Disturbances of conduction within or between the ventricles are termed intra and inter ventricular conduction disturbances respectively. They may be due to diffuse prolongation or slowing of conduction throughout the ventricles, to changes in the sequence of activation within the ventricles or to both. Diffuse slowing of conduction without a change in the sequence of activation can occur when the sodium inward current, the current most responsible for cellular depolarization, is decrease which. This can be caused by drugs which block the sodium channels within the cell membrane such as lidocaine and quinidine, or by an increase in the extracellular potassium concentration, which leads to a less negative resting membrane potential and inactivation of the sodium current. These changes, which will be discussed in the chapter entitled “Electrolyte Abnormalities, Drug Effects and the Long QT Syndromes”, will cause diffuse widening (i.e. prolongation) of the QRS complex, most often without a change in activation sequence or QRS morphology . Diffuse slowing of conduction without a change in the sequence of activation can also occur when cell to cell transmission of the impulse is slowed or blocked. The QRS widening associated with ventricular hypertrophy may be attributed in part to this mechanism and will also be discussed later. In this chapter, we will focus on the electrocardiographic manifestations of the conduction disturbances caused by the changes in the sequence of activation. Included in this category are the left and right bundle branch block, the fascicular blocks and ventricular pre-excitation. We will also discuss the changes in the sequence of activation that may accompany abrupt changes in cycle length. This often occurs in normal hearts and represents a functional rather than a pathologic or anatomic abnormality and is referred to as "aberrant ventricular conduction". Included in this discussion will be the secondary changes in the sequence of repolarization that must occur in association with the changes in the sequence of depolarization and that often result in T wave abnormalities. The ECG manifestations of the ventricular conduction disturbances associated with ischemia and the cardiomyopathies will be discussed in later chapters.