Effects of tidal volume and positive end-expiratory pressure during resuscitation of very premature lambs

Inflating Pressure and Not Expiratory Pressure Initiates Lung Injury at Birth in Preterm Lambs

Rationale: Inflation is essential for aeration at birth, but current inflating pressure settings are without an evidence base. Objectives: To determine the role of inflating pressure (ΔP), and its relationship with positive end-expiratory pressure (PEEP), in initiating early lung injury pathways in the preterm lamb lung. Methods: Preterm (124 to 127 d) steroid-exposed lambs (n = 45) were randomly allocated (8-10 per group) to 15 minutes of respiratory support with placental circulation and 20 or 30 cm H2O ΔP, with an initial high PEEP (maximum, 20 cm H2O) recruitment maneuver known to facilitate aeration (dynamic PEEP), and compared with dynamic PEEP with no ΔP or 30 cm H2O ΔP and low (4 cm H2O) PEEP. Lung mechanics and aeration were measured throughout. After an additional 30 minutes of apneic placental support, lung tissue and bronchoalveolar fluid were analyzed for regional lung injury, including proteomics. Measurements and Main Results: The 30 cm H2O ΔP and dynamic PEEP strategies resulted in quicker aeration and better compliance but higher tidal volumes (often >8 ml/kg, all P < 0.0001; mixed effects) and injury. ΔP 20 cm H2O with dynamic PEEP resulted in the same lung mechanics and aeration, but less energy transmission (tidal mechanical power), as ΔP 30 cm H2O with low PEEP. Dynamic PEEP without any tidal inflations resulted in the least lung injury. Use of any tidal inflating pressures altered metabolic, coagulation and complement protein pathways within the lung. Conclusions: Inflating pressure is essential for the preterm lung at birth, but it is also the primary mediator of lung injury. Greater focus is needed on strategies that identify the safest application of pressure in the delivery room.

Investigating Pathways of Ventilation Induced Brain Injury on Cerebral White Matter Inflammation and Injury After 24 h in Preterm Lambs

Initiation of respiratory support in the delivery room increases the risk and severity of brain injury in preterm neonates through two major pathways: an inflammatory pathway and a haemodynamic pathway. The relative contribution of each pathway on preterm brain injury is not known. We aimed to assess the role of the inflammatory and haemodynamic pathway on ventilation-induced brain injury (VIBI) in the preterm lamb. Fetal lambs (125 ± 1 day gestation) were exteriorised, instrumented and ventilated with a high tidal-volume (V_T) injurious strategy for 15 min either with placental circulation intact to induce the inflammatory pathway only (INJ_INF; n = 7) or umbilical cord occluded to induce both the inflammatory and haemodynamic pathways (INJ_INF+HAE; n = 7). Sham controls were exteriorised but not ventilated (SHAM; n = 5) while unoperated controls (UNOP; n = 7) did not undergo fetal instrumentation. Fetuses were returned in utero following intervention and the ewe allowed to recover. Arterial blood gases and plasma were sampled periodically. Twenty-four hours following intervention, lambs were delivered and maintained on non-injurious ventilation for ∼40 min then brains were collected post-mortem for immunohistochemistry and RT-qPCR to assess inflammation, vascular pathology and cell death within white matter regions. Compared to INJ_INF lambs, INJ_INF+HAE lambs achieved a consistently higher V_T during injurious ventilation and carotid blood flow was significantly lower than baseline by the end of ventilation. Throughout the 24 h recovery period, systemic arterial IL-6 levels of INJ_INF+HAE lambs were significantly higher than SHAM while there was no difference between INJ_INF and SHAM animals. At 24 h, mRNA expression levels of pro-inflammatory cytokines, tight junction proteins, markers of cell death, and histological injury indices of gliosis, blood vessel protein extravasation, oligodendrocyte injury and cell death were not different between groups. Injurious ventilation, irrespective of strategy, did not increase brain inflammation or injury 24 h later when compared to control animals. However, the haemodynamic pathway did influence carotid blood flow adaptations during injurious ventilation and increased systemic arterial IL-6 that may underlie long-term pathology. Future studies are required to further characterise the pathways and their long-term effects on VIBI.

Improving Newborn Respiratory Outcomes With a Sustained Inflation: A Systematic Narrative Review of Factors Regulating Outcome in Animal and Clinical Studies

Respiratory support is critically important for survival of newborns who fail to breathe spontaneously at birth. Although there is no internationally accepted definition of a sustained inflation (SI), it has commonly been defined as a positive pressure inflation designed to establish functional residual capacity and applied over a longer time period than normally used in standard respiratory support (SRS). Outcomes vary distinctly between studies and to date there has been no comprehensive investigation of differences in SI approach and study outcome in both pre-clinical and clinical studies. A systematic literature search was performed and, after screening, identified 17 animal studies and 17 clinical studies evaluating use of a SI in newborns compared to SRS during neonatal resuscitation. Study demographics including gestational age, SI parameters (length, repetitions, pressure, method of delivery) and study outcomes were compared. Animal studies provide mechanistic understanding of a SI on the physiology underpinning the cardiorespiratory transition at birth. In clinical studies, there is considerable difference in study quality, delivery of SIs (number, pressure, length) and timing of primary outcome evaluation which limits direct comparison between studies. The largest difference is method of delivery, where the role of a SI has been observed in intubated animals, as the inflation pressure is directly applied to the lung, bypassing the obstructed upper airway in an apnoeic state. This highlights a potential limitation in clinical use of a SI applied non-invasively. Further research is required to identify if a SI may have greater benefits in subpopulations of newborns.

Delivery of Positive End-Expiratory Pressure Using Self-Inflating Bags during Newborn Resuscitation Is Possible Despite Mask Leak

BACKGROUND

Ventilation is the key intervention to resuscitate non-breathing newborns. Positive end-expiratory pressure (PEEP) may facilitate lung-liquid clearance and help establish functional residual capacity.

OBJECTIVES

The aim of this study was to describe how mask leak and ventilation rates affect delivered PEEP and tidal volumes during newborn resuscitations using a self-inflating bag with an integrated PEEP valve.

METHODS

This was an observational study including near-term/term newborns who received bag-mask ventilation (BMV) with a new self-inflating bag with a novel 6 mbar PEEP valve, without external gas flow, between October 1, 2016 and June 30, 2018 in rural Tanzania. Helping Babies Breathe-trained midwives performed most of the resuscitations. Pressures and flow were continuously measured and recorded by resuscitation monitors.

RESULTS

In total, 198 newborns with a median gestation of 39 weeks (25th, 75th percentiles 37, 40) and birth weight of 3,100 g (2,580, 3,500) were included. The median delivered PEEP and expired (tidal) volume at different levels of mask leak were 6.0 mbar and 11.3 mL/kg at 0-20% mask leak, 5.5 mbar and 9.3 mL/kg at 20-40%, 5.2 mbar and 7.8 mL/kg at 40-60%, 4.6 mbar and 5.0 mL/kg at 60-80%, and 1.0 mbar and 0.6 mL/kg at 80-100% mask leak. A high ventilation rate (>60/min) nearly halved expired volumes compared to <60/min for 0-60% leak. The BMV rate had a negligible effect on peak inflation pressure (PIP) and PEEP.

CONCLUSIONS

Mask leak up to 80% did not impair the provision of recommended PEEP or tidal volumes during BMV with a self-inflating bag. High or low ventilation rates did not significantly affect PIP or PEEP. Expired volumes were reduced at ventilation rates >60/min.

The interrelationship of recruitment maneuver at birth, antenatal steroids, and exogenous surfactant on compliance and oxygenation in preterm lambs

BACKGROUND

To describe the interrelationship between antenatal steroids, exogenous surfactant, and two approaches to lung recruitment at birth on oxygenation and respiratory system compliance (Cdyn) in preterm lambs.

METHODS

Lambs (n = 63; gestational age 127 ± 1 d) received either surfactant at 10-min life (Surfactant), antenatal corticosteroids (Steroid), or neither (Control). Within each epoch lambs were randomly assigned to a 30-s 40 cmH2O sustained inflation (SI) or an initial stepwise positive end-expiratory pressure (PEEP) open lung ventilation (OLV) maneuver at birth. All lambs then received the same management for 60-min with alveolar-arterial oxygen difference (AaDO2) and Cdyn measured at regular time points.

RESULTS

Overall, the OLV strategy improved Cdyn and AaDO2 (all epochs except Surfactant) compared to SI (all P < 0.05; two-way ANOVA). Irrespective of strategy, Cdyn was better in the Steroid group in the first 10 min (all P < 0.05). Thereafter, Cdyn was similar to Steroid epoch in the OLV + Surfactant, but not SI + Surfactant group. OLV influenced the effect of steroid and surfactant (P = 0.005) on AaDO2 more than SI (P = 0.235).

CONCLUSIONS

The antenatal state of the lung influences the type and impact of a recruitment maneuver at birth. The effectiveness of surfactant maybe enhanced using PEEP-based time-dependent recruitment strategies rather than approaches solely aimed at initial lung liquid clearance.

Pressure-limited sustained inflation vs. gradual tidal inflations for resuscitation in preterm lambs

Support of the mechanically complex preterm lung needs to facilitate aeration while avoiding ventilation heterogeneities: whether to achieve this gradually or quickly remains unclear. We compared the effect of gradual vs. constant tidal inflations and a pressure-limited sustained inflation (SI) at birth on gas exchange, lung mechanics, gravity-dependent lung volume distribution, and lung injury in 131-day gestation preterm lambs. Lambs were resuscitated with either 1) a 20-s, 40-cmH2O pressure-limited SI (PressSI), 2) a gradual increase in tidal volume (Vt) over 5-min from 3 ml/kg to 7 ml/kg (IncrVt), or 3) 7 ml/kg Vt from birth. All lambs were subsequently ventilated for 15 min with 7 ml/kg Vt with the same end-expiratory pressure. Lung mechanics, gas exchange and spatial distribution of end-expiratory volume (EEV), and tidal ventilation (electrical impedance tomography) were recorded regularly. At 15 min, early mRNA tissue markers of lung injury were assessed. The IncrVt group resulted in greater tissue hysteresivity at 5 min (P = 0.017; two-way ANOVA), higher alveolar-arterial oxygen difference from 10 min (P < 0.01), and least uniform gravity-dependent distribution of EEV. There were no other differences in lung mechanics between groups, and the PressSI and 7 ml/kg Vt groups behaved similarly throughout. EEV was more uniformly distributed, but Vt least so, in the PressSI group. There were no differences in mRNA markers of lung injury. A gradual increase in Vt from birth resulted in less recruitment of the gravity-dependent lung with worse oxygenation. There was no benefit of a SI at birth over mechanical ventilation with 7 ml/kg Vt.

Unraveling the Links Between the Initiation of Ventilation and Brain Injury in Preterm Infants

The initiation of ventilation in the delivery room is one of the most important but least controlled interventions a preterm infant will face. Tidal volumes (V T) used in the neonatal intensive care unit are carefully measured and adjusted. However, the V Ts that an infant receives during resuscitation are usually unmonitored and highly variable. Inappropriate V Ts delivered to preterm infants during respiratory support substantially increase the risk of injury and inflammation to the lungs and brain. These may cause cerebral blood flow instability and initiate a cerebral inflammatory cascade. The two pathways increase the risk of brain injury and potential life-long adverse neurodevelopmental outcomes. The employment of new technologies, including respiratory function monitors, can improve and guide the optimal delivery of V Ts and reduce confounders, such as leak. Better respiratory support in the delivery room has the potential to improve both respiratory and neurological outcomes in this vulnerable population.

Effect of sustained inflation vs. stepwise PEEP strategy at birth on gas exchange and lung mechanics in preterm lambs

BACKGROUND

Sustained inflation (SI) at birth facilitates establishment of functional residual capacity (FRC) in the preterm lung, but the ideal lung recruitment strategy is unclear. We have compared the effect of SI and a stepwise positive end-expiratory pressure (PEEP; SEP) strategy in a preterm model.

METHODS

127 d gestation lambs received either 20-s SI (n = 9) or 2 cmH2O stepwise PEEP increases to 20 cmH2O every 10 inflations, and then decreases to 6 cmH2O (n = 10). Ventilation continued for 70 min, with surfactant administered at 10 min. Alveolar-arterial oxygen gradient (AaDO2), compliance (C(dyn)), end-expiratory thoracic volume (EEVRIP; respiratory inductive plethysmography), and EEV and C(dyn) in the gravity-dependent and nondependent hemithoraces (electrical impedance tomography) were measured throughout. Early mRNA markers of lung injury were analyzed using quantitative real-time PCR.

RESULTS

From 15 min of life, AaDO2 was lower in SEP group (P < 0.005; two-way ANOVA). SEP resulted in higher and more homogeneous C(dyn) (P < 0.0001). Mean (SD) EEVRIP at 5 min was 18 (9) ml/kg and 6 (5) ml/kg following SEP and SI, respectively (P = 0.021; Bonferroni posttest); this difference was due to a greater nondependent hemithorax EEV. There was no difference in markers of lung injury.

CONCLUSION

An SEP at birth improved gas exchange, lung mechanics, and EEV, without increasing lung injury, compared to the SI strategy used.

Establishing lung gas volumes at birth: interaction between positive end-expiratory pressures and tidal volumes in preterm rabbits

BACKGROUND

We investigated the effects of positive end-expiratory pressure (PEEP) and tidal volume (VT) on lung aeration, pulmonary mechanics, and the distribution of ventilation immediately after birth using a preterm rabbit model.

METHODS

Sixty preterm rabbits (27 d) received volume-targeted positive pressure ventilation from birth, with one of the 12 combinations of PEEP (0, 5, 8, or 10 cm H2O) and VT (4, 8, or 12 ml/kg). Outcomes included functional residual capacity (FRC), peak inflating pressure (PIP), dynamic compliance (Cd), and distribution of ventilation.

RESULTS

Increasing PEEP from 0 to 10 cm H2O increased FRC by 4 ml/kg, increased Cd by 0.2 ml/kg/cm H2O, and reduced PIP by 5 cm H2O. Increasing VT from 4 to 12 ml/kg increased FRC by 2 ml/kg, increased Cd by 0.3 ml/kg/cm H2O, and increased PIP by 4 cmH2O. No effect of VT on FRC occurred at 0 or 5 PEEP, and no effect of PEEP occurred at VT = 4 ml/kg. At 0 PEEP, increasing VT increased the proportion of gas entering the smaller apical regions, whereas at 10 PEEP, increasing VT increased the proportion of gas entering basal regions, from 47% to 63%.

CONCLUSION

Both PEEP and VT have independent, additive effects on FRC, lung mechanics, and the distribution of ventilation during the immediate newborn period.

Early biomarkers and potential mediators of ventilation-induced lung injury in very preterm lambs

Background

Bronchopulmonary dysplasia (BPD) is closely associated with ventilator-induced lung injury (VILI) in very preterm infants. The greatest risk of VILI may be in the immediate period after birth, when the lungs are surfactant deficient, still partially filled with liquid and not uniformly aerated. However, there have been very few studies that have examined this immediate post-birth period and identified the initial injury-related pathways that are activated. We aimed to determine if the early response genes; connective tissue growth factor (CTGF), cysteine rich-61 (CYR61) and early growth response 1 (EGR1), were rapidly induced by VILI in preterm lambs and whether ventilation with different tidal volumes caused different inflammatory cytokine and early response gene expression.

Methods

To identify early markers of VILI, preterm lambs (132 d gestational age; GA, term ~147 d) were resuscitated with an injurious ventilation strategy (V_T 20 mL/kg for 15 min) then gently ventilated (5 mL/kg) for 15, 30, 60 or 120 min (n = 4 in each). To determine if early response genes and inflammatory cytokines were differentially regulated by different ventilation strategies, separate groups of preterm lambs (125 d GA; n = 5 in each) were ventilated from birth with a V_T of 5 (VG5) or 10 mL/kg (VG10) for 135 minutes. Lung gene expression levels were compared to levels prior to ventilation in age-matched control fetuses.

Results

CTGF, CYR61 and EGR1 lung mRNA levels were increased ~25, 50 and 120-fold respectively (p < 0.05), within 30 minutes of injurious ventilation. VG5 and VG10 caused significant increases in CTGF, CYR61, EGR1, IL1-β, IL-6 and IL-8 mRNA levels compared to control levels. CTGF, CYR61, IL-6 and IL-8 expression levels were higher in VG10 than VG5 lambs; although only the IL-6 and CYR61 mRNA levels reached significance.

Conclusion

CTGF, CYR61 and EGR1 may be novel early markers of lung injury and mechanical ventilation from birth using relatively low tidal volumes may be less injurious than using higher tidal volumes.

Positive end-expiratory pressure and tidal volume during initial ventilation of preterm lambs

Positive end-expiratory pressure (PEEP) protects the lung from injury during sustained ventilation, but its role in protecting the lung from injury during the initiation of ventilation in the delivery room is not established. We aimed to evaluate whether PEEP and/or tidal volume (VT) within the first 15-min of ventilation are protective against lung injury. Operatively delivered preterm lambs (133 +/- 1 d gestation) were randomly assigned to unventilated controls or to one of four 15 min ventilation interventions: 1) VT15 mL/kg, PEEP 0 cm H2O; 2) VT15 mL/kg, PEEP 5 cm H2O; 3) VT8 mL/kg, PEEP 0 cm H2O; and 4) VT8 mL/kg, PEEP 5 cm H2O. Each group was subsequently ventilated with VT 10 mL/kg, PEEP 5 cm H2O for 1 h 45 min. Lung function was assessed and measurements of lung injury were evaluated postmortem. After the 15 min ventilation maneuver, the VT15 groups were hypocarbic, had higher oxygenation, and required lower pressures than the VT8 groups; no consistent effect of PEEP was found. Markers of lung injury were significantly elevated in all ventilation groups compared with unventilated controls; no effect of PEEP was found. Ventilation resulted in localization of IL-6 to the small airways. Initial ventilation of preterm lambs with PEEP and/or VT of 8 mL/kg did not prevent an inflammatory injury to the lung.