Coarctation of the Aorta – CoA
You have read something of congenital heart defects inside the heart. Now let us consider a defect that is outside the heart itself, in one of the great arteries of the body – the aorta.
What is COARCTATION of the aorta ?
Coarctation of the aorta (CoA) is an area of localized narrowing of the large artery called the aorta. (“Coarctatio” – Latin : a drawing or pressing together). The narrowing may be caused by a “shelf” of tissue inside the blood vessel which reduces its area. Alternately, it may be caused by under-development of a portion of the aorta itself, which causes a longer area of reduced diameter.
Where does CoA occur ?
The narrowing that occurs in CoA is most commonly seen at a portion called the ISTHMUS.
But what is the isthmus ?
The aorta gives off arterial branches that supply the rest of the body with blood containing life-giving oxygen . The earliest branches are the coronary arteries, that supply the heart itself.
The next group of three arteries is called the brachio-cephalic group of vessels. This is because they supply blood to the upper limbs (“brachio-“) and the head and brain (“cephalic“).
The first of these three is the innominate artery, which supplies the right arm and right side of the face and head. The next is the left common carotid (LCC) artery, which provides blood flow to the left half of the head. The third is the left subclavian artery (LSA), which supplies the left arm.
(Yes, this is the same artery used in the Blalock-Taussig Shunt operation for the Tetralogy of Fallot).
The portion of the aorta immediately beyond this third branch (left subclavian artery) is called the isthmus. It is the narrowest portion of the aorta even normally, and in CoA it becomes even more restrictive.
Rarely, narrowing is present in other parts of the aorta – before the area of the ductus arteriosus (“pre-ductal coarctation” or “infantile” coarctation), between the carotid artery and subclavian artery, or even lower down in the aorta.
What happens when there is a Coarctation of the aorta ?
The sequence of events is like any condition where there is an obstruction to smooth blood flow.
When the aorta is narrow, blood flow across the coarctation is obstructed. As a result, the blood pressure builds up in the arterial branches before (or proximal) to the CoA. At the same time, blood flow is decreased in the part of the aorta beyond the area of CoA. Let’s discuss these two effects separately.
Pressure builds up proximal to the CoA. The left ventricle, which is the heart chamber that pumps blood into the aorta and other branch arteries, now has to work against this high pressure. In other words, it has to do more work than normal.
As we have seen in other conditions, the heart cannot sustain this increased effort forever without showing abnormal changes. At first, the wall of the left ventricle becomes thicker, in order to pump blood with more force. This is called hypertrophy of the ventricle.
For awhile, this helps the ventricle pump blood against high pressure and resistance – but not for very long. When the limit of tolerance is reached, the ventricle begins to “fail”. This means it can no longer pump blood efficiently, and it begins to become large (dilated) and weak.
This is a very bad sign, for if the left ventricle fails, the rest of the body will not receive enough blood and oxygen to carry on working normally. It is manifested by the patient developing breathlessness on exertion (or even at rest) and early fatigue.
And the left ventricle is not the only structure to be affected by this narrowing of the aorta. The portion of the aorta proximal to the CoA, and its branches are also exposed to this high pressure. This is one of the reasons why children with CoA have a high blood pressure recorded in their upper limbs.
Because of this high blood pressure, changes occur in the walls of these blood vessels. The changes are similar to those in older adults with hypertension – or high blood pressure. They include thickening and deposition of fats in the artery wall, and hardening of the arteries.
If allowed to continue without correction of CoA, this will eventually result in narrowing and complete block of the branch arteries, with a catastrophic outcome.
Another risk, especially in the blood vessels inside the brain, is that they may suddenly burst from the high pressure inside them. This may result in a stroke.
Also, the coronary arteries that supply the heart with blood may be narrowed. This hastens the onset of heart failure, and may even cause a “heart attack” at a young age.
And what about the parts of the body beyond the coarctation area?
They receive less blood than normal, and at a lower pressure than normal. And this also is not good.
When the lower half of the body gets less blood than it requires, it cannot grow and develop normally. In the extreme case, the lower limbs may be thin and weak.
The ability to walk, run or play is limited. This is because, during exercise, the muscles of the legs demand more oxygen and blood flow. This cannot be provided when there is a CoA. So, the muscles get tired sooner, and limit exercise tolerance. The child complains of weakness or pain in both legs – called claudication.
A more serious consequence of the reduced blood flow is its effect on the kidneys. The kidneys are organs that make urine, and through it remove many waste materials from the body. It also plays an important role in keeping the blood pressure in the body within normal limits.
When the kidneys receive low blood flow, they send signals to the rest of the body which makes the blood pressure higher still. This is another mechanism for the hypertension of CoA. If left alone for long, there is permanent damage to the kidneys. This may cause the blood pressure to remain high even after successful repair of CoA!
So how does the body compensate for this state of affairs ?
Nature is wonderful. For most defects, Nature has provided a method of compensation, which reduces or even sometimes totally abolishes the ill-effects produced by that disease. It is true of CoA also.
In coarctation of the aorta, we have an interesting situation. The upper half of the body has too HIGH a blood pressure, and the lower part has a very LOW pressure.
How can this be made equal?
Simply, by opening up channels that connect the portions of the aorta above the CoA with the portion below it.
Normally, many small blood vessels connect branches from the upper body with the branches to the lower body. Since in a normal person the blood pressure is equal in both parts, these small channels are insignificant.
But in CoA, when there is such a great difference in pressure between the two halves of the body, these small channels become larger and dilated. They act as Nature’s “bypass channels” to divert blood from the upper half to the low pressure lower half.
These channels are called collateral arteries. They may be absent or small at birth. But as the child grows, these collaterals become larger and more important. In an older child, these collaterals may even be visible! They produce pulsations over the back.
So, more the collateral supply, the better – Right ?
Not always. Or atleast, not to the surgeon.
While operating for CoA, the chest is opened at the left side. If collateral arteries are very large, they may be injured causing profuse bleeding. It makes the operation more difficult. That is why surgery is advised early in life, before collaterals become very prominent.
What other defects may be seen along with CoA ?
Some heart defects are common in association with CoA. These include a Ventricular Septal Defect (VSD) and a bicuspid aortic valve (which is a condition where the aortic valve has only two leaflets, instead of the normal three).
Dilated portions of arteries – called aneurysms – are more frequent inside the brain in patients with CoA. Other defects may also occur in combination with CoA.
When should Coarctation of the Aorta be repaired ?
As in most congenital heart defects, there is no simple answer. There is still a lot of controversy about which patients need intervention, and about what method of treatment is right.
Children with severe CoA need early repair. A severe CoA would be identified by a pressure difference of more than 50 mm.Hg across the area of narrowing (but even this figure is not universally accepted).
Milder types of CoA may need individual assessment. Any patient who has symptoms or complications due to the CoA needs repair.
So which is the ideal age for repair ?
It was previously recommended that CoA be repaired before 10 years of age. The reasons were that:
- The left ventricle is not exposed to a high work-load for too long
- Collaterals do not become too large
- Changes in the kidney do not become permanent, and blood pressure will return to normal after surgery
Recently, with increasing safety of correction in smaller children, there is a trend towards earlier repair. Some suggest that the benefits are maximum when surgery is offered before two years of age. But the last word on this topic has not been said.
What are the methods of treatment available ?
There are two main methods of therapy – Surgery and Balloon Dilation.
What are the surgical options ?
Surgery for coarctation was first successfully performed by Drs.Crafoord and Gross in 1945. Many different operations have been described for CoA, each with its specific advantages and drawbacks.
1. RESECTION AND ANASTOMOSIS
In this operation, the area of narrowing is removed by excision (Resection) and the portions of normal aorta on either side are stitched to one another directly (Anastomosis). This is possible only if the area of CoA is short and can be excised totally.
2. EXTENDED RESECTION AND ANASTOMOSIS
The advantage of a “resection and anastomosis” operation is the excellent relief of obstruction. Directly sewing together two arteries, which have the potential to grow as the child grows, prevents recurrent narrowing. Because of these benefits, surgeons today attempt to resect even longer segments of CoA, and connect the healthy aorta directly after freeing it over a longer distance.
3. PATCH GRAFTING
In this technique, a length-wise (longitudinal) cut is made across the area of narrowing, opening it up. A shelf inside the aorta is removed, if present. The “opened-up” segment is then closed using a patch of fabric like Dacron or PTFE (Poly tetra-fluoro ethylene), so that the lumen (or cavity) of the aorta is no longer narrow.
This method provides good relief from obstruction. The problems with it are that the patch does not grow as the child does. Also, there is a chance of dilatation of the aorta – or aneurysm formation – at this site later in life.
4. SUBCLAVIAN FLAP PLASTY
Using a fabric patch to widen the aorta prevents future growth. To avoid this, Dr.Waldhausen introduced a new method which used the Left Subclavian Artery (LSA) as a patch to close the coarcted segment after a length-wise incision.
Since the subclavian artery retains its growth potential, the chances of late recurrent narrowing are lower.
5. TUBE GRAFT REPLACEMENT
In this method, the area of CoA is excised totally. To restore continuity, a tube made of a fabric like Dacron or PTFE is used. This tube graft is sewn on both sides to healthy aorta.
Some surgeons use an aortic homograft for this purpose. An aortic homograft is taken from a human cadaver, and specially prepared to make it absolutely sterile and preserve its structure and strength. It is an excellent material for many replacement operations, but has the drawback of becoming hard and laden with calcium over many years.
The method is preferred in older children, or in re-operations for recurrent coarctation. In young kids, since the graft cannot grow, a second operation may be needed later to replace it with a larger sized graft.
What are the risks of surgery ?
Surgery for CoA provides excellent relief of obstruction. If performed at an early age, it can prevent all the complications of CoA.
But surgical repair is not without risks. The most disastrous complication is that of paralysis of the lower half of the body – paraplegia. This is a direct result of the reduced blood flow to the part of the aorta beyond the CoA.
The nerve impulses that control sensation and power of the muscles in the lower half of the body run in a special bundle of nerve fibers called the spinal cord. This is a very important structure, and is protected by a bony tunnel, the vertebral column (or the spine).
The spinal cord receives its blood supply from arteries which are branches from the aorta. The nerve fibers are very delicate structures which cannot live without blood supply for more than a few minutes. A further problem is that once they die, nerve fibers cannot grow back again. So any injury produced is permanent.
In CoA, the aorta is temporarily blocked using clamps during the operation to give the surgeon better visibility. When the aorta is clamped, the branches that supply the spinal cord do not receive any blood flow. The nerves are in danger of injury.
Many factors determine the risk of damage. The duration for which the aorta is clamped and the extent of development of collateral arteries are the most important. The “safe” period for surgery depends upon these factors. In addition, the surgeon employs many measures to reduce the chances of injury to the spinal cord.
Some of these methods include monitoring spinal cord evoked potentials (SEP) during surgery, and using shunts or the heart-lung machine to provide an alternate source of blood supply to the spinal cord. Cooling the patient to decrease the energy demands of the nerves, and using many drugs which protect the spinal cord from injury, have made surgery for CoA safer over the years.
The risk of paralysis – paraplegia – is below 1% in many large centers, and with development of better protective techniques, may approach zero. In very small sick babies operated in the first month of life, the risk is a higher.
Persistent hypertension – high blood pressure – is likely when surgery has been deferred to an older age. 80 to 90% of patients have normal blood pressure five years after surgery.
What about the long term outcome ?
Patients operated for CoA have a near normal lifestyle. They have a slightly higher risk for developing hypertension than normal persons.
Recurrent coarctation – obstruction developing after a successful operation – or Residual coarctation – incomplete relief of obstruction after operation – is seen in 7 to 12% of patients. The risk of re-developing an obstruction is least with the subclavian flap operation or resection and anastomosis.
And a word about Balloon Dilatation
Recently, some alternative methods have been tried. These are “experimental“, in the sense that their effectiveness in the long run has not been proved. The advantages with these new procedures is that they are less painful, make hospital stay shorter, and avoid a surgical scar.
One of these is trans-catheter balloon dilatation. A catheter is a special thin tube passed into the blood vessels through a small “needle-stick” in the groin. Through this catheter, a special balloon device is passed into the aorta. The “balloon” device is positioned across the area of coarctation and inflated. The inflation is observed on an x-ray machine. When the obstruction has been relieved, the balloon is deflated and the catheter removed.
Balloon dilatation of CoA is extremely useful in patients who develop recurrent CoA after prior surgery. Surgical repair for the second time is more risky because the first operation leaves the area scarred. A difficult operation is avoided by balloon catheter dilatation. For primary treatment of CoA, the use of balloon dilation is still controversial.
There are a few dangers with balloon dilatation too. Injury to the artery in the groin may occur. Rarely, the aorta may rupture at the site of balloon inflation, and require emergency surgery. The tip of the catheter may penetrate the artery wall partially, producing a condition called aortic dissection, which can become a serious problem.