Case # 2: This is a routine preoperative ECG taken on a 66 year old woman scheduled for cataract surgery. She denies all cardiovascular symptoms.
Stuart A. Winston, D.O.
Michael H. Lehmann, M.D.
Richard D. Judge, M.D.
1. Does her ECG suggest that she has had a "silent" inferior myocardial infarct?
2. Should she have further evaluation, eg: a stress thallium study, prior to operation?
Despite the Q wave and the negative T wave in lead III, this ECG would be considered within normal limits.No further cardiac evaluation is needed.
This isolated Q wave in lead III with a negative T wave is a common normal variant. Remember that lead III is "BIPOLAR", that is, it is the instantaneous difference in voltage recorded between the left leg and left arm. Therefore it has no special geographic significance in and of itself e.g.: it does not necessarily reflect the inferior wall of the heart.
Notice that when you record lead III, the positive pole of your galvanometer (or ECG recorder) is attached to the left leg and the negative pole is attached to the left arm. Therefore, lead III represents the instantaneous difference in voltage between these two points of the body.
III = aVF - aVL
The voltage recorded at aVL is being continuously subtracted from aVF.
Another way of representing lead III (remember your algebra?) is: III = aVF + (-aVL)
Putting it visually:
So an initial R wave in aVL which is quite common will translate into a Q wave in lead III. This Q wave can be very prominent at times but should not be misinterpreted as due to an inferior infarct. The same is true of the negative T wave in lead III. Notice in this case that a normal positive T wave in aVL is responsible for a negative T wave in lead III.
Beware of lead III! For diagnosing an inferior infarct look first to lead aVF which DOES HAVE true spatial significance and after that to leads III and II to which aVF contributes, but only indirectly.
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