Shown here are the settings in which right bundle branch block most commonly occurs. The association with atrial septal defects and with Ebstein's anomaly, in which there is displacement of the tricuspid valve into a poorly developed right ventricle, is in the range of 90%. Thus, these diagnoses should be questioned if RBBB is not present. The functional category includes the occurrence of RBBB in association with early premature beats and sudden changes in heart rate. Note also that RBBB may occur in the absence of any recognized cause.