Coarctation of the Aorta
Definition
Coarctation of the aorta is caused by the discrete narrowing of the distal segment of the aorta beyond the left subclavian artery arising from the aortic arch. This includes the deformity of the media of the aorta and is represented by a curtain-like infolding of the outer wall of the aorta which causes a narrowing. The coarctation is due to an inbalance in fetal circulation within the aorta and pulmonary arteries that interferes with normal hemodynamic molding of these arteries.
Coarctation of the aorta occurs in approximately 8% of congenital heart defects.
Anatomy
The coarctation occurs after the left subclavian which arises from the aortic arch. It begins as an indentation of the posterior wall. When the ductus arteriosus is patent, the aorta is not significantly obstructed; however, if the ductus arteriosus begins to close after birth the aorta is no longer widened by it and this results in aortic obstruction.
The coarctation may be preductal, where the narrowing is proximal to the ductus arteriosus or ligamentum arteriosum.
The coarctation may be postductal, where the narrowing is distal to the ductus arteriosus or ligamentum arteriosum.
Collateral circulation connects the proximal and the distal aspects of the vessels over time. This collateral circulation will develop mainly from the subclavian, scapular, internal thoracic, and intercostal arteries. Collateral circulation is divided into anterior and posterior systems.
In the anterior system, the internal mammary arteries and the epigastric arteries join to form collaterals which supply the abdominal wall and the lower extremities.
In the posterior system, the parascapular arteries connect with the intercostal arteries to form collaterals which supply the distal aortic compartment and primarily the abdominal viscera.
The left subclavian may form collaterals through linkage to the IMA's and intercostals to give blood supply distally. The right subclavian will join with the vertebral, spinal, cervical, and scapular branches, and will eventually provide blood supply to the intercostal circuit.
The coarctation of the aorta may also involve the whole contour of the aorta which is known as hypoplasia segmentaria.
Pathophysiology
The coarctation is due to intracardiac abnormalities where the increase in pulmonary artery and ductal flow and the decrease in aortic flow are present. The area of the aorta which is affected remains under development and narrows. This usually develops into the preductal type.
When there is a defect which is only significant in the fetal state (i.e. narrow foramen ovale), and ductal flow exceeds the flow across the aortic valve, the flow to the ductus arteriosus will begin to supply a portion of the distal brachiocephalic circulation. This biforcated circulation causes an indentation in the aorta beyond the ductal opening. The ductus will continue to close and cause infolds in the lumen, producing a smaller opening. This usually results in the postductal type.
The principle cardiac anatomy is left ventricular hypertrophy due to increased afterload, since tension is increased when the heart tries to pump against it causing the ventricle to become hypertrophied. It is usually accompinied with venous congestion. Dilatations of the aorta above and below the coarctation are common. Aneurysms of the aortic arch, the Circle of Willis, and the descending aorta are common. The rupture of these aneurysms is the primary cause of death in adult patients with coarctation of the aorta.
Shunts
Left to right shunt: occurs when a postductal coarctation and a PDA are found together. Manifestations include right diastolic overload, varying degrees of pulmonary hypertension, right ventricular hypertrophy, and eventually Eisenmeiger Syndrome. In this case, there is good formation of collateral circulation at birth. This case is also more benign than preductal coarctation in association with a PDA.
Right to left shunt: allows for a continuous blood flow from the pulmonary artery to the aorta. Occurs when a preductal coarctation and a PDA are found together. Manifestations include right ventricular hypertrophy, pulmonary hypertension, aortic and left ventricular hypoplasia. Most of these patients die within a few months of birth due to poor collateral circulation at birth causing heart failure, pulmonary hypertension, and the effects associated when deoxygenated blood reaches descending aorta.
When a postductal COA is found with a closed ductus arteriosus the extent of the problems depends upon the severity of the stenosis and the development of the collateral circulation.
When a preductal COA is found with a closed PDA there is usually good collateral circulation present. The closing of the PDA causes changes in pulmonary arterial blood flow that increase diastolic overload and cause LV failure from the sudden overload of blood in the left ventricle. This leads to 88% mortality rate.
Associated Abnormalities
1) Bicuspid Aortic Valve (85%)
2) Mitral Valve Malformation (30%)
3) PDA (22%)
4) Congestive Heart Failure (10%)
5) Bacterial Endocarditis (may result in cerebral vascular accident due to increased turbulence of blood)
Hemodynamics
Arterial Hypertension: 150/100 due to systolic overload.
Pulmonary Hypertension: large PDA, left to right shunt.
Distal Arterial Hypotension: deficit of blood flow distally to COA.
Local Blood Turbulence: bacterial endocarditis.
Aortic and LV Hypoplasia: pulmonary hypertension, RV hypertrophy.
Clinical Manifestations
LV and Systolic Overload: dyspnea, CHF, left to right shunt.
Abnormal Hemodynamics: aortic aneurysms.
Large PDA and Pulmonary Hypertension: dyspnea, RV failure.
Prognosis and Medical Intervention
Medical management is important for the treatment or prevention of bacterial endocarditis. It is also important to treat newborns who present with severe heart failure with intravenous prostaglandins to reopen or keep ductus arteriosus in a patent state. However, in case of closed PDA, surgical intervention should be given an immediate consideration. Optional surgery may be postponed until 15 years of age when the aorta is fully developed.
Surgical treatment results in end to end anastomosis of the aorta. If this is not possible due to the presentation of tubular hypoplasia, a dacron tube prosthesis may be used. If the aorta has degenerative lesions and resection is hazardous due to a tortuous aorta, the prosthesis should be implanted around the area of the coarctation. In cases of localized coarctation, the area of the infolded aortic lumen can be excised and a patch can be placed over the area where the wall was cut open to form the continuous lumen once again.
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