The relative simplicity of the clinical presentation and management of an atrial septal defect belies the complexity of the developmental pathogenesis. Here, we describe the anatomic development of the atrial septum and the venous return to the atrial chambers. Experimental models suggest how mutations and naturally occurring genetic variation could affect developmental steps to cause a defect within the oval fossa, the so-called secundum defect, or other interatrial communications, such as the sinus venosus defect or ostium primum defect.

Although atrioventricular septal defects are categorized according to the anatomical atrioventricular orifice, the location of the intracardiac shunt in atrioventricular septal defects is important from a surgical perspective. Herein, we report three cases of atrioventricular septal defects with a small or no ostium primum defect. Case 1 (3-month-old girl) was diagnosed preoperatively with a ventricular septal defect, secundum atrial septal defect, and mitral valve cleft. After the operation, the diagnosis was corrected to an atrioventricular septal defect and was repaired completely. Case 2 (9-year-old girl) underwent pulmonary artery banding for a ventricular septal defect with a straddling mitral valve. After the experience with Case 1, we realized similarities between Cases 1 and 2. Therefore, we corrected the diagnosis to atrioventricular septal defect and achieved definitive repair. Based on these experiences, we accurately diagnosed Case 3 (3-month-old boy) with an atrioventricular septal defect. This variant is poorly known; however, proper morphological understanding is necessary to facilitate anatomical repair and prevent postoperative atrioventricular blocks. Some cases of this variant may be diagnosed as a ventricular septal defect with straddling mitral valve and are unable to receive definitive repair. The direction of the cleft, absence of atrioventricular valve offsetting, and trileaflet of the left atrioventricular valve all seem useful for making a diagnosis, and these can be easily confirmed by echocardiography.

Trifoliate left atrioventricular (AV) valve with common atrioventricular junction is considered part of the spectrum of atrioventricular septal defect. This valve morphology is typically associated with defects in the AV septum resulting in communication at the atrial or ventricular level, but has also been described as an isolated defect in the setting of a common AV junction without AV septal defect. Trifoliate left AV valve exhibits a line of apposition between the bridging leaflets that is directed toward the inlet interventricular septum, distinguishing it from isolated mitral valve cleft in which the orientation of the bridging leaflets are toward the left ventricular outflow tract. The echocardiographic findings of four dogs with trifoliate left AV valve are described; two with intact septal structures and two with large ostium primum defects. Three dogs underwent open surgical repair using different approaches depending on the presence or absence of a septal defect. One of these underwent concurrent surgical repair for right AV valve dysplasia. One dog with intact septal structures underwent interventional closure of a concurrent patent ductus arteriosus. Current terminology associated with trileaflet left AV valve malformations is reviewed.

Atrial septal defects are common congenital heart defects, characterized by insufficient/absent tissue at the interatrial septum. An unrepaired defect may be associated with right heart volume overload, atrial arrhythmia or pulmonary arterial hypertension. The 3 major types of atrial septal defect are: ostium secundum defect, ostium primum defect, and sinus venosus. Characteristic physical findings include a midsystolic pulmonary flow or ejection murmur, accompanied by a fixed split-second heart sound. Small defects may spontaneously close; larger defects may persist and result in hemodynamic and clinical sequelae requiring percutaneous or surgical intervention. Severe pulmonary arterial hypertension is a contraindication to closure.

Atrial septal defect (ASD) is a common congenital abnormality that occurs in the form of ostium secundum, ostium primum, sinus venosus, and rarely, coronary sinus defects. Pathophysiologic consequences of ASDs typically begin in adulthood, and include arrhythmia, paradoxical embolism, cerebral abscess, pulmonary hypertension, and right ventricular failure. Two-dimensional (2D) transthoracic echocardiography with Doppler is a central aspect of the evaluation. This noninvasive imaging modality often establishes the diagnosis and provides critical information guiding intervention. A comprehensive echocardiogram includes evaluation of anatomical ASD characteristics, flow direction, associated abnormalities (eg, anomalous pulmonary veins), right ventricular anatomy and function, pulmonary pressures, and the pulmonary/systemic flow ratio. The primary indication for ASD closure is right heart volume overload, whether symptoms are present or not. ASD closure may also be reasonable in other contexts, such as paradoxical embolism. ASD type and local clinical expertise guide choice of a percutaneous versus surgical approach to ASD closure.