The frontal plane QRS loop in the normal heart and in diaphragmatic myocardial infarction.

The efferent limb of the frontal plane QRS loop of the vectorcardiogram was studied quantitatively in 100 normal subjects and in 50 patients with electrocardiographic QRS evidence of diaphragmatic infarction. Efferent limb analysis consisted of determination of the position of the .02-second and maximal vectors in the triaxial reference frame and of the ratio born by the magnitude of the .02-second vector to that of the maximal vector. This ratio was not found to be greater than 0.16 in the normal heart with clockwise loop inscription and a superiorly directed .02-second vector. When 20 patients with only ST-T changes of diaphragmatic infarction in the electrocardiogram were similarly studied, 13 showed significant QRS loop evidence of this infarction.