Depolarization of the heart has been divided into five phases (Figure 7, Table 11): The first is atrial (both right & left) depolarization where the mean vector is (+) 60 degrees. Following this phase, is AV, His bundle, and Purkinje fiber conduction to the ventricles which is electrically silent.
The second phase is septal depolarization, where the vector swings sequentially from (+) 180 degrees toward (+) 60 degrees, the mean vector being about (+) 150 degrees. This produces a Q wave in some leads.
The third phase is depolarization of the heart apex, giving a vector of about (+) 60 degrees.
Following this, depolarization proceeds up the heart wall toward the base, and consequently the vector changes from (+) 60 degrees to as much as (-) 30 degrees. Depolarization of the ventricle wall produces an R wave in most leads, while late depolarization often produces an S wave.
The overall mean angle of depolarization of the normal heart (i.e. electrical axis) is approximately (+) 60 degrees, with a range from (-) 20 degrees to (+) 105 degrees (Figure 8). The relative effect of the electrical changes in any lead at any time depends both on the vector and on the mass of tissue depolarizing or repolarizing. The large shift toward the left in Phase 4 is due to the much greater mass of the left ventricle relative to the right ventricle.
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