Comparative Study on the Outcomes of Right Ventricular Outflow Tract
Stenting 
The management of symptomatic infants with tetralogy of Fallot (ToF) has undergone
changes over the years. Temporary palliation remains a crucial option for managing
cyanotic patients with low birth weight or complex anatomy in most centers, as
complete repair in these cases carries significantly higher risks compared to repair
at an older age  . Independent
risk factors for complete neonatal repair and palliation with a modified
Blalock-Taussig shunt (mBTs) include low birth weight and small pulmonary arteries
(PAs)  . Limited reports have assessed pulmonary arterial
growth after the mBTs procedure [ - ] . Several studies have indicated
enhanced pulmonary arterial growth after the Norwood procedure in hypoplastic left
heart syndrome, utilizing a right ventricle to PA conduit compared to an
mBTs  . On the other hand, relieving the right ventricular
outflow tract (RVOT) obstruction in patients with Fallot-type lesions through stent
implantation may promote better pulmonary arterial growth compared to an mBTs.
Surgical strategies such as modified mBTs or primary neonatal repair might lead to
potential complications, such as pulmonary over-circulation or even PA
stenosis  . These surgical
strategies have been associated with relatively high mortality rates, reported as
6.2% and 7.8%, respectively  . An
alternative approach to palliation, such as RVOT stenting, has emerged due to the
accepted optimum timing for complete repair being between three and 11
months  . Previous
studies on this technique have consistently reported favorable results,
demonstrating excellent outcomes with a low incidence of complications. Several
studies have demonstrated that in patients with hypoplastic left heart syndrome,
pulmonary atresia, and ventricular septal defect (VSD) (types A and B according to
Tchervenkov classification), reconstruction of the RVOT yields better outcomes in
terms of PA rehabilitation compared to the formation of an mBTs [ - ] . Therefore, we expected that the resolution of significant
obstruction in the RVOT in ToF may result in more efficient growth of the central PA
compared to the utilization of the mBTs. The aim of our study was to compare the
outcomes of two different palliative approaches, specifically RVOT stenting vs. modified mBTs, in the context of a two-stage correction of
ToF. 

Between June 2018 and August 2022, a total of 42 consecutive patients with
symptomatic ToF were included in the study. This study followed a multicenter
approach and utilized a prospective, open, and randomized design with two arms. The
study was carried out in accordance with the Declaration of Helsinki and was
approved by the institutional review board (Nº 9 from 17 May 2018). According to the
inclusion and exclusion criteria ( ), a
total of 42 infants who underwent staged correction of the defect were included in
the study out of the initial 58 patients. Informed consent was obtained from each
patient (from their legal representatives). After randomization, the children were
divided into two groups (21 patients in each group) based on the method of
palliative treatment: either stenting of the RVOT or creation of an mBTs. The
patients’ baseline characteristics are given in . Infants diagnosed with ToF, aged < 4 months, scheduled to
undergo palliative surgery, and meeting the inclusion criteria of having confluent
PA, a Nakata index of ≥ 150 mm 2 /m 2 , and a PA z score of < -2 were included in the study. Patients who had
discordant atrioventricular and/or ventriculoarterial connections, contaminant
cardiac pathologies including common atrioventricular canal (CAVC), multiple VSDs,
or coronary anomalies, genetic pathologies, or those who had undergone other
surgical approaches such as complete repair, central shunt placement, or patent
ductus arteriosus stenting were excluded from the study. Around 55% of the patients
underwent initial palliation due to either significant spells or severe
desaturation, or the presence of hypoplastic PA with a z score of
< -2. At the time of intervention, over 70% of the patients were receiving
medical treatment, including the administration of beta-blockers or prostaglandin
infusion. Complete repair beyond early infancy was postponed in cases where there
were other relevant comorbidities, such as syndromes, prematurity, or low birth
weight, and if the patient was clinically stable. This study aims to compare the degree of development of the central PAs following
palliative intervention, describe complications in the interstage period, and
evaluate the outcomes of delayed anatomical correction. The primary endpoint was the
degree of development of the central PAs at three months following the palliative
operation. The effectiveness of the palliative treatment was considered achieved if
the Nakata index was ≥ 200 mm 2 /m 2 (evaluated using
binary logistic regression). The secondary endpoints included evaluating early and mid-term postoperative
complications such as shunt thrombosis, bleeding, infective endocarditis, stent
thrombosis, and reinterventions. The growth dynamics of the pulmonary valve, main PA, and right and left branches of
the PA at the level of bifurcation were evaluated using a series of
echocardiographic examinations conducted prior to the palliative treatment, during
the interstage period on an outpatient basis, and at the time of delayed anatomical
correction. The results were analyzed using indexed z score
values  . To assess the
anatomy of the RVOT, degree of PA hypoplasia, and coronary artery anatomy prior to
the palliative intervention and at three months post-intervention, contrast-enhanced
multislice computed tomography (MSCT) was performed. The degree of PA hypoplasia was
evaluated using the Nakata index, McGoon ratio, and Reddy index  . Surgical Techniques Modified Blalock-Taussig Shunt Technique All mBTs procedures were performed through a sternotomy. Following a complete
sternotomy, the excision of the right lobe of the thymus was performed, and
the course of the brachiocephalic trunk was dissected up to its bifurcation.
Additionally, the dissection of the right PA was carried out up to the
hilum. A bolus of 100 IU/kg crystalline heparin was administered. The
brachiocephalic trunk was clamped with a Cooley clamp, and the distal right
subclavian artery, temporarily with a ligaclip. A longitudinal arteriotomy
was then made at the previously marked undersurface of the continuity
between the truncus and subclavian artery. The thin-wall Gore-Tex stretch
vascular graft (W. L. Gore & Associates, Inc., Arizona, United States of
America) was sutured to the arteriotomy in an oblique fashion, with one end
of the graft connected end-to-side. Clamps were released and a good shunt
flow through the anastomosis ascertained. The Cooley clamp was placed again
to exclude the proximal anastomosis from the circulation. The shunt length
was trimmed so as to avoid it being too long. The right PA was excluded from
circulation using two vascular clamps. The transversely fashioned distal end
of the graft was anastomosed to the right PA. Right Ventricular Outflow Tract Stenting Procedure All cardiac catheterizations were performed under general anaesthesia with
fluoroscopic guidance. Catheterization was done through the right or left
femoral venous approach. The patients were given 50-100 IU/kg of heparin in
two doses (50 IU/kg first and 50 IU/kg half hour later) to maintain an
activated clotting time of > 200 s after catheterization. Prophylactic
intravenous antibiotic with 30 mg/kg of cefazolin was administrated at the
beginning of the procedure and in two subsequent doses every eight hours
during the following 24 hours. The stenting procedure involved the use of
bi-plane angiography and accessing the RVOT through the femoral veins. A
right ventricular cineangiogram is performed through a pigtail or any other
diagnostic catheter placed within the apex of the right ventricle. A
telescopic configuration of catheters was used, along with a guide wire, to
cross the RVOT. A coronary wire was securely positioned in the distal
right/left PA. After stabilization of the guide wire, balloon angioplasty of
the RVOT and the main PA was performed to alleviate the difficulties of
stent implementation into the RVOT ( ). The stents were selected based on the size of the infants,
RVOT infundibular length, and anticipated length of palliation. After the
selection of the stent, the appropriate delivery sheath or guide catheter is
used. Peripheral stents with nominal diameters that were 1-2 mm greater than
the annular dimension were selected and implanted in the RVOT. 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. Given these anatomical
features, longer stents, measuring 14-16 mm in length, were used in such
cases. In instances where marked pulmonary valve hypoplasia was not
observed, and future valve-sparing procedures were anticipated, stents were
exclusively implanted in the RVOT. In these cases, the stent diameter was
chosen to be 2-3 mm larger than the RVOT. The stents were 5-7 mm in diameter
and 12-16 mm in length. The stent position is verified using the side arm of
the sheath on the check angiogram ( ). The final angiogram records the stent position, opacification
of the branch PAs, and pulmonary valve movements. Before removing the
catheter, echocardiography was performed to confirm the position of the
stent, assess any impact on tricuspid valve function, and check for signs of
effusion. The results indicated a significant increase in pulmonary blood
flow and oxygen saturation. Complete Correction Complete correction surgery was performed in all cases under conditions of
cardiopulmonary bypass, moderate hypothermia, and with the use of
Custodiol® cardioplegic solution. The choice of the method for
reconstructing the RVOT was based on the assessment of the risk of
developing right ventricular insufficiency. Statistical Analysis The sample size calculation was performed using G*Power 3.1 ( http://gpower.hhu.de ), considering an anticipated difference of
40% in mean deposition, with a statistical power of 80%. To account for
potential incomplete observations during the follow-up period, the calculated
sample size was increased by 10% to adequately compensate for any missing data.
Continuous variables are expressed as median (25 th ; 75 th percentile), unless otherwise specified. Categorical variables are presented as
numbers (%). The Mann-Whitney U test, chi-square test, and Fisher’s exact test
were used for intergroup comparisons. Multiple logistic regression analysis was
performed to detect risk factors for the occurrence of complications (residual
shunts). The multivariate model considered the significant variables
( P <0.05) in univariate analysis, and odds ratios and
their 95% confidence intervals were calculated. P <0.05 was
considered statistically significant. Statistical analysis was performed by
using Stata 14 (StataCorp LP, College Station, Texas, United States of America)
and MS Excel 2022 software (Microsoft Corporation, Redmond, Washington, United
States of America). 

Modified Blalock-Taussig Shunt Technique All mBTs procedures were performed through a sternotomy. Following a complete
sternotomy, the excision of the right lobe of the thymus was performed, and
the course of the brachiocephalic trunk was dissected up to its bifurcation.
Additionally, the dissection of the right PA was carried out up to the
hilum. A bolus of 100 IU/kg crystalline heparin was administered. The
brachiocephalic trunk was clamped with a Cooley clamp, and the distal right
subclavian artery, temporarily with a ligaclip. A longitudinal arteriotomy
was then made at the previously marked undersurface of the continuity
between the truncus and subclavian artery. The thin-wall Gore-Tex stretch
vascular graft (W. L. Gore & Associates, Inc., Arizona, United States of
America) was sutured to the arteriotomy in an oblique fashion, with one end
of the graft connected end-to-side. Clamps were released and a good shunt
flow through the anastomosis ascertained. The Cooley clamp was placed again
to exclude the proximal anastomosis from the circulation. The shunt length
was trimmed so as to avoid it being too long. The right PA was excluded from
circulation using two vascular clamps. The transversely fashioned distal end
of the graft was anastomosed to the right PA. Right Ventricular Outflow Tract Stenting Procedure All cardiac catheterizations were performed under general anaesthesia with
fluoroscopic guidance. Catheterization was done through the right or left
femoral venous approach. The patients were given 50-100 IU/kg of heparin in
two doses (50 IU/kg first and 50 IU/kg half hour later) to maintain an
activated clotting time of > 200 s after catheterization. Prophylactic
intravenous antibiotic with 30 mg/kg of cefazolin was administrated at the
beginning of the procedure and in two subsequent doses every eight hours
during the following 24 hours. The stenting procedure involved the use of
bi-plane angiography and accessing the RVOT through the femoral veins. A
right ventricular cineangiogram is performed through a pigtail or any other
diagnostic catheter placed within the apex of the right ventricle. A
telescopic configuration of catheters was used, along with a guide wire, to
cross the RVOT. A coronary wire was securely positioned in the distal
right/left PA. After stabilization of the guide wire, balloon angioplasty of
the RVOT and the main PA was performed to alleviate the difficulties of
stent implementation into the RVOT ( ). The stents were selected based on the size of the infants,
RVOT infundibular length, and anticipated length of palliation. After the
selection of the stent, the appropriate delivery sheath or guide catheter is
used. Peripheral stents with nominal diameters that were 1-2 mm greater than
the annular dimension were selected and implanted in the RVOT. 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. Given these anatomical
features, longer stents, measuring 14-16 mm in length, were used in such
cases. In instances where marked pulmonary valve hypoplasia was not
observed, and future valve-sparing procedures were anticipated, stents were
exclusively implanted in the RVOT. In these cases, the stent diameter was
chosen to be 2-3 mm larger than the RVOT. The stents were 5-7 mm in diameter
and 12-16 mm in length. The stent position is verified using the side arm of
the sheath on the check angiogram ( ). The final angiogram records the stent position, opacification
of the branch PAs, and pulmonary valve movements. Before removing the
catheter, echocardiography was performed to confirm the position of the
stent, assess any impact on tricuspid valve function, and check for signs of
effusion. The results indicated a significant increase in pulmonary blood
flow and oxygen saturation. Complete Correction Complete correction surgery was performed in all cases under conditions of
cardiopulmonary bypass, moderate hypothermia, and with the use of
Custodiol® cardioplegic solution. The choice of the method for
reconstructing the RVOT was based on the assessment of the risk of
developing right ventricular insufficiency. 

All mBTs procedures were performed through a sternotomy. Following a complete
sternotomy, the excision of the right lobe of the thymus was performed, and
the course of the brachiocephalic trunk was dissected up to its bifurcation.
Additionally, the dissection of the right PA was carried out up to the
hilum. A bolus of 100 IU/kg crystalline heparin was administered. The
brachiocephalic trunk was clamped with a Cooley clamp, and the distal right
subclavian artery, temporarily with a ligaclip. A longitudinal arteriotomy
was then made at the previously marked undersurface of the continuity
between the truncus and subclavian artery. The thin-wall Gore-Tex stretch
vascular graft (W. L. Gore & Associates, Inc., Arizona, United States of
America) was sutured to the arteriotomy in an oblique fashion, with one end
of the graft connected end-to-side. Clamps were released and a good shunt
flow through the anastomosis ascertained. The Cooley clamp was placed again
to exclude the proximal anastomosis from the circulation. The shunt length
was trimmed so as to avoid it being too long. The right PA was excluded from
circulation using two vascular clamps. The transversely fashioned distal end
of the graft was anastomosed to the right PA. 

All cardiac catheterizations were performed under general anaesthesia with
fluoroscopic guidance. Catheterization was done through the right or left
femoral venous approach. The patients were given 50-100 IU/kg of heparin in
two doses (50 IU/kg first and 50 IU/kg half hour later) to maintain an
activated clotting time of > 200 s after catheterization. Prophylactic
intravenous antibiotic with 30 mg/kg of cefazolin was administrated at the
beginning of the procedure and in two subsequent doses every eight hours
during the following 24 hours. The stenting procedure involved the use of
bi-plane angiography and accessing the RVOT through the femoral veins. A
right ventricular cineangiogram is performed through a pigtail or any other
diagnostic catheter placed within the apex of the right ventricle. A
telescopic configuration of catheters was used, along with a guide wire, to
cross the RVOT. A coronary wire was securely positioned in the distal
right/left PA. After stabilization of the guide wire, balloon angioplasty of
the RVOT and the main PA was performed to alleviate the difficulties of
stent implementation into the RVOT ( ). The stents were selected based on the size of the infants,
RVOT infundibular length, and anticipated length of palliation. After the
selection of the stent, the appropriate delivery sheath or guide catheter is
used. Peripheral stents with nominal diameters that were 1-2 mm greater than
the annular dimension were selected and implanted in the RVOT. 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. Given these anatomical
features, longer stents, measuring 14-16 mm in length, were used in such
cases. In instances where marked pulmonary valve hypoplasia was not
observed, and future valve-sparing procedures were anticipated, stents were
exclusively implanted in the RVOT. In these cases, the stent diameter was
chosen to be 2-3 mm larger than the RVOT. The stents were 5-7 mm in diameter
and 12-16 mm in length. The stent position is verified using the side arm of
the sheath on the check angiogram ( ). The final angiogram records the stent position, opacification
of the branch PAs, and pulmonary valve movements. Before removing the
catheter, echocardiography was performed to confirm the position of the
stent, assess any impact on tricuspid valve function, and check for signs of
effusion. The results indicated a significant increase in pulmonary blood
flow and oxygen saturation. 

Complete correction surgery was performed in all cases under conditions of
cardiopulmonary bypass, moderate hypothermia, and with the use of
Custodiol® cardioplegic solution. The choice of the method for
reconstructing the RVOT was based on the assessment of the risk of
developing right ventricular insufficiency. 

The sample size calculation was performed using G*Power 3.1 ( http://gpower.hhu.de ), considering an anticipated difference of
40% in mean deposition, with a statistical power of 80%. To account for
potential incomplete observations during the follow-up period, the calculated
sample size was increased by 10% to adequately compensate for any missing data.
Continuous variables are expressed as median (25 th ; 75 th percentile), unless otherwise specified. Categorical variables are presented as
numbers (%). The Mann-Whitney U test, chi-square test, and Fisher’s exact test
were used for intergroup comparisons. Multiple logistic regression analysis was
performed to detect risk factors for the occurrence of complications (residual
shunts). The multivariate model considered the significant variables
( P <0.05) in univariate analysis, and odds ratios and
their 95% confidence intervals were calculated. P <0.05 was
considered statistically significant. Statistical analysis was performed by
using Stata 14 (StataCorp LP, College Station, Texas, United States of America)
and MS Excel 2022 software (Microsoft Corporation, Redmond, Washington, United
States of America). 

Pulmonary Development A comparative assessment of the growth dynamics of the central PAs was conducted
from the time of palliative intervention to delayed anatomical correction using
indexed parameters. In the intergroup comparison, there were no significant
differences in the degree of development of the central PAs between the stent
and mBTs groups prior to palliative treatment ( z score growth
2.2 in stent group [ P <0.01], z score growth
1.3 in mBTs group [ P <0.01]). By the time of complete
correction, no significant growth of the pulmonary valve was observed in the
study groups ( P =0.5 in the stent group, P =0.1
in the mBTs group). However, there was noted development of the branches of the
PA. During the observation period, the median z score of the
right PA increased from -2.3 to 0.08 in the stent group
( P <0.01), and from -2.3 to -1.5 in the mBTS group
( P <0.01), respectively. The median z score of the left PA in the stent group increased from -2.1 to 0.5
( P <0.01), while in the mBTs group, it increased from
-1.9 to -0.25 ( P <0.01). displays all the measurements of the PA branches based on
echocardiography data. The growth dynamics of the PAs in terms of z scores during the observation period are presented in and . According to contrast-enhanced MSCT data obtained
three months after palliative treatment, there was also noted growth of the PA
based on the Nakata index. In the stent group, the mean Nakata index increased
from 121.17 mm 2 /m 2 to 208.6 mm 2 /m 2 ( P <0.01), and in the mBTs group, it increased from
135.81 mm 2 /m 2 to 169.4 mm 2 /m 2 ( P <0.01). By the time of complete correction, in the
intergroup comparison, the z scores of the right and left PAs, as well as the
Nakata index, were significantly higher in the stent group compared to the mBTs
group ( P <0.01, P =0.049, and P <0.01, respectively). To assess and compare the growth rate of the central PAs in the study groups, a
mixed-effects model analysis was performed ( ). As a result, it was found that stenting of the pulmonary
valve leads to a growth rate of the right PA at 2.05*10-2 z score per day ( P <0.01), which is 3.01 times higher than the
growth rate of 0.68*10-2 z score per day in the mBTs group. The
growth rate of the left PA in the stent group was 2.23*10-2 z score per day, which is 1.47 times higher than the growth rate of 1.56*10-2 z score per day in the mBTs group ( P -value
< 0.01). None of the independent predictors (prematurity, neonatal status,
weight before palliative treatment) significantly influenced the growth rate of
the PAs. In order to determine the factors influencing the achievement of a Nakata index
≥ 200 mm 2 /m 2 in the study groups, binary logistic
regression was performed. The results are presented in . As a result, it was found that stenting of the RVOT
increases the odds of achieving a Nakata index ≥ 200
mm 2 /m 2 by 7.78 times compared to the formation of an
mBTs. Binary logistic regression was performed to assess the factors influencing the
achievement of a Nakata index ≥ 200 mm 2 /m 2 by the
time of delayed anatomical correction. Interstage Period In one patient (4.8%) after 76 days following the stenting of the RVOT, a severe
infectious process with sepsis occurred, leading to a fatal outcome. No other
complications requiring hospitalization during the interstage period or repeat
interventions were observed in the remaining cases. Complete Correction Delayed complete correction was performed on 20 patients in the stent group with
an average of 97 (92;118) days after palliative intervention and on 21 infants
in the mBTs group after 109 (106;128) days. General characteristics of the
patients before the surgical treatment are presented in . In patients of the stent group, the stent was located throughout the RVOT with
ingrowth of the structures of the bare metal stent into the hypertrophied
septoparietal trabeculae of RVOT. In 12 out of 20 children (55%), the stent
crossed the pulmonary valve, while in the remaining nine cases (45%), it was
isolated in the RVOT. In both groups, after palliative treatment, pulmonary
valve hypoplasia was preserved with a median z score of -3.2 in
the stent group and a median z score of -3.1 in the mBTs group;
the leaflets of the valve were mostly represented by fibrotically altered cusps.
Due to the persistent marked hypoplasia, the native valve was transected in all
cases during complete repair. Intraoperatively, during the revision, the
previously implanted stents were not deformed, not patent, and not covered with
fibrous tissue around their perimeter. During the process of explantation, the
stent was gradually dissected from the RVOT and extracted from the PA trunk
without significant technical problems. In cases where the stent was
intra-annularly positioned, the fibrotic leaflets of the pulmonary valve were
resected simultaneously. Transannular plasties of the RVOT were performed in 12
(60%) out of 20 patients in the stent group and in 15 (71.4%) out of 21 children
in the mBTs group ( P =0.52). Valve-sparing reconstruction of the
PA outflow with the Contegra™ No. 12 xenograft conduit (Medtronic, United
States of America) was performed in eight children (40%) in the stent group and
in six (28.6%) in the mBTs group ( P =0.52). Due to the
deformation and stenosis of the PA at the site of the previously formed shunt,
the reconstruction of the PA outflow was complemented by the plasties of the
stenosed branch. In the mBTs group, the duration of the operation was significantly longer
( P =0.046) compared to the stent group, which was due to the
adhesive process in the pericardial cavity ( ). However, considering the additional time spent on
explanting the stent from the RVOT, the duration of aortic occlusion and the
duration of cardiopulmonary bypass were significantly longer in the stent group
( P <0.01 and P <0.01, respectively).
Due to the development of severe right ventricular insufficiency in the early
postoperative period, one patient in the stent group (5%) and another patient in
the mBTs group (4.8%) after transannular plasty required the establishment of
extracorporeal membrane oxygenation (for 10 days in the stent group and for five
days in the mBTs group), with subsequent favorable outcomes. Additionally, one
case of stent placement in the branches of the PAs after prosthetics of the PA
was registered in each of the studied groups. In the mBTs group, diaphragm
plication was required in one case (4.8%) on the eighth day after the operation.
No fatalities were reported during the hospitalization period
( P >0.99). 

A comparative assessment of the growth dynamics of the central PAs was conducted
from the time of palliative intervention to delayed anatomical correction using
indexed parameters. In the intergroup comparison, there were no significant
differences in the degree of development of the central PAs between the stent
and mBTs groups prior to palliative treatment ( z score growth
2.2 in stent group [ P <0.01], z score growth
1.3 in mBTs group [ P <0.01]). By the time of complete
correction, no significant growth of the pulmonary valve was observed in the
study groups ( P =0.5 in the stent group, P =0.1
in the mBTs group). However, there was noted development of the branches of the
PA. During the observation period, the median z score of the
right PA increased from -2.3 to 0.08 in the stent group
( P <0.01), and from -2.3 to -1.5 in the mBTS group
( P <0.01), respectively. The median z score of the left PA in the stent group increased from -2.1 to 0.5
( P <0.01), while in the mBTs group, it increased from
-1.9 to -0.25 ( P <0.01). displays all the measurements of the PA branches based on
echocardiography data. The growth dynamics of the PAs in terms of z scores during the observation period are presented in and . According to contrast-enhanced MSCT data obtained
three months after palliative treatment, there was also noted growth of the PA
based on the Nakata index. In the stent group, the mean Nakata index increased
from 121.17 mm 2 /m 2 to 208.6 mm 2 /m 2 ( P <0.01), and in the mBTs group, it increased from
135.81 mm 2 /m 2 to 169.4 mm 2 /m 2 ( P <0.01). By the time of complete correction, in the
intergroup comparison, the z scores of the right and left PAs, as well as the
Nakata index, were significantly higher in the stent group compared to the mBTs
group ( P <0.01, P =0.049, and P <0.01, respectively). To assess and compare the growth rate of the central PAs in the study groups, a
mixed-effects model analysis was performed ( ). As a result, it was found that stenting of the pulmonary
valve leads to a growth rate of the right PA at 2.05*10-2 z score per day ( P <0.01), which is 3.01 times higher than the
growth rate of 0.68*10-2 z score per day in the mBTs group. The
growth rate of the left PA in the stent group was 2.23*10-2 z score per day, which is 1.47 times higher than the growth rate of 1.56*10-2 z score per day in the mBTs group ( P -value
< 0.01). None of the independent predictors (prematurity, neonatal status,
weight before palliative treatment) significantly influenced the growth rate of
the PAs. In order to determine the factors influencing the achievement of a Nakata index
≥ 200 mm 2 /m 2 in the study groups, binary logistic
regression was performed. The results are presented in . As a result, it was found that stenting of the RVOT
increases the odds of achieving a Nakata index ≥ 200
mm 2 /m 2 by 7.78 times compared to the formation of an
mBTs. Binary logistic regression was performed to assess the factors influencing the
achievement of a Nakata index ≥ 200 mm 2 /m 2 by the
time of delayed anatomical correction. 

In one patient (4.8%) after 76 days following the stenting of the RVOT, a severe
infectious process with sepsis occurred, leading to a fatal outcome. No other
complications requiring hospitalization during the interstage period or repeat
interventions were observed in the remaining cases. 

Delayed complete correction was performed on 20 patients in the stent group with
an average of 97 (92;118) days after palliative intervention and on 21 infants
in the mBTs group after 109 (106;128) days. General characteristics of the
patients before the surgical treatment are presented in . In patients of the stent group, the stent was located throughout the RVOT with
ingrowth of the structures of the bare metal stent into the hypertrophied
septoparietal trabeculae of RVOT. In 12 out of 20 children (55%), the stent
crossed the pulmonary valve, while in the remaining nine cases (45%), it was
isolated in the RVOT. In both groups, after palliative treatment, pulmonary
valve hypoplasia was preserved with a median z score of -3.2 in
the stent group and a median z score of -3.1 in the mBTs group;
the leaflets of the valve were mostly represented by fibrotically altered cusps.
Due to the persistent marked hypoplasia, the native valve was transected in all
cases during complete repair. Intraoperatively, during the revision, the
previously implanted stents were not deformed, not patent, and not covered with
fibrous tissue around their perimeter. During the process of explantation, the
stent was gradually dissected from the RVOT and extracted from the PA trunk
without significant technical problems. In cases where the stent was
intra-annularly positioned, the fibrotic leaflets of the pulmonary valve were
resected simultaneously. Transannular plasties of the RVOT were performed in 12
(60%) out of 20 patients in the stent group and in 15 (71.4%) out of 21 children
in the mBTs group ( P =0.52). Valve-sparing reconstruction of the
PA outflow with the Contegra™ No. 12 xenograft conduit (Medtronic, United
States of America) was performed in eight children (40%) in the stent group and
in six (28.6%) in the mBTs group ( P =0.52). Due to the
deformation and stenosis of the PA at the site of the previously formed shunt,
the reconstruction of the PA outflow was complemented by the plasties of the
stenosed branch. In the mBTs group, the duration of the operation was significantly longer
( P =0.046) compared to the stent group, which was due to the
adhesive process in the pericardial cavity ( ). However, considering the additional time spent on
explanting the stent from the RVOT, the duration of aortic occlusion and the
duration of cardiopulmonary bypass were significantly longer in the stent group
( P <0.01 and P <0.01, respectively).
Due to the development of severe right ventricular insufficiency in the early
postoperative period, one patient in the stent group (5%) and another patient in
the mBTs group (4.8%) after transannular plasty required the establishment of
extracorporeal membrane oxygenation (for 10 days in the stent group and for five
days in the mBTs group), with subsequent favorable outcomes. Additionally, one
case of stent placement in the branches of the PAs after prosthetics of the PA
was registered in each of the studied groups. In the mBTs group, diaphragm
plication was required in one case (4.8%) on the eighth day after the operation.
No fatalities were reported during the hospitalization period
( P >0.99). 

RVOT stenting is a potentially viable alternative to the creation of an mBTs,
offering advantages such as antegrade pulsatile flow of venous blood into the
pulmonary bed with high oxygen saturation of arterial blood and rapid stabilization
of hemodynamics and organ perfusion due to the absence of a decrease in diastolic
arterial pressure [ , - ] . Within the scope of our prospective, two-center randomized study, a comparative
evaluation of the outcomes of two palliative treatment methods - stenting of the
RVOT and creation of an mBTs - was conducted. The study aimed to investigate their
impact on pulmonary flow development, describe major cardiovascular events in the
interstage period, and assess the immediate outcomes of complete correction. A number of studies confirm that stenting of the RVOT in critical forms of ToF is an
effective and safe procedure, with a mortality rate of 0% (Sandoval J. P. et
al.  ) and 1.7% (Quandt
D, et al.  ) in the early
postoperative period. The results of delayed surgical treatment after stenting of
the RVOT are comparable to the outcomes of primary complete correction in older age
groups. In our study, the mortality rate in the early postoperative period after
palliative treatment was also 0%. Only one lethality outcome (4.8%) was observed in
the interstage period in the stent group, which was associated with non-cardiac
pathology. In other aspects, no major cardiovascular events were identified in the
study groups during the interstage period. However, literature describes cases of
persistent desaturation after RVOT stenting, caused by the development of
obstruction in the proximal part of the RVOT, which can deform the corresponding
section of the stent. In such cases, balloon angioplasty or the placement of another
stent with coverage of the previous one has been carried out to address the
resulting obstruction in the proximal part of the RVOT [ , - ] . Several authors have reported
cases of persistent desaturation after RVOT stenting, necessitating additional
creation of an mBTs  . In turn, after the creation of
mBTs, cases of thrombosis have been observed, requiring reformation of the mBTs or
stenting of the pathway to the PA  . In a study
assessing the impact of both palliative treatment methods on PA growth, it was shown
that stenting of the RVOT leads to faster, significant, and more uniform development
of the PA compared to the creation of an mBTs. Over the observation period, the
stent group showed a significant increase in the right PA (∆ z score) of 2.2, which was significantly higher than the mBTs group with a score of
1.1. A similar pattern was observed in the left PA, with a ∆ z score
of 2.6 in the stent group, significantly exceeding the score of 1.6 in the mBTs
group. Using a mixed-effects model, it was also found that the rate of growth of the
right and left PAs was significantly higher in the stent group. In retrospective
studies by several authors [ , , , ] , similar results
have been reported, confirming faster and more uniform growth of the branches of the
PA after stenting of the RVOT. In studies conducted by a group of authors from
Toronto, it was described that in 80% of patients at the time of stenting of RVOT
the Nakata index was < 100 mm 2 /m 2 , while at the time of
complete correction, only 16% of children had a Nakata index remaining < 100
mm 2 /m 2  . In the study by Dohlen et al.  , an increase in the Nakata index from a mean of
56 mm 2 /m 2 (range 21-77) to 150 mm 2 /m 2 (range 123-231) was described. In our study, the stent group showed a ∆ (increase)
in the Nakata index of 105.2 mm 2 /m 2 , which was significantly
higher than the mBTs group with a ∆ of 58.2 mm 2 /m 2 . Some
authors use the difference in Nakata index as a measure for evaluating the growth of
the PA. In our analysis, we also used the difference in the Nakata index  to estimate the PA growth rate. The
mean index of growth (mm 2 /m 2 /month), representing the speed of
growth of the PA, was 35.9 mm 2 /m 2 /month in the RVOT stent
group and 22.1 mm 2 /m 2 /month in the group of mBTs. Binary logistic regression analysis revealed that stenting increased the odds of
achieving a Nakata index ≥ 200 mm 2 /m 2 by 7.78 times
compared to the formation of a modified mBTs. Another important consideration is the
fact that when assessing the growth of the PAs using the Nakata and the z score indices, the interventions on the PAs themselves during
surgical procedures could potentially alter the data used to estimate the growth
rate. This is why it is likely that the use of the lower lobe index in these
patients for assessing PA growth is more reliable since the lower lobe branches of
the PAs remain intact. Another interesting hypothesis is that during stenting of the RVOT, the development
of the PA improves due to the balanced growth of both the right and left PAs. This
is because the PAs themselves are not directly affected during the procedure. Daniel
Quandt et al. demonstrated in their study that patients who underwent initial RVOT
stenting exhibited improved and more consistent growth of the branch PAs compared to
those who underwent an mBTs  .
Indeed, our study partially addressed this issue, although it was not the primary
endpoint. We were able to assess this by measuring the lower lobe index, as the
lower branch remains intact from the surgical intervention in mBTs procedures.
However large multicenter studies needed to validate this hypothesis. One of the important questions is the reconstruction of the RVOT during delayed
complete correction. Colleagues from Birmingham describe in a single-center
retrospective study that the type of reconstruction of the RVOT after stenting of
the pulmonary valve or creation of an mBTs was similar  . In 58% of cases after pulmonary valve stenting
or creation of an mBTs, transannular patch repair of the RVOT was performed. In 30%
after stenting and 31% after mBTs creation, PA prosthetic replacement was done. It
was also noted that the number of plastic interventions on the PA branches after
pulmonary valve stenting was lower (15%) compared to the results after forming the
mBTs (31%). According to the results of our study, the type of reconstruction of the outflow
tract to the PA did not statistically differ between the groups. Transannular patch
repair of the RVOT was performed in 12 (60%) cases out of 20 in the stent group and
in 15 (71.4%) out of 21 cases in the mBTs group. Eight children (40%) in the stent
group and six (28.6%) in the mBTs group underwent prosthetic reconstruction of the
outflow tract to the PA. The choice of reconstruction method was determined by the
assessment of the risk of right ventricular insufficiency. All patients in the mBTs
group required additional plastic intervention on the stenosed PA after mBTs
formation. In one case in the stent group, after prosthetic replacement of the PA,
stenoses of the branch PAs were detected, requiring additional stenting. In the intergroup comparison, the duration of the complete correction surgery in the
stent group was significantly shorter. The duration of the complete correction
procedure in patients from the stent group was considered to be significantly
shorter primarily due to factors such as the absence of adhesion formation and a
reduced occurrence of intraoperative complications. However, the duration of aortic
occlusion and artificial circulation was significantly higher in the pulmonary valve
stenting group, which can be explained by the additional time required for
explantation of the stent from the RVOT. Furthermore, it should also be emphasized that despite the positive results
associated with RVOT stenting, there are still potential complications to be aware
of, including perforation of the distal PA, different forms of PA injury, migration
or fracture of the stent, arrhythmias, coronary artery compression, and the
development of postoperative infectious complications. After acquiring proficiency
and overcoming the learning curve, it is expected that there will be a reduction in
the occurrence of complications. Limitations Considering the very low percentage of children with severe forms of tetralogy of
Fallot, the sample size in a prospective randomized study is limited. This may
be insufficient to determine the presence or absence of differences in certain
studied parameters and may limit the significance of the obtained results. 

Considering the very low percentage of children with severe forms of tetralogy of
Fallot, the sample size in a prospective randomized study is limited. This may
be insufficient to determine the presence or absence of differences in certain
studied parameters and may limit the significance of the obtained results. 

Stenting of the RVOT and creating an mBTs both reduce the severity of PA hypoplasia,
as assessed by the Nakata index. However, in intergroup comparison, the increase in
the Nakata index after stenting is more significant. Therefore, both stenting of the
RVOT and mBTs are important tools in the arsenal of pediatric cardiologists and
surgeons. RVOT stenting provides hemodynamic stabilization and symmetric growth of
the PA compared to mBTs. 

