The aorta is the largest artery of the human body, starting at its origin from the left ventricle (separated from the heart by the aptly named aortic valve) and making a U-shaped turn in the upper chest and then coursing down the length of the thorax and abdomen just to the left of the spinal column.  The aorta gives rise to all of the arteries that provide oxygen and nutrients to the brain, extremities, and the organs of the thoracic and abdominal cavities, finally terminating at the bifurcation of the iliac arteries within the pelvic girdle. 

Under normal conditions, the aorta has an internal diameter that remains the same until the arteries begin branching off.  In some instances, diseases can cause alterations of this diameter, such as aneurysms, which are abnormal increases in diameter with ballooning of a segment of the aorta, complete with the weakening of the vascular layers and a high likelihood of rupturing (aneurysms are like a bubble that forms on a weakened segment of a garden hose).  Conversely are congenital (present at birth) or acquired conditions in which there is a narrowing of a segment of the artery, such as coarctation of the aorta, in which a shelf of tissue restricts blood flow through this obstruction to the distal abdominal and lower extremity vasculature.  Acquired causes of this narrowing can include dissection or trauma to the particular segment.  A schematic representation of this condition can be seen at www.lucinafoundation.org/birthdefects-coarctation.html

In the past, there were two basic categorizations for the narrowed lesion associated with coarctation; the type is based on the location with reference to the ductus arteriosus (the thin vessel that direct blood away from the collapsed lungs of the fetus and into the aorta; this tube normally closes within about one week of life to form the ligamentum arteriosum, which helps suspend the heart within the thoracic cavity).¹  When the narrowing occurs before the ductus, it is termed preductal or infantile types; those involving obstruction after the ductus arteriosus are considered to be postductal or adult types.  Current consensus considers all coarctation cases to be considered to simply be juxtaductal, or arising just beyond the ligamentum arteriosum.

The most serious consequence in this disease process is a complete disruption of the aorta, called an interrupted aorta.  In this case, there is no flow through the aorta to the distal aspects of the body and blood flow must find alternative methods of providing oxygen and nutrients, as will be discussed below.  The incidence of this form of aortic malformation represents approximately 1% of all cases of congenital heart defects.²  Interrupted aortic arch is rarely an isolated lesion, but typically part of a more complex defect, such as a ventricular septal defect (abnormal formation of a hole between the two ventricles) and narrowing of the subaortic arch segment.³  There are three basic categories of this condition, based on the location of the interruption of the aorta.  Type A lesions involve disruption of this vessel distal to the left subclavian artery.  Type B interruptions occur between the left subclavian artery and the left common carotid.  Type C involves division of the aorta between the innominate (sometimes called the brachiocephalic) artery and the left common carotid.  Type B is the most common variety, causing up to 50% of cases.  Because there is connection of the ductus arteriosus distal to the site of the interruption in types A and B, infants with these forms are completely dependent on this in utero conduit to provide blood flow to the thoracic and abdominal organs and the lower extremities, thus it is imperative to maintain this vessel in a state of patency (such as with the use of prostaglandin E₁ infusion) until surgical correction of the defect is accomplished.

Likewise, a condition known as hypoplastic left heart syndrome, involves underdevelopment of the left ventricle (hypoplasia), either severe mitral valvular stenosis (restriction of valvular opening) or atresia (complete absence of valvular tissue), either severe aortic valvular stenosis or atresia, and hypoplasia of the ascending aorta and aortic arch.²  It is also not uncommon for there to be coarctation of the aorta as well.  As is the case with interrupted aortic arch, patients with hypoplastic left heart syndrome are completely dependent on perfusion via the ductus arteriosus and it is imperative to maintain patency of this vessel, such as with the use of a prostaglandin E₁ infusion.  This condition may be part of a chromosomal complication, such as trisomies 13 or 18.³  It can also be seen in Turner syndrome, a chromosomal disorder in which there is the absence of one X chromosome and characterized by ovarian failure, genital hypoplasia, dwarfism, shortened metacarpals (bones that make up the palms of the hands), and a shield-like chest formation, as well as the cardiac malformations.⁴  Infants born with this condition typically present with mild heart failure, possibly with murmurs of valvular regurgitation.  Severe cases typically have more pronounced heart failure as well as evidence of end-organ damage (elevations of liver and kidney enzymes, as well as manifestations of circulatory collapse (cardiogenic shock).  These findings become more severe as the ductus arteriosus begins to close at around one week post-delivery.

Causes:

The exact mechanism by which coarctation or interruption of the aorta occurs is not clearly understood.  There are two hypotheses that are used to explain this phenomenon.¹  The first, called the hemodynamic theory, describes an abnormality in the blood flow just before the ductus arteriosus or an exaggeration in the angle of the descending aorta between the ductus and the aorta, compromising blood flow down this normal route and forcing blood across the ductus.  When the ductus arteriosus closes off within the first 7 days after delivery of the newborn, there is an increase in the compromised delivery of blood below this narrowing.

The second theory, termed ectopic ductal tissue theory, described the abnormal deposition of ductal tissue into the aorta.¹  This creates a shelf of tissue that, when combined with the eventual closure of the ductus arteriosus, creates an obstruction to perfusion to of the abdomen and lower extremities.  For many researchers and practitioners, this theory does not provide adequate explanation for the wide varieties of obstruction seen and the impaired development of the aortic arch (hypoplasia).

Prevalence and Associated Conditions:

Coarctation of the aorta accounts for approximately 8% of all congenital heart defects in India.⁵  It is common for this defect to be noted in the first year of life due to the early onset of symptoms.¹  There does not appear to be a racial predilection for this condition, though a gender predominance is seen with the ratio of males presenting with this condition double that of females.

It is also not uncommon for there to be other congenital malformations present concomitant with coarctation.  The most common of these anomalies is bicuspid aortic valve, in which there are two aortic valve leaflets vice the normal three; this is found to occur in greater than 50% of cases of patients with coarctation.  Another relatively common finding is a patent ductus arteriosus.  This occurs in up to 50% of patients with coarctation and is thought to be a result of the increased pressure within the aortic segment proximal to the narrowed region forcing this vessel to remain open.  It is also possible for coarctation of the aorta to be part of a more complex genetic problem, such as Turner syndrome, as well as prematurity, diaphragmatic hernia (segments of the stomach and/or intestines are pushed up into the thoracic cavity through an opening in the diaphragm), and tracheo-esophageal fistula (abnormal connection between the windpipe and the esophagus).⁶  Coarctation of the aorta is the most common cardiac anomaly found in patients with Turner syndrome.

Pathophysiology:

One of the consequences of either coarctation of the aorta or interrupted aortic arch is the development of obstructive shock, in which inhibition of blood flow to key organs results in impaired oxygenation and delivery of nutrients.⁷  This form of impaired tissue perfusion should be suspected when an infant has a history of poor feedings, severe lethargy, oliguria (decreased urine output) or anuria (absence of urine output), decreased or absent distal pulses (such as the femoral pulses), and the progression of acid-base imbalance, in particular metabolic acidosis.

Proximal to the obstruction, the heart is forced to work harder to overcome the increased afterload (resistance to flow out of the ventricle when it contracts) created by the narrowing.¹  The compensatory response to this enhanced workload is the development of ventricular hypertrophy (thickening of the muscle cells and overall muscle mass).  This increase in ventricular wall stress and oxygen demand can lead to congestive heart failure and shock.  This heart failure is exacerbated by the fixed lesion within the aorta and other compensatory mechanisms that occur below the site of the narrowing, leading to a backup in pressure from the left ventricle and left atrial, backwards through the lungs (leading to pulmonary edema – fluid within the air sacs of the lungs – and other problems with breathing), and ultimately to an excessive workload within the right side of the heart.

Below the site of narrowing within the aorta, the kidneys are underperfused.  It is commonly postulated that the compromised perfusion within the kidneys causes stimulation of one of the body’s means of retaining fluid to increase blood pressure – the renin-angiotensin-aldosterone system.  Within this complex series of humoral reactions, the conversion of inactive angiotensin I to the active angiotensin II can cause the body to retain sodium (and thus water) as well as cause constriction of blood vessels within the body, including those above the level of the coarctation.¹  This leads to an increase in blood pressure, especially within the vessels above the level of the coarctation.  This can be worsened by the effects of the mechanical obstruction, such as the need for the heart to pump blood at an exaggerated intensity to adequately perfuse beyond the narrowing.

A normal physiologic compensation that occurs in response to diminished blood flow due to such an obstruction is the sprouting of blood vessels to bypass the narrowed segment, a process called collateralization.  This adaptation is particularly necessary in the extreme form of aortic disruption found with complete aortic interruption.  The elevated pressure, due to the various mechanisms stated above, can cause pronounced dilation of these collateral vessels, resulting in visible pulsations across the back of the patient as noted during physical examination or notching of the ribs when evaluating a chest radiograph.⁸ 

 Clinical Presentation:

A large percentage of individuals who have coarctation of the aorta are asymptomatic.  It is typically those with severe degrees of obstruction that present with various symptoms, with other factors such as the age of the individual and the presence of other accompanying congenital defects that further worsen the condition.⁹  The enhanced perfusion above the lesion can lead to complaints of headaches, nosebleeds, and exertional dyspnea (difficulty breathing).⁸  The elevated pressures can lead to an increase the risk of stroke, aortic dissection or aneurysms, and congestive heart failure.  The differences in perfusion above and below the narrowed segment can lead to a phenomenon of differential cyanosis, in which there is a bluish discoloration of the body below the lesion.  The patient may complain of claudication (aching pain and fatigue within the muscles that increases with exertion, as a result of inadequate delivery of oxygen and nutrients).

One potential finding on physical examination is a differential in the blood pressures between the two arms or between the arms and legs, depending on the location of the obstruction.  The systolic pressures will be higher in the segments above the lesion; a difference of greater than 20 mmHg is considered significant and highly suspicious of underlying coarctation¹⁰.  Auscultation may reveal a harsh systolic murmur best heard along the left sternal border or along the back; the intensity of the murmur is directly proportional to the degree of narrowing (the more intense the murmur, the more severe the restriction).  A systolic click may also be heard along the right upper sternal border, owing to the strong likelihood of a concomitant bicuspid aortic valve.  As a result of the decreased pressures within the lower extremities, pulses will be weaker there on palpation.  Evaluation of the interior of the eye via ophthalmascopy may reveal dilation of the retinal arteries as a result of the sustained hypertension within the upper body.¹¹

Electrocardiography reveals non-specific findings associated with the enlargement of the heart due to increased workload.⁸  Typical features include findings of left ventricular hypertrophy and left atrial enlargement.  It is also not uncommon for the patient to have arrhythmias such as atrial fibrillation due to the dilation of the atrial tissue with subsequent stretching of the normal intra-atrial tracts.

Imaging:

Chest radiography is particularly helpful, potentially revealing notching of the inferior surfaces of the posterior ribs, a finding that occurs as the collateral circulation rubs and wears away a notch within the bone.¹²  It may also be possible to visualize the narrowed aortic segment, which causes the finding of the “3 Sign,” formed by the dilated left subclavian artery (upper notch of the 3) and the dilated distal aorta (the lower aspect of the notch).⁸  More advanced imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI), can better elucidate the indented segment, as well as the presence of collateral blood vessels.

Echocardiography can be useful in identifying not only the level of coarctation, but also delineate associated congenital findings (for example, bicuspid aortic valve or patent ductus arteriosus) and compensatory left ventricular hypertrophy.  The suprasternal window (with the transducer placed in the notch at the top of the breastbone and aimed slightly inferiorly) is particularly useful for locating narrowing within the aortic arch and descending aorta, with color flow Doppler potentially showing the increased velocity of blood moving through the constriction.¹³  Pulsed-wave Doppler can be used to identify the location of the narrowing by the step-up in velocity with continuous wave Doppler showing the systolic gradient.  The use of transesophageal echocardiography (in which a patient is given mild sedation and then has a probe placed behind the heart via the esophagus) may allow for more exact definition of the location, extent (how long the narrowing is), and degree of obstruction.  Infection involving any of the heart valves is a particular concern and any patient who exhibits a fever and worsening of their murmur should have echocardiographic evaluation to look for valvular vegetations (conglomerations of bacteria, debris, and tissue that cling to the tips of the valve) seen with bacterial endocarditis.  Coarctation and various concomitant anomalies are all high risk conditions for inflammation and infection.⁸

Aortography (the placement of a catheter inserted into the proximal aorta from either the femoral or radial arteries with the subsequent injection of a large volume of radioopaque contrast to fill the aorta) can be diagnostic, although it is seldom indicated for definitive diagnosis.¹¹  When used, it can show the narrowing by giving the aorta a characteristic sausage-link appearance.  This contrast injection can also be helpful in visualizing the presence of collateral circulation to the parts of the body distal to the coarctation.  The passage of the catheter across the narrowed segment can also be used to evaluate the pressure gradient, which can then be utilized to determine the degree of the narrowing.  Pressure gradients exceeding 20 mmHg are considered significant and typically warrants surgical intervention.

Clinical Management:

The definitive management for coarctation of the aorta is either surgical removal of the diseased segment or the use of a specialized catheter with a balloon, inserted via the femoral or radial artery, with a balloon that dilates the narrowing when inflated.¹²  The surgical option most commonly used today involves excising the segment with the lesion and either joining the two portions of disease-free aorta in an end-to-end anastomosis or the use of a Dacron or other synthetic graft to connect the gap.⁶  This procedure is performed via a left lateral thoracotomy (opening the chest cavity along the left side of the rib cage rather than through the front of the chest, as would be used for bypass surgery or valvular repair/replacement).⁴  Other potential methods of surgically correcting this problem include patch aortoplasty, in which a patch derived of synthetic material is secured to the aorta near the narrowed segment in order to widen the area.¹  Another surgical correction, called a subclavian flap aortoplasty, involves removal of the coarctation segment and closure of the area using a section of the left subclavian artery, which has previously been ligated (cut and sutured shut).  These last two methods have not been used very often recently due to the higher incidence of complications, such as aneurysms.¹¹

Percutaneous balloon dilatation remains a viable alternative to surgery, with the placement of a self-expanding or balloon-expanding stents helping to reduce the incidence of recurrence of the coarctation.¹⁴  This method typically has a lower rate of post-procedure complications or mortality, with most of the problems arising from the percutaneous nature of the procedure itself (such as infection, bleeding, vasospasm, or retroperitoneal hemorrhage during the initial needle and catheter insertion if performed via the femoral route).

Even following such repair methods there is a rather elevated risk of subsequent complications.  According to a study by Oliver and associates, there was a 16% incidence of complications such as premature coronary artery disease, left ventricular outflow tract abnormalities, and problems with the aorta, such as the development of aneurysms, dissections, and fistulas.¹⁵  Aneurysms occurring at the site of the surgical repair were noted in up to 50% of individuals, with the type of surgical intervention weighing heavily in the overall likelihood of such risks, with end-to-end anastomosis carrying the least potential for such aneurysm formation.  Another potential problem is continuation of the pre-surgical hypertension, with the associated complications that arise from sustained elevations in blood pressure (for example, stroke, myocardial ischemia, damage to vessels within the eyes and kidneys, and impotence).  The prevalence of such residual systemic hypertension is approximately 75% at 30 years.⁴  Even patients who undergo non-invasive balloon dilatation have a 5-10% incidence of re-coarctation.⁸

Besides the aforementioned surgical and transcatheter modalities for restoring and improving perfusion to tissues distal to the obstruction, other pharmacologic agents to maximize perfusion and minimize complications may be required for acutely ill patients who are awaiting more definitive treatment.  Patients in shock may be treated with boluses of crystalloids (normal saline or Lactated Ringer's solution), although this may not be effective in improving distal perfusion due to the inflexible narrowing.⁷  Inotropic agents (those that increase the squeeze, or contractility, of the heart to enhance forward blood flow), such as dobutamine, may be of more benefit than fluids alone.  Depending on the severity of the aortic narrowing, this drug can potentially lead to significant hypertension (high blood pressure) above the level of the narrowing, with the possibility of intracranial bleeding or overworking of the heart.  Prostaglandin E₁ (PGE₁) is an agent that can be used to help keep the ductus arteriosus open, thereby keeping flow to the lungs possible, as well as enhancing perfusion below the obstruction.  Side effects may include episodic breathing and apnea (cessation of breathing for potentially prolonged periods of time) and peripheral vasodilation (opening of the distal arterioles with subsequent decreases in blood pressure and possibly tissue perfusion). 

Prognosis:

Uncorrected coarctation carries a mortality rate of approximately 50% by age 30 and up to 90% by the age of 60.⁸  The primary determinants that enable the patient to survive to older ages are the age at the time of surgery and the presence and severity of associated lesions.  If surgery is performed before the age of 20 and concomitant anomalies are not significant, the person can expect to live a normal life expectancy.  For those patients who wait until they are 20 to 40 years of age before undergoing surgery typically have a 25-year survival rate of 75%, with the percentage and the length of life expectancy dropping rather dramatically the longer they wait for correction of the coarctation and associated complications.  Approximately 10% of patients may need additional surgery, typically to correct either re-coarctation or associated problems (such as replacement of a bicuspid aortic valve).

References and Further Reading

A schematic representation of this condition can be seen at www.lucinafoundation.org/birthdefects-coarctation.html

1.                  Syamasundar Rao, P. & Seib, P.M. (2009).  "Coarctation of the Aorta."  Accessed on November 22, 2010 from emedicine.medscape.com.

2.                  Karlsen, K.A. & Tani, L.Y. (2003).  S.T.A.B.L.E. – Cardiac Module:  Recognition and Stabilization of Neonates with Severe CHD.  The S.T.A.B.L.E. Program, Park City, UT.

3.                  Taeusch, H.W., Ballard, R.A., & Gleason, C.A. (2005).  Avery's Diseases of the Newborn, 8^th ed.  Elsevier, Philadelphia.

4.                  Murphy, J.G. & Lloyd. M. A. (2007).  Mayo Clinic Cardiology:  Concise Textbook, 3^rd ed.  Mayo Foundation for Medical Education and Research, Rochester, MO.

5.                  Saxena, A (2005).  "Congenital Heart Disease in India:  A Status Report."  Indian Journal of Pediatrics, 72 (July, 2005):  595-598.

6.                  Oman, M. & Wolf, A. (2009) “Coarctation of the Aorta in a Neonate” in Murphy, P.J., Marriage, S.C., & Davis, P.J.  Case Studies in Pediatric Critical Care.  Cambridge University Press, Cambridge, NY, et al.

7.                  McLean, B & Zimmerman, J.L. (editors) (2007).  Fundamental Critical Care Support, 4^th ed.  Society of Critical Care Medicine, Mount Prospect, IL.

8.                  Crawford, M.H., Srivathson, K., & McGlothlin, D.P. (2006).  Current Consult Cardiology.  Lange Medical Books / McGraw-Hill, New York, et al.

9.                  Peacock, W.F. & Tiffany, B.R. (2006).  Cardiac Emergencies.  McGraw-Hill, New York, et al.

10.              Brickner, M.E. (2000).  "Congenital Heart Disease in Adults (First of Two Parts)."  New England Journal of Medicine, 342 (4), 256-263.

11.              Wu, J.C. & Child, J.S. (2004).  "Common Congenital Heart Disorders in Adults."  Current Problems in Cardiology 2004, 29, 641-700.

12.              Lilly, LS (editor) (2007).  Pathophysiology of Heart Disease:  A Collaborative Project of Medical Students and Faculty, 4^th ed.  Lippincott, Williams, & Wilkins, Philadelphia.

13.              Otto, C. M. (2007).  The Practice of Clinical Echocardiography, 3^rd ed.  Saunders / Elsevier, Philadelphia.

14.              Ebeid, M.R., Prieto, L.R., & Latson, L.A. (1997).  "Use of Balloon-Expandable Stents for Coarctation of the Aorta:  Initial Results and Intermediate-Term Follow-Up."  Journal of the American College of Cardiology, 30 (7), 1847-1852.

15.              Oliver, J.M., Gallego, P., Gonzalez, A., Aroca, A., Bret, M., & Mesa, J.M. (2004).  "Risk Factors for Aortic Complications in Adults with Coarctation of the Aorta."  Journal of the American College of Cardiology, 44 (8), 1641-1647.

Contributor:

Sean Marcus Hancock, RDCS (AE), RCS, RCSA, RCIS, CCT

Staff Cardiovascular Technician and Clinical Educator

Naval Hospital Bremerton, Bremerton, Washington

April 2012