pulmonary venous and vestibular components of the divided left atrium There is continuous flow at high velocity, suggesting severe stenosis and pulmonary venous hypertension secondary to peak and mean gradients of 44 and 19 mm Hg, respectively Differential Diagnosis In infants and children, this lesion must be considered when there is unexplained pulmonary hypertension or pulmonary congestion, although careful echocardiographic assessment usually establishes the diagnosis promptly In older children, adolescents, and adults, in whom echo windows may be more challenging, either transesophageal echocardiography or tomographic imaging should be considered if the diagnosis cannot be excluded Treatment The only appropriate treatment is surgical The dividing partition is resected on cardiopulmonary bypass This is usually achieved by a right atrial incision, visualizing the partition through the oval foramen or an atrial septal defect The shelf can also be removed by a left atrial incision Anecdotally, surgery carries an excellent prognosis, although reports of long-term outcome are lacking Division of the Morphologically Right Atrium (Video 30.1) Hemodynamically significant division of the morphologically right atrium is considerably rarer than that of the left It results from persistence of the valves of the embryonic systemic venous sinus.1,10 The dividing partition is therefore placed between the systemic venous sinus and the distal part of the right atrium, made up of the vestibule and appendage The embryonic valvar structures, whose significance during fetal life is to direct the richly oxygenated inferior caval venous blood across to the left atrium and thence to the aorta, normally regress in late fetal life and early childhood They persist as the eustachian and thebesian valves, the valves of the inferior caval vein and coronary sinus, respectively These valves can retain their fetal proportions in abnormal conditions and divide the right atrium The dividing partition can itself be fenestrated and not produce major obstruction to the flow of blood This is termed a Chiari network and usually does not cause problems (Fig 30.6) If, in contrast, the persisting valvar structures are not fenestrated, they can produce partitions within the right atrium Most examples seen in postnatal life have coexisted with atresia or stenosis of the pulmonary valve or with tricuspid atresia (Fig 30.7).10 FIG 30.6 Heart showing valves of the embryonic venous sinus that have persisted in postnatal life, but have become fenestrated, producing the socalled Chiari network