Before discussing the hemodynamic abnormalities of left-to-right shunt lesions, knowledge of the model that will be used throughout this section is helpful. Figure 9-1 is a block diagram of a normal heart in which one arrow represents a “unit” of normal cardiac output. It is assumed that the cardiac chambers and great arteries and veins indicated by one arrow are normal in size. If a cardiac chamber or great artery has more than one arrow in it, that chamber or blood vessel is going to be dilated. A diagram of a normal cardiac roentgenogram is presented in Chapter 4 (see Fig. 4-2 ). Modifications in appearance of chest roentgenogram secondary to enlargement or reduction of cardiac chambers or great vessels are presented in diagrammatic drawings to aid in the interpretation of chest radiograph films.
Atrial Septal Defect
In acyanotic patients with atrial septal defects (ASDs), the direction of the shunt is from left to right and the magnitude of the left-to-right shunt is determined by the size of the defect and the relative compliance of the right ventricle (RV) and left ventricle (LV). Because the compliance of the RV is greater than that of the LV, a left-to-right shunt is present. The magnitude of the shunt is reflected in the degree of cardiac enlargement. Let it be assumed that there is a left-to-right shunt of one arrow at the atrial level. As seen in Figure 9-2 , the right atrium (RA), RV, and main pulmonary artery (PA) and its branches have two arrows and are therefore dilated. These findings are translated into the chest radiographs ( Fig. 9-3 ), which reveal enlargement of the RA, RV, and PA, as well as an increase in pulmonary vascular markings. Note that the left atrium (LA) is not enlarged (see Figs. 9-2 and 9-3 ). This is because the increased pulmonary venous return to the LA does not stay in that chamber; rather, it is shunted immediately to the RA. The absence of LA enlargement is one of the helpful radiographic signs for differentiating an ASD from a ventricular septal defect (VSD) in patients with increased pulmonary vascularity.
The dilated RV cavity prolongs the time required for depolarization of the RV because of its longer pathway, producing either complete or incomplete right bundle branch block (RBBB) pattern (with rsR′ in V1) in the electrocardiogram (ECG). The RBBB pattern in children with ASDs is not the result of actual block in the right bundle. If the duration of the QRS complex is not abnormally prolonged, the ECG may be read as mild right ventricular hypertrophy (RVH). Therefore, either (complete or incomplete) RBBB pattern or mild RVH is seen on the ECG of children with ASD.
The heart murmur in ASD is not caused by the shunt at the atrial level. Because the pressure gradient between the atria is so small and the shunt occurs throughout the cardiac cycle, both in systole and diastole, the left-to-right shunt is silent. The heart murmur in ASD originates from the pulmonary valve because of the increased blood flow (denoted by two arrows) passing through this normal-sized valve, producing a relative stenosis of the pulmonary valve. Therefore, the murmur is systolic in timing and is maximal at the pulmonary valve area (i.e., at the upper left sternal border). When the shunt is large, increased blood flow through the tricuspid valve (denoted by two arrows) results in a relative stenosis of this valve, producing a mid-diastolic murmur at the tricuspid valve area (i.e., lower left sternal border). The widely split S2 that is a characteristic finding in ASD results partly from RBBB. The RBBB delays both the electrical depolarization of the RV and the ventricular contraction, resulting in delayed closure of the pulmonary valve. In addition, the large atrial shunt tends to abolish respiration-related variations in systemic venous return to the right side of the heart, resulting in a fixed S2.
It should be noted that infants and small children rarely manifest with clinical findings described above even in the presence of a moderately large ASD (proved by echocardiographic studies) until they are 3 to 4 years of age. It is because the compliance of the RV improves slowly so that any significant shunt does not occur until that age.
Children with ASD rarely experience congestive heart failure (CHF) even in the presence of a large left-to-right shunt. The PAs can handle an increased amount of blood flow for a long time without developing pulmonary hypertension or CHF because there is no direct transmission of the systemic pressure to the PA, and PA pressure remains normal. However, CHF and pulmonary hypertension eventually develop in the third and fourth decades of life if the shunt is large.
Ventricular Septal Defect
The direction of the shunt in acyanotic VSD is left to right. The magnitude of the shunt is determined by the size, not the location, of the defect and the level of pulmonary vascular resistance (PVR). With a small defect, a large resistance to the left-to-right shunt occurs at the defect, and the shunt does not depend on the level of PVR. A decrease in the PVR occurs normally in this situation. With a large VSD, the resistance offered by the defect is minimal, and the left-to-right shunt depends largely on the level of PVR. The lower the PVR, the greater the magnitude of the left-to-right shunt. This type of left-to-right shunt is called a dependent shunt (in contrast to obligatory shunt , to be discussed later in this chapter). Even in the presence of a large VSD in a newborn, the PVR remains elevated, and therefore a large shunt does not occur until the infant reaches 6 to 8 weeks of age, when the shunt increases and CHF may develop.
In a VSD of moderate size, the cardiac chambers or vessels with two arrows enlarge, resulting in enlargement of the main PA, LA, and LV, as well as an increase in pulmonary vascular markings ( Fig. 9-4 ). In VSD, it is the LV that does volume overwork, not the RV. This results in LV enlargement; the RV does not enlarge. Because the shunt of VSD occurs mainly during systole when the RV also contracts, the shunted blood goes directly to the PA rather than remaining in the RV cavity. Therefore, there is no significant volume overload to the RV, and the RV remains relatively normal in size in a VSD with moderate shunt ( Figs. 9-4 and 9-5 ). It should be noted that LA enlargement is present only with VSD but not with ASD. It should also be noted that both VSD and PDA produce an enlargement of the LA and LV.
