• Heart is positioned such that the the inferior border is facing towards your left. View of inflow tract of rt. V..

• Note the thin transluscent tricuspid valve between the right ventricle below and atrium above.

• Note the 2 defects (arrow) just above the T.valve ring.

• Note the curved rim of the fossa ovalis just above the handle of the arrow. This is the inferior margin of the septum secundum, with the septum below it being of septum primum origin.

• This is therefore a defect of the septum primum type.

•Unknown in > 90% of cases. Identifiable chromosomal abnormalities in 5% of cases, i.e. trisomies 18 & 21.

•Clear that etiology not entirely genetic, since only one of monozygotic twins may develop an anomaly, and rubella in the mother during the first trimester leads to congenital heart disease (CHD) and a variety of defects in other systems, including immunodeficiencies.

•Definite evidence of radiation, and chemicals also being etiologic factors.

•Septum primum, septum secundum, and the sinus venosus are involved anlagen in three types of defects.

•Failure of S. primum to form completely results in a low ASD adjoining AV valve.

•Failure of normally partially formed S. secundum to reach antero-inferiorly far enough, &/or excessive S. primum resorption results in fossa ovalis ASD^s.

•Primitive sinus venosus contributes to atrial walls, septum, ostia of superior and inferior vena cavas and pulmonary veins.

•Defects form high or low in septum near V. cava ostia.

•ASD found in 5% of all cases of CHD.

•CHD most common form of heart disease in childhood.

•Incidence is 6-8/1000 live full term births.

•Incidence higher in stillborns and premature births.

•90% of ASD^s are of the septum secundum type, and 5% of the septum primum type.

•A left to right atrial shunt increases pulmonary blood flow, but may be asymptomatic throughout life if the defect is much less than 1 cm in diameter.

•Arrhythmias and a murmur developing in the third decade with larger defects prompt evaluations for corrective low risk surgery to prevent later potential pulmonary hypertension.

•Chronic 2-4 X normal pulmonary blood flow may provoke pulmonary hypertension with right to left shunt and heart failure in less than 10% of cases.

•Septum primum defects are adjacent to AV valve ring, antero-inferior to fossa ovalis.

•Septum secundum defects form within the fossa ovalis or along its superior margin if S. secundum development is incomplete.

•Sinus venosus defects are located immediately adjacent to the ostia of the superior or inferior vena cavas, and are often associated with anomalies of pulmonary venous drainage into the right atrium or superior vena cava.

•Individual layers of involved structures are histologically normal.

• Cotran RS et. al.: Robbins Pathologic Basis of Disease. 5th edition. Philadelphia, W.B. Saunders, 1994, pp. 573-4.

• Harrison^s Principles of Internal Medicine, 13th Edition: Isselbach et. al. (eds). New York, McGraw-Hill, 1994, pp. 1040-1.

• Current literature from PubMed at National Library of Medicine