Page 12, Electrocardiography, Dr. D. Penney

The ECG (continued...)

Analysis: (continued..)

A method of measuring mean electrical axis of the ventricles more precisely involves using vector analysis. Leads I and aVF are (conveniently) at 90 degrees to each other and so form good bases for vector analysis. Leads I and III may also be used (see Scheidt, 1983).

a) Measure the net positive deflection of the QRS waves in leads I and aVF (Figure 9). Net positive deflection = height above baseline - height below.

b) Construct two perpendicular lines representing leads I and aVF (Figure 10).

c) Mark the net positive deflection height for each lead on the respective vector line (these may be scaled up).

d) Construct perpendicular lines at these points such that they intersect.

e) The line joining the intersection of the lead lines with the intersection of the constructed lines is the axis of depolarization of the ventricles.

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