**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 [1–3]. 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 [4–8].

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 mm<sup>2</sup> /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, 11–13], 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.
