Activation of the hypertrophic right ventricle in the dog

was produced in six dogs following systolic overload of the right ventricle by surgical banding of the pulmonary trunk. Activation of the myocardium and specialized conducting tissue of the right ventricle was studied using intramural multi-electrodes or an exploring electrode, first with the heart in situ and then with the heart in a modified Langendorff perfusion circuit. Normal epicardial and intramural activation patterns were found in RVH, and the prolonged excitation time was found to be due to the increased muscle mass. No delay in activation was found in any part of the specialized conducting tissue of the right ventricle. Late activated Purkinje fibres were found in the outflow tract of the right ventricle in the dogs with hypertrophy and in a control series of normal dogs. The present electrocardiographic criteria for complete and incomplete right bundle branch block (RBBB) are based on widening of the QRS coniplex and the rSR pattern. This type of change can occur also in RVH. Because it has been shown here that no delay occurs in the specialized conducting system of the right ventricle in RVH secondary to systolic loading, the application of criteria to electrocardiograms that also fulfil the criteria for RVH may be misleading. I t is suggested that in such cases the diagnosis of a conduction disturbance must also depend upon other methods, and that the terms ‘incomplete ’ and ‘complete ’ RBBB should be handled with care i n this context. In patients with proven right ventricular hyper-trophy ( R V H) the electrocardiogram frequently fulfils the criteria for both RVH and right bundle branch block (RBBB). This has clearly cause