Matthew J Adams, Ryan M Serrano, Michael W Johansen Department of Pediatric Cardiology, Riley Hospital for Children at Indiana University, Indianapolis, IN, USA Infants with congenital diaphragmatic hernia (CDH) remain a population with high morbidity and mortality rates near 30% despite advances in risk stratification as well as advances in medical care. This is secondary to pulmonary dysfunction from lung hypoplasia and cardiac dysfunction associated with pulmonary hypertension. The vasoactive-inotropic score (VIS) assigns a numeric value to the number and dosage of vasoactive infusions and has been shown to predict poor outcomes in postoperative congenital heart disease. The goal of our study was to determine if VIS can be used as an additional tool to help more accurately risk stratify patients with CDH. A retrospective chart review was performed on patients admitted with CDH to a single academic tertiary care medical center from January 2010 to December 2020. Patient clinical course and outcomes throughout hospital admission as well as hourly doses of all continuous vasoactive medications during the first 72 h were recorded. Specific outcomes included in-hospital mortality, duration of hospitalization, duration of mechanical ventilation, use of extra-corporeal membrane oxygenation (ECMO, use of pulmonary vasodilators, and measures of cardiac function and shunting on echocardiogram. Patients with a VIS score greater than 10 were significantly more likely to have pulmonary hypertension needing pulmonary vasodilators, have decreased right ventricular function, require the use of ECMO, have prolonged mechanical ventilation, require longer hospitalization, and have altered feeding requiring tube feeding at discharge. Further, those with a VIS greater than 10 were over 11 times more likely to die. In infants with CDH, the amount of hemodynamic support needed in the first 72 h after admission as measured with VIS was highly predictive of multiple markers of morbidity and mortality. We believe that VIS may be an easily calculated and useful adjunct marker to further risk stratify these sick neonates. Annabelle Barnes, Eleni Tamvaki, Lynsay MacDonald, Shahin Moledina Great Ormond Street Hospital and UCL Institute of Cardiovascular Sciences, London, UK The National Cohort study of Idiopathic and Heritable Pulmonary Arterial Hypertension (COHORT) aims to set up a national cohort of phenotyped and biobanked patients (adult and child) and their relatives. We sought to investigate reasons for the study's high recruitment rate in paediatrics and simultaneously explore reasons for declined participation into COHORT. We gathered data from patients and child relatives approached for recruitment into the COHORT study from July 2020 to present. Reasons for decline, if shared, were recorded at the time of approaching patients and families. 118 patients were approached regarding the COHORT study. 81% were successfully recruited, 11% declined, and 8% are currently considering the study information. Of the 13 patients who declined, 54% did so because they did not wish to provide bloods for research. This was either due to concerns around extra volume of blood (n = 5), needle phobia (n = 1) or preference for phlebotomy location (n = 1). Other reasons for declining participation were stress of a new pulmonary hypertension diagnosis (n = 1), not interested in research (n = 1), not wanting genetic testing (n = 1) and no reason given (n = 3). The following features did not predict recruitment into this small study: presence of additional learning needs, past participation in research, and approach before site visit. This study indicates that recruitment into paediatric research can be highly successful. Our findings suggest that concerns around blood sampling present a barrier to paediatric research participation. In future, thematic analysis of a paediatric patient focus group may provide valuable insight into motivations for research participation and inform future study designs. Bjorkman KR, Aggarwal M, Mohammad Nijres B Division of Pediatric Cardiology, University of Iowa, Iowa City, IA, USA; Division of Pediatric Cardiology, Washington University, St. Louis, MO, USA The prostacyclin receptor agonist, selexipag, provides an oral option targeting the prostacyclin pathway as a part of triple-therapy for pulmonary arterial hypertension (PAH) with a phosphodiesterase-5 inhibitor and an endothelin receptor antagonist. The aim was to review selexipag dosing in young children and infants, as guidelines have not been established for this group. We reviewed all published literature and our center's experience on selexipag use in children <5 years old. Patient characteristics were recorded including diagnoses, dosing, additional medication regimens, outcomes, and adverse events. Four published works were reviewed with 30 total patients—7 who initiated therapy <5 years of age. These 7 children were 0.6–2.8 years old, weighed 5–13.4 kg, and 5/7 were female. An additional patient at our institution was diagnosed with heritable-PAH (heterozygous ACVRL1)—baseline reduced right ventricular (RV) function, NTproBNP 32,869 pg/mL, supra-systemic RV systolic pressure (RVSP) by cath and echo-estimated tricuspid valve regurgitation velocity, and pulmonary vascular resistance 44.3 woods units*m2. Pulmonary vasodilator therapy was initiated and increased to sildenafil 1 mg/kg three times daily, bosentan 2 mg/kg twice daily, and subcutaneous treprostinil 120 ng/kg/min with improvement of RVSP to ½ systemic and normalization of RV function and NTproBNP. At 2-years 8-months old,12.4 kg, Treprostinil was gradually weaned to 70 ng/kg/min before planned transition to oral selexipag for ease of at-home medical management. He started selexipag 100 µg twice daily and increased to 600 µg dosing with wean off treprostinil, experiencing symptoms of jaw pain, headaches, and diarrhea. Dosing was decreased to 500 µg (40 µg/kg), and adverse symptoms resolved. On follow-up, NTproBNP remained normal (122 pg/mL), and echo demonstrated stable RVSP near ½ systemic. This review of selexipag use in children <5 years old for treatment of PAH found max dosing with acceptable outcomes and side effects between 40 and 70 µg/kg twice daily. Laura Carmona, Ana Gimeno, Maria Alvarez, Paloma Lopez-Ortego, Alejando Avila, Luis Arruza, Dolores Elorza, Cristina Gonzalez-Menchen, Carlos Labrandero, María Lozano, Laura Garcia Bermejo, Laura Moreno, Gustavo Melen, Manuel Ramirez Orellana, Amparo Moya, Tomás Sanchez-Tamayo, Antonio Pavón, Max Vento, Maria Jesus del Cerro, on behalf of “Pulmescel” investigators. Neonatology Department “La Fe” University Hospital, Valencia; Pediatric Cardiology, “Ramón y Cajal” University Hospital, Madrid; Neonatology Department, “La Paz” University Hospital, Madrid; Neonatology department, CHUAC University Hospital, A Coruña; Biomarkers and Therapeutic Targets Laboratory, Ramon y Cajal University Hospital, Madrid; Pharmacology Department, Medicine, Complutense University, Madrid; Cell Therapy Laboratory, Niño Jesus University Hospital, Madrid; Pediatric Cardiology La Fe University Hospital, Neonatology Carlos Haya University Hospital, Malaga; Neonatology Virgen del Rocio University Hospital, Sevilla, Spain. In animal models, MSCs have shown beneficial effects in BPD prevention. There is lack of data about the safety of repeated intravenous (iv) doses of MSCs in preterm newborns. Our objective was to determine feasibility and safety of IV administration of repeated doses of UCd-MSCs in preterm newborns. We conducted a Multicentric, phase I clinical trial, including preterm newborns with birth weight (BW) ≤ 1,250 grams and <28 weeks gestational age (GA), on mechanical ventilation with FiO2 ≥ 30% at postnatal day 14. Intervention: Three doses of 5×106/kg of UCd-MSCs, at weekly intervals (expanded at Cell Therapy Laboratory of Niño Jesus Hospital). The outcomes of treated newborns were retrospectively compared with a cohort of ELGAN, with similar characteristics, and RSS at postnatal day +14, born in Spain in 2019-2020. From April 2019 to July 2020, 10 patients (6 males, 4 females) were recruited, with mean GA 25 weeks (SD = 1.03) and BW 659 grams (SD = 153) (range 455-930). mean Respiratory Severity Score at 14 days of life of 5.37 (SD: 3.2, range: 2.3-12.4). All patients completed the 3 doses (one dose/week), the first one at 16 (SD:2.4) postnatal days. All patients were discharged from the hospital. No deaths or any adverse effects related with the ucd-MSCs were observed during administration or at follow up (mean 24 months, SD 5.4). According to Bancalari definition, all treated patients were diagnosed with BPD: mild (n = 1), moderate (n = 4), severe (n = 5)); using Janssen classification: no BPD (n = 1), grade 1 (n = 1), grade 2 (n = 7), grade 3 (n = 1). The incidence of prematurity-related morbidities, including BPD, was similar in “MSCs-treatment” and in the “comparison” groups, with 0% mortality in the MSCs treated group, and 15% in the control group. In this pilot trial, repeated intravenous infusion of UCd-MSCs in ELGAN with was safe in the short and midterm follow-up. Cherestal B, Singh A, Finn P, Birnbaum B, Aly D Children's Mercy Hospital, Kansas City, MO, USA Scimitar syndrome is a rare congenital condition with a wide range of anomalies, primarily involving right pulmonary venous, arterial, and parenchymal development. We discuss a patient with a rare form of Scimitar syndrome presenting with severe pulmonary hypertension (PHN) as a neonate. Case: Full term 15-day-old female, presented to the cardiology clinic with increased work of breathing and a systolic murmur. Echocardiogram suggested supra-systemic pulmonary hypertension, moderate to severe right ventricular dilation and dysfunction and otherwise normal cardiovascular anatomy. Chest x-ray showed mild cardiomegaly and perihilar atelectasis/edema. Capillary blood gas demonstrated normal oxygenation, but significant hypercarbia (PCO2 67-75 mmHg). Sildenafil and Lasix were initiated. Computed tomography angiography (CTA) demonstrated hypoplastic right lower lung lobe (RLL) and significant left bronchial extrinsic compression by cardiomegaly. A large anomalous arterial supply to the RLL from the celiac trunk was observed. Pulmonary veins, one right and two left, entered the left atrium. Diagnostic cardiac catheterization showed Qp:Qs 2:1, normal pulmonary vascular resistance (PVRi) of 1.65 WUxm2, a single arterial supply of the RLL by a systemic collateral off the celiac trunk and anomalous RLL venous drainage through a network of pulmonary venous channels connecting normally to the left atrium (consistent with intralobar pulmonary sequestration). Various types of noninvasive ventilation were attempted, with no significant impact on patient's hypercarbia. Follow-up echocardiograms and blood gases revealed improved PHN, decreased cardiomegaly, and gradually improved PCO2 levels. Patient is being considered for coiling of the anomalous arterial supply and lobectomy when clinically indicated (persistent or recurrent infections, pulmonary over circulation and/or failure to thrive). Conclusion: We presented an atypical form of Scimitar syndrome with single anomalous arterial supply to a sequestered RLL, and otherwise normal pulmonary venous connection. This was unveiled by the work up of unexplained severe PHN in a full-term newborn. Colglazier E, Farrell R, Parker C, Austin E, Fineman J Departments of Pediatrics and Nursing, UCSF Benioff Children's Hospital, San Francisco, CA; Department of Pediatrics, Vanderbilt University Medical Center, Nashville, TN, USA The molecular genetic basis of heritable pulmonary arterial hypertension (HPAH) is evolving and heterogenous, with at least 26 genes currently displaying varying levels of evidence for disease causality. In fact, current data suggests that genetic factors contribute to >40% of pediatric-onset PAH. Caveolin 1 (CAV1) is a protein found in the plasma membrane of endothelial cells that is essential for the formation of caveolae. Caveolae are flask-shaped invaginations of the plasma membrane, whose function modulates many signaling cascades associated with vascular homeostasis, such as the nitric oxide cascade, G-protein coupled receptors, and the TGF beta superfamily. Although animal and human studies suggest that aberrations in CAV1 signaling participate in the development of pulmonary vascular disorders, to date there is only one report of HPAH associated with CAV1 genetic mutations. A case report of a two-year-old female with late recognition of HPAH from CAV 1 mutation [CAV1 c.474del, p. (Leu159Serfs*22)], which ultimately resulted in her death due to cardiac failure in December 2019. A pedigree of the child's affected family members is presented and shows a female sibling, mother and maternal grandfather with the same CAV1 mutation but no clinical or echocardiographic evidence of PAH. This case report demonstrates the transmission pattern of CAV1 suggests that additional genetic or environmental factors contribute to or modify the genetic predisposition in development of clinically detectable PAH. As a prior case study on CAV1 mutations reports, HPAH from CAV1 mutation in childhood presents with severe disease and poor survival. Genetic testing and the discovery of rare genetic alterations in PAH may aid in identifying disease etiologies, guide therapeutic decisions, and ultimately identify novel therapeutic targets. A. Cruz-Utrilla, N. Gallego, J. Tenorio, I. Guillén, A. Torrent-Vernetta, A. Moya, C. Labrandero, Elvira Garrido-Lestache, A Moreno, P. Escribano-Subias, MJ del Cerro, on behalf of the REHIPED registry investigators Pulmonary Hypertension Unit, Department of Cardiology, Hospital Universitario 12 de Octubre, Madrid, Spain; Centro de Investigación Biomédica en Red de Enfermedades Cardiovasculares (CIBERCV), Madrid, Spain; Instituto de Genética Médica y Molecular (INGEMM), Hospital Universitario La Paz, Madrid, Spain; CIBERER, Centro de Investigación en Red de Enfermedades Raras, Instituto de Salud Carlos III, Madrid, Spain; ITHACA, European Reference Network on Rare Congenital Malformations and Rare Intellectual Disability; Pediatric cardiology Unit, Department of Pediatrics, Hospital Universitario Virgen del Rocío, Sevilla, Spain; Pediatric Pneumology and Lung transplant Unit, Department of Pediatrics, Hospital Vall d'Hebron, Barcelona, Spain; Pediatric cardiology Unit, Department of Pediatrics Hospital Universitario La Fe, Valencia; Pediatric pulmonary Hypertension Unit, Pediatric Cardiology Department, Hospital Universitario Ramón y Cajal, Madrid, Spain; Irycis, CIBERER The genetic background of pulmonary arterial hypertension (PAH) in the Spanish pediatric population could be different from the genetic background described in other countries. Besides, clinical implications of a positive genetic result in the PAH pediatric population are not fully understood yet, and could result in the “reclassification” of patients from one PAH group to another type among the current pediatric classification. Patients up to 18 years at diagnosis included in the Spanish registry of Pulmonary Hypertension (REHIPED) from January 2011 to December 2021 were included. Clinical variables and genetical results were recorded. Successive NGS panels involving up to 35 genes were used. After the results of the genetic testing, we analyzed differences in survival, and if patients “moved” to another category in the current PH classification. In the selected cohort of 98 patients (56.1% female), median age at diagnosis was 7.1 years (IQr 1.5-14.7), and ethnicity as follows: Caucasian (81.6%), Romani (8.2%), others (10.2%). Before the genetic testing, patients had been classified as Idiopathic (53.1%), Congenital Heart Disease-PAH (30.6%), Heritable (5.1%), Pulmonary veno-occlusive disease (PVOD) 6.1%, and multisystemic disorder associated with PH (5.1%). Pathogenic or likely pathogenic variants were found in 44 of the screened patients (44.9%): BMPR2 (12 cases), EIF2AK4 (9), TBX4 (n = 4), MECP2 (n = 3), KCNK3 (n = 2), FOXF1 (n = 2), NFU1 (N = 4), ACVRL1 (n = 1), BMPR1B (n = 1), CLBCI (n = 1), GBE1 (n = 1), GDF2 (n = 1), SOX17 (n = 1), VHL (n = 1), and digenic pathogenic variant in ABCC8/SMAD1 (n = 1). After genetic analysis, 28 patients (28.6%) were “reclassified”, with heritable PAH, PVOD and multisystemic disorders increasing up to 18.4%, 8.2%, and 12.2%, respectively. Worse survival from death or lung transplantation was observed in PVOD and multisystemic disorders. The Spanish pediatric PAH population showed higher prevalence of EIF2AK4 than other pediatric registries. Genetic testing resulted in the “reclassification” of a significant number of patients. A. Cruz-Utrilla, N. Gallego, J. Tenorio, I. Guillén, A. Torrent-Vernetta, P. Escribano-Subias, MJ del Cerro Pulmonary Hypertension Unit, Department of Cardiology, Hospital Universitario 12 de Octubre, Madrid, Spain; Centro de Investigación Biomédica en Red de Enfermedades Cardiovasculares (CIBERCV), Madrid, Spain; Instituto de Genética Médica y Molecular (INGEMM), Hospital Universitario La Paz, Madrid, Spain; CIBERER, Centro de Investigación en Red de Enfermedades Raras, Instituto de Salud Carlos III, Madrid, Spain; ITHACA, European Reference Network on Rare Congenital Malformations and Rare Intellectual Disability; Pediatric cardiology Unit, Department of Pediatrics, Hospital Universitario Virgen del Rocío, Sevilla, Spain; Pediatric Pneumology Unit, Department of Pediatrics, Hospital Vall d'Hebron, Barcelona, Spain; Pediatric Cardiology Unit, Department of Pediatrics, Hospital Universitario Ramón y Cajal, Madrid, Spain. Development of pulmonary arterial hypertension (PAH) after the correction of d-transposition of great arteries (d-TGA) is a known entity. The mechanisms under this disease are not fully elucidated yet, although genetic factors have been proposed as a possible mechanism. The aim of this study was to describe the results of genetic testing in a series of patients with d-TGA and associated PAH. Patients were included in the Spanish Registry of Pediatric Pulmonary Hypertension (REHIPED). Diagnosis of PAH was done by right heart catheterization except clinical instability. A NGS customized panel including 21 genes associated with PAH was carried out. Classification of variants was done according the ACMG guidelines. A written informed consent was obtained from parents or guardian in all cases. Between 2011 and 2020 six patients were included among 9 children with PAH associated with d-TGA. Of them, 5 patients had a previous arterial switch operation (ASO), and one a Mustard surgery (patient 5). All of them except the former were operated during their first month of life. 4 patients were male and 2 females (patients 3 and 4). 3 patients had an intact ventricular septum (patients 2,3 and 5). Two gene variants were encountered among the 6 included patients. A variant initially classified of unknown significance (VUS) in ENG in patient 2 (NM_0001118.2:c.79 A > G. p. Thr27Ala), and a variant originally catalogued as likely pathogenic in BMPR2 in patient 3 (NM_001204.6:c.2598 G > C). After family screening, both gene variants were reclassified in likely benign and VUS, respectively. This is the first study describing an ample genetic analysis in a cohort of patients with PAH associated with d-TGA. Although results are limited by sample size, no variants were found in genes related with the development of PAH. Further works should evaluate other candidate genes in this rare condition. Alejandro Cruz Utrilla, Amparo Moya Bonora, Alejandro Rodriguez Ogando, Ana Isabel Fernandez Avila, Ana Diez Izquierdo, Alberto Mendoza, Maria Jesús del Cerro Marin, Antonio Moreno Galdó, on behalf of “REHIPED” investigators Pulmonary Hypertension Unit, Cardiology Department, Hospital Universitario 12 de Octubre, Madrid, Spain; Department of Pediatric Cardiology, Hospital Universitari I Politècnic La Fe, Valencia, Spain; Pediatric Cardiology Department, Hospital Universitario Gregorio Marañón, Madrid, Spain; Department of Genetics, Hospital Universitario Gregorio Marañón, Madrid, Spain; Pediatric neumology Department, Hospital Vall d'Hebron, Barcelona, Spain; epartment of Pediatric Cardiology, Hospital Universitario 12 de Octubre, Madrid, Spain; Pediatric Cardiology Department. Ramón y Cajal University Hospital. Madrid, Spain; Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain; Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), Instituto de Salud Carlos III (ISCIII), Madrid, Spain MECP2 duplication syndrome is a rare, X-linked neurodevelopmental disorder characterized by intellectual disability, early-onset hypotonia, psychomotor and behavioral features. Occasionally, pulmonary hypertension (PH) is also present. Nevertheless, the prevalence of MECP2 variants in infants with a previous PH diagnosis has not been yet reported. To estimate the prevalence of MECP2 syndrome in infants with PH/PAH diagnosis, we selected patients diagnosed of PH under 2 years from 2010 to 2021. Clinical and hemodynamic data were retrospectively reviewed. Three infants were included, characterized by a severe clinical and hemodynamical disease, and treated with multiple pulmonary vasodilators since the diagnosis. Two of them died during their first year. Autopsies demonstrated typical vascular remodeling resembling PAH in one case and associated capillary hemangiomatosis in the other. Only one patient was diagnosed with PH before the recognition of additional systemic findings. Genetically, two patients had a long duplication in the long arm of chromosome X including MECP gene. A missense mutation in the exon 3 of the gene was found in the remaining patient (c.952 C > T). Prevalence of MECP2 in infants under 2 years diagnosed with PAH was 2.6%, and 1.2% considering all PH diagnoses in infants under 2 years. PH in MECP2 has histological features of PAH, and a dismal prognosis, often refractory to PAH targeted therapies. MECP2 syndrome should be ruled out in infants with a diagnosis of PH, especially if hypotonia, psychomotor retardation, dysmorphic features, recurrent infections, or associated congenital heart defects are present. MECP2 prevalence in Spanish infants diagnosed with PH was as high as 2.6%. MECP2 duplication should be incorporated in the genetic studies for PH in the pediatric population. Ronald W. Day University of Utah and Primary Children's Hospital, Salt Lake City, USA Systemic venous collaterals may cause an inordinate amount of hypoxemia or an unsustainable oxygen requirement after single ventricle palliation with a superior cavopulmonary anastomosis. Anterior venous collaterals are easily identified by injections of contrast in the superior vena cava or innominate vein (venography) during heart catheterization, while posterior venous collaterals may be overlooked. This report describes how well posterior systemic venous collaterals were identified by venography and CT angiography in six affected patients after a superior cavopulmonary anastomosis. The medical records and images of patients with evidence of prominent posterior systemic venous collaterals after a superior cavopulmonary anastomosis were retrospectively reviewed. Six patients with prominent posterior systemic venous collaterals were identified between 2008 and 2021. Posterior venous collaterals were identified by venography after being identified by CT angiography in one patient; by CT angiography when overlooked by venography in four patients; and by repeat venography when overlooked by venography during an initial heart catheterization. Three patients survived following closure of posterior venous collaterals. The collateral veins were occluded with access through vessels entering the inferior vena cava in two patients and a vessel originating from the innominate vein in one patient. Two patients died without closure of posterior venous collaterals. One patient survived without closure of posterior venous collaterals after treatment with extracorporeal membrane oxygenation and orthotopic heart transplantation. All patients were treated with pulmonary vasodilators without sufficient improvement to prevent a need for intervention, death, or transplantation. Posterior systemic venous collaterals may be associated with undesirable levels of hypoxemia or an unsustainable oxygen requirement despite treatment with pulmonary vasodilators after a superior cavopulmonary anastomosis. Posterior venous collaterals were identified by CT angiography when overlooked by venography. CT angiograms may also define the course of collaterals and provide guidance for interventions during heart catheterization. Domingo LT, Ivy DD, Abman, SH, Grenolds, AM, MacLean JT, Breaux JA, Minford KJ, Frank BS Children's Hospital of Colorado, Aurora, CO, USA Riociguat, an oral soluble guanylate cyclase stimulator, has been approved for use in adults with pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension. However, there is limited data on its therapeutic use in children. We report the case of two infants with severe suprasystemic pulmonary hypertension who were successfully treated with riociguat after failure to wean off inhaled nitric oxide (iNO) despite combination PAH therapy. Case 1 is a 4-month-old term male with compound heterozygous SLC25A26 mutation and severe pulmonary hypertension. Echocardiogram showed persistent suprasystemic right ventricular systolic pressures and moderate RV dysfunction despite sildenafil therapy. Cardiac catheterization showed mPAP 88 mmHg and PVRi 28.2 WU*m2 (oxygen), decreased to mPAP 62 mmHg, and PVRi 12 WU*m2 (iNO+oxygen). Bosentan, IV remodulin and iNO were added. He failed multiple attempts to wean off iNO over the following weeks with tachypnea, hypoxemia, and worsening pulmonary hypertension on echocardiogram despite uptitration of remodulin. Case 2 is a 6-month-old term male with TBX4 deletion and severe pulmonary hypertension. Initial cardiac catheterization showed PVRi 15.5 WU*m2 (iNO+O2 and sildenafil) and PVRi 10.2 WU*m2 with the addition of epoprostenol. He was on combination PAH therapy including sildenafil, bosentan and intravenous treprostinil but sildenafil was discontinued due to parental request. He failed multiple iNO wean attempts with increased right to left ductal shunting and tachypnea. Given the clinically observed high level of sensitivity to iNO, both infants were started on riociguat with initial dose of 0.05 mg/dose after sildenafil was discontinued. Both tolerated slow riociguat advance to 1 mg/dose and successfully weaned off iNO thereafter. No adverse effects in the titration period or follow up were observed in either case. Riociguat may be considered as an adjuvant therapeutic agent in selected children with severe PAH who are unable to wean from iNO therapy. Duggal M., Moore S. S., Simoneau J., Von Oettingen J. E., Santanna G. and Altit G McGill University Health Centre – Montreal Children's Hospital. Neonatology Division. Pediatric Cardiology Division. Division of Pediatric Endocrinology. Faculty of Medicine and Health Sciences, McGill University, Montreal, QC, Canada Diazoxide inhibits insulin release and is used for neonatal hyperinsulinemic hypoglycemia. Its use in neonates has been associated with pulmonary hypertension (PH) and more recently with necrotizing enterocolitis (NEC). These outcomes are of concern given their risk of morbidity and mortality but remain insufficiently studied. Retrospective cohort of infants ≥31+6/7 weeks admitted between January 2014 and June 2020 to a tertiary NICU for hypoglycemia and exposed to diazoxide. Clinically acquired ECHO following diazoxide treatment were reviewed by a masked data extractor. PH was defined as systolic pulmonary arterial pressure (sPAP) ≥40 mmHg, tricuspid valve regurgitation ≥35 mmHg and/or left ventricular (LV) end-systolic eccentricity index (EI) ≥ 1.3 (a measure of septal flattening). NEC or suspected NEC was defined as pneumatosis on radiography or symptoms leading to feeds cessation and initiation of antibiotics. Of 63 infants, 13% (n = 8) had evidence of suspected (n = 7) or confirmed (n = 1) NEC. Of 36 ECHO (median age of 14 days [8-37]), 33% (n = 12) had evidence of PH, deemed severe (≥ 2/3 systemic) in 19% (n = 7). Right ventricle (RV) dysfunction occurred in 4 of 63 newborns (low tricuspid annular plane systolic excursion) and LV dysfunction (ejection fraction by Simpson's <55%) in 19 (30%). In total, NEC or PH was found in 20/63 infants (32%). We found an increased pulmonary artery diameter (0.89 ± 0.22 vs 1.26 ± 0.19 cm, p = .004), and a decreased LV peak circumferential strain rate in those with PH (-1.53 ± 0.33 vs -1.90 ± 0.53, p = .03). The use of diazoxide in neonates was associated with a high incidence of PH and suspected or confirmed NEC. No specific clinical features were discriminative of those developing adverse outcomes. LV compromise was frequent, potentially affecting intestinal perfusion and pulmonary vasculature by post-capillary congestion. Until future studies can better determine safety of this drug, diazoxide in neonates should be used with caution. Carolyn Fall, Heidi Ostler, Howaida El Said, Henri Justino, Shylah Haldeman, Jeanne Carroll, Rohit Rao Rady Children's Hospital, San Diego, CA, USA; UC San Diego Health, San Diego, CA, USA Type 1 pulmonary arterial hypertension (PAH) outside of PPHN is rarely described in the neonatal population. Heritable PAH has been previously associated with variants of the SOX17 gene in the pediatric population but has not previously been reported in a neonate. A full-term female neonate developed cyanosis shortly after birth and was found to have severe pulmonary hypertension on echo with no other Structural Congenital Heart Dise
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