EKG - Axis

Wonder Me! Summary:

III - Axis:           

Lead I –   QRS points upwards        = positive             = to the Left arm               = plot on X axis of Lt arm of pt

QRS points downward       = negative            = to the Right arm              = plot on –Xaxis of Rt arm of pt

Lead aVF – QRS points upwards     = positive             = down to the Left foot      = plot on downward Y axis of the Lt foot of pt

    QRS points downward   = negative            = to the head                      = plot on upward Y axis of head of pt

“Two thumbs up” rule = QRS upwards in both Lead I & lead aVF = Normal Axis = Vector to the left and downwards

Calculation of the net vector of QRS:

Lead I –

. Add the number of positive boxes (above the horizontal axis) and subtract the number of negative boxes (below).

. A positive number falls to the RIGHT of the center and a negative number falls to the LEFT of the center.

Lead aVF –  

. Add the number of positive boxes (above the horizontal axis) and subtract the number of negative boxes (below).

. Plot this on the Y axis with positive aVF pointing DOWN and negative aVF pointing UP.   

Results: Now look at these two vectors.  If they are approximately the same size, then the addition of these two vectors is around 45 degrees.  If the horizontal vector is double the size of the vertical vector, then the axis is around 30 degrees.

The next piece of information to acquire is the AXIS of the QRS complex.  Many books and physicians suggest different complex formulas and logical progressions to accomplish this task.  However, many only answer the question “Is the axis ‘normal?’”  According to the way that I was taught to read EKG’s, this was not adequate and a single number should be derived when asked to calculate the axis of the QRS complex.

To calculate the AXIS of the QRS complex simply look at leads I and aVF.  Remember that lead I points horizontally to the right (on the paper, left on the body) and lead aVF points downward on the page.  These are the only two leads that you will need to look at for this simple computation.  Figure out the “net vector” of the QRS complex in lead I.  To do this, add the number of positive boxes (above the horizontal axis) and subtract the number of negative boxes (below).  Most of the time you should end up with a positive number.  Plot your net number of boxes along an X axis.  Keep in mind that a positive number falls to the RIGHT of the center and a negative number falls to the LEFT of the center.   Then do the same thing and calculate the “net vector” for the QRS complex in lead aVF.  Plot this on the Y axis with positive aVF pointing DOWN and negative aVF pointing UP.  Now look at these two vectors.  If they are approximately the same size, then the addition of these two vectors is around 45 degrees.  If the horizontal vector is double the size of the vertical vector, then the axis is around 30 degrees.  Remember that lead I is designated to be 0 degrees.  Positive aVF is designated as 90 degrees and negative aVF (pointing up) is designated to be –90 degrees.  Using this simple method, you can calculate the AXIS of any QRS complex.  One fact to point out is that if the “net vector” turns out to be 0 (iso-electric) in either I or aVF, then you do not need to plot that on the axis.  Your final QRS axis will be either 0 degrees, 180 degrees or +/- 90 degrees depending which lead had a net vector of zero.

Source: https://kchemekg.wordpress.com/ekg-test-home/