Background: Persistent pulmonary hypertension of the newborn (PPHN) is a cause of neonatal hypoxic respiratory failure due to the failed transition of the pulmonary vasculature after birth. Mechanisms of disease are unknown, but we hypothesize they are directly related to insults in the intrauterine environment. The objective was to describe and compare placentas of PPHN infants to understand significant preceding factors from the maternal fetal environment. Methods: We conducted a case control study of mother infant dyads;35 weeks gestation who delivered at a tertiary care center between 2020 to 2025. Cases were infants diagnosed with PPHN and treated with inhaled nitric oxide; controls were infants without congenital anomalies. Placentas underwent blinded histopathologic review using standardized criteria. Results: 106 placentas were analyzed (53 PPHN, 53 controls). Placental lesions were significantly more common in PPHN, including maternal vascular malperfusion (30.2% vs 9.4%, p<0.01), fetal vascular malperfusion (34.0% vs 17.0%, p=0.05), placental inflammation (66.0% vs 37.7%, p<0.01), meconium (43.4% vs 15.2%, p<0.01), and chorangiosis (7.6% vs 0%, p=0.04). Conclusion: PPHN placentas demonstrate lesions of malperfusion, inflammation, and chronic meconium exposure, suggesting a complex interplay between intrauterine hypoxia and inflammation as a mechanism for the abnormal pulmonary vascular reactivity see in PPHN. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded by T32HL160508-01A1 [SMT] ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: IRB of University of California, San Francisco gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors

OBJECTIVETo describe the course and outcomes of children under 18 with left-to-right (LR) shunts and pulmonary arterial hypertension (PAH) undergoing one of two management approaches: PAH treatment prior to LR shunt repair (Treat First) and LR shunt repair first, with or without subsequent PAH treatment (Repair First).METHODSRetrospective single center study, conducted from September 2015 to September 2021, of children LR shunts and PAH (defined as indexed pulmonary vascular resistance (PVRi) ≥ 4WU*m2) but without Eisenmenger physiology. Patient characteristics, longitudinal hemodynamics data, PAH management, LR shunt repair, and outcomes were reviewed.RESULTSOf 768 patients evaluated for LR shunt closure, 51 (6.8%) had LR shunt associated PAH [median age 1.1 (0.37,5) years, median PVRi 6 (5.2,8.7) WU*m2]. Of the "Treat First" group (n=33, 65%), 27 (82%) underwent LR shunt closure and 6 (18%) did not respond to PAH therapy and did not undergo LR shunt closure. In the "Repair First" group (n=18, 35%), 12 (67%) received PAH therapy and 6 (33%) did not. Mortality rates were 6% in "Treat First" and 11% in "Repair First", follow-up of 3.4 and 2.5 years, respectively. After LR shunt closure, there was no significant change in PVRi over a median follow-up of 2 years after surgery (p=0.77).CONCLUSIONSIn children with LR shunts and associated PAH, treatment with PAH-targeted therapy before defect repair does not appear to endanger the subjects and may have some benefit. The response to PAH-targeted therapy before shunt closure persists 2-3 years post-closure, providing valuable insights into the long-term management of these patients.

OBJECTIVES:To evaluate disparities in 1-year mortality in infants with congenital diaphragmatic hernia (CDH) by maternal race/ethnicity in a US population-based dataset, and to quantify mediation effects of socioeconomic status (SES) and maternal and neonatal medical factors. STUDY DESIGN:We identified infants with CDH from the US Natality Database (2014-2019) that links birth and death certificates to 1-year of age. Primary outcome was 1-year mortality. Primary predictor was maternal race/ethnicity. Candidate mediators (SES [defined by maternal education level and public insurance status] and neonatal and maternal factors) were assessed using structural equation modeling. RESULTS:Among 2589 infants with CDH, 1-year mortality was 28.4% (n = 734). Infant mortality differed by maternal race/ethnicity: Non-Hispanic White 25.5%, Black 39.1%, Hispanic 31.2%. Mortality differences persisted in multivariate analysis. Mediation analyses showed that SES accounted for 54.1% (20.4-68.6%) of the disparity between infants of Non-Hispanic White and Hispanic mothers. In contrast, SES accounted for only 17.1% (3.6-37.4%) of the disparity between infants of Non-Hispanic White and Black mothers, while the unmeasured remaining effect of race/ethnicity contributed 75.9% (49.5-91.1%). CONCLUSIONS:In CDH, SES accounts for a large proportion of the mortality disparity for infants of Hispanic mothers while the remaining effect of race, aside from SES and neonatal factors, accounts for a large proportion of the mortality disparity for infants of Black mothers. The remaining effect of maternal race may be due to unmeasured aspects of SES, such as social determinants of health, and structural and systemic racism and bias.

ObjectiveTo determine the incidence, risk factors, and outcomes of pulmonary hypertension (PH) in the pediatric intensive care unit (PICU) after pediatric hematopoietic stem cell transplant (HCT).MethodsThis was a retrospective study of pediatric patients who underwent allogeneic HCT between January 2008-December 2014 at a center contributing to the Center for International Blood and Marrow Transplant Research data registry. Incidence of PH was assessed from PICU diagnostic codes from records merged from the Virtual Pediatric Systems database. Regression and survival analyses identified factors associated with post-HCT PH. Additional post-HCT morbidities and survival after PH were also assessed.ResultsAmong 6,995 HCT recipients, there were 29 cases of PH, a cumulative incidence of 0.42% (95% CI 0.27%-0.57%) at 60 months post-HCT. In the sub-cohort of 1,067 patients requiring intensive care after HCT, this accounted for a PH prevalence of 2.72% (95% CI 1.74–3.69%). There was an increased risk of developing PH associated with Black/African American race, metabolic disorders, partially HLA-matched or cord blood allografts, graft-versus-host prophylaxis regimen, and lower pre-HCT functional status. Patients who developed PH had significant PICU comorbidities including heart failure, pulmonary hemorrhage, respiratory failure, renal failure, and infections. Survival at 6 months after diagnosis of post-HCT PH was 51.7% (95% CI 32.5%-67.9%).ConclusionsPH is a rare but serious complication in the pediatric post-HCT population. A significant burden of additional comorbidities, procedural interventions, and risk of mortality is associated with its development. Close monitoring and prompt intervention for this severe complication are necessary in this vulnerable population.

Pediatric pulmonary hypertension is a heterogeneous disease associated with significant morbidity and mortality. MicroRNAs have been implicated as both pathologic drivers of disease and potential therapeutic targets in pediatric pulmonary hypertension. We sought to characterize the circulating microRNA profiles of a diverse array of pediatric patients with pulmonary hypertension using high-throughput sequencing technology. Peripheral blood samples were drawn from patients recruited at the time of a clinically indicated cardiac catheterization, and microRNA sequencing followed by differential expression and target/pathway enrichment analyses were performed. Among 63 pediatric patients with pulmonary hypertension, we identified specific microRNA signatures that uniquely classified patients by disease subtype, correlated with indicators of disease severity including invasive hemodynamic metrics, and changed over the course of treatment for pulmonary hypertension. These microRNA profiles include a number of specific microRNA molecules known to function in signaling pathways critical to pulmonary vascular biology and disease, including transforming growth factor-β (TGF-β), VEGF, PI3K/Akt, cGMP-PKG, and HIF-1 signaling. Circulating levels of miR-122-5p, miR-124-3p, miR-204-5p, and miR-9-5p decreased over the course of treatment in a subset of patients who had multiple samples drawn during the study period. Our findings support the further investigation of specific microRNAs as mechanistic mediators, biomarkers, and therapeutic targets in pulmonary hypertension.NEW & NOTEWORTHY We present novel insight into the circulating microRNA profiles of pediatric patients with pulmonary hypertension. Our findings support the utility of microRNAs as both useful biomarkers of disease severity and potential therapeutic targets in pediatric pulmonary hypertension.

Objective To determine the association between fertility treatment, socioeconomic status (SES), and neonatal and post-neonatal mortality. Study design Retrospective cohort study of all births (19,350,344) and infant deaths from 2014–2018 in the United States. The exposure was mode of conception—spontaneous vs fertility treatment. The outcome was neonatal (<28d), and post-neonatal (28d–1y) mortality. Multivariable logistic models were stratified by SES. Result The fertility treatment group had statistically significantly higher odds of neonatal mortality (high SES OR 1.59; CI [1.5, 1.68], low SES OR 2.11; CI [1.79, 2.48]) and lower odds of post-neonatal mortality (high SES OR 0.87, CI [0.76, 0.996], low SES OR 0.6, CI [0.38, 0.95]). SES significantly modified the effect of ART/NIFT on neonatal and post-neonatal mortality. Conclusions Fertility treatment is associated with higher neonatal and lower post-neonatal mortality and SES modifies this effect. Socioeconomic policies and support for vulnerable families may help reduce rates of infant mortality.

OBJECTIVES:To describe the typical clinical course of reversible persistent pulmonary hypertension of the newborn (PPHN) from perinatal etiologies and compare that with the clinical course of PPHN due to underlying fetal developmental etiologies. STUDY DESIGN:This was a single-center, retrospective cohort study of liveborn newborns either born or transferred to our facility for higher level of care between 2015 and 2020 with gestational age ≥35 weeks and a clinical diagnosis of PPHN in the electronic health record. Newborns with complex congenital heart disease and congenital diaphragmatic hernia were excluded. Using all data available at time of collection, newborns were stratified into 2 groups by PPHN etiology - perinatal and fetal developmental causes. Primary outcomes were age at initiation, discontinuation, and total duration of extracorporeal life support, mechanical ventilation, supplemental oxygen, inhaled nitric oxide, inotropic support, and prostaglandin E1. Our secondary outcome was age at echocardiographic resolution of pulmonary hypertension. Groups were compared by t-test. Time-to-event Kaplan Meier curves described and compared (log-rank test) discontinuation of each therapy. RESULTS:Sixty-four (72%) newborns had perinatal etiologies whereas 24 (28%) had fetal developmental etiologies. The resolution of perinatal PPHN was more rapid compared with fetal developmental PPHN. By 10 days of age, more neonates were off inotropes (98% vs 29%, P < .01), decannulated from extracorporeal life support (100% vs 0%, P < .01), extubated (75% vs 37%, P < .01), and had echocardiographic resolution of PH (35% vs 7%, P = .02). CONCLUSIONS:An atypical PPHN course, characterized by persistent targeted therapies in the second week of life, warrants further work-up for fetal developmental causes.

Chronic neonatal respiratory illness is marked by the intermediate outcome of bronchopulmonary dysplasia (BPD) and progresses through stages of elevated respiratory resource utilization and wheezing illness in infancy and early childhood, to chronic obstructive lung disease and wheeze and asthma in adolescence. Risk factors for BPD reflect impaired lung development at birth, including immaturity due to low gestational age and the consequences of an adverse fetal environment, associated with impaired placental function and fetal growth. Postnatal lung development in preterm newborns born with these vulnerabilities may be further impeded by exposure to supplemental oxygen and invasive mechanical ventilation, yet application of strategies to limit these exposures in the neonatal period have not translated broadly to later term benefit with respect to respiratory morbidity and carry some adverse consequences. Patterns of respiratory resource utilization and elevated prevalence of persistent wheezing illness in infancy in former preterm newborns portend later persistence of obstructive lung disease. Immune deficits related to degree of prematurity, infant lung dysfunction at hospital discharge, and well-known risks following discharge from environmental exposures such as tobacco smoke and pollutants likely confer susceptibility to lower respiratory tract infection and chronic obstructive airway disease in former premature newborns. Those infants and children born at extremely low gestational age and with more severe neonatal respiratory illness are at greatest risk for both near-term (BPD) and long-term consequences of dysplastic lung and airway development.

AbstractCardiac catheterization remains the gold standard for the diagnosis and management of pediatric pulmonary hypertension (PH). There is lack of consensus regarding optimal anesthetic and airway regimen. This retrospective study describes the anesthetic/airway experience of our single center cohort of pediatric PH patients undergoing catheterization, in which obtaining hemodynamic data during spontaneous breathing is preferential. A total of 448 catheterizations were performed in 232 patients. Of the 379 cases that began with a natural airway, 274 (72%) completed the procedure without an invasive airway, 90 (24%) received a planned invasive airway, and 15 (4%) required an unplanned invasive airway. Median age was 3.4 years (interquartile range [IQR] 0.7–9.7); the majority were either Nice Classification Group 1 (48%) or Group 3 (42%). Vasoactive medications and cardiopulmonary resuscitation were required in 14 (3.7%) and eight (2.1%) cases, respectively; there was one death. Characteristics associated with use of an invasive airway included age <1 year, Group 3, congenital heart disease, trisomy 21, prematurity, bronchopulmonary dysplasia, WHO functional class III/IV, no PH therapy at time of case, preoperative respiratory support, and having had an intervention (p < 0.05). A composite predictor of age <1 year, Group 3, prematurity, and any preoperative respiratory support was significantly associated with unplanned airway escalation (26.7% vs. 6.9%, odds ratio: 4.9, confidence interval: 1.4–17.0). This approach appears safe, with serious adverse event rates similar to previous reports despite the predominant use of natural airways. However, research is needed to further investigate the optimal anesthetic regimen and respiratory support for pediatric PH patients undergoing cardiac catheterization.

It is increasingly recognised that diverse genetic respiratory disorders present as severe pulmonary hypertension (PH) in the neonate and young infant, but many controversies and uncertainties persist regarding optimal strategies for diagnosis and management to maximise long-term outcomes. To better define the nature of PH in the setting of developmental lung disease (DEVLD), in addition to the common diagnoses of bronchopulmonary dysplasia and congenital diaphragmatic hernia, we established a multidisciplinary group of expert clinicians from stakeholder paediatric specialties to highlight current challenges and recommendations for clinical approaches, as well as counselling and support of families. In this review, we characterise clinical features of infants with DEVLD/DEVLD-PH and identify decision-making challenges including genetic evaluations, the role of lung biopsies, the use of imaging modalities and treatment approaches. The importance of working with team members from multiple disciplines, enhancing communication and providing sufficient counselling services for families is emphasised to create an interdisciplinary consensus.

This review provides a comprehensive summary of the current understanding of pulmonary hypertension (PH) in congenital diaphragmatic hernia, outlining the underlying pathophysiologic mechanisms, methods for assessing PH severity, optimal management strategies, and prognostic implications.

Abstract Inhaled iloprost (iILO) has shown efficacy in treating patients with hypoxic lung disease and pulmonary hypertension, inducing selective pulmonary vasodilation and improvement in oxygenation. However, its short elimination half‐life of 20–30 min necessitates frequent intermittent dosing (6–9 times per day). Thus, the administration of iILO via continuous nebulization represents an appealing method of drug delivery in the hospital setting. The objectives are: (1) describe our continuous iILO delivery methodology and safety profile in mechanically ventilated pediatric pulmonary hypertension patients; and (2) characterize the initial response of iILO in these pediatric patients currently receiving iNO. Continuous iILO was delivered and well tolerated (median 6 days; range 1–94) via tracheostomy or endotracheal tube using the Aerogen® mesh nebulizer system coupled with a Medfusion® 400 syringe pump. No adverse events or delivery malfunctions were reported. Initiation of iILO resulted in an increase in oxygen saturation from 81.4 ± 8.6 to 90.8 ± 4.1%, p < 0.05. Interestingly, prior iNO therapy for >1 day resulted in a higher response rate to iILO (as defined as a ≥ 4% increase in saturations) compared to those receiving iNO <1 day (85% vs. 50%, p = 0.06). When the use of iILO is considered, continuous delivery represents a safe, less laborious alternative and concurrent treatment with iNO should not be considered a contraindication. However, given the retrospective design and small sample size, this study does not allow the evaluation of the efficacy of continuous iILO on outcomes beyond the initial response. Thus, a prospective study designed to evaluate the efficacy of continuous iILO is necessary.

BackgroundNeonates with persistent pulmonary hypertension of the newborn (PPHN) can present with hypoxia and right ventricular dysfunction with resultant inadequate oxygen delivery and end-organ damage. This study describes the use of prostaglandin-E1 (PGE) for ductal patency to preserve right ventricular systolic function and limit afterload in newborns with PPHN.MethodsThis is a retrospective cohort study that follows the hemodynamics, markers of end-organ perfusion, length of therapeutics, and echocardiographic variables of 57 newborns who used prostglandin-E1 in the setting of PPHN.ResultsTachycardia, lactic acidosis, and supplemental oxygen use improved following PGE initiation. Fractional area change (FAC), to assess right ventricular systolic function, and pulmonary arterial acceleration time indexed to right ventricular ejection time (PAAT/RVET), to assess right ventricular afterload, also improved over three time points relative to PGE use (before, during, and after).ConclusionsOverall, we described the safety and utility of PGE in newborns with severe PPHN for stabilization while allowing natural disease progression.

Objective:To investigate the complex interplay between fertility treatment, multiple gestations, and prematurity.Design:Retrospective cohort study linking the national Center for Disease Control and Prevention infant birth and death data from 2014 to 2018.Setting:National database from Center of Disease Control and Prevention.Patients:In total, 19,454,155 live-born infants with gestational ages 22-44 weeks, 114,645 infants born using non IVF fertility treatment (NIFT), and 179,960 via assisted reproductive technology (ART).Intervention:Noninvasive fertility treatment or ART vs. spontaneously conceived pregnancies.Main Outcome Measures:The main outcome assessed was prematurity. Formal mediation analysis was conducted to calculate the percentage mediated by multiple gestations.Results:Newborns born using NIFT or ART compared with those with no fertility treatment had a higher incidence of multiple gestation (no fertility treatment = 3.0%; NIFT = 24.7%; ART = 32.7%; P<.001) and prematurity (no fertility treatment = 11.2%; NIFT = 23.4%; ART = 28.4%; P<.001). Mediation analysis demonstrates that 76.8% (95% confidence interval [CI], 75.2%-78.1%) of the effect of NIFT on prematurity was mediated through multiple gestations. Similarly, 71.2% (95% CI, 70.8%-72.7%) of the effect of ART on prematurity is mediated through multiple gestation. However, the direct effect of NIFT on prematurity is 20.4% (95% CI, 19.0%-22.0%). The direct effect of ART was 24.7% (95% CI, 23.7%-25.6%).Conclusion:A significant proportion of prematurity associated with fertility treatment is mediated by the treatment itself, independent of multiple gestations.

ObjectivesWe sought to investigate how race, ethnicity, and socioeconomic status relate to tracheostomy insertion and post-tracheostomy mortality among infants with bronchopulmonary dysplasia (BPD). MethodsThe Vizient Clinical Database/Resource Manager was queried to identify infants born <= 32 weeks with BPD admitted to US hospitals from January 2012 to December 2020. Markers of socioeconomic status were linked to patient records from the Agency for Healthcare Research and Quality's Social Determinants of Health Database. Regression models were used to assess trends in annual tracheostomy insertion rate and odds of tracheostomy insertion and post-tracheostomy mortality, adjusting for sociodemographic and clinical factors. ResultsThere were 40,021 ex-premature infants included in the study, 1614 (4.0%) of whom received a tracheostomy. Tracheostomy insertion increased from 2012 to 2017 (3.1%-4.1%), but decreased from 2018 to 2020 (3.3%-1.6%). Non-Hispanic Black infants demonstrated a 25% higher odds (aOR 1.25, 1.09-1.43) and Hispanic infants demonstrated a 20% lower odds (aOR 0.80, 0.65-0.96) of tracheostomy insertion compared with non-Hispanic White infants. Patients receiving public insurance had increased odds of tracheostomy insertion (aOR 1.15, 1.03-1.30), but there was no relation between other metrics of socioeconomic status and tracheostomy insertion within our cohort. In-hospital mortality among the tracheostomy-dependent was 14.1% and was not associated with sociodemographic factors. ConclusionsDisparities in tracheostomy insertion are not accounted for by differences in socioeconomic status or the presence of additional neonatal morbidities. Post-tracheostomy mortality does not demonstrate the same relationships. Further investigation is needed to explore the source and potential mitigators of the identified disparities.

Objective: To investigate the complex interplay between fertility treatment, multiple gestations, and prematurity. Design: Retrospective cohort study linking the national Center for Disease Control and Prevention infant birth and death data from 2014 to 2018. Setting: National database from Center of Disease Control and Prevention. Patients: In total, 19,454,155 live-born infants with gestational ages 22-44 weeks, 114,645 infants born using non IVF fertility treatment (NIFT), and 179,960 via assisted reproductive technology (ART). Intervention: Noninvasive fertility treatment or ART vs. spontaneously conceived pregnancies. Main Outcome Measures: The main outcome assessed was prematurity. Formal mediation analysis was conducted to calculate the percentage mediated by multiple gestations. Results: Newborns born using NIFT or ART compared with those with no fertility treatment had a higher incidence of multiple gestation (no fertility treatment = 3.0%; NIFT = 24.7%; ART = 32.7%; P< .001) and prematurity (no fertility treatment = 11.2%; NIFT = 23.4%; ART = 28.4%; P< .001). Mediation analysis demonstrates that 76.8% (95% confidence interval [CI], 75.2%-78.1%) of the effect of NIFT on prematurity was mediated through multiple gestations. Similarly, 71.2% (95% CI, 70.8%-72.7%) of the effect of ART on prematurity is mediated through multiple gestation. However, the direct effect of NIFT on prematurity is 20.4% (95% CI, 19.0%- 22.0%). The direct effect of ART was 24.7% (95% CI, 23.7%-25.6%). Conclusion: A significant proportion of prematurity associated with fertility treatment is mediated by the treatment itself, independent of multiple gestations. (Fertil Steril Rep (R) 2023;4:313-20. (c) 2023 by American Society for Reproductive Medicine.)

This review provides a comprehensive summary of the current understanding of pulmonary hypertension (PH) in congenital diaphragmatic hernia , outlining the underlying pathophysiologic mechanisms, methods for assessing PH severity, optimal management strategies, and prognostic implications.

ObjectivesIn fetuses with left-sided congenital diaphragmatic hernia (CDH), left heart structures may appear small, but usually normalize after birth in the absence of structural cardiac anomalies. To decrease the possibility of an erroneous diagnosis of structural heart disease, we identify normal values for left heart structures in the presence of left CDH and secondarily investigate the relationship of left heart size and survival to neonatal hospital discharge.MethodsLeft heart structures [mitral valve (MV) and aortic valve (AoV) annulus diameter, left ventricle (LV) length and width] were measured by fetal echocardiogram in fetuses with left CDH and no congenital heart disease. We generated linear regression models to establish the relationship of gestational age for each left heart structure using data from fetuses who survived after birth. We calculated z-scores (normalized to gestational age), and assessed the relationship of survival to the size of each structure.ResultsOne hundred forty-two fetuses underwent fetal echocardiogram (median 25 weeks' gestation, IQR 23, 27 weeks). Left heart structures were deemed small when using published normative data from unaffected fetuses (z-scores: MV −1.09 ± 1.35, AoV −2.12 ± 1.16, LV length −1.36 ± 1.24, LV width −4.79 ± 0.79). CDH-specific models derived from log-transformed values yielded left-shifted distributions, reflecting the small structures (mean z-score lower by: MV 0.99 ± 0.30, AoV 2.04 ± 0.38, LV length 1.30 ± 0.36, and LV width 4.69 ± 0.28; p < 0.0001 for all comparisons). Non-survivors had worse z-scores than survivors for all measurements, but this did not reach statistical significance.ConclusionsLog-transformed linear models generated new normative data for fetal left heart structures in left CDH, which may be used to allay antenatal concerns regarding structural left heart anomalies. There were no significant differences in z-scores between survivors and non-survivors, suggesting that in the absence of true structural disease, cardiac evaluation is not predictive in isolation and that causes of mortality are likely multifactorial in this population.

Pulmonary artery acceleration time (PAAT) is considered useful for the non‐invasive evaluation of pulmonary artery pressure (PAP) and pulmonary vascular resistance (PVR). PAAT is dependent on PAP, PVR, pulmonary artery compliance, stroke volume, and heart rate. Its relative dependency on these determinants may differ between young and older children, raising uncertainty regarding its utility in young children. We aim to identify the primary determinants of the PAAT in children less than 36 months undergoing cardiac catheterization and its utility for the diagnosis of elevated PVR.