Most secundum and primum atrial septal defects (ASD) are easily visualized by echocardiography, although sinus venous defects are often difficult to detect without TEE.559,560 Apical echocardiographic views often show artifactual "dropout" in the region of the fossa ovalis, since the interatrial septum is thin in this area and runs parallel to the ultrasound beam. Therefore, the subcostal view provides the optimal imaging plane to detect lesions of the atrial septum.561 Ostium secundum defects are the most common form of ASD, and 2D imaging shows a localized absence of septal tissue in the midportion of the interatrial septum (Q-hB; Fig. 13-11 OA Plate 69). Lack of any interatrial septal tissue between the defect and the base of the interventricular septum characterizes an ostium primum defect (&> Figure 13-110,5). Although ostium secundum defects are usually isolated, ostium primum (or partial AV canal) defects are often accompanied by other lesions, such as cleft anterior mitral valve leaflet, MR, and atrioventricular canal ventricular septal defect.562 Sinus venosus defects are strongly associated with partial anomalous pulmonary venous return (for example, drainage of the right upper pulmonary vein into the right atrium or superior vena cava) (Fig. 13-111). Rarely, the atrial septum may be completely absent (B+;0i Fig. 13-112). With all but small ASDs, the right atrium is enlarged and RV volume overload is present, with a dilated RV and paradoxical septal motion.563
Intravenous contrast injection generally demonstrates shunting across the ASD, frequently with bidirectional flow.564 Therefore, "negative jets" of unopacified flow from the left atrium into the contrast-filled right atrium may alternate with the appearance of contrast bubbles flowing through the defect into the LA. When an ASD is present, contrast should appear quickly (within three to five heartbeats) in the LA after entering the right atrium. Delayed appearance of contrast in the LA may indicate an intrapulmonary shunt rather than an ASD.
Color Doppler imaging is also useful for detecting flow through ASDs (B-hB; Fig. 13-11 OA Plate 69), although the pressure drop between atria often does not produce turbulence. Inflow from the inferior vena cava and right-sided pulmonary veins may be prominent in normals and can be misinterpreted as a shunt.565,566 Pulsed-wave Doppler recordings usually reveal continuous flow, which peaks in late systole. Pulmonary-to-systemic flow ratios can be estimated in ASD (and ventricular septal defects) by comparing volumetric flow measurements through the LV and RV outflow tracts. Such calculations are only moderately accurate in adults.567,568
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.