Impact of surgery and anesthesia during early brain development: A perfect storm.

Neonatal surgery and concomitant anesthesia coincide with a timeframe of rapid brain development. The speed and complexity of early brain development superimposed on immature regulatory mechanisms that include incomplete cerebral autoregulation, insufficient free radical scavenging and an immature immune response puts the brain at risk. Brain injury may have long-term consequences for multiple functional domains including cognition, learning skills, and behavior. Neurodevelopmental follow-up studies have noted mild-to-moderate deficits in children who underwent major neonatal surgery and related anesthesia. The present review evaluates neonatal surgery against the background of neurobiological processes that unfold at a pace unparalleled by any other period of human brain development. First, a structured summary of early brain development is provided in order to establish theoretical groundwork. Next, literature on brain injury and neurodevelopmental outcome after neonatal surgery is discussed. Special attention is given to recent findings of structural brain damage reported after neonatal surgery. Notably, high-quality imaging data acquired before surgery are currently lacking. Third, mechanisms of injury are interrogated taking the perspective of early brain development into account. We propose a novel disease model that constitutes a triad of inflammation, vascular immaturity, and neurotoxicity of prolonged exposure to anesthetic drugs. With each of these components exacerbating the other, this amalgam incites the perfect storm, resulting in brain injury. When examining the brain, it seems intuitive to distinguish between neonates (i.e., <60 postconceptional weeks) and more mature infants, multiple and/or prolonged anesthesia exposure and single, short surgery. This review culminates in an outline of anesthetic considerations and future directions that we believe will help move the field forward.

Effects of early nutrition and growth on brain volumes, white matter microstructure, and neurodevelopmental outcome in preterm newborns.

BackgroundThis study aimed to investigate the effect of nutrition and growth during the first 4 weeks after birth on cerebral volumes and white matter maturation at term equivalent age (TEA) and on neurodevelopmental outcome at 2 years' corrected age (CA), in preterm infants.MethodsOne hundred thirty-one infants born at a gestational age (GA) <31 weeks with magnetic resonance imaging (MRI) at TEA were studied. Cortical gray matter (CGM) volumes, basal ganglia and thalami (BGT) volumes, cerebellar volumes, and total brain volume (TBV) were computed. Fractional anisotropy (FA) in the posterior limb of internal capsule (PLIC) was obtained. Cognitive and motor scores were assessed at 2 years' CA.ResultsCumulative fat and enteral intakes were positively related to larger cerebellar and BGT volumes. Weight gain was associated with larger cerebellar, BGT, and CGM volume. Cumulative fat and caloric intake, and enteral intakes were positively associated with FA in the PLIC. Cumulative protein intake was positively associated with higher cognitive and motor scores (all P<0.05).ConclusionOur study demonstrated a positive association between nutrition, weight gain, and brain volumes. Moreover, we found a positive relationship between nutrition, white matter maturation at TEA, and neurodevelopment in infancy. These findings emphasize the importance of growth and nutrition with a balanced protein, fat, and caloric content for brain development.

Mild cerebellar injury does not significantly affect cerebral white matter microstructural organization and neurodevelopmental outcome in a contemporary cohort of preterm infants.

BackgroundPreterm birth is associated with an increased risk of cerebellar injury. The aim of this study was to assess the impact of cerebellar hemorrhages (CBH) on cerebral white matter microstructural tissue organization and cerebellar volume at term-equivalent age (TEA) in extremely preterm infants. Furthermore, we aimed to evaluate the association between CBH and neurodevelopmental outcome in late infancy.MethodsA total of 24 preterm infants with punctate CBH were included and each matched to two preterm control infants. T1-, T2-weighted images and diffusion-weighted imaging were acquired on a 3T magnetic resonance imaging (MRI) system. Regions of interest were drawn on a population-specific neonatal template and automatically registered to individual fractional anisotropy (FA) maps. Brain volumes were automatically computed. Neurodevelopmental outcome was assessed using the Bayley scales of Infant and Toddler Development at 2 years of corrected age.ResultsCBHs were not significantly related to FA in the posterior limb of the internal capsule and corpus callosum or to cerebellar volume. Infants with CBH did not have poorer neurodevelopmental outcome compared with control infants.ConclusionThese findings suggest that the impact of mild CBH on early macroscale brain development may be limited. Future studies are needed to assess the effects of CBH on long-term neurodevelopment.

Early human brain development: insights into macroscale connectome wiring.

BACKGROUND: Early brain development is closely dictated by distinct neurobiological principles. Here, we aimed to map early trajectories of structural brain wiring in the neonatal brain. METHODS: We investigated structural connectome development in 44 newborns, including 23 preterm infants and 21 full-term neonates scanned between 29 and 45 postmenstrual weeks. Diffusion-weighted imaging data were combined with cortical segmentations derived from T2 data to construct neonatal connectome maps. RESULTS: Projection fibers interconnecting primary cortices and deep gray matter structures were noted to mature faster than connections between higher-order association cortices (fractional anisotropy (FA) F = 58.9, p < 0.001, radial diffusivity (RD) F = 28.8, p < 0.001). Neonatal FA-values resembled adult FA-values more than RD, while RD approximated the adult brain faster (F = 358.4, p < 0.001). Maturational trajectories of RD in neonatal white matter pathways revealed substantial overlap with what is known about the sequence of subcortical white matter myelination from histopathological mappings as recorded by early neuroanatomists (mean RD 68 regions r = 0.45, p = 0.008). CONCLUSION: Employing postnatal neuroimaging we reveal that early maturational trajectories of white matter pathways display discriminative developmental features of the neonatal brain network. These findings provide valuable insight into the early stages of structural connectome development.

Severe retinopathy of prematurity is associated with reduced cerebellar and brainstem volumes at term and neurodevelopmental deficits at 2 years.

BackgroundTo evaluate the association between severe retinopathy of prematurity (ROP), measures of brain morphology at term-equivalent age (TEA), and neurodevelopmental outcome.MethodsEighteen infants with severe ROP (median gestational age (GA) 25.3 (range 24.6-25.9 weeks) were included in this retrospective case-control study. Each infant was matched to two extremely preterm control infants (n=36) by GA, birth weight, sex, and brain injury. T2-weighted images were obtained on a 3 T magnetic resonance imaging (MRI) at TEA. Brain volumes were computed using an automatic segmentation method. In addition, cortical folding metrics were extracted. Neurodevelopment was formally assessed at the ages of 15 and 24 months.ResultsInfants with severe ROP had smaller cerebellar volumes (21.4±3.2 vs. 23.1±2.6 ml; P=0.04) and brainstem volumes (5.4±0.5 ml vs. 5.8±0.5 ml; P=0.01) compared with matched control infants. Furthermore, ROP patients showed a significantly lower development quotient (Griffiths Mental Development Scales) at the age of 15 months (93±15 vs. 102±10; P=0.01) and lower fine motor scores (10±3 vs. 12±2; P=0.02) on Bayley Scales (Third Edition) at the age of 24 months.ConclusionSevere ROP was associated with smaller volumes of the cerebellum and brainstem and with poorer early neurodevelopmental outcome. Follow-up through childhood is needed to evaluate the long-term consequences of our findings.

The emergence of functional architecture during early brain development.

Early human brain development constitutes a sequence of intricate processes resulting in the ontogeny of functionally operative neural circuits. Developmental trajectories of early brain network formation are genetically programmed and can be modified by epigenetic and environmental influences. Such alterations may exert profound effects on neurodevelopment, potentially persisting throughout the lifespan. This review focuses on the critical period of fetal and early postnatal brain development. Here we collate findings from neuroimaging studies, with a particular focus on functional MRI research that interrogated early brain network development in both health and high-risk or disease states. First, we will provide an overview of the developmental processes that take place from the embryonic period through early infancy in order to contextualize brain network formation. Second, functional brain network development in the typically developing brain will be discussed. Third, we will touch on prenatal and perinatal risk factors that may interfere with the trajectories of functional brain wiring, including prenatal substance exposure, maternal mental illness and preterm birth. Collectively, studies have revealed the blueprint of adult human brain organization to be present in the neonatal brain. Distinct attributes of human brain architecture have even been detected in the developing fetal brain from as early as 24 postconceptional weeks. During postnatal brain development, the brain's wiring pattern is further sculpted and modulated to become the full facsimile of the adult human brain, with functional brain network refinement being more rigorous than structural brain network maturation. Advances in neuroimaging techniques have paved the way towards a comprehensive understanding of the maturational pathways of brain network development and of how early developmental adversity may affect these trajectories. Such insights are fundamental for our understanding of human brain functioning, for early identification of infants at risk, as well as for future neuroprotective strategies.

Neonatal Surgery for Noncardiac Congenital Anomalies: Neonates at Risk of Brain Injury.

OBJECTIVE: To evaluate the incidence of brain injury after neonatal surgery for noncardiac congenital anomalies using magnetic resonance imaging (MRI). STUDY DESIGN: An MRI was obtained in 101 infants at 7 days [range: 1-115] after neonatal surgery for major noncardiac congenital anomalies. Brain injury was assessed using T1, T2, diffusion weighted imaging, and susceptibility-weighted imaging. RESULTS: Thirty-two preterm infants (<37 weeks of gestation) and 69 full-term infants were included. MRI abnormalities were found in 24 (75%) preterm and 40 (58%) full-term infants. Parenchymal lesions were noted in 23 preterm (72%) and 29 full-term infants (42%). These consisted of punctate white matter lesions (n = 45), punctate cerebellar lesions (n = 17), thalamic infarction (n = 5), and periventricular hemorrhagic infarction (n = 4). Nonparenchymal abnormalities were found in 9 (28%) preterm and 26 (38%) full-term infants. These included supra- and infratentorial subdural hemorrhages (n = 30), intraventricular hemorrhage grade II (n = 7), and asymptomatic sinovenous thrombosis (n = 1). A combination of parenchymal lesions was present in 21 infants. Of infants who had an MRI within 10 days after surgery, punctate white matter lesions were visible on diffusion weighted imaging in 22 (61%), suggestive of recent ischemic origin. Type of congenital anomaly and prematurity were most predictive of brain injury. CONCLUSIONS: Infants who have neonatal surgery for noncardiac congenital anomalies are at risk of brain injury, potentially accounting for the neurodevelopmental delay frequently observed in this population. Further research is warranted into potential mechanisms of brain injury and its timing of onset. Long-term neurodevelopmental follow-up is needed in this vulnerable population.

White matter maturation in the neonatal brain is predictive of school age cognitive capacities in children born very preterm.

AIM: To investigate the association between white matter organization in the neonatal brain and cognitive capacities at early school age in children born very preterm. METHOD: Thirty children born very preterm (gestational age median 27.5wks, interquartile range [IQR] 25.5-29.5; 18 males, 12 females) were included in this retrospective observational cohort study. Diffusion-weighted imaging (DWI) had been performed on a 3T system in the neonatal period (median 41.3 [IQR 40.0-42.6]wks) and cognitive functioning was formally assessed at age 5 years and 7 months (IQR 5.4-5.9y) using the Wechsler Preschool and Primary Scale of Intelligence. Structural connectivity maps were reconstructed from the DWI data using deterministic streamline tractography. Network metrics of global and local communication and mean fractional anisotropy of white matter pathways were related to IQ and processing speed at age 5 years using linear regression analyses. RESULTS: Mean fractional anisotropy was significantly related to Performance IQ at age 5 years (F=8.48, p=0.007). Findings persisted after adjustment for maternal education level. INTERPRETATION: Our findings provide evidence that the blueprint of later cognitive achievement is already present at term-equivalent age and suggest that white matter connectivity strength may be a valuable predictor for long-term cognitive functioning.

Brain Volumes at Term-Equivalent Age in Preterm Infants: Imaging Biomarkers for Neurodevelopmental Outcome through Early School Age.

OBJECTIVE: To evaluate the relationship between brain volumes at term and neurodevelopmental outcome through early school age in preterm infants. STUDY DESIGN: One hundred twelve preterm infants (born mean gestational age 28.6 ± 1.7 weeks) were studied prospectively with magnetic resonance imaging (imaged at mean 41.6 ± 1.0 weeks). T2- and T1-weighted images were automatically segmented, and volumes of 6 tissue types were related to neurodevelopmental outcome assessed using the Bayley Scales of Infant and Toddler Development, Third Edition (cognitive, fine, and gross motor scores) at 24 months corrected age (n = 112), Griffiths Mental Development Scales (developmental quotient) at age 3.5 years (n = 98), Movement Assessment Battery for Children, Second Edition (n = 85), and Wechsler Preschool and Primary Scale of Intelligence, Third Edition at age 5.5 years (n = 44). Corrections were made for intracranial volume, maternal education, and severe brain lesions. RESULTS: Ventricular volumes were negatively related to neurodevelopmental outcome at age 24 months and 3.5 years, as well as processing speed at age 5.5 years. Unmyelinated white matter (UWM) volume was positively associated with motor outcome at 24 months and with processing speed at age 5.5 years. Cortical gray matter (CGM) volume demonstrated a negative association with motor performance and cognition at 24 months and with developmental quotient at age 3.5 years. Cerebellar volume was positively related to cognition at these time points. Adjustment for brain lesions attenuated the relations between cerebellar and CGM volumes and cognition. CONCLUSIONS: Brain volumes of ventricles, UWM, CGM, and cerebellum may serve as biomarkers for neurodevelopmental outcome in preterm infants. The relationship between larger CGM volumes and adverse neurodevelopment may reflect disturbances in neuronal and/or axonal migration at the UWM-CGM boundary and warrants further investigation.

The Neonatal Connectome During Preterm Brain Development.

The human connectome is the result of an elaborate developmental trajectory. Acquiring diffusion-weighted imaging and resting-state fMRI, we studied connectome formation during the preterm phase of macroscopic connectome genesis. In total, 27 neonates were scanned at week 30 and/or week 40 gestational age (GA). Examining the architecture of the neonatal anatomical brain network revealed a clear presence of a small-world modular organization before term birth. Analysis of neonatal functional connectivity (FC) showed the early formation of resting-state networks, suggesting that functional networks are present in the preterm brain, albeit being in an immature state. Moreover, structural and FC patterns of the neonatal brain network showed strong overlap with connectome architecture of the adult brain (85 and 81%, respectively). Analysis of brain development between week 30 and week 40 GA revealed clear developmental effects in neonatal connectome architecture, including a significant increase in white matter microstructure (P < 0.01), small-world topology (P < 0.01) and interhemispheric FC (P < 0.01). Computational analysis further showed that developmental changes involved an increase in integration capacity of the connectivity network as a whole. Taken together, we conclude that hallmark organizational structures of the human connectome are present before term birth and subject to early development.

Neonatal brain oxygenation during thoracoscopic correction of esophageal atresia.

BACKGROUND: Little is known about the effects of carbon dioxide (CO2) insufflation on cerebral oxygenation during thoracoscopy in neonates. Near-infrared spectroscopy can measure perioperative brain oxygenation [regional cerebral oxygen saturation (rScO2)]. AIMS: To evaluate the effects of CO2 insufflation on rScO2 during thoracoscopic esophageal atresia (EA) repair. METHODS: This is an observational study during thoracoscopic EA repair with 5 mmHg CO2 insufflation pressure. Mean arterial blood pressure (MABP), arterial oxygen saturation (SaO2), partial pressure of arterial carbon dioxide (paCO2), pH, and rScO2 were monitored in 15 neonates at seven time points: baseline (T0), after anesthesia induction (T1), after CO2-insufflation (T2), before CO2-exsufflation (T3), and postoperatively at 6 (T4), 12 (T5), and 24 h (T6). RESULTS: MABP remained stable. SaO2 decreased from T0 to T2 [97 ± 3-90 ± 6 % (p < 0.01)]. PaCO2 increased from T0 to T2 [41 ± 6-54 ± 15 mmHg (p < 0.01)]. pH decreased from T0 to T2 [7.33 ± 0.04-7.25 ± 0.11 (p < 0.05)]. All parameters recovered during the surgical course. Mean rScO2 was significantly higher at T1 compared to T2 [77 ± 10-73 ± 7 % (p < 0.05)]. Mean rScO2 levels never dropped below a safety threshold of 55 %. CONCLUSION: The impact of neonatal thoracoscopic repair of EA with insufflation of CO2 at 5 mmHg was studied. Intrathoracic CO2 insufflation caused a reversible decrease in SaO2 and pH and an increase in paCO2. The rScO2 was higher at anesthesia induction but remained stable and within normal limits during and after the CO2 pneumothorax, which suggest no hampering of cerebral oxygenation by the thoracoscopic intervention. Future studies will focus on the long-term effects of this surgery on the developing brain.

Impact of nutrition on brain development and its neuroprotective implications following preterm birth.

The impact of nutrition on brain development in preterm infants has been increasingly appreciated. Early postnatal growth and nutrient intake have been demonstrated to influence brain growth and maturation with subsequent effects on neurodevelopment that persist into childhood and adolescence. Nutrition could also potentially protect against injury. Inflammation and perinatal infection play a crucial role in the pathogenesis of white matter injury, the most common pattern of brain injury in preterm infants. Therefore, nutritional components with immunomodulatory and/or anti-inflammatory effects may serve as neuroprotective agents. Moreover, growing evidence supports the existence of a microbiome-gut-brain axis. The microbiome is thought to interact with the brain through immunological, endocrine, and neural pathways. Consequently, nutritional components that may influence gut microbiota may also exert beneficial effects on the developing brain. Based on these properties, probiotics, prebiotic oligosaccharides, and certain amino acids are potential candidates for neuroprotection. In addition, the amino acid glutamine has been associated with a decrease in infectious morbidity in preterm infants. In conclusion, early postnatal nutrition is of major importance for brain growth and maturation. Additionally, certain nutritional components might play a neuroprotective role against white matter injury, through modulation of inflammation and infection, and may influence the microbiome-gut-brain axis.

The effects of CO2-insufflation with 5 and 10 mmHg during thoracoscopy on cerebral oxygenation and hemodynamics in piglets: an animal experimental study.

OBJECTIVE: To evaluate the effect of CO2-insufflation with 5 and 10 mmHg on cerebral oxygenation and hemodynamics in neonates. BACKGROUND: An increasing percentage of surgical interventions in neonates are performed by minimal invasive techniques. Recently, concerns have been raised regarding a decrease of cerebral oxygenation in neonates during thoracoscopy as a result of CO2-insufflation. METHODS: This was an animal experimental study. Piglets were anesthetized, intubated, ventilated, and surgically prepared for CO2-insufflation. Insufflation was done with 5 or 10 mmHg CO2 during 1 h. Arterial saturation (SaO2), heart rate (HR), mean arterial blood pressure (MABP), and cerebral oxygenation (rScO2) were monitored. CFTOE, an estimator of cerebral oxygen extraction ((SaO2 - rScO2)/SaO2)), was calculated. Arterial blood gases were drawn every 15': pre (T0), during (T1-T4) and after CO2-insufflation (T5). RESULTS: Ten piglets (4 kg) were randomized for 5 (P5) and 10 (P10) mmHg CO2-insufflation. Two P10 piglets needed resuscitation after insufflation, none P5. Linear mixed-effect modeling of paCO2, pH, and SaO2 showed that values were dependent on time and time squared (p < 0.001) but were not different between the 5 and 10 mmHg groups. Analysis demonstrated significant changes over time in heart rate and MABP between the 5 and 10 mmHg groups, with a significant higher heart rate and lower blood pressure in the 10 mmHg group (p < 0.001). For rScO2 and cFTOE, no group differences could be demonstrated, but a significant effect of time was found: rScO2 increased and cFTOE decreased (p < 0.001). CONCLUSIONS: Insufflation of CO2 during thoracoscopy with 10 mmHg caused more severe hemodynamic instability and seems to be related with a decrease of cerebral perfusion as represented by a higher oxygen extraction. CO2-insufflation of 5 mmHg for thoracoscopy seems to have no adverse effects on cerebral oxygenation.

Weight gain velocity in very low-birth-weight infants: effects of exposure to biological maternal sounds.

OBJECTIVE: To examine the effects of biological maternal sounds (BMS) on weight gain velocity in very low-birth-weight (VLBW) infants (≤ 1,500 g). STUDY DESIGN: An exploratory study with a matched-control design. A prospective cohort of VLBW infants exposed to attenuated recordings of BMS during their neonatal intensive care unit hospitalization were compared with retrospective controls matched 1:1 for sex, birth weight, gestational age, scores for neonatal acute physiology and perinatal extension (SNAPPE - II) scores (n = 32). RESULTS: A linear mixed model controlling for gestational age, chronic lung disease, and days to regain birth weight revealed that infants receiving BMS significantly improved their weight gain velocity compared matched controls (p < 0.001) during the neonatal period. No differences were found on days spent nothing by mouth (p = 0.18), days until full enteral feeds (p = 0.51), total fluid intake (p = 0.93), or caloric intake (p = 0.73). CONCLUSION: Exposure to BMS may improve weight gain velocity in VLBW infants. Further research is needed to evaluate the effectiveness of this noninvasive intervention during the neonatal period.

Cortical magnetization transfer abnormalities and connectome dysconnectivity in schizophrenia.

Macroscale dysconnectivity in schizophrenia is associated with neuropathological abnormalities. The extent to which alterations in cortical myelination as revealed in vivo by magnetization transfer ratio (MTR) are related to macroscale dysconnectivity remains unknown. We acquired magnetization transfer imaging (MTI) data and diffusion weighted imaging (DWI) data from 78 schizophrenia patients and 93 healthy controls for MTR extraction and connectome reconstruction to examine the possible link between cortical myelination and macroscale dysconnectivity. Our findings showed significant cortical MTR disruptions in several prefrontal areas in schizophrenia patients, including bilateral rostral middle frontal areas, right pars orbitalis, and right frontal pole. Furthermore, cortical MTR alterations between patients and controls were significantly correlated with the level of regional disconnectivity. Together, our findings provide evidence that microstructural neuropathological abnormalities in schizophrenia are predominately present in prefrontal areas of the cortex and are associated with alterations in structural connectome architecture at the whole brain network level.

Prediction of cognitive and motor outcome of preterm infants based on automatic quantitative descriptors from neonatal MR brain images.

This study investigates the predictive ability of automatic quantitative brain MRI descriptors for the identification of infants with low cognitive and/or motor outcome at 2-3 years chronological age. MR brain images of 173 patients were acquired at 30 weeks postmenstrual age (PMA) (n = 86) and 40 weeks PMA (n = 153) between 2008 and 2013. Eight tissue volumes and measures of cortical morphology were automatically computed. A support vector machine classifier was employed to identify infants who exhibit low cognitive and/or motor outcome (<85) at 2-3 years chronological age as assessed by the Bayley scales. Based on the images acquired at 30 weeks PMA, the automatic identification resulted in an area under the receiver operation characteristic curve (AUC) of 0.78 for low cognitive outcome, and an AUC of 0.80 for low motor outcome. Identification based on the change of the descriptors between 30 and 40 weeks PMA (n = 66) resulted in an AUC of 0.80 for low cognitive outcome and an AUC of 0.85 for low motor outcome. This study provides evidence of the feasibility of identification of preterm infants at risk of cognitive and motor impairments based on descriptors automatically computed from images acquired at 30 and 40 weeks PMA.

Brain oxygenation during laparoscopic correction of hypertrophic pyloric stenosis.

BACKGROUND: Concern remains about the safety of carbon dioxide (CO2) pneumoperitoneum (PP) in young infants having surgery for pyloric stenosis via laparoscopy. Interests here mainly focus on possible jeopardized organ perfusion and in particular brain oxygenation with possible adverse neurodevelopmental outcomes. The aim of this study was to investigate the intraoperative effects of CO2 gas PP on cerebral oxygenation during laparoscopic surgery for hypertrophic pyloric stenosis in young infants. PATIENTS AND METHODS: In this single-center prospective observational study, we investigated brain oxygenation in 12 young infants receiving laparoscopic pyloromyotomy with CO2 PP, with a pressure of 8 mm Hg and a flow rate of 5 L/minute. Intraoperative hemodynamic parameters and transcranial near-infrared spectroscopy to assess regional cerebral oxygen saturation (rScO2) were monitored continuously during the whole procedure. Parameters were analyzed in four intervals: before insufflation (T0), during (start [T1] and end [T2]), and after cessation (T3) of the CO2 PP. RESULTS: Blood pressure and end-tidal CO2 (etCO2) increased during the procedure: mean arterial pressure, 35±5 mm Hg at T0 to 43±9 mm Hg at T2; etCO2, 35±4 mm Hg at T0 to 40±3 mm Hg at T3. The rScO2 remained stable throughout the whole anesthetic period. In none of the patients did the rScO2 drop below the safety threshold of 55% (rScO2, 68±14% at T0 to 71±9% at T3). CONCLUSIONS: Our results indicate that a laparoscopic procedure with a CO2 PP of 8 mm Hg can be performed under safe anesthetic conditions in the presence of gradually increasing blood pressure and etCO2 without altering regional brain oxygenation levels.