Estimation of frontal-plane cardiac vectors in heart disease

Frontal-plane vector angle and magnitude values were clinically estimated in 100 patients with cardiac disease. Maximal deviations from isopotential in standard limb leads I and II were summed and mathematically transformed into estimated vector magnitudes (mV) and estimated angles (degrees). These values were then compared against reference vectors (previously shown to be excellent standards) defined from the same QRS complexes. The estimated magnitude values reached a degree of accuracy of +/- .25 mV in 78% of the cases, and the estimated angle values reached a degree of accuracy of +/- 15 degrees in 80% of the cases. The lowest estimate errors occurred in the infarct group, whereas the largest errors occurred in the chamber-enlargement and axis-deviation groups.