Coarctation of the Aorta
and Related Aortic Anomalies
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, 8th ed.
Elsevier, Philadelphia.
4.
Murphy, J.G. & Lloyd. M. A. (2007). Mayo Clinic Cardiology: Concise Textbook, 3rd 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, 4th 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, 4th ed.
Lippincott, Williams, & Wilkins, Philadelphia.
13.
Otto, C. M. (2007). The Practice of Clinical Echocardiography,
3rd 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