Physiology lecture on cardiovascular system physiology explains Electrical axis of the heart or cardiac axis is average direction of depolarisation of heart.
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It is generally determined from ECG’s of lead I and lead aVF.
For calculation, we need to
1)Determine voltage of QRS complex in these leads by subtracting the amplitude of R wave with most negative wave (which can be either Q wave or R wave)
2) Mark them on a paper along with lead direction and determine the electrical axis
3)Drop a perpendicular from the marked points this points. These perpendiculars will meet at some point. Now from the centre draw an arrow to meet the point of the intersection.
The angle of this central line from the horizontal is the electrical axis of heart.
Normal range of this cardiac axis is -30 degrees to +110 degrees. If the axis is more negative than -30 degrees, it is left axis deviation and if it is more positive than +110 degrees, it means right axis deviation.
As a shortcut,
If both lead I and avF QRS complexes are positive..it is normal axis
If Lead I -ve, avF +ve, it is RAD
If Both -ve, extreme axis
If Lead I +ve and avF -ve… look at lead II.. if lead II is positive, it is normal axis, if -ve it is left axis deviation.
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