Cerebrovascular reactivity measured by near-infrared spectroscopy

Detection of Impaired Cerebral Autoregulation Using Selected Correlation Analysis: A Validation Study

Multimodal brain monitoring has been utilized to optimize treatment of patients with critical neurological diseases. However, the amount of data requires an integrative tool set to unmask pathological events in a timely fashion. Recently we have introduced a mathematical model allowing the simulation of pathophysiological conditions such as reduced intracranial compliance and impaired autoregulation. Utilizing a mathematical tool set called selected correlation analysis (sca), correlation patterns, which indicate impaired autoregulation, can be detected in patient data sets (scp). In this study we compared the results of the sca with the pressure reactivity index (PRx), an established marker for impaired autoregulation. Mean PRx values were significantly higher in time segments identified as scp compared to segments showing no selected correlations (nsc). The sca based approach predicted cerebral autoregulation failure with a sensitivity of 78.8% and a specificity of 62.6%. Autoregulation failure, as detected by the results of both analysis methods, was significantly correlated with poor outcome. Sca of brain monitoring data detects impaired autoregulation with high sensitivity and sufficient specificity. Since the sca approach allows the simultaneous detection of both major pathological conditions, disturbed autoregulation and reduced compliance, it may become a useful analysis tool for brain multimodal monitoring data.

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.

Optimizing Cerebral Autoregulation May Decrease Neonatal Regional Hypoxic-Ischemic Brain Injury

BACKGROUND

Therapeutic hypothermia provides incomplete neuroprotection for neonatal hypoxic-ischemic encephalopathy (HIE). We examined whether hemodynamic goals that support autoregulation are associated with decreased brain injury and whether these relationships are affected by birth asphyxia or vary by anatomic region.

METHODS

Neonates cooled for HIE received near-infrared spectroscopy autoregulation monitoring to identify the mean arterial blood pressure with optimized autoregulatory function (MAPOPT). Blood pressure deviation from MAPOPT was correlated with brain injury on MRI after adjusting for the effects of arterial carbon dioxide, vasopressors, seizures, and birth asphyxia severity.

RESULTS

Blood pressure deviation from MAPOPT related to neurologic injury in several regions independent of birth asphyxia severity. Greater duration and deviation of blood pressure below MAPOPT were associated with greater injury in the paracentral gyri and white matter. Blood pressure within MAPOPT related to lesser injury in the white matter, putamen and globus pallidus, and brain stem. Finally, blood pressures that exceeded MAPOPT were associated with reduced injury in the paracentral gyri.

CONCLUSIONS

Blood pressure deviation from optimal autoregulatory vasoreactivity was associated with MRI markers of brain injury that, in many regions, were independent of the initial birth asphyxia. Targeting hemodynamic ranges to optimize autoregulation has potential as an adjunctive therapy to hypothermia for HIE.

Cerebral Autoregulation Real-Time Monitoring

Cerebral autoregulation is a mechanism which maintains constant cerebral blood flow (CBF) despite changes in mean arterial pressure (MAP). Assessing whether this mechanism is intact or impaired and determining its boundaries is important in many clinical settings, where primary or secondary injuries to the brain may occur. Herein we describe the development of a new ultrasound tagged near infra red light monitor which tracks CBF trends, in parallel, it continuously measures blood pressure and correlates them to produce a real time autoregulation index. Its performance is validated in both in-vitro experiment and a pre-clinical case study. Results suggest that using such a tool, autoregulation boundaries as well as its impairment or functioning can be identified and assessed. It may therefore assist in individualized MAP management to ensure adequate organ perfusion and reduce the risk of postoperative complications, and might play an important role in patient care.

Gradient adjustment method for better discriminating correlating and non-correlating regions of physiological signals: application to the partitioning of impaired and intact zones of cerebral autoregulation

Cerebral blood flow (CBF) is regulated over a range of systemic blood pressures by the cerebral autoregulation (CA) control mechanism. This range lies within the lower and upper limits of autoregulation (LLA, ULA), beyond which blood pressure drives CBF, and CA function is considered impaired. A standard method to determine autoregulation limits noninvasively using NIRS technology is via the COx measure: a moving correlation index between mean arterial pressure and regional oxygen saturation. In the intact region, there should be no correlation between these variables whereas in the impaired region, the correlation index should approximate unity. In practice, however, the data may be noisy and/or the intact region may often exhibit a slightly positive relationship. This positive relationship may render traditional autoregulation limit calculations difficult to perform, resulting in the need for manual interpretation of the data using arbitrary thresholds. Further, the underlying mathematics of the technique are asymmetric in terms of the results produced for impaired and intact regions and are, in fact, not computable for the ideal case within the intact region. In this work, we propose a novel gradient adjustment method (GACOx) to enhance the differences in COx values observed in the intact and impaired regions. Results from a porcine model (N = 8) are used to demonstrate that GACOx is successful in determining LLA values where traditional methods fail. It is shown that the derived GACOx indices exhibit a mean difference between the intact/impaired regions of 1.54 ± 0.26 (mean ± SD), compared to 0.14 ± 0.10 for the traditional COx method. The GACOx effectively polarizes the COx data in order to better differentiate the intact and impaired zones and, in doing so, makes the determination of the LLA and ULA points a simpler and more consistent task. The method lends itself to the automation of the robust determination of autoregulation zone limits.

Noninvasive methods of detecting increased intracranial pressure

The detection of elevated intracranial pressure (ICP) is of paramount importance in the diagnosis and management of a number of neurologic pathologies. The current gold standard is the use of intraventricular or intraparenchymal catheters; however, this is invasive, expensive, and requires anesthesia. On the other hand, diagnosing intracranial hypertension based on clinical symptoms such as headaches, vomiting, and visual changes lacks sensitivity. As such, there exists a need for a noninvasive yet accurate and reliable method for detecting elevated ICP. In this review, we aim to cover both structural modalities such as computed tomography (CT), magnetic resonance imaging (MRI), ocular ultrasound, fundoscopy, and optical coherence tomography (OCT) as well as functional modalities such as transcranial Doppler ultrasound (TCD), visual evoked potentials (VEPs), and near-infrared spectroscopy (NIRS).

Cerebral blood flow and autoregulation: current measurement techniques and prospects for noninvasive optical methods

Cerebral blood flow (CBF) and cerebral autoregulation (CA) are critically important to maintain proper brain perfusion and supply the brain with the necessary oxygen and energy substrates. Adequate brain perfusion is required to support normal brain function, to achieve successful aging, and to navigate acute and chronic medical conditions. We review the general principles of CBF measurements and the current techniques to measure CBF based on direct intravascular measurements, nuclear medicine, X-ray imaging, magnetic resonance imaging, ultrasound techniques, thermal diffusion, and optical methods. We also review techniques for arterial blood pressure measurements as well as theoretical and experimental methods for the assessment of CA, including recent approaches based on optical techniques. The assessment of cerebral perfusion in the clinical practice is also presented. The comprehensive description of principles, methods, and clinical requirements of CBF and CA measurements highlights the potentially important role that noninvasive optical methods can play in the assessment of neurovascular health. In fact, optical techniques have the ability to provide a noninvasive, quantitative, and continuous monitor of CBF and autoregulation.

Monitoring of cerebral autoregulation

Pressure autoregulation is an important hemodynamic mechanism that protects the brain against inappropriate fluctuations in cerebral blood flow in the face of changing cerebral perfusion pressure (CPP). Static autoregulation represents how far cerebrovascular resistance changes when CPP varies, and dynamic autoregulation represents how fast these changes happen. Both have been monitored in the setting of neurocritical care to aid prognostication and contribute to individualizing CPP targets in patients. Failure of autoregulation is associated with a worse outcome in various acute neurological diseases. Several studies have used transcranial Doppler ultrasound, intracranial pressure (ICP with vascular reactivity as surrogate measure of autoregulation), and near-infrared spectroscopy to continuously monitor the impact of spontaneous fluctuations in CPP on cerebrovascular physiology and to calculate derived variables of autoregulatory efficiency. Many patients who undergo such monitoring demonstrate a range of CPP in which autoregulatory efficiency is optimal. Management of patients at or near this optimal level of CPP is associated with better outcomes in traumatic brain injury. Many of these studies have utilized the concept of the pressure reactivity index, a correlation coefficient between ICP and mean arterial pressure. While further studies are needed, these data suggest that monitoring of autoregulation could aid prognostication and may help identify optimal CPP levels in individual patients.

A pilot cohort study of cerebral autoregulation and 2-year neurodevelopmental outcomes in neonates with hypoxic-ischemic encephalopathy who received therapeutic hypothermia

Background

Neurodevelopmental disabilities persist in survivors of neonatal hypoxic-ischemic encephalopathy (HIE) despite treatment with therapeutic hypothermia. Cerebrovascular autoregulation, the mechanism that maintains cerebral perfusion during changes in blood pressure, may influence outcomes. Our objective was to describe the relationship between acute autoregulatory vasoreactivity during treatment and neurodevelopmental outcomes at 2 years of age.

Methods

In a pilot study of 28 neonates with HIE, we measured cerebral autoregulatory vasoreactivity with the hemoglobin volume index (HVx) during therapeutic hypothermia, rewarming, and the first 6 h of normothermia. The HVx, which is derived from near-infrared spectroscopy, was used to identify the individual optimal mean arterial blood pressure (MAP_OPT) at which autoregulatory vasoreactivity is greatest. Cognitive and motor neurodevelopmental evaluations were completed in 19 children at 21–32 months of age. MAP_OPT, blood pressure in relation to MAP_OPT, blood pressure below gestational age + 5 (ga + 5), and regional cerebral oximetry (rSO₂) were compared to the neurodevelopmental outcomes.

Results

Nineteen children who had HIE and were treated with therapeutic hypothermia performed in the average range on cognitive and motor evaluations at 21–32 months of age, although the mean performance was lower than that of published normative samples. Children with impairments at the 2-year evaluation had higher MAP_OPT values, spent more time with blood pressure below MAP_OPT, and had greater blood pressure deviation below MAP_OPT during rewarming in the neonatal period than those without impairments. Greater blood pressure deviation above MAP_OPT during rewarming was associated with less disability and higher cognitive scores. No association was observed between rSO₂ or blood pressure below ga + 5 and neurodevelopmental outcomes.

Conclusion

In this pilot cohort, motor and cognitive impairments at 21–32 months of age were associated with greater blood pressure deviation below MAP_OPT during rewarming following therapeutic hypothermia, but not with rSO₂ or blood pressure below ga + 5. This suggests that identifying individual neonates’ MAP_OPT is superior to using hemodynamic goals based on gestational age or rSO₂ in the acute management of neonatal HIE.

Electronic supplementary material

The online version of this article (doi:10.1186/s12883-015-0464-4) contains supplementary material, which is available to authorized users.

A pilot cohort study of cerebral autoregulation and 2-year neurodevelopmental outcomes in neonates with hypoxic-ischemic encephalopathy who received therapeutic hypothermia

BACKGROUND

Neurodevelopmental disabilities persist in survivors of neonatal hypoxic-ischemic encephalopathy (HIE) despite treatment with therapeutic hypothermia. Cerebrovascular autoregulation, the mechanism that maintains cerebral perfusion during changes in blood pressure, may influence outcomes. Our objective was to describe the relationship between acute autoregulatory vasoreactivity during treatment and neurodevelopmental outcomes at 2 years of age.

METHODS

In a pilot study of 28 neonates with HIE, we measured cerebral autoregulatory vasoreactivity with the hemoglobin volume index (HVx) during therapeutic hypothermia, rewarming, and the first 6 h of normothermia. The HVx, which is derived from near-infrared spectroscopy, was used to identify the individual optimal mean arterial blood pressure (MAPOPT) at which autoregulatory vasoreactivity is greatest. Cognitive and motor neurodevelopmental evaluations were completed in 19 children at 21-32 months of age. MAPOPT, blood pressure in relation to MAPOPT, blood pressure below gestational age + 5 (ga + 5), and regional cerebral oximetry (rSO2) were compared to the neurodevelopmental outcomes.

RESULTS

Nineteen children who had HIE and were treated with therapeutic hypothermia performed in the average range on cognitive and motor evaluations at 21-32 months of age, although the mean performance was lower than that of published normative samples. Children with impairments at the 2-year evaluation had higher MAPOPT values, spent more time with blood pressure below MAPOPT, and had greater blood pressure deviation below MAPOPT during rewarming in the neonatal period than those without impairments. Greater blood pressure deviation above MAPOPT during rewarming was associated with less disability and higher cognitive scores. No association was observed between rSO2 or blood pressure below ga + 5 and neurodevelopmental outcomes.

CONCLUSION

In this pilot cohort, motor and cognitive impairments at 21-32 months of age were associated with greater blood pressure deviation below MAPOPT during rewarming following therapeutic hypothermia, but not with rSO2 or blood pressure below ga + 5. This suggests that identifying individual neonates' MAPOPT is superior to using hemodynamic goals based on gestational age or rSO2 in the acute management of neonatal HIE.

A pilot cohort study of cerebral autoregulation and 2-year neurodevelopmental outcomes in neonates with hypoxic-ischemic encephalopathy who received therapeutic hypothermia

BACKGROUND

Neurodevelopmental disabilities persist in survivors of neonatal hypoxic-ischemic encephalopathy (HIE) despite treatment with therapeutic hypothermia. Cerebrovascular autoregulation, the mechanism that maintains cerebral perfusion during changes in blood pressure, may influence outcomes. Our objective was to describe the relationship between acute autoregulatory vasoreactivity during treatment and neurodevelopmental outcomes at 2 years of age.

METHODS

In a pilot study of 28 neonates with HIE, we measured cerebral autoregulatory vasoreactivity with the hemoglobin volume index (HVx) during therapeutic hypothermia, rewarming, and the first 6 h of normothermia. The HVx, which is derived from near-infrared spectroscopy, was used to identify the individual optimal mean arterial blood pressure (MAPOPT) at which autoregulatory vasoreactivity is greatest. Cognitive and motor neurodevelopmental evaluations were completed in 19 children at 21-32 months of age. MAPOPT, blood pressure in relation to MAPOPT, blood pressure below gestational age + 5 (ga + 5), and regional cerebral oximetry (rSO2) were compared to the neurodevelopmental outcomes.

RESULTS

Nineteen children who had HIE and were treated with therapeutic hypothermia performed in the average range on cognitive and motor evaluations at 21-32 months of age, although the mean performance was lower than that of published normative samples. Children with impairments at the 2-year evaluation had higher MAPOPT values, spent more time with blood pressure below MAPOPT, and had greater blood pressure deviation below MAPOPT during rewarming in the neonatal period than those without impairments. Greater blood pressure deviation above MAPOPT during rewarming was associated with less disability and higher cognitive scores. No association was observed between rSO2 or blood pressure below ga + 5 and neurodevelopmental outcomes.

CONCLUSION

In this pilot cohort, motor and cognitive impairments at 21-32 months of age were associated with greater blood pressure deviation below MAPOPT during rewarming following therapeutic hypothermia, but not with rSO2 or blood pressure below ga + 5. This suggests that identifying individual neonates' MAPOPT is superior to using hemodynamic goals based on gestational age or rSO2 in the acute management of neonatal HIE.

Trending autoregulatory indices during treatment for traumatic brain injury

Our goal is to use automatic data monitoring for reliable prediction of episodes of intracranial hypertension in patients with traumatic brain injury. Here we test the validity of our method on retrospective patient data. We developed the Continuous Hemodynamic Autoregulatory Monitor (CHARM), that siphons and stores signals from existing monitors in the surgical intensive care unit (SICU), efficiently compresses them, and standardizes the search for statistical relationships between any proposed index and adverse events. CHARM uses an automated event detector to reliably locate episodes of elevated intracranial pressure (ICP), while eliminating artifacts within retrospective patient data. A graphical user interface allowed data scanning, selection of criteria for events, and calculating indices. The pressure reactivity index (PRx), defined as the least square linear regression slope of intracranial pressure versus arterial BP, was calculated for a single case that spanned 259 h. CHARM collected continuous records of ABP, ICP, ECG, SpO2, and ventilation from 29 patients with TBI over an 18-month period. Analysis of a single patient showed that PRx data distribution in the single hours immediately prior to all 16 intracranial hypertensive events, significantly differed from that in the 243 h that did not precede such events (p < 0.0001). The PRx index, however, lacked sufficient resolution as a real-time predictor of IH in this patient. CHARM streamlines the search for reliable predictors of intracranial hypertension. We report statistical evidence supporting the predictive potential of the pressure reactivity index.

Monitoring cerebral autoregulation after brain injury: multimodal assessment of cerebral slow-wave oscillations using near-infrared spectroscopy

BACKGROUND

Continuous monitoring of cerebral autoregulation might provide novel treatment targets and identify therapeutic windows after acute brain injury. Slow oscillations of cerebral hemodynamics (0.05-0.003 Hz) are visible in multimodal neuromonitoring and may be analyzed to provide novel, surrogate measures of autoregulation. Near-infrared spectroscopy (NIRS) is an optical neuromonitoring technique, which shows promise for widespread clinical applicability because it is noninvasive and easily delivered across a wide range of clinical scenarios. The aim of this study is to identify the relationship between NIRS signal oscillations and multimodal neuromonitoring, examining the utility of near infrared derived indices of cerebrovascular reactivity.

METHODS

Twenty-seven sedated, ventilated, brain-injured patients were included in this observational study. Intracranial pressure, transcranial Doppler-derived flow velocity in the middle cerebral artery, and ipsilateral cerebral NIRS variables were continuously monitored. Signals were compared using wavelet measures of phase and coherence to examine the spectral features involved in reactivity index calculations. Established indices of autoregulatory reserve such as the pressure reactivity index (PRx) and mean velocity index (Mx) and the NIRS indices such as total hemoglobin reactivity index (THx) and tissue oxygen reactivity index (TOx) were compared using correlation and Bland-Altman analysis.

RESULTS

NIRS indices correlated significantly between PRx and THx (rs = 0.63, P < 0.001), PRx and TOx (r = 0.40, P = 0.04), and Mx and TOx (r = 0.61, P = 0.004) but not between Mx and THx (rs = 0.26, P = 0.28) and demonstrated wide limits between these variables: PRx and THx (bias, -0.06; 95% limits, -0.44 to 0.32) and Mx and TOx (bias, +0.15; 95% limits, -0.34 to 0.64). Analysis of slow-wave activity throughout the intracranial pressure, transcranial Doppler, and NIRS recordings revealed statistically significant interrelationships, which varied dynamically and were nonsignificant at frequencies <0.008 Hz.

CONCLUSIONS

Although slow-wave activity in intracranial pressure, transcranial Doppler, and NIRS is significantly similar, it varies dynamically in both time and frequency, and this manifests as incomplete agreement between reactivity indices. Analysis informed by a priori knowledge of physiology underpinning NIRS variables combined with sophisticated analysis techniques has the potential to deliver noninvasive surrogate measures of autoregulation, guiding therapy.

A Review of Wavelet Transform Time-Frequency Methods for NIRS-Based Analysis of Cerebral Autoregulation

Near-infrared spectroscopy (NIRS) has been proposed as a suitable technique for the analysis of cerebral autoregulation as it provides a simpler acquisition methodology and more artifact-free signal. A number of sophisticated wavelet transform methods have recently emerged to quantify the cerebral autoregulation mechanism using NIRS and blood pressure signals. These provide an enhanced partitioning of signal information via the time-frequency plane, which facilitates improved extraction of the components of interest. This area is reviewed, and enhancements to this form of analysis are suggested.

An observational near-infrared spectroscopy study on cerebral autoregulation in post-cardiac arrest patients: time to drop 'one-size-fits-all' hemodynamic targets?

AIMS

A subgroup of patients with ROSC after cardiac arrest (CA) with disturbed cerebral autoregulation might benefit from higher mean arterial pressures (MAP). We aimed to (1) phenotype patients with disturbed autoregulation, (2) investigate whether these patients have a worse prognosis, (3) define an individual optimal MAP per patient and (4) investigate whether time under this individual optimal MAP is associated with outcome.

METHODS

Prospective observational study in 51 post-CA patients monitored with near infrared spectroscopy.

RESULTS

(1) 18/51 patients (35%) had disturbed autoregulation. Phenotypically, a higher proportion of patients with disturbed autoregulation had pre-CA hypertension (31±47 vs. 65±49%, p=0.02) suggesting that right shifting of autoregulation is caused by chronic adaptation of cerebral blood flow to higher blood pressures. (2) In multivariate analysis, patients with preserved autoregulation (n=33, 65%) had a significant higher 180-days survival rate (OR 4.62, 95% CI [1.06:20.06], p=0.04]. Based on an index of autoregulation (COX), the average COX-predicted optimal MAP was 85 mmHg in patients with preserved and 100 mmHg in patients with disturbed autoregulation. (3) An individual optimal MAP could be determined in 33/51 patients. (4) The time under the individual optimal MAP was negatively associated with survival (OR 0.97, 95% CI [0.96:0.99], p=0.02). The time under previously proposed fixed targets (65, 70, 75, 80 mmHg) was not associated with a differential survival rate.

CONCLUSION

Cerebral autoregulation showed to be disturbed in 35% of post-CA patients of which a majority had pre-CA hypertension. Disturbed cerebral autoregulation within the first 24h after CA is associated with a worse outcome. In contrast to uniform MAP goals, the time spent under a patient tailored optimal MAP, based on an index of autoregulation, was negatively associated with survival.

Development of Point of Care Testing Device for Neurovascular Coupling From Simultaneous Recording of EEG and NIRS During Anodal Transcranial Direct Current Stimulation

This paper presents a point of care testing device for neurovascular coupling (NVC) from simultaneous recording of electroencephalogram (EEG) and near infrared spectroscopy (NIRS) during anodal transcranial direct current stimulation (tDCS). Here, anodal tDCS modulated cortical neural activity leading to hemodynamic response can be used to identify the impaired cerebral microvessels functionality. The impairments in the cerebral microvessels functionality may lead to impairments in the cerebrovascular reactivity (CVR), where severely reduced CVR predicts the chances of transient ischemic attack and ipsilateral stroke. The neural and hemodynamic responses to anodal tDCS were studied through joint imaging with EEG and NIRS, where NIRS provided optical measurement of changes in tissue oxy-([Formula: see text] and deoxy-([Formula: see text]) hemoglobin concentration and EEG captured alterations in the underlying neuronal current generators. Then, a cross-correlation method for the assessment of NVC underlying the site of anodal tDCS is presented. The feasibility studies on healthy subjects and stroke survivors showed detectable changes in the EEG and the NIRS responses to a 0.526 A/[Formula: see text] of anodal tDCS. The NIRS system was bench tested on 15 healthy subjects that showed a statistically significant (p < 0.01) difference in the signal-to-noise ratio (SNR) between the ON- and OFF-states of anodal tDCS where the mean SNR of the NIRS device was found to be 42.33 ± 1.33 dB in the ON-state and 40.67 ± 1.23 dB in the OFF-state. Moreover, the clinical study conducted on 14 stroke survivors revealed that the lesioned hemisphere with impaired circulation showed significantly (p < 0.01) less change in [Formula: see text] than the nonlesioned side in response to anodal tDCS. The EEG study on healthy subjects showed a statistically significant (p < 0.05) decrease around individual alpha frequency in the alpha band (8-13 Hz) following anodal tDCS. Moreover, the joint EEG-NIRS imaging on 4 stroke survivors showed an immediate increase in the theta band (4-8 Hz) EEG activity after the start of anodal tDCS at the nonlesioned hemisphere. Furthermore, cross-correlation function revealed a significant (95% confidence interval) negative cross correlation only at the nonlesioned hemisphere during anodal tDCS, where the log-transformed mean-power of EEG within 0.5-11.25 Hz lagged [Formula: see text] response in one of the stroke survivors with white matter lesions. Therefore, it was concluded that the anodal tDCS can perturb the local neural and the vascular activity (via NVC) which can be used for assessing regional NVC functionality where confirmatory clinical studies are required.

The ontogeny of cerebrovascular pressure autoregulation in premature infants

OBJECTIVE

To quantify cerebrovascular autoregulation as a function of gestational age (GA) and across the phases of the cardiac cycle.

STUDY DESIGN

The present study is a hypothesis-generating re-analysis of previously published data. Premature infants (n=179) with a GA range of 23 to 33 weeks were monitored with umbilical artery catheters and transcranial Doppler insonation of the middle cerebral artery for 1-h sessions over the first week of life. Autoregulation was quantified by three methods, as a moving correlation coefficient between: (1) systolic arterial blood pressure (ABP) and systolic cerebral blood flow (CBF) velocity (Sx); (2) mean ABP and mean CBF velocity (Mx); and (3) diastolic ABP and diastolic CBF velocity (Dx). Comparisons of individual and cohort cerebrovascular pressure autoregulation were made across GA for each aspect of the cardiac cycle.

RESULTS

Systolic, mean and diastolic ABP increased with GA (r=0.3, 0.4 and 0.4; P<0.0001). Systolic CBF velocity was pressure-passive in infants with the lowest GA, and Sx decreased with advancing GA (r=-0.3; P<0.001), indicating increased capacity for cerebral autoregulation during systole during development. By contrast, Dx was elevated, indicating dysautoregulation, in all subjects and showed minimal change with advancing GA (r=-0.06; P=0.05). Multivariate analysis confirmed that both GA (P<0.001) and 'effective cerebral perfusion pressure' (ABP minus critical closing pressure (CrCP); P<0.01) were associated with Sx.

CONCLUSION

Premature infants have low and usually pressure-passive diastolic CBF velocity. By contrast, the regulation of systolic CBF velocity by pressure autoregulation developed in this cohort between 23 and 33 weeks GA. Elevated effective cerebral perfusion pressure derived from the CrCP was associated with dysautoregulation.

Ozone autohemotherapy induces long-term cerebral metabolic changes in multiple sclerosis patients

Ozone autohemotherapy is an emerging therapeutic technique that is gaining increasing importance in treating neurological disorders. A validated and standard methodology to assess the effect of such therapy on brain metabolism and circulation is however still lacking. We used a near-infrared spectroscopy (NIRS) system to monitor the cerebral metabolism and a transcranial Doppler (TCD) to monitor the blood flow velocity in the middle cerebral arteries. Fifty-four subjects (32 neurological patients and 22 controls) were tested before, during, and after ozone autohemotherapy. We monitored the concentration changes in the level of oxygenated and deoxygenated haemoglobin, and in the level of the Cytochrome-c-oxidase (CYT-c). As a primary endpoint of the work, we showed the changes in the brain metabolism and circulation of the entire population. The concentration of oxygenated haemoglobin increased after the reinjection of the ozoned blood and remained higher than the beginning for another 1.5 hours. The concentration of the deoxygenated haemoglobin decreased during the therapy and the CYT-c concentration markedly increased about 1 hour after the reinjection. No significant changes were observed on the blood flow velocity. As secondary endpoint, we compared the NIRS metabolic pattern of 20 remitting-relapsing multiple sclerosis (MS) patients against 20 controls. We showed that by using only 7 NIRS variables it was possible to characterize the metabolic brain pattern of the two groups of subjects. The MS subjects showed a marked increase of the CYT-c activity and concentration about 40 minutes after the end of the autohemotherapy, possibly revealing a reduction of the chronic oxidative stress level typical of MS sufferers. From a technical point of view, this preliminary study showed that NIRS could be useful to show the effects of ozone autohemotherapy at cerebral level, in a long-term monitoring. The clinical result of this study is the quantitative measurement of the CYT-c level changes in MS induced by ozone autohemotherapy.

Apparent diffusion coefficient scalars correlate with near-infrared spectroscopy markers of cerebrovascular autoregulation in neonates cooled for perinatal hypoxic-ischemic injury

BACKGROUND AND PURPOSE

Neurologic morbidity remains high in neonates with perinatal hypoxic-ischemic injury despite therapeutic hypothermia. DTI provides qualitative and quantitative information about the microstructure of the brain, and a near-infrared spectroscopy index can assess cerebrovascular autoregulation. We hypothesized that lower ADC values would correlate with worse autoregulatory function.

MATERIALS AND METHODS

Thirty-one neonates with hypoxic-ischemic injury were enrolled. ADC scalars were measured in 27 neonates (age range, 4-15 days) in the anterior and posterior centrum semiovale, basal ganglia, thalamus, posterior limb of the internal capsule, pons, and middle cerebellar peduncle on MRI obtained after completion of therapeutic hypothermia. The blood pressure range of each neonate with the most robust autoregulation was identified by using a near-infrared spectroscopy index. Autoregulatory function was measured by blood pressure deviation below the range with optimal autoregulation.

RESULTS

In neonates who had MRI on day of life ≥10, lower ADC scalars in the posterior centrum semiovale (r = -0.87, P = .003, n = 9) and the posterior limb of the internal capsule (r = -0.68, P = .04, n = 9) correlated with blood pressure deviation below the range with optimal autoregulation during hypothermia. Lower ADC scalars in the basal ganglia correlated with worse autoregulation during rewarming (r = -0.71, P = .05, n = 8).

CONCLUSIONS

Blood pressure deviation from the optimal autoregulatory range may be an early biomarker of injury in the posterior centrum semiovale, posterior limb of the internal capsule, and basal ganglia. Optimizing blood pressure to support autoregulation may decrease the risk of brain injury in cooled neonates with hypoxic-ischemic injury.

A pilot study of cerebrovascular reactivity autoregulation after pediatric cardiac arrest

AIM

Improved survival after cardiac arrest has placed greater emphasis on neurologic resuscitation. The purpose of this pilot study was to evaluate the relationship between cerebrovascular autoregulation and neurologic outcomes after pediatric cardiac arrest.

METHODS

Children resuscitated from cardiac arrest had autoregulation monitoring during the first 72h after return of circulation with an index derived from near-infrared spectroscopy in a pilot study. The range of mean arterial blood pressure (MAP) with optimal vasoreactivity (MAPOPT) was identified. The area under the curve (AUC) of the time spent with MAP below MAPOPT and MAP deviation below MAPOPT was calculated. Neurologic outcome measures included placement of a new tracheostomy or gastrostomy, death from a primary neurologic etiology (brain death or withdrawal of support for neurologic futility), and change in the Pediatric Cerebral Performance Category score (ΔPCPC).

RESULTS

Thirty-six children were monitored. Among children who did not require extracorporeal membrane oxygenation (ECMO), children who received a tracheostomy/gastrostomy had greater AUC during the second 24h after resuscitation than those who did not (P=0.04; n=19). Children without ECMO who died from a neurologic etiology had greater AUC during the first 48h than did those who lived or died from cardiovascular failure (P=0.04; n=19). AUC below MAPOPT was not associated with ΔPCPC when children with or without ECMO were analyzed separately.

CONCLUSIONS

Deviation from the blood pressure with optimal autoregulatory vasoreactivity may predict poor neurologic outcomes after pediatric cardiac arrest. This experimental autoregulation monitoring technique may help individualize blood pressure management goals after resuscitation.

Intraoperative blood pressure and cerebral perfusion: strategies to clarify hemodynamic goals

Blood pressure can vary considerably during anesthesia. If blood pressure falls outside the limits of cerebrovascular autoregulation, children can become at risk of cerebral ischemic or hyperemic injury. However, the blood pressure limits of autoregulation are unclear in infants and children, and these limits can shift after brain injury. This article will review autoregulation, considerations for the hemodynamic management of children with brain injuries, and research on autoregulation monitoring techniques.

Cerebrovascular autoregulation and neurologic injury in neonatal hypoxic-ischemic encephalopathy

BACKGROUND

Neonates with hypoxic-ischemic encephalopathy (HIE) are at risk of cerebral blood flow dysregulation. Our objective was to describe the relationship between autoregulation and neurologic injury in HIE.

METHODS

Neonates with HIE had autoregulation monitoring with the hemoglobin volume index (HVx) during therapeutic hypothermia, rewarming, and the first 6 h of normothermia. The 5-mm Hg range of mean arterial blood pressure (MAP) with best vasoreactivity (MAPOPT) was identified. The percentage of time spent with MAP below MAPOPT and deviation in MAP from MAPOPT were measured. Neonates received brain magnetic resonance imaging (MRI) 3-7 d after treatment. MRIs were coded as no, mild, or moderate/severe injury in five regions.

RESULTS

HVx identified MAPOPT in 79% (19/24), 77% (17/22), and 86% (18/21) of the neonates during hypothermia, rewarming, and normothermia, respectively. Neonates with moderate/severe injury in paracentral gyri, white matter, basal ganglia, and thalamus spent a greater proportion of time with MAP below MAPOPT during rewarming than neonates with no or mild injury. Neonates with moderate/severe injury in paracentral gyri, basal ganglia, and thalamus had greater MAP deviation below MAPOPT during rewarming than neonates without injury.

CONCLUSION

Maintaining MAP within or above MAPOPT may reduce the risk of neurologic injuries in neonatal HIE.

Continuous cerebrovascular reactivity monitoring and autoregulation monitoring identify similar lower limits of autoregulation in patients undergoing cardiopulmonary bypass

OBJECTIVES

Cerebrovascular autoregulation can be monitored with a moving linear correlation of blood pressure to cerebral blood flow velocity (mean velocity index, Mx) during cardiopulmonary bypass (CPB). Vascular reactivity can be monitored with a moving linear correlation of blood pressure to cerebral blood volume trended with near-infrared spectroscopy (hemoglobin volume index, HVx). We hypothesized that the lower limits of autoregulation (LLA) and the optimal blood pressure (ABPopt) associated with the most active autoregulation could be determined by HVx in patients undergoing CPB.

METHODS

Adult patients (n = 109) who underwent CPB for cardiac surgery had monitoring of both autoregulation (Mx) and vascular reactivity (HVx). Individual curves of Mx and HVx were constructed by placing each in 5 mmHg bins. The LLA and ABPopt for each subject were then identified by both methods and compared for agreement by correlation analysis and Bland-Altman.

RESULTS

The average LLA defined by Mx compared to HVx were comparable (66±13 and 66±12 mmHg). Correlation between the LLA defined by Mx and HVx was significant (Pearson r = 0.2867; P = 0.0068). The average ABPopt with the most robust autoregulation by Mx was comparable to HVx (75±11 and 74±13 mmHg) with significant correlation (Pearson r = 0.5915; P < or =0.0001).

DISCUSSION

Autoregulation and vascular reactivity monitoring are expected to be distinct, as flow and volume have different phasic relationships to pressure when cerebrovascular autoregulation is active. However, the two metrics have good agreement when identifying the LLA and optimal blood pressure in patients during CPB.

Cerebrovascular autoregulation after rewarming from hypothermia in a neonatal swine model of asphyxic brain injury

After hypoxic brain injury, maintaining blood pressure within the limits of cerebral blood flow autoregulation is critical to preventing secondary brain injury. Little is known about the effects of prolonged hypothermia or rewarming on autoregulation after cardiac arrest. We hypothesized that rewarming would shift the lower limit of autoregulation (LLA), that this shift would be detected by indices derived from near-infrared spectroscopy (NIRS), and that rewarming would impair autoregulation during hypertension. Anesthetized neonatal swine underwent sham surgery or hypoxic-asphyxic cardiac arrest, followed by 2 h of normothermia and 20 h of hypothermia, with or without rewarming. Piglets were further divided into cohorts for cortical laser-Doppler flow (LDF) measurements during induced hypotension or hypertension. We also tested whether indices derived from NIRS could identify the LDF-derived LLA. The LLA did not differ significantly among groups with sham surgery and hypothermia (29 ± 8 mmHg), sham surgery and rewarming (34 ± 7 mmHg), arrest and hypothermia (29 ± 10 mmHg), and arrest and rewarming (38 ± 11 mmHg). The LLA was not affected by arrest (P = 0.60), temperature (P = 0.08), or interaction between arrest and temperature (P = 0.73). The NIRS-derived indices detected the LLA accurately, with the area under the receiver-operator characteristic curves of 0.81-0.96 among groups. In groups subjected to arrest and hypothermia, with or without rewarming, the slope of LDF relative to cerebral perfusion pressure during hypertension was not significantly different from zero (P > 0.10). In conclusion, rewarming did not shift the LLA during hypotension or affect autoregulation during hypertension after asphyxic cardiac arrest. The NIRS-derived autoregulation indices identified the LLA accurately.

Cerebral blood flow autoregulation is preserved after continuous-flow left ventricular assist device implantation

OBJECTIVE

To compare cerebral blood flow (CBF) autoregulation in patients undergoing continuous-flow left ventricular assist device (LVAD) implantation with that in patients undergoing coronary artery bypass grafting (CABG).

DESIGN

Prospective, observational, controlled study.

SETTING

Academic medical center.

PARTICIPANTS

Fifteen patients undergoing LVAD insertion and 10 patients undergoing CABG.

MEASUREMENTS AND MAIN RESULTS

Cerebral autoregulation was monitored with transcranial Doppler and near-infrared spectroscopy. A continuous Pearson correlation coefficient was calculated between mean arterial pressure (MAP) and CBF velocity and between MAP and near-infrared spectroscopic data, rendering the variables mean velocity index (Mx) and cerebral oximetry index (COx), respectively. Mx and COx approach 0 when autoregulation is intact (no correlation between CBF and MAP), but approach 1 when autoregulation is impaired. Mx was lower during and immediately after cardiopulmonary bypass in the LVAD group than in the CABG group, indicating better-preserved autoregulation. Based on COx monitoring, autoregulation tended to be better preserved in the LVAD group than in the CABG group immediately after surgery (p = 0.0906). On postoperative day 1, COx was lower in the LVAD group than in the CABG group, indicating preserved CBF autoregulation (p = 0.0410). Based on COx monitoring, 3 patients (30%) in the CABG group had abnormal autoregulation (COx ≥0.3) on the first postoperative day but no patient in the LVAD group had this abnormality (p = 0.037).

CONCLUSIONS

These data suggest that CBF autoregulation is preserved during and immediately after surgery in patients undergoing LVAD insertion.

Physiological monitoring of the severe traumatic brain injury patient in the intensive care unit

Traumatic brain injury (TBI) is a major cause of morbidity and mortality worldwide. Despite encouraging animal research, pharmacological agents and neuroprotectants have disappointed in the clinical environment. Current TBI management therefore is directed towards identification, prevention, and treatment of secondary cerebral insults that are known to exacerbate outcome after injury. This strategy is based on a variety of monitoring techniques that include the neurological examination, imaging, laboratory analysis, and physiological monitoring of the brain and other organ systems used to guide therapeutic interventions. Recent clinical series suggest that TBI management informed by multimodality monitoring is associated with improved patient outcome, in part because care is provided in a patient-specific manner. In this review we discuss physiological monitoring of the brain after TBI and the emerging field of neurocritical care bioinformatics.

Cerebrovascular autoregulation in pediatric moyamoya disease

BACKGROUND

Moyamoya syndrome carries a high risk of cerebral ischemia, and impaired cerebrovascular autoregulation may play a critical role. Autoregulation indices derived from near-infrared spectroscopy (NIRS) may clarify hemodynamic goals that conform to the limits of autoregulation.

OBJECTIVES

The aims of this pilot study were to determine whether the NIRS-derived indices could identify blood pressure ranges that optimize autoregulation and whether autoregulatory function differs between anatomic sides in patients with unilateral vasculopathy.

METHODS

Pediatric patients undergoing indirect surgical revascularization for moyamoya were enrolled sequentially. NIRS-derived autoregulation indices, the cerebral oximetry index (COx) and the hemoglobin volume index (HVx), were calculated intraoperatively and postoperatively to measure autoregulatory function. The 5-mmHg ranges of optimal mean arterial blood pressure (MAPOPT ) with best autoregulation and the lower limit of autoregulation (LLA) were identified.

RESULTS

Of seven enrolled patients (aged 2-16 years), six had intraoperative and postoperative autoregulation monitoring and one had only intraoperative monitoring. Intraoperative MAPOPT was identified in six (86%) of seven patients with median values of 60-80 mmHg. Intraoperative LLA was identified in three (43%) patients with median values of 55-65 mmHg. Postoperative MAPOPT was identified in six (100%) of six patients with median values of 70-90 mmHg. Patients with unilateral disease had higher intraoperative HVx (P = 0.012) on side vasculopathy.

CONCLUSIONS

NIRS-derived indices may identify hemodynamic goals that optimize autoregulation in pediatric moyamoya.

Detection of neurologic injury using vascular reactivity monitoring and glial fibrillary acidic protein

New noninvasive methods for monitoring cerebrovascular pressure reactivity coupled with a blood-based assay for brain-specific injury in preterm infants could allow early diagnosis of brain injury and set the stage for improved timing and effectiveness of interventions. Using an adaptation of near-infrared spectroscopy, we report a case of a very low birth weight infant undergoing hemoglobin volume index monitoring as a measure of cerebrovascular pressure reactivity. During the monitoring period, this infant demonstrated significant disturbances in cerebrovascular pressure reactivity that coincided with elevation of serum glial fibrillary acidic protein and new findings of brain injury on head ultrasound. This case report demonstrates the potential of emerging noninvasive monitoring methods to assist in both detection and therapeutic management to improve neurologic outcomes of the very low birth weight neonate.

Validation of a stand-alone near-infrared spectroscopy system for monitoring cerebral autoregulation during cardiac surgery

BACKGROUND

Individualizing arterial blood pressure (ABP) targets during cardiopulmonary bypass (CPB) based on cerebral blood flow (CBF) autoregulation monitoring may provide a more effective means for preventing cerebral hypoperfusion than the current standard of care. Autoregulation can be monitored in real time with transcranial Doppler (TCD). We have previously demonstrated that near-infrared spectroscopy (NIRS)-derived regional cerebral oxygen saturation (rS(c)O(2)) provides a clinically suitable surrogate of CBF for autoregulation monitoring. The purpose of this study was to determine the accuracy of a stand-alone "plug-and-play" investigational system for autoregulation monitoring that uses a commercially available NIRS monitor with TCD methods.

METHODS

TCD monitoring of middle cerebral artery CBF velocity and NIRS monitoring were performed in 70 patients during CPB. Indices of autoregulation were computed by both a personal computer-based system and an investigational prototype NIRS-based monitor. A moving linear correlation coefficient between slow waves of ABP and CBF velocity (mean velocity index [Mx]) and between ABP and rS(c)O(2) (cerebral oximetry index [COx]) were calculated. When CBF is autoregulated, there is no correlation between CBF and ABP; when CBF is dysregulated, Mx and COx approach 1 (i.e., CBF and ABP are correlated). Linear regression and bias analysis were performed between time-averaged values of Mx and COx derived from the personal computer-based system and from COx measured with the prototype monitor. Values for Mx and COx were categorized in 5 mm Hg bins of ABP for each patient. The lower limit of CBF autoregulation was defined as the ABP where Mx incrementally increased to ≥0.4.

RESULTS

There was correlation and good agreement between COx derived from the prototype monitor and Mx (r = 0.510; 95% confidence interval, 0.414-0.595; P < 0.001; bias, -0.07 ± 0.19). The correlation and bias between the personal computer-based COx and the COx from the prototype NIRS monitor were r = 0.957 (95% confidence interval, 0.945-0.966; P < 0.001 and 0.06 ± 0.06, respectively). The average ABP at the lower limit of autoregulation was 63 ± 11 mm Hg (95% prediction interval, 52-74 mm Hg). Although the mean ABP at the COx-determined lower limit of autoregulation determined with the prototype monitor was statistically different from that determined by Mx (59 ± 9 mm Hg; 95% prediction interval, 50-68 mm Hg; P = 0.026), the difference was not likely clinically meaningful.

CONCLUSIONS

Monitoring CBF autoregulation with an investigational stand-alone NIRS monitor is correlated and in good agreement with TCD-based methods. The availability of such a device would allow widespread autoregulation monitoring as a means of individualizing ABP targets during CPB.

Positive end-expiratory pressure oscillation facilitates brain vascular reactivity monitoring

The pressure reactivity index (PRx) identifies optimal cerebral perfusion pressure after traumatic brain injury. We describe a method to improve PRx precision by induced variations in arterial blood pressure (ABP) using positive end-expiratory pressure (PEEP) modulation ( iPRx). Neonatal swine ( n = 10) were ventilated with static PEEP and then with PEEP oscillated between 5 and 10 cmH2O at a frequency of 1/min. PRx was recorded as a moving correlation coefficient between ABP and intracranial pressure (ICP) from spontaneous ABP activity (0.05-0.003 Hz) during static PEEP. iPRx was similarly recorded with PEEP oscillation-induced ABP waves. The lower limit of autoregulation (LLA) was delineated with continuous cortical laser Doppler flux monitoring. PEEP oscillation increased autoregulation-monitoring precision. The ratios of median absolute deviations to range of possible values for the PRx and iPRx were 9.5% (8.3–13.7%) and 6.2% (4.2–8.7%), respectively ( P = 0.006; median, interquartile range). The phase-angle difference between ABP and ICP above LLA was 161° (150°–166°) and below LLA, −31° (−42° to 12°, P < 0.0001). iPRx above LLA was −0.42 (−0.67 to −0.29) and below LLA, 0.32 (0.22–0.43, P = 0.0004). A positive iPRx was 97% specific and 91% sensitive for perfusion pressure below LLA. PEEP oscillation caused stable, low-frequency ABP oscillations that reduced noise in the PRx. Safe translation of these findings to clinical settings is expected to yield more accurate and rapid delineation of individualized optimal perfusion-pressure goals for patients.

Modelling cerebrovascular reactivity: a novel near-infrared biomarker of cerebral autoregulation?

Understanding changes in cerebral oxygenation, haemodynamics and metabolism holds the key to individualised, optimised therapy after acute brain injury. Near-infrared spectroscopy (NIRS) offers the potential for non-invasive, continuous bedside measurement of surrogates for these processes. Interest has grown in applying this technique to interpret cerebrovascular pressure reactivity (CVPR), a surrogate of the brain's ability to autoregulate blood flow. We describe a physiological model-based approach to NIRS interpretation which predicts autoregulatory efficiency from a model parameter k_aut. Data from three critically brain-injured patients exhibiting a change in CVPR were investigated. An optimal value for k_aut was determined to minimise the difference between measured and simulated outputs. Optimal values for k_aut appropriately tracked changes in CVPR under most circumstances. Further development of this technique could be used to track CVPR providing targets for individualised management of patients with altered vascular reactivity, minimising secondary neurological insults.

Renovascular reactivity measured by near-infrared spectroscopy

Hypotension and shock are risk factors for death, renal insufficiency, and stroke in preterm neonates. Goal-directed neonatal hemodynamic management lacks end-organ monitoring strategies to assess the adequacy of perfusion. Our aim is to develop a clinically viable, continuous metric of renovascular reactivity to gauge renal perfusion during shock. We present the renovascular reactivity index (RVx), which quantifies passivity of renal blood volume to spontaneous changes in arterial blood pressure. We tested the ability of the RVx to detect reductions in renal blood flow. Hemorrhagic shock was induced in 10 piglets. The RVx was monitored as a correlation between slow waves of arterial blood pressure and relative total hemoglobin (rTHb) obtained with reflectance near-infrared spectroscopy (NIRS) over the kidney. The RVx was compared with laser-Doppler measurements of red blood cell flux, and renal laser-Doppler measurements were compared with cerebral laser-Doppler measurements. Renal blood flow decreased to 75%, 50%, and 25% of baseline at perfusion pressures of 60, 45, and 40 mmHg, respectively, whereas in the brain these decrements occurred at pressures of 30, 25, and 15 mmHg, respectively. The RVx compared favorably to the renal laser-Doppler data. Areas under the receiver operator characteristic curves using renal blood flow thresholds of 50% and 25% of baseline were 0.85 (95% CI, 0.83–0.87) and 0.90 (95% CI, 0.88–0.92). Renovascular autoregulation can be monitored and is impaired in advance of cerebrovascular autoregulation during hemorrhagic shock.

Multimodal intracranial monitoring: implications for clinical practice

This article presents an overview of intracranial monitoring techniques during the perioperative and intensive care management of neurologic patients. Various regional and global brain monitors are available; some modalities are well established whereas others are new to the clinical arena and their indications are still being evaluated. Indications for monitoring are reviewed, modalities critically evaluated, and future directions identified.

Non-invasive methods of estimating intracranial pressure

Non-invasive measurement of intracranial pressure can be invaluable in the management of critically ill patients. We performed a comprehensive review of the literature to evaluate the different methods of measuring intracranial pressure. Several methods have been employed to estimate intracranial pressure, including computed tomography, magnetic resonance imaging, transcranial Doppler sonography, near-infrared spectroscopy, and visual-evoked potentials. In addition, multiple techniques of measuring the optic nerve and the optic nerve sheath diameter have been studied. Ultrasound measurements of the optic nerve sheath diameter and Doppler flow are especially promising and may be useful in selected settings.

Noninvasive autoregulation monitoring in a swine model of pediatric cardiac arrest

BACKGROUND

Cerebrovascular autoregulation after resuscitation has not been well studied in an experimental model of pediatric cardiac arrest. Furthermore, developing noninvasive methods of monitoring autoregulation using near-infrared spectroscopy (NIRS) would be clinically useful in guiding neuroprotective hemodynamic management after pediatric cardiac arrest. We tested the hypotheses that the lower limit of autoregulation (LLA) would shift to a higher arterial blood pressure between 1 and 2 days of recovery after cardiac arrest and that the LLA would be detected by NIRS-derived indices of autoregulation in a swine model of pediatric cardiac arrest. We also tested the hypothesis that autoregulation with hypertension would be impaired after cardiac arrest.

METHODS

Data on LLA were obtained from neonatal piglets that had undergone hypoxic-asphyxic cardiac arrest and recovery for 1 day (n = 8) or 2 days (n = 8), or that had undergone sham surgery with 2 days of recovery (n = 8). Autoregulation with hypertension was examined in a separate cohort of piglets that underwent hypoxic-asphyxic cardiac arrest (n = 5) or sham surgery (n = 5) with 2 days of recovery. After the recovery period, piglets were reanesthetized, and autoregulation was monitored by standard laser-Doppler flowmetry and autoregulation indices derived from NIRS (the cerebral oximetry [COx] and hemoglobin volume [HVx] indices). The LLA was determined by decreasing blood pressure through inflation of a balloon catheter in the inferior vena cava. Autoregulation during hypertension was evaluated by inflation of an aortic balloon catheter.

RESULTS

The LLAs were similar between sham-operated piglets and piglets that recovered for 1 or 2 days after arrest. The NIRS-derived indices accurately detected the LLA determined by laser-Doppler flowmetry. The area under the curve of the receiver operator characteristic curve for cerebral oximetry index was 0.91 at 1 day and 0.92 at 2 days after arrest. The area under the curve for hemoglobin volume index was 0.92 and 0.89 at the respective time points. During induced hypertension, the static rate of autoregulation, defined as the percentage change in cerebrovascular resistance divided by the percentage change in cerebral perfusion pressure, was not different between postarrest and sham-operated piglets. At 2 days recovery from arrest, piglets exhibited neurobehavioral deficits and histologic neuronal injury.

CONCLUSIONS

In a swine model of pediatric hypoxic-asphyxic cardiac arrest with confirmed brain damage, the LLA did not differ 1 and 2 days after resuscitation. The NIRS-derived indices accurately detected the LLA in comparison with laser-Doppler flow measurements at those time points. Autoregulation remained functional during hypertension.

What comes first? The dynamics of cerebral oxygenation and blood flow in response to changes in arterial pressure and intracranial pressure after head injury

BACKGROUND

Brain tissue partial oxygen pressure (Pbt(O(2))) and near-infrared spectroscopy (NIRS) are novel methods to evaluate cerebral oxygenation. We studied the response patterns of Pbt(O(2)), NIRS, and cerebral blood flow velocity (CBFV) to changes in arterial pressure (AP) and intracranial pressure (ICP).

METHODS

Digital recordings of multimodal brain monitoring from 42 head-injured patients were retrospectively analysed. Response latencies and patterns of Pbt(O(2)), NIRS-derived parameters [tissue oxygenation index (TOI) and total haemoglobin index (THI)], and CBFV reactions to fluctuations of AP and ICP were studied.

RESULTS

One hundred and twenty-one events were identified. In reaction to alterations of AP, ICP reacted first [4.3 s; inter-quartile range (IQR) -4.9 to 22.0 s, followed by NIRS-derived parameters and CBFV (10.9 s; IQR: -5.9 to 39.6 s, 12.1 s; IQR: -3.0 to 49.1 s, 14.7 s; IQR: -8.8 to 52.3 s for THI, CBFV, and TOI, respectively), with Pbt(O(2)) reacting last (39.6 s; IQR: 16.4 to 66.0 s). The differences in reaction time between NIRS parameters and Pbt(O(2)) were significant (P<0.001). Similarly when reactions to ICP changes were analysed, NIRS parameters preceded Pbt(O(2)) (7.1 s; IQR: -8.8 to 195.0 s, 18.1 s; IQR: -20.6 to 80.7 s, 22.9 s; IQR: 11.0 to 53.0 s for THI, TOI, and Pbt(O(2)), respectively). Two main patterns of responses to AP changes were identified. With preserved cerebrovascular reactivity, TOI and Pbt(O(2)) followed the direction of AP. With impaired cerebrovascular reactivity, TOI and Pbt(O(2)) decreased while AP and ICP increased. In 77% of events, the direction of TOI changes was concordant with Pbt(O(2)).

CONCLUSIONS

NIRS and transcranial Doppler signals reacted first to AP and ICP changes. The reaction of Pbt(O(2)) is delayed. The results imply that the analysed modalities monitor different stages of cerebral oxygenation.

Predicting the limits of cerebral autoregulation during cardiopulmonary bypass

BACKGROUND

Mean arterial blood pressure (MAP) targets are empirically chosen during cardiopulmonary bypass (CPB). We have previously shown that near-infrared spectroscopy (NIRS) can be used clinically for monitoring cerebral blood flow autoregulation. The hypothesis of this study was that real-time autoregulation monitoring using NIRS-based methods is more accurate for delineating the MAP at the lower limit of autoregulation (LLA) during CPB than empiric determinations based on age, preoperative history, and preoperative blood pressure.

METHODS

Two hundred thirty-two patients undergoing coronary artery bypass graft and/or valve surgery with CPB underwent transcranial Doppler monitoring of the middle cerebral arteries and NIRS monitoring. A continuous, moving Pearson correlation coefficient was calculated between MAP and cerebral blood flow velocity and between MAP and NIRS data to generate mean velocity index and cerebral oximeter index. When autoregulated, there is no correlation between cerebral blood flow and MAP (i.e., mean velocity and cerebral oximetry indices approach 0); when MAP is below the LLA, mean velocity and cerebral oximetry indices approach 1. The LLA was defined as the MAP at which mean velocity index increased with declining MAP to ≥ 0.4. Linear regression was performed to assess the relation between preoperative systolic blood pressure, MAP, MAP in 10% decrements from baseline, and average cerebral oximetry index with MAP at the LLA.

RESULTS

The MAP at the LLA was 66 mm Hg (95% prediction interval, 43 to 90 mm Hg) for the 225 patients in which this limit was observed. There was no relationship between preoperative MAP and the LLA (P = 0.829) after adjusting for age, gender, prior stroke, diabetes, and hypertension, but a cerebral oximetry index value of >0.5 was associated with the LLA (P = 0.022). The LLA could be identified with cerebral oximetry index in 219 (94.4%) patients. The mean difference in the LLA for mean velocity index versus cerebral oximetry index was -0.2 ± 10.2 mm Hg (95% CI, -1.5 to 1.2 mm Hg). Preoperative systolic blood pressure was associated with a higher LLA (P = 0.046) but only for those with systolic blood pressure ≤ 160 mm Hg.

CONCLUSIONS

There is a wide range of MAP at the LLA in patients during CPB, making estimation of this target difficult. Real-time monitoring of autoregulation with cerebral oximetry index may provide a more rational means for individualizing MAP during CPB.

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

OBJECTIVE

Knowledge remains limited regarding cerebral blood flow autoregulation after cardiac arrest and during postresuscitation hypothermia. We determined the relationship of cerebral blood flow to cerebral perfusion pressure in a swine model of pediatric hypoxic-asphyxic cardiac arrest during normothermia and hypothermia and tested novel measures of autoregulation derived from near-infrared spectroscopy.

DESIGN

Prospective, balanced animal study.

SETTING

Basic physiology laboratory at an academic institution.

SUBJECTS

Eighty-four neonatal swine.

INTERVENTIONS

Piglets underwent hypoxic-asphyxic cardiac arrest or sham surgery and recovered for 2 hrs with normothermia followed by 4 hrs of either moderate hypothermia or normothermia. In half of the groups, blood pressure was slowly decreased through inflation of a balloon catheter in the inferior vena cava to identify the lower limit of cerebral autoregulation at 6 hrs postresuscitation. In the remaining groups, blood pressure was gradually increased by inflation of a balloon catheter in the aorta to determine the autoregulatory response to hypertension. Measures of autoregulation obtained from standard laser-Doppler flowmetry and indices derived from near-infrared spectroscopy were compared.

MEASUREMENTS AND MAIN RESULTS

Laser-Doppler flux was lower in postarrest animals compared to sham-operated controls during the 2-hr normothermic period after resuscitation. During the subsequent 4-hr recovery, hypothermia decreased laser-Doppler flux in both the sham surgery and postarrest groups. Autoregulation was intact during hypertension in all groups. With arterial hypotension, postarrest, hypothermic piglets had a significant decrease in the perfusion pressure lower limit of autoregulation compared to postarrest, normothermic piglets. The near-infrared spectroscopy-derived measures of autoregulation accurately detected loss of autoregulation during hypotension.

CONCLUSIONS

In a pediatric model of cardiac arrest and resuscitation, delayed induction of hypothermia decreased cerebral perfusion and decreased the lower limit of autoregulation. Metrics derived from noninvasive near-infrared spectroscopy accurately identified the lower limit of autoregulation during normothermia and hypothermia in piglets resuscitated from arrest.

Relationship between cerebrovascular dysautoregulation and arterial blood pressure in the premature infant

OBJECTIVE

To evaluate cerebrovascular autoregulation as a function of arterial blood pressure (ABP) in the critically ill, premature infant.

STUDY DESIGN

A prospective observational pilot study was conducted in two tertiary care Neonatal Intensive-Care Units. Premature infants (n=23, ≤30 weeks estimated gestational age with invasive ABP monitoring) were enrolled and received routine care while undergoing continuous autoregulation monitoring, using the cerebral oximetry index (COx). The COx is a moving, linear correlation coefficient between cortical reflectance oximetry and ABP. COx values were stratified as a function of ABP for individual subject recordings and for the cohort.

RESULT

The mean duration of autoregulation monitoring was 3.2 days (median: 2.97, range: 0.61-3.99). A total of 10 of 23 (43%) developed intraventricular hemorrhage and 1 of 23 (4%) developed periventricular leukomalacia by head ultrasound. No association was found between neurologic injury and percentage of the monitoring periods with autoregulation impairment (defined as COx>0.5). Lower ABP was associated with dysautoregulation (higher COx values, P<0.01). The percentage of time with impaired autoregulation was greater with lower ABP (P=0.013, Spearman r=0.51).

CONCLUSION

All infants studied had periods with intact and periods with impaired cerebrovascular autoregulation, measured with the COx. Low ABP was associated with impaired autoregulation.

Brain oxygen tension monitoring following penetrating ballistic-like brain injury in rats

While brain oxygen tension (PbtO(2)) monitoring is an important parameter for evaluating injury severity and therapeutic efficiency in severe traumatic brain injury (TBI) patients, many factors affect the monitoring. The goal of this study was to identify the effects of FiO(2) (fraction of inspired oxygen) on PbtO(2) in uninjured anesthetized rats and measure the changes in PbtO(2) following penetrating ballistic-like brain injury (PBBI). Continuous PbtO(2) monitoring in uninjured anesthetized rats showed that PbtO(2) response was positively correlated with FiO(2) (0.21-0.35) but PbtO(2) remained stable when FiO(2) was maintained at ∼0.26. Importantly, although increasing FiO(2) from 0.21 to 0.35 improved P(a)O(2), it concomitantly reduced pH levels and elevated P(a)CO(2) values out of the normal range. However, when the FiO(2) was maintained between 0.26 and 0.30, the pH and P(a)O(2) levels remained within the normal or clinically acceptable range. In PBBI rats, PbtO(2) was significantly reduced by ∼40% (16.9 ± 1.2 mm Hg) in the peri-lesional region immediately following unilateral, frontal 10% PBBI compared to sham rats (28.6 ± 1.7 mm Hg; mean ± SEM, p<0.05) and the PBBI-induced reductions in PbtO(2) were sustained for at least 150 min post-PBBI. Collectively, these results demonstrate that FiO(2) affects PbtO(2) and that PBBI produces acute and sustained hypoxia in the peri-lesional region of the brain injury. This study provides important information for the management of PbtO(2) monitoring in this brain injury model and may offer insight for therapeutic strategies targeted to improve the hypoxia/ischemia state in the penetrating-type brain injury.

The limitations of near-infrared spectroscopy to assess cerebrovascular reactivity: the role of slow frequency oscillations

BACKGROUND

A total hemoglobin reactivity index (THx) derived from near-infrared spectroscopy (NIRS) has recently been introduced to assess cerebrovascular reactivity noninvasively. Analogously to the pressure reactivity index (PRx), THx is calculated as correlation coefficient with arterial blood pressure (ABP). However, the reliability of THx in the injured brain is uncertain. Although slow oscillations have been described in NIRS signals, their significance for assessment of autoregulation remains unclear. In the current study, we investigated the role of slow oscillations of total hemoglobin for NIRS-based cerebrovascular reactivity monitoring.

METHODS

This study was based on a retrospective analysis of data that were consecutively recorded for a different project published previously. Thirty-seven patients with traumatic brain injury and admitted to Addenbrooke's Neurosciences Critical Care Unit between June 2008 and June 2009 were included. After artifact removal, we performed spectral analysis of the tissue hemoglobin index (THI, a measure of oxy- and deoxygenated hemoglobin) and intracranial pressure (ICP) signal. PRx and THx were calculated as moving correlations between ICP and ABP, and THI and ABP, respectively. The agreement between PRx and THx as a function of normalized power of slow oscillations (0.015-0.055 Hz) contained in the input signals was assessed performing between-subject and within-subject correlation analyses. Furthermore, the correlation between the THx values derived from the right and left sides was analyzed.

RESULTS

The agreement between PRx and THx depended on the power of slow oscillations in the input signals. Between-subject comparisons revealed a significant correlation between THx and PRx (r = 0.80, 95% confidence interval 0.53-0.92, P < 0.01) for patients with normalized slow wave activity >0.4 in the THI signal, compared with r = 0.07 (95% confidence interval -0.40 to 0.51, P = 0.79) in the remaining files. Furthermore, within-subject comparisons suggested that THx may be used as a substitute for PRx only when there is an at least moderate agreement (r = 0.36) between the THx values derived from the right and left sides.

CONCLUSIONS

Our results suggest that the NIRS-based cerebrovascular reactivity index THx can be used as a noninvasive substitute for PRx, but only during phases with sufficient slow wave power in the input signal. Furthermore, a good agreement between the THx measures on both sides seems to be a prerequisite for comparison of a global (PRx) versus the more local (THx) index. Nevertheless, noninvasive assessment of cerebrovascular reactivity may be desirable in patients without ICP monitoring and help to guide ABP management in these patients.