PSIO 532 Study Guide - Midterm Guide: Left Axis Deviation, Left Ventricular Hypertrophy, Ventricular Hypertrophy
Graduate Physiology PSL
Mean Electrical Axis
➔ Explain the functional significance of the mean electrical axis and means to identify it
Identifying the Mean Electrical Axis
▪ The orientation of the wave of depolarization (as well as repolarization)
can be represented with an arrow called a vector
▪ The ea electrical ais is the average of all the electrical activit of
the cardiac chamber muscle mass (average of all the individual vectors); we
know that for ventricular depolarization, the main vector is travelling towards
the positive electrode (Lead II) → measures the average electricity of all the
tissues
▪ We use the LIMB leads to help determine the Mean Electrical Axis
▪ These arrows are used to create an axis of the heart; all the arrows are shifted
towards where the heart would actually be; so this would be the reference
point we are going to be using
▪ Each of these leads will be represented by the angle by which it
approximates the position of the heart
▪ In general, the normal QRS mean electrical axis is between -30⁰ and
+90⁰
▪ However, there are conditions that can cause physical change in the
position of the heart and the mass of the tissue that is depolarizing will also
affect this axis
▪ Less than -30°= left axis deviation → be pathologic and occur in left ventricular
hypertrophy
▪ Greater than +90°= right axis deviation → can also be pathologic in cases of right
ventricular hypertrophy especially when due to severe pressure overload
▪ Look at the limb leads: I, II, III, aVR, aVL, aVF
▪ Enision the leads as a circle lying on the patients’ chest; the circle is diided into
12 segments
▪ Each segment is 30 degrees
▪ In order to figure out the Mean Electrical Axis (MEA):
□ Look for two leads, one with the most isoelectric lead
□ Isoelectric lead: isoelectric equally positive & negative; equal in
magnitude up and down; looking for leads that are mostly just going
up and down
□ So when you find that first isoelectric lead, for example Lead III, you
dra a line through that lead on the patient’s circle and then draw a
line exactly perpendicular to that Lead III line
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