Cerebral blood flow and cerebrovascular autoregulation in a swine model of pediatric cardiac arrest and hypothermia

Temperature Control in the Era of Personalized Medicine: Knowledge Gaps, Research Priorities, and Future Directions

Hypoxic-ischemic brain injury (HIBI) is the leading cause of death and disability after cardiac arrest. To date, temperature control is the only intervention shown to improve neurologic outcomes in patients with HIBI. Despite robust preclinical evidence supporting hypothermia as neuroprotective therapy after cardiac arrest, there remains clinical equipoise regarding optimal core temperature, therapeutic window, and duration of therapy. Current guidelines recommend continuous temperature monitoring and active fever prevention for at least 72 h and additionally note insufficient evidence regarding temperature control targeting 32 °C-36 °C. However, population-based thresholds may be inadequate to support the metabolic demands of ischemic, reperfused, and dysregulated tissue. Promoting a more personalized approach with individualized targets has the potential to further improve outcomes. This review will analyze current knowledge and evidence, address research priorities, explore the components of high-quality temperature control, and define critical future steps that are needed to advance patient-centered care for cardiac arrest survivors.

Monitoring of cerebral blood flow autoregulation: physiologic basis, measurement, and clinical implications

Cerebral blood flow (CBF) autoregulation is the physiologic process whereby blood supply to the brain is kept constant over a range of cerebral perfusion pressures ensuring a constant supply of metabolic substrate. Clinical methods for monitoring CBF autoregulation were first developed for neurocritically ill patients and have been extended to surgical patients. These methods are based on measuring the relationship between cerebral perfusion pressure and surrogates of CBF or cerebral blood volume (CBV) at low frequencies (<0.05 Hz) of autoregulation using time or frequency domain analyses. Initially intracranial pressure monitoring or transcranial Doppler assessment of CBF velocity was utilised relative to changes in cerebral perfusion pressure or mean arterial pressure. A more clinically practical approach utilising filtered signals from near infrared spectroscopy monitors as an estimate of CBF has been validated. In contrast to the traditional teaching that 50 mm Hg is the autoregulation threshold, these investigations have found wide interindividual variability of the lower limit of autoregulation ranging from 40 to 90 mm Hg in adults and 20-55 mm Hg in children. Observational data have linked impaired CBF autoregulation metrics to adverse outcomes in patients with traumatic brain injury, ischaemic stroke, subarachnoid haemorrhage, intracerebral haemorrhage, and in surgical patients. CBF autoregulation monitoring has been described in both cardiac and noncardiac surgery. Data from a single-centre randomised study in adults found that targeting arterial pressure during cardiopulmonary bypass to above the lower limit of autoregulation led to a reduction of postoperative delirium and improved memory 1 month after surgery compared with usual care. Together, the growing body of evidence suggests that monitoring CBF autoregulation provides prognostic information on eventual patient outcomes and offers potential for therapeutic intervention. For surgical patients, personalised blood pressure management based on CBF autoregulation data holds promise as a strategy to improve patient neurocognitive outcomes.

Cerebral Autoregulation: A Target for Improving Neurological Outcomes in Extracorporeal Life Support

Despite improvements in survival after illnesses requiring extracorporeal life support, cerebral injury continues to hinder successful outcomes. Cerebral autoregulation (CA) is an innate protective mechanism that maintains constant cerebral blood flow in the face of varying systemic blood pressure. However, it is impaired in certain disease states and, potentially, following initiation of extracorporeal circulatory support. In this review, we first discuss patient-related factors pertaining to venovenous and venoarterial extracorporeal membrane oxygenation (ECMO) and their potential role in CA impairment. Next, we examine factors intrinsic to ECMO that may affect CA, such as cannulation, changes in pulsatility, the inflammatory and adaptive immune response, intracranial hemorrhage, and ischemic stroke, in addition to ECMO management factors, such as oxygenation, ventilation, flow rates, and blood pressure management. We highlight potential mechanisms that lead to disruption of CA in both pediatric and adult populations, the challenges of measuring CA in these patients, and potential associations with neurological outcome. Altogether, we discuss individualized CA monitoring as a potential target for improving neurological outcomes in extracorporeal life support.

Intracranial Pressure-Derived Cerebrovascular Reactivity Indices and Their Critical Thresholds: A Canadian High Resolution-Traumatic Brain Injury Validation Study

Current neurointensive care guidelines recommend intracranial pressure (ICP) and cerebral perfusion pressure (CPP) centered management for moderate-severe traumatic brain injury (TBI) because of their demonstrated associations with patient outcome. Cerebrovascular reactivity metrics, such as the pressure reactivity index (PRx), pulse amplitude index (PAx), and RAC index, have also demonstrated significant prognostic capabilities with regard to outcome. However, critical thresholds for cerebrovascular reactivity indices have only been identified in two studies conducted at the same center. In this study, we aim to determine the critical thresholds of these metrics by leveraging a unique multi-center database. The study included a total of 354 patients from the CAnadian High-Resolution TBI (CAHR-TBI) Research Collaborative. Based on 6-month Glasgow Outcome Scores, patients were dichotomized into alive versus dead and favorable versus unfavorable. Chi-square values were then computed for incrementally increasing values of each physiological parameter of interest against outcome. The values that generated the greatest chi-squares for each parameter were considered to be the thresholds with the greatest outcome discriminatory capacity. To confirm that the identified thresholds provide prognostic utility, univariate and multivariable logistical regression analyses were performed adjusting for the International Mission for Prognosis and Analysis of Clinical Trials (IMPACT) variables. Through the chi-square analysis, a lower limit CPP threshold of 60 mm Hg and ICP thresholds of 18 mm Hg and 22 mm Hg were identified for both survival and favorable outcome predictions. For the cerebrovascular reactivity metrics, different thresholds were identified for the two outcome dichotomizations. For survival prediction, thresholds of 0.35, 0.25, and 0 were identified for PRx, PAx, and RAC, respectively. For favorable outcome prediction, thresholds of 0.325, 0.20, and 0.05 were found. Univariate logistical regression analysis demonstrated that the time spent above/below thresholds were associated with outcome. Further, multivariable logistical regression analysis found that percent time above/below the identified thresholds added additional variance to the IMPACT core model for predicting both survival and favorable outcome. In this study, we were able to validate the results of the previous two works as well as to reaffirm the ICP and CPP guidelines from the Brain Trauma Foundation (BTF) and the Seattle International Severe Traumatic Brain Injury Consensus Conference (SIBICC).

Prognostic value of near-infrared spectroscopy regional oxygen saturation and cerebrovascular reactivity index in acute traumatic neural injury: a CAnadian High-Resolution Traumatic Brain Injury (CAHR-TBI) Cohort Study

BACKGROUND

Near-infrared spectroscopy regional cerebral oxygen saturation (rSO2) has gained interest as a raw parameter and as a basis for measuring cerebrovascular reactivity (CVR) due to its noninvasive nature and high spatial resolution. However, the prognostic utility of these parameters has not yet been determined. This study aimed to identify threshold values of rSO2 and rSO2-based CVR at which outcomes worsened following traumatic brain injury (TBI).

METHODS

A retrospective multi-institutional cohort study was performed. The cohort included TBI patients treated in four adult intensive care units (ICU). The cerebral oxygen indices, COx (using rSO2 and cerebral perfusion pressure) as well as COx_a (using rSO2 and arterial blood pressure) were calculated for each patient. Grand mean thresholds along with exposure-based thresholds were determined utilizing sequential chi-squared analysis and univariate logistic regression, respectively.

RESULTS

In the cohort of 129 patients, there was no identifiable threshold for raw rSO2 at which outcomes were found to worsen. For both COx and COx_a, an optimal grand mean threshold value of 0.2 was identified for both survival and favorable outcomes, while percent time above - 0.05 was uniformly found to have the best discriminative value.

CONCLUSIONS

In this multi-institutional cohort study, raw rSO2was found to contain no significant prognostic information. However, rSO2-based indices of CVR, COx and COx_a, were found to have a uniform grand mean threshold of 0.2 and exposure-based threshold of - 0.05, above which clinical outcomes markedly worsened. This study lays the groundwork to transition to less invasive means of continuously measuring CVR.

Regional disparity in continuously measured time-domain cerebrovascular reactivity indices: a scoping review of human literature

Objective: Cerebral blood vessels maintaining relatively constant cerebral blood flow (CBF) over wide range of systemic arterial blood pressure (ABP) is referred to as cerebral autoregulation (CA). Impairments in CA expose the brain to pressure-passive flow states leading to hypoperfusion and hyperperfusion. Cerebrovascular reactivity (CVR) metrics refer to surrogate metrics of pressure-based CA that evaluate the relationship between slow vasogenic fluctuations in cerebral perfusion pressure/ABP and a surrogate for pulsatile CBF/cerebral blood volume.Approach: We performed a systematically conducted scoping review of all available human literature examining the association between continuous CVR between more than one brain region/channel using the same CVR index.Main Results: In all the included 22 articles, only handful of transcranial doppler (TCD) and near-infrared spectroscopy (NIRS) based metrics were calculated for only two brain regions/channels. These metrics found no difference between left and right sides in healthy volunteer, cardiac surgery, and intracranial hemorrhage patient studies. In contrast, significant differences were reported in endarterectomy, and subarachnoid hemorrhage studies, while varying results were found regarding regional disparity in stroke, traumatic brain injury, and multiple population studies.Significance: Further research is required to evaluate regional disparity using NIRS-based indices and to understand if NIRS-based indices provide better regional disparity information than TCD-based indices.

High spatial and temporal resolution cerebrovascular reactivity for humans and large mammals: A technological description of integrated fNIRS and niABP mapping system

Introduction: The process of cerebral vessels maintaining cerebral blood flow (CBF) fairly constant over a wide range of arterial blood pressure is referred to as cerebral autoregulation (CA). Cerebrovascular reactivity is the mechanism behind this process, which maintains CBF through constriction and dilation of cerebral vessels. Traditionally CA has been assessed statistically, limited by large, immobile, and costly neuroimaging platforms. However, with recent technology advancement, dynamic autoregulation assessment is able to provide more detailed information on the evolution of CA over long periods of time with continuous assessment. Yet, to date, such continuous assessments have been hampered by low temporal and spatial resolution systems, that are typically reliant on invasive point estimations of pulsatile CBF or cerebral blood volume using commercially available technology. Methods: Using a combination of multi-channel functional near-infrared spectroscopy and non-invasive arterial blood pressure devices, we were able to create a system that visualizes CA metrics by converting them to heat maps drawn on a template of human brain. Results: The custom Python heat map module works in "offline" mode to visually portray the CA index per channel with the use of colourmap. The module was tested on two different mapping grids, 8 channel and 24 channel, using data from two separate recordings and the Python heat map module was able read the CA indices file and represent the data visually at a preselected rate of 10 s. Conclusion: The generation of the heat maps are entirely non-invasive, with high temporal and spatial resolution by leveraging the recent advances in NIRS technology along with niABP. The CA mapping system is in its initial stage and development plans are ready to transform it from "offline" to real-time heat map generation.

Early evolution of glial morphology and inflammatory cytokines following hypoxic-ischemic injury in the newborn piglet brain

Neuroinflammation is a hallmark of hypoxic-ischemic injury and can be characterized by the activation of glial cells and the expression of inflammatory cytokines and chemokines. Interleukin (IL)-1β and tumor necrosis factor (TNF)α are among the best-characterized early response cytokines and are often expressed concurrently. Several types of central nervous system cells secrete IL-1β and TNFα, including microglia, astrocytes, and neurons, and these cytokines convey potent pro-inflammatory actions. Chemokines also play a central role in neuroinflammation by controlling inflammatory cell trafficking. Our aim was to characterise the evolution of early neuroinflammation in the neonatal piglet model of hypoxic-ischemic encephalopathy (HIE). Piglets (< 24 h old) were exposed to HI insult, and recovered to 2, 4, 8, 12 or 24H post-insult. Brain tissue from the frontal cortex and basal ganglia was harvested for assessment of glial cell activation profiles and transcription levels of inflammatory markers in HI piglets with comparison to a control group of newborn piglets. Fluorescence microscopy was used to observe microglia, astrocytes, neurons, degenerating neurons and possibly apoptotic cells, and quantitative polymerase chain reaction was used to measure gene expression of several cytokines and chemokines. HI injury was associated with microglial activation and morphological changes to astrocytes at all time points examined. Gene expression analyses of inflammation-related markers revealed significantly higher expression of pro-inflammatory cytokines tumor necrosis factor-α (TNFα) and interleukin 1 beta (IL-1β), chemokines cxc-chemokine motif ligand (CXCL)8 and CXCL10, and anti-inflammatory cytokine transforming growth factor (TGF)β in every HI group, with some region-specific differences noted. No significant difference was observed in the level of C-X-C chemokine receptor (CCR)5 over time. This high degree of neuroinflammation was associated with a reduction in the number of neurons in piglets at 12H and 24H in the frontal cortex, and the putamen at 12H. This reduction of neurons was not associated with increased numbers of degenerating neurons or potentially apoptotic cells. HI injury triggered a robust early neuroinflammatory response associated with a reduction in neurons in cortical and subcortical regions in our piglet model of HIE. This neuroinflammatory response may be targeted using novel therapeutics to reduce neuropathology in our piglet model of neonatal HIE.

Association of prehospital hypotension depth and dose with survival following out-of-hospital cardiac arrest

INTRODUCTION

Hypotension following resuscitation from out-of-hospital cardiac arrest (OHCA) may cause harm by exacerbating secondary brain injury; however, limited research has explored this relationship. Our objective was to examine the association between duration and depth of prehospital post return of spontaneous circulation (ROSC) hypotension and survival.

METHODS

We utilized the 2019 and 2020 ESO Data Collaborative public use research data sets for this study (ESO, Austin, TX). Hypotension dose (mmHg*min.), average prehospital systolic blood pressure (SBP), and lowest recorded prehospital SBP were calculated. The association of these measures with survival to home (STH) and rearrest were explored using multivariable logistic regression. Time to hypotension resolution analyses by hypotension management strategy (push dose vasopressors, vasopressor infusion, or fluid only) were conducted using adjusted Cox proportional hazards models.

RESULTS

17,280 OHCA patients met inclusion criteria, of which 3,345 had associated hospital outcome data. Over one-third (37.8%; 6,526/17,280) of all patients had at least one recorded SBP below 90 mmHg. When modeled continuously, average prehospital SBP (1.19 [1.15, 1.23] per 10 mmHg), lowest prehospital SBP (1.20 [1.17, 1.24] per 10 mmHg), and hypotension dose (0.995 [0.993, 0.996] per mmHg*min.) were independently associated with STH. Differences in hypotension management were not associated with differences in survival or time to hypotension resolution.

CONCLUSION

Severity and duration of hypotension were significantly associated with worse outcomes in this dataset. Defining a threshold for hypotension requiring treatment above the classical SBP threshold of 90 mmHg may be warranted in the setting of prehospital post-resuscitation care.

The utility of therapeutic hypothermia on cerebral autoregulation

Cerebral autoregulation (CA) dysfunction is a strong predictor of clinical outcome in patients with acute brain injury (ABI). CA dysfunction is a potential pathologic defect that may lead to secondary injury and worse functional outcomes. Early therapeutic hypothermia (TH) in patients with ABI is controversial. Many factors, including patient selection, timing, treatment depth, duration, and rewarming strategy, impact its clinical efficacy. Therefore, optimizing the benefit of TH is an important issue. This paper reviews the state of current research on the impact of TH on CA function, which may provide the basis and direction for CA-oriented target temperature management.

Cerebral Autoregulation Monitoring in Traumatic Brain Injury: An Overview of Recent Advances in Personalized Medicine

Impaired cerebral autoregulation (CA) in moderate/severe traumatic brain injury (TBI) has been identified as a strong associate with poor long-term outcomes, with recent data highlighting its dominance over cerebral physiologic dysfunction seen in the acute phase post injury. With advances in bedside continuous cerebral physiologic signal processing, continuously derived metrics of CA capacity have been described over the past two decades, leading to improvements in cerebral physiologic insult detection and development of novel personalized approaches to TBI care in the intensive care unit (ICU). This narrative review focuses on highlighting the concept of continuous CA monitoring and consequences of impairment in moderate/severe TBI. Further, we provide a comprehensive description and overview of the main personalized cerebral physiologic targets, based on CA monitoring, that are emerging as strong associates with patient outcomes. CA-based personalized targets, such as optimal cerebral perfusion pressure (CPPopt), lower/upper limit of regulation (LLR/ULR), and individualized intra-cranial pressure (iICP) are positioned to change the way we care for TBI patients in the ICU, moving away from the "one treatment fits all" paradigm of current guideline-based therapeutic approaches, towards a true personalized medicine approach tailored to the individual patient. Future perspectives regarding research needs in this field are also discussed.

Autoregulation of Cerebral Blood Flow During 3-h Continuous Cardiopulmonary Resuscitation at 27°C

Introduction: Victims of accidental hypothermia in hypothermic cardiac arrest (HCA) may survive with favorable neurologic outcome if early and continuous prehospital cardiopulmonary resuscitation (CPR) is started and continued during evacuation and transport. The efficacy of cerebral autoregulation during hypothermic CPR is largely unknown and is aim of the present experiment. Methods: Anesthetized pigs (n = 8) were surface cooled to HCA at 27°C before 3 h continuous CPR. Central hemodynamics, cerebral O₂ delivery (DO₂) and uptake (VO₂), cerebral blood flow (CBF), and cerebral perfusion pressure (CPP) were determined before cooling, at 32°C and at 27°C, then at 15 min after the start of CPR, and hourly thereafter. To estimate cerebral autoregulation, the static autoregulatory index (sARI), and the CBF/VO₂ ratio were determined. Results: After the initial 15-min period of CPR at 27°C, cardiac output (CO) and mean arterial pressure (MAP) were reduced significantly when compared to corresponding values during spontaneous circulation at 27°C (-66.7% and -44.4%, respectively), and remained reduced during the subsequent 3-h period of CPR. During the first 2-h period of CPR at 27°C, blood flow in five different brain areas remained unchanged when compared to the level during spontaneous circulation at 27°C, but after 3 h of CPR blood flow in 2 of the 5 areas was significantly reduced. Cooling to 27°C reduced cerebral DO₂ by 67.3% and VO₂ by 84.4%. Cerebral VO₂ was significantly reduced first after 3 h of CPR. Cerebral DO₂ remained unaltered compared to corresponding levels measured during spontaneous circulation at 27°C. Cerebral autoregulation was preserved (sARI > 0.4), at least during the first 2 h of CPR. Interestingly, the CBF/VO₂ ratio during spontaneous circulation at 27°C indicated the presence of an affluent cerebral DO₂, whereas after CPR, the CBF/VO₂ ratio returned to the level of spontaneous circulation at 38°C. Conclusion: Despite a reduced CO, continuous CPR for 3 h at 27°C provided sufficient cerebral DO₂ to maintain aerobic metabolism and to preserve cerebral autoregulation during the first 2-h period of CPR. This new information supports early start and continued CPR in accidental hypothermia patients during rescue and transportation for in hospital rewarming.

Continuous Time-Domain Cerebrovascular Reactivity Metrics and Discriminate Capacity for the Upper and Lower Limits of Autoregulation: A Scoping Review of the Animal Literature

Over a wide range of systemic arterial pressures, cerebral blood flow (CBF) is regulated fairly constantly by the cerebral vessels in a process termed cerebral autoregulation (CA), which is depicted by the Lassen autoregulatory curve. After traumatic brain injury (TBI), CA can get impaired and these impairments manifest in changes of the Lassen autoregulatory curve. Continuous surrogate metrics of pressure-based CA, termed cerebrovascular reactivity (CVR) metrics, evaluate the relationship between slow vasogenic fluctuations in a driving pressure for cerebral blood flow, and the most commonly studied and utilized measures are based in the time domain and have been increasingly applied in bedside TBI care and have sparked the investigation of individualized cerebral perfusion pressure targets. However, not all CVR metrics have been validated as true measures of autoregulation in the pre-clinical setting. We reviewed all available pre-clinical animal literature that assessed the association between continuous time-domain metrics of CVR and some aspect of the Lassen autoregulatory curve. All 15 articles found associated the evaluated continuous metrics to the lower limit of autoregulation curve whereas none looked at the upper limit. Most of the evaluated metrics showed the ability to discriminate the lower limit of autoregulation with various methods of perturbation. Further work is required to evaluate the utility of such surrogate measures against the upper limit of autoregulation, while also providing validation to the existing literature supporting specific indices and their ability to discriminate the lower limit.

Early Use of Transcranial Doppler Ultrasonography to Stratify Neonatal Encephalopathy

BACKGROUND

The dynamic nature of neonatal hypoxic-ischemic encephalopathy (HIE) after birth necessitates reliable biomarkers to identify infants with evolving brain injury. This prospective cohort aims to use serial Doppler ultrasonography (US) to measure cerebral blood flow velocity and resistance index (RI) to help detect the time and evolution of the clinical encephalopathy.

METHODS

A total of 60 neonates were enrolled all ≥36 weeks' gestation with perinatal acidemia, defined as a blood gas pH ≤ 7.0 or base deficit ≥16 mmol/L and encephalopathy including a matched control group without encephalopathy. Each neonate received one to three serial Doppler recordings starting at six to 24 hours of life. Mean RI ≤ 0.55 was considered abnormal.

RESULTS

Mean RIs obtained shortly after birth were significantly lower with increasing severity of encephalopathy. On the first Doppler recordings, abnormal mean RIs were seen in 11 of 18 (61%) neonates with mild, 13 of 17 (76%) with moderate, and two of two (100%) with severe HIE. Of the neonates with mild HIE and abnormal mean RIs, congruity abnormal amplitude electroencephalography (45%), brain magnetic resonance imaging (45%), and abnormal head ultrasound (44%) are here reported.

CONCLUSIONS

Doppler measurements can provide bedside adjunct biomarkers indicating the time and severity of neonatal HIE.

Cerebral autoregulation in anoxic brain injury patients treated with targeted temperature management

Background

Little is known about the prevalence of altered CAR in anoxic brain injury and the association with patients’ outcome. We aimed at investigating CAR in cardiac arrest survivors treated by targeted temperature management and its association to outcome.

Methods

Retrospective analysis of prospectively collected data. Inclusion criteria: adult cardiac arrest survivors treated by targeted temperature management (TTM). Exclusion criteria: trauma; sepsis, intoxication; acute intra-cranial disease; history of supra-aortic vascular disease; severe hemodynamic instability; cardiac output mechanical support; arterial carbon dioxide partial pressure (PaCO₂) > 60 mmHg; arrhythmias; lack of acoustic window. Middle cerebral artery flow velocitiy (FV) was assessed by transcranial Doppler (TCD) once during hypothermia (HT) and once during normothermia (NT). FV and blood pressure (BP) were recorded simultaneously and Mxa calculated (MATLAB). Mxa is the Pearson correlation coefficient between FV and BP. Mxa > 0.3 defined altered CAR. Survival was assessed at hospital discharge. Cerebral Performance Category (CPC) 3–5 assessed 3 months after CA defined unfavorable neurological outcome (UO).

Results

We included 50 patients (Jan 2015–Dec 2018). All patients had out-of-hospital cardiac arrest, 24 (48%) had initial shockable rhythm. Time to return of spontaneous circulation was 20 [10–35] min. HT (core body temperature 33.7 [33.2–34] °C) lasted for 24 [23–28] h, followed by rewarming and NT (core body temperature: 36.9 [36.6–37.4] °C). Thirty-one (62%) patients did not survive at hospital discharge and 36 (72%) had UO. Mxa was lower during HT than during NT (0.33 [0.11–0.58] vs. 0.58 [0.30–0.83]; p = 0.03). During HT, Mxa did not differ between outcome groups. During NT, Mxa was higher in patients with UO than others (0.63 [0.43–0.83] vs. 0.31 [− 0.01–0.67]; p = 0.03). Mxa differed among CPC values at NT (p = 0.03). Specifically, CPC 2 group had lower Mxa than CPC 3 and 5 groups. At multivariate analysis, initial non-shockable rhythm, high Mxa during NT and highly malignant electroencephalography pattern (HMp) were associated with in-hospital mortality; high Mxa during NT and HMp were associated with UO.

Conclusions

CAR is frequently altered in cardiac arrest survivors treated by TTM. Altered CAR during normothermia was independently associated with poor outcome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40560-021-00579-z.

A method to evaluate dynamic cerebral pressure-flow relationships in the conscious rat

The classic dogma of cerebral autoregulation is that cerebral blood flow is steadily maintained across a wide range of perfusion pressures. This has been challenged by recent studies suggesting little to no "autoregulatory plateau" in the relationship between cerebral blood flow and blood pressure (BP). Therefore, the mechanisms underlying the cerebral pressure-flow relationship still require further understanding. Here, we present a novel approach to examine dynamic cerebral autoregulation in conscious Wistar rats (n = 16) instrumented to measure BP and internal carotid blood flow (iCBF), as an indicator of cerebral blood flow. Transient reductions in BP were induced by occluding the vena cava via inflation of a chronically implanted intravascular silicone balloon. Falls in BP were paralleled by progressive decreases in iCBF, with no evidence of a steady-state plateau. No significant changes in internal carotid vascular resistance (iCVR) were observed. In contrast, intravenous infusions of the vasoactive drug sodium nitroprusside (SNP) produced a similar fall in BP but increases in iCBF and decreases in iCVR were observed. These data suggest a considerable confounding influence of vasodilatory drugs such as SNP on cerebrovascular tone in the rat, making them unsuitable to investigate cerebral autoregulation. We demonstrate that our technique of transient vena cava occlusion produced reliable and repeatable depressor responses, highlighting the potential for our approach to permit assessment of the dynamic cerebral pressure-flow relationship over time in conscious rats.NEW & NOTEWORTHY We present a novel technique to overcome the use of vasoactive agents when studying cerebrovascular dynamics in the conscious rat. Our method of vena cava occlusion to reduce BP was associated with decreased iCBF and no change in iCVR. In contrast, comparable BP falls with intravenous SNP increased iCBF and reduced iCVR. Thus, the dynamic cerebral pressure-flow relationship shows a narrower, less level autoregulatory plateau than conventionally thought. We confirm our method allows repeatable assessment of cerebrovascular dynamics in conscious rats.

Bedside and laboratory neuromonitoring in neonatal encephalopathy

Several bedside and laboratory neuromonitoring tools are currently used in neonatal encephalopathy (NE) to assess 1) brain function [amplitude-integrated electroencephalogram (aEEG) and EEG], 2) cerebral oxygenation delivery and consumption [near-infrared spectroscopy (NIRS)] and 3) blood and cerebrospinal fluid biomarkers. The aim of the review is to provide the role of neuromonitoring in understanding the development of brain injury in these newborns and better predict their long-term outcome. Simultaneous use of these monitoring modalities may improve our ability to provide meaningful prognostic information regarding ongoing treatments. Evidence will be summarized in this review for each of these modalities, by describing (1) the methods, (2) the clinical evidence in context of NE both before and with hypothermia, and (3) the research and future directions.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Determining Thresholds for Three Indices of Autoregulation to Identify the Lower Limit of Autoregulation During Cardiac Surgery

OBJECTIVES

Monitoring cerebral autoregulation may help identify the lower limit of autoregulation in individual patients. Mean arterial blood pressure below lower limit of autoregulation appears to be a risk factor for postoperative acute kidney injury. Cerebral autoregulation can be monitored in real time using correlation approaches. However, the precise thresholds for different cerebral autoregulation indexes that identify the lower limit of autoregulation are unknown. We identified thresholds for intact autoregulation in patients during cardiopulmonary bypass surgery and examined the relevance of these thresholds to postoperative acute kidney injury.

DESIGN

A single-center retrospective analysis.

SETTING

Tertiary academic medical center.

PATIENTS

Data from 59 patients was used to determine precise cerebral autoregulation thresholds for identification of the lower limit of autoregulation. These thresholds were validated in a larger cohort of 226 patients.

METHODS AND MAIN RESULTS

Invasive mean arterial blood pressure, cerebral blood flow velocities, regional cortical oxygen saturation, and total hemoglobin were recorded simultaneously. Three cerebral autoregulation indices were calculated, including mean flow index, cerebral oximetry index, and hemoglobin volume index. Cerebral autoregulation curves for the three indices were plotted, and thresholds for each index were used to generate threshold- and index-specific lower limit of autoregulations. A reference lower limit of autoregulation could be identified in 59 patients by plotting cerebral blood flow velocity against mean arterial blood pressure to generate gold-standard Lassen curves. The lower limit of autoregulations defined at each threshold were compared with the gold-standard lower limit of autoregulation determined from Lassen curves. The results identified the following thresholds: mean flow index (0.45), cerebral oximetry index (0.35), and hemoglobin volume index (0.3). We then calculated the product of magnitude and duration of mean arterial blood pressure less than lower limit of autoregulation in a larger cohort of 226 patients. When using the lower limit of autoregulations identified by the optimal thresholds above, mean arterial blood pressure less than lower limit of autoregulation was greater in patients with acute kidney injury than in those without acute kidney injury.

CONCLUSIONS

This study identified thresholds of intact and impaired cerebral autoregulation for three indices and showed that mean arterial blood pressure below lower limit of autoregulation is a risk factor for acute kidney injury after cardiac surgery.

Brain monitoring using near-infrared spectroscopy to predict outcome after cardiac arrest: a novel phenotype in a rat model of cardiac arrest

Improving neurological outcomes after cardiac arrest (CA) is the most important patient-oriented outcome for CA research. Near-infrared spectroscopy (NIRS) enables a non-invasive, real-time measurement of regional cerebral oxygen saturation. Here, we demonstrate a novel, non-invasive measurement using NIRS, termed modified cerebral oximetry index (mCOx), to distinguish the severity of brain injury after CA. We aimed to test the feasibility of this method to predict neurological outcome after asphyxial CA in rats. Our results suggest that mCOx is feasible shortly after resuscitation and can provide a surrogate measure for the severity of brain injury in a rat asphyxia CA model.

The association between early impairment in cerebral autoregulation and outcome in a pediatric swine model of cardiac arrest

AIMS

Evaluate cerebral autoregulation (CAR) by intracranial pressure reactivity index (PRx) and cerebral blood flow reactivity index (CBFx) during the first four hours following return of spontaneous circulation (ROSC) in a porcine model of pediatric cardiac arrest. Determine whether impaired CAR is associated with neurologic outcome.

METHODS

Four-week-old swine underwent seven minutes of asphyxia followed by ventricular fibrillation induction and hemodynamic-directed CPR. Those achieving ROSC had arterial blood pressure, intracranial pressure (ICP), and microvascular cerebral blood flow (CBF) monitored for 4 h. Animals were assigned an 8 -h post-ROSC swine cerebral performance category score (1 = normal; 2-4=abnormal neurologic function). In this secondary analytic study, we calculated PRx and CBFx using a continuous, moving correlation coefficient between mean arterial pressure (MAP) and ICP, and between MAP and CBF, respectively. Burden of impaired CAR was the area under the PRx or CBFx curve using a threshold of 0.3 and normalized as percentage of monitoring duration.

RESULTS

Among 23 animals, median PRx was 0.14 [0.06,0.25] and CBFx was 0.36 [0.05,0.44]. Median burden of impaired CAR was 21% [18,27] with PRx and 30% [17,40] with CBFx. Neurologically abnormal animals (n = 10) did not differ from normal animals (n = 13) in post-ROSC MAP (63 vs. 61 mmHg, p = 0.74), ICP (15 vs. 14 mmHg, p = 0.78) or CBF (274 vs. 397 Perfusion Units, p = 0.12). CBFx burden was greater among abnormal than normal animals (45% vs. 24%, p = 0.001), but PRx burden was not (25% vs. 20%, p = 0.38).

CONCLUSION

CAR is impaired early after ROSC. A greater burden of CAR impairment measured by CBFx was associated with abnormal neurologic outcome.CHOP Institutional Animal Care and Use Committee protocol 19-001327.

Exploratory Assessment of the Relationship Between Hemoglobin Volume Phase Index, Magnetic Resonance Imaging, and Functional Outcome in Neonates with Hypoxic-Ischemic Encephalopathy

BACKGROUND/OBJECTIVE

Near-infrared spectroscopy (NIRS)-based measures of cerebral autoregulation (CAR) can potentially identify neonates with hypoxic-ischemic encephalopathy (HIE) who are at greatest risk of irreversible brain injury. However, modest predictive abilities have precluded previously described metrics from entering clinical care. We previously validated a novel autoregulation metric in a piglet model of induced hypotension called the hemoglobin volume phase index (HVP). The objective of this study was to evaluate the clinical ability of the HVP to predict adverse outcomes neonates with HIE.

METHODS

This is a prospective study of neonates with HIE who underwent therapeutic hypothermia (TH) at a level 4 neonatal intensive care unit (NICU). Continuous cerebral NIRS and mean arterial blood pressure (MAP) from indwelling arterial catheters were measured during TH and through rewarming. Multivariate autoregressive process was used to calculate the coherence between MAP and the sum total of the oxy- and deoxygenated Hb densities (HbT), a surrogate measure of cerebral blood volume (CBV). The HVP was calculated as the cosine-transformed phase shift at the frequency of maximal MAP-HbT coherence. Brain injury was assessed by neonatal magnetic resonance imaging (MRI), and developmental outcomes were assessed by the Bayley Scales of Infant Development (BSID-III) at 15-30 months. The ability of the HVP to predict (a) death or severe brain injury by MRI and (b) death or significant developmental delay was assessed using logistic regression analyses.

RESULTS

In total, 50 neonates with moderate or severe HIE were monitored. Median HVP was higher, representing more dysfunctional autoregulation, in infants who had adverse outcomes. After adjusting for sex and encephalopathy grade at presentation, HVP at 21-24 and 24-27 h of life predicted death or brain injury by MRI (21-24 h: OR 8.8, p = 0.037; 24-27 h: OR 31, p = 0.011) and death or developmental delay at 15-30 months (21-24 h: OR 11.8, p = 0.05; 24-27 h: OR 15, p = 0.035).

CONCLUSIONS

Based on this pilot study of neonates with HIE, HVP merits further study as an indicator of death or severe brain injury on neonatal MRI and neurodevelopmental delay in early childhood. Larger studies are warranted for further clinical validation of the HVP to evaluate cerebral autoregulation following HIE.

Mean arterial pressure is associated with the neurological function in patients who survived after cardiopulmonary resuscitation: A retrospective cohort study

Background

About 18% to 40% of the survivors have moderate to severe neurological dysfunction. At present, studies on mean arterial pressure (MAP) and neurological function of patients survived after cardiopulmonary resuscitation (CPR) are limited and conflicted.

Hypothesis

The higher the MAP of the patient who survived after CPR, the better the neurological function.

Method

A retrospective cohort study was conducted to detect the relationship between MAP and the neurological function of patients who survived after CPR by univariate analysis, multivariate regression analysis, and subgroup analysis.

Results

From January 2007 to December 2015, a total of 290 cases met the inclusion criteria and were enrolled in this study. The univariate analysis showed that MAP was associated with the neurological function of patients who survived after CPR; its OR value was 1.03 (1.01, 1.04). The multi‐factor regression analysis also showed that MAP was associated with the neurological function of patients survived after CPR in the four models, the adjusted OR value of the four models were 1.021 (1.008, 1.035); 1.028 (1.013, 1.043); 1.027 (1.012, 1.043); and 1.029 (1.014, 1.044), respectively. The subgroups analyses showed that when 65 mm Hg ≤ MAP<100 mm Hg and when patients with targeted temperature management or without extracorporeal membrane oxygenation, with the increase of MAP, the better neurological function of patients survived after CPR.

Conclusion

This study found that the higher MAP, the better the neurological function of patients who survived after CPR. At the same time, the maintenance of MAP at 65 to 100 mm Hg would improve the neurological function of patients who survived after CPR.

Haemodynamics and vasopressor support during prolonged targeted temperature management for 48 hours after out-of-hospital cardiac arrest: a post hoc substudy of a randomised clinical trial

BACKGROUND

Comatose patients admitted after out-of-hospital cardiac arrest frequently experience haemodynamic instability and anoxic brain injury. Targeted temperature management is used for neuroprotection; however, targeted temperature management also affects patients' haemodynamic status. This study assessed the haemodynamic status of out-of-hospital cardiac arrest survivors during prolonged (48 hours) targeted temperature management at 33°C.

METHODS

Analysis of haemodynamic and vasopressor data from 311 patients included in a randomised, clinical trial conducted in 10 European hospitals (the TTH48 trial). Patients were randomly allocated to targeted temperature management at 33°C for 24 (TTM24) or 48 (TTM48) hours. Vasopressor and haemodynamic data were reported hourly for 72 hours after admission. Vasopressor load was calculated as norepinephrine (µg/kg/min) plus dopamine(µg/kg/min/100) plus epinephrine (µg/kg/min).

RESULTS

After 24 hours, mean arterial pressure (mean±SD) was 74±9 versus 75±9 mmHg (P=0.19), heart rate was 57±16 and 55±14 beats/min (P=0.18), vasopressor load was 0.06 (0.03-0.15) versus 0.08 (0.03-0.15) µg/kg/min (P=0.22) for the TTM24 and TTM48 groups, respectively. From 24 to 48 hours, there was no difference in mean arterial pressure (Pgroup=0.32) or lactate (Pgroup=0.20), while heart rate was significantly lower (average difference 5 (95% confidence interval 2-8) beats/min, Pgroup<0.0001) and vasopressor load was significantly higher in the TTM48 group (Pgroup=0.005). In a univariate Cox regression model, high vasopressor load was associated with mortality in univariate analysis (hazard ratio 1.59 (1.05-2.42) P=0.03), but not in multivariate analysis (hazard ratio 0.77 (0.46-1.29) P=0.33).

CONCLUSIONS

In this study, prolonged targeted temperature management at 33°C for 48 hours was associated with higher vasopressor requirement but no sign of any detrimental haemodynamic effects.

Comparison of wavelet and correlation indices of cerebral autoregulation in a pediatric swine model of cardiac arrest

Existing cerebrovascular blood pressure autoregulation metrics have not been translated to clinical care for pediatric cardiac arrest, in part because signal noise causes high index time-variability. We tested whether a wavelet method that uses near-infrared spectroscopy (NIRS) or intracranial pressure (ICP) decreases index variability compared to that of commonly used correlation indices. We also compared whether the methods identify the optimal arterial blood pressure (ABPopt) and lower limit of autoregulation (LLA). 68 piglets were randomized to cardiac arrest or sham procedure with continuous monitoring of cerebral blood flow using laser Doppler, NIRS and ICP. The arterial blood pressure (ABP) was gradually reduced until it dropped to below the LLA. Several autoregulation indices were calculated using correlation and wavelet methods, including the pressure reactivity index (PRx and wPRx), cerebral oximetry index (COx and wCOx), and hemoglobin volume index (HVx and wHVx). Wavelet methodology had less index variability with smaller standard deviations. Both wavelet and correlation methods distinguished functional autoregulation (ABP above LLA) from dysfunctional autoregulation (ABP below the LLA). Both wavelet and correlation methods also identified ABPopt with high agreement. Thus, wavelet methodology using NIRS may offer an accurate vasoreactivity monitoring method with reduced signal noise after pediatric cardiac arrest.

Resuscitating the Globally Ischemic Brain: TTM and Beyond

Cardiac arrest (CA) afflicts ~ 550,000 people each year in the USA. A small fraction of CA sufferers survive with a majority of these survivors emerging in a comatose state. Many CA survivors suffer devastating global brain injury with some remaining indefinitely in a comatose state. The pathogenesis of global brain injury secondary to CA is complex. Mechanisms of CA-induced brain injury include ischemia, hypoxia, cytotoxicity, inflammation, and ultimately, irreversible neuronal damage. Due to this complexity, it is critical for clinicians to have access as early as possible to quantitative metrics for diagnosing injury severity, accurately predicting outcome, and informing patient care. Current recommendations involve using multiple modalities including clinical exam, electrophysiology, brain imaging, and molecular biomarkers. This multi-faceted approach is designed to improve prognostication to avoid "self-fulfilling" prophecy and early withdrawal of life-sustaining treatments. Incorporation of emerging dynamic monitoring tools such as diffuse optical technologies may provide improved diagnosis and early prognostication to better inform treatment. Currently, targeted temperature management (TTM) is the leading treatment, with the number of patients needed to treat being ~ 6 in order to improve outcome for one patient. Future avenues of treatment, which may potentially be combined with TTM, include pharmacotherapy, perfusion/oxygenation targets, and pre/postconditioning. In this review, we provide a bench to bedside approach to delineate the pathophysiology, prognostication methods, current targeted therapies, and future directions of research surrounding hypoxic-ischemic brain injury (HIBI) secondary to CA.

Effects of therapeutic hypothermia on cerebral tissue oxygen saturation in a swine model of post-cardiac arrest

Since the introduction of therapeutic hypothermia (TH), trends have changed in the monitoring indicators used during and after cardiac arrest. During hypothermia, the cerebral metabolic rate of oxygen is reduced, which leads to uncertainty in regional cerebral tissue oxygen saturation (S_ctO₂). The aim of the present study was to evaluate the effect of TH on changes in S_ctO₂ using near-infrared spectroscopy. A total of 23 male domestic pigs were randomized into three groups: TH (n=9), normothermia (NT; n=9) and control (n=5). Animals in the control group underwent surgical preparation only. The animal models were established using 8 min of ventricular fibrillation and 5 min of cardiopulmonary resuscitation. In the TH group, at 5 min after resuscitation, the animals were cooled with a cooling blanket and ice packs for 24 h. S_ctO₂ was recorded throughout the experiment. In all groups, The mean arterial pressure, arterial carbon dioxide partial pressure, arterial oxygen partial pressure, lactate, neuron-specific enolase (NSE) and S100B were measured at baseline and at 1, 3, 6, 12, 24 and 30 h after resuscitation. S_ctO₂ significantly decreased after ventricular fibrillation, compared with the baseline. Following resuscitation, the S_ctO₂ values gradually increased to 55.6±3.8% of baseline in the TH group and 51.2±3.5% in the NT group (P=0.039). Significant differences between the two groups were observed, starting at 6 h after cardiac arrest. Throughout the hypothermic period, NSE and S100B showed an increasing trend, then decreased during rewarming in the TH and NT groups. NSE and S100B showed greater improvement in the TH group compared with the NT group at 6 and 24 h after resuscitation. Following cardiac arrest, therapeutic hypothermia could increase S_ctO₂ after resuscitation and could improve neurological outcome. In conclusion, S_ctO₂ may be a feasible marker for use in the early assessment of brain damage during and after cardiac arrest.

Dissociation of Cerebral Blood Flow and Femoral Artery Blood Pressure Pulsatility After Cardiac Arrest and Resuscitation in a Rodent Model: Implications for Neurological Recovery

Background

Impaired neurological function affects 85% to 90% of cardiac arrest (CA) survivors. Pulsatile blood flow may play an important role in neurological recovery after CA. Cerebral blood flow (CBF) pulsatility immediately, during, and after CA and resuscitation has not been investigated. We characterized the effects of asphyxial CA on short‐term (<2 hours after CA) CBF and femoral arterial blood pressure (ABP) pulsatility and studied their relationship to cerebrovascular resistance (CVR) and short‐term neuroelectrical recovery.

Methods and Results

Male rats underwent asphyxial CA followed by cardiopulmonary resuscitation. A multimodal platform combining laser speckle imaging, ABP, and electroencephalography to monitor CBF, peripheral blood pressure, and brain electrophysiology, respectively, was used. CBF and ABP pulsatility and CVR were assessed during baseline, CA, and multiple time points after resuscitation. Neuroelectrical recovery, a surrogate for neurological outcome, was assessed using quantitative electroencephalography 90 minutes after resuscitation. We found that CBF pulsatility differs significantly from baseline at all experimental time points with sustained deficits during the 2 hours of postresuscitation monitoring, whereas ABP pulsatility was relatively unaffected. Alterations in CBF pulsatility were inversely correlated with changes in CVR, but ABP pulsatility had no association to CVR. Interestingly, despite small changes in ABP pulsatility, higher ABP pulsatility was associated with worse neuroelectrical recovery, whereas CBF pulsatility had no association.

Conclusions

Our results reveal, for the first time, that CBF pulsatility and CVR are significantly altered in the short‐term postresuscitation period after CA. Nevertheless, higher ABP pulsatility appears to be inversely associated with neuroelectrical recovery, possibly caused by impaired cerebral autoregulation and/or more severe global cerebral ischemia.

Effect of Body Temperature on Cerebral Autoregulation in Acutely Comatose Neurocritically Ill Patients

OBJECTIVES

Impaired cerebral autoregulation following neurologic injury is a predictor of poor clinical outcome. We aimed to assess the relationship between body temperature and cerebral autoregulation in comatose patients.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurocritical care unit of the Johns Hopkins Hospital.

PATIENTS

Eighty-five acutely comatose patients (Glasgow Coma Scale score of ≤ 8) admitted between 2013 and 2017.

INTERVENTIONS

None.

MEASUREMENT AND MAIN RESULTS

Cerebral autoregulation was monitored using multimodal monitoring with near-infrared spectroscopy-derived cerebral oximetry index. Cerebral oximetry index was calculated as a Pearson correlation coefficient between low-frequency changes in regional cerebral oxygenation saturation and mean arterial pressure. Patients were initially analyzed together, then stratified by temperature pattern over the monitoring period: no change (< 1°C difference between highest and lowest temperatures; n = 11), increasing (≥ 1°C; n = 9), decreasing (≥ 1°C; n = 9), and fluctuating (≥ 1°C difference but no sustained direction of change; n = 56). Mixed random effects models with random intercept and multivariable logistic regression analysis were used to assess the association between hourly temperature and cerebral oximetry index, as well as between temperature and clinical outcomes. Cerebral oximetry index showed a positive linear relationship with temperature (β = 0.04 ± 0.10; p = 0.29). In patients where a continual increase or decrease in temperature was seen during the monitoring period, every 1°C change in temperature resulted in a cerebral oximetry index change in the same direction by 0.04 ± 0.01 (p < 0.001) and 0.02 ± 0.01 (p = 0.12), respectively, after adjusting for PaCO2, hemoglobin, mean arterial pressure, vasopressor and sedation use, and temperature probe location. There was no significant difference in mortality or poor outcome (modified Rankin Scale score of 4-6) between temperature pattern groups at discharge, 3, or 6 months.

CONCLUSIONS

In acute coma patients, increasing body temperature is associated with worsening cerebral autoregulation as measured by cerebral oximetry index. More studies are needed to clarify the impact of increasing temperature on cerebral autoregulation in patients with acute brain injury.

Correlation Between the Number of Lenticulostriate Arteries and Imaging of Cerebral Small Vessel Disease

Background and purpose: Hypoperfusion plays an important role in the pathophysiology of cerebral small vessel disease (SVD). Lenticulostriate arteries (LSAs) are some of the most important cerebral arterial small vessels. This study aimed to investigate whether the number of LSAs was associated with the cerebral perfusion in SVD patients and determine the correlation between the number of LSAs and SVD severity.

Methods: Five hundred and ninety-four consecutive patients who underwent digital subtraction angiography were enrolled in this study. The number of LSAs was determined. Computed tomography perfusion (CTP) was used to calculate the cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), and time to peak (TTP). Magnetic resonance imaging (MRI) was performed to assess cerebral infarct, cerebral microbleeds (CMBs), white matter hyperintensities (WMHs), enlarged perivascular spaces (EPVSs), and lacunes. An SVD compound score was calculated to express the level of cerebral SVD load.

Results: The SVD scores were negatively correlated with the number of the LSAs (P < 0.001, r_s = −0.44). The number of LSAs was inversely associated with the presence of any type of SVD (P < 0.001). The adjusted ORs of the SVD severity were 0.31 for LSA group 1 (LSA > 20) vs. group 2 (LSA = 10–20) and 0.47 for LSA group 2 (LSA = 10–20) vs. group 3 (LSA < 10). MTT and TTP were significantly higher and CBF was significantly lower when the number of LSAs was between 5 and 10 on each side of the basal ganglia (P < 0.001, <0.001, and <0.001, respectively). The CBV was slightly lower when the number of LSAs was between 5 and 10, while it was significantly lower when the number was <5 on each side of the basal ganglia (P < 0.05, <0.0001, respectively).

Conclusion: LSA count was lower in SVD patients than the non-SVD participants and there was a positive correlation between the cerebral perfusion and the number of LSAs. The LSA number was negatively associated with SVD severity, hypoperfusion might play an important role. This finding may have potentially important clinical implications for monitoring LSA in SVD patients.

Arterial blood pressure during targeted temperature management after out-of-hospital cardiac arrest and association with brain injury and long-term cognitive function

OBJECTIVES

During targeted temperature management after out-of-hospital cardiac arrest infusion of vasoactive drugs is often needed to ensure cerebral perfusion pressure. This study investigated mean arterial pressure after out-of-hospital cardiac arrest and the association with brain injury and long-term cognitive function.

METHODS

Post-hoc analysis of patients surviving at least 48 hours in the biobank substudy of the targeted temperature management trial with available blood pressure data. Patients were stratified in three groups according to mean arterial pressure during targeted temperature management (4-28 hours after admission; <70 mmHg, 70-80 mmHg, >80 mmHg). A biomarker of brain injury, neuron-specific enolase, was measured and impaired cognitive function was defined as a mini-mental state examination score below 27 in 6-month survivors.

RESULTS

Of the 657 patients included in the present analysis, 154 (23%) had mean arterial pressure less than 70 mmHg, 288 (44%) had mean arterial pressure between 70 and 80 mmHg and 215 (33%) had mean arterial pressure greater than 80 mmHg. There were no statistically significant differences in survival (P=0.35) or neuron-specific enolase levels (P=0.12) between the groups. The level of target temperature did not statistically significantly interact with mean arterial pressure regarding neuron-specific enolase (P_{interaction_MAP*TTM}=0.58). In the subgroup of survivors with impaired cognitive function (n=132) (35%) mean arterial pressure during targeted temperature management was significantly higher (P_group=0.03).

CONCLUSIONS

In a large cohort of comatose out-of-hospital cardiac arrest patients, low mean arterial pressure during targeted temperature management was not associated with higher neuron-specific enolase regardless of the level of target temperature (33°C or 36°C for 24 hours). In survivors with impaired cognitive function, mean arterial pressure during targeted temperature management was significantly higher.

Comparison of Frequency- and Time-Domain Autoregulation and Vasoreactivity Indices in a Piglet Model of Hypoxia-Ischemia and Hypothermia

INTRODUCTION

The optimal method to detect impairments in cerebrovascular pressure autoregulation in neonates with hypoxic-ischemic encephalopathy (HIE) is unclear. Improving autoregulation monitoring methods would significantly advance neonatal neurocritical care.

METHODS

We tested several mathematical algorithms from the frequency and time domains in a piglet model of HIE, hypothermia, and hypotension. We used laser Doppler flowmetry and induced hypotension to delineate the gold standard lower limit of autoregulation (LLA). Receiver operating characteristics curve analyses were used to determine which indices could distinguish blood pressure above the LLA from that below the LLA in each piglet.

RESULTS

Phase calculation in the frequency band with maximum coherence, as well as the correlation between mean arterial pressure (MAP) and near-infrared spectroscopy relative total tissue hemoglobin (HbT) or regional oxygen saturation (rSO2), accurately discriminated functional from dysfunctional autoregulation. Neither hypoxia-ischemia nor hypothermia affected the accuracy of these indices. Coherence alone and gain had low diagnostic value relative to phase and correlation.

CONCLUSION

Our findings indicate that phase shift is the most accurate component of autoregulation monitoring in the developing brain, and it can be measured using correlation or by calculating phase when coherence is maximal. Phase and correlation autoregulation indices from MAP and rSO2 and vasoreactivity indices from MAP and HbT are accurate metrics that are suitable for clinical HIE studies.

Cerebral microcirculatory alterations and the no-reflow phenomenon in vivo after experimental pediatric cardiac arrest

Decreased cerebral blood flow (CBF) after cardiac arrest (CA) contributes to secondary ischemic injury in infants and children. We previously reported cortical hypoperfusion with tissue hypoxia early in a pediatric rat model of asphyxial CA. In order to identify specific alterations as potential therapeutic targets to improve cortical hypoperfusion post-CA, we characterize the CBF alterations at the cortical microvascular level in vivo using multiphoton microscopy. We hypothesize that microvascular constriction and disturbances of capillary red blood cell (RBC) flow contribute to cortical hypoperfusion post-CA. After resuscitation from 9 min asphyxial CA, transient dilation of capillaries and venules at 5 min was followed by pial arteriolar constriction at 30 and 60 min (19.6 ± 1.3, 19.3 ± 1.2 µm at 30, 60 min vs. 22.0 ± 1.2 µm at baseline, p < 0.05). At the capillary level, microcirculatory disturbances were highly heterogeneous, with RBC stasis observed in 25.4% of capillaries at 30 min post-CA. Overall, the capillary plasma mean transit time was increased post-CA by 139.7 ± 51.5%, p < 0.05. In conclusion, pial arteriolar constriction, the no-reflow phenomenon and increased plasma transit time were observed post-CA. Our results detail the microvascular disturbances in a pediatric asphyxial CA model and provide a powerful platform for assessing specific vascular-targeted therapies.

Spatial T-maze identifies cognitive deficits in piglets 1 month after hypoxia-ischemia in a model of hippocampal pyramidal neuron loss and interneuron attrition

Neonatal brain injury from hypoxia-ischemia (HI) causes major morbidity. Piglet HI is an established method for testing neuroprotective treatments in large, gyrencephalic brain. Though many neurobehavior tests exist for rodents, such tests and their associations with neuropathologic injury remain underdeveloped and underutilized in large, neonatal HI animal models. We examined whether spatial T-maze and inclined beam tests distinguish cognitive and motor differences between HI and sham piglets and correlate with neuropathologic injury. Neonatal piglets were randomized to whole-body HI or sham procedure, and they began T-maze and inclined beam testing 17 days later. HI piglets had more incorrect T-maze turns than did shams. Beam walking time did not differ between groups. Neuropathologic evaluations at 33 days validated the injury with putamen neuron loss after HI to below that of sham procedure. HI decreased the numbers of CA3 pyramidal neurons but not CA1 pyramidal neurons or dentate gyrus granule neurons. Though the number of hippocampal parvalbumin-positive interneurons did not differ between groups, HI reduced the number of CA1 interneuron dendrites. Piglets with more incorrect turns had greater CA3 neuron loss, and piglets that took longer in the maze had fewer CA3 interneurons. The number of putamen neurons was unrelated to T-maze or beam performance. We conclude that neonatal HI causes hippocampal CA3 neuron loss, CA1 interneuron dendritic attrition, and putamen neuron loss at 1-month recovery. The spatial T-maze identifies learning and memory deficits that are related to loss of CA3 pyramidal neurons and fewer parvalbumin-positive interneurons independent of putamen injury.

Continuous monitoring of cerebrovascular reactivity through pulse transit time and intracranial pressure

OBJECTIVE

Cerebrovascular reactivity (CR) is a mechanism that maintains stable blood flow supply to the brain. Pressure reactivity index (PRx), the correlation coefficient between slow waves of invasive arterial blood pressure (ABP) and intracranial pressure (ICP) has been validated for CR assessment. However, in clinical ward, not every subarachnoid hemorrhage (SAH) patient has invasive ABP monitoring. Pulse transit time (PTT), the propagation time of a pulse wave travelling from the heart to peripheral arteries, has been suggested as a surrogate measure of ABP. In this study, we proposed to use PTT instead of invasive ABP to monitor CR.

APPROACH

Forty-five SAH patients with simultaneous recordings of invasive ABP, ICP, oxygen saturation level (SpO2) and electrocardiograph (ECG) were included. PTT was calculated as the time from the ECG R-wave peak to the onset of SpO2. PTT based pressure reactivity index (tPRx) was calculated as the correlation coefficient between slow waves of PTT and ICP. Wavelet tPRx (wtRx) was calculated as the cosine of wavelet phase shift between PTT and ICP. Meanwhile, PRx and wPRx were also calculated using invasive ABP and ICP as input.

MAIN RESULTS

The result showed a negative relationship between PTT and ABP (r  =  -0.58, p   <  0.001). tPRx negatively correlated with PRx (r  =  -0.51, p   =  0.003). Wavelet method correlated well with correlation method demonstrated through positive relationship between wPRx and PRx (r  =  0.82, p   <  0.001) as well as wtPRx and tPRx (r  =  0.84, p   <  0.001).

SIGNIFICANCE

PTT demonstrates great potential as a useful tool for CR assessment when invasive ABP is unavailable. Key points • Pulse transit time (PTT), defined as the propagation time of a pulse wave travelling from the heart to the peripheral arteries, has been proposed as a surrogate measure of ABP. The relationship between PTT and ABP in SAH patients remains unknown. • Cerebrovascular reactivity (CR) assessment through PTT has advantages over invasive ABP, as it avoids bleeding and infection risk, and can be used outside of the ICU. • We introduced a new method to assess CR using PTT and ICP through correlation based method and wavelet based method. • We found that beat-to-beat PTT was negatively related with invasive ABP in SAH patients. A significant linear relationship exists between PTT-based CR parameter and a well validated method, PRx. PTT demonstrates great potential as a useful tool for CR assessment when invasive ABP is unavailable in SAH patients.

Neonatal cerebrovascular autoregulation

Cerebrovascular pressure autoregulation is the physiologic mechanism that holds cerebral blood flow (CBF) relatively constant across changes in cerebral perfusion pressure (CPP). Cerebral vasoreactivity refers to the vasoconstriction and vasodilation that occur during fluctuations in arterial blood pressure (ABP) to maintain autoregulation. These are vital protective mechanisms of the brain. Impairments in pressure autoregulation increase the risk of brain injury and persistent neurologic disability. Autoregulation may be impaired during various neonatal disease states including prematurity, hypoxic-ischemic encephalopathy (HIE), intraventricular hemorrhage, congenital cardiac disease, and infants requiring extracorporeal membrane oxygenation (ECMO). Because infants are exquisitely sensitive to changes in cerebral blood flow (CBF), both hypoperfusion and hyperperfusion can cause significant neurologic injury. We will review neonatal pressure autoregulation and autoregulation monitoring techniques with a focus on brain protection. Current clinical therapies have failed to fully prevent permanent brain injuries in neonates. Adjuvant treatments that support and optimize autoregulation may improve neurologic outcomes.

Effects of moderate and severe arterial hypotension on intracerebral perfusion and brain tissue oxygenation in piglets

BACKGROUND

Hypotension is common in anaesthetised children, and its impact on cerebral oxygenation is unknown. The goal of the present study was to investigate the effects of moderate systemic arterial hypotension (mHT) and severe hypotension (sHT) on cerebral perfusion and brain tissue oxygenation in piglets.

METHODS

Twenty-seven anaesthetised piglets were randomly allocated to a control group, mHT group, or sHT group. Cerebral monitoring comprised a tissue oxygen partial pressure ( [Formula: see text] ) and laser Doppler (LD) perfusion probe advanced into the brain tissue, and a near-infrared spectroscopy sensor placed over the skin measuring regional oxygen saturation (rSO₂). Arterial hypotension was induced by blood withdrawal and i.v. nitroprusside infusion [target MAP: 35-38 (mHT) and 27-30 (sHT) mm Hg]. Data were analysed at baseline, and every 20 min during and after treatment.

RESULTS

Compared with control, [Formula: see text] decreased equally with mHT and sHT [mean (SD) after 60 min: control: 17.1 (6.4); mHT: 6.4 (3.6); sHT: 7.2 (4.3) mm Hg]. No differences between groups were detected for rSO₂ and LD during treatment. However, in the sHT group, rSO₂ increased after restoring normotension [from 49.3 (9.5) to 58.9 (8.9)% Post60]. sHT was associated with an increase in blood lactate [from 1.5 (0.4) to 2.4 (0.9) mmol L^-1], and a decrease in bicarbonate [28 (2.4) to 25.8 (2.6) mmol L^-1] and base excess [4.7 (1.9) to 2.0 (2.7) mmol L^-1] between baseline and 60 min after the start of the experiment.

CONCLUSIONS

Induction of mHT and sHT by hypovolaemia and nitroprusside infusion caused alterations in brain tissue oxygenation in a piglet model, but without detectable changes in brain tissue perfusion and regional oxygen saturation.

Validation of Intracranial Pressure-Derived Cerebrovascular Reactivity Indices against the Lower Limit of Autoregulation, Part II: Experimental Model of Arterial Hypotension

The aim of this work was to explore the relationship between intracranial pressure (ICP)-derived indices of cerebrovascular reactivity and the lower limit of autoregulation (LLA) during arterial hypotension. We retrospectively reviewed recorded physiological data from piglets that underwent controlled hypotension. Hypotension was induced by inflation of a balloon catheter in the inferior vena cava. ICP, cortical laser Doppler flowmetry (LDF), and arterial blood pressure (ABP) monitoring was conducted. ICP-derived indices were calculated: pressure reactivity index (PRx; correlation between ICP and mean arterial pressure [MAP]); pulse amplitude index (PAx; correlation between pulse amplitude of ICP [AMP] and MAP); and RAC (correlation between AMP and cerebral perfusion pressure [CPP]). LLA was estimated by piece-wise linear regression of CPP versus LDF. We produced error bar plots for PRx, PAx, and RAC against 5-mm Hg bins of CPP, displaying the relationship with the LLA. We compared CPP values at clinically relevant thresholds of PRx, PAx, and RAC to CPP measured at the LLA. Receiver operating curve (ROC) analysis was performed for each index across the LLA using 5-mm Hg bins for CPP. Mean LLA was 36.2 ± 10.5 mm Hg. Error bar plots demonstrated that PRx, PAx, and RAC increased, with CPP decreasing below the LLA. CPP at clinically relevant thresholds for PRx, PAx, and RAC displayed weak associations with the LLA, indicating that thresholds defined in TBI may not apply to a model of arterial hypotension. ROC analysis indicated that PRx, PAx, and RAC predicted the LLA, with AUCs of: 0.806 (95% confidence interval [CI], 0.750-0.863; p < 0.0001), 0.726 (95% CI, 0.664-0.789; p < 0.0001), and 0.710 (95% CI, 0.646-0.775; p < 0.0001), respectively. Three ICP-derived continuous indices of cerebrovascular reactivity, PRx, PAx, and RAC, were validated against the LLA within this experimental model of arterial hypotension, with PRx being superior.

The Effects of Induction and Treatment of Intracranial Hypertension on Cerebral Autoregulation: An Experimental Study

Background

This study aimed to analyse cerebral autoregulation (CA) during induction and treatment of intracranial hypertension (ICH) in an experimental model.

Materials and Methods

Landrace and Duroc piglets were divided into mild and severe ICH groups. Four or seven millilitres of saline solution was infused into paediatric bladder catheter inserted in the parietal lobe (balloon inflation). After 1.5 h, a 3% saline solution was infused via venous catheter, and 30 min later, the bladder catheter balloon was deflated (surgery). The cerebral static autoregulation (sCA) index was evaluated using cerebral blood flow velocities (CBFV) obtained with Doppler ultrasound.

Results

Balloon inflation increased ICP in both groups. The severe ICH group showed significantly lower sCA index values (p=0.001, ANOVA) after balloon inflation (ICH induction) and a higher sCA index after saline injection (p=0.02) and after surgery (p=0.04). ICP and the sCA index were inversely correlated (r=-0.68 and p<0.05). CPP and the sCA index were directly correlated (r=0.74 and p<0.05).

Conclusion

ICH was associated with local balloon expansion, which triggered CA impairment, particularly in the severe ICH group. Moreover, ICP-reducing treatments were associated with improved CA in subjects with severe ICH.

Intracranial Hypertension and Cerebral Autoregulation: A Systematic Review and Meta-Analysis

OBJECTIVE

To present a systematic review and meta-analysis to establish the relation between cerebral autoregulation (CA) and intracranial hypertension.

METHODS

An electronic search using the term "Cerebral autoregulation and intracranial hypertension" was designed to identify studies that analyzed cerebral blood flow autoregulation in patients undergoing intracranial pressure (ICP) monitoring. The data were used in meta-analyses and sensitivity analyses.

RESULTS

A static CA technique was applied in 10 studies (26.3%), a dynamic technique was applied in 25 studies (65.8%), and both techniques were used in 3 studies (7.9%). Static CA studies using the cerebral blood flow technique revealed impaired CA in patients with an ICP ≥20 (standardized mean difference [SMD] 5.44%, 95% confidence interval [CI] 0.25-10.65, P = 0.04); static CA studies with transcranial Doppler revealed a tendency toward impaired CA in patients with ICP ≥20 (SMD -7.83%, 95% CI -17.52 to 1.85, P = 0.11). Moving correlation studies reported impaired CA in patients with ICP ≥20 (SMD 0.06, 95% CI 0.07-0.14, P < 0.00001). A comparison of CA values and mean ICP revealed a correlation between greater ICP and impaired CA (SMD 5.47, 95% CI 1.39-10.1, P = 0.01). Patients with ICP ≥20 had an elevated risk of impaired CA (OR 2.27, 95% CI 1.20-4.31, P = 0.01).

CONCLUSIONS

A clear tendency toward CA impairment was observed in patients with increased ICP.

Individualized perfusion targets in hypoxic ischemic brain injury after cardiac arrest

Secondary injury is a major determinant of outcome in hypoxic ischemic brain injury (HIBI) after cardiac arrest and may be mitigated by optimizing cerebral oxygen delivery (CDO₂). CDO₂ is determined by cerebral blood flow (CBF), which is dependent upon mean arterial pressure (MAP). In health, CBF remains constant over the MAP range through cerebral autoregulation. In HIBI, the zone of intact cerebral autoregulation is narrowed and varies for each patient. Maintaining MAP within the intact autoregulation zone may mitigate ischemia, hyperemia and secondary injury. The optimal MAP in individual patients can be determined using real time autoregulation monitoring techniques.

Impairment of cerebral autoregulation in pediatric extracorporeal membrane oxygenation associated with neuroimaging abnormalities

Extracorporeal membrane oxygenation (ECMO) is a life-supporting therapy for critically ill patients with severe respiratory and/or cardiovascular failure. Cerebrovascular impairment can result in hemorrhagic and ischemic complications commonly seen in the patients supported on ECMO. We investigated the degree of cerebral autoregulation impairment during ECMO as well as whether it is predictive of neuroimaging abnormalities. Spontaneous fluctuations of mean arterial pressure (MAP) and cerebral tissue oxygen saturation ([Formula: see text]) were continuously measured during the ECMO run. The dynamic relationship between the MAP and [Formula: see text] fluctuations was assessed based on wavelet transform coherence (WTC). Neuroimaging was conducted during and/or after ECMO as standard of care, and the abnormalities were evaluated based on a scoring system that had been previously validated among ECMO patients. Of the 25 patients, 8 (32%) had normal neuroimaging, 7 (28%) had mild to moderate neuroimaging abnormalities, and the other 10 (40%) had severe neuroimaging abnormalities. The degrees of cerebral autoregulation impairment quantified based on WTC showed significant correlations with the neuroimaging scores ([Formula: see text]; [Formula: see text]). Evidence that cerebral autoregulation impairment during ECMO was related to the patients' neurological outcomes was provided.

Impaired autophagosome clearance contributes to neuronal death in a piglet model of neonatal hypoxic-ischemic encephalopathy

To examine the temporal relationship of cortical autophagic flux with delayed neuronal cell death after hypoxia-ischemia (HI) in neonatal piglets. HI was produced with 45-min hypoxia and 7-min airway occlusion in 3-5-day-old piglets. Markers of autophagic, lysosomal and cell death signaling were studied via immunohistochemistry, immunoblotting, and histochemistry in piglet brains. In vitro, autophagy was impaired in cultured mouse cortical neurons treated with chloroquine with or without rapamycin for 1 d in the presence of Z-VAD-fmk, cyclosporine A, or vehicle control, and cell viability was assessed with the MTT assay. In vivo, neuronal cell death of sensorimotor cortex was delayed by 1-2 days after HI, whereas LC3-II, Beclin-1, PI3KC3, ATG12-ATG-5, and p-ULK1 increased by 1.5-6 h. Autophagosomes accumulated in cortical neurons by 1 d owing to enhanced autophagy and later to decreased autophagosome clearance, as indicated by LC3, Beclin-1, and p62 accumulation. Autophagy flux impairment was attributable to lysosomal dysfunction, as indicated by low lysosomal-associated membrane protein 2, cathepsin B, and cathepsin D levels at 1 d. Ubiquitin levels increased at 1 d. Autophagosome and p62 accumulated predominantly in neurons at 1 d, with p62 puncta occurring in affected cells. Beclin-1 colocalized with markers of caspase-dependent and caspase-independent apoptosis and necrosis in neurons. In vitro, mouse neonatal cortical neurons treated with rapamycin and chloroquine showed increased autophagosomes, but not autolysosomes, and increased cell death that was attenuated by cyclosporine A. Neonatal HI initially increases autophagy but later impairs autophagosome clearance, coinciding with delayed cortical neuronal death.

The Anesthesiologist's Role in Treating Abusive Head Trauma

Abusive head trauma (AHT) is the most common cause of severe traumatic brain injury (TBI) in infants and the leading cause of child abuse-related deaths. For reasons that remain unclear, mortality rates after moderate AHT rival those of severe nonintentional TBI. The vulnerability of the developing brain to injury may be partially responsible for the poor outcomes observed after AHT. AHT is mechanistically more complex than nonintentional TBI. The acute-on-chronic nature of the trauma along with synergistic injury mechanisms that include rapid rotation of the brain, diffuse axonal injury, blunt force trauma, and hypoxia-ischemia make AHT challenging to treat. The anesthesiologist must understand the complex injury mechanisms inherent to AHT, as well as the pediatric TBI treatment guidelines, to decrease the risk of persistent neurologic disability and death. In this review, we discuss the epidemiology of AHT, differences between AHT and nonintentional TBI, the severe pediatric TBI treatment guidelines in the context of AHT, anesthetic considerations, and ethical and legal reporting requirements.

New Wavelet Neurovascular Bundle for Bedside Evaluation of Cerebral Autoregulation and Neurovascular Coupling in Newborns with Hypoxic-Ischemic Encephalopathy

Neonatal encephalopathy (NE) resulting from birth asphyxia constitutes a major global public health burden for millions of infants every year, and despite therapeutic hypothermia, half of these neonates have poor neurological outcomes. As new neuroprotective interventions are being studied in clinical trials, there is a critical need to establish physiological surrogate markers of therapeutic efficacy, to guide patient selection and/or to modify the therapeutic intervention. The challenge in the field of neonatal brain injury has been the difficulty of clinically discerning NE severity within the short therapeutic window after birth or of analyzing the dynamic aspects of the cerebral circulation in sick NE newborns. To address this roadblock, we have recently developed a new "wavelet neurovascular bundle" analytical system that can measure cerebral autoregulation (CA) and neurovascular coupling (NVC) at multiple time scales under dynamic, nonstationary clinical conditions. This wavelet analysis may allow noninvasive quantification at the bedside of (1) CA (combining metrics of blood pressure and cerebral near-infrared spectroscopy, NIRS) and (2) NVC (combining metrics obtained from NIRS and EEG) in newborns with encephalopathy without mathematical assumptions of linear and stationary systems. In this concept paper, we present case examples of NE using the proposed physiological wavelet metrics of CA and NVC. The new approach, once validated in large NE studies, has the potential to optimize the selection of candidates for therapeutic decision-making, and the prediction of neurocognitive outcomes.

High-Dose Cannabidiol Induced Hypotension after Global Hypoxia-Ischemia in Piglets

BACKGROUND

Cannabidiol (CBD) is considered a promising neuroprotectant after perinatal hypoxia-ischemia (HI). We have previously studied the effects of CBD 1 mg/kg in the early phase after global HI in piglets. In contrast to prior studies, we found no evidence of neuroprotection and hypothesized that higher doses might be required to demonstrate efficacy in this animal model.

OBJECTIVE

To assess the safety and potential neuroprotective effects of high-dose CBD.

METHODS

Anesthetized newborn piglets underwent global HI by ventilation with 8% O2 until the point of severe metabolic acidosis (base excess -20 mmol/L) and/or hypotension (mean arterial blood pressure ≤20 mm Hg). Piglets were randomized to intravenous treatment with vehicle (n = 9) or CBD (n = 13). The starting dose, CBD 50 mg/kg, was reduced if adverse effects occurred. The piglets were euthanized 9.5 h after HI and tissue was collected for analysis.

RESULTS

CBD 50 mg/kg (n = 4) induced significant hypotension in 2 out of 4 piglets, and 1 out of 4 piglets suffered a fatal cardiac arrest. CBD 25 mg/kg (n = 4) induced significant hypotension in 1 out of 4 piglets, while 10 mg/kg (n = 5) was well tolerated. A significant negative correlation between the plasma concentration of CBD and hypotension during drug infusion was observed (p < 0.005). Neuroprotective effects were evaluated in piglets that did not display significant hypotension (n = 9) and CBD did not alter the degree of neuronal damage as measured by a neuropathology score, levels of the astrocytic marker S100B in CSF, magnetic resonance spectroscopy markers (Lac/NAA and Glu/NAA ratios), or plasma troponin T.

CONCLUSIONS

High-dose CBD can induce severe hypotension and did not offer neuroprotection in the early phase after global HI in piglets.

Alterations in Cerebral Blood Flow after Resuscitation from Cardiac Arrest

Greater than 50% of patients successfully resuscitated from cardiac arrest have evidence of neurological disability. Numerous studies in children and adults, as well as in animal models have demonstrated that cerebral blood flow (CBF) is impaired after cardiac arrest. Stages of cerebral perfusion post-resuscitation include early hyperemia, followed by hypoperfusion, and finally either resolution of normal blood flow or protracted hyperemia. At the level of the microcirculation the blood flow is heterogeneous, with areas of no flow, low flow, and increased flow. CBF directed therapies in animal models of cardiac arrest improved neurological outcome, and therefore, the alterations in CBF after cardiac arrest likely contribute to the development of hypoxic ischemic encephalopathy. Current intensive care after cardiac arrest is centered upon maintaining systemic oxygenation, normal blood pressure values for age, maintaining general homeostasis, and avoiding hyperthermia. Assessment of CBF and oxygenation is not routinely performed after cardiac arrest. Currently available and underutilized techniques to assess cerebral perfusion include transcranial doppler, near-infrared spectroscopy, and arterial spin labeling magnetic resonance imaging. Limited clinical studies established the role of CBF and oxygenation monitoring in prognostication after cardiac arrest and few studies suggest that guiding critical care post-resuscitation to mean arterial pressures above the minimal autoregulatory range might improve outcome. Important knowledge gaps thus remain in cerebral monitoring and CBF and oxygen goal-directed therapies post-resuscitation from cardiac arrest.