Effect of caval occlusion on the magnitude of unipolar atrial electrograms in dogs

Studies were done on eight mongrel dogs anesthetized with pentobarbital sodium to assess the effects of acute changes in heart volume on the magnitude of electrocardiographic potentials recorded from the right atrium. Following midsternotomy, acute changes in heart volume were produced by occlusion of the superior vena cava (SVC), inferior vena cava (IVC), or both vena cavae while heart rate was controlled by pacing the right atrium. P-wave area, defined by integrating absolute value of the P wave over time, increased by 8.2 +/- 7.4% during SVC occlusion, 13.3 +/- 8.5% during IVC occlusion, and 23.6 +/- 16.2% during occlusion of both vena cavae. Areas of ventricular complexes appearing in atrial electrograms decreased by 6.6 +/- 6.2% during SVC occlusion, 11.1 +/- 6.7% during IVC occlusion, and 15.2 +/- 9.2% during occlusion of both vena cavae. These results are compatible with predominately tangential activation of the atrium and radial excitation of the ventricle and provide experimental confirmation of the "Brody effect."