These tracings were recorded from the same 75 year old patient. The QRS complexes and T waves in the tracing on the left are normal. In the tracing on the right, the shape and duration of the QRS complexes are altered because of changes in the sequence of depolarization associated with the development of left bundle branch block (to be discussed in the next section). Note also the changes in the T waves. These T wave changes reflect the changes in the sequence of repolarization that are the necessary result of, and thus secondary to, the changes in the sequence of depolarization. They do not indicate a primary change in either the shape or duration of the ventricular action potential, although this cannot be excluded.