Optimal Oxygen Targets in Term Lambs with Meconium Aspiration Syndrome and Pulmonary Hypertension

Achieved oxygen saturations and risk for bronchopulmonary dysplasia with pulmonary hypertension in preterm infants

OBJECTIVE

Characterisation of oxygen saturation (SpO2)-related predictors that correspond with both bronchopulmonary dysplasia-associated pulmonary hypertension (BPD-PH) development and survival status in infants with BPD-PH may improve patient outcomes. This investigation assessed whether (1) infants with BPD-PH compared with infants with BPD alone, and (2) BPD-PH non-survivors compared with BPD-PH survivors would (a) achieve lower SpO2 distributions, (b) have a higher fraction of inspired oxygen (FiO2) exposure and (c) have a higher oxygen saturation index (OSI).

DESIGN

Case-control study between infants with BPD-PH (cases) and BPD alone (controls) and by survival status within cases.

SETTING

Single-centre study in the USA.

PATIENTS

Infants born at <29 weeks' gestation and on respiratory support at 36 weeks' postmenstrual age.

EXPOSURES

FiO2 exposure, SpO2 distributions and OSI were analysed over the week preceding BPD-PH diagnosis.

MAIN OUTCOMES AND MEASURES

BPD-PH, BPD alone and survival status in infants with BPD-PH.

RESULTS

40 infants with BPD-PH were compared with 40 infants with BPD alone. Infants who developed BPD-PH achieved lower SpO2 compared with infants with BPD (p<0.001), were exposed to a higher FiO2 (0.50 vs 0.34; p=0.02) and had a higher OSI (4.3 vs 2.6; p=0.03). Compared with survivors, infants with BPD-PH who died achieved a lower SpO2 (p<0.001) and were exposed to a higher FiO2 (0.70 vs 0.42; p=0.049).

CONCLUSIONS

SpO2-related predictors differed between infants with BPD-PH and BPD alone and among infants with BPD-PH by survival status. The OSI may provide a non-invasive predictor for BPD-PH in preterm infants.

Oxygen Targets in Neonatal Pulmonary Hypertension: Individualized, "Precision-Medicine" Approach

Oxygen is a specific pulmonary vasodilator. Hypoxemia causes pulmonary vasoconstriction, and normoxia leads to pulmonary vasodilation. However, hyperoxia does not enhance pulmonary vasodilation but causes oxidative stress. There are no clinical trials evaluating optimal oxygen saturation or Pao2 in pulmonary hypertension. Data from translational studies and case series suggest that oxygen saturation of 90% to 97% or Pao2 between 50 and 80 mm Hg is associated with the lowest pulmonary vascular resistance.

Paneth cell ontogeny in term and preterm ovine models

Introduction

Paneth cells are critically important to intestinal health, including protecting intestinal stem cells, shaping the intestinal microbiome, and regulating host immunity. Understanding Paneth cell biology in the immature intestine is often modeled in rodents with little information in larger mammals such as sheep. Previous studies have only established the distribution pattern of Paneth cells in healthy adult sheep. Our study aimed to examine the ontogeny, quantification, and localization of Paneth cells in fetal and newborn lambs at different gestational ages and with perinatal transient asphyxia. We hypothesized that ovine Paneth cell distribution at birth resembles the pattern seen in humans (highest concentrations in the ileum) and that ovine Paneth cell density is gestation-dependent.

Methods

Intestinal samples were obtained from 126-127 (preterm, with and without perinatal transient asphyxia) and 140-141 (term) days gestation sheep. Samples were quantified per crypt in at least 100 crypts per animal and confirmed as Paneth cells through in immunohistochemistry.

Results

Paneth cells had significantly higher density in the ileum compared to the jejunum and were absent in the colon.

Discussion

Exposure to perinatal transient asphyxia acutely decreased Paneth cell numbers. These novel data support the possibility of utilizing ovine models for understanding Paneth cell biology in the fetus and neonate.

Femoral Occlusion during Neonatal Cardiopulmonary Resuscitation Improves Outcomes in an Ovine Model of Perinatal Cardiac Arrest

BACKGROUND

The goal of chest compressions during neonatal resuscitation is to increase cerebral and coronary blood flow leading to the return of spontaneous circulation (ROSC). During chest compressions, bilateral femoral occlusion may increase afterload and promote carotid and coronary flow, an effect similar to epinephrine. Our objectives were to determine the impact of bilateral femoral occlusion during chest compressions on the incidence and timing of ROSC and hemodynamics.

METHODOLOGY

In this randomized study, 19 term fetal lambs in cardiac arrest were resuscitated based on the Neonatal Resuscitation Program guidelines and randomized into two groups: femoral occlusion or controls. Bilateral femoral arteries were occluded by applying pressure using two fingers during chest compressions.

RESULTS

Seventy percent (7/10) of the lambs in the femoral occlusion group achieved ROSC in 5 ± 2 min and three lambs (30%) did not receive epinephrine. ROSC was achieved in 44% (4/9) of the controls in 13 ± 6 min and all lambs received epinephrine. The femoral occlusion group had higher diastolic blood pressures, carotid and coronary blood flow.

CONCLUSION

Femoral occlusion resulted in faster and higher incidence of ROSC, most likely due to attaining increased diastolic pressures, coronary and carotid flow. This is a low-tech intervention that can be easily adapted in resource limited settings, with the potential to improve survival and neurodevelopmental outcomes.

Meconium aspiration syndrome: a comprehensive review

Meconium aspiration syndrome (MAS) is a complex respiratory disease that continues to be associated with significant morbidities and mortality. The pathophysiological mechanisms of MAS include airway obstruction, local and systemic inflammation, surfactant inactivation and persistent pulmonary hypertension of the newborn (PPHN). Supplemental oxygen and non-invasive respiratory support are the main therapies for many patients. The management of the patients requiring invasive mechanical ventilation could be challenging because of the combination of atelectasis and air trapping. While studies have explored various ventilatory modalities, evidence to date does not clearly support any singular modality as superior. Patient's pathophysiology, symptom severity, and clinician/unit expertise should guide the respiratory management. Early identification and concomitant management of PPHN is critically important as it contributes significantly to mortality and morbidities.

Vitality in Newborn Farm Animals: Adverse Factors, Physiological Responses, Pharmacological Therapies, and Physical Methods to Increase Neonate Vigor

Vitality is the vigor newborn animals exhibit during the first hours of life. It can be assessed by a numerical score, in which variables, such as heart rate, respiratory rate, mucous membranes' coloration, time the offspring took to stand up, and meconium staining, are monitored. Vitality can be affected by several factors, and therapies are used to increase it. This manuscript aims to review and analyze pharmacological and physical therapies used to increase vitality in newborn farm animals, as well as to understand the factors affecting this vitality, such as hypoxia, depletion of glycogen, birth weight, dystocia, neurodevelopment, hypothermia, and finally, the physiological mechanism to achieve thermostability. It has been concluded that assessing vitality immediately after birth is essential to determine the newborn's health and identify those that need medical intervention to minimize the deleterious effect of intrapartum asphyxia. Vitality assessment should be conducted by trained personnel and adequate equipment. Evaluating vitality could reduce long-term neonatal morbidity and mortality in domestic animals, even if it is sometimes difficult with the current organization of some farms. This review highlights the importance of increasing the number of stock people during the expected days of parturitions to reduce long-term neonatal morbidity and mortality, and thus, improve the farm's performance.

Intermittent Hypoxemia and Bronchopulmonary Dysplasia with Pulmonary Hypertension in Preterm Infants

Rationale: Bedside biomarkers that allow early identification of infants with bronchopulmonary dysplasia-associated pulmonary hypertension (BPD-PH) are critically important, given the higher risk of death in these infants. Objectives: We hypothesized that infants with BPD-PH have patterns of intermittent hypoxemia (IH) that differ from infants with BPD without PH. Methods: We conducted a matched case-control study of extremely preterm infants from 22 weeks 0 days to 28 weeks 6 days born between 2018 and 2020 at the University of Alabama at Birmingham. BPD-PH status was determined using echocardiographic data performed after postnatal Day 28. Physiologic data were compared between infants with BPD-PH (cases) and BPD alone (control subjects). Receiver operating characteristic (ROC) analysis estimated the predictive ability of cumulative hypoxemia, desaturation frequency, and duration of intermittent hypoxemic events in the week preceding echocardiography to discriminate between cases and control subjects. Measurements and Main Results: Forty infants with BPD-PH were compared with 40 infants with BPD alone. Infants with and without PH had a similar frequency of IH events, but infants with PH had more prolonged hypoxemic events for desaturations below 80% (7 s vs. 6 s; P = 0.03) and 70% (105 s vs. 58 s; P = 0.008). Among infants with BPD-PH, infants who died had longer hypoxemic events below 70% (145 s vs. 72 s; P = 0.01). Using the duration of hypoxemic events below 70%, the areas under the ROC curves for diagnosis of BPD-PH and death in BPD-PH infants were 0.71 and 0.77, respectively. Conclusions: Longer duration of intermittent hypoxemic events was associated both with a diagnosis of BPD-PH and with death among infants with BPD-PH.

Meconium Aspiration Syndrome in Animal Models: Inflammatory Process, Apoptosis, and Surfactant Inactivation

Meconium Aspiration Syndrome is a condition that causes respiratory distress in newborns due to occlusion and airway inflammation, and surfactant inactivation by meconium. This condition has been described in animal species such as canids, sheep, cattle, horses, pigs, and marine mammals. In its pathogenesis, the pulmonary epithelium activates a limited inflammatory response initiated by cytokines causing leukocyte chemotaxis, inhibition of phagocytosis, and pathogen destruction. Likewise, cytokines release participates in the apoptosis processes of pneumocytes due to the interaction of angiotensin with cytokines and the caspase pathway. Due to these reactions, the prevalent signs are lung injury, hypoxia, acidosis, and pneumonia with susceptibility to infection. Given the importance of the pathophysiological mechanism of meconium aspiration syndrome, this review aims to discuss the relevance of the syndrome in veterinary medicine. The inflammatory processes caused by meconium aspiration in animal models will be analyzed, and the cellular apoptosis and biochemical processes of pulmonary surfactant inactivation will be discussed.

Initial Use of 100% but Not 60% or 30% Oxygen Achieved a Target Heart Rate of 100 bpm and Preductal Saturations of 80% Faster in a Bradycardic Preterm Model

Background: Currently, 21−30% supplemental oxygen is recommended during resuscitation of preterm neonates. Recent studies have shown that 58% of infants < 32 week gestation age are born with a heart rate (HR) < 100 bpm. Prolonged bradycardia with the inability to achieve a preductal saturation (SpO2) of 80% by 5 min is associated with mortality and morbidity in preterm infants. The optimal oxygen concentration that enables the achievement of a HR ≥ 100 bpm and SpO2 of ≥80% by 5 min in preterm lambs is not known. Methods: Preterm ovine model (125−127 d, gestation equivalent to human neonates < 28 weeks) was instrumented, and asphyxia was induced by umbilical cord occlusion until bradycardia. Ventilation was initiated with 30% (OX30), 60% (OX60), and 100% (OX100) for the first 2 min and titrated proportionately to the difference from the recommended preductal SpO2. Our primary outcome was the incidence of the composite of HR ≥ 100 bpm and SpO2 ≥ 80%, by 5 min. Secondary outcomes were to evaluate the time taken to achieve the primary outcome, gas exchange, pulmonary/systemic hemodynamics, and the oxidative injury. Results: Eighteen lambs (OX30-6, OX60-5. OX100-7) had an average HR < 91 bpm with a pH of <6.92 before resuscitation. Sixty seven percent achieved the primary outcome in OX100, 40% in OX60, and none in OX30. The time taken to achieve the primary outcome was significantly shorter with OX100 (6 ± 2 min) than with OX30 (10 ± 3 min) (* p = 0.04). The preductal SpO2 was highest with OX100, while the peak pulmonary blood flow was lowest with OX30, with no difference in O2 delivery to the brain or oxidative injury by 10 min. Conclusions: The use of 30%, 60%, and 100% supplemental O2 in a bradycardic preterm ovine model did not demonstrate a significant difference in the composite primary outcome. The current recommendation to use 30% oxygen did not achieve a preductal SpO2 of 80% by 5 min in any preterm lambs. Clinical studies to optimize supplemental O2 in depressed preterm neonates not requiring chest compressions are warranted.

Pulmonary hypertension and oxidative stress: Where is the link?

Oxidative stress can be associated with hyperoxia and hypoxia and is characterized by an increase in reactive oxygen (ROS) and nitrogen (RNS) species generated by an underlying disease process or by supplemental oxygen that exceeds the neutralization capacity of the organ system. ROS and RNS acting as free radicals can inactive several enzymes and vasodilators in the nitric oxide pathway promoting pulmonary vasoconstriction resulting in persistent pulmonary hypertension of the newborn (PPHN). Studies in animal models of PPHN have shown high ROS/RNS that is further increased by hyperoxic ventilation. In addition, antioxidant therapy increased PaO2 in these models, but clinical trials are lacking. We recommend targeting preductal SpO2 between 90 and 97%, PaO2 between 55 and 80 mmHg and avoiding FiO2 > 0.6-0.8 if possible during PPHN management. This review highlights the role of oxidative and nitrosative stress markers on PPHN and potential therapeutic interventions that may alleviate the consequences of increased oxidant stress during ventilation with supplemental oxygen.

Factors to Consider to Study Preductal Oxygen Saturation Targets in Neonatal Pulmonary Hypertension

There are potential benefits and risks to the infant with higher and lower oxygen saturation (SpO₂) targets, and the ideal range for infants with pulmonary hypertension (PH) remains unknown. Targeting high SpO₂ can promote pulmonary vasodilation but cause oxygen toxicity. Targeting lower SpO₂ may increase pulmonary vascular resistance, especially in the presence of acidosis and hypothermia. We will conduct a randomized pilot trial to compare two ranges of target preductal SpO₂ in late-preterm and term infants with hypoxic respiratory failure (HRF) and acute pulmonary hypertension (aPH) of the newborn. We will assess the reliability of a newly created HRF/PH score that could be used in larger trials. We will assess trial feasibility and obtain preliminary estimates of outcomes. Our primary hypothesis is that in neonates with PH and HRF, targeting preductal SpO₂ of 95-99% (intervention) will result in lower pulmonary vascular resistance and pulmonary arterial pressures, and lower the need for pulmonary vasodilators (inhaled nitric oxide-iNO, milrinone and sildenafil) compared to targeting SpO₂ at 91-95% (standard). We also speculate that a higher SpO₂ target can potentially induce oxidative stress and decrease response to iNO (oxygenation and pulmonary vasodilation) for those patients that still require iNO in this range. We present considerations in planning this trial as well as some of the details of the protocol design (Clinicaltrials.gov (NCT04938167)).

Supplemental Oxygen in the Newborn: Historical Perspective and Current Trends

Oxygen is the final electron acceptor in aerobic respiration, and a lack of oxygen can result in bioenergetic failure and cell death. Thus, administration of supplemental concentrations of oxygen to overcome barriers to tissue oxygen delivery (e.g., heart failure, lung disease, ischemia), can rescue dying cells where cellular oxygen content is low. However, the balance of oxygen delivery and oxygen consumption relies on tightly controlled oxygen gradients and compartmentalized redox potential. While therapeutic oxygen delivery can be life-saving, it can disrupt growth and development, impair bioenergetic function, and induce inflammation. Newborns, and premature newborns especially, have features that confer particular susceptibility to hyperoxic injury due to oxidative stress. In this review, we will describe the unique features of newborn redox physiology and antioxidant defenses, the history of therapeutic oxygen use in this population and its role in disease, and clinical trends in the use of therapeutic oxygen and mitigation of neonatal oxidative injury.

Randomized trial of oxygen weaning strategies following chest compressions during neonatal resuscitation

BACKGROUND

The Neonatal Resuscitation Program (NRP) recommends using 100% O2 during chest compressions and adjusting FiO2 based on SpO2 after return of spontaneous circulation (ROSC). The optimal strategy for adjusting FiO2 is not known.

METHODS

Twenty-five near-term lambs asphyxiated by umbilical cord occlusion to cardiac arrest were resuscitated per NRP. Following ROSC, lambs were randomized to gradual decrease versus abrupt wean to 21% O2 followed by FiO2 titration to achieve NRP SpO2 targets. Carotid blood flow and blood gases were monitored.

RESULTS

Three minutes after ROSC, PaO2 was 229 ± 32 mmHg in gradual wean group compared to 57 ± 13 following abrupt wean to 21% O2 (p < 0.001). PaO2 remained high in the gradual wean group at 10 min after ROSC (110 ± 10 vs. 67 ± 12, p < 0.01) despite similar FiO2 (~0.3) in both groups. Cerebral O2 delivery (C-DO2) was higher above physiological range following ROSC with gradual wean (p < 0.05). Lower blood oxidized/reduced glutathione ratio (suggesting less oxidative stress) was observed with abrupt wean.

CONCLUSION

Weaning FiO2 abruptly to 0.21 with adjustment based on SpO2 prevents surge in PaO2 and C-DO2 and minimizes oxidative stress compared to gradual weaning from 100% O2 following ROSC. Clinical trials with neurodevelopmental outcomes comparing post-ROSC FiO2 weaning strategies are warranted.

IMPACT

In a lamb model of perinatal asphyxial cardiac arrest, abrupt weaning of inspired oxygen to 21% prevents excessive oxygen delivery to the brain and oxidative stress compared to gradual weaning from 100% oxygen following return of spontaneous circulation. Clinical studies assessing neurodevelopmental outcomes comparing abrupt and gradual weaning of inspired oxygen after recovery from neonatal asphyxial arrest are warranted.

Pathophysiology and Management of Persistent Pulmonary Hypertension of the Newborn

Persistent pulmonary hypertension of the newborn (PPHN) is a disorder of circulatory transition resulting in high pulmonary vascular resistance with extrapulmonary right-to-left shunts causing hypoxemia. There has been substantial gain in understanding of pathophysiology of PPHN over the past 2 decades, and biochemical pathways responsible for abnormal vasoconstriction of pulmonary vasculature are now better understood. Easy availability of bedside echocardiography helps in establishing early definitive diagnosis, understanding the pathophysiology and hemodynamic abnormalities, monitoring the disease process, and response to therapeutic intervention. There also has been significant advancement in specific management of PPHN targeted at deranged biochemical pathways and hemodynamic instability.

Randomized Trial of Oxygen Saturation Targets during and after Resuscitation and Reversal of Ductal Flow in an Ovine Model of Meconium Aspiration and Pulmonary Hypertension

Neonatal resuscitation (NRP) guidelines suggest targeting 85-95% preductal SpO₂ by 10 min after birth. Optimal oxygen saturation (SpO₂) targets during resuscitation and in the post-resuscitation management of neonatal meconium aspiration syndrome (MAS) with persistent pulmonary hypertension (PPHN) remains uncertain. Our objective was to compare the time to reversal of ductal flow from fetal pattern (right-to-left), to left-to-right, and to evaluate pulmonary (Q_PA), carotid (Q_CA)and ductal (Q_DA) blood flows between standard (85-94%) and high (95-99%) SpO₂ targets during and after resuscitation. Twelve lambs asphyxiated by endotracheal meconium instillation and cord occlusion to induce MAS and PPHN were resuscitated per NRP guidelines and were randomized to either standard (85-94%) or high (95-99%) SpO₂ targets. Out of twelve lambs with MAS and PPHN, six each were randomized to standard and high SpO₂ targets. Median [interquartile range] time to change in direction of blood flow across the ductus arteriosus from right-to-left, to left-to-right was significantly shorter with high SpO₂ target (7.4 (4.4-10.8) min) compared to standard SpO₂ target (31.5 (21-66.2) min, p = 0.03). Q_PA was significantly higher during the first 10 min after birth with higher SpO₂ target. At 60 min after birth, the Q_PA, Q_CA and Q_DA were not different between the groups. To conclude, targeting SpO₂ of 95-99% during and after resuscitation may hasten reversal of ductal flow in lambs with MAS and PPHN and transiently increase Q_PA but no differences were observed at 60 min. Clinical studies comparing low and high SpO₂ targets assessing hemodynamics and neurodevelopmental outcomes are warranted.

Differential Alveolar and Systemic Oxygenation during Preterm Resuscitation with 100% Oxygen during Delayed Cord Clamping

OBJECTIVE

 Delayed cord clamping (DCC) and 21 to 30% O₂ resuscitation is recommended for preterm infants but is commonly associated with low pulmonary blood flow (Qp) and hypoxia. 100% O₂ supplementation during DCC for 60 seconds followed by 30% O₂ may increase Qp and oxygen saturation (SpO₂).

STUDY DESIGN

 Preterm lambs (125-127 days of gestation) were resuscitated with 100% O₂ with immediate cord clamping (ICC, n = 7) or ICC + 30% O₂, and titrated to target SpO₂ (n = 7) or DCC + 100% O₂ for 60 seconds, which followed by cord clamping and 30% O₂ titration (n = 7). Seven preterm (23-27 weeks of gestation) human infants received continuous positive airway pressure (CPAP) + 100% O₂ for 60 seconds during DCC, cord clamping, and 30% O₂ supplementation after cord clamping.

RESULTS

 Preterm lambs in the ICC + 100% O₂ group resulted in PaO₂ (77 ± 25 mmHg), SpO₂ (77 ± 11%), and Qp (27 ± 9 mL/kg/min) at 60 seconds. ICC + 30% O₂ led to low Qp (14 ± 3 mL/kg/min), low SpO₂ (43 ± 26%), and PaO₂ (19 ± 7 mmHg). DCC + 100% O₂ led to similar Qp (28 ± 6 mL/kg/min) as ICC + 100% O₂ with lower PaO₂. In human infants, DCC + CPAP with 100% O₂ for 60 seconds, which followed by weaning to 30% resulted in SpO₂ of 92 ± 11% with all infants >80% at 5 minutes with 100% survival without severe intraventricular hemorrhage.

CONCLUSION

 DCC + 100% O₂ for 60 seconds increased Qp probably due to transient alveolar hyperoxia with systemic normoxia due to "dilution" by umbilical venous return. Larger translational and clinical studies are warranted to confirm these findings.

KEY POINTS

· Transient alveolar hyperoxia during delayed cord clamping can enhance pulmonary vasodilation.. · Placental transfusion buffers systemic oxygen tension and limits hyperoxia.. · Use of 100% oxygen for 60 seconds during DCC was associated with SpO2 ≥80% by 5 minutes..

How Do We Monitor Oxygenation during the Management of PPHN? Alveolar, Arterial, Mixed Venous Oxygen Tension or Peripheral Saturation?

Oxygen is a pulmonary vasodilator and plays an important role in mediating circulatory transition from fetal to postnatal period. Oxygen tension (PO₂) in the alveolus (PAO₂) and pulmonary artery (PaO₂) are the main factors that influence hypoxic pulmonary vasoconstriction (HPV). Inability to achieve adequate pulmonary vasodilation at birth leads to persistent pulmonary hypertension of the newborn (PPHN). Supplemental oxygen therapy is the mainstay of PPHN management. However, optimal monitoring and targeting of oxygenation to achieve low pulmonary vascular resistance (PVR) and optimizing oxygen delivery to vital organs remains unknown. Noninvasive pulse oximetry measures peripheral saturations (SpO₂) and a target range of 91-95% are recommended during acute PPHN management. However, for a given SpO₂, there is wide variability in arterial PaO₂, especially with variations in hemoglobin type (HbF or HbA due to transfusions), pH and body temperature. This review evaluates the role of alveolar, preductal, postductal, mixed venous PO₂, and SpO₂ in the management of PPHN. Translational and clinical studies suggest maintaining a PaO₂ of 50-80 mmHg decreases PVR and augments pulmonary vasodilator management. Nevertheless, there are no randomized clinical trials evaluating outcomes in PPHN targeting SpO₂ or PO₂. Also, most critically ill patients have umbilical arterial catheters and postductal PaO₂ may not be an accurate assessment of oxygen delivery to vital organs or factors influencing HPV. The mixed venous oxygen tension from umbilical venous catheter blood gas may assess pulmonary arterial PO₂ and potentially predict HPV. It is crucial to conduct randomized controlled studies with different PO₂/SpO₂ target ranges for the management of PPHN and compare outcomes.

Bidirectional Ductal Shunting and Preductal to Postductal Oxygenation Gradient in Persistent Pulmonary Hypertension of the Newborn

BACKGROUND

The aim was to evaluate the relationship between the direction of the patent ductus arteriosus (PDA) shunt and the pre- and postductal gradient for arterial blood gas (ABG) parameters in a lamb model of meconium aspiration syndrome (MAS) with persistent pulmonary hypertension of the newborn (PPHN).

METHODS

PPHN was induced by intermittent umbilical cord occlusion and the aspiration of meconium through the tracheal tube. After delivery, 13 lambs were ventilated and simultaneous 129 pairs of pre- and postductal ABG were drawn (right carotid and umbilical artery, respectively) while recording the PDA and the carotid and pulmonary blood flow.

RESULTS

Meconium aspiration resulted in hypoxemia. The bidirectional ductal shunt had a lower postductal partial arterial oxygen tension ([PaO₂] with lower PaO₂/FiO₂ ratio-97 ± 36 vs. 130 ± 65 mmHg) and left pulmonary flow (81 ± 52 vs. 133 ± 82 mL/kg/min). However, 56% of the samples with a bidirectional shunt had a pre- and postductal saturation gradient of < 3%.

CONCLUSIONS

The presence of a bidirectional ductal shunt is associated with hypoxemia and low pulmonary blood flow. The absence of a pre- and postductal saturation difference is frequently observed with bidirectional right-to-left shunting through the PDA, and does not exclude a diagnosis of PPHN in this model.