Embryology

A basic understanding of cardiac embryology is helpful to understand the complex spectrum of abnormalities that may be encountered in CHD. By definition, cardiac embryology reflects the events that occur in the development of the human heart from conception to the eighth week of gestation. The heart, unlike any other organ, must be able to function while it is undergoing embryologic development.

The major events of cardiac development occur between the third and fifth weeks of gestation. The cardiovascular system arises from the mesoderm as the cardiogenic crescent on the 18th day of fetal life. A straight heart tube develops by the 20th day of gestation, and it is at this point that the heart begins to beat. As cells continue to be added to the heart tube, the tube undergoes the looping process. Typically the heart tube loops to the right, which is referred to as D-looping. The heart may also loop to the left, which is referred to as L-looping. The looping of the heart plays a major role in the eventual spatial relationship of the right ventricle (RV) and left ventricle (LV) [ Fig. 3.1 ] .

Looping of the primitive heart tube during embryologic development.

Diagram depicting the straight heart tube in the center with a common entrance of the venous system into the common atrium ( A ) that continues to the developing ventricle ( V ), with a single outflow called the bulbous cordis ( BC ) that connects to the truncus arteriosus ( TA ). The heart tube can loop to the right (D-looping) to produce normally related ventricles, or to the left (L-looping) to produce inverted ventricles. After looping, there will be further remodeling and septation to create left (LA) and right atria (RA) and left ( LV ) and right ventricles ( RV ), and septation of the TA to the aorta ( Ao ) and pulmonary artery ( PA ).

Once looping is complete during the fourth week of gestation, circulation through the heart is in series from the right atrium (RA) to the left atrium (LA), then the LV to the RV to the truncus arteriosus, which serves as a common outflow tract to the aorta and main pulmonary artery (MPA). During this time, the embryologic aortic arches are also developing into the great arteries and their major branches, as well as the ductus arteriosus. The process of convergence, which aligns the RA with the developing RV and the aorta with the developing LV, follows ventricular looping. The heart then undergoes septation during the fifth week of fetal life. The interventricular septum grows from apex to base to septate the ventricles, and the formation of the septum primum and secundum separates the LA and RA. The endocardial cushion tissue lining the atrioventricular canal (AVC) then septates the common AV valve into separate tricuspid and mitral valves. At the same time, the common outflow tract from the ventricles, or the truncus arteriosus, septates to separate the ascending aorta and the MPA. The infundibulum, or conus of muscle forming the right ventricular outflow tract beneath the pulmonary valve, together with the swirling blood flowing out of the developing ventricles, causes the left and right ventricular outflow tracts and aorta and pulmonary artery (PA) to appear wrapped around one another when they are normally developed (i.e., not parallel in configuration) [ Fig. 3.2 ] . Separation of the right and left heart is nearly complete by the seventh week of fetal life, but does not completely finish until the final closure of the ductus arteriosus and the interatrial septum after birth.

Types of conus or infundibulum.

A, The normal heart has a subpulmonary conus as part of the right ventricle (RV), and the left ventricle (LV) normally has no conus when aligned with the aorta. B, A subaortic conus occurs when the RV is aligned with the aorta in transposition of the great arteries. C, There is both a subaortic and a subpulmonary conus when the RV gives rise to both great arteries, as in double-outlet RV. D, Rarely, the conus is entirely absent, as in the setting of double-outlet LV.

In summary, the major events in cardiac embryology occur between the third and seventh weeks of fetal life, and include the formation of a straight heart tube with common venous inflow to the developing LV and common arterial outflow from the developing RV. This is followed by looping of the tube to the right (D) or to the left (L), differentiation of morphologic chambers with convergence to provide inflow to the RV and outflow to the LV, valve formation, and chamber separation. Abnormal arrest of this process or defects during any stage of development may result in the wide spectrum of CHDs that can be detected at birth.

Identification of Cardiac Chambers and Great Vessels

The first step in the analysis of CHD by cross-sectional imaging is to accurately identify the morphology of the cardiac chambers and vessels. The RA is identified as the chamber that receives the insertion of the inferior vena cava (IVC) and coronary sinus, and also by a broad-based triangular appendage with characteristic pectinate muscle morphology extending to the AV junction. The LA is identified by a narrow-based, tubular, finger-like appendage. The pulmonary vein insertion is not reliable for identification of the LA, because the pulmonary veins are often partially or totally anomalous. The situs of the atria can be solitus (S), inversus (I), or ambiguous (A) [ Fig. 3.3 ].

Types of atrial situs.

Diagram depicting the three possibilities for situs of the left (LA) and right atria (RA) as solitus, inversus, or ambiguous. The atrial situs does not always correlate with the visceral situs in the abdomen. Atrial situs ambiguous is associated with a single common atrium because of absence of the interatrial septum and is most often seen in patients with heterotaxy syndrome.

The RV is identified by the following features:

• 1.
  

  A muscular attachment between the free wall and the interventricular septum called the moderator band

  
  
• 2.
  

  An AV valve that is slightly more apical in position than the AV valve of the LV

  
  
• 3.
  

  The presence of an infundibulum (or conus), which creates a muscular outflow tract that separates the tricuspid valve and the semilunar valve arising from the RV [ Fig. 3.4 ]

  
  

  
  

  
  

  

  
  

  
  

  Morphologic characteristics of the normal right (RV) and left ventricles (LV).

  
  

  A, Four-chamber steady-state free precession (SSFP) magnetic resonance image shows the moderator band ( arrow ) of a normal RV. The RV is positioned to the right of and anterior to the LV, and the LV has a smooth superior septal surface. B, Long-axis sagittal oblique SSFP image through the RV profiles the normal tricuspid and pulmonary valves, which are separated by the conus or infundibulum ( arrow ). C, Vertical long-axis SSFP image profiles the normal aortic and mitral valves, which are in fibrous continuity because there is no subaortic conus. Ao, Aorta; LA, left atrium; RA, right atrium.

  
  

The LV is identified by its smooth superior septal surface, which is devoid of any muscular attachments, and by the absence of a conus, so that the mitral valve and the semilunar valve arising from the LV are in fibrous continuity.

The PA is recognized as the vessel that supplies branches to the lungs, but not to the body. The aorta is identified as the vessel that supplies the coronary arteries and systemic arteries to the body.

Segmental Approach to Heart Disease

Any combination of atrial, ventricular, and great vessel morphology may occur in CHD. One must take a simple, logical, step-by-step approach to understand and describe disease before initiating treatment. The segmental approach to CHD described by Richard van Praagh [ Fig. 3.5 ] involves determination of the following factors:

• 1.
  

  The morphology of the three major cardiac segments: Viscero-atrial situs, ventricular looping, and the relationship of the great arteries

  
  
• 2.
  

  How each segment is connected to the adjacent segment: This determination involves analysis of the AVC, which connects the atria to the ventricles, and evaluation of the ventriculo-arterial connection, including the conus (also called the infundibulum ), which connects the ventricles to the great arteries. Potential AV connections are concordant connection (LA to LV and RA to RV), discordant connection (LA to RV and RA to LV), common atrioventricular canal (CAVC), double-inlet LV, unilateral AV valve atresia, straddling AV valve, and overriding AV valve. The options at the level of the conus are normal connection, transposition of the great arteries (TGA), truncus arteriosus, double-outlet right ventricle (DORV), and rarely, double-outlet LV [ Fig. 3.2 ]. Description of the conal muscle is important, because this muscle may narrow the outflow tracts and influence subsequent surgical management

  
  
• 3.
  

  The presence of associated malformations involving the atrial and ventricular septum or the extracardiac vasculature: Examples include but are not limited to the following: ASD, VSD, partial or total anomalous pulmonary venous connection (TAPVC), bilateral superior vena cava (SVC), and coarctation

  
  
• 4.
  

  How segmental combinations and connections, along with the associated malformations, function

Steps in the segmental approach to congenital heart disease.

As an example, using the segmental approach, a patient would be described as “congenitally corrected” {S,L,L} TGA with pulmonary stenosis and a VSD [ Fig. 3.6 ]. Breaking this down to the individual components, there is (1) S-solitus atria, L-looped ventricles, and L-malposed great arteries; (2) a discordant AV connection (LA to RV and RA to LV) and a discordant ventriculo-arterial connection (RV to aorta and LV to PA); (3) pulmonary stenosis and a VSD; and (4) a balanced (physiologically corrected) circulation.

{S,L,L} Transposition of the great arteries with ventricular septal defect (VSD) and pulmonary stenosis, illustrating the steps in the segmental approach to congenital heart disease.

A, Coronal maximum intensity projection (MIP) image from 3D magnetic resonance angiography (MRA) demonstrating the superior and inferior vena cavae connecting to a right-sided right atrium ( RA ). B, Axial image at the level of the atria demonstrating the left atrium ( LA ) positioned to the left and receiving the pulmonary veins. C, Four-chamber steady-state free precession (SSFP) MR image showing the right ventricle ( RV ) with a trabeculated septum positioned to the left of the left ventricle ( LV ), which has a smooth septal surface. The heart is L-looped. D, Axial MIP image from a 3D SSFP showing the aorta ( Ao ) positioned to the left and anterior to the main pulmonary artery ( MPA ), or L-malposed. E, Sagittal oblique SSFP image showing the aorta arising from the RV, with a subaortic conus ( arrow ) separating the aortic valve from the tricuspid valve. F, Coronal oblique SSFP image through the outflow tracts shows the LV aligned with the MPA, narrowing of the subpulmonary outflow tract, and a conoventricular VSD ( asterisk ).

Heterotaxy

Accurate anatomic and physiologic diagnosis of CHD using a structured approach has led to significant improvement in patient outcomes. This method is associated with a more appropriate selection of therapeutic options, whether they be medical, surgical, or both. This is especially true as applied to the patient with complex CHD in the setting of heterotaxy syndrome, or abnormal sidedness of the thoracic and abdominal viscera. With normal development, there is lateralization of the internal thoracic and abdominal organs with respect to the left-right axis, so that the heart is in the left chest with the apex pointing to the left, the stomach and spleen are located in the left upper quadrant, and the liver is located in the right upper quadrant. This is referred to as situs solitus. Situs inversus totalis refers to complete mirror image reversal of the normal position of the thoracic and abdominal viscera and vasculature. Patients with heterotaxy have situs ambiguous or variable lateralization of the thoracic and abdominal viscera and vasculature that does not follow either situs solitus or situs inversus totalis. The exact incidence of heterotaxy is unknown, but it is thought to occur in 1 in 10,000 to 40,000 births, including 3% of all neonates with CHD and 30% who have a cardiac malposition. The prognosis of patients with heterotaxy is primarily based on the severity of the associated cardiovascular malformations.

Heterotaxy encompasses a spectrum of cardiac, vascular, and visceral abnormalities of early embryonic development of the thorax and abdomen that occur in the fifth week of gestation. This spectrum includes abnormalities in septation of the conotruncus, growth of the endocardial cushions, septation between the LA and pulmonary venous plexus, formation of the spleen, rotation of the bowel, and lobation of the lungs. The defining features of heterotaxy therefore include conotruncal anomalies (such as TGA or DORV), endocardial cushion or CAVC defects, anomalous pulmonary and systemic venous connections, symmetry of the atrial appendages with partial or complete absence of the atrial septum, asplenia or polysplenia, malrotation of the bowel, and symmetry of lung lobation (bilateral right- or left-sided morphology).

There is no one feature that is pathognomonic in heterotaxy syndrome. To be able to recognize the numerous different congenital cardiovascular anomalies that may be present, it is helpful to understand that heterotaxy syndrome represents a spectrum between complete bilateral right-sidedness (also called right isomerism or heterotaxy with asplenia ) and bilateral left-sidedness (also called left isomerism or heterotaxy with polysplenia ). Most patients fall somewhere in between these two extremes [ Figs. 3.7 and 3.8 ].

Heterotaxy syndrome.

Diagram depicting the extremes of bilateral right isomerism (asplenia) versus bilateral left isomerism (polysplenia) of the abdominal viscera and lungs. IVC , inferior vena cava; PA , pulmonary artery; SVC , superior vena cava.

Heterotaxy examples.

A, Coronal image from magnetic resonance angiography (MRA) showing bilateral right-sided bronchi and minor fissures, and a right-sided stomach in a patient with asplenia. B, Coronal computed tomography (CT) image showing bilateral left-sided bronchi in a patient with polysplenia. C, Four-chamber steady-state free precession image in a patient with polysplenia showing a balanced common atrioventricular canal defect with a common atrium ( CA ) with no interatrial septum and a large inlet ventricular septal defect ( asterisk ). D, Coronal maximum intensity projection image from an MRA in the same patient as ( C ) showing a midline liver, a right-sided stomach, and a cluster of splenules ( SP ) in the right upper quadrant. In addition, there are bilateral superior vena cavae ( asterisks ) and an unusual left-sided inferior vena cava ( arrow ) that extends to the right to enter the undersurface of the atria with the hepatic veins. LV, left ventricle; RV, right ventricle.

Right isomerism or heterotaxy with asplenia is characterized by bilateral right eparterial bronchi and trilobed lungs, absence of a spleen, a midline transverse liver, and a stomach in variable position. Heterotaxy with asplenia may be associated with complex forms of cyanotic heart disease including complete absence of the atrial septum resulting in a common atrium, CAVC, and DORV and/or pulmonary atresia.

Left isomerism or heterotaxy with polysplenia is associated with bilateral left hyparterial bronchi and bilobed lungs, a midline transverse liver, multiple spleens or splenules that can be located on either side of the abdomen, and a stomach in variable position. Heterotaxy with polysplenia may be associated with intracardiac defects similar to those seen in the setting of asplenia, but they tend to be less severe.

Patients with heterotaxy may have numerous different anomalies of the systemic and pulmonary venous connections, including partial or TAPVC and a bilateral SVC. An interrupted IVC at the level of the liver with azygous continuation is more often seen in polysplenia (bilateral left-sidedness), because the IVC is normally a right-sided structure.

Applying the segmental approach to CHD is crucial in the setting of heterotaxy. This method facilitates accurate characterization of complex and variable morphology, and guides decision making in the best approach for management and surgical repair.

Physiologic Subgroups

Chest radiographs play an important role in evaluating the physiology of heart disease and determining palliative therapy at birth by distinguishing between lesions associated with increased pulmonary blood flow (left-to-right shunts and intermixing states with unobstructed pulmonary blood flow), lesions associated with decreased pulmonary blood flow due to right-sided obstruction, and lesions associated with pulmonary venous hypertension due to systemic (left) sided obstruction [ Fig. 3.9 ]. Intermixing states are usually due to the presence of a large shunt in lesions associated with a common atrial or single ventricular chamber. Clinically, cyanosis is present due to tremendous mixing of deoxygenated blood with oxygenated blood before it enters into the systemic circulation.

Physiology of congenital heart disease delineated by chest radiography.

A, Mild cardiomegaly with an upturned cardiac apex, a concave main pulmonary artery segment, and symmetric, severely diminished pulmonary blood flow in a 4-year-old with tetralogy of Fallot/pulmonary atresia. B, Moderate cardiomegaly and symmetric, increased pulmonary blood flow in a 3-month-old with a large atrial septal defect and ventricular septal defect. C, Moderate cardiomegaly with interstitial edema in an 8-day-old with critical aortic stenosis.

With the current diagnostic accuracy of echocardiography for CHD, chest radiography now plays a lesser role in diagnosing the morphology of CHD. In the past, chest radiographs were analyzed together with physiologic information gleaned from clinical examination (murmurs, cyanosis, etc.) to construct a differential diagnosis that would determine the initial treatment needed by the patient at birth. Radiographic classification of CHD may be based on pulmonary blood flow as follows:

• I.
  

  Increased pulmonary vascularity

  
  
  
  
  • A.
    

    Central left-to-right shunts (acyanotic)

    
    
    
    
    • 1.
      

      Ventricular septal defect (VSD)

      
      
    • 2.
      

      Atrial septal defect (ASD)

      
      
    • 3.
      

      Atrioventricular septal defect/common atrioventricular canal (CAVC)

      
      
    • 4.
      

      Patent ductus arteriosus (PDA)

      
      
    • 5.
      

      Less common lesions: partial anomalous pulmonary venous connection (PAPVC), aortopulmonary window, coronary-cameral fistula

    
    
    
    
  • B.
    

    Peripheral left-to-right shunts

    
    
    
    
    • 1.
      

      Vein of Galen malformation

      
      
    • 2.
      

      Hepatic hemangioendothelioma

      
      
    • 3.
      

      Large extremity arteriovenous malformations

    
    
    
    
  • C.
    

    Hyperdynamic states, such as thyrotoxicosis and anemia

    
    
  • D.
    

    Intermixing states without obstruction to pulmonary blood flow (cyanotic)

    
    
    
    
    • 1.
      

      Transposition of the great arteries (TGA)

      
      
    • 2.
      

      Truncus arteriosus

      
      
    • 3.
      

      Double outlet right ventricle (DORV)

      
      
    • 4.
      

      Single ventricle

      
      
    • 5.
      

      TAPVC (unobstructed)

    
    

  
  
  
  
• II.
  

  Decreased pulmonary vascularity

  
  
  
  
  • A.
    

    Tetralogy of Fallot (TOF)

    
    
  • B.
    

    Pulmonary atresia with VSD

    
    
  • C.
    

    Intermixing states with obstructed pulmonary blood flow (cyanotic)

    
    
    
    
    • 1.
      

      TGA with pulmonary stenosis

      
      
    • 2.
      

      Truncus arteriosus with pulmonary stenosis

      
      
    • 3.
      

      DORV with pulmonary stenosis/atresia

    
    

  
  
  
  
• III.
  

  Pulmonary venous congestion and edema

  
  
  
  
  • A.
    

    Left-sided obstructive lesions

    
    
    
    
    • 1.
      

      Cor triatriatum

      
      
    • 2.
      

      Hypoplastic left heart syndrome (HLHS)

      
      
    • 3.
      

      Shone syndrome

      
      
    • 4.
      

      Bicuspid aortic valve and aortic stenosis

      
      
    • 5.
      

      Aortic coarctation

    
    
    
    
  • B.
    

    Hyperdynamic states

    
    
  • C.
    

    Severe left-to-right shunts

  
  
  
  
• IV.
  

  Normal pulmonary vascularity

  
  
  
  
  • A.
    

    Mild left-sided obstructive lesions

    
    
  • B.
    

    Intermixing states with balanced pulmonary blood flow

    
    
  • C.
    

    Mild right-sided obstruction—pulmonary valve stenosis

  
  

Treatment of Congenital Heart Disease

Immediate Palliation

The need for immediate palliative therapy of CHD at birth depends on the patient’s clinical and hemodynamic status. The type of palliation is determined by echocardiographic and radiographic findings. For example, in a patient with increased pulmonary vascularity, PA banding to diminish pulmonary blood flow may be considered. In a patient with decreased pulmonary vascularity caused by outflow tract obstruction, pulmonary blood flow can be augmented by surgical relief of the obstruction or placement of a systemic arterial-to-pulmonary arterial shunt such as a modified Blalock-Taussig shunt (BTS) [ Fig. 3.10 ]. Older adults with CHD may have had the classic BTS or Waterston or Potts shunts, which are no longer routinely used. Intermixing states are treated in the neonatal period to balance and optimize pulmonary and systemic blood flow. Pulmonary venous hypertension caused by left-sided obstruction is treated through relief of the cause of obstruction.

Diagram depicting the various forms of palliative shunts used to augment pulmonary blood flow for patients with cyanosis. See section “Named” Pediatric Cardiac Surgical Procedures in the appendix.

Permanent Palliation

After immediate palliation, decisions regarding permanent palliation are based on the number of functioning ventricles and the feasibility of separating deoxygenated and oxygenated blood flows. When there are two well-formed, normal-sized, functioning ventricles in conditions such as TOF, TGA, truncus arteriosus, and some forms of CAVC and DORV that are amenable to a two-ventricle repair, the anatomy is rearranged to restore physiologic venous return and unobstructed systemic and pulmonary arterial outflows.

When there is only one normal-sized, well-functioning RV or LV, this ventricle will be used as the systemic ventricle, and the pulmonary circulation will be palliated by establishing direct connections between the systemic veins and pulmonary arteries or cavopulmonary connections. This is called single-ventricle repair [ Fig. 3.11 ]. Conditions that can be palliated with single-ventricle repair include unbalanced CAVC, tricuspid atresia, HLHS, double-inlet LV, DORV, and pulmonary atresia with an intact ventricular septum. Occasionally, conditions with two functioning ventricles may end up in the single-ventricle pathway because of an inability to separate the deoxygenated and oxygenated circulations. This may occur with some forms of DORV, straddling AV valves, and crisscross AV valves. A list of named surgical procedures is provided in the appendix to this chapter (see section “Named” Pediatric Cardiac Surgical Procedures ).

Steady-state free precession magnetic resonance images showing examples of single-ventricle hearts often requiring single-ventricle palliation.

A, Tricuspid atresia. Note the hypoplastic right ventricle (RV) and fat filling the anterior atrioventricular (AV) groove in the expected location of the tricuspid valve. B, Double-inlet left ventricle (LV). Both the mitral and the tricuspid valves connect to a single LV. C, Unbalanced right dominant AV canal. A single AV valve is aligned with the RV, and a hypoplastic LV is seen posteriorly. D, Double-outlet RV. Both the aorta and the main pulmonary artery arise from the RV, and a bilateral conus is seen. Asterisks indicate Fontan pathway in cross section. RA, right atrium.

Indications for Imaging by Computed Tomography and Magnetic Resonance Imaging

In the preoperative period, CT and MRI are used to evaluate CHD when complete characterization of pathology by echocardiography is inadequate due to suboptimal acoustic windows. Evaluation by echocardiography may be suboptimal with abnormalities involving the extracardiac vasculature, such as aortic coarctation, anomalous pulmonary venous connection, systemic venous anomalies, branch PA stenosis, and anomalous coronary artery origin.

In the postoperative period, the goals of imaging change considerably. Information about cardiac function and flow and potential postoperative complications is needed in addition to morphologic assessment. The goals of postoperative imaging are to evaluate ventricular and valvular function, survey the patency of surgically created grafts, conduits, and baffles, and gather information that will aid in the timing of necessary surgical or catheter-based reintervention. Typical indications for postoperative imaging that will help guide subsequent management include the evaluation for recurrent coarctation, the determination of timing of catheter-based or surgical pulmonary valve replacement in repaired TOF, the investigation of the cause of cyanosis after the Fontan operation, the evaluation of the systemic RV after intraatrial repair of TGA, and screening evaluation before staged single-ventricle repair. The role of MRI and CT increases in the postoperative period and in older patients when acoustic windows diminish.

Left-to-Right Shunts

Communications between the systemic and pulmonary circuits that do not result in cyanosis include intracardiac left-to-right shunts and large systemic arteriovenous connections. Common examples of the former are VSD, ASD, AVC defect, and PDA. Less common examples are partial anomalous pulmonary venous connection, aortopulmonary window, and coronary-cameral fistula. Examples of peripheral left-to-right shunts are infantile hepatic hemangioendotheliomas and large systemic arteriovenous malformations such as the vein of Galen malformation in the brain and Parkes-Weber syndrome affecting the extremities.

Ventricular Septal Defect

After bicuspid aortic valve, VSD is the second most common congenital intracardiac lesion in children, accounting for 20% to 30% of cases of CHD. Five percent of VSDs are associated with chromosomal abnormalities such as trisomies 13, 18, and 21. Many small VSDs close spontaneously. The classification of VSDs on the basis of location [ Fig. 3.12 ] and size is important in deciding whether medical (expectant) or surgical treatment is indicated.

Diagram illustrating the types of atrial ( ASD ) and ventricular septal defects ( VSD ) as viewed from the ( A ) posterior and ( B ) rightward aspects of the heart. RUL, right upper lobe.

(Courtesy of Dr. Andrew Phelps.)

Muscular VSDs are defects in the muscular septum [ Fig. 3.13 ]. They may be classified according to their location in the septum (midmuscular, apical muscular). “Swiss cheese septum” describes multiple muscular VSDs.

Ventricular septal defect (VSD).

A, Chest radiograph at birth showing cardiomegaly with increased pulmonary vascularity. B, The same patient at 3 years old, with marked enlargement of the main and central branch pulmonary arteries. Note the pruning of the pulmonary arteries as they extend into the periphery of the lungs, consistent with pulmonary artery hypertension. C, Long-axis bright blood magnetic resonance (MR) image shows a defect in the mid muscular septum, or a muscular VSD. D, Four-chamber MR image of a different patient demonstrates an inlet or atrioventricular (AV) canal type VSD in continuity with the plane of the AV valves.

An AVC-type VSD is a defect in the AVC portion of the septum [ Fig. 3.13 ]. Also known as an inlet VSD, this condition is commonly seen in the setting of heterotaxy.

Several different terms are used for outflow tract VSDs or defects in the interventricular septum associated with the outlet or subaortic or subpulmonary region of the heart. Conoventricular VSDs are a form of outlet VSD occurring within or at the junction between the conal septum and the muscular septum. Conoventricular VSDs may be in close proximity to the semilunar valves, and are also referred to as subarterial VSDs. Clinically, it is important to evaluate for associated aortic valve prolapse and aortic regurgitation.

The strictest definition of a membranous VSD is a defect of only the membranous septum, which would result in a relatively small defect. In day-to-day use, the term perimembranous or paramembranous VSD usually implies that the defect includes the region of the membranous septum, but also extends into the muscular septum, conal or outflow tract septum, or AVC septum. Outflow tract VSDs can be associated with malalignment, in which the conal septum of the RV outflow tract is displaced. This may create a substrate for outflow tract obstruction. Anterior malalignment of the conal septum often results in the RV outflow obstruction seen in TOF (described later), and posterior malalignment can lead to a VSD with LV outflow tract obstruction and an arch abnormality such as coarctation or complete arch interruption.

A VSD classically presents with a prominent, blowing, pansystolic murmur heard best at the left lower sternal border. Affected patients may have congestive heart failure (CHF), repeated respiratory infections, or failure to thrive. Presentation is usually after the first month of life when pulmonary vascular resistance normalizes and the degree of shunting increases. In an older child, a large, untreated VSD may lead to elevated pulmonary vascular resistance and pulmonary hypertension, which can resolve if the shunt is closed. In rare cases, pulmonary vascular obstructive disease or irreversible pulmonary hypertension can develop, and may even cause reversal of blood flow. This leads to a right-to-left shunt across the septal defect (formerly known as “Eisenmenger physiology”), and eventually to cyanosis. In most cases, corrective surgery is performed before flow reversal across the shunt occurs.

Conventional chest radiograph findings of a left-to-right shunt are usually evident when the ratio of pulmonary to systemic blood flow is greater than 2 : 1. Pulmonary vascularity is increased, and there is dilatation of the pulmonary arteries and cardiomegaly [ Fig. 3.13 ]. A VSD generally results in volume loading of the LA and LV, which may appear prominent on chest radiographs. When pulmonary vascular resistance normalizes after the first month of life, pulmonary blood flow may further increase, resulting in radiographic signs of pulmonary vascular congestion and edema.

Up to 50% of VSDs close spontaneously. Those that do not may be treated with surgical closure of the defect. VSDs that are difficult to reach surgically can be treated with device closure at cardiac catheterization.