Samantha K. Barton

Initiation of Resuscitation with High Tidal Volumes Causes Cerebral Hemodynamic Disturbance, Brain Inflammation and Injury in Preterm Lambs

Aims Preterm infants can be inadvertently exposed to high tidal volumes (VT) in the delivery room, causing lung inflammation and injury, but little is known about their effects on the brain. The aim of this study was to compare an initial 15 min of high VT resuscitation strategy to a less injurious resuscitation strategy on cerebral haemodynamics, inflammation and injury. Methods Preterm lambs at 126 d gestation were surgically instrumented prior to receiving resuscitation with either: 1) High VT targeting 10–12 mL/kg for the first 15 min (n = 6) or 2) a protective resuscitation strategy (Prot VT), consisting of prophylactic surfactant, a 20 s sustained inflation and a lower initial VT (7 mL/kg; n = 6). Both groups were subsequently ventilated with a VT 7 mL/kg. Blood gases, arterial pressures and carotid blood flows were recorded. Cerebral blood volume and oxygenation were assessed using near infrared spectroscopy. The brain was collected for biochemical and histologic assessment of inflammation, injury, vascular extravasation, hemorrhage and oxidative injury. Unventilated controls (UVC; n = 6) were used for comparison. Results High VT lambs had worse oxygenation and required greater ventilatory support than Prot VT lambs. High VT resulted in cerebral haemodynamic instability during the initial 15 min, adverse cerebral tissue oxygenation index and cerebral vasoparalysis. While both resuscitation strategies increased lung and brain inflammation and oxidative stress, High VT resuscitation significantly amplified the effect (p = 0.014 and p<0.001). Vascular extravasation was evident in the brains of 60% of High VT lambs, but not in UVC or Prot VT lambs. Conclusion High VT resulted in greater cerebral haemodynamic instability, increased brain inflammation, oxidative stress and vascular extravasation than a Prot VT strategy. The initiation of resuscitation targeting Prot VT may reduce the severity of brain injury in preterm neonates.

Single Sustained Inflation followed by Ventilation Leads to Rapid Cardiorespiratory Recovery but Causes Cerebral Vascular Leakage in Asphyxiated Near-Term Lambs

Background A sustained inflation (SI) rapidly restores cardiac function in asphyxic, bradycardic newborns but its effects on cerebral haemodynamics and brain injury are unknown. We determined the effect of different SI strategies on carotid blood flow (CaBF) and cerebral vascular integrity in asphyxiated near-term lambs. Methods Lambs were instrumented and delivered at 139 ± 2 d gestation and asphyxia was induced by delaying ventilation onset. Lambs were randomised to receive 5 consecutive 3 s SI (multiple SI; n = 6), a single 30 s SI (single SI; n = 6) or conventional ventilation (no SI; n = 6). Ventilation continued for 30 min in all lambs while CaBF and respiratory function parameters were recorded. Brains were assessed for gross histopathology and vascular leakage. Results CaBF increased more rapidly and to a greater extent during a single SI (p = 0.01), which then decreased below both other groups by 10 min, due to a higher cerebral oxygen delivery (p = 0.01). Blood brain barrier disruption was increased in single SI lambs as indicated by increased numbers of blood vessel profiles with plasma protein extravasation (p = 0.001) in the cerebral cortex. There were no differences in CaBF or cerebral oxygen delivery between the multiple SI and no SI lambs. Conclusions Ventilation with an initial single 30 s SI improves circulatory recovery, but is associated with greater disruption of blood brain barrier function, which may exacerbate brain injury suffered by asphyxiated newborns. This injury may occur as a direct result of the initial SI or to the higher tidal volumes delivered during subsequent ventilation.

Respiratory support for premature neonates in the delivery room: effects on cardiovascular function and the development of brain injury

The transition to newborn life in preterm infants is complicated by immature cardiovascular and respiratory systems. Consequently, preterm infants often require respiratory support immediately after birth. Although aeration of the lung underpins the circulatory transition at birth, positive pressure ventilation can adversely affect cardiorespiratory function during this vulnerable period, reducing pulmonary blood flow and left ventricular output. Furthermore, pulmonary volutrauma is known to initiate pulmonary inflammatory responses, resulting in remote systemic involvement. This review focuses on the downstream consequences of positive pressure ventilation, in particular, interactions between cardiovascular output and the initiation of a systemic inflammatory cascade, on the immature brain. Recent studies have highlighted that positive pressure ventilation strategies are precursors of cerebral injury, probably mediated through cerebral blood flow instability. The presence of, or initiation of, an inflammatory cascade accentuates adverse cerebral blood flow, in addition to being a direct source of brain injury. Importantly, the degree of brain injury is dependent on the nature of the initial ventilation strategy used.

Early Detection of Ventilation-Induced Brain Injury Using Magnetic Resonance Spectroscopy and Diffusion Tensor Imaging: An In Vivo Study in Preterm Lambs

Background and Aim High tidal volume (VT) ventilation during resuscitation of preterm lambs results in brain injury evident histologically within hours after birth. We aimed to investigate whether magnetic resonance spectroscopy (MRS) and/or diffusion tensor imaging (DTI) can be used for early in vivo detection of ventilation-induced brain injury in preterm lambs. Methods Newborn lambs (0.85 gestation) were stabilized with a “protective ventilation” strategy (PROT, n = 7: prophylactic Curosurf, sustained inflation, VT 7 mL/kg, positive end expiratory pressure (PEEP) 5 cmH2O) or an initial 15 minutes of “injurious ventilation” (INJ, n = 10: VT 12 mL/kg, no PEEP, late Curosurf) followed by PROT ventilation for the remainder of the experiment. At 1 hour, lambs underwent structural magnetic resonance imaging (Siemens, 3 Tesla). For measures of mean/axial/radial diffusivity (MD, AD, RD) and fractional anisotropy (FA), 30 direction DTI was performed. Regions of interests encompassed the thalamus, internal capsule, periventricular white matter and the cerebellar vermis. MRS was performed using a localized single-voxel (15×15×20 mm3, echo time 270 ms) encompassing suptratentorial deep nuclear grey matter and central white matter. Peak-area ratios for lactate (Lac) relative to N-acetylaspartate (NAA), choline (Cho) and creatine (Cr) were calculated. Groups were compared using 2-way RM-ANOVA, Mann-Whitney U-test and Spearmans correlations. Results No cerebral injury was seen on structural MR images. Lambs in the INJ group had higher mean FA and lower mean RD in the thalamus compared to PROT lambs, but not in the other regions of interest. Peak-area lactate ratios >1.0 was only seen in INJ lambs. A trend of higher mean peak-area ratios for Lac/Cr and Lac/Cho was seen, which correlated with lower pH in both groups. Conclusion Acute changes in brain diffusion measures and metabolite peak-area ratios were observed after injurious ventilation. Early MRS/DTI is able to detect the initiation of ventilation-induced brain injury.

Exposure to intrauterine inflammation leads to impaired function and altered structure in the preterm heart of fetal sheep.

Intrauterine inflammation is a major contributor to preterm birth and has adverse effects on preterm neonatal cardiovascular physiology. Cardiomyocyte maturation occurs in late gestation in species such as humans and sheep. We tested the hypothesis that intrauterine inflammation has deleterious effects on cardiac function in preterm sheep which might be explained by altered cardiomyocyte proliferation and maturation. Pregnant ewes received an ultrasound-guided intra-amniotic injection of lipopolysaccharide (LPS) or saline 7 days prior to delivery at day 127 of pregnancy (term 147 days). Cardiac contractility was recorded in spontaneously beating hearts of the offspring, perfused in a Langendorff apparatus. Saline-filled latex balloons were inserted into the left ventricle (LV) and right ventricle (RV). Responsiveness to isoprenaline and stop-flow/reperfusion was assessed. In other experiments, hearts were perfusion-fixed, and cardiomyocyte nuclearity, volume and number were determined. β-Adrenoceptor mRNA levels were determined in unfixed tissue. In hearts of LPS-exposed fetuses, contractility in the LV and RV was suppressed by ~40% and cardiomyocyte numbers were reduced by ~25%. Immature mono-nucleated cardiomyocytes had lower volumes (~18%), whereas mature bi-nucleated cardiomyocyte volume was ~77% greater. Although basal coronary flow was significantly increased by 21±7% in LPS-exposed hearts, following ischaemia/reperfusion (IR), end-diastolic pressure was increased 2.4±0.3-fold and infarct area was increased 3.2±0.6-fold compared with those in controls. Maximum responsiveness to isoprenaline was enhanced by LPS, without an increase in β-adrenoceptor mRNA, suggesting altered second messenger signalling. Intrauterine inflammation altered cardiac growth, suppressed contractile function and enhanced responsiveness to stress. Although these effects may ensure immediate survival, they probably contribute to the increased vulnerability of organ perfusion in preterm neonates.

Human Amnion Epithelial Cells Modulate Ventilation-Induced White Matter Pathology in Preterm Lambs

Background: Preterm infants can be inadvertently exposed to high tidal volumes (VT) during resuscitation in the delivery room due to limitations of available equipment. High VT ventilation of preterm lambs produces cerebral white matter (WM) pathology similar to that observed in preterm infants who develop cerebral palsy. We hypothesized that human amnion epithelial cells (hAECs), which have anti-inflammatory and regenerative properties, would reduce ventilation-induced WM pathology in neonatal late preterm lamb brains. Methods: Two groups of lambs (0.85 gestation) were used, as follows: (1) ventilated lambs (Vent; n = 8) were ventilated using a protocol that induces injury (VT targeting 15 ml/kg for 15 min, with no positive end-expiratory pressure) and were then maintained for another 105 min, and (2) ventilated + hAECs lambs (Vent+hAECs; n = 7) were similarly ventilated but received intravenous and intratracheal administration of 9 × 107 hAECs (18 × 107 hAECs total) at birth. Oxygenation and ventilation parameters were monitored in real time; cerebral oxygenation was measured using near-infrared spectroscopy. qPCR (quantitative real-time PCR) and immunohistochemistry were used to assess inflammation, vascular leakage and astrogliosis in both the periventricular and subcortical WM of the frontal and parietal lobes. An unventilated control group (UVC; n = 5) was also used for qPCR analysis of gene expression. Two-way repeated measures ANOVA was used to compare physiological data. Students t test and one-way ANOVA were used for immunohistological and qPCR data comparisons, respectively. Results: Respiratory parameters were not different between groups. Interleukin (IL)-6 mRNA levels in subcortical WM were lower in the Vent+hAECs group than the Vent group (p = 0.028). IL-1β and IL-6 mRNA levels in periventricular WM were higher in the Vent+hAECs group than the Vent group (p = 0.007 and p = 0.001, respectively). The density of Iba-1-positive microglia was lower in the subcortical WM of the parietal lobes (p = 0.010) in the Vent+hAECs group but not in the periventricular WM. The number of vessels in the WM of the parietal lobe exhibiting protein extravasation was lower (p = 0.046) in the Vent+hAECs group. Claudin-1 mRNA levels were higher in the periventricular WM (p = 0.005). The density of GFAP-positive astrocytes was not different between groups. Conclusions: Administration of hAECs at the time of birth alters the effects of injurious ventilation on the preterm neonatal brain. Further studies are required to understand the regional differences in the effects of hAECs on ventilation-induced WM pathology and their net effect on the developing brain.

Ventilation-induced brain injury in preterm neonates: A review of potential therapies

Mechanical ventilation is a risk factor for cerebral inflammation and brain injury in preterm neonates. The risk increases proportionally with the intensity of treatment. Recent studies have shown that cerebral inflammation and injury can be initiated in the delivery room. At present, initiation of intermittent positive pressure ventilation (IPPV) in the delivery room is one of the least controlled interventions a preterm infant will likely face. Varying pressures and volumes administered shortly after birth are sufficient to trigger pathways of ventilation-induced lung and brain injury. The pathways involved in ventilation-induced brain injury include a complex inflammatory cascade and haemodynamic instability, both of which have an impact on the brain. However, regardless of the strategy employed to deliver IPPV, any ventilation has the potential to have an impact on the immature brain. This is particularly important given that preterm infants are already at a high risk for brain injury simply due to immaturity. This highlights the importance of improving the initial respiratory support in the delivery room. We review the mechanisms of ventilation-induced brain injury and discuss the need for, and the most likely, current therapeutic agents to protect the preterm brain. These include therapies already employed clinically, such as maternal glucocorticoid therapy and allopurinol, as well as other agents, such as erythropoietin, human amnion epithelial cells and melatonin, already showing promise in preclinical studies. Their mechanisms of action are discussed, highlighting their potential for use immediately after birth.

Prophylactic erythropoietin exacerbates ventilation‐induced lung inflammation and injury in preterm lambs

Erythropoietin (EPO) has been suggested as a potential treatment for bronchopulmonary dysplasia (BPD) in preterm infants. Ventilation‐induced lung injury (VILI) is a major cause of BPD in preterm neonates. We investigated whether early high‐dose EPO (i.v. 5000 IU kg−1) administration can reduce lung inflammation and injury resultant from VILI in ventilated preterm lambs. Early high‐dose EPO administration increased mRNA expression of early markers of lung inflammation and injury and systemic injury controls. Early high‐dose EPO worsened histological assessment of inflammation, airway wall thickness, haemorrhage and total injury compared to controls. Early high‐dose EPO may increase the incidence and severity of respiratory disease in ventilated, preterm neonates.

Protective ventilation of preterm lambs exposed to acute chorioamnionitis does not reduce ventilation-induced lung or brain injury

Background The onset of mechanical ventilation is a critical time for the initiation of cerebral white matter (WM) injury in preterm neonates, particularly if they are inadvertently exposed to high tidal volumes (VT) in the delivery room. Protective ventilation strategies at birth reduce ventilation-induced lung and brain inflammation and injury, however its efficacy in a compromised newborn is not known. Chorioamnionitis is a common antecedent of preterm birth, and increases the risk and severity of WM injury. We investigated the effects of high VT ventilation, after chorioamnionitis, on preterm lung and WM inflammation and injury, and whether a protective ventilation strategy could mitigate the response. Methods Pregnant ewes (n = 18) received intra-amniotic lipopolysaccharide (LPS) 2 days before delivery, instrumentation and ventilation at 127±1 days gestation. Lambs were either immediately euthanased and used as unventilated controls (LPSUVC; n = 6), or were ventilated using an injurious high VT strategy (LPSINJ; n = 5) or a protective ventilation strategy (LPSPROT; n = 7) for a total of 90 min. Mean arterial pressure, heart rate and cerebral haemodynamics and oxygenation were measured continuously. Lungs and brains underwent molecular and histological assessment of inflammation and injury. Results LPSINJ lambs had poorer oxygenation than LPSPROT lambs. Ventilation requirements and cardiopulmonary and systemic haemodynamics were not different between ventilation strategies. Compared to unventilated lambs, LPSINJ and LPSPROT lambs had increases in pro-inflammatory cytokine expression within the lungs and brain, and increased astrogliosis (p<0.02) and cell death (p<0.05) in the WM, which were equivalent in magnitude between groups. Conclusions Ventilation after acute chorioamnionitis, irrespective of strategy used, increases haemodynamic instability and lung and cerebral inflammation and injury. Mechanical ventilation is a potential contributor to WM injury in infants exposed to chorioamnionitis.

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.