Therapeutic cardiac catheterization in children

DAVID F. WAX, MD

aFall 1999

Pediatric therapeutic catheterization  refers to the non-surgical treatment of structural heart disease by using specially designed catheters and implantable devices that are delivered through catheters. Although there are many different congenital heart defects, therapeutic catheterization is largely confined to treating two basic categories of problems: opening things that are closed and closing things that are open.

More specifically, blood vessels and heart valves that are too small can be enlarged using balloon catheters and/or implantable devices known as stents. In addition, abnormal connections within the heart (VSDs and ASDs) and abnormal blood vessels (PDAs and collaterals) can be closed using a variety of innovative devices that are designed to fold or coil inside of a catheter and then open up when properly positioned within the defect or blood vessel.

For some lesions, transcatheter therapy has long been available and is now widely agreed to be the treatment of choice. For example, for pulmonary stenosis, the first balloon valvuloplasty was performed in 1982 and is currently the standard of care. For other lesions such as unrepaired coarctation of the aorta, improvements in surgical results have made transcatheter therapy more controversial.

Implantable devices that have been approved by the FDA include intravascular stents used for maintaining vessel patency and Gianturco coils for occluding abnormal vascular connections. Devices for closure of VSDs, ASDs and larger PDAs are in various phases of FDA-approved trials in this country, and information on the trials that are enrolling patients often can be found by searching the Web.

This article addresses in more detail three interventional procedures.

TRANSCATHETER CLOSURE OF THE
PATENT DUCTUS ARTERIOSUS

Isolated persistence of the ductus arteriosus or PDA accounts for approximately 5% of congenital heart disease although it is seen even more frequently in association with other complex heart lesions. A PDA is a source of left-to-right shunting causing increased pulmonary blood flow and increased pulmonary venous return. If it is large, it may cause heart failure, failure to thrive and pulmonary hypertension. More common is the small PDA presenting as a continuous murmur in an asymptomatic patient. These are closed because of the risk of bacterial endarteritis. Standard surgical therapy consists of a thoracic incision and ligation and division of the ductus.

Porstmann first reported transcatheter closure of the PDA in 1967 using a foam plug delivered through a large catheter. Although clinically effective, the very large catheter size prevented wide acceptance of the procedure.

Using a technique first reported by Cambier in 1992, most transcatheter PDA occlusions are now performed using Gianturco vascular occlusion coils. The small stainless steel or platinum coils with integral synthetic fibres are inexpensive, readily available and have been used in the vascular space since the mid 1970s. The coils are usually delivered via a very small (1.3 mm) arterial catheter and positioned as shown in Figure 1. A small length of coil anchors the coil in the pulmonary artery, and the rest of the loops are positioned within the aortic ampulla causing clot formation, obstruction to blood flow and ultimately endothelialization. The procedure is performed with deep sedation, and all but the smallest children are discharged the same day.

FIGURE 1. Retrograde occlusion of a PDA with a Gianturco coil. From A to E: The catheter is passed from the aorta to the pulmonary artery across the PDA; a small portion of coil is advanced to anchor the coil in the pulmonary artery; the catheter is carefully withdrawn and the rest of the coil is delivered into the aortic ampulla.

The incidence of minor complications is quite low (approximately 3% to 5%) and includes coil embolization, incomplete closure, mild left pulmonary artery stenosis (in the smallest infants) and very rarely hemolysis (secondary to incomplete closure). No deaths or cerebral accidents have been reported. Very large ducti and very small infants are still probably best treated surgically, but for ducti 4 mm and smaller (the majority) and children 6 kg and larger, this procedure can be performed safely and with about a 95% to 97% effectiveness rate.

A promising new device for closing larger PDAs (and in smaller children) is the Amplatz PDA occluder, which is beginning Phase II clinical trials in this country. This device is similar in construction to the ASD occluder discussed below. The Amplatz PDA occluder is delivered through a very small catheter and will expand to form a large plug similar to the original Portsman device.

TRANSCATHETER CLOSURE OF ATRIAL SEPTAL DEFECT

Isolated atrial septal defect accounts for approximately 6% of CHD. The pathophysiology is that of a low-pressure left-to-right shunt with right heart dilation and pulmonary overcirculation. Symptoms are rare before the third decade and include CHF, pulmonary hypertension and atrial arrhythmia. Rarely a very large defect may cause CHF with growth failure in a young child. It is difficult to predict which patients will go on to develop symptoms. Not all symptoms are reversible if the defect is repaired in adulthood. For these reasons, moderate to large defects found in young children are usually closed before the child starts school or as soon as they are found in older children.

Surgical closure performed since 1948 enjoys excellent results and very low morbidity and mortality. A sternal or more uncommonly thoracic incision is required as is cardio-pulmonary bypass and a three-day to a one-week hospitalization.

King et al. reported the first transcatheter closure of an ASD in 1976 using a device that required an extremely large sheath (or catheter) for implantation. Since then, several different designs have undergone and continue to undergo clinical testing with generally good results. As of this writing, there are several active clinical trials in the United States and abroad using devices such as the Amplatzer, the CardioSeal, and the Angel Wings. Although varying in specific details of construction, most devices have in common two discs connected together in the center and designed so that they can collapse to fit inside a catheter and expand again when positioned in the heart.

In the typical "clamshell" design, two large discs positioned on either side of the defect are responsible for closure of the defect. Potential disadvantages of this design include the need for large discs relative to the size of the defect and—because of the lack of a centering mechanism—the technical difficulty in getting the device to sit properly within the defect. The Amplatzer ASD Occluder is an example of the "self-centering design" where the connection between the two discs is larger in diameter and serves to center the device within the defect and also aids in occlusion (Figure 2).

FIGURE 2. Implantation of the Amplatzer ASD Occluder. From A to E: The delivery catheter is positioned across the atrial defect; the left atrial disc with the self-centering connecting stalk is delivered; the device is withdrawn so that the connecting stalk is within the ASD and the left disc is firm against the atrial septum; the right atrial disc is delivered, and the delivery cable is disconnected from the device. Until the delivery cable is disconnected, the device can be withdrawn back into the catheter and removed from the body.

The Amplatzer has been used in over 2,700 patients world wide since 1995 and is the device being used in a clinical trial at CMMC. The Amplatzer offers several advantages over competing designs. Because of its Nitinol (memory wire) construction and unique design it can be withdrawn back into the catheter and removed from the body if proper positioning in the heart is not possible. The device is available in a range of sizes from 4 mm to 32 mm; the appropriate size allows the connecting stalk to fill the ASD, center the device, ensure complete closure and allow the atrial discs to be relatively smaller than those of competing designs.

Figure 2 demonstrates the technique for inserting the Amplatzer. The procedure is currently performed under general anesthesia to allow for transesophageal echo during the procedure. The patient is observed overnight and discharged the next morning. Patients take a baby aspirin daily for six months until endothelialization of the device is complete. Follow-up consists of an echocardiogram and a chest x-ray at six months and one year. Possible complications include infection at the catheter site, arrhythmia, stroke, cardiac perforation, device embolization and incomplete closure.

Most, but not all, secundum atrial septal defects are amenable to closure by the Amplatzer device. Currently criteria include a defect size of less than or equal to 32 mm and a 4 mm rim of atrial septal tissue surrounding the defect. These anatomical details can usually be ascertained by a careful transthoracic echocardiogram. Occasionally, in larger patients a transesphogeal echo is needed to properly visualize the defect. If this is required, we offer patients the option of having the TEE under general anesthesia with catheter closure of the defect to follow if appropriate.

At the last reporting of U.S. results, 186 patients had atrial defects closed with the Amplatzer device with no major complications. There were 8 (4.3%) minor complications including 4 device embolizations (2 removed percutaneously and 2 removed surgically) and 4 instances of arrhythmia (3 transient, 1 persistent complete heart block). The closure rate for the 121 patients with 6 months follow-up was 99%.

THE USE OF STENTS IN PEDIATRIC THERAPEUTIC CATHERIZATION

Endovascular stents are devices implanted by transcatheter technique that expand within the vascular space and are usually used to maintain vessel patency after balloon angioplasty. The stent prevents recoil of the vessel providing better acute results and considerably less re-stenosis than with balloon angioplasty alone. A variety of design choices are available; the type most commonly used in pediatrics is the Palmaz stent (Johnson & Johnson). The Palmaz stent is a stainless steel tube with laser-etched slots. The stent is positioned over an angioplasty balloon and advanced to the site of the stenosis. When the balloon is inflated, the struts of the stent open up, and the stent remains fixed in the expanded position within the vessel (Figure 3).

FIGURE 3. Implantation of Palmaz stent to right pulmonary artery. From A to E: The balloon catheter with the stent mounted is advanced through a previously placed sheath to the right pulmonary artery. The sheath is withdrawn exposing the stent, and the balloon is inflated expanding the stent and relieving the stenosis. The balloon is deflated and withdrawn leaving the stent in place. The guidewire is removed after the insuring proper positioning of the stent.

After placement, the vascular endothelium grows over the struts of the stent over several months, functionally incorporating the stent into the vessel wall. Stents have been used effectively to treat stenosis in virtually every vessel in the circulation. However, the most common use of stents in the pediatric population is to treat peripheral pulmonary artery stenosis (PPS). PPS may be seen as an isolated diagnosis or more commonly as a component of complex congenital heart disease like tetralogy of Fallot, pulmonary atresia, and hypoplastic left heart syndrome.

The stenosis may be native or iatrogenic, secondary to surgical manipulation of the pulmonary arteries and scar formation. The consequences of severe bilateral PPS are elevated right ventricular pressure with ultimate right heart failure. Unilateral PPS more usually causes decreased flow to the affected lung and may cause ventilation/perfusion mismatch and exercise intolerance.

Surgical therapy for peripheral pulmonary artery stenosis is often unrewarding. Balloon angioplasty alone has a success rate of approximately 50% with a 16% recurrence rate. In contrast, stent placement yields an acute angiographic success rate of close to 100% with a recurrence rate of only 2–3%.

The mechanism of recurrent stenosis after stent placement is different from that seen after primary angioplasty. With the stent in place, recoil of the vessel is essentially impossible, however the endothelialization process may go awry resulting in a thick neointimal layer causing a functional stenosis. This neointimal hyperplasia is usually seen at the junction of two stents positioned in series or in stents that are overdilated and made larger than the normal "reference" vessel on either side of the stent. This intimal buildup can usually be treated with additional angioplasty.

Stents are ideally used in situations where the reference vessel is at its adult size. In this instance, the stent can be safely dilated to its final size at the initial procedure. Stents implanted in a younger child may have to be redilated at a later date to account for vessel growth. While this has been demonstrated in animals and humans, it is not always possible. In addition, the catheters and sheaths required to implant stents are larger than for angioplasty alone, making implantation in a small child difficult. For these reasons, stent implantation is usually only performed in a small child when no other alternative exists.

Promising new applications for stents include their use in the older child and adult with unrepaired coarctation of the aorta. Initial studies have demonstrated excellent acute results with a very low rate of re-stenosis. It is hoped that the incidence of late aneurysm formation, a recognized complication of balloon angioplasty, will be lower with stent placement. There is also active work in the development of biodegradable stents, which would eliminate the concerns about using smaller stents in a growing child and the as-yet unknown long-term consequences of a foreign body in the vascular space.

The three procedures discussed in this article represent some of the more exciting possibilities in pediatric therapeutic catheterization. Advances in bio-materials and industrial design should lead to a wider range of therapeutic options especially with regards to miniaturization and the ability to effectively and safely treat lesions in smaller infants. For children with complex congenital heart disease, transcatheter intervention is often used in combination with surgical therapy to achieve the best possible result. Pediatricians caring for patients with congenital heart disease should discuss with the child's cardiologist the role of therapeutic catheterization in the management of that child's condition.

FOR FURTHER READING

1. King TD, Thompson SL, Steiner C, Mills NL: Secundum atrial septal defect: Nonoperative closure during cardiac catheterization. JAMA 1976; 235:2506–09.

2. Cambier PA, Kirby WC, Wortham DC, Moore JW: Percutaneous closure of the small (less than 2.5 mm) patent ductus arteriosus using coil embolization. Am J Cardiol 1992;69:815–816.

3. Shim D, Fedderly RT, Beekman RH III, Ludomirsky A, Young ML, Schork MA, Lloyd TR. Follow-up of coil occlusion of patent ductus arteriosus. J Am Coll Cardiol 1996;28:207–211.

4. O'Laughlin MP, Perry SB, Lock JE, Mullins CE: Use of endovascular stents in congenital heart disease. Circulation 1991;83:1923–1939.

5. Stanger P, Cassidy SC, Girod DA, Kan JS, Lababidi Z, Shapiro SR: Balloon pulmonary valvuloplasty: Results of the Valvuloplasty and Angioplasty of Congenital Anomalies Registry. Am J Cardiol 1990;65:775–783.

6. Masura J, Lange PE, Hijazi ZM et al: US/International Multicenter Trial of Atrial Septal Catheter Closure Using the Amplatzer© Septal Occluder: Initial Results [Abstract 804-1]. J Am Coll Cardiol 1998;31(2) 57A.

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