Blood gases and pulmonary blood flow during resuscitation of very preterm lambs treated with antenatal betamethasone and/or Curosurf: effect of positive end-expiratory pressure

Higher CPAP levels improve functional residual capacity at birth in preterm rabbits

BACKGROUND

Preterm infants are commonly supported with 4-8 cm H₂O continuous positive airway pressures (CPAP), although higher CPAP levels may improve functional residual capacity (FRC).

METHODS

Preterm rabbits delivered at 29/32 days (~26-28 weeks human) gestation received 0, 5, 8, 12, 15 cm H₂O of CPAP or variable CPAP of 15 to 5 or 15 to 8 cm H₂O (decreasing ~2 cm H₂O/min) for up to 10 min after birth.

RESULTS

FRC was lower in the 0 (6.8 (1.0-11.2) mL/kg) and 5 (10.1 (1.1-16.8) mL/kg) compared to the 15 (18.8 (10.9-22.4) mL/kg) cm H₂O groups (p = 0.003). Fewer kittens achieved FRC > 15 mL/kg in the 0 (20%), compared to 8 (36%), 12 (60%) and 15 (73%) cm H₂O groups (p = 0.008). While breathing rates were not different (p = 0.096), apnoea tended to occur more often with CPAP < 8 cm H₂O (p = 0.185). CPAP belly and lung bulging rates were similar whereas pneumothoraces were rare. Lowering CPAP from 15 to 5, but not 15 to 8 cm H₂O, decreased FRC and breathing rates.

CONCLUSION

In all, 15 cm H₂O of CPAP improved lung aeration and reduced apnoea, but did not increase the risk of lung over-expansion, pneumothorax or CPAP belly immediately after birth. FRC and breathing rates were maintained when CPAP was decreased to 8 cm H₂O.

IMPACT

Although preterm infants are commonly supported with 4-8 cm H₂O CPAP at birth, preclinical studies have shown that higher PEEP levels improve lung aeration. In this study, CPAP levels of 15 cm H₂O improved lung aeration and reduced apnoea in preterm rabbit kittens immediately after birth. In all, 15 cm H₂O CPAP did not increase the risk of lung over-expansion (indicated by bulging between the ribs), pneumothorax, or CPAP belly. These results can be used when designing future studies on CPAP strategies for preterm infants in the delivery room.

Comparison of Respiratory Support After Delivery in Infants Born Before 28 Weeks' Gestational Age: The CORSAD Randomized Clinical Trial

IMPORTANCE

Establishing stable breathing is a key event for preterm infants after birth. Delivery of pressure-stable continuous positive airway pressure and avoiding face mask use could be of importance in the delivery room.

OBJECTIVE

To determine whether using a new respiratory support system with low imposed work of breathing and short binasal prongs decreases delivery room intubations or death compared with a standard T-piece system with a face mask.

DESIGN, SETTING, AND PARTICIPANTS

In this unblinded randomized clinical trial, mothers threatening preterm delivery before week 28 of gestation were screened. A total of 365 mothers were enrolled, and 250 infants were randomized before birth and 246 liveborn infants were treated. The trial was conducted in 7 neonatal intensive care units in 5 European countries from March 2016 to May 2020. The follow-up period was 72 hours after intervention.

INTERVENTIONS

Infants were randomized to either the new respiratory support system with short binasal prongs (n = 124 infants) or the standard T-piece system with face mask (n = 122 infants). The intervention was providing continuous positive airway pressure for 10 to 30 minutes and positive pressure ventilation, if needed, with the randomized system.

MAIN OUTCOMES AND MEASURES

The primary outcome was delivery room intubation or death within 30 minutes of birth. Secondary outcomes included respiratory and safety variables.

RESULTS

Of 246 liveborn infants treated, the mean (SD) gestational age was 25.9 (1.3) weeks, and 127 (51.6%) were female. A total of 41 infants (33.1%) receiving the new respiratory support system were intubated or died in the delivery room compared with 55 infants (45.1%) receiving standard care. The adjusted odds ratio was statistically significant after adjusting for stratification variables (adjusted odds ratio, 0.53; 95% CI, 0.30-0.94; P = .03). No significant differences were seen in secondary outcomes or safety variables.

CONCLUSIONS AND RELEVANCE

In this study, using the new respiratory support system reduced delivery room intubation in extremely preterm infants. Stabilizing preterm infants with a system that has low imposed work of breathing and binasal prongs as interface is safe and feasible.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02563717.

Sustained Lung Inflations During Neonatal Resuscitation at Birth: A Meta-analysis

CONTEXT

The International Liaison Committee on Resuscitation prioritized review of sustained inflation (SI) of the lung at birth.

OBJECTIVE

To complete a systematic review and meta-analysis comparing strategies using 1 or more SI ≥1 second with intermittent inflations <1 second for newborns at birth.

DATA SOURCES

Medline, Embase, and Evidence-Based Medicine Reviews were searched from January 1, 1946, to July 20, 2020.

STUDY SELECTION

Studies were selected by pairs of independent reviewers in 2 stages.

DATA EXTRACTION

Reviewers extracted data, appraised risk of bias, and assessed certainty of evidence for each outcome.

RESULTS

Ten trials enrolling 1502 preterm newborns were included. Five studies included newborns who did not receive assisted ventilation at the outset. There were no differences between SI and control groups for death before discharge or key morbidities. For death within the first 2 days, comparing SI with the controls, risk ratio was 2.42 (95% confidence interval = 1.15-5.09). In subgroup analysis of preterm infants ≤28 + 0 weeks' gestation, for death before discharge, risk ratio was 1.38 (95% confidence interval = 1.00-1.91). Together, these findings suggest the potential for harm of SI.

LIMITATIONS

The certainty of evidence was very low for death in the delivery room and low for all other outcomes.

CONCLUSIONS

In this systematic review, we did not find benefit in using 1 or more SI >5 seconds for preterm infants at birth. SI(s) may increase death before discharge among the subgroup born ≤28 + 0 weeks' gestation. There is insufficient evidence to determine the likely effect of SI(s) on other key morbidities.

Increased peak end-expiratory pressure in ventilated preterm lambs changes cerebral microvascular perfusion: direct synchrotron microangiography assessment

Positive end-expiratory pressure (PEEP) improves oxygenation in mechanically ventilated preterm neonates by preventing lung collapse. However, high PEEP may alter cerebral blood flow secondarily to the increased intrathoracic pressure, predisposing to brain injury. The precise effects of high PEEP on cerebral hemodynamics in the preterm brain are unknown. We aimed to assess the effect of PEEP on microvessels in the preterm brain by using synchrotron radiation (SR) microangiography, which enables in vivo real-time high-resolution imaging of the cerebral vasculature. Preterm lambs (0.8 gestation, n = 4) were delivered via caesarean section, anesthetized, and ventilated. SR microangiography of the right cerebral hemisphere was performed with iodine contrast administered into the right carotid artery during PEEP ventilation of 5 and 10 cmH₂O. Carotid blood flow was measured using an ultrasonic flow probe placed around the left carotid artery. An increase of PEEP from 5 to 10 cmH₂O increased the diameter of small cerebral vessels (<150 µm) but decreased the diameter of larger cerebral vessels (>500 µm) in all four lambs. Additionally, the higher PEEP increased the cerebral contrast transit time in three of the four lambs. Carotid blood flow increased in two lambs, which also had increased carbon dioxide levels during PEEP 10. Our results suggest that PEEP of 10 cmH₂O alters the preterm cerebral hemodynamics, with prolonged cerebral blood flow transit and engorgement of small cerebral microvessels likely due to the increased intrathoracic pressure. These microvascular changes are generally not reflected in global assessment of cerebral blood flow or oxygenation.NEW & NOTEWORTHY An increase of positive end-expiratory pressure (PEEP) from 5 to 10 cmH₂O increased the diameter of small cerebral vessels (<150 µm) but decreased the diameter of larger cerebral vessels (>500 µm). This suggests increased intrathoracic pressure due to high PEEP can drive microvessel engorgement in the preterm brain, which may play a role in cerebrovascular injury.

High-CPAP Does Not Impede Cardiovascular Changes at Birth in Preterm Sheep

Objective: Continuous positive airway pressures (CPAP) used to assist preterm infants at birth are limited to 4-8 cmH₂O due to concerns that high-CPAP may cause pulmonary overexpansion and adversely affect the cardiovascular system. We investigated the effects of high-CPAP on pulmonary (PBF) and cerebral (CBF) blood flows and jugular vein pressure (JVP) after birth in preterm lambs. Methods: Preterm lambs instrumented with flow probes and catheters were delivered at 133/146 days gestation. Lambs received low-CPAP (LCPAP: 5 cmH₂O), high-CPAP (HCPAP: 15 cmH₂O) or dynamic HCPAP (15 decreasing to 8 cmH₂O at ~2 cmH₂O/min) for up to 30 min after birth. Results: Mean PBF was lower in the LCPAP [median (Q1-Q3); 202 (48-277) mL/min, p = 0.002] compared to HCPAP [315 (221-365) mL/min] and dynamic HCPAP [327 (269-376) mL/min] lambs. CBF was similar in LCPAP [65 (37-78) mL/min], HCPAP [73 (41-106) mL/min], and dynamic HCPAP [66 (52-81) mL/min, p = 0.174] lambs. JVP was similar at CPAPs of 5 [8.0 (5.1-12.4) mmHg], 8 [9.4 (5.3-13.4) mmHg], and 15 cmH₂O [8.6 (6.9-10.5) mmHg, p = 0.909]. Heart rate was lower in the LCPAP [134 (101-174) bpm; p = 0.028] compared to the HCPAP [173 (139-205)] and dynamic HCPAP [188 (161-207) bpm] groups. Ventilation or additional caffeine was required in 5/6 LCPAP, 1/6 HCPAP, and 5/7 dynamic HCPAP lambs (p = 0.082), whereas 3/6 LCPAP, but no HCPAP lambs required intubation (p = 0.041), and 1/6 LCPAP, but no HCPAP lambs developed a pneumothorax (p = 0.632). Conclusion: High-CPAP did not impede the increase in PBF at birth and supported preterm lambs without affecting CBF and JVP.

Fetal growth restriction is associated with an altered cardiopulmonary and cerebral hemodynamic response to surfactant therapy in preterm lambs

BACKGROUND

Efficacy of surfactant therapy in fetal growth restricted (FGR) preterm neonates is unknown.

METHODS

Twin-bearing ewes underwent surgery at 105 days gestation to induce FGR in one twin by single umbilical artery ligation. At 123-127 days, catheters and flow probes were implanted in pulmonary and carotid arteries to measure flow and pressure. Lambs were delivered, intubated and mechanically ventilated. At 10 min, surfactant (100 mg kg^-1) was administered. Ventilation, oxygenation, and hemodynamic responses were recorded for 1 h before euthanasia at 120 min. Lung tissue and bronchoalveolar lavage fluid was collected for analysis of surfactant protein mRNA and phosphatidylcholines (PCs).

RESULTS

FGR preterm lambs were 26% lighter than appropriate for gestational age (AGA) lambs and had baseline differences in lung mechanics and pulmonary blood flows. Surfactant therapy reduced ventilator and oxygen requirements and improved lung mechanics in both groups, although a more rapid improvement in compliance and tidal volume was observed in AGA lambs. Surfactant administration was associated with decreased mean pulmonary and carotid blood flow in FGR but not AGA lambs. No major differences in surfactant protein mRNA or PC levels were noted.

CONCLUSIONS

Surfactant therapy was associated with an altered pulmonary and cerebral hemodynamic response in preterm FGR lambs.

Gestational Age Influences the Early Microarchitectural Changes in Response to Mechanical Ventilation in the Preterm Lamb Lung

Background: Preterm birth is associated with abnormal lung architecture, and a reduction in pulmonary function related to the degree of prematurity. A thorough understanding of the impact of gestational age on lung microarchitecture requires reproducible quantitative analysis of lung structure abnormalities. The objectives of this study were (1) to use quantitative histological software (ImageJ) to map morphological patterns of injury resulting from delivery of an identical ventilation strategy to the lung at varying gestational ages and (2) to identify associations between gestational age-specific morphological alterations and key functional outcomes. Method: Lung morphology was compared after 60 min of a standardized ventilation protocol (40 cm H₂O sustained inflation and then volume-targeted positive pressure ventilation with positive end-expiratory pressure 8 cm H₂O) in lambs at different gestations (119, 124, 128, 133, 140d) representing the spectrum of premature developmental lung states and the term lung. Age-matched controls were compared at 124 and 128d gestation. Automated and manual functions of Image J were used to measure key histological features. Correlation analysis compared morphological and functional outcomes in lambs aged ≤128 and >128d. Results: In initial studies, unventilated lung was indistinguishable at 124 and 128d. Ventilated lung from lambs aged 124d gestation exhibited increased numbers of detached epithelial cells and lung tissue compared with 128d lambs. Comparing results from saccular to alveolar development (120-140d), lambs aged ≤124d exhibited increased lung tissue, average alveolar area, and increased numbers of detached epithelial cells. Alveolar septal width was increased in lambs aged ≤128d. These findings were mirrored in the measures of gas exchange, lung mechanics, and molecular markers of lung injury. Correlation analysis confirmed the gestation-specific relationships between the histological assessments and functional measures in ventilated lambs at gestation ≤128 vs. >128d. Conclusion: Image J allowed rapid, quantitative assessment of alveolar morphology, and lung injury in the preterm lamb model. Gestational age-specific patterns of injury in response to delivery of an identical ventilation strategy were identified, with 128d being a transition point for associations between morphological alterations and functional outcomes. These results further support the need to develop individualized respiratory support approaches tailored to both the gestational age of the infant and their underlying injury response.

Supporting breathing of preterm infants at birth: a narrative review

Most very preterm infants have difficulty aerating their lungs and require respiratory support at birth. Currently in clinical practice, non-invasive ventilation in the form of continuous positive airway pressure (CPAP) and positive pressure ventilation (PPV) is applied via facemask. As most very preterm infants breathe weakly and unnoticed at birth, PPV is often administered. PPV is, however, frequently ineffective due to pressure settings, mask leak and airway obstruction. Meanwhile, high positive inspiratory pressures and spontaneous breathing coinciding with inflations can generate high tidal volumes. Evidence from preclinical studies demonstrates that high tidal volumes can be injurious to the lungs and brains of premature newborns. To reduce the need for PPV in the delivery room, it should be considered to optimise spontaneous breathing with CPAP. CPAP is recommended in guidelines and commonly used in the delivery room after a period of PPV, but little data is available on the ideal CPAP strategy and CPAP delivering devices and interfaces used in the delivery room. This narrative review summarises the currently available evidence for why PPV can be inadequate at birth and what is known about different CPAP strategies, devices and interfaces used the delivery room.

Comparison of Two Respiratory Support Strategies for Stabilization of Very Preterm Infants at Birth: A Matched-Pairs Analysis

Objective: Respiratory support for stabilizing very preterm infants at birth varies between centers. We retrospectively compared two strategies that involved either increasing continuous positive airway pressures (CPAP), or increasing oxygen supplementation. Methods: Matched-pairs of infants (<28 weeks of gestation) were born either at the Leiden University Medical Center [low-pressure: CPAP 5-8 cmH₂O and/or positive pressure ventilation (PPV) and fraction of inspired oxygen (FiO₂) 0.3-1.0; n = 27], or at the University Hospital of Cologne (high-pressure: CPAP 12-35 cmH₂O, no PPV and FiO₂ 0.3-0.4; n = 27). Respiratory support was initiated non-invasively via facemask at both units. Infants (n = 54) were matched between centers for gestational age and birth weight, to compare physiological and short-term clinical outcomes. Results: In the low-pressure group, 20/27 (74%) infants received 1-2 sustained inflations (20, 25 cm H₂O) and 22/27 (81%) received PPV (1:19-3:01 min) using pressures of 25-27 cm H₂O. Within 3 min of birth [median (IQR)], mean airway pressures [12 (6-15) vs. 19 (16-23) cmH₂O, p < 0.001] and FiO₂ [0.30 (0.28-0.31) vs. 0.22 (0.21-0.30), p < 0.001] were different in low- vs. high-pressure groups, respectively. SpO₂ and heart rates were similar. After 3 min, higher FiO₂ levels [0.62 (0.35-0.98) vs. 0.28 (0.22-0.38), p = 0.005] produced higher SpO₂ levels [77 (50-92) vs. 53 (42-69)%, p < 0.001] in the low-pressure group, but SpO₂/FiO₂ and heart rates were similar. While intubation rates during admission were significantly different (70 vs. 30%, p = 0.013), pneumothorax rates (4 vs. 19%, p = 0.125) and the occurrence of spontaneous intestinal perforations (0 vs. 15%, p = 0.125) were similar between groups. Conclusion: Infants (<28 weeks) can be supported non-invasively at birth with either higher or lower pressures and while higher-pressure support may require less oxygen, it does not eliminate the need for oxygen supplementation. Future studies need to examine the effect of high pressures and pressure titration in the delivery room.

Early Use of Inhaled Nitric Oxide in Preterm Infants: Is there a Rationale for Selective Approach?

Background Inhaled nitric oxide (iNO) is being increasingly used in preterm infants < 34 weeks with hypoxemic respiratory failure (HRF) and/or pulmonary hypertension (PH). Objective To evaluate the risk factors, survival characteristics, and lung histopathology in preterm infants with PH/HRF. Methods Retrospective chart review was conducted to determine characteristics of 93 preterm infants treated with iNO in the first 28 days and compared with 930 matched controls. Factors associated with survival with preterm HRF and smooth muscle actin from nine autopsies were evaluated. Results Preterm neonates treated with iNO had a higher incidence of preterm prolonged rupture of membrane (pPROM ≥ 18 hours), oligohydramnios and delivered by C-section. In infants treated with iNO, antenatal steroids (odds ratio [OR],3.7; confidence interval [CI], 1.2-11.3; p = 0.02), pPROM (OR, 1.001; CI, 1.0-1.004; p = 0.3), and oxygenation response to iNO (OR, 3.7; CI, 1.08-13.1; p = 0.037) were associated with survival. Thirteen infants with all three characteristics had 100% (13/13) survival without severe intraventricular hemorrhage (IVH)/periventricular leukomalacia (PVL) compared with 48% survival (12/25, p = 0.004) and 16% severe IVH/PVL without any of these factors. Severity of HRF correlated with increased smooth muscle in pulmonary vasculature. Conclusion Preterm infants with HRF exposed to antenatal steroids and pPROM had improved oxygenation with iNO and survival without severe IVH/PVL. Precisely targeting this subset may be beneficial in future trials of iNO.

Sustained Lung Inflation: Physiology and Practice

Lung aeration is the most critical task newborns must accomplish after birth. Almost all extremely preterm infants require respiratory support during this process, but the best method to promote lung aeration in preterm infants is unknown. The current standard practice is intermittent positive pressure ventilation with positive end-expiratory pressure. Sustained inflation is a promising alternative strategy for lung liquid clearance and aeration. Here we review the physiologic rationale for sustained inflation and the available clinical evidence for sustained inflation in preterm infants.

The timing of umbilical cord clamping at birth: physiological considerations

While it is now recognized that umbilical cord clamping (UCC) at birth is not necessarily an innocuous act, there is still much confusion concerning the potential benefits and harms of this common procedure. It is most commonly assumed that delaying UCC will automatically result in a time-dependent net placental-to-infant blood transfusion, irrespective of the infant's physiological state. Whether or not this occurs, will likely depend on the infant's physiological state and not on the amount of time that has elapsed between birth and umbilical cord clamping (UCC). However, we believe that this is an overly simplistic view of what can occur during delayed UCC and ignores the benefits associated with maintaining the infant's venous return and cardiac output during transition. Recent experimental evidence and observations in humans have provided compelling evidence to demonstrate that time is not a major factor influencing placental-to-infant blood transfusion after birth. Indeed, there are many factors that influence blood flow in the umbilical vessels after birth, which depending on the dominating factors could potentially result in infant-to-placental blood transfusion. The most dominant factors that influence umbilical artery and venous blood flows after birth are lung aeration, spontaneous inspirations, crying and uterine contractions. It is still not entirely clear whether gravity differentially alters umbilical artery and venous flows, although the available data suggests that its influence, if present, is minimal. While there is much support for delaying UCC at birth, much of the debate has focused on a time-based approach, which we believe is misguided. While a time-based approach is much easier and convenient for the caregiver, ignoring the infant's physiology during delayed UCC can potentially be counter-productive for the infant.

New concepts in neonatal resuscitation

PURPOSE OF REVIEW

There has been a substantial increase in the number of studies of neonatal resuscitation and it is timely to review the accumulating evidence.

RECENT FINDINGS

There have been major changes in the way that newly born infants are managed in the delivery room. Colour is no longer recommended as a useful indicator of oxygenation or effectiveness of resuscitation. Pulse oximetry provides rapid, continuous and accurate information on both oxygenation and heart rate. Resuscitation of term infants should begin with air, with the provision of blended oxygen to maintain oxygen saturations similar to those of term infants requiring no resuscitation. Positive end-expiratory pressure during initial ventilation aids lung aeration and establishment of functional residual capacity. Respiratory function monitoring allows operators to identify factors adversely affecting ventilation, including leak around the face mask and airway obstruction. Clamping of the umbilical cord should be delayed for at least 1 min for infants not requiring resuscitation.

SUMMARY

The International Liaison Committee on Resuscitation guidelines on the management of newborn infants were updated in 2010 and incorporate much of the newly available evidence. The use of intensive care techniques in the delivery room is promising but requires further evaluation. Monitoring techniques and interventions need to be adapted for use in developing countries.

Inflammation in utero exacerbates ventilation-induced brain injury in preterm lambs

Cerebral blood flow disturbance is a major contributor to brain injury in the preterm infant. The initiation of ventilation may be a critical time for cerebral hemodynamic disturbance leading to brain injury in preterm infants, particularly if they are exposed to inflammation in utero. We aimed to determine whether exposure to inflammation in utero alters cardiopulmonary hemodynamics, resulting in cerebral hemodynamic disturbance and related brain injury during the initiation of ventilation. Furthermore, we aimed to determine whether inflammation in utero alters the cerebral hemodynamic response to challenge induced by high mean airway pressures. Pregnant ewes received intra-amniotic lipopolysaccharide (LPS) or saline either 2 or 4-days before preterm delivery (at 128 ± 1 days of gestation). Lambs were surgically instrumented for assessment of pulmonary and cerebral hemodynamics before delivery and positive pressure ventilation. After 30 min, lambs were challenged hemodynamically by incrementing and decrementing positive end-expiratory pressure. Blood gases, arterial pressures, and blood flows were recorded. The brain was collected for biochemical and histological assessment of inflammation, brain damage, vascular extravasation, hemorrhage, and oxidative injury. Carotid arterial pressure was higher and carotid blood flow was more variable in 2-day LPS lambs than in controls during the initial 15 min of ventilation. All lambs responded similarly to the hemodynamic challenge. Both 2- and 4-day LPS lambs had increased brain interleukin (IL)-1β, IL-6, and IL-8 mRNA expression; increased number of inflammatory cells in the white matter; increased incidence and severity of brain damage; and vascular extravasation relative to controls. Microvascular hemorrhage was increased in 2-day LPS lambs compared with controls. Cerebral oxidative injury was not different between groups. Antenatal inflammation causes adverse cerebral hemodynamics and increases the incidence and severity of brain injury in ventilated preterm lambs.

The cerebral critical oxygen threshold of ventilated preterm lambs and the influence of antenatal inflammation

Perinatal inflammation is associated with adverse neurodevelopmental outcomes, which may be partly due to changes in the cerebral oxygen delivery/consumption relationship. We aimed to determine the critical oxygen delivery threshold of the brain of preterm, ventilated lambs and to determine whether the critical threshold is affected by exposure to inflammation in utero. Pregnant ewes received intra-amniotic injection of lipopolysaccharide or saline at 125 or 127 days of gestation. Pulmonary and systemic flow probes and catheters were surgically positioned in the fetus immediately before delivery at 129 days of gestation. After delivery, lambs were ventilated for 90 min using a positive end-expiratory pressure recruitment strategy. Cardio-respiratory variables and blood gases were measured regularly. Systemic and cerebral oxygen delivery, consumption (Fick), and extraction were calculated, and the relationship between cerebral delivery and consumption analyzed. Linear regression was used to define the transition or "critical" oxygen threshold as the point at which the slope of the oxygen delivery/consumption curve changed to be > 10°. Four subgroups were defined according to the calculated critical threshold. A total of 150 measurements were recorded in 18 lambs. Fetal cerebral oxygen consumption was increased by antenatal lipopolysaccharide (P < 0.05). The postnatal critical oxygen threshold was 3.6 ml·kg⁻¹·min⁻¹, corresponding to cerebral oxygen consumption of 0.73 ml·kg⁻¹·min⁻¹. High oxygen delivery and consumption were associated with increased pulmonary and carotid blood flow and systemic extraction compared with low oxygen delivery and consumption. No postnatal effect of antenatal inflammation was observed. Inflammation in utero increases fetal, but not postnatal, cerebral oxygen consumption. Adverse alterations to pulmonary blood flow can result in reduced cerebral blood flow, oxygen delivery, and consumption. Regardless of exposure to inflammation, there is a consistent postnatal relationship between cerebral oxygen delivery and consumption.

High positive end-expiratory pressure during high-frequency jet ventilation improves oxygenation and ventilation in preterm lambs

Increasing positive end-expiratory pressure (PEEP) is advocated to recruit alveoli during high-frequency jet ventilation (HFJV), but its effect on cardiopulmonary physiology and lung injury is poorly documented. We hypothesized that high PEEP would recruit alveoli and reduce lung injury but compromise pulmonary blood flow (PBF). Preterm lambs of anesthetized ewes were instrumented, intubated, and delivered by cesarean section after instillation of surfactant. HFJV was commenced with a PEEP of 5 cm H2O. Lambs were allocated randomly at delivery to remain on constant PEEP (PEEPconst, n = 6) or to recruitment via stepwise adjustments in PEEP (PEEPadj, n = 6) to 12 cm H2O then back to 8 cm H2O over the initial 60 min. PBF was measured continuously while ventilatory parameters and arterial blood gases were measured at intervals. At postmortem, in situ pressure-volume deflation curves were recorded, and bronchoalveolar lavage fluid and lung tissue were obtained to assess inflammation. PEEPadj lambs had lower pressure amplitude, fractional inspired oxygen concentration, oxygenation index, and PBF and more compliant lungs. Inflammatory markers were lower in the PEEPadj group. Adjusted PEEP during HFJV improves oxygenation and lung compliance and reduces ventilator requirements despite reducing pulmonary perfusion.

Monitoring oxygen saturation and heart rate in the early neonatal period

Pulse oximetry is commonly used to assist clinicians in assessment and management of newly born infants in the delivery room (DR). In many DRs, pulse oximetry is now the standard of care for managing high risk infants, enabling immediate and dynamic assessment of oxygenation and heart rate. However, there is little evidence that using pulse oximetry in the DR improves short and long term outcomes. We review the current literature on using pulse oximetry to measure oxygen saturation and heart rate and how to apply current evidence to management in the DR.

Acute AT(1)-receptor blockade reverses the hemodynamic and baroreflex impairment in adult sheep exposed to antenatal betamethasone

To accelerate lung development and protect neonates from other early developmental problems, synthetic steroids are administered maternally in the third trimester, exposing fetuses that are candidates for premature delivery to them. However, steroid exposure at this point of gestation may lead to elevated blood pressure [mean arterial pressure (MAP)] during adolescence. We hypothesize that fetal exposure to steroids activates the renin-angiotensin system, inducing an elevation in blood pressure and attenuation of baroreflex sensitivity (BRS) that is angiotensin II dependent in early adulthood. To test this hypothesis, fetal sheep were exposed to betamethasone (Beta) or vehicle (control) administered to ewes at day 80 of gestation and delivered at full term. At 1.8 yr of age, male offspring were instrumented for conscious recording of MAP, heart rate, and measurement of BRS [as low-frequency-alpha, high-frequency-alpha, sequence (seq) UP, seq DOWN, and seq TOTAL]. Beta-exposed sheep (n = 6) had higher MAP than control sheep (n = 5) (93 + or - 2 vs. 84 + or - 2 mmHg, P < 0.01). Acute blockade of angiotensin type 1 receptors with candesartan (0.3 mg/kg iv) normalized MAP in Beta-exposed sheep (85 + or - 4 mmHg), with no effect in control sheep (82 + or - 3 mmHg). Before angiotensin type 1 blockade, BRS maximum gain was significantly lower in Beta-exposed vs. control sheep (11 + or - 3 vs. 26 + or - 3 ms/mmHg, P < 0.0.01). However, 45 min after candesartan injection, BRS was increased in Beta-exposed (21 + or - 5 ms/mmHg) and control (35 + or - 4 ms/mmHg) sheep. Heart rate variability (HRV) and blood pressure variability (BPV) revealed lower HRV (SD of beat-to-beat interval and root mean square of successive beat-to-beat differences in R-R interval duration) and higher BPV (SD of MAP, systolic arterial pressure in low-frequency range) in Beta-exposed sheep. Candesartan partially restored HRV in Beta-exposed sheep and fully corrected BPV. Thus, in utero exposure to synthetic glucocorticoids causes long-lasting programming of the cardiovascular system via renin-angiotensin system-dependent mechanisms.

Intrauterine inflammation causes pulmonary hypertension and cardiovascular sequelae in preterm lambs

Chorioamnionitis increases the risk and severity of persistent pulmonary hypertension of the newborn in preterm infants. Exposure of preterm fetal lambs to intra-amniotic (IA) lipopolysaccharide (LPS) induces chorioamnionitis, causes hypertrophy of pulmonary resistance arterioles, and alters expression of pulmonary vascular growth proteins. We investigated the cardiopulmonary and systemic hemodynamic consequences of IA LPS in preterm lambs. Pregnant ewes received IA injection of LPS ( n = 6) or saline (controls; n = 8) at 122 days gestation, 7 days before exteriorization, instrumentation, and delivery of the fetus with pulmonary and systemic flow probes and catheters at 129 days gestation. Newborn lambs were ventilated, targeting a tidal volume of 6–7 ml/kg and a positive end-expiratory pressure (PEEP) of 4 cmH2O. At 30 min, all lambs underwent a PEEP challenge: PEEP was increased by 2 cmH2O at 10-min intervals to 10 cmH2O and then decreased similarly to 4 cmH2O. Ventilation parameters, arterial blood flows, and pressures were recorded in real-time for 90 min. LPS lambs had higher total protein in bronchoalveolar lavage fluid ( P < 0.002), increased medial thickness of arteriolar walls ( P = 0.013), and right ventricular hypertrophy ( P = 0.012). Compared with controls, LPS lambs had worse oxygenation ( P < 0.001), decreased pulmonary blood flow ( P = 0.05), and higher pulsatility index ( P < 0.001) and pulmonary ( P < 0.001) and systemic arterial pressures ( P = 0.005) than controls. Intra-amniotic LPS increased right-to-left shunting across the ductus arteriosus ( P = 0.018) and decreased left ventricular output ( P < 0.001). We conclude that inflammation and pulmonary remodeling induced by IA LPS adversely alters pulmonary hemodynamics with subsequent cardiovascular and systemic sequelae, which may predispose the preterm lamb to persistent pulmonary hypertension of the newborn.

Antenatal corticosteroids increase fetal, but not postnatal, pulmonary blood flow in sheep

The lungs of very preterm infants have immature airways and gas exchange structures and are usually surfactant deficient. Antenatal corticosteroids are commonly used to enhance fetal lung maturation in preterm infants, but little is known of their effects on pulmonary blood flow (PBF) before and immediately after birth. Our aim was to determine the effects of antenatal betamethasone on PBF before birth and during the postnatal transition in very preterm lambs. Antenatal betamethasone treatment significantly increased mean fetal PBF from 20.2 +/- 5.1 to 84.3 +/- 18.3 mL/min at 30 h after administration; the PBF waveform was also significantly altered. Mean diastolic PBF increased from -38.5 +/- 4.9 pretreatment to -10.2 +/- 11.0 mL/min at approximately 36 h after the initial betamethasone dose (negative values indicate retrograde flow away from the lungs). Within 10 min after delivery, PBF was similar in control and betamethasone-treated lambs. These data demonstrate that antenatal betamethasone significantly increases fetal PBF and alters the PBF waveform but has little effect on postnatal PBF.

Dynamic changes in the direction of blood flow through the ductus arteriosus at birth

Major cardiovascular changes occur at birth, including increased pulmonary blood flow (PBF) and closure of the ductus arteriosus (DA), which acts as a low resistance shunt between the fetal pulmonary and systemic circulations. Although the pressure gradient between these circulations reverses after birth, little is known about DA blood flow changes and whether reverse DA flow contributes to PBF after birth. Our aim was to describe the changes in PBF and DA flow before, during and after the onset of pulmonary ventilation at birth. Flow probes were implanted on the left pulmonary artery (LPA) and DA in preterm fetal sheep (n = 8) approximately 3 days before they were delivered and ventilated. Blood flow was measured in the LPA and DA, before and after umbilical cord occlusion (UCO) and for 2 h after ventilation onset. Following UCO, DA flow decreased from 534 +/- 57 ml min(1) to 237 +/- 29 ml min(1) which reflected a similar reduction in right ventricular output. Within 5 min of ventilation onset, PBF increased from 11 +/- 6 ml min(1) to 230 +/- 13 ml min(1) whereas DA flow decreased to 172 +/- 54 ml min(1); negative values indicate reverse DA flow (left-to-right shunting). Reverse flow through the DA contributed up to 50% of total PBF at 30 min and a decrease in this contribution accounted for 71 +/- 13% of the time-related decrease in PBF after birth. DA blood flow is very dynamic after birth and depends upon the pressure gradient between the pulmonary and systemic circulations. Following ventilation, reverse DA flow provided a significant contribution to total PBF after birth.

Cardiovascular and pulmonary consequences of airway recruitment in preterm lambs

Increases in positive end-expiratory pressure (PEEP) improve arterial oxygenation in preterm infants, but the effects on cardiopulmonary hemodynamics are understood poorly. We aimed to determine the effect of increased PEEP on cardiopulmonary hemodynamics and to compare measurements from indwelling flow probes with Doppler echocardiography. Preterm lambs (129 ± 1 days) were ventilated initially with a tidal volume of 7 ml/kg and 4 cmH2O of PEEP. In ramp lambs ( n = 7), PEEP was increased by 2-cmH2O increments to 10 cmH2O and then in decrements back to 4 cmH2O. PEEP was unchanged in controls ( n = 6). Doppler echocardiographic flow measurements in the left pulmonary artery (LPA) and ductus arteriosus (DA) were correlated with flow probe measurements. Compared with controls, high PEEP reduced LPA flow from baseline (10-cmH2O PEEP: 43 ± 8% vs. control: 83 ± 21%; P = 0.029). High PEEP increased the proportion of right-to-left (R-L) shunting through the DA, with a trend to an increased oxygenation index compared with controls (oxygenation index: 44.5 ± 13.5 at 10-cmH2O PEEP vs. 19.4 ± 4.5 in controls; P = 0.07). Increasing PEEP decreased heart rate (17 beats/min; P = 0.03) and tended to lower systolic arterial pressure (5.0 mmHg; P = 0.052) compared with controls. Doppler echocardiography measurement of LPA flows correlated strongly with indwelling flow probe ( r2 = 0.73, P < 0.001), except during highly turbulent flows. Increases in PEEP have significant cardiopulmonary consequences in preterm lambs, including reduced LPA flow and increased R-L shunt through the DA. These changes are likely due to the concomitant increase in downstream pulmonary vascular resistance and increased cardiovascular constraint induced by PEEP.

Continuous positive airway pressure: scientific and clinical rationale

PURPOSE OF REVIEW

To present recent data on the role of noninvasive ventilation in the respiratory management of newborn infants.

RECENT FINDINGS

Noninvasive ventilation is growing in popularity but is applied using widely varying devices and settings. Although short-term physiological advantages were reported for bubble and variable-flow continuous positive airways pressure, neither has convincingly shown superior clinically important outcomes. Continuous positive airways pressure may be used as the initial mode of support for very preterm infants but increased rates of pneumothorax in infants not receiving surfactant are a concern. Methods of administering surfactant without endotracheal intubation deserve further study. Nasal intermittent positive-pressure ventilation shows promise as a primary treatment for respiratory distress syndrome. Optimal pressure settings for continuous positive airways pressure and nasal intermittent positive-pressure ventilation remain uncertain.

SUMMARY

Noninvasive ventilation has partially fulfilled its promise as a gentler alternative to ventilation via an endotracheal tube. Appropriately designed randomized clinical trials are required to determine the best nasal interfaces and pressure generators.