Abstract
Persistent truncus arteriosus is a relatively rare cardiac anomaly, occurring in 0.4% to 4% of individuals with congenital heart disease. The condition is characterized by a single arterial trunk arising from the heart, overriding the ventricular septum and receiving blood from both ventricles. This persistent truncal artery supplies blood to the systemic, coronary, and pulmonary circulations. This chapter will discuss the natural history, embryology, anatomy, and physiology of truncus arteriosus as well as the clinically relevant aspects of classification, presentation, diagnosis, surgical correction, and postoperative management.
Key Words
truncus arteriosus, congenital heart disease, truncus valve insufficiency
Persistent truncus arteriosus is a relatively rare cardiac anomaly, occurring in 0.4% to 4% of individuals with congenital heart disease. The condition is characterized by a single arterial trunk arising from the heart, overriding the ventricular septum and receiving blood from both ventricles. This persistent truncal artery supplies blood to the systemic, coronary, and pulmonary circulations. This chapter will discuss the natural history, embryology, anatomy, and physiology of truncus arteriosus as well as the clinically relevant aspects of classification, presentation, diagnosis, surgical correction, and postoperative management.
Natural History
The prognosis of unrepaired persistent truncus arteriosus is poor, with mortality rates in the first year of life exceeding 70%. The majority of deaths that occur during infancy result from severe congestive heart failure (CHF). As pulmonary vascular resistance (PVR) decreases shortly after birth, excessive pulmonary blood flow results, leading to pulmonary edema, left ventricular (LV) volume overload, and ultimately death from CHF in most patients. Rarely, sudden cardiac death has been reported in preoperative patients with truncus arteriosus. Some of these cases are linked with significant truncal valve stenosis, whereas ventricular arrhythmia induced by myocardial ischemia has been implicated in others. Unrepaired patients who survive infancy generally develop severe pulmonary vascular occlusive disease early in childhood. Case reports have documented patients with unrepaired truncus arteriosus surviving into adulthood. Survival in these patients has been attributed to pulmonary artery stenosis or increased PVR, which limits the pulmonary blood flow and slows the development of CHF.
Embryology
Truncal ridges form in the truncus arteriosus during week 5 of gestation and become continuous with the conal septum superiorly. These ridges eventually fuse to separate the truncus arteriosus into two channels, the aorta and pulmonary trunk. The spiral formation of these ridges results in the normal orientation of the aorta and pulmonary artery (PA), with the aorta positioned posteriorly and to the right of the PA. The conus cordis gives rise to the LV and right ventricular (RV) outflow tracts when the conal septum is complete. The truncal and conal septa then fuse, creating RV-to-PA and LV-to-aortic continuity. Persistent truncus arteriosus results from failure of the truncal ridges and aortopulmonary septum to develop and divide into the aorta and pulmonary trunk.
The mechanism for failure of the truncal septation remains unclear. Deficiencies in neural crest development and migration have been implicated as a possible mechanism for conotruncal anomalies. Truncus arteriosus and other arch anomalies have been associated with deletion of chromosome 22q11. DiGeorge syndrome (velocardiofacial syndrome), a monoallelic microdeletion of chromosome 22q11, is characterized by conotruncal heart defects, hypoplasia of the thymus and parathyroid gland, craniofacial dysmorphisms, and developmental delay. A microdeletion of chromosome 22q11 has been identified in 20% to 40% of cases of truncus arteriosus.
Anatomy
The anatomy of truncus arteriosus is best described by failure of the PA to separate from the aorta during development, leading to a large common arterial trunk that serves as outflow for both ventricles. The systemic, coronary, and pulmonary blood flow all arise from this common arterial trunk ( Fig. 55.1 ). The segmental anatomy of truncus arteriosus can be described as situs solitus with a D -looping ventricle ( Fig. 55.2 ). The common artery arises from a common semilunar valve that overrides a large ventricular septal defect (VSD), frequently with malalignment to the right. The common semilunar valve, or truncal valve, may have a variable number of cusps, with three cusps occurring most frequently.
Collett and Edwards established a classification system based on the origins of the PAs from the truncal artery in 1949. Van Praagh and Van Praagh classified truncus arteriosus based on the morphology of the conotruncal septum and the presence of associated anomalies. Russell et al. recently proposed a “simplified” categorization for the common arterial trunk that places emphasis on the nature of the systemic pathways. In this system, groups are assigned with either aortic or pulmonary dominance of the common arterial trunk.
Classification
The system established by Collett and Edwards describes type I as an arterial trunk originating from the common semilunar valve with its immediate bifurcation into a PA and ascending aorta ( Fig. 55.3 ). Collett and Edwards type I therefore has common origin of the right and left PAs. Type II defects refer to the separate origin of the left and right PAs from the posterior wall of the truncal artery. Type III describes anatomy similar to that of type II but with the right and left PAs originating farther apart. In type IV, often referred to as pseudotruncus, the main PA is absent, with the lungs receiving their blood supply through aortopulmonary collaterals. Most would agree that this entity should not be described as a truncus defect but rather is a form of pulmonary atresia with VSD and major aortopulmonary collateral arteries.
Richard Van Praagh and Stella Van Praagh classified truncus arteriosus based on the presence or absence of the conotruncal septum. When the conal septum fails to form, a conal-type VSD results. The system uses the designation A to represent the presence of a VSD and B for the absence of a VSD. The variable development of the truncal septum defines their specific category. In Van Praagh type 1 the truncal septum is partially developed so that a PA and aorta coexist. Type A1 is identical to type I of Collett and Edwards. In Van Praagh type 2, complete absence of the truncal septum is seen, with the main PAs originating from the truncal artery separately. This includes Collett and Edwards type II and most cases of type III. Van Praagh type 3 is characterized by the absence of one PA originating from the truncal artery. Most commonly, type 3 will include the right PA originating from the common trunk, with pulmonary blood supply to the left lung provided by a PA arising from the aortic arch (a subtype of Collett and Edwards type III) or by systemic to pulmonary collaterals. The term hemitruncus is sometimes used when one PA takes origin from the ascending aorta ( Fig. 55.4 ). Van Praagh type 4 describes any type of truncus associated with an interrupted arch defect. The arch anomaly is usually a type B interruption with the descending aorta receiving its blood supply from a large patent ductus arteriosus (PDA) and the PAs originating from the truncal artery. The Van Praagh system allows a clearer anatomic description of the defect, allowing better preoperative planning for repair. It also eliminates those defects without at least one PA originating from the truncus.
The classification system proposed by Russell et al. assigns groups with either aortic or pulmonary dominance ( Fig. 55.5 ). In this system, truncus arteriosus with aortic dominance describes hearts with a common arterial trunk with both PAs coming from truncus and an unobstructed aortic arch. Therefore truncus arteriosus with aortic dominance includes Collett and Edwards types 1, 2, and 3 and also includes Van Praagh types 1, 2, and 3. Meanwhile, truncus arteriosus with pulmonary dominance describes hearts with an obstructed aortic arch, including hearts with coarctation, hypoplastic aortic arch, and interrupted aortic arch (Van Praagh type 4). Russell et al. therefore describe that pulmonary dominance is found only when the aortic component of the trunk is hypoplastic and an arterial duct supplies the majority of flow to the descending aorta. In this setting (with pulmonary dominance) the aortic component is discrete from the pulmonary component within the pericardial cavity, and the PAs arise from the sides of the major pathway. In cases with aortic dominance the common trunk itself supplies the arch vessels and descending aorta; the PAs arise close together from the dorsal surface of the arterial trunk.
Associated Anomalies
Various cardiovascular anomalies have been described in association with truncus arteriosus, many of which have important implications in management and outcome. Variations in coronary artery anatomy can add significant complexity and risk to repair and are relatively common, occurring in 15% to 49% of cases. Coronary ostia can originate in various positions from within the truncal artery or as a single coronary with a variable epicardial course. There is a strong tendency for the left coronary artery to arise from a more posterior level than it does normally from the aorta. Anomalous origin of the left coronary artery from the right coronary will cross the RV outflow tract (RVOT). This is of particular importance when planning the construction of the RV-to-PA conduit. When the left main coronary ostium originates more superiorly in the commissure, care must be taken to avoid injury during separation of the PAs.
Structural abnormalities of the truncal valve may have significant clinical implications and must be considered during management. The truncal valve may have a variable number of cusps, with three cusps reported in 64% of cases, four cusps in 27%, and two cusps in 8%. Truncal valve incompetence has long been linked to poor outcomes, and a recent analysis of Congenital Heart Surgeons’ Society data confirms this in a series of 572 patients in which mortality for repair of truncus arteriosus with truncal valve repair was 30% versus 10% for isolated truncus repair. Truncal valve insufficiency leads to ventricular dilation and low diastolic coronary perfusion. Truncal valve stenosis is often mild or even exaggerated because of the excessive left-to-right shunt and volume overload. It is unusual to have severe truncal valve stenosis that requires intervention at the time of initial repair. These scenarios in combination with increased diastolic pulmonary runoff can lead to significant myocardial ischemia.
The VSD in truncus arteriosus is similar to the malalignment defect of tetralogy of Fallot and results from the failure of the conal septum to develop and rotate. The defect is often large, nonrestrictive, with the superior border being formed by the truncal valve. The inferior and anterior borders are formed by the two limbs of the septomarginalis trabeculation (SMT). In two-thirds of cases the posterior arm of the SMT and the ventriculoinfundibular fold join to separate the VSD from the septal leaflet of the tricuspid valve. This separation places the conduction system away from the inferior border of the VSD and less likely to be injured during repair. In one-third of cases, a deficiency of the ventriculoinfundibular fold and posterior division of the SMT exists, with the defect extending to the annulus of the tricuspid valve. This leaves the conduction system close to the inferior edge of the VSD and vulnerable to injury during closure.
Another abnormality associated with truncus arteriosus is interruption of the aortic arch; most commonly type B. Approximately 10% of patients with truncus arteriosus will have an interruption of their aortic arch. The highest-mortality group for truncus arteriosus repair was among patients who underwent concomitant repair of interrupted aortic arch and truncal valve repair (60% mortality). Other defects of surgical significance occurring with truncus arteriosus include right aortic arch (18% to 36%), left superior vena cava, aberrant subclavian artery, and atrial septal defect. Pulmonary artery branch stenosis can also occur. Noncardiac anomalies are present in approximately 20% of cases and may contribute to death. DiGeorge syndrome is associated with truncus arteriosus in 30% of cases. Less common defects associated with truncus arteriosus include tethered-cord syndrome, unilateral renal agenesis, and anal atresia.
Physiology
Truncus arteriosus is a complete admixture lesion. Blood from both the LV and the RV is ejected through the single semilunar valve and into the truncal artery. Relative blood flow, and thus oxygen saturation, is dependent on the relative resistance of each circulation. Pulmonary blood flow may be limited by stenosis of the PAs, but this is uncommon. In most instances, pulmonary blood flow (and thus arterial oxygen saturation) is determined by the resistance to flow in the pulmonary vascular bed. In the perinatal period, elevated PVR will limit pulmonary blood flow, and patients will frequently be cyanotic with oxygen saturations between 75% and 80%. After the second week of life, PVR decreases, and the pulmonary-to-systemic (Q p :Q s ) flow ratio exceeds 1. Increased pulmonary blood flow results in decreased cyanosis with oxygen saturations in the low 90% range. Pulmonary blood flow may, however, become torrential, leading to volume overload of the LV, pulmonary edema, and decreased systemic oxygen delivery. Left-to-right shunting of blood from the common trunk into the PAs occurs both in systole and diastole, increasing the Q p :Q s relative to other shunt lesions like an isolated large VSD. The increased pulmonary diastolic runoff may also contribute to decreased coronary artery perfusion as well as the development of intestinal ischemia and necrotizing enterocolitis. The development of CHF is accelerated in the presence of an insufficient truncal valve. In most cases of truncus arteriosus signs of CHF are present by the second to third weeks of life. Over time the continued exposure of the pulmonary vascular bed to excessive blood flow leads to the development of pulmonary vascular obstructive disease. The PVR increases, and the pulmonary-to-systemic ratio approaches 1 or less. A point is reached at which these changes become irreversible.
Clinical Presentation
Truncus arteriosus is generally diagnosed in the neonatal period, with a presenting sign of mild cyanosis. As the child develops CHF, symptoms of dyspnea, diaphoresis, and failure to thrive will become apparent. Physical examination usually reveals a systolic thrill and murmur over the left third and fourth intercostal spaces parasternally. The patient has a jerky, collapsing arterial pulse due to the rapid runoff from the truncal artery into the pulmonary circulation. The apical impulse is prominent, and signs of cardiomegaly are noted. The second heart sound is single and accentuated. When truncal valve incompetence is present, a diastolic murmur follows the second heart sound. Hepatomegaly is often present.
Diagnosis
The first diagnostic test usually obtained is a chest radiograph, which reveals cardiomegaly with biventricular enlargement and increased pulmonary vasculature. The aortic arch is to the right in 20% of patients, and the left PA may be elevated from the normal position. The superior mediastinum may be narrow due to the absent main PA. The electrocardiogram findings are nonspecific and usually indicate biventricular hypertrophy.
Echocardiogram is currently the gold standard for diagnosis of truncus arteriosus. In most cases, echocardiogram will detail all relevant anatomy for surgical planning. The parasternal long-axis views will demonstrate a single great vessel overriding the ventricular septum ( and ). The parasternal short-axis views allow evaluation of truncus valve morphology, whereas apical and subcostal views will give the best assessment of truncal valve stenosis and insufficiency. Characterization of truncal valve function using color and spectral Doppler to evaluate for insufficiency or stenosis is critical. In addition to evaluating the outflow VSD, a diligent search for any additional VSDs must be performed. The origin of the left and right PAs must be established and investigation for pulmonary stenosis performed. Coronary artery anomalies are frequent in truncus arteriosus, and thus coronary anatomy must be fully delineated. The origin of the coronary ostia is of particular importance because the left main ostia may originate more superiorly in the common trunk and must be avoided when separating the PAs. Additionally, anomalous origin of the left coronary artery from the right coronary will cross the RVOT and is susceptible to injury during construction of the RV-to-PA conduit. Finally, the aortic arch through the proximal descending aorta must be evaluated for interruption
Cardiac catheterization ( Fig. 55.6 ) is reserved for cases in which anatomy could not be clearly defined on echocardiogram, additional information is needed concerning the truncal valve, or the PVR warrants further assessment. Pulmonary vascular resistance is usually only mildly elevated (2 to 4 Wood units/m 2 ) in infants younger than 3 months. In the setting of elevated or fixed PVR, operative correction may not be tolerated.
