Atrial Septal Defects

Objectives
 
At the completion of this chapter the user will be able to:
 
  • Describe Anatomic Features of Common Adult Congenital Heart Defects
  • Discuss Pathophysiology of Common Adult Congenital Heart Defects
  • Evaluate the Echocardiographic Findings of Common Adult Congenital Heart Defects
 
Introduction
 

Transesophageal echocardiography (TEE) is an important intraoperative diagnostic imaging tool for the perioperative management of adult congenital heart defects. Information obtained from TEE allows for confirmation of preoperative diagnoses, detection of unsuspected findings, and assessment of the surgical repair. This chapter will review the anatomic and hemodynamic TEE assessment of common adult congenital heart defects such as atrial septal communications, ventricular septal defects, left ventricular outflow tract obstruction, patent ductus arteriosus and coarctation of the aorta.

 

Atrial Septal Defects
 
Embryology of the Atrial and Ventricular Septums
 

Within the atria, the migration of the atrial septum creates a separate right and left atria leads to the characteristic appearance of the foramen ovale, with the possibility of a patent foramen ovale. The septum primum and ventricular septum grow towards and fuse with the endocardial cushion.  The septum primum obliterates the fossa ovalis.  The septum primum base dissolves forming an ostium secundum.  The septum secundum grows towards the endocardial cushion and obliterates the ostium secundum.   Failure of the septum primum and septum secundum to fuse forms a patent foramen ovale.  A patent foramen ovale is present in 25-30% of patients. An ostium primum defect forms when the septum primum fails to fuse with the endocardial cushion.  An ostium secundum defect forms when the septum secundum fails to form. 

The interventricular septum grows and fuses with the endocardial cushion.  Failure of fusion of the ventricular septum with the endocardial cushion or failure to form anywhere along it's growth path will cause a ventricular septal defect.

Embryologic Formation of Atrial Septums
  

Anatomic Features
 
Atrial septal communications account for up to one-third of all cases of congenital heart defects in the adult population. Atrial septal defects (ASD) are communications that occur in interatrial septum and are associated with variable degrees of interatrial shunting. Anatomically, the defects are classified according to their embryonic origin and include: ostium secundum, ostium primum or sinus venosus defects.
Atrial Septal Defects
 
Ostium Secundum Defect
 
Ostium secundum defects are the most common of the ASD, comprising 75% of all defects.   The ostium secundum defect is located in the central part of the atrial septum in the region of the fossa ovalis.   In some cases, this defect is associated with mitral valve prolapse and mitral regurgitation.
Ostium Secundum Defect 1X 2X 3X 4X
Normal Interatrial Septum 1X 2X 3X 4X

Ostium Primum Defect

  
Ostium primum defects comprise 15% of ASD. The ostium primum defect can be categorized in the grouping of the atrioventricular (AV) septal defects also known as partial atrioventricular septal defect, atrioventricular canal, or endocardial cushion defects. These defects are due to an absence of tissue in the inferior aspect of the atrial septum and may include abnormal development of the atrioventricular septal region and valves. Abnormalities associated with primum defects include a cleft anterior mitral leaflet with variable degrees of mitral regurgitation, inlet ventricular defect, and partial attachment of the septal leaflet of the mitral valve to the interventricular septum. Most primum ASD are relatively large and lead to right heart dilation.
Ostium Primum Defect 1X 2X 3X 4X   
Normal Interatrial Septum 1X 2X 3X 4X

Sinus Venosus Defect

  
Sinus venosus defects are usually located high in the atrial septum where the superior vena cava enters the right atrium and make up 10% of all atrial septal communications. The superior vena cava may override the interatrial septum. This defect is frequently associated with partial anomalous pulmonary venous return of the right pulmonary veins to the superior vena cava or right atrium. Sinus venosus defects may be superior or inferior vena caval type, however superior vena caval types are the most common. The uncommon inferior vena cava form of the sinus venosus defect is anatomically located at the inferior vena caval-atrial junction.
Sinus Venosus Defect 1X 2X 3X 4X
Normal Heart Images 1X 2X 3X 4X
  

Coronary Sinus Septal Defect

  
The 'unroofed' coronary sinus septal defect is a defect between the coronary sinus and the left atrium leading to left-to-right shunting through an 'unroofed' coronary sinus. This frequently is found in association with a persistent left superior vena cava.
script
Unroofed Coronary Sinus Septal Defect
Pathophysiology
 

Regardless of the type of ASD, physiologic consequences result from interatrial shunting of blood. Direction and amount of shunting depends on the defect size, filling properties of the ventricles and the pulmonary and systemic vascular resistance. For example, defects which are considered small (those less than 0.5 cm in diameter) are associated with little shunting and almost no hemodynamic sequelae.  Whereas, larger defects (those greater than 2 cm in diameter) may be associated with considerable shunting and hemodynamic sequelae.

Shunt direction is typically directed from left to right because the right ventricle is more compliant in the adult patient. As a consequence of the shunting there is increased pulmonary blood flow and subsequent dilation of right sided structures. Increased pulmonary vascular resistance and pulmonary artery pressures may develop over time with the augmented pulmonary blood flow. Changes in right ventricular compliance over time may lead to decrements in left-to-right shunting and right-to-left shunting may ensue.

A left-to-right atrial shunt is considered significant when the pulmonary to systemic flow (Qp/Qs) ratio is greater than 1.5/1.0 or if it causes right sided dilation. An ASD with a ratio of pulmonary-to-systemic flow of 1.5 or more, significant right ventricular dilation, and progressive pulmonary hypertension are indications for closure. The presence of high pulmonary vascular resistance however, may be a contraindication to surgery ( > 10 Woods units/m2 or >7 Woods units/ m2 with vasodilators).


(Woods Unit = (Mean PAP - PCWP ) / CO)