Subvalvar obstruction is usually muscular in nature and is not generally amenable to catheter intervention (Figure 35.1A). Residual or recurrent “infundibular” obstruction is most commonly found in patients who have undergone repair of tetralogy of Fallot. Double-chambered right ventricle (DCRV) also features hypertrophy of the infundibular muscle, causing it to closely appose the free wall of the RV in systole and narrow the outflow tract below the pulmonary valve. In children, this lesion may be associated with small membranous ventricular septal defects (VSDs) or membranous subaortic obstruction. Over time, the membranous ventricular septal defect may close spontaneously as the infundibular obstruction progresses, so that DCRV may present in the adult as an apparently isolated subvalvar obstruction.

Pulmonary valve annular hypoplasia may also present in the setting of significant right ventricular outflow obstruction and can be found in patients as residua of surgically corrected congenital heart disease (CHD). This lesion is not generally amenable to transcatheter intervention owing to the fibrous ring of the hypoplastic annulus that resists balloon dilation.

Except when scarring from previous surgical interventions produces narrowing of the MPA (i.e., following arterial reconstruction in neonatal transposition of the great arteries, or tetralogy of Fallot), supravalvar obstruction generally occurs at the sinotubular junction and is congenital. Sinotubular junction stenosis can be confused echocardiographically with valvar disease, because the normal leaflets are limited in their forward motion by the distal ridge of muscular tissue (Figure 35.1B), appear to dome, and exhibit turbulent flow at the leaflet tips on Doppler, just as is seen in valvar pulmonary stenosis (PS). Balloon dilation of supravalvar obstructions is largely unsuccessful owing to the muscular/elastic nature of the arterial wall in the MPA, and has been associated with MPA rupture. Stents have been placed successfully to relieve obstruction in the supravalvar region, but risk stenting open the pulmonary valve and causing severe insufficiency, most often a poor trade-off for moderate PS.

Valvar PS is a common congenital lesion. Similar in nature to congenital aortic stenosis (AS), it is typically caused by commissural fusion that precludes the leaflets separating fully (Figure 35.1C). The result is a diminished valve orifice/valve area resulting in increased RV afterload.

The natural history studies of isolated valvar PS in children indicate that a pressure gradient >50 mmHg is associated with poor long-term outcomes, including RV myocardial dysfunction, ventricular arrhythmia, and sudden death,¹, ², ³ whereas pressure gradients <30 mmHg are not associated with symptoms, changes in lifestyle, or life expectancy. As treatment shifted from surgical to catheter-based therapy in the 1980s, the indications for intervention have changed. Currently, any patient with a peak-to-peak valvar gradient ≥40 mmHg should be considered for balloon valvuloplasty.

Percutaneous pulmonary valvuloplasty in children was first reported by Semb et al.¹¹ in 1979. However, the static balloon technique, reported by Kan et al.¹² in 1982, was the first to be applied widely. Results have demonstrated the safety and effectiveness¹³, ¹⁴, ¹⁵, ¹⁶ of this technique and have established it as the treatment of choice for children and adults with isolated pulmonary valve stenosis.

Balloon angioplasty for hemodynamically important branch PA lesions can be accomplished with small peripheral vascular balloon catheters. Specially designed high-pressure balloon catheters or bladed cutting balloons may be used for more resistant lesions. Placement of intravascular stents has become the treatment of choice in older children and adults who have completed or nearly completed their growth. Particularly in the proximal, more muscular branches of the pulmonary tree, stents more reliably improve vessel size, overcoming the recoil of these elastic vessels, and reduce the need for oversized balloons. With increased experience demonstrating that later stent reexpansion is possible/safe, stents are now being used in even the youngest patients in critical situations.

Later experience with bladed cutting balloon catheters has shown improved outcomes for the treatment of what were previously considered undilatable lesions. Small studies, under compassionate use protocols, have shown significant improvement in vessel size using oversized cutting balloons that cut through intimal and medial layers.³⁰,³¹ After cutting-balloon inflation, subsequent use of high-pressure balloons and/or intravascular stents has further improved outcome with success rates reported in up to 92% of vessels.³²

Intravascular stents were first used for elastic/resistant branch PAs in 1989²⁶, ²⁷ and are now the most commonly used in CHD. Although balloon angioplasty remains the treatment of choice in peripheral lesions and in infants/small children (growth issues), stent implantation in the PA bed has become first-line therapy for proximal obstructions, lesions owing to surgical distortion, external compression, inadequate results of balloon angioplasty, and obstructive intimal flaps following balloon angioplasty. Infants who undergo surgical repairs such as the arterial switch for transposition of the great arteries, repair of tetralogy of Fallot with branch PA plasty, and palliations such as shunts or PA banding may have acute postoperative obstructions that create hemodynamic instability or place the obstructed lung at risk for long-term hypoperfusion. Stent usage in the immediate postoperative period, as an alternative to repeat surgical intervention, can be lifesaving and is more reliable than balloon angioplasty alone. Since early stent use in the branch PAs, technology has improved both in the stents themselves and in the delivery balloons. The use of stents has improved the success rate of branch PA interventions to >90%.

Most experience with stent implantation in branch PAs is with the balloon expandable Palmaz and Genesis stents (Johnson & Johnson, New Brunswick, NJ), though newer stent designs may have significant advantages including trackability, conformability to vessel course, reduction in balloon and vessel damage owing to smoother device edges, diminished stent shortening with expansion, and improved visibility with alternative materials. Innovations in balloon design include a marked reduction in catheter shaft diameter and slip-/scratch-resistant balloon surfaces (to reduce the incidence of the stent slipping off or puncturing the balloon). The BIB (balloon-in-balloon) balloon dilation catheter system (Nu-Med, Hopkinton, NY) is being used for many indications and helps minimize the risks of stent malposition.

Heparinization and prophylactic antibiotic use is recommended in these typically long procedures. Predilation with a balloon is optional but is not necessary for most branch PA lesions. Stent and balloon sizes are selected for the individual lesion. The stent is crimped onto the balloon and can be delivered using one of two techniques. With a standard approach, an oversized long sheath or guiding catheter is passed over a stiff exchange-length guidewire (with a short floppy tip), such that the sheath tip is distal to the lesion to be stented. The balloon/stent combination is then advanced over the guidewire through the sheath to the implantation site. Alternatively, the front-loading technique involves passing the balloon catheter fully through the sheath outside the body, crimping the stent onto the balloon, and pulling it back into the front of the sheath before introducing this assembly over the wire. Combined techniques are also used in which the long sheath is advanced over the wire to the inferior vena cava (IVC), the stent then mounted on the balloon, and advanced over the wire to the tip of the sheath prior to advancing the entire system over the wire to the target lesion. The stent is deployed as described in Chapter 31. Antiplatelet therapy is the norm, for a period of 3 to 6 months to prevent in-stent thrombosis. For patients with normal pulsatile flow in the PAs, aspirin alone should be sufficient. For patients with nonpulsatile flow (bidirectional Glenn shunt or Fontan), dual antiplatelet therapy or warfarin is warranted, though there is little data available on which to base such a decision. There is a very low rate of restenosis in the branch PAs except for the relative stenosis that develops as the patient grows.³²

Stents may also be used to extend the life of surgical conduits inserted between the heart and the branch PAs. With rapid growth of the infant, such conduits may kink and develop intraluminal obstruction. Balloon angioplasty alone in this setting has been largely unsuccessful because the conduit will reassume its kinked course on balloon deflation. In several patients, we have been able to hold off conduit replacement for several years using this approach, removing the stent with the explanted conduit at later surgery. This may allow for placement of an adult-sized conduit at the next surgery, and future placement of transcatheter valve implants (see above). For patients with obstructions in full-sized conduits, a pulmonary valve implant would be the current procedure of choice.

Balloon aortic valvuloplasty for congenital AS in a child was first reported in 1983.³⁶ It has been performed subsequently in large numbers of patients with both congenital and acquired valvar stenoses.³⁶, ³⁷, ³⁸, ³⁹, ⁴⁰, ⁴¹, ⁴² In acquired calcific valve stenosis, acute relief of gradient is possible. However, the rapid rate of restenosis, the risk of cerebral embolization, the risk of disrupting the valve leaflets and causing severe valvar insufficiency, along with unpredictable and suboptimal degrees of obstruction relief has left balloon valvuloplasty for calcified valves as an emergency option only, when surgical valve replacement is not an option. The recent successes of transcatheter aortic valve replacement⁷,⁴³ are changing the paradigms for treatment of older adults with severe aortic valve disease.

In children, adolescents, and young adults with congenital valvar AS, however, balloon valvuloplasty remains an excellent alternative to surgical valvotomy or to valve replacement, since the pathology involves more commissural fusion and less leaflet rigidity seen in adult patients with senescent and densely calcified aortic valves.

Physiologically, critical AS is a different entity from severe AS in older children, since the valve may be virtually atretic and the left ventricular cavity may be moderately to severely hypoplastic. Unlike hypoplasia of the right ventricle in critical PS (in which right atrial blood may cross the PFO to maintain LV preload and cardiac output), the size of the left ventricle has a direct impact on survival. LV hypoplasia, reduced LV compliance, and reduced emptying in the face of extraordinary afterload lead to increased left atrial and pulmonary venous pressures. While the PFO (or ASD) may act as a pop-off valve for the left atrium (LA), reducing pulmonary venous pressures and congestion, any blood that shunts from the LA to the RA, reduces LV preload, and as a result further reduces left ventricular output. Low cardiac output and pulmonary venous congestion may be incompatible with postnatal life. If LV hypoplasia is a concern, the alternative is to assign these patients to a stage I palliation for hypoplastic left heart syndrome. A retrospective analysis of a group of patients with critical AS undergoing surgical valvotomy or balloon dilation at Boston Children’s Hospital led to a scoring system (based on echocardiographic measurement of left-sided structures) that can be used to triage such patients.⁴⁴

The neonate with critical AS may be asymptomatic immediately after birth because of the normal remnants of the fetal circulation. Left atrial return crosses the PFO to the RA, rather than entering the LV. The volume-loaded RV ejects blood into the MPA, where flow is distributed between the lungs and the systemic circulation via the ductus arteriosus, based on the relative resistance of each pathway. If a normal
blood volume reaches the aorta, there are no clinical problems. However, as pulmonary resistance falls and as the ductus arteriosus starts to close, RV blood preferentially flows to the lungs. A corresponding fall in systemic blood flow results in diminished tissue oxygen delivery, anaerobic respiration at the cellular level, and profound metabolic acidosis. Reestablishing and maximizing systemic flow is the key to resuscitating the acidotic newborn. Prostaglandin is required to reopen the ductus, and elevation of pulmonary vascular resistance is desirable (minimize FiO₂, allow pCO₂ to rise).

When most of the systemic flow arises from the ductus arteriosus, rather than the LV, the measured valvar pressure gradient across the stenotic valve is not a meaningful number, as the flow across the valve can approach zero. Prior to ductal closure, critical AS is manifested by right-to-left systolic flow at the ductus arteriosus with equal systemic arterial desaturation in all four limbs.

Using routine sedation, a femoral vein and artery are entered percutaneously and the patient is heparinized. The venous catheter is used to measure right heart pressures and cardiac output (when no shunts are present) before and after dilation. In older patients, aortic valves can be dilated from the femoral vein using a transseptal approach, but this risks entrapping/stenting open/injuring the mitral valve, and the retrograde approach from the femoral artery remains the most common technique. The typical method for crossing the stenotic aortic valve from the aorta is to advance the soft end of a straight wire out of a steerable catheter (multipurpose or Judkins Right curve) and use it to probe for the valve orifice. We often use a hydrophilic guidewire, as the reduced friction of its surface allows for more rapid in and out movements, but probing must be done gently to avoid perforating a cusp or damaging the coronary arteries. When the LV is entered, a transvalvar gradient is measured by simultaneously recording pressure from the catheter and from the femoral sheath (one French size larger than the catheter). An aortogram may be done to define the anatomic landmarks if the time for crossing becomes prolonged. We do not routinely do a baseline aortic angiogram, as the amount of aortic regurgitation should have been documented by echo as part of the precatheterization evaluation. A pigtail catheter is exchanged over a wire, a left ventriculogram may be performed and the aortic annulus can be measured at the hinge points of the valve.

In contrast to the pulmonary valvuloplasty, the balloon diameter is chosen to be only 75% to 90% of the annulus diameter. Animal and clinical studies demonstrate that aortic valvuloplasty with a balloon/annulus ratio greater than 1.0 is more likely to be associated with aortic regurgitation.⁴⁵,⁴⁶ A double-balloon technique was used in the past to allow for the use of smaller sheaths, when the annulus was larger, and there was concern for femoral artery injury in small children. However with balloons on shafts as small as 3F and 4F today, there is little indication for such an approach. The pigtail catheter is exchanged for the balloon dilation catheter, which is centered across the valve, inflated and deflated rapidly, and pulled back to the descending aorta. The gradient and the cardiac output are remeasured following dilation, and an aortogram is performed to look for aortic regurgitation. If the residual gradient is >55 mmHg and an aortogram shows no or only mild regurgitation, a larger balloon may be used. It is far better, however, to leave a residual gradient than to cause significant aortic insufficiency in the small child, as surgical backout options are limited (Ross procedure).

It can often be difficult to keep the balloon positioned in the valve during inflation, against the force of left ventricular ejection. A stiff catheter shaft, long balloon, and extra stiff exchange wire will help stabilize the position, or the balloon can be advanced so that it lies along the top of the aortic arch rather than around the underside of the arch. An increasingly common technique, suggested by Cribier,⁴⁷ involves the use of a transvenous pacing catheter positioned in the RV to rapidly pace the ventricle during balloon inflation to reduce ventricular ejection and provide a more stable balloon position. This technique is a critical component of the transcatheter aortic valve implantation procedure.

Before the catheterization begins, the baby’s hemodynamic status must be optimized. We perform this catheter procedure with the baby intubated and paralyzed and receiving a prostaglandin infusion. The FiO₂ is turned down to 21%, and pCO₂ is allowed to rise to the mid-40s to increase pulmonary vascular resistance and maximize systemic flow across the PDA. Body temperature is carefully monitored.

The umbilical artery can usually be used in the first week of life. Catheter manipulation is more difficult from the umbilical artery owing to the inferoposterior loop in its course before it enters the descending aorta, but its use avoids damage to the femoral artery in these very small infants. Surgical cutdown at the carotid artery for catheter access has also been employed. This approach simplifies crossing the valve⁴⁸ because of the straight path to the valve from the neck. As with older patients, a transseptal approach can be used from either the femoral or umbilical vein, with no puncture required since all neonates have a PFO. But this approach is rarely used since it carries a much higher risk of mitral valve injury than in the older population.

Balloon valvuloplasty for congenital AS has been an effective palliation in the pediatric population with excellent gradient reduction and increase in valve area. But the procedure is complicated by the development of significant aortic regurgitation in 10% to 30% either immediately or within weeks of the procedure (inversely related to balloon/annulus ratio); femoral arterial complications in 30% to 40% (inversely related to the size of the patient), and restenosis.⁴⁹, ⁵⁰, ⁵¹ Survival with
freedom from reintervention was seen in only 50% of patients at 14.4 years in one cohort of 269 patients⁵¹ and in 60% at 12 years in a group of 74 patients.⁵⁰ Transient left bundle branch block occurred in 15% and ventricular arrhythmias requiring cardioversion in 3%.⁵¹ In a 25-year review of balloon aortic valvuloplasty at Texas Children’s Hospital,⁴² death or transplant was significantly correlated with poor baseline LV systolic function, and the need for recurrent intervention was significantly higher in neonates than in older patients (34% versus 9%).