Right and left ventricular pressure curves were recorded simultaneously with the electrocardiogram in the dog. Control observations were made during sinus tachycardia with normal intraventricular conduction, and repeated after production of right and left bundle-branch block and after production of complete heart block with idioventricular rhythm. The mode relationship between onset of isometric contraction in the two ventricles was 0.00 seconds during sinus tachycardia. During right bundle-branch block, left ventricular contraction started 0.01 second prior to the right; during left bundle-branch block, right ventricular contraction started 0.02 second prior to the left. During left ventricular idioventricular rhythm, left ventricular isometric contraction started 0.01 second prior to the right; during right ventricular idioventricular rhythm, right ventricular contraction started 0.01 second prior to the left. These changes in relative onset of ventricular contraction are far less than those expected on theoretical grounds. Submitted on March 26, 1959
