Atrial Septal Defect

Introduction

Atrial septal defects (ASDs) occur in 0.1% of the population and represent the largest group of congenital defects in the adult population. Echocardiographic evaluation of ASDs should include characterization of the defect, evaluation for additional associated lesions, and description of the physiologic effect of the ASD. Here we characterize a standardized approach as well as highlight some common pitfalls.

Anatomy/Embryology

Atrial septation occurs early in embryologic formation and is nearly complete by 2 months’ gestation. Embryologic formation of the atrial septum is a complex series of changes but is simplified here. Septation occurs with the formation of two membranes. The first membrane that occurs on the left atrium (LA) side is called the septum primum, and the second membrane is the septum secundum. Initially, these membranes function to allow for continuous inferior vena cava (IVC)-LA flow in utero (via the foramen ovale). After birth, the membranes fuse (in the majority of patients) to form the fossa ovalis.

Patent Foramen Ovale

In most patients, the foramen ovale closes by the second month of life. In adulthood, persistence of a patent foramen ovale (PFO) is a variant existing in 20%–25% of the general population. Although the clinical importance of PFO is unclear, there are some specific circumstances when diagnosis is important. Examples include patients with right atrial hypertension (e.g., pulmonary hypertension), pre-cardiopulmonary bypass, in anticipation of procedures that require interatrial access (such as left atrial ablation), and in patients with recurrent embolic events. On transthoracic echocardiogram (TTE), diagnosis can be made with either color Doppler or agitated saline contrast.

• 1.

  Color Doppler: On TTE, the interatrial septum is best imaged in the apical four-chamber view, subcostal four-chamber, and the parasternal short-axis view at the aortic valve level. However, because of the frequently poor color Doppler signal in the subcostal four-chamber position in adults, this image can have low sensitivity, particularly in the setting of abdominal obesity. Color flow typically has a tunnel-like appearance. On transesophageal imaging, the atrial septum is best imaged in the bicaval view. Typically, a clockwise rotation of the probe, which scans the septum and fossa ovalis from left to right, is needed to detect and identify the exact location of the PFO. In both TTE and transesophageal echocardiography (TEE), a low Nyquist limit with a high frame rate is required to detect the low velocity, intermittent flow of a PFO. Video 43.1 shows an example on TTE. (A TEE example is shown in Chapter 17 , Fig. 17.8 , and Video 17.2 .)

• 2.

  Agitated saline contrast: using the same views for color Doppler, saline contrast can be used to determine the presence of an interatrial connections. Imaging should begin prior to the opacification of the right atrium (RA). PFO flow typically occurs 3–5 beats after right ventricle (RV) opacification. In contrast, ASD flow is even faster, almost instantaneous ( Video 43.2 ). In contrast, extracardiac shunting occurs after five beats. In patients with very low right atrial pressure, additional maneuvers such as Valsalva are requisite to increase the RA pressure enough to force right to left flow of the saline contrast across the septum.

Clinical Presentation of Atrial Level Defects

Unrepaired atrial level defects presenting in adulthood can be found incidentally, as many patients are asymptomatic. If symptomatic, the most common complaints include dyspnea, fatigue, palpitations, and chest pain. Clinical findings suggestive of an unrepaired atrial level defect include a pulmonary flow murmur and fixed split S2. On electrocardiogram (ECG) patients often have right bundle branch block.