Hemitruncus arteriosus

Hemitruncus Arteriosus: A Comprehensive Review

Introduction

Hemitruncus arteriosus, also known as anomalous origin of one pulmonary artery from the aorta (AOPA), is a rare congenital cardiovascular malformation in which one branch pulmonary artery (either right or left) arises anomalously from the ascending aorta while the other pulmonary artery maintains its normal origin from the main pulmonary artery and right ventricular outflow tract. This condition represents a distinct clinical entity that, despite its rarity, carries significant morbidity and mortality if left untreated due to the rapid development of severe pulmonary vascular disease and heart failure.^{[1][2][3][4][5][6][7][8][9]}

The term “hemitruncus” derives from the embryological relationship to truncus arteriosus, as the condition is believed to result from abnormal aorticopulmonary septation during early cardiac development. However, unlike true truncus arteriosus (where a single arterial trunk gives rise to the systemic, pulmonary, and coronary circulations with two separate semilunar valves absent), hemitruncus features two distinct semilunar valves—a normally positioned aortic valve and a pulmonary valve—with anomalous origin of only one pulmonary artery branch.^{[5][10][11][8][12][13]}

Epidemiology and Prevalence

Hemitruncus arteriosus is an extremely rare congenital cardiac anomaly, accounting for only 0.1% to 0.12% of all congenital heart defects. In a large single-center study of 5,729 patients with congenital heart disease evaluated by multidetector CT angiography, AOPA was identified in only 19 patients, yielding a prevalence of 0.33% among patients with known or suspected congenital heart disease.^{[8][14][15][16][1][5]}

Regarding laterality, anomalous origin of the right pulmonary artery from the aorta (AORPA) is 4 to 8 times more common than anomalous origin of the left pulmonary artery (AOLPA). Studies consistently report AORPA in approximately 85-89% of cases and AOLPA in approximately 11-15%.^{[14][15][16][5][8]}

The condition affects males and females without clear gender predilection, though some series report slight male predominance. Without surgical intervention, the estimated 1-year survival is only approximately 30%, with most patients succumbing to heart failure and pulmonary vascular disease in infancy.^[17][8][14]

Embryology and Etiology

The embryological basis of hemitruncus arteriosus involves abnormal development of the aortic arch system and conotruncal septation during the critical period of cardiac morphogenesis.^{[11][18][13][8]}

Embryological Mechanisms

Several distinct embryological mechanisms underlie the different subtypes of AOPA:^[8]

Proximal AORPA (most common form):

• Results from abnormal aorticopulmonary septation and/or incomplete leftward migration of the right pulmonary artery during its formation^[13][8]
• Commonly associated with aortopulmonary septal defects due to the shared developmental pathway^[8]
• The combination of AORPA with aortopulmonary window further promotes development of interrupted aortic arch or coarctation through hemodynamic effects^[8]

Distal AORPA (approximately 15% of AORPA cases):

• Originates near the innominate artery or aortic arch^[5][8]
• Results from persistence of the right fifth aortic arch with disappearance of the proximal pulmonary artery and distal right sixth aortic arch^[8]
• This mechanism explains the distal narrowing containing ductal tissue sometimes observed in this variant^[8]

AOLPA:

• Essentially an aortic arch anomaly rather than a conotruncal defect^[8]
• Results from failure of development of the left sixth aortic arch with or without persistence of the left fifth arch^[8]
• In the absence of the left sixth aortic arch, the left pulmonary artery fails to connect to the main pulmonary artery, and the aortic sac connection persists^[8]
• Commonly associated with tetralogy of Fallot and right aortic arch^[14][8]

Genetic Associations

While the precise genetic etiology remains incompletely characterized, hemitruncus arteriosus is associated with conditions affecting neural crest cell migration:^[8]

• 22q11.2 deletion (DiGeorge/CATCH 22 syndrome): May be warranted for screening when corroborative clinical features are present^[8]
• Abnormalities in neural crest cell migration contribute to both conotruncal anomalies and aortic arch malformations^[12][8]

Classification

Hemitruncus arteriosus is classified based on the side of anomalous origin (right vs. left) and the site of origin from the aorta:^[18][14][8]

By Laterality

Type	Description	Frequency
AORPA	Right pulmonary artery from ascending aorta	85-89%
AOLPA	Left pulmonary artery from ascending aorta	11-15%

By Site of Origin (Cutsche and Van Mierop Classification)

Type	Site of Origin	Characteristics
Type I (Proximal)	Posterolateral wall of ascending aorta, close to aortic valve	Most common (~85%); associated with PDA, aortopulmonary window
Type II (Distal)	Near innominate artery or aortic arch	~15% of AORPA; requires more extensive surgical mobilization
Type III	Descending aorta	Rare; reported primarily for AOLPA

More recent classifications describe AOPA as:^[18]

• Type I: Proximal ascending aorta
• Type II: Aortic arch or innominate artery
• Type III: Descending aorta

Pathophysiology

The hemodynamic consequences of hemitruncus arteriosus are severe and rapidly progressive:^{[4][19][17][8]}

Hemodynamic Derangement

Affected (Ipsilateral) Lung:

• Receives blood directly from the aorta at systemic arterial pressure^[19][17][8]
• Experiences severe pressure overload leading to rapid development of pulmonary vascular obstructive disease^[17][8]
• Pulmonary arterial hypertension develops as early as 3 months of life without intervention^[8]

Contralateral Lung:

• Receives the entire right ventricular output^[17][8]
• Experiences significant volume overload as all pulmonary blood flow is directed to one lung^[8]
• Both volume and pressure overload contribute to progressive pulmonary vascular disease^[17]

Cardiac Effects:

• Right ventricular volume overload: The entire cardiac output to the lungs must pass through the main pulmonary artery to one lung^[17]
• Left ventricular volume overload: Increased pulmonary venous return from excessive pulmonary blood flow^[17]
• Congestive heart failure: Develops rapidly in infancy as pulmonary vascular resistance falls^[7][4][19]
• Biventricular failure: Progressive without intervention^[17]

Natural History Without Treatment

Without surgical correction:^[1][17][8]

• Severe pulmonary arterial hypertension develops within the first few months of life
• Irreversible pulmonary vascular obstructive disease may develop as early as 6 months of age
• Heart failure is the predominant cause of death in infancy
• 1-year survival: Approximately 30%^[8]
• Most patients do not survive beyond the first year of life^[17]
• Rare adult survivors typically have associated lesions (such as tetralogy of Fallot) that protected one lung from excessive blood flow^[2][13]

Associated Cardiac Anomalies

Hemitruncus arteriosus is frequently associated with other congenital cardiovascular malformations, occurring in up to 95% of cases:^[14][8]

Common Associations

With AORPA (Right-Sided AOPA):

• Patent ductus arteriosus (PDA): Most common association (71% of AORPA cases)^[13][14]
• Aortopulmonary window: Frequently associated with proximal AORPA^[8]
• Interrupted aortic arch or coarctation: May occur due to hemodynamic effects^[8]
• Ventricular septal defect^[13]

With AOLPA (Left-Sided AOPA):

• Right aortic arch: Very common (100% association in some series)^[14][8]
• Tetralogy of Fallot: Frequently associated^[14][8]
• Other conotruncal anomalies^[8]

Less Common Associations

• Atrial septal defect^[20]
• Double outlet right ventricle^[8]
• Pulmonary stenosis^[8]
• Ventricular septal defect^[20][8]
• Subaortic stenosis
• Coronary artery anomalies

The presence of associated lesions significantly impacts surgical planning and outcomes.^[8]

Clinical Presentation

The clinical presentation varies based on age at diagnosis, associated anomalies, and degree of pulmonary vascular disease.^{[4][7][19][8]}

Neonatal Presentation

Most patients present in the first weeks to months of life with:^{[3][7][1][4][8]}

Symptoms:

• Respiratory distress: Tachypnea, retractions, grunting
• Feeding difficulties: Poor feeding, failure to thrive
• Congestive heart failure: As pulmonary vascular resistance drops postnatally
• Cyanosis: Often mild initially, worsening with heart failure

Physical Examination Findings:

• Tachypnea and tachycardia
• Bounding peripheral pulses: Due to aortic runoff into the pulmonary circulation (similar to PDA)
• Widened pulse pressure
• Systolic murmur: Often loud, along left sternal border
• Continuous murmur: May be present if associated PDA
• Hepatomegaly: With progressive heart failure
• Signs of heart failure (gallop rhythm, rales, edema)

Presentation Beyond Infancy

Adult presentation is extremely rare and typically occurs only when associated lesions provide some degree of pulmonary protection:^[21][2][13]

• Exercise intolerance
• Dyspnea on exertion
• Signs of established pulmonary hypertension
• Cyanosis (if Eisenmenger physiology develops)
• Right heart failure

Diagnostic Evaluation

Echocardiography

Two-dimensional transthoracic echocardiography with color Doppler is the primary diagnostic modality:^{[22][4][14][8]}

Key Findings:

• Anomalous origin of one pulmonary artery branch from the ascending aorta
• Normal origin of the contralateral pulmonary artery from the right ventricular outflow tract
• Two separate semilunar valves (aortic and pulmonary)
• Associated cardiac anomalies (PDA, VSD, etc.)
• Assessment of ventricular function
• Estimation of pulmonary artery pressures

Limitations:

• May fail to detect AOPA in up to 15-21% of cases^[22][14]
• Distal origin types are particularly challenging to visualize^[22]
• Associated anomalies may be missed^[14]

Computed Tomography Angiography (CTA)

Multidetector CT angiography (MDCTA) provides excellent anatomical delineation and is increasingly used:^{[15][19][18][14][8]}

Advantages:

• Superior visualization of pulmonary artery anatomy and origin
• Accurate identification of AOPA subtype and origin site
• Detection of associated anomalies (PDA, aortic arch anomalies)
• Three-dimensional reconstruction aids surgical planning^[20]
• Detects anomalies missed by echocardiography (21% additional diagnoses in one series)^[14]

Indications:

• Confirmation of suspected AOPA
• Surgical planning (especially for distal origin types)
• Evaluation of associated aortic arch anomalies
• Postoperative assessment for anastomotic stenosis^[8]

Cardiac Catheterization

Role is primarily reserved for:^[12][8]

• Interventional procedures: Balloon angioplasty or stenting for pulmonary artery stenosis
• Assessment of pulmonary vascular resistance: In late-presenting cases where operability is questionable
• Hemodynamic evaluation: When non-invasive assessment is inconclusive

Limitations:

• Application of Fick’s principle may be erroneous due to admixture to the affected lung^[8]

Cardiac MRI

Cardiac magnetic resonance imaging may be useful for:^[8]

• Accurate quantification of pulmonary blood flow to each lung using phase contrast sequences
• Assessment of ventricular function
• Evaluation of pulmonary vascular resistance when combined with catheterization data

Assessment of Operability in Late Presenters

For patients presenting beyond 6 months of age, assessment of operability becomes crucial:^[8]

Favorable Echocardiographic Features:

• Increased pulmonary venous return to left atrium
• Flow reversal in descending aorta
• Sub-systemic mean and diastolic pulmonary artery pressures
• Predominant left-to-right shunt across patent foramen ovale

Multimodality Assessment:

• Combined cardiac MRI (for flow quantification) and catheterization (for pressure measurement) may help derive pulmonary vascular resistance in each lung bed^[8]
• Theoretically, surgical reimplantation would reduce overall PVR based on parallel resistance principles^[8]

Treatment and Surgical Management

Principles of Management

Early surgical repair is imperative to prevent irreversible pulmonary vascular disease:^{[23][24][1][4][8]}

• Surgery should ideally be performed within the first 3-6 months of life^[14][8]
• Surgical correction before age 1 year enables normalization of pulmonary artery pressure in 92% of patients^[14]
• Delayed repair risks development of severe, potentially irreversible pulmonary vascular obstructive disease^[8]

Surgical Techniques

Standard Approach:

• Median sternotomy
• Cardiopulmonary bypass with bicaval cannulation
• Cold blood cardioplegia
• AOPA is clamped and excised from aorta^[8]

Direct Reimplantation (Most Common Technique):

• The anomalous pulmonary artery is directly reimplanted into the main pulmonary artery^{[9][22][20][8]}
• Employed when AOPA arises close to the MPA (posterolateral or posterior aortic origin)^[8]
• Requires extensive mobilization of the anomalous PA to hilar branches^[8]
• Aortic defect is closed directly or with pericardial/prosthetic patch^[8]

Key Technical Considerations to Prevent Anastomotic Stenosis:^[8]

• Extensive mobilization of anomalous PA to hilar branches
• Harvest PA from aorta with generous cuff
• Adequate mobilization of ascending aorta
• Mobilization of MPA and normal branch PA
• PDA ligation and division
• Pericardial augmentation anteriorly if any anastomotic tension
• Ascending aorta transection in difficult cases for visualization
• Lecompte maneuver if RPA compressed after reconstruction

Alternative Techniques for Distal Origin or Length Discrepancy:

• Single aortic flap technique^[8]
• Double flap technique: Anteriorly-based aortic flap and posteriorly-based MPA flap to elongate PA before anastomosis^[8]
• Interposition conduit: When length discrepancy is very significant (higher reintervention rates)^[8]

Concomitant Repairs:

• Associated cardiac anomalies are typically repaired during the same operation^[8]
• PDA ligation
• VSD closure
• Interrupted aortic arch repair
• Aortopulmonary window closure

Surgical Outcomes

Early Outcomes:

• Surgical mortality: 0-20% in contemporary series, depending on complexity of associated anomalies^[23][8]
• Higher mortality associated with:
  • Complex associated anomalies (interrupted aortic arch, aortopulmonary window)^[8]
  • Distal origin types requiring more complex reconstruction^[8]
  • Late presentation with established pulmonary vascular disease

Long-Term Outcomes:

• Early repair results in excellent hemodynamic and anatomic results^[24][23]
• Survival is excellent with low incidence of reoperation or reintervention when surgery is performed early^[24][23]
• Normalization of pulmonary artery pressures achievable in majority when surgery performed before age 1 year^[14]

Complications and Reintervention

Anastomotic Stenosis

The most common postoperative complication requiring reintervention:^[8]

• Incidence: 12.5% to 36% in various series^[8]
• Often amenable to catheter-based intervention (balloon angioplasty, stenting)
• Redo surgery rarely required

Long-Term Considerations

• Reduced perfusion to affected lung: Has been reported several years after surgery, necessitating long-term follow-up^[8]
• Pulmonary artery growth: Must be monitored, especially in patients repaired as neonates
• Residual pulmonary hypertension: Possible in late-presenting cases

Prognosis

With Early Surgical Repair

The prognosis is excellent when diagnosis is made early and surgical repair performed within the first months of life:^[23][24][14]

• Near-normal survival with appropriate early intervention
• Normalization of pulmonary pressures in >90% when repaired before age 1 year^[14]
• Low reintervention rates with proper surgical technique^[23]

Without Surgical Repair

Prognosis is dismal without intervention:^[17][8]

• 1-year survival: ~30%
• Most patients die in infancy from heart failure
• Development of irreversible Eisenmenger syndrome if survival to childhood/adulthood

Factors Affecting Prognosis

Favorable Factors:

• Early diagnosis and surgical repair
• Isolated AOPA without complex associated anomalies
• Proximal origin type (easier surgical correction)
• Absence of established pulmonary vascular disease

Unfavorable Factors:

• Delayed diagnosis and late presentation
• Complex associated anomalies (interrupted aortic arch, aortopulmonary window)
• Established pulmonary vascular obstructive disease
• Distal origin requiring complex reconstruction

Follow-Up Recommendations

Long-term follow-up is essential for all patients with repaired hemitruncus arteriosus:^[8]

Clinical Monitoring:

• Regular clinical assessment for symptoms of heart failure, exercise intolerance, or arrhythmias
• Assessment of growth and development

Imaging Surveillance:

• Serial echocardiography to assess:
  • Anastomotic site gradient
  • Pulmonary artery pressures
  • Ventricular function
  • Development of anastomotic stenosis
• CT angiography if anastomotic complications suspected or for detailed anatomical assessment^[8]

Timing:

• Close follow-up in the first postoperative year
• Regular cardiology follow-up throughout childhood and into adulthood
• Transition to adult congenital heart disease care when appropriate

Conclusion

Hemitruncus arteriosus is a rare but potentially life-threatening congenital cardiac malformation characterized by anomalous origin of one pulmonary artery branch from the ascending aorta. Without surgical intervention, the condition leads to rapid development of severe pulmonary vascular disease, heart failure, and death in infancy in the majority of cases.

Early diagnosis through echocardiography, supplemented by CT angiography when needed, enables timely surgical intervention. Surgical repair, typically involving direct reimplantation of the anomalous pulmonary artery into the main pulmonary artery, yields excellent outcomes when performed within the first months of life. The key to successful surgical outcomes lies in extensive mobilization, tension-free anastomosis, and appropriate management of associated cardiac anomalies.

Long-term follow-up is essential to monitor for anastomotic complications and ensure adequate pulmonary artery growth. With advances in surgical techniques and perioperative care, the prognosis for patients with early-diagnosed and appropriately managed hemitruncus arteriosus has improved dramatically, with most achieving excellent long-term survival and quality of life.

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