Right Ventricular Outflow Tract Stenting

1. Introduction

Cyanotic congenital heart defects continue to be a cause of significant morbidity and mortality among infants and are an important contributor to neonatal mortality. In the new paediatric cardiac programmes, the majority of these defects are being corrected very early in life, but the repair, whether corrective or palliative, sometimes is difficult to achieve, especially in neonates with complex anatomy or those with risk factors. Right ventricular outflow stenting (RVOT) has emerged as the main bridging procedure in infants who require early interventions, such as neonates who are duct dependent or who are severely cyanosed and have poor anatomy of the right ventricular outflow tract and/or pulmonary arterial tree (hypoplastic branch PAs).

2. Indications

Neonates with risk factors such as prematurity, low birth weight, infection (sepsis), necrotizing enterocolitis, cerebrovascular event, pulmonary diseases and other conditions requiring noncardiac surgery, which included tracheoesophageal fistula and gastrointestinal anomalies.

Infants and older children with anatomy are not suitable for primary repair or as a bail out procedure in emergent conditions like uncontrolled cyanotic spells.

As reported by various authors, co-morbidities increase the risk of or may delay, primary cardiac repair [1, 2, 3]. Blalock–Taussig (BT) shunts performed in infants with these co-morbidities have an increased rate of complications, besides an unpredictable post-op course such as pulmonary over circulation and distortion of pulmonary artery anatomy [4, 5, 6, 7, 8].

The unpredictability of pulmonary balloon valvuloplasty combined with the high morbidity/mortality of both modified BT shunt placement and primary repair signalled the need for an alternative palliation option—the RVOT stent. RVOT stenting was initially described by Gibbs et al. [9] but the initial results were not encouraging, and it was re-introduced from 2010 onwards as a mode of palliation.

Sandoval et al., at the Hospital for Sick Children in Toronto [1], performed a detailed retrospective review of their experience managing infants with Tetralogy of Fallot(TOF). Infants were treated in 1 of 4 ways: those with early cyanosis (<3 months of age) were treated with either primary repair (early-PS group in those with pulmonary stenosis, and early-PA group in those with pulmonary atresia) or RVOT stenting (stent group); whereas those without early cyanosis had primary repair electively at an age deemed optimal between 3 and 11 months (surg>3mo group). Risk factors for primary repair were defined as low weight (<2.5 kg), prematurity (<37 weeks’ gestational age), pulmonary artery hypoplasia (Z score < −2) and significant noncardiac co-morbidities. The decision to perform early primary repair versus RVOT stenting was based largely on the presence of risk factors. Aortopulmonary shunts and ductal stents were reserved for infants with whom RVOT patency could not be established; that is, RVOT muscular atresia and unsuccessful RVOT intervention, or those with nonconfluent central pulmonary arteries. The early-PS, early-PA, and stent groups all had similar cumulative and postoperative lengths of stay, all significantly longer than that for the surgery >3mo group. Most importantly, the stent group at baseline had significantly smaller pulmonary arteries than the other 3 groups at a median Nakata index of 79 mm²/m². After RVOT stenting, there was significant catch-up growth of the pulmonary arteries. Although they remained somewhat smaller at the time of ultimate anatomic repair (median 147 mm²/mm² compared with 167 mm²/m² in the surg>3mo group), this difference was not statistically significant.

The ideal palliations as described by Glatz in an editorial in 2016 [10] are as follows: 1. Providing a stable and balanced source of pulmonary blood flow; 2. Allowing growth of pulmonary vessels 3. Providing adequate time for subsidence of co-morbidities and to gain weight and 4. Leaving no residue.

Transcatheter techniques in the initial palliation of these patients have previously been attempted [9, 11, 12, 13], but did not gain widespread acceptance. Qureshi et al. attempted balloon dilatation as initial palliation in 15 infants with modest results with 10 requiring more than one dilatation [11]. Similarly, Sluysmans T et al. in 1995, published the result of BPV in 19 infants and concluded that it leads to a 30–40% reduction in the need for transannular patches at the time of corrective surgery [12]. Similarly, Gibbs first described RVOT stenting as a palliative procedure in 4 patients with associated co-morbidities [9]. Ballooning of the right ventricular outflow tract has gone out of favour because of dynamic obstruction in patients of TOF.

PDA stenting is gaining popularity in some of the centres but is a technically challenging procedure, besides establishment of pulmonary blood flow is unpredictable and sometimes leads to flooding of pulmonary circulation. Transcatheter RVOT stenting is gaining popularity as this results in a more physiological flow to pulmonary arteries and encourages equal growth of small pulmonary arteries providing a better surgical substrate for subsequent repair.

3. Procedure

The procedure as described by Quandt and Stumper et al. [13] include detailed pre-procedure work up and is usually performed under general anaesthesia and mechanical ventilation, since the children are usually hypoxic and sick and can deteriorate fast during the procedure.

Ambient temperature should be maintained by using Bair Hugger to prevent hypothermia.

Prostaglandin infusions are usually continued and all the emergency drugs should be available.

The child is to be positioned on the table with the arms elevated and the area to be painted and draped.

Access is usually via the right femoral vein in a majority of cases but sometimes an internal jugular venous approach is preferred if crossing of RVOT is difficult, especially in smaller children. A right femoral artery cannula is inserted for continuous blood pressure monitoring and for blood gas analysis. Once the right femoral venous sheath (usually 5F) is inserted, 50–100 IU/Kg of Heparin is given. The child also receives a prophylactic antibiotic (Cefazolin) dose.

A right ventricular cineangiogram is performed through an NIH or any other diagnostic catheter placed within the apex of the right ventricle; 30° RAO with 20° cranial tilt and a straight lateral projection are used. Some centres prefer to do angiograms in LAO view instead of lateral view. The intent is to delineate the RVOT, its length and diameter, the diameter of pulmonary valve annulus and the size of branch pulmonary arteries.

Selection of the size and the type of stent to be implanted is guided by the size of the patient, the dimensions of the outflow tract and the anticipated length of palliation.

For smaller children and neonates with short term palliation- coronary stent is preferred.

For older children or those who required medium to longer-term palliation- a bare metal peripheral vascular stent, preferably Cook Formula pre-mounted 414 or 418 stent, may be used.

The advantage of Cooks Formula stent is that it can be re-dilated if required and provided long term palliation. Sometimes, the availability of the specific stent is an issue; in such situations, any peripheral stent may be used. Balloon mounted stents are preferred. However, these stents may require thicker wire (0.035) either Amplatz superstiff or even Teflon wire for the stent delivery. Another disadvantage is that the stiffer wire may precipitate RVOT spasm and the child may have significant desaturation during the procedure.

After the selection of the stent, the appropriate delivery sheath or guide catheter is used. For coronary stents, a 4 French (F) Flexor sheath (Cook Europe, Bjaeverskov, Denmark) or a 60 cm 6 F right Judkins guide catheter (Cordis Corp, Miami Lakes FL) may be used.

A 0.014″ coronary wire is advanced across the RVOT via an end-hole catheter and a stable position is achieved by placing the wire in distal branch pulmonary arteries (PAs). Once the coronary wire is stable in the distal branch pulmonary artery, the selected delivery sheath or guide catheter replaces the diagnostic catheter. In older children where a stiff wire is required, firstly, a softer catheter such as a Glide catheter is passed into the distal branch PAs and then advance a 0.032 Terumo wire and final diagnostic catheter like (Judkins Right) JR or Multipurpose catheter may be used for replacing the Terumo wire with the stiff wires like Amplatz superstiff. The whole exercise is done due to hypertrophied infundibular area which may not allow the diagnostic catheter to pass over the coronary wire.

Cook Formula stents are implanted through either 5 or 6 F Flexor sheaths or sometimes Mullins sheath may be needed. The disadvantage of using a stiffer sheath is that it normally does not easily crosses the infundibular area, especially in older children who present late and have very hypertrophied RVOT. In these cases, the sheath is placed just below the infundibular area with multiple side arm injections; the stent is negotiated across the RVOT avoiding the annulus. However, if the annulus is small or if there is supravalvular PA narrowing, the stent may be placed across the pulmonary valve achieving a two-point fixation, one at infundibular and another at valve annulus level.

In infants and young children, the pre-mounted stent is placed over the wire but within the delivery sheath and advanced to the intended position within the RVOT and the stent is fully uncovered after checking with test angiograms. When the position of the stent appears satisfactory, (confirming it on echocardiography when necessary), the balloon is inflated. Following placement of the stent, the balloon is slowly deflated whilst the delivery sheath was advanced over the balloon, so as to re-sheath it. The position of the stent is confirmed on the check angiogram (Figures 1 and 2) via the side arm of the sheath. The position of the stent, opacification of the branch PAs and pulmonary valve movements are recorded on the final angiogram.

Echocardiography is performed for confirmation of the position of the stent, ventricular function, any interference with tricuspid valve function and evidence for effusion. A repeat blood gas analysis is obtained and improvement in PO2 is recorded. When we are confident of the implanted stent, the coronary wire along with the delivery sheath is removed under fluoroscopic monitoring and manual haemostasis is achieved.

The infant/child is transferred to the Neonatal/Paediatric ICU, as appropriate and vital signs are monitored. Patients who experienced an increase of oxygen saturation in excess of 20% are commenced on twice-daily diuretics. Chest X-ray is performed for any evidence of flooding of the lungs. In our experience, a peak gradient in excess of 40 mm Hg across RVOT on Doppler echocardiography post stenting usually does not have over circulation. Heparin infusion is continued and replaced with Aspirin (3–5 mg/kg) once the child starts to take it orally and the aspirin is continued till the child undergoes complete repair. RVOT stenting usually leads to a uniform growth of branch pulmonary arteries as shown in Figure 2 where a repeat angiogram one year post procedure showed adequate sized pulmonary arteries amiable for complete repair.

4. Modifications

The use of a long sheath as described by Quandt et al. [13] is mainly performed to minimise the tricuspid valve apparatus injury and to achieve a stable stent position by repeated test angiograms. However, the use of a long delivery sheath may be associated with haemodynamic instability, particularly in smaller and sicker patients due to tricuspid valve splinting open leading to compromised cardiac output and extreme cyanosis.

Linnane et al. have used periventricular approach in 4 patients weighing ≤2 kg through a small subxiphoid incision [14]. This approach provided a more direct route to the RVOT and the stent was deployed under Trans Thoracic Echocardiography (TTE) guidance.

Linnane et al. [15] also described a method for avoiding long delivery sheath during the stent deployment by crossing the tricuspid valve and RVOT with an angled glide catheter to facilitate placement of the guidewire in the branch pulmonary arteries and doing 3–4 clockwise rotations to create some backwards tension on the wire during stent advancement. The authors utilised TTE to guide the stent placement rather than angiography.

5. Complications

Malposition or migration- of the stent may occur due to more proximal deployment of the stent. If the stent does not achieve 2 point fixation, it remains unstable. The commonest site of embolization is descending aorta (Figure 3). The use of a long delivery sheath for confirming the position and preventing the slippage of the stent minimises the risk. Sometimes, distal deployment of the stent can lead to migration of stent into branch pulmonary arteries.

Other Complications include balloon rupture, dissection, stent induced pulmonary oedema, arrhythmias, injury to adjoining structures and injury to tricuspid valve leading to tricuspid regurgitation, hypotension and hemodynamic instability during the procedure.

6. Challenges

The procedure is a technically difficult procedure with a significant learning curve and is difficult to execute in a new program.

Requirement of re-interventions, especially if performed in early neonatal period with very hypoplastic pulmonary arteries.

Difficult corrective surgery post stenting- Removal of stent can be challenging with increased cardiopulmonary bypass time, sometimes the posterior aspect of stent is left in situ, and concerns of injury to adjoining structures like aortic and tricuspid valve can complicate the post-op recovery.

Long term data on RVOT stenting is lacking, especially RV dilatation, growth of pulmonary vessels and need for re-interventions.

7. Outcomes

The immediate outcome in children undergoing RVOT stenting is quite favourable with saturations improving immediately after the procedure. In a yet unpublished data from the authors more than 30 children had undergone RVOT stenting majority of which were more than one year of age, there was significant improvement in the saturations and children were shifted out of PICU within 24 hrs. There were two instances of stent embolization which happened in the initial phase of the learning curve with no in-hospital mortality. The children were discharged on antiplatelets and no episode of stent thrombosis or fracture was noted on follow up.

In a retrospective study by Sandoval et al. at the Hospital for Sick Children in Toronto [1] which divided the children undergoing treatment for TOF into four categories as described earlier, it was found that the RVOT stent group had significantly smaller pulmonary arteries as compared to other 3 groups (median Nakata index of 79 mm²/m²) with a comparable post-operative stay, thereby implying that the procedure can be done in children with very unfavourable anatomy.

8. Summary

Transcatheter RVOT stenting is increasingly preferred over other palliative procedures as aorto-pulmonary shunts have a very unpredictable course post-operatively, especially neonatal BT shunts. The risks of shunt thrombosis or pulmonary over circulation can influence the post-operative recovery, shunt leads to disruption and distortion of the pulmonary arteries which can significantly impact the definitive repair at a later date. RVOT stenting establishes a more physiological flow to pulmonary arteries and encourages equal growth of small pulmonary arteries providing a better surgical substrate for subsequent repair.

9. Conclusion

With increasing expertise and modifications like avoidance of long sheath and reducing procedure time, RVOT stenting is emerging as a safer alternative to other palliative procedures. However, many centres are opting for early primary repair. The RVOT stenting procedure is likely to gain more and more acceptance among other palliative options as the institutes increase their experience.