
Original articles (abstract) 
Volume
 47 No. 2 : 2003 

Indian
J Physiol Pharmacol 2003; 212  216
Simplified
calculation of mean QRS vector (mean electrical axis of heart)
of electrocardiogram
P. N. Singh* and M. Sajjad Athar**
* Department of Physiology,
Faculty of Medicine,
and
** Department of Physics,
Faculty of Science,
A.M.U. Aligarh – 202 002
( Received on April 16, 2002 )
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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 10°. 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 Swave 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 LeadI, 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 Swave 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 LeadI and LeadIII.
The formula derived is as follows, , where I and III represent
net voltage in LeadI and III, q = angle subtended with the
axis LeadI. The value of q can be found by using scientific
calculator or the table. In case net voltage of QRS complex
in LeadI being negative, the value of the q should be subtracted
from 180° to find the angle of mean QRS vector.



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