Simplified [correction of Simlified] calculation of mean QRS vector (mean electrical axis of heart) of electrocardiogram.

ABSTRACT

In clinical practice assessment of the mean QRS axis (MQRSA) provides information related either with hypertrophy of the ventricles or conduction blocks. The method adopted by clinicians i.e. the inspection of the QRS voltage in six of the limb leads has inherent element of subjectivity of approximately 10degrees. Moreover, in certain condition, when there is ambiguity about differentiation of left axis deviation assessed by inspection method in to either hypertrophy of left ventricles or complete/hemi block of the left bundle branches, accurate measurement of the axis becomes necessary to arrive at the correct diagnosis. Though a formula based on area under R wave and S-wave of the same QRS complex has been derived for accurate measurement of axis, considering its use in the computer software, working with ordinary electrocardiograph the only method for accurate measurement of the QRS axis is plotting method i. e. the net voltages in Lead-I, and III on their respective axes which is not practicable in clinical settings. Although, calculation of MQRSA by area method gives an accurate assessment of MQRSA, some authors prefer measurement of axis by voltage method, as in cases of the right ventricular hypertrophy with a broad S-wave calculation of axis by area method may give erroneous results. Hence, to obtain correct measurement of MQRSA, we have derived a simplified formula based on the net voltage of QRS complexes in Lead-I and Lead-III. The formula derived is as follows, Tan(theta) =(I + 2III) divided by sqrt [3I], where I and III represent net voltage in Lead-I and III, theta = angle subtended with the axis Lead-I. The value of theta can be found by using scientific calculator or the table. In case net voltage of QRS complex in Lead-I being negative, the value of the theta should be subtracted from 180degrees to find the angle of mean QRS vector.