Determining the QRS axis: visual estimation is equal to calculation

The QRS axis of the electrocardiogram (ECG) is often considered in clinical practice, but its determination is frequently limited to a rough estimation, such as "normal", with left or right deviation, and superior or inferior in the case of premature ventricular complexes (PVCs). However, a more exact determination of the QRS axis may be warranted in certain scenarios, such as to determine the origin of PVCs more precisely, and is attainable by visual estimation using the hexaxial reference system.

The aim of this study was to determine how well such an estimation of the QRS axis would correlate with the axis calculated by formulas.

A PVC database from 2012-2020 was used to extract 12-lead ECGs of patients with outflow tract PVCs and analyze the QRS axes of regularly conducted beats as well as PVCs. QRS axes were determined visually by two physicians with the help of the hexaxial reference system to an accuracy of 10° on the one hand, and were calculated using three previously described formulas based on QRS voltages on the other.

A total of 216 QRS complexes from 108 patients were analyzed (108 regularly conducted beats and 108 PVCs). Estimated QRS axes of regularly conducted beats and PVCs were 39 ± 40° and 88 ± 15°, respectively. Calculated QRS axes of regularly conducted beats according to the three formulas were 37 ± 40°, 36 ± 40° and 35 ± 38°, respectively. Calculated QRS axes of PVCs according to the three formulas were 87 ± 15°, 87 ± 14° and 86 ± 16°, respectively. Correlation coefficients showed strong correlations between the estimated and the three calculated values for regularly conducted beats (0.98, 0.97 and 0.98) and PVCs (0.94, 0.94 and 0.94).

A sophisticated visual estimation of the QRS axis correlates well with voltage-based calculations and can therefore be considered sufficient for most purposes.