The genesis of indeterminate axis: a quantitative vectorcardiographic analysis.

The vectorcardiographic basis of indeterminate QRS axis has not been elucidated. Accordingly, Frank lead VCGs were obtained on ten consecutive subjects with biphasic QRS complexes in the six extremity leads. The frontal plane vector in each case showed a very narrow loop inscribed in a symmetrical fashion around the E point. The maximal width of the frontal plane loop (0.20+/-0.10 mV, mean +/-SD) was significantly smaller (P less than 0.02) than previously published normal values (0.29+/-0.15 mV, n=100). Terminal forces were oriented to the right and superiorly. The transverse plane vector also showed a characteristic pattern with posterior and rightward orientation of terminal forces; the 50 ms QRS vector was significantly (P less than 0.0005) more posterior and rightward (258+/-29 degrees) in subjects with indeterminate axis compared with established normal values (307+/-28 degrees, n=510). The precordial leads reflecting these terminal, rightward forces showed prominent S waves in lead V5 (0.86+/-0.48 mV) and V6 (0.38+/-0.27 mV) which were significantly greater (p less than 0.0005) than the amplitude of S waves in lead V5 (0.24+/-0.20 mV) and V6 (0.06+/-0.09 mV) established for normals (n=424). These findings suggest that: 1) the indeterminate nature of the frontal plane QRS axis is an epiphenomenon resulting from orientation of the plane of the QRS loop relatively perpendicular to the frontal plane; and 2) the underlying factor is the posterior, rightward and superior orientation of terminal QRS forces, which may result from a number of causes.