Calculator estimation of cardiac vectors in the frontal plane

A simple quantitative method is described that permits the rapid and accurate estimation of frontal plane QRS vector characteristics on an inexpensive programmable calculator. The derived formulas, based upon ECG amplitudes in standard leads I and II, are modified from those of Einthoven and are suitable for all possible angle and magnitude measurements. The accuracy of these measurements was verified in 100 normal human volunteers. Good agreement was found between vector estimates and absolute references describing the frontal plane vector loop. Thus, the estimated vector magnitude closely approximated the maximum instantaneous vector magnitude, and the estimated vector angle closely approximated the mean electrical axis of the heart. The technique compares favorably with other estimation procedures, not sacrificing accuracy for speed of implementation. For this reason, the calculator approach may be of practical interest for routine vector analysis in a clinical setting, including ICUs. All pertinent equations are derived and an example calculator program is provided.