Discontinuation of the Aortic Arch
Introduction
Congenital cardiac defects are often easiest to understand by asking three simple questions:
Where is the anatomical abnormality?
How does it alter blood flow?
Is enough oxygenated blood reaching the systemic circulation?
This provides the basis for the useful, if simplified, distinction between the “pink baby” and the “blue baby.”
A pink baby has enough oxygenated blood reaching the body to avoid obvious central cyanosis. A blue baby has significant deoxygenated blood entering the systemic circulation.
Interrupted Aortic Arch does not fit neatly into either category.
It is fundamentally an obstructive lesion of the systemic circulation, but because the lower body is often supplied through the ductus arteriosus, oxygenation can differ above and below the interruption.
This makes it an excellent example of why congenital heart disease is best understood through anatomy, pressure, resistance and direction of blood flow, rather than colour alone.
Body
What is Interrupted Aortic Arch?
Interrupted Aortic Arch — IAA — is a congenital abnormality in which there is complete loss of continuity between portions of the aortic arch.
It can reasonably be considered the extreme end of the spectrum of aortic arch obstruction.
In coarctation of the aorta, there is a narrowed pathway.
In Interrupted Aortic Arch, there is no continuous pathway at all.
The ascending aorta and vessels arising before the interruption are supplied by the left ventricle.
The descending aorta, however, is physically separated from this route.
In the newborn, blood reaching the descending aorta therefore depends upon the patent ductus arteriosus connecting the pulmonary artery to the distal aorta.
This makes Interrupted Aortic Arch a duct-dependent systemic circulation.
Without the ductus, there may be no effective route for blood to reach a substantial proportion of the body.
Normal Aortic Arch Anatomy
Blood leaves the left ventricle through the aortic valve and enters the ascending aorta.
The aorta then curves posteriorly to form the aortic arch.
Three major arterial branches normally arise from the arch:
1. Brachiocephalic artery
This subsequently divides into the right common carotid and right subclavian arteries.
2. Left common carotid artery
This supplies the left side of the head and neck.
3. Left subclavian artery
This principally supplies the left upper limb.
After the origin of the left subclavian artery, the arch continues into the descending thoracic aorta.
The descending aorta subsequently supplies the abdominal organs, kidneys, spinal circulation, pelvis and lower limbs.
In the normal circulation this represents one continuous arterial pathway from the left ventricle to the systemic circulation.
In Interrupted Aortic Arch, that continuity is broken.
Development of the Aortic Arch
The mature aortic arch develops from a complex embryological system involving the primitive aortic arches.
These paired vascular structures appear, remodel and regress during early development.
Different parts of the mature great vessels are derived from different embryological segments.
Normal formation therefore depends not only upon vessels developing, but upon the correct portions persisting while others disappear.
Interrupted Aortic Arch results when part of the developing arch fails to form or regresses abnormally.
The precise anatomical site at which continuity is lost determines the type of Interrupted Aortic Arch.
The Three Main Types
Interrupted Aortic Arch is traditionally classified according to the Celoria and Patton classification.
Type A
In Type A, the interruption occurs distal to the left subclavian artery.
All three normal branches of the aortic arch arise proximal to the interruption.
The interruption then lies between the left subclavian artery and the descending aorta.
Type B
In Type B, the interruption occurs between the left common carotid artery and the left subclavian artery.
The left subclavian artery therefore arises from the distal portion of the circulation rather than from the proximal arch.
Type B is the most common form.
The left subclavian artery has not disappeared.
It remains present but lies on the opposite side of the interruption from the brachiocephalic and left common carotid arteries.
Type C
In Type C, the interruption occurs between the brachiocephalic artery and the left common carotid artery.
The brachiocephalic artery arises proximal to the interruption, while the left common carotid and left subclavian arteries arise distally.
Type C is the rarest form.
Why the Ductus Arteriosus is Essential
During foetal life, Interrupted Aortic Arch may be relatively well tolerated.
This is because the foetal circulation already contains a normal vascular bypass around much of the aortic arch.
That bypass is the ductus arteriosus.
The ductus connects the pulmonary artery to the descending aorta.
Because pulmonary vascular resistance is high before birth, much of the right ventricular output passes from the pulmonary artery through the ductus into the descending aorta.
Consequently, despite complete interruption of the aortic arch, the lower body can continue to receive blood before birth.
Birth changes everything.
What Happens After Birth?
With the first breaths, the lungs expand.
Pulmonary vascular resistance falls.
The placenta is removed from the circulation and systemic vascular resistance rises.
Under normal circumstances, the ductus arteriosus begins to constrict because it is no longer required.
In Interrupted Aortic Arch, however, the ductus is not redundant.
It is the principal route supplying the descending aorta.
As the ductus constricts, systemic blood flow below the interruption progressively decreases.
This means blood supply can deteriorate to:
- the abdominal organs;
- kidneys;
- spinal circulation;
- bowel;
- pelvis;
- and lower limbs.
A newborn who initially appeared relatively stable can therefore deteriorate dramatically as ductal closure progresses.
Interrupted Aortic Arch is consequently a classic duct-dependent congenital cardiac emergency.
Where Does the Descending Aortic Blood Come From?
Normally:
Left ventricle → Aorta → Aortic arch → Descending aorta
In IAA this pathway is interrupted.
Instead:
Right ventricle → Pulmonary artery → Ductus arteriosus → Descending aorta
This explains why the ductus must remain open and why closure can cause cardiovascular collapse.
It also explains why oxygen saturation in the lower body may differ from saturation in the upper body.
The Importance of Mixing
Many babies with Interrupted Aortic Arch also have a Ventricular Septal Defect (VSD).
This is often central to the physiology.
The VSD allows blood to mix between the ventricles.
The right ventricle can then eject mixed blood into the pulmonary artery.
Some travels to the lungs.
Some passes through the ductus arteriosus and into the descending aorta.
The neonatal circulation therefore depends upon a balance between:
intracardiac mixing;
pulmonary blood flow;
systemic blood flow;
pulmonary vascular resistance;
systemic vascular resistance;
and
ductal patency.
Differential Cyanosis
Interrupted Aortic Arch demonstrates why one oxygen saturation value does not always describe the whole circulation.
Blood reaching the upper body may come directly from the left ventricle through the proximal aorta.
Blood reaching the descending aorta may arrive through the pulmonary artery and ductus arteriosus.
The lower body may therefore receive blood with a lower oxygen content.
The baby can consequently appear relatively pink above and more cyanosed below.
This is termed differential cyanosis.
Pre-ductal and post-ductal oxygen saturations can therefore provide different information.
A right-hand saturation is commonly used as a pre-ductal measurement.
A foot saturation provides a post-ductal measurement.
Presentation
A baby with Interrupted Aortic Arch may initially appear surprisingly well while the ductus remains open.
As ductal constriction progresses, the infant can deteriorate rapidly.
Clinical features may include:
- tachypnoea;
- respiratory distress;
- poor feeding;
- lethargy;
- weak peripheral pulses;
- cool lower limbs;
- poor capillary refill;
- oliguria or anuria;
- metabolic acidosis;
- increasing lactate;
- heart failure;
- and cardiogenic shock.
The presentation may resemble neonatal sepsis, making recognition of the cardiovascular physiology particularly important.
Upper and Lower Body Perfusion
The site of interruption determines which vessels remain connected to the proximal ascending aorta.
This means blood pressure and pulse findings can vary according to whether the lesion is Type A, B or C.
For example, in Type B:
The brachiocephalic artery and left common carotid artery originate proximal to the interruption.
The left subclavian artery originates distal to it.
Consequently, perfusion of the left arm may differ from perfusion of the right arm.
This is why accurate anatomical diagrams are particularly important when teaching IAA.
Associated Ventricular Septal Defect
A VSD is one of the most common abnormalities associated with Interrupted Aortic Arch.
The anatomy can also be associated with narrowing of the left ventricular outflow tract — LVOT.
The surgeon must therefore consider more than simply reconnecting the two ends of the aorta.
The entire pathway from:
left ventricle → LVOT → aortic valve → ascending aorta → reconstructed arch
must be capable of supporting systemic blood flow.
Association With 22q11.2 Deletion
Interrupted Aortic Arch, particularly Type B, has an important association with 22q11.2 deletion syndrome.
This condition has historically been known by several names, including DiGeorge syndrome and velocardiofacial syndrome.
The deletion can affect development of structures derived from the pharyngeal arches and pouches and is associated with several conotruncal and aortic arch abnormalities.
Affected children may also have abnormalities involving:
- calcium regulation;
- immune function;
- the palate;
- and other developmental systems.
For the perioperative team, recognition of possible hypocalcaemia is particularly important.
Diagnosis
Echocardiography
Echocardiography is central to diagnosis.
It can identify:
- the site of interruption;
- the relationship of the head and neck vessels;
- ductal patency;
- ventricular septal defects;
- ventricular function;
- aortic and pulmonary valve anatomy;
- and possible left ventricular outflow obstruction.
CT and MRI
Cross-sectional imaging can provide detailed information about the aortic arch and branch vessels.
CT angiography can be particularly useful for demonstrating the spatial relationship between:
- the brachiocephalic artery;
- left common carotid;
- left subclavian;
- ductus arteriosus;
- descending aorta;
- and any aberrant vessels.
Initial Management
Once Interrupted Aortic Arch is suspected, maintaining systemic blood flow becomes the immediate priority.
The key treatment is prostaglandin E1 to maintain or restore ductal patency.
Other management may include:
- respiratory support;
- correction of acidosis;
- inotropic support;
- careful fluid management;
- correction of hypoglycaemia or electrolyte disturbance;
- and urgent transfer to a congenital cardiac centre.
The aim is to stabilise the circulation until definitive surgical repair can be performed.
Surgical Repair
Definitive treatment requires reconstruction of the aortic arch.
The exact technique depends upon:
- the type of interruption;
- the distance between the proximal and distal aorta;
- the size of the arch;
- associated VSD;
- LVOT anatomy;
- and other cardiac defects.
The general objective is straightforward:
restore continuity between the ascending and descending aorta.
This may involve direct anastomosis between the distal arch and descending aorta or more extensive reconstruction where the arch is hypoplastic.
If a VSD is present, it may be repaired during the same operation.
Because these are often neonates with complex associated anatomy, repair commonly requires cardiopulmonary bypass.
Cardiopulmonary Bypass and Cerebral Perfusion
Arch reconstruction creates a particular problem.
The surgeon needs to operate on the major arterial pathway supplying both the brain and the rest of the body.
Various perfusion strategies may therefore be used to protect the brain and other organs while the arch is reconstructed.
These may include:
- hypothermia;
- selective cerebral perfusion;
- and carefully controlled periods of reduced systemic flow.
The exact approach varies between centres and according to anatomy.
For the theatre practitioner, understanding the principle is more important than memorising one technique:
while the arch is being reconstructed, normal systemic circulation cannot simply continue unchanged.
Anaesthetic and Theatre Considerations
Interrupted Aortic Arch is fundamentally a problem of systemic perfusion.
A normal-looking oxygen saturation does not necessarily mean the circulation is adequate.
Important considerations include:
- pre-ductal and post-ductal saturation;
- upper- and lower-body perfusion;
- invasive arterial pressure monitoring;
- lactate;
- acid-base status;
- urine output;
- temperature;
- glucose;
- calcium;
- and cerebral oxygenation where available.
Maintaining ductal patency before repair is critical.
Sudden loss of ductal flow can result in profound systemic hypoperfusion.
The anaesthetic and surgical teams must therefore understand exactly which vascular territories are being supplied by which part of the circulation.
After Repair
Successful reconstruction restores continuity between the ascending and descending aorta.
However, long-term follow-up remains essential.
Potential complications include:
- recurrent arch obstruction;
- residual stenosis;
- systemic hypertension;
- left ventricular outflow obstruction;
- aortic valve disease;
- and complications related to associated defects.
Patients therefore require continued congenital cardiac surveillance.
Interrupted Aortic Arch in One Sentence
Interrupted Aortic Arch is a complete break in continuity of the aortic arch in which lower-body systemic blood flow after birth is usually dependent upon a patent ductus arteriosus.
And the most important practical question is:
What happens when the ductus closes?
The answer explains why the condition is a neonatal cardiac emergency.
Glossary of Terms
Aorta
The principal systemic artery carrying oxygenated blood from the left ventricle to the body.
Aortic Arch
The curved portion of the aorta connecting the ascending and descending aorta and giving rise to the major vessels supplying the head, neck and upper limbs.
Brachiocephalic Artery
The first major branch of the normal aortic arch. It subsequently divides into the right common carotid and right subclavian arteries.
Cardiopulmonary Bypass — CPB
A technique in which a heart-lung machine temporarily maintains circulation and gas exchange during cardiac surgery.
Cyanosis
Bluish discoloration of the skin or mucous membranes caused by increased quantities of deoxygenated haemoglobin in the circulation.
Differential Cyanosis
A difference in oxygenation between different parts of the body, commonly between pre-ductal and post-ductal circulation.
Ductus Arteriosus
A normal foetal vascular connection between the pulmonary artery and descending aorta.
Duct-Dependent Systemic Circulation
A circulation in which adequate systemic blood flow depends upon continued patency of the ductus arteriosus.
Interrupted Aortic Arch is a classic example.
Hypocalcaemia
An abnormally low concentration of calcium in the blood.
This can be relevant in patients with 22q11.2 deletion syndrome.
Interrupted Aortic Arch — IAA
Complete discontinuity between portions of the aortic arch.
Left Common Carotid Artery
One of the major branches of the aortic arch, supplying the left side of the head and neck.
Left Subclavian Artery
One of the major branches of the aortic arch, principally supplying the left upper limb.
Left Ventricular Outflow Tract — LVOT
The pathway by which blood leaves the left ventricle and enters the aorta.
Patent Ductus Arteriosus — PDA
Persistence of the ductus arteriosus after birth.
In IAA, ductal patency is essential to maintaining lower-body systemic blood flow before surgical repair.
Post-Ductal
Refers to circulation or measurements distal to the point at which the ductus arteriosus enters the aorta.
A foot oxygen saturation is commonly used as a post-ductal measurement.
Pre-Ductal
Refers to circulation or measurements proximal to the ductus arteriosus.
A right-hand oxygen saturation is commonly used as a pre-ductal measurement.
Prostaglandin E1 — PGE1
A medication used to maintain or restore patency of the ductus arteriosus in duct-dependent congenital heart disease.
Pulmonary Vascular Resistance — PVR
The resistance to blood flow through the pulmonary circulation.
Systemic Vascular Resistance — SVR
The resistance to blood flow through the systemic circulation.
Ventricular Septal Defect — VSD
An abnormal communication between the right and left ventricles.
VSD is frequently associated with Interrupted Aortic Arch.
22q11.2 Deletion Syndrome
A chromosomal deletion syndrome associated with a range of congenital abnormalities, including conotruncal and aortic arch defects.
It has historically included conditions described as DiGeorge syndrome and velocardiofacial syndrome.
