Abstract
Congenital heart disease (CHD) is the most common congenital malformation, causing up to 10% of infant deaths and 50% of deaths due to a congenital malformation, while extracardiac congenital anomalies are also present in up to 25% of neonates. Approximately 25% of congenital heart lesions can be considered more complex, and one-third will require intervention during infancy. A thorough understanding of the complete anatomic substrate related to the patient’s specific congenital cardiac lesion and resultant hemodynamic pathophysiology is essential for early recognition, diagnosis, and planning of optimal intensive care unit management of critically ill pediatric patients by the pediatric intensivist, both before and after surgical- and/or catheterization-based interventions. A systematic method to diagnose and assess CHD is necessary, as exemplified by the sequential segmental approach for describing the malformed heart. Congenital cardiac lesions can then be divided into four pathophysiologically distinct groups, as detailed in this chapter: congenital heart defects with dominant left-to-right shunt, including anomalous pulmonary venous connections; cyanotic congenital heart disease; obstructive congenital heart disease; and complex congenital heart disease. These are described in some detail, emphasizing the morphologic features and variants that may impact patients’ health care in the intensive care setting.
Key Words
Congenital heart disease, Cardiac morphology, Cyanotic heart disease, Obstructive congenital heart disease
Over 90% of cardiac pathologic conditions in the pediatric population in the developed world are congenital in origin, in that it is present at birth even if undetected until older; in contrast, adult heart disease is largely classified as being acquired. In less developed countries, rheumatic fever is still prevalent. The incidence of congenital heart disease is 6 to 8 per thousand live births (approximately 1 in 130 to 145 live births), whereas at 20 weeks’ gestation this figure is 5 to 10 times higher, due to associated lethal chromosomal abnormalities. Approximately 25% of congenital heart lesions can be considered more complex, and one-third will require intervention during infancy. These figures do not include the presence of a bicuspid, nonstenotic aortic valve (2%) or the association of a patent arterial duct in those born prematurely. The incidence of the various lesions is detailed in Table 12.1 .
| Ventricular septal defect | 32% |
| Atrial septal defect | 8% |
| Pulmonary stenosis | 7% |
| Patent arterial duct | 7% |
| Tetralogy of Fallot | 5% |
| Coarctation of the aorta | 5% |
| Transposition of the great arteries | 4% |
| Aortic stenosis | 4% |
| Atrioventricular septal defect | 4% |
| Hypoplastic left heart syndrome | 3% |
| Complex functionally univentricular lesions (double-inlet ventricle, tricuspid or mitral atresia) | 3% |
| Pulmonary atresia with intact ventricular septum | 2% |
| Double-outlet right ventricle | 2% |
| Common arterial trunk | 1% |
| Totally anomalous pulmonary venous connections | 1% |
| Miscellaneous | 13% |
An understanding of the anatomy of congenital cardiovascular malformations is essential for early recognition, diagnosis, and timely management of critically ill pediatric patients. Critical congenital heart defects lead to death in the neonatal period or in early infancy if left untreated. The morphologic characteristics and variations related to these lesions form the basis for understanding the clinical presentation, results of clinical and laboratory assessment, and responses to treatment. Moreover, even relatively simple, well-tolerated, and otherwise benign congenital cardiovascular defects (e.g., muscular ventricular septal defect) may have a profound impact on other organ function and recovery (e.g., the lungs) when an extracardiac organ is adversely affected by another disease (e.g., infection).
Sequential Segmental Approach
Before describing the lesions found within the congenitally malformed heart, it is important to be aware of three fundamental conventions used to describe the components of the normal heart. First, when cardiac chambers are described, the qualifiers right and left refer only to the morphologic characteristics of the chambers usually designated as being right and left, and not their position in the thorax, as originally proposed by Lev in 1954. If a spatial frame of reference is required, then this first convention mandates that the terms right-sided, left-sided, anterior, and posterior are used. When dealing with cardiovascular structures other than cardiac chambers, in contrast, right and left do refer to the spatial position of these structures within the thorax, and not to their morphologic counterparts. Thus the right superior caval vein (vena cava) refers to the caval vein on the right side of the body.
The second convention, known as the morphologic method, states that variable features within the heart should be defined in terms of their own intrinsic morphologic characteristics, and not on the basis of other features that are themselves variable.
The final convention, known as the segmental approach, as described by Van Praagh in the early 1970s, is now established as fundamental to imparting the full nature of a cardiac defect, particularly when the malformation is complex and involves multiple cardiac segments. When using this approach, the analyses and descriptions of the malformed heart are made in a logical sequence, permitting the anomalies to be described with precision and in unambiguous fashion. The heart is approached in terms of three major building blocks: the atria, the ventricular mass, and the arterial trunks. There is limited potential for variation in each segment. Segmental analysis therefore depends upon the recognition of the topologic arrangement of the three cardiac segments.
Subsequently, two systems of classification evolved to describe the malformed heart. The sequential segmental variant of the system, emanating from the so-called European School led by Anderson and colleagues in the 1970s, combines this morphologic information with the ways in which the segments are joined, or not joined, to each other ( Fig. 12.1 ), and then to their relationships. To avoid ambiguity the system is designed so that each segment can be described according to how it is linked to the subsequent one, while fully describing each segment and the nature of the junctional connections.
The alternative classification system, emanating from the so-called Boston School, uses independent descriptions of three major segments: viscero-atrial situs, ventricular loop and great arterial situs, based originally on embryologic assumptions. Adjacent segments are related to each other by intersegmental alignments, which may not equate to connections.
For the purposes of this chapter, the so-called European approach has been adopted when describing congenital heart malformations. Therefore when describing a malformed heart, particularly when a complex lesion is present, the sequence used begins with the position and orientation of the heart and then follows the flow of blood through the heart in a sequential manner from the great mediastinal veins to the atria, then ventricles via the atrioventricular (AV) valves, before reaching the arterial trunks via the ventriculoarterial semilunar valves. Additional lesions such as septal defects and arterial obstruction are then described.
From the view of the pediatric cardiac intensivist, this segmental approach allows for structured and reproducible clinical and noninvasive laboratory (x-ray examination, echocardiography, computed tomography, magnetic resonance imaging) assessment of congenital pediatric cardiovascular anomalies. Finding common hemodynamic features and the impact of various anomalies makes their understanding easier and more relevant to the practicing intensivist.
Congenital cardiac lesions can be divided on this basis into:
- 1.
Congenital heart defects with dominant left-to-right shunt, including anomalous pulmonary venous connections
- 2.
Cyanotic congenital heart disease
- 3.
Obstructive congenital heart disease
- 4.
Complex congenital heart disease
Congenital Heart Defects With Dominant Left-to-Right Shunt
Patent Ductus Arteriosus (Patent Arterial Duct)
The ductus arteriosus (arterial duct) joins the pulmonary trunk with the descending aorta. It is part of the normal fetal circulation as it carries most of the blood reaching the pulmonary trunk to the aorta, bypassing the lungs ( Fig. 12.2 ). The direction of blood flow through the patent ductus arteriosus (PDA) reverses postnatally. There is spontaneous ductal closure in most neonates. Patency beyond 1 month of age (3 months in premature infants) is considered abnormal and is common in preterm neonates. The PDA is mostly left sided but right-sided or bilateral PDA can be present. It is the only source of pulmonary blood supply in some complex congenital heart defects (e.g., pulmonary atresia with intact ventricular septum), but it can be absent in others (e.g., common arterial trunk). The risk of developing pulmonary vascular disease is high if a large PDA is left untreated into childhood.
Interatrial Communications (Including Atrial Septal Defect)
Various parts of the interatrial septum can be affected by defects. Interatrial communications are described according to their developmental locations: secundum atrial septal defect (ASD) (within oval fossa and its vicinity), primum ASD (part of atrioventricular septal defect [AVSD]; see later discussion), sinus venosus defect (superior or inferior adjacent to the orifice of superior or inferior caval vein), or at the coronary sinus ( Fig. 12.3 ). The size of secundum ASDs depends on the extent of deficiency of the fossa valve. Small defects (less than 3 to 4 mm) are often considered part of the spectrum of a patent foramen ovale and often undergo spontaneous closure. Secundum ASDs may be associated with other congenital heart defects (e.g., pulmonary valve stenosis). Transcatheter device closure of larger defects is dependent upon there being sufficient rims for the device to affix to the surround of the defect and size of the defect in relation to size of the patient. A superior sinus venosus defect is commonly associated with partial anomalous pulmonary venous drainage that involves the right upper pulmonary vein joining the superior caval vein (vena cava). The volume of left-to-right shunt depends on the size of the defect and the relative compliance of right and left ventricles. The low kinetic energy of blood reaching the pulmonary arteries makes any increase of pulmonary vascular resistance associated with isolated ASD in childhood very rare. Large ASDs left untreated until adulthood are associated with risk of atrial dysrhythmias, congestive heart failure (right heart failure), and pulmonary vascular disease and are therefore closed in childhood even in asymptomatic individuals. Paradoxical embolism may occur regardless of ASD size but is rare in children.
Ventricular Septal Defect
The ventricular septal defect (VSD) is the most common congenital heart defect (30%) and can be defined as a hole or pathway between the ventricular chambers. The normal ventricular septum is composed of muscle apart from a very small portion that is a fibrous membrane at the border between the aortic and tricuspid valves. The categorization of VSDs depends on their position relative to this membranous septum and its geographic location with respect to the right ventricular side of the interventricular septum, that is, the ventricular inlet, outlet, or trabecular (apical) portion ( Fig. 12.4 ). Perimembranous VSDs have a border that abuts the remnant of the membranous septum, that is, they are situated in a central geographic location and there is atrioventricular valvar and arterial valvar continuity (the aortic and tricuspid valve in the normally connected heart). Of importance is that the conduction system is positioned along the posteroinferior rim of the defect, making it vulnerable to injury during a therapeutic procedure to close such a hole. The VSD in this central perimembranous position may involve the ventricular inlet (perimembranous inlet VSD) or the ventricular outlet (perimembranous outlet) portions of the septum. Perimembranous inlet defects have two independent AV valves and AV junctions and must be distinguished from hearts with a common AV junction and valve with associated AVSD (see later discussion). Perimembranous outlet defects almost invariably are associated with a varying degree of outlet septal malalignment with respect to the trabecular muscular septum, as classically seen in tetralogy of Fallot (TOF), where there is marked anterior septal malalignment and aortic override.
The second group of VSDs is the muscular defects. These have exclusively muscular rims around the hole through the ventricular septum and are remote from the major bundles of the cardiac conduction system. They are then further described based on their geographic location within the interventricular septum as opening to the ventricular inlet, outlet, or specific parts of the trabecular portion (midseptal, anterosuperior, posteroinferior, and apical).
When the VSD is in the outlet portion and there is no muscular separation between the aortic and pulmonary valves (absent or purely fibrous outlet septum) such that the valves are in fibrous continuity with each other at the defect’s cranial border, this is termed a doubly committed juxta-arterial VSD. These may also have perimembranous extension with a fibrous posteroinferior rim, or a muscular posteroinferior rim, which results in some protection to the conduction bundle. Outlet VSDs of all types may have a degree of anterior or posterior malalignment of the outlet septum with respect to the trabecular muscular septum, posterior deviation being associated with aortic arch obstruction, whereas anterior deviation as seen in TOF is associated with right ventricular outlet narrowing or even atresia in extreme cases.
The position of the VSD has implications for spontaneous closure rate (highest in muscular defects) and function of surrounding valves (aortic and pulmonary valvar regurgitation in doubly committed defects, aortic and tricuspid valvar regurgitation associated with perimembranous defects). Multiple defects can be present most often in the muscular part of interventricular septum. The majority of VSDs are small (<3 mm diameter) with a high rate of spontaneous closure, especially in isolated small defects (up to 90% by 6 years of age). The size of the defect is only one factor influencing the volume of left-to-right blood shunting across the defect. The relative right and left ventricular pressure and compliance, and ratio of pulmonary and systemic vascular resistance all play an important role too. The high kinetic energy of blood reaching the pulmonary circulation due to a large left-to-right shunt through the VSD represents a risk for early onset of pulmonary vascular disease, and timely VSD closure (before 2 years of age, or much earlier if associated with other congenital heart lesions such as transposition of the great arteries) is advocated. There is a frequent association with other congenital heart defects (e.g., aortic coarctation), or the defect is an integral part of more complex congenital cardiac malformation (e.g., TOF, functionally univentricular heart). There is a significant risk of infective endocarditis in association with small VSDs.
Atrioventricular Septal Defect (Atrioventricular Canal Defect)
The AVSD is characterized by a common AV junction owing to a lack of division of the embryonic AV canal into separate left and right parts together with a failure of fusion between the down-growing atrial septum and the up-growing ventricular septum. The valve guarding the common AV junction can have a common valvar orifice guarded by five leaflets, inclusive of superior and inferior bridging leaflets that have commitment to both ventricles, or discrete left and right orifices that are separated by conjoined leaflet tissues ( Fig. 12.5 ). In the latter variant, there are three leaflets in the left-sided valve. AVSDs are usually associated with near equal commitment of each AV valve to each ventricle with equal ventricular sizes (AVSD with balanced ventricles), or there may be ventricular imbalance with relative hypoplasia of either ventricle, which may be so severe as to result in a functionally univentricular circulation.
