Cerebral blood flow and oxygenation in infants after birth asphyxia. Clinically useful information?

Cerebral Blood Flow of the Neonatal Brain after Hypoxic-Ischemic Injury

OBJECTIVE

Hypoxic-ischemic encephalopathy (HIE) in infants can have long-term adverse neurodevelopmental effects and markedly reduce quality of life. Both the initial hypoperfusion and the subsequent rapid reperfusion can cause deleterious effects in brain tissue. Cerebral blood flow (CBF) assessment in newborns with HIE can help detect abnormalities in brain perfusion to guide therapy and prognosticate patient outcomes.

STUDY DESIGN

The review will provide an overview of the pathophysiological implications of CBF derangements in neonatal HIE, current and emerging techniques for CBF quantification, and the potential to utilize CBF as a physiologic target in managing neonates with acute HIE.

CONCLUSION

The alterations of CBF in infants during hypoxia-ischemia have been studied by using different neuroimaging techniques, including nitrous oxide and xenon clearance, transcranial Doppler ultrasonography, contrast-enhanced ultrasound, arterial spin labeling MRI, 18F-FDG positron emission tomography, near-infrared spectroscopy (NIRS), functional NIRS, and diffuse correlation spectroscopy. Consensus is lacking regarding the clinical significance of CBF estimations detected by these different modalities. Heterogeneity in the imaging modality used, regional versus global estimations of CBF, time for the scan, and variables impacting brain perfusion and cohort clinical characteristics should be considered when translating the findings described in the literature to routine practice and implementation of therapeutic interventions.

KEY POINTS

· Hypoxic-ischemic injury in infants can result in adverse long-term neurologic sequelae.. · Cerebral blood flow is a useful biomarker in neonatal hypoxic-ischemic injury.. · Imaging modality, variables affecting cerebral blood flow, and patient characteristics affect cerebral blood flow assessment..

A Scoping Review of Cerebral Doppler Arterial Waveforms in Infants

Cerebral Doppler ultrasound has been an important tool in pediatric diagnostics and prognostics for decades. Although the Doppler spectrum can provide detailed information on cerebral perfusion, the measured spectrum is often reduced to simple numerical parameters. To help pediatric clinicians recognize the visual characteristics of disease-associated Doppler spectra and identify possible areas for future research, a scoping review of primary studies on cerebral Doppler arterial waveforms in infants was performed. A systematic search in three online bibliographic databases yielded 4898 unique records. Among these, 179 studies included cerebral Doppler spectra for at least five infants below 1 y of age. The studies describe variations in the cerebral waveforms related to physiological changes (43%), pathology (62%) and medical interventions (40%). Characteristics were typically reported as resistance index (64%), peak systolic velocity (43%) or end-diastolic velocity (39%). Most studies focused on the anterior (59%) and middle (42%) cerebral arteries. Our review highlights the need for a more standardized terminology to describe cerebral velocity waveforms and for precise definitions of Doppler parameters. We provide a list of reporting variables that may facilitate unambiguous reports. Future studies may gain from combining multiple Doppler parameters to use more of the information encoded in the Doppler spectrum, investigating the full spectrum itself and using the possibilities for long-term monitoring with Doppler ultrasound.

Cerebral ischemia in the developing brain

Brain ischemia affects all ages, from neonates to the elderly population, and is a leading cause of mortality and morbidity. Multiple preclinical rodent models involving different ages have been developed to investigate the effect of ischemia during different times of key brain maturation events. Traditional models of developmental brain ischemia have focused on rodents at postnatal day 7-10, though emerging models in juvenile rodents (postnatal days 17-25) indicate that there may be fundamental differences in neuronal injury and functional outcomes following focal or global cerebral ischemia at different developmental ages, as well as in adults. Here, we consider the timing of injury in terms of excitation/inhibition balance, oxidative stress, inflammatory responses, blood brain barrier integrity, and white matter injury. Finally, we review translational strategies to improve function after ischemic brain injury, including new ideas regarding neurorestoration, or neural repair strategies that restore plasticity, at delayed time points after ischemia.

Diastolic Dysfunction in Neonates With Hypoxic-Ischemic Encephalopathy During Therapeutic Hypothermia: A Tissue Doppler Study

Diastolic dysfunction often complicates myocardial ischemia with increased mortality rates. However, less is known about diastolic function after perinatal asphyxia in neonates with hypoxic-ischemic encephalopathy (HIE) during therapeutic hypothermia (TH) and rewarming.

AIM

The aim of this study was to assess diastolic function with tissue Doppler imaging (TDI) in neonates with moderate-severe HIE during TH and rewarming.

METHOD

Newborns at >36 weeks' gestation with moderate-severe HIE treated with TH were evaluated with targeted neonatal echocardiography (TNE), including TDI, within 24 h of TH initiation (T1), at 48-72 h of treatment (T2), and after rewarming (T3). These retrospective data were collected and compared with a control group of healthy babies at >36 weeks' gestation that was prospectively evaluated following the same protocol.

RESULTS

A total of 21 patients with HIE + TH and 15 controls were included in the study. Myocardial relaxation before the onset of biventricular filling was prolonged in the HIE + TH group during TH with significantly longer isovolumic relaxation time (IVRT') in the left ventricle (LV), the septum, and the right ventricle (RV). This was associated with slower RV early diastolic velocity (e') and prolonged filling on T1. Total isovolumic time (t-IVT; isovolumic contraction time [IVCT'] + IVRT') and myocardial performance index (MPI') were globally increased in asphyxiated neonates. All these differences persisted after correction for heart rate (HR) and normalized after rewarming. TDI parameters assessing late diastole (a' velocity or e'/a' and E/e' ratios) did not differ between groups.

CONCLUSION

TDI evaluation in our study demonstrated a pattern of early diastolic dysfunction during TH that normalized after rewarming, whereas late diastole seemed to be preserved. Our data also suggest a possible involvement of impaired twist/untwist motion and dyssynchrony. More studies are needed to investigate the impact and therapeutic implication of diastolic dysfunction in these babies, as well as to clarify the role of TH in these findings.

Neuroprotection of the Perinatal Brain by Early Information of Cerebral Oxygenation and Perfusion Patterns

Abnormal patterns of cerebral perfusion/oxygenation are associated with neuronal damage. In preterm neonates, hypoxemia, hypo-/hypercapnia and lack of cerebral autoregulation are related to peri-intraventricular hemorrhages and white matter injury. Reperfusion damage after perinatal hypoxic ischemia in term neonates seems related with cerebral hyperoxygenation. Since biological tissue is transparent for near infrared (NIR) light, NIR-spectroscopy (NIRS) is a noninvasive bedside tool to monitor brain oxygenation and perfusion. This review focuses on early assessment and guiding abnormal cerebral oxygenation/perfusion patterns to possibly reduce brain injury. In term infants, early patterns of brain oxygenation helps to decide whether or not therapy (hypothermia) and add-on therapies should be considered. Further NIRS-related technical advances such as the use of (functional) NIRS allowing simultaneous estimation and integrating of heart rate, respiration rate and monitoring cerebral autoregulation will be discussed.

Cerebral blood flow velocity and oxygenation correlate predominantly with right ventricular function in cooled neonates with moderate-severe hypoxic-ischemic encephalopathy

The relationship between right ventricular (RV) function and cerebral blood flow (CBF) velocity and cerebral oxygenation was assessed in neonates with hypoxic-ischemic encephalopathy (HIE) treated with therapeutic hypothermia (TH). Echocardiographic, transcranial Doppler, and hemodynamic data from 37 neonates with moderate-severe HIE + TH were reviewed. Twenty healthy newborns served as controls. Cardiac dysfunction in HIE + TH was characterized by a predominant RV dysfunction, with concomitantly reduced CBF velocity. A significant correlation was found between CBF velocity and tricuspid annular plane systolic excursion (TAPSE), RV output (RVO), and stroke volume (SV_RV), as well as with left ventricular output and stroke volume. Brain oxygenation (rSO2) correlated significantly with RVO, SV_RV, TAPSE, ejection fraction, and fractional shortening, whereas cerebral fractional tissue oxygen extraction (FTOEc) correlated with RVO, SV_RV, RV myocardial performance index, and superior vena cava flow. CBF velocity and cerebral NIRS correlations were stronger with parameters of right ventricular performance.Conclusion: CBF velocity and brain oxygenation correlate predominantly with RV function in HIE + TH. This suggests a preferential contribution of RV performance to cerebral hemodynamics in this context. What is Known: • Neonates with hypoxic ischemic encephalopathy frequently exhibit alterations of cardiac function and cerebral blood flow. • These are considered organ-specific consequences of perinatal asphyxia. What is New: • We show that cerebral blood flow velocity and brain oxygenation are correlated predominantly with right ventricular function during therapeutic hypothermia. • This suggests a potential direct contribution of right ventricular performance to cerebral hemodynamics in this context.

Anoxic Brain Injury Detection with the Normalized Diffusion to ASL Perfusion Ratio: Implications for Blood-Brain Barrier Injury and Permeability

BACKGROUND AND PURPOSE

Anoxic brain injury is a result of prolonged hypoxia. We sought to describe the nonquantitative arterial spin-labeling perfusion imaging patterns of anoxic brain injury, characterize the relationship of arterial spin-labeling and DWI, and evaluate the normalized diffusion-to-perfusion ratio to differentiate patients with anoxic brain injury from healthy controls.

MATERIALS AND METHODS

We identified all patients diagnosed with anoxic brain injuries from 2002 to 2019. Twelve ROIs were drawn on arterial spin-labeling with coordinate-matched ROIs identified on DWI. Linear regression analysis was performed to examine the relationship between arterial spin-labeling perfusion and diffusion signal. Normalized diffusion-to-perfusion maps were generated using a custom-built algorithm.

RESULTS

Thirty-five patients with anoxic brain injuries and 34 healthy controls were identified. Linear regression analysis demonstrated a significant positive correlation between arterial spin-labeling and DWI signal. By means of a combinatory cutoff of slope of >0 and R² of > 0.78, linear regression using arterial spin-labeling and DWI showed a sensitivity of 0.86 (95% CI, 0.71-0.94) and specificity of 0.82 (95% CI, 0.66-0.92) for anoxic brain injuries. A normalized diffusion-to-perfusion color map demonstrated heterogeneous ratios throughout the brain in healthy controls and homogeneous ratios in patients with anoxic brain injuries.

CONCLUSIONS

In anoxic brain injuries, a homogeneously positive correlation between qualitative perfusion and DWI signal was identified so that areas of increased diffusion signal showed increased ASL signal. By exploiting this relationship, the normalized diffusion-to-perfusion ratio color map may be a valuable imaging biomarker for diagnosing anoxic brain injury and potentially assessing BBB integrity.

Changes in brain perfusion in successive arterial spin labeling MRI scans in neonates with hypoxic-ischemic encephalopathy

The primary objective of this study was to evaluate changes in cerebral blood flow (CBF) using arterial spin labeling MRI between day 4 of life (DOL4) and day 11 of life (DOL11) in neonates with hypoxic-ischemic encephalopathy (HIE) treated with hypothermia. The secondary objectives were to compare CBF values between the different regions of interest (ROIs) and between infants with ischemic lesions on MRI and infants with normal MRI findings.

We prospectively included all consecutive neonates with HIE admitted to the neonatal intensive care unit of our institution who were eligible for therapeutic hypothermia. Each neonate systematically underwent two MRI examinations as close as possible to day 4 (early MRI) and day 11 (late MRI) of life. A custom processing pipeline of morphological and perfusion imaging data adapted to neonates was developed to perform automated ROI analysis.

Twenty-eight neonates were included in the study between April 2015 and December 2017. There were 16 boys and 12 girls. Statistical analysis was finally performed on 37 MRIs, 17 early MRIs and 20 late MRIs. Eleven neonates had both early and late MRIs of good quality available. Eight out of 17 neonates (47%) had an abnormal on late MRI as performed and 7/20 neonates (35%) had an abnormal late MRI. CBF values in the basal ganglia and thalami (BGT) and temporal lobes were significantly higher on DOL4 than on DOL11. There were no significant differences between DOL4 and DOL11 for the other ROIs. CBF values were significantly higher in the BGT vs. the cortical GM, on both DOL4 and DOL11. On DOL4, the CBF was significantly higher in the cortical GM, the BGT, and the frontal and parietal lobes in subjects with an abnormal MRI compared to those with a normal MRI. On DOL11, CBF values in each ROI were not significantly different between the normal MRI group and the abnormal MRI group, except for the temporal lobes.

This article proposes an innovative processing pipeline for morphological and ASL data suited to neonates that enable automated segmentation to obtain CBF values over ROIs. We evaluate CBF on two successive scans within the first 15 days of life in the same subjects. ASL imaging in asphyxiated neonates seems more relevant when used relatively early, in the first days of life. The correlation of intra-subject changes in cerebral perfusion between early and late MRI with neurodevelopmental outcome warrants investigation in a larger cohort, to determine whether the CBF pattern change can provide prognostic information beyond that provided by visible structural abnormalities on conventional MRI.

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A processing pipeline suited to neonates was developed for automated ROI analysis.

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Basal ganglia and thalamic CBF values were significantly higher on DOL4 vs. DOL11.

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Neonates with abnormal morphological MRI had hyperperfusion in grey matter on DOL4.

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No perfusion differences were found on DOL11 between normal and abnormal MRIs.

The fetus at the tipping point: modifying the outcome of fetal asphyxia

Brain injury around birth is associated with nearly half of all cases of cerebral palsy. Although brain injury is multifactorial, particularly after preterm birth, acute hypoxia-ischaemia is a major contributor to injury. It is now well established that the severity of injury after hypoxia-ischaemia is determined by a dynamic balance between injurious and protective processes. In addition, mothers who are at risk of premature delivery have high rates of diabetes and antepartum infection/inflammation and are almost universally given treatments such as antenatal glucocorticoids and magnesium sulphate to reduce the risk of death and complications after preterm birth. We review evidence that these common factors affect responses to fetal asphyxia, often in unexpected ways. For example, glucocorticoid exposure dramatically increases delayed cell loss after acute hypoxia-ischaemia, largely through secondary hyperglycaemia. This critical new information is important to understand the effects of clinical treatments of women whose fetuses are at risk of perinatal asphyxia.

Applications of near infrared spectroscopy in the neonate

PURPOSE OF REVIEW

There has been a significant increase in the utilization of NIRS in neonatal care over the last few years, with some centers now routinely utilizing this monitoring technique for direct intervention at the bedside. In this review, we provide a summary of the most up-to-date evidence on near infrared spectroscopy utilization, with particular emphasis on measurement of cerebral oxygenation in preterm infants.

RECENT FINDINGS

There have been significant advances in the technology, leading to an increase in the number of available devices and in the use of this monitoring tool to reduce cerebral injury in preterm infants. The role of NIRS in assessing cerebral autoregulation in preterm and term infants, in evaluating somatic oxygenation, and in the management of newborns with hypoxic ischaemic encephalopathy is discussed.

SUMMARY

Two recent pilot randomized controlled trials highlight the potential of cerebral oxygenation monitoring to direct management in the delivery room and the neonatal intensive care unit. However, we urge caution against routine use and await the results of further studies in this area before considering this type of monitoring as standard of care.

Monitoring Cerebral Oxygenation in Neonates: An Update

Cerebral oxygenation is not always reflected by systemic arterial oxygenation. Therefore, regional cerebral oxygen saturation (rScO₂) monitoring with near-infrared spectroscopy (NIRS) is of added value in neonatal intensive care. rScO₂ represents oxygen supply to the brain, while cerebral fractional tissue oxygen extraction, which is the ratio between rScO₂ and systemic arterial oxygen saturation, reflects cerebral oxygen utilization. The balance between oxygen supply and utilization provides insight in neonatal cerebral (patho-)physiology. This review highlights the potential and limitations of cerebral oxygenation monitoring with NIRS in the neonatal intensive care unit.

Neuroimaging and Other Neurodiagnostic Tests in Neonatal Encephalopathy

Hypoxic-ischemic encephalopathy is associated with a high risk of morbidity and mortality in the neonatal period. Long-term neurodevelopmental disability is also frequent in survivors. Conventional MRI defines typical patterns of injury that reflect specific pathophysiologic mechanisms. Advanced magnetic resonance techniques now provide unique perspectives on neonatal brain metabolism, microstructure, and connectivity. The application of these imaging techniques has revealed that brain injury commonly occurs at or near the time of birth and evolves over the first weeks of life. Amplitude-integrated electroencephalogram and near-infrared spectroscopy are increasingly used as bedside tools in neonatal intensive care units to monitor brain function.

Cerebral oximetry in preterm infants: an agenda for research with a clear clinical goal

Preterm birth constitutes a major cause of death before 5 years of age and it is a major cause of neurodevelopmental impairment across the world. Preterm infants are most unstable during the transition between fetal and newborn life during the first days of life and most brain damage occurs in this period. The brain of the preterm infant is accessible for tissue oximetry by near-infrared spectroscopy. Cerebral oximetry has the potential to improve the long-term outcome by helping to tailor the support of respiration and circulation to the individual infant's needs, but the evidence is still lacking. The goals for research include testing the benefit and harms of cerebral oximetry in large-scale randomized trials, improved definition of the hypoxic threshold, better understanding the effects of intensive care on cerebral oxygenation, as well as improved precision of oximeters and calibration among devices or standardization of values in the hypoxic range. These goals can be pursued in parallel.

In vivo Monitoring of Cerebral Hemodynamics in the Immature Rat: Effects of Hypoxia-Ischemia and Hypothermia

BACKGROUND

Neonatal hypoxic-ischemic (HI) encephalopathy occurs in 1-4 per 1,000 live term births and can cause devastating neurodevelopmental disabilities. Currently, therapeutic hypothermia (TH) is the only treatment with proven efficacy. Since TH is associated with decreased cerebral metabolism and cerebral blood flow (CBF), it is important to assess CBF at the bedside. Diffuse correlation spectroscopy (DCS) has emerged as a promising optical modality to noninvasively assess an index of CBF (CBFi) in both humans and animals. In this initial descriptive study, we employ DCS to monitor the evolution of CBFi following HI with or without TH in immature rats. We investigate potential relationships between CBF and subsequent cerebral damage.

METHODS

HI was induced on postnatal day 10 or 11 rat pups by right common carotid artery ligation followed by 60-70 min hypoxia (8% oxygen). After HI, the pups recovered for 4 h under hypothermia (HI-TH group, n = 23) or normothermia (HI-N group, n = 23). Bilateral measurements of hemispheric CBFi were made with DCS in unanesthetized animals at baseline, before HI, and 0, 1, 2, 3, 4, 5, and 24 h after HI. The animals were sacrificed at either 1 or 4 weeks, and brain injury was scored on an ordinal scale of 0-5 (0 = no injury).

RESULTS

Carotid ligation caused moderate bilateral decreases in CBFi. Following HI, an initial hyperemia was observed that was more prominent in the contralateral hemisphere. After initiation of TH, CBFi dropped significantly below baseline levels and remained reduced for the duration of TH. In contrast, CBFi in the HI-N group was not significantly decreased from baseline levels. Reductions in CBFi after 4 h of TH were not associated with reduced damage at 1 or 4 weeks. However, elevated ipsilateral CBFi and ipsilateral-to-contralateral CBFi ratios at 24 h were associated with worse outcome at 1 week after HI.

CONCLUSIONS

Both HI and TH alter CBFi, with significant differences in CBFi between hypothermic and normothermic groups after HI. CBFi may be a useful biomarker of subsequent cerebral damage.