Slowing of the rate at which QRS complexes appear on the body surface electrocardiogram, or the presence of unexpected pauses between QRS complexes, is usually due to a decrease in the rate at which the pacemaker cells spontaneously depolarize or a failure of supraventricular impulses to propagate to the ventricles. Such failures of impulse propagation may occur in the region surrounding the sinus node, in which case they are referred to as sino-atrial or SA block, or in the atrio-ventricular conducting system comprised of the AV node and the His-Purkinje system, in which case they are referred to as atrio-ventricular or AV block.