Park's Pediatric Cardiology for Practitioners, 6th Ed.

Pathophysiology of Left-to-Right Shunt Lesions

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.

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FIGURE 9-1 Block diagram of a normal heart. One arrow represents a unit of normal cardiac output. AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; PV, pulmonary vein; RA, right atrium; RV, right ventricle; VC, vena cava.

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FIGURE 9-2 Block diagram of an atrial septal defect. The number of arrows in each chamber represents the amount of blood to be handled by that particular chamber. When one redraws the chambers with two arrows larger than normal, one can predict which chambers will be enlarged.

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FIGURE 9-3 Diagram of posteroanterior and lateral views of chest roentgenograms. Enlargement of the right atrium (RA) and pulmonary artery (PA) segment and increased pulmonary vascular markings are present in the posteroanterior view. The right ventricular enlargement is best seen in the lateral view. AO, aorta; IVC, inferior vena cava; LA, left atrium; LAA, left atrial appendage; LPA, left pulmonary artery; LV, left ventricle; RPA, right pulmonary artery; RV, right ventricle; SVC, superior vena cava.

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. 9-4and 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.

Figure 9-6 summarizes the hemodynamics of VSDs of varying sizes and helps in the understanding of clinical manifestations. The size of a cardiac chamber directly relates to the amount of blood (or the number of arrows) handled by the chamber. The total number of arrows in the heart diagram also determines the overall size of the heart. Let us examine Figure 9-6 for varying sizes of VSDs.

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FIGURE 9-4 Block diagram of ventricular septal defect that shows the chambers and vessels that will be enlarged. There is an enlargement of the left atrium and left ventricle. The pulmonary artery is prominent, and the pulmonary vascularity is increased. Note the absence of right ventricular enlargement (see text for explanation).

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FIGURE 9-5 Diagrams of posteroanterior and lateral views of chest roentgenograms of a moderate ventricular septal defect. Enlargement of the left atrium (LA), left ventricle (LV), and pulmonary artery (PA) and increased pulmonary vascular markings are present. Note the presence of LA enlargement, which is absent in atrial septal defect. Other abbreviations are the same as those in Figure 9-3.

With a small VSD, there is only half an arrow coming from the LV to the main PA. In addition, the degree of pulmonary vascular congestion and chamber enlargement is either minimal or too small to result in a significant change in the chest x-ray films (see Fig. 9-6). The degree of volume work imposed on the LV is also too small to produce left ventricular hypertrophy (LVH) on the ECG. The shunt itself produces a heart murmur (regurgitant systolic), and the intensity of the P2 is normal because the PA pressure remains normal.

With a VSD of moderate size, one arrow shunts from the LV to the RV, and all the chambers that are enlarged handle two arrows. Therefore, the degree of cardiomegaly on the radiographic film is significant. The volume overwork done by the LV is significant, so the ECG shows LVH of the “volume overload” type. Although the shunt is large, the RV is not significantly dilated, and the pressure in this chamber is elevated only slightly (see Fig. 9-6). In other words, in a moderate VSD, the RV is not under significant volume or pressure overload; therefore, ECG signs of RVH are absent. As in a small VSD, a heart murmur (regurgitant systolic type) is produced by the left-to-right shunt. The normal-sized mitral valve handles two arrows. This relative mitral stenosis produces a mid-diastolic rumble at the apex. The PA pressure is mildly elevated; therefore, the intensity of the P2 may increase slightly.

With a large VSD, the overall heart size is larger than that seen with a moderate VSD because there is a much greater shunt. Because there is direct transmission of the LV pressure through the large defect to the RV, in addition to a much greater shunt, the RV becomes enlarged and hypertrophied. Therefore, the chest radiograph shows biventricular enlargement, left atrial enlargement, and greatly increased pulmonary vascularity (see Fig. 9-6). The ECG shows biventricular hypertrophy (BVH) and sometimes left atrial hypertrophy (LAH). A large VSD usually results in CHF in early infancy.

When a large VSD is left untreated, irreversible changes take place in the pulmonary arterioles, producing pulmonary vascular obstructive disease (or Eisenmenger’s syndrome). It may take years to develop this condition. When Eisenmenger’s syndrome occurs, striking changes take place in the heart size, ECG, and clinical findings. Because the PVR is notably elevated at this stage, approaching the systemic level, the magnitude of the left-to-right shunt decreases. This results in removal of the volume overload placed on the LV as well as the LA. Therefore, the size of the LV and the overall heart size decrease, and the ECG evidence of LVH disappears, leaving only RVH because of the persistence of pulmonary hypertension. Although the heart size becomes small, the PA segment remains enlarged because of persistent pulmonary hypertension. In other words, with the development of pulmonary vascular obstructive disease, the heart size returns to normal except for a prominent PA segment, and a pure RVH on the ECG results. A bidirectional shunt causes cyanosis. Because the shunt is small, the loudness of the murmur decreases, or it may even disappear. The S2 is loud and single owing to pulmonary hypertension.

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FIGURE 9-6 Diagrammatic summary of the pathophysiology of ventricular septal defect. Most of the radiographic and electrocardiographic findings can be deduced from this diagram (see text for full description). LAH, left atrial hypertrophy; LVH, left ventricular hypertrophy; LVP, left ventricular pressure; PVR, pulmonary vascular resistance; RVH, right ventricular hypertrophy; RVP, right ventricular pressure.

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FIGURE 9-7 Block diagram of the heart in patent ductus arteriosus (PDA). Note the similarities between PDA and ventricular septal defect as to chamber enlargement. There is enlargement of the aorta to the level of the ductus arteriosus.

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FIGURE 9-8 Diagrams of the posteroanterior and lateral chest radiographs of patent ductus arteriosus (PDA). Note the similarities between PDA and ventricular septal defect. Abbreviations are the same as those in Figure 9-3.

Patent Ductus Arteriosus

The hemodynamics of PDA are similar to those of VSD. The magnitude of the left-to-right shunt is determined by the resistance offered by the ductus (i.e., diameter, length, and tortuosity) when the ductus is small and by the level of PVR when the ductus is large (i.e., dependent shunt). Therefore, the onset of CHF with PDA is similar to that with VSD.

The chambers and vessels that enlarge are the same as those in VSD except for an enlarged aorta to the level of the PDA (i.e., enlarged ascending aorta and transverse arch), which also handles an increased amount of blood flow (Fig. 9-7). Therefore, in PDA, chest radiographic films show enlargement of the LA and LV, a large ascending aorta and PA, and an increase in pulmonary vascular markings (Fig. 9-8). Although the aorta is enlarged, it usually does not produce an abnormal cardiac silhouette because the ascending aorta does not normally form the cardiac silhouette. Therefore, chest radiographic films of PDA are indistinguishable from those of VSD.

Hemodynamic consequences of PDA are similar to those of VSD. In PDA with a small shunt, the LV enlargement is minimal; therefore, the ECG and chest radiographic findings are close to normal. Because there is a significant pressure gradient between the aorta and the PA in both systole and diastole, the left-to-right shunt occurs in both phases of the cardiac cycle, thereby producing the characteristic continuous murmur of this condition. With a small shunt, the intensity of the P2 is normal because the PA pressure is normal.

In PDA with a moderately large shunt, the heart size is moderately enlarged with increased pulmonary blood flow. The chambers enlarged are the LA, LV, and PA segments. The ECG shows LVH as in moderate VSD. In addition to the characteristic continuous murmur, there may be an apical diastolic flow rumble as a result of relative stenosis of the mitral valve. The P2 slightly increases in intensity if it can be separated from the loud heart murmur.

In a large PDA, marked cardiomegaly and increased pulmonary vascular markings are present. The volume overload is on the LV and LA, which produces LVH and occasional LAH on the ECG. The free transmission of the aortic pressure to the PA produces pulmonary hypertension and RV hypertension, with resulting RVH on the ECG. Therefore, the ECG shows BVH and LAH, as in a large VSD. The continuous murmur is present, with an apical diastolic rumble owing to relative mitral stenosis. The P2 is accentuated in intensity due to pulmonary hypertension.

An untreated large PDA can also produce pulmonary vascular obstructive disease, with a resulting bidirectional (i.e., right-to-left and left-to-right) shunt at the ductus level. The bidirectional shunt may produce cyanosis only in the lower half of the body (i.e., differential cyanosis). As in VSD with Eisenmenger’s syndrome, the heart size returns to normal because of the reduced magnitude of the shunt. The peripheral pulmonary vascularity decreases, but the central hilar vessels and the main PA segment are greatly dilated owing to severe pulmonary hypertension. The ECG shows pure RVH because the LV is no longer under volume overload. Auscultation no longer reveals the continuous murmur or the apical rumble as a result of the shunt reduction. The S2 is single and loud due to pulmonary hypertension.

Endocardial Cushion Defect

During fetal life, the endocardial cushion tissue contributes to the closure of both the lower part of the atrial septum (i.e., ostium primum) and the upper part of the ventricular septum in addition to the formation of the mitral and tricuspid valves. The failure of normal development of this tissue may be either complete or partial. A simple way of understanding the complete form of endocardial cushion defect (ECD) is that the tissue in the center of the heart is missing, with resulting VSD, the primum type of ASD, and clefts in the mitral and tricuspid valves. In the partial form of the defect, only an ASD is present in the ostium primum septum (primum type of ASD), often associated with a cleft in the mitral valve.

Hemodynamic abnormalities of primum-type ASD are similar to those of secundum-type ASD, in which the RA and RV are dilated with increased pulmonary blood flow (Fig. 9-9). These changes are expressed in the chest radiographs (see Fig. 9-3). The cleft mitral valve is usually insignificant from a hemodynamic point of view because blood regurgitated into the LA is immediately shunted to the RA, thereby decompressing the LA. The physical findings are also similar to those of secundum ASD: a widely split and fixed S2, a systolic ejection murmur at the upper left sternal border, and a mid-diastolic rumble of relative tricuspid stenosis at the lower left sternal border. In addition, a systolic murmur of mitral regurgitation (MR) is occasionally present. The ECG findings are also similar: RBBB (with rsR′ in V1) or mild RVH. One exception, which is important in differentiating between the two types of ASDs, is the presence of a “superior” QRS axis or left anterior hemiblock (with the QRS axis in the range of −20 to −150 degrees) in primum-type ASD. The abnormal QRS axis seen in ECD (both partial and complete forms) is not the result of axis deviation or any of the hemodynamic abnormalities mentioned; rather, the abnormal QRS axis occurs as a result of the primary abnormality in the development of the bundle of His and the bundle branches.

Hemodynamic changes seen with complete ECD are the sum of the changes seen in ASD and VSD. There is volume overload of the LA and LV as in VSD and partially due to MR. In addition, it has volume overload of the RA and RV as in ASD (see Fig. 9-9). The result is biatrial and biventricular enlargement (Fig. 9-10). The magnitude of the left-to-right shunt in complete ECD is determined by the level of PVR (i.e., dependent shunt). The ECG also reflects these changes as BVH and occasional biatrial hypertrophy (BAH). “Superior” QRS axis is also characteristic of ECD as discussed earlier. Physical examination is characterized by a hyperactive precordium and regurgitant systolic murmurs of VSD and MR, loud and narrowly split S2 (because of pulmonary hypertension), apical or tricuspid diastolic rumble (or both), and signs of CHF. Those who survive infancy may develop pulmonary vascular obstructive disease, as already discussed for large VSD and large PDA.

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FIGURE 9-9 Hemodynamic changes in different types of endocardial cushion defects (ECD). Hemodynamics of the ostium primum type of atrial septal defect (ASD) are identical to those of the secundum type of ASD. The cleft mitral valve is usually not significant from a hemodynamic point of view, and its effect is not shown here. In complete ECD, the hemodynamic changes are the sum of those of ventricular septal defect (VSD) and ASD, resulting in enlargement of all four cardiac chambers and increased pulmonary blood flow. The shunt depends on the level of the pulmonary vascular resistance (PVR) (e.g., dependent shunt). In the left ventricle–to–right atrial (LV-RA) shunt, the shunt depends not on the level of PVR but on the size of the defect (obligatory shunt). Therefore, congestive heart failure may occur within the first weeks of life. BAH, biatrial hypertrophy; LAHB, left anterior hemiblock; LVH, left ventricular hypertrophy; ↑ PR, prolongation of the PR interval on electrocardiography; RBBB, right bundle branch block; RVH, right ventricular hypertrophy.

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FIGURE 9-10 Diagrams of chest roentgenograms in the complete form of endocardial cushion defect. All four cardiac chambers are enlarged, with increased pulmonary vascular markings. Abbreviations are the same as those in Figure 9-3.

A direct communication between the LV and RA may occur as part of ECD (or as an isolated defect unrelated to ECD). The direction of the shunt is from the high-pressure LV to the low-pressure RA. The magnitude of the shunt is determined by the size of the defect, regardless of the state of PVR; blood shunted to the RA must go forward through the lungs even if the PVR is high. This type of shunt, which is independent of the status of PVR, is called an obligatory shunt (see Fig. 9-9). When an LV-RA shunt is present as part of complete ECD, CHF may occur within a few weeks, which is earlier than in the usual VSD. The enlarged chambers are identical to those of the complete form of ECD. Therefore, the chest radiographs and ECG findings are similar to those seen in complete ECD. Physical findings also resemble those of complete ECD, although the holosystolic murmur (resulting from the LV-RA shunt) may be more prominent at the mid-right sternal border.