Truncus Arteriosus Imaging

Truncus arteriosus is a rare, congenital heart disease characterized by a single great artery that leaves the base of the heart, giving rise to the coronary, pulmonary, and systemic arteries. Without surgical intervention, death in infancy is the rule. Long-term surgical outcomes are good, but there are often residual and potential complications that require regular, long-term cardiology follow-up. ^[1]

Chest radiography usually is the first investigation performed in the neonatal period. Cardiomegaly is frequently present at birth. Chest radiographic findings usually reveal an increase in pulmonary arterial blood flow manifesting as increased pulmonary vascular markings. The combination of a right-sided aortic arch, cardiomegaly, and increased pulmonary vascularity strongly suggests truncus arteriosus; however, further diagnostic investigations are always needed to confirm the diagnosis. ^{[1, 2, 3]}

Echocardiography is the modality of choice for diagnosing truncus arteriosus. The other investigations noted in this section are complementary. Echocardiography has markedly changed the evaluation, diagnosis, and management of congenital heart disease. Echocardiographic findings, usually diagnostic, demonstrate the origin and configuration of the pulmonary arteries, the ventricular septal defect, the truncus arteriosus, and the aortic arch, as well as the status of the truncal valve. Complete echocardiographic examination can be performed to establish goal-directed treatment, to almost eliminate the need for confirmatory cardiac catheterization, and to provide a cost-effective, feasible tool for close follow-up observation of patients after surgery. ^{[4, 5, 6, 7]}

Echocardiography confirms the diagnosis and can delineate the anatomy in great detail. Cardiac magnetic resonance imaging (MRI), catheterization, and angiography can be used to further assess anatomy and cardiac function if needed for presurgical planning and postsurgical evaluation. Genetic testing is recommended for all patients born with truncus arteriosus due to its frequent association with 22q11 genetic mutations. ^[1]  To make a definitive prenatal diagnosis of truncus arteriosus by fetal echocardiography, visualization of a single arterial outflow tract, presence of a ventricular septal defect, and absence of a pulmonary valve are required. ^{[1, 3]}

For postnatal diagnosis, truncus arteriosus is suggested by the history and physical findings, along with abnormal results on the critical congenital heart disease (CCHD) screening test in the first few days after birth, showing preductal and postductal oxygen saturation less than 95% with mild or unnoticeable cyanosis. Unscreened infants present within the first 2 weeks for evaluation of a heart murmur, or with symptoms of congestive heart failure resulting from increased blood flow to the lungs. These infants exhibit poor feeding, lethargy, tachypnea, costal-sternal retractions, grunting, nasal flaring, tachycardia, or hepatomegaly. ^{[1, 3, 7]}

Computed tomography (CT) scanning is another imaging modality that can be used to assess infants with complex congenital heart disease. ^{[8, 9, 10, 11, 12]} Standard CT scans are useful for evaluating suggested anomalies of the aortic arch, a double aortic arch, and retroesophageal vascular structures indicating an anomalous origin of the subclavian artery. Electron-beam CT (EBCT) scanning accurately defines systemic and pulmonary venous connections, while demonstrating other anomalies associated with truncus arteriosus, including abnormalities of the pulmonary artery.

Contrast-enhanced EBCT and MRI are the noninvasive procedures of choice for diagnosis and exclusion of anomalies of the origin and course of the coronary arteries. Standard CT, EBCT, and MRI are useful, noninvasive techniques for visualizing cardiovascular anatomy in patients with congenital heart disease. EBCT and MRI can also enable assessment of cardiovascular function. MRI appears to be the most suitable of these techniques for detecting congenital heart disease. ^{[13, 14]}

When angiographic and 2-dimensional (2D) echocardiograms are compared, MRI enables accurate anatomic diagnosis of complex congenital heart disease. In some instances, use of MRI can replace the need for invasive cardiac catheterization or can reduce the number of catheterizations required in the care of patients with complex congenital heart disease.

For complex congenital heart disease, MRI is necessary for preoperative assessment in adults and in infants; these results influence surgical planning by providing information about anatomic topography of the vascular malformation and its relation to the bronchial system. MRI is reliable in classifying truncus arteriosus by showing the anatomy of the pulmonary artery. The size of the pulmonary artery and its branches can be measured in transverse, coronal, and sagittal planes. ^{[15, 16]}

Cardiac catheterization is usually performed to confirm anatomic details, to obtain physiologic data regarding pulmonary vasculature, and to accurately calculate pulmonary vascular resistance. ^[2] Cardiac catheterization is important in decision-making regarding the time and type of surgery to be performed (palliative vs corrective).

The role of nuclear imaging in the diagnosis of truncus arteriosus is not well established. However, all other modalities, including cardiac angiography, constitute the standard of care for diagnosing this complex congenital cardiac disease with all of its associated anomalies.

Although imaging modalities have improved tremendously, some limitations remain. One fairly common pitfall with imaging techniques is the suggested presence of a partially formed aorticopulmonary septum, and thus the presence of a main pulmonary segment. At surgery, however, branch pulmonary artery orifices may be found adjacent to one another in the left posterolateral aspect of the common arterial trunk. The surgeon may be unable to excise a main pulmonary artery segment from the common arterial trunk, even when the segment was depicted on images, because images may reveal no actual length. ^[17]

(The images below show radiographic and MRI characteristics of truncus arteriosus.)

In truncus arteriosus, 4 anatomic types are recognized on the basis of the anatomic origin of pulmonary arteries, according to Collette and Edwards. ^[18] Types I-III are shown in the image below.

In type I, a common form, a short pulmonary trunk arises from the truncus arteriosus, giving rise to both pulmonary arteries.

In type II, each pulmonary artery arises separately from, but close to, the other, from the posterior aspect of the truncus.

In type III, each pulmonary artery arises from the lateral aspect of the truncus.

Type IV, referred to as pseudotruncus (seen in the image below), is currently considered to represent a form of pulmonary atresia with ventricular septal defect— that is, a severe form of tetralogy of Fallot—rather than truncus arteriosus. ^[19]

In 1965, Van Praagh introduced a new classification with 4 subtypes, which are demonstrated in the images below.

Type 1 is similar to type I as described by Collette and Edwards. ^[18]

Type 2 is made up mostly of types II and III in the Collette and Edwards classification, for which proximity of the origin of the pulmonary arteries is not specified.

In type 3, a single pulmonary artery branch does not arise from the common pulmonary trunk, instead originating from the ductus arteriosus or directly from the aorta.

In type 4, the aortic arch is hypoplastic or interrupted, and a large patent ductus arteriosus is present.

The Van Praagh classification specifies the presence (subtype A) or absence (subtype B) of ventricular septal defect. Each case is accordingly assigned a nomenclature that includes a letter and a number. ^[20]

Although both classifications have found wide application in clinical cardiology and cardiac surgery, each has limitations. Collette and Edwards type IV is probably a misnomer because it describes a separate entity with different therapeutic and prognostic implications. In addition, cardiothoracic surgeons often refer to a type 1½, which is similar to type I but with a shallow aorticopulmonary segment. This fairly common entity is not included in either classification. ^[17]

The term hemitruncus has fallen out of use, but it refers to a rare anomaly in which a single pulmonary artery branch, usually the right branch, arises from the ascending aorta just above the aortic sinuses, while the main pulmonary artery and the other pulmonary artery branch arise in their normal positions.

Pathophysiology

To understand the pathophysiology of truncus arteriosus, it is first important to understand the path of blood flow and anatomy. In patients with truncus arteriosus, systemic venous blood normally returns to the right atrium and flows into the right ventricle. Pulmonary venous blood from the lungs normally flows through the pulmonary veins into the left atrium and to the left ventricle. The ventricular septal defect allows oxygenated and deoxygenated blood to mix before it is ejected through a common truncal valve to a single great artery, subsequently supplying the coronary, pulmonary, and systemic circulations. Mixing of pulmonary and systemic blood before departure from the heart, combined with the degree of pulmonary blood flow and pulmonary vascular resistance, drives the pathophysiology and the clinical picture of truncus arteriosus. ^[1]

Truncus arteriosus results when proper embryologic processes fail to create a truncoconal septal wall and the single truncal root does not divide into separate aortic and pulmonic outflow tracts, which inhibits the proper creation of separate aortic and pulmonary valves, resulting in a single truncal valve. ^[1]

Although no direct cause is known, truncus arteriosus is frequently associated with 22q11 genetic mutations. ^[1]

Coronary artery anomalies are known to be associated with truncus arteriosus (common arterial trunk). Preoperative delineation of coronary anatomy is important for avoiding intraoperative and postoperative complications. ^[21]

Epidemiology

Truncus arteriosus is seen with an annual incidence of 7 per 100,000 live births. Although it accounts for less than 1% of all congenital heart lesions, it is the cause of 4% of critical congenital heart defects. ^[1]

Truncus arteriosus carries a very poor prognosis. In the absence of early correction, only 12% of patients born with this anomaly survive beyond 1 year. ^[22]

Treatment

Initial management of truncus arteriosus revolves around stabilizing the patient and balancing blood flow through pulmonary and systemic circuits. Care is typically provided in a neonatal or cardiac intensive care setting. ^[1]

Surgery is the only definitive treatment, but repeat interventions are ubiquitous, so long-term follow-up with a congenital heart disease specialist is advised. Without surgical intervention, most patients would die before their first birthday. Few complications are associated with truncus arteriosus before surgical intervention. Most complications occur during the first 48 hours postoperatively. Complications requiring surgical re-intervention in the immediate postoperative period include mediastinal bleeding, pleural effusion, pneumothorax, and cardiac tamponade. ^[21]

Surgery for truncus arteriosus has an early mortality of 3-20%, with long-term survival of approximately 75% at 20 years. Nowadays, truncus arteriosus repair is done most often in the neonatal period, together with single-stage repair of concomitant cardiovascular anomalies. There are many challenging subgroups of patients with truncus arteriosus, including those with clinically significant truncal valve insufficiency, an interrupted aortic arch, or a coronary artery anomaly. In fact, truncal valve competency appears to be the most important factor influencing outcomes after truncus arteriosus repair. ^[23]

Use of a conduit during truncus arteriosus repair invariably requires reoperation on the right ventricular outflow tract. Because of improvement in perioperative techniques over time, many children are now living well into adulthood after repair of truncus arteriosus, albeit with a high rate of reoperation. Despite this, long-term outcomes of truncus arteriosus repair are good, with many patients asymptomatic and enjoying quality of life comparable to that of the general population. ^[23]

Children with repaired truncus arteriosus have increased ascending aortic stiffness and altered left ventricular function, as measured by cardiac MRI. Longitudinal studies and advanced cardiac MRI assessments are warranted to better determine the long-term potential for late aortic complications and to optimize both medical and surgical treatment of these patients after truncus arteriosus repair. ^[24]

Prognosis

Surgery for truncus arteriosus has an early mortality of 3-20%, with long-term survival of approximately 75% at 20 years. Nowadays, truncus arteriosus repair is done most often in the neonatal period, together with single-stage repair of concomitant cardiovascular anomalies. There are many challenging subgroups of patients with truncus arteriosus, including those with clinically significant truncal valve insufficiency, an interrupted aortic arch, or a coronary artery anomaly. In fact, truncal valve competency appears to be the most important factor influencing outcomes after truncus arteriosus repair. ^[23]  

As the result of improvements in perioperative techniques over time, many children are now living well into adulthood after repair of truncus arteriosus, albeit with a high rate of reoperation. Despite this, long-term outcomes of truncus arteriosus repair are good, with many patients asymptomatic and enjoying a quality of life comparable to that of the general population. ^[23]  

To improve outcomes, an interprofessional approach to truncus arteriosus is recommended. Team-based healthcare delivery, including high-risk obstetricians and fetal cardiac imaging specialists, starts very early after conception for these patients. After delivery, the timing of surgical re-intervention and/or transcatheter intervention is important for enhancing outcomes. Surgery is the only definitive treatment, but repeat interventions are ubiquitous, so long-term follow-up with a congenital heart disease specialist is advised. Without surgical intervention, most patients would die before their first birthday. ^[1]

Truncus arteriosus is a congenital heart defect that is associated with high resource use, cost, and mortality. In a multicenter study, Johnson et al found that among neonates undergoing truncus arteriosus repair, costs are lower and outcomes better at high-volume centers, thus resulting in higher value at all ages of repair. Value-based interventions should be considered to improve outcomes and decrease costs in truncus arteriosus care. ^[25]

Echocardiography is one of the most frequently used techniques for diagnosing cardiovascular disease. ^[26] This diagnostic modality facilitates comprehensive evaluation of the cardiovascular system. Standard echocardiographic views (long and short axes, 2- and 4-chamber views) are usually obtained in parasternal, apical, and subcostal positions. Extended echocardiographic examination can be performed with more views as necessary.

Use of 2D echocardiography and Doppler echocardiography, including color-flow techniques, has greatly revolutionized the clinician's ability to accurately discern cardiac anatomy and, in some patients, hemodynamics in malformations of the conotruncus. ^{[27, 28, 29]} At some centers, if echocardiography reveals straightforward anatomy, the patient undergoes repair without the need for angiocardiography. ^[2]

Echocardiography reveals the origin and configuration of pulmonary arteries. It also shows the relationship of the truncus to the left and right ventricles, identifies a ventricular septal defect, defines the morphology and functional derangement of the truncal valve, and assesses the physiologic consequences. ^[5]

(See the image below.)

When visualized from the parasternal view, 3 defects with relatively similar echocardiographic appearances can be confused: truncus arteriosus, pulmonary atresia with ventricular septal defect, and tetralogy of Fallot. However, when a high parasternal short-axis view is obtained by scanning superiorly from the semilunar valve, direct visualization of the origin of the pulmonary arteries usually helps in differentiating the 3 lesions and reveals the origin of the pulmonary artery arising directly from the truncal root in truncus arteriosus. ^[30]

Color-flow imaging further delineates truncus and pulmonary arterial arrangements. Visualization of the truncal origin of the truncal artery or its branches is a major requirement for echocardiographic diagnosis of truncus arteriosus. The truncal valve and its leaflet morphology can be interrogated by using the short-axis view. Color-flow imaging in the long-axis or 4-chamber view is done to establish the presence and degree of truncal valve regurgitation.

Continuous-wave Doppler helps to reveal the presence and the degree of truncal valve stenosis.

Two-dimensional imaging provides information on biventricular function. ^[31]

High-resolution echocardiography can be used to diagnose truncus arteriosus in utero. This allows prenatal counseling and planning of pregnancy, delivery, and prenatal care. ^[32]

As a result of its superb visualization of cardiovascular structures, transesophageal echocardiography (TEE) is increasingly used in the diagnosis of congenital heart disease. ^[6] With a small probe, TEE can be performed in infants and young children. However, TEE is required less often for pediatric patients than for adults. General anesthesia is usually needed to perform TEE in children younger than 9 years. ^[32]

Proper technique and cognitive skills are required for optimal application of echocardiography and for interpretation of its results. Echocardiography is an operator-dependent modality, even more so than other cardiovascular techniques.

Chest radiography is usually the first investigation performed in the neonatal period. Cardiomegaly (seen in the image below) is frequently present at birth. As pulmonary vascular resistance decreases, usually after the second or third day of life, the increase in pulmonary arterial blood flow is considerable and manifests as increased pulmonary vascular markings.

 

In 50% of patients, the left atrium enlarges to accommodate increased pulmonary venous return. This enlargement is best identified beneath the left bronchus on a lateral image. In addition, volume overload on the left side of the heart results in dilatation of the left ventricle. The pulmonary vascular pattern shows evidence of venous congestion due to left ventricular failure with cardiomegaly. Enlargement of the right ventricle and the right atrium emerges when congestive heart failure develops, or when the right ventricle selectively receives regurgitant blood flow across the truncal valve. ^[2]

One third of patients have a right-sided aortic arch. A dilated truncal root (large aortic shadow) is not uncommon.

A superiorly located proximal left pulmonary artery, as seen in type I truncus arteriosus, can be identified on frontal chest radiographs in 10% of patients. The right hilum is elevated in 30% (waterfall or hilar comma sign). The hilar comma sign is especially evident on the opposite side of the aortic arch. ^[33]

In truncus arteriosus with an absent pulmonary artery, ^[34]  pulmonary vascular markings are diminished on the side of the absent pulmonary artery, which usually coincides with the side of the aortic arch. The result is a concave pulmonary segment that is best appreciated on the right anterior oblique view. This finding is noted in 50% of patients when separate pulmonary arterial branches arise directly from the truncus. ^[31]

Among late survivors with high pulmonary vascular resistance, the lungs are oligemic, the main pulmonary artery and the right and left branches are increased in prominence with pruning of the peripheral vascular tree, and the size of the left ventricle is almost normal unless clinically significant truncal valve regurgitation or stenosis occurs. ^[31]

The combination of right-sided aortic arch, cardiomegaly, and increased pulmonary vascularity strongly suggests truncus arteriosus; however, further diagnostic investigations are always needed to confirm the diagnosis. ^[2]

Computed tomography (CT) scanning is another imaging modality that can be used to evaluate the heart. Although it has some disadvantages when compared to MRI, such as lower contrast resolution, inability to image in multiple planes, use of ionizing radiation, and often use of an iodinated contrast agent, CT offers advantages, such as relatively fast imaging time and ability to depict calcification.

Standard CT scans are useful for evaluating suggested anomalies of the aortic arch. ^[13] Contrast-enhanced CT is usually required to demonstrate vascular tissue surrounding the trachea in the presence of a double aortic arch, and it is useful for evaluating the retroesophageal vascular structure, which indicates an anomalous origin of the subclavian artery. The presence of 4 paratracheal vessels arranged symmetrically at the cervicothoracic junction suggests a double aortic arch. ^[35]

Electron-beam CT (EBCT) scans accurately define systemic and pulmonary venous connections and reveal atrial and ventricular septal defects. Normal and abnormal atrioventricular valves can be demonstrated on EBCT. EBCT scans obtained at the base of the heart effectively show congenital anomalies of the arteries. ^[36]

EBCT effectively delineates other anomalies associated with truncus arteriosus, including abnormalities of the pulmonary artery (eg, congenital absence of the artery, as seen in the images below), peripheral coarctations, and hypoplasias. However, multiplanar MRI is the most effective technique for assessing pulmonary arterial anomalies. ^[35]

Contrast-enhanced EBCT and MRI are probably the noninvasive procedures of choice for diagnosis and exclusion of anomalies of the origin and course of coronary arteries. These studies are especially important for showing a course in the ventricular septum or between the base of the aorta and the pulmonary artery when the left anterior descending coronary artery arises from the right coronary artery.

Computed tomography (CT) scanning plays an important supplementary role in assessing patients with truncus arteriosus. Fast multisection spiral CT with high-quality 2D and 3D multiplanar reformatted images can be performed to accurately and systematically evaluate mediastinal vessels, cardiac chambers, and ventricular-arterial connections, as well as coronary artery and valves, through a step-by-step approach. ^[37]

Contrast-enhanced CT of anomalies of the mediastinal vessels is known to be more than 90% accurate. CT demonstration of abnormalities of the great vessels, such as positional anomalies, atresias, and hypoplasias, is equivalent to angiocardiographic depiction, and CT is usually superior to 2D echocardiography.

Although echocardiography showed a ventricular septal defect in a 26-year-old female patient, CT revealed a single arterial trunk overriding the interventricular septum, with a ventricular septal defect and the descending aorta giving rise to the pulmonary arteries, leading to diagnosis of truncus arteriosus type IV. ^[12]

Magnetic resonance imaging (MRI) scans, based on proton-density and proton-relaxation dynamics, differ from images produced by x-rays, which are associated with absorption of x-ray energy. MRI dynamics vary according to the tissue under examination and reflect the tissue's physical and chemical properties.

MRI has evolved sufficiently to be recognized as a useful noninvasive method that is complementary to echocardiography for evaluation of congenital heart disease. In some cases, MRI is superior to other imaging modalities, particularly in evaluating thoracic aortic anomalies, in defining the anatomy of central pulmonary arteries, and in characterizing the morphology of the truncus.

(MRI characteristics of truncus arteriosus are evident in the images below.)

MRI is the procedure of choice for postoperative follow-up observation of patients with congenital heart disease. Technological advances now permit not only morphologic evaluation (with spin-echo and magnetic resonance angiographic techniques) but also collection of functional and flow information (with fast cine gradient-echo and velocity-encoded sequences). As a result, pediatric cardiologists and cardiac surgeons recognize MRI as an essential technique for preoperative and postoperative evaluation of some congenital heart diseases. ^[38]

The volume of shunts, valvular function, and pressure gradients across valves and conduits can be estimated by using velocity-encoded cine MRI (velocity-flow mapping). However, widespread application of echocardiography and Doppler techniques for many of the same purposes influences the clinical use of MRI capabilities. As a consequence, the current clinical role of MRI is to supplement information acquired via echocardiography.

In a study of 27 adult patients (median age, 26 yr) over 5.2 years with repaired truncus arteriosus, MRI revealed that decreased right ventricular ejection fraction and a smaller ascending aorta were associated with adverse events after repair. ^[16]

Degree of confidence

MRI can be used with high diagnostic accuracy for assessment of morphologic and functional features of congenital heart disease. ^[39] MRI is particularly valuable for imaging the heart and great vessels because flowing blood produces a unique signal. Therefore, no contrast medium is required to define cardiac chambers and the lumina nor to reveal locations of the great vessels. Cardiac evaluation requires either electrocardiogram (ECG)-gated MRI or cine MRI.

Several centers have reported effectiveness ratings of MRI for evaluation of congenital heart disease in children and adults. ^{[38, 40]} In several studies in which the results of MRI were corroborated with angiography and/or 2D echocardiography, accurate anatomic diagnosis of anomalies was achieved with MRI in more than 90% of patients. ^{[15, 41]} Diagnostic accuracy of MRI exceeds 90% for abnormalities of atrioventricular connections.

After congenital heart disease is surgically corrected, patients must be monitored for extended periods because morphologic and functional abnormalities may remain or develop. Therefore, a noninvasive imaging tool is mandatory for timely detection of such abnormalities. Echocardiography may be hampered in these patients because scar tissue and thoracic deformities limit the acoustic window. MRI is advantageous in follow-up imaging of postsurgical patients; several techniques can be used for evaluation of morphologic and functional abnormalities and for patient follow-up over time. ^{[15, 40]}

Nuclear medicine studies have had limited usefulness in the diagnosis and treatment of truncus arteriosus; at times, however, nuclear medicine comes into play. Patients with truncus arteriosus may show a discrepancy in pulmonary blood pressure between arteries because of ostial stenosis or previous pulmonary artery banding, and radioisotope lung scanning can help in detecting selective pulmonary arterial resistance.

This resistance cannot be separately estimated in each lung by angiocardiography unless blood flow to each lung is assessed. Furthermore, when the pulmonary artery is absent, perfusion lung scanning can be done to confirm the absence of 1 of the pulmonary arteries and to ascertain the status of peripheral pulmonary arterial flow. ^[30]

Echocardiographic findings are usually diagnostic. These images demonstrate the origin and the configuration of pulmonary arteries, a ventricular septal defect, truncus arteriosus, and the aortic arch, as well as the status of the truncal valve.

Cardiac catheterization (shown in the images below) is usually performed to confirm anatomic details, to obtain physiologic data regarding the pulmonary vasculature, and to accurately calculate pulmonary vascular resistance. ^[2] Cardiac catheterization must be performed with the patient in stable condition, particularly in terms of acid-base balance, if meaningful data are to be obtained.

Since the introduction of flow-directed balloon catheters (Swan-Ganz catheters), which have greatly facilitated entrance to pulmonary arteries in the truncal root to reach the aortic arch and the descending aorta, all necessary information can be obtained by using the venous approach alone. However, the retrograde arterial approach is occasionally chosen, particularly if angiocardiography of the truncal root is used to assess insufficiency of the truncal valve. ^[30]

The cranially angulated angiocardiographic view facilitates visualization of proximal pulmonary arteries.

For patients in whom previous operations were performed and the pericardial space entered, epicardial adhesions may obscure direct visualization of coronary arteries at the time of surgery. For these patients, preoperative selective coronary angiography should be performed if truncal root injection does not satisfactorily provide the answer. ^[30]

For patients with an absent pulmonary artery, pulmonary wedge injection or selective injection into the systemic collateral arteries can be used to identify the pulmonary arterial tree on the affected side. ^[30]

For patients with an interrupted aortic arch, identification of the exact site of interruption in relation to aortic branches is important for planning the appropriate corrective procedure. ^[30]

Use of oximetry has allowed clinicians to identify cardiac shunts at different levels and to accurately calculate the severity of the shunt.

Cardiac catheterization is important for decision-making regarding the time and type of surgery (palliative vs corrective).

Beyond early infancy, pulmonary vascular resistance must be assessed accurately for proper selection of corrective surgery. ^[42] This resistance can be calculated indirectly by dividing mean driving pressure across the pulmonary bed by the total pulmonary flow index.

Patients with truncus arteriosus who have 2 pulmonary arteries and pulmonary arterial resistance greater than 8 units/m² have higher operative and postoperative mortality risks when compared to patients with low resistance. ^{[43, 44]} This difference is due to progression of pulmonary vascular obstructive disease with secondary severe pulmonary hypertension and right ventricular failure.

Angiocardiography tends to cause underestimation of the severity of truncal valve insufficiency. Because pulmonary arteries arise from the truncal root, preferential runoff into the pulmonary circulation may mask insufficiency of the truncal valve, leading to difficulty in assessment. ^[30]