The initial vector that is responsible for the normal Q wave in lead I (a horizontal lead) and for either an initial R or Q wave in lead III (a lead that has a dominantly vertical orientation at 120°) is due to the initial forces that result from depolarization of the interventricular septum from left to right. It is well known that Q waves infrequently are not present in lead III.1 The origin of this Q wave is the initial septal vector that is directed superiorly in such patients.2 Luisada and Slodki have noted that a Q wave in lead III commonly is found in a normal heart when the electrical axis is near horizontal.3 Similarly, Friedberg has stated that "the presence of a Q3 and inverted T3 in themselves may not signify myocardial infarction for they may be produced by a transverse position of
Halfant RH, Bodenheimer MM. Clarification of Q Waves in Lead III-Reply. Arch Intern Med. 1977;137(10):1486. doi:10.1001/archinte.1977.03630220110034
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