The limb leads from the tracing shown on the previous page are shown again here. Note that the QRS duration is significantly prolonged and measures 0.16 seconds. The initial septal forces are represented by the small Q wave in leads I and aVL and the small R waves in III and aVF. The frontal plane spatial vector of these forces (labeled 1 on the above figure), has an axis of approximately +110 degrees. The main QRS vector( labeled 2) reflects the R wave in lead I and the S wave in lead III. It is directed superiorly and to the left with an axis of approximately -30 degrees. The terminal vector (labeled 3) reflects the S waves in leads I and aVL and the R’ waves in III and aVR. It represents the unopposed right ventricular forces liberated by the right bundle branch block and is directed to the right with an axis of approximately +170 degrees.