The left-hand column shows the development of the form of the QRS complex in VG in lett bundle branch block. The direction of depolarisation of the interventricular septum is from right to left instead ol the normal left to right. As a result. the normal (septal. i.e. Phase 1) q wave is absent from Vg. Since the initial ventricular depolarisation does not follow the usual rapid conducting pathways. the total ORS duration is prolonged. When left bundle branch block is complicated by inlarction ol the interventricular septum (right-hand column). no voltage is produced during spread of the depolarisation through the septum ( in the same way as the R wave is lost lrom precordial leads in anterior inlarction). There is. therefore. no deflection corresponding to Phase 1 of ventricular