Septal Qs are normal in I, F, V5 and V6 (left or lateral leads). Small Qs are also generally innocent in lead III and lead V1 if no other abnormality is seen.
Q waves are significant if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave. Significant Q waves indicate either myocardial infarction or obstructive septal hypertrophy (IHSS).
A Q wave in lead III alone is not diagnostic of infarction, even if it is otherwise significant in size and width. Qs in III are ignored unless other abnormalities are seen.
In transmural myocardial infarction, significant Q waves (1 box wide or 1/4 the R) appear in the leads looking at the area of infarction: II, III, and F for inferior infarction; I, L, and V5-V6 for lateral; and V2-V4 for anterior.
Q waves that occur in the setting of LBBB or LVH are less reliable for diagnosis of myocardial infarction.
The presence of ST or T wave abnormality in the same lead(s) as borderline Qs makes these Qs suspicious for infarction.
These significant Qs of IHSS are almost always accompanied by evidence of marked left ventricular hypertrophy.
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