Page 12, Electrocardiography, Dr. D. Penney
The ECG (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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