Analysis of dynamic autoregulation assessed by the cuff deflation method

Correlation and asynchronization of electroencephalogram and cerebral blood flow in active and passive stimulations

Objective.Real-time brain monitoring is of importance for intraoperative surgeries and intensive care unit, in order to take timely clinical interventions. Electroencephalogram (EEG) is a conventional technique for recording neural excitations (e.g. brain waves) in the cerebral cortex, and near infrared diffuse correlation spectroscopy (DCS) is an emerging technique that can directly measure the cerebral blood flow (CBF) in microvasculature system. Currently, the relationship between the neural activities and cerebral hemodynamics that reflects the vasoconstriction features of cerebral vessels, especially under both active and passive situation, has not been elucidated thus far, which triggers the motivation of this study.Approach.We used the verbal fluency test as an active cognitive stimulus to the brain, and we manipulated blood pressure changes as a passive challenge to the brain. Under both protocols, the CBF and EEG responses were longitudinally monitored throughout the cerebral stimulus. Power spectrum approaches were applied the EEG signals and compared with CBF responses.Main results.The results show that the EEG response was significantly faster and larger in amplitude during the active cognitive task, when compared to the CBF, but with larger individual variability. By contrast, CBF is more sensitive when response to the passive task, and with better signal stability. We also found that there was a correlation (p< 0.01,r= 0.866,R2= 0.751) between CBF and EEG in initial response during the active task, but no significant correlation (p> 0.05) was found during the passive task. The similar relations were also found between regional brain waves and blood flow.Significance.The asynchronization and correlation between the two measurements indicates the necessity of monitoring both variables for comprehensive understanding of cerebral physiology. Deep exploration of their relationships provides promising implications for DCS/EEG integration in the diagnosis of various neurovascular and psychiatric diseases.

Investigation of Cerebral Autoregulation Using Time-Frequency Transformations

The authors carried out the study of the state of systemic and cerebral hemodynamics in normal conditions and in various neurosurgical pathologies using modern signal processing methods. The results characterize the condition for the mechanisms of cerebral circulation Institute of Computer Science and Control, Higher School of Cyber-Physical Systems and Control regulation, which allows for finding a solution to fundamental and specific clinical problems for the effective treatment of patients with various pathologies. The proposed method is based on the continuous wavelet transform of systemic arterial pressure and blood flow velocity signals in the middle cerebral artery recorded by non-invasive methods of photoplethysmography and transcranial doppler ultrasonography. The study of these signals in real-time in the frequency range of Mayer waves makes it possible to determine the cerebral autoregulation state in certain diseases before and after surgical interventions. The proposed method uses a cross-wavelet spectrum, which helps obtain wavelet coherence and a phase shift between the wavelet coefficients of systemic arterial pressure signals and blood flow velocity in the Mayer wave range. The obtained results enable comparing the proposed method with that based on the short-time Fourier transform. The comparison showed that the proposed method has higher sensitivity to changes in cerebral autoregulation and better localization of changes in time and frequency.

Utility of Transcranial Doppler in Moderate and Severe Traumatic Brain Injury: A Narrative Review of Cerebral Physiologic Metrics

Since its creation in the 1980s, transcranial Doppler (TCD) has provided a method of non-invasively monitoring cerebral physiology and has become an invaluable tool in neurocritical care. In this narrative review, we examine the role TCD has in the management of the moderate and severe traumatic brain injury (TBI) patient. We examine the principles of TCD and the ways in which it has been applied to gain insight into cerebral physiology following TBI, as well as explore the clinical evidence supporting these applications. Its usefulness as a tool to non-invasively determine intracranial pressure, detect post-traumatic vasospasm, predict patient outcome, and assess the state of cerebral autoregulation are all explored.

Venous sinus stenting improves cerebral autoregulation in a patient with venous sinus stenosis: a case report

BACKGROUND

Venous sinus stenosis (VSS) is a type of cerebral venous vascular disease. Cerebral autoregulation is an indicator of cerebral arterial function. The cerebral circulatory system is composed of the venous system and arterial system. Impaired venous function may affect arterial function. Thus, cerebral venous stenosis may influence cerebral autoregulation.

CASE PRESENTATION

In this case, a 50-year-old woman with transient blindness and headache was admitted to the hospital. The patient was diagnosed with VSS. A stent was placed at the stenosis. The stent released the intravenous pressure and remitted the patient's symptoms. Measurements of dynamic cerebral autoregulation (dCA) were performed at 3 time points: before stenting, after stenting, and 3 months later. The dCA gradually improved after stenting.

CONCLUSION

VSS may have an influence on cerebral autoregulation, and effective treatment improves cerebral autoregulation in patients with VSS.

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.

Dynamic autoregulation of cerebral blood flow measured non-invasively with fast diffuse correlation spectroscopy

Cerebral autoregulation (CA) maintains cerebral blood flow (CBF) in the presence of systemic blood pressure changes. Brain injury can cause loss of CA and resulting dysregulation of CBF, and the degree of CA impairment is a functional indicator of cerebral tissue health. Here, we demonstrate a new approach to noninvasively estimate cerebral autoregulation in healthy adult volunteers. The approach employs pulsatile CBF measurements obtained using high-speed diffuse correlation spectroscopy (DCS). Rapid thigh-cuff deflation initiates a chain of responses that permits estimation of rates of dynamic autoregulation in the cerebral microvasculature. The regulation rate estimated with DCS in the microvasculature (median: 0.26 s-1, inter quartile range: 0.19 s-1) agrees well (R = 0.81, slope = 0.9) with regulation rates measured by transcranial Doppler ultrasound (TCD) in the proximal vasculature (median: 0.28 s-1, inter quartile range: 0.10 s-1). We also obtained an index of systemic autoregulation in concurrently measured scalp microvasculature. Systemic autoregulation begins later than cerebral autoregulation and exhibited a different rate (0.55 s-1, inter quartile range: 0.72 s-1). Our work demonstrates the potential of diffuse correlation spectroscopy for bedside monitoring of cerebral autoregulation in the microvasculature of patients with brain injury.

Comparing Models of Spontaneous Variations, Maneuvers and Indexes to Assess Dynamic Cerebral Autoregulation

OBJECTIVE

We analyzed the performance of linear and nonlinear models to assess dynamic cerebral autoregulation (dCA) from spontaneous variations in healthy subjects and compared it with the use of two known maneuvers to abruptly change arterial blood pressure (BP): thigh cuffs and sit-to-stand.

MATERIALS AND METHODS

Cerebral blood flow velocity and BP were measured simultaneously at rest and while the maneuvers were performed in 20 healthy subjects. To analyze the spontaneous variations, we implemented two types of models using support vector machine (SVM): linear and nonlinear finite impulse response models. The classic autoregulation index (ARI) and the more recently proposed model-free ARI (mfARI) were used as measures of dCA. An ANOVA analysis was applied to compare the different methods and the coefficient of variation was calculated to evaluate their variability.

RESULTS

There are differences between indexes, but not between models and maneuvers. The mfARI index with the sit-to-stand maneuver shows the least variability.

CONCLUSIONS

Support vector machine modeling of spontaneous variation with the mfARI index could be used for the assessment of dCA as an alternative to maneuvers to introduce large BP fluctuations.

Pressure Autoregulation Measurement Techniques in Adult Traumatic Brain Injury, Part I: A Scoping Review of Intermittent/Semi-Intermittent Methods

The purpose of this study was to perform a systematic, scoping review of commonly described intermittent/semi-intermittent autoregulation measurement techniques in adult traumatic brain injury (TBI). Nine separate systematic reviews were conducted for each intermittent technique: computed tomographic perfusion (CTP)/Xenon-CT (Xe-CT), positron emission tomography (PET), magnetic resonance imaging (MRI), arteriovenous difference in oxygen (AVDO₂) technique, thigh cuff deflation technique (TCDT), transient hyperemic response test (THRT), orthostatic hypotension test (OHT), mean flow index (Mx), and transfer function autoregulation index (TF-ARI). MEDLINE^®, BIOSIS, EMBASE, Global Health, Scopus, Cochrane Library (inception to December 2016), and reference lists of relevant articles were searched. A two tier filter of references was conducted. The total number of articles utilizing each of the nine searched techniques for intermittent/semi-intermittent autoregulation techniques in adult TBI were: CTP/Xe-CT (10), PET (6), MRI (0), AVDO₂ (10), ARI-based TCDT (9), THRT (6), OHT (3), Mx (17), and TF-ARI (6). The premise behind all of the intermittent techniques is manipulation of systemic blood pressure/blood volume via either chemical (such as vasopressors) or mechanical (such as thigh cuffs or carotid compression) means. Exceptionally, Mx and TF-ARI are based on spontaneous fluctuations of cerebral perfusion pressure (CPP) or mean arterial pressure (MAP). The method for assessing the cerebral circulation during these manipulations varies, with both imaging-based techniques and TCD utilized. Despite the limited literature for intermittent/semi-intermittent techniques in adult TBI (minus Mx), it is important to acknowledge the availability of such tests. They have provided fundamental insight into human autoregulatory capacity, leading to the development of continuous and more commonly applied techniques in the intensive care unit (ICU). Numerous methods of intermittent/semi-intermittent pressure autoregulation assessment in adult TBI exist, including: CTP/Xe-CT, PET, AVDO₂ technique, TCDT-based ARI, THRT, OHT, Mx, and TF-ARI. MRI-based techniques in adult TBI are yet to be described, with the main focus of MRI techniques on metabolic-based cerebrovascular reactivity (CVR) and not pressure-based autoregulation.

Dynamic Cerebrovascular and Intracranial Pressure Reactivity Assessment of Impaired Cerebrovascular Autoregulation in Intracranial Hypertension

We previously suggested that the discrepancy between a critical cerebral perfusion pressure (CPP) of 30 mmHg, obtained by increasing intracranial pressure (ICP), and 60 mmHg, obtained by decreasing arterial pressure, was due to pathological microvascular shunting at high ICP [1], and that the determination of the critical CPP by the static cerebral blood flow (CBF) autoregulation curve is not valid with intracranial hypertension. Here, we demonstrated that induced dynamic ICP reactivity (iPRx), and cerebrovascular reactivity (CVRx) tests accurately identify the critical CPP in the hypertensive rat brain, which differs from that obtained by the static autoregulation curve. Step changes in CPP from 70 to 50 and 30 mmHg were made by increasing ICP using an artificial cerebrospinal fluid reservoir connected to the cisterna magna. At each CPP, a transient 10-mmHg increase in arterial pressure was induced by bolus intravenous dopamine. iPRx and iCVRx were calculated as ΔICP/Δ mean arterial pressure (MAP) and as ΔCBF/ΔMAP, respectively. The critical CPP at high ICP, obtained by iPRx and iCVRx, is 50 mmHg, where compromised capillary flow, transition of blood flow to nonnutritive microvascular shunts, tissue hypoxia, and brain-blood barrier leakage begin to occur, which is higher than the 30 mmHg determined by static autoregulation.

Critical cerebral perfusion pressure at high intracranial pressure measured by induced cerebrovascular and intracranial pressure reactivity

OBJECTIVES

The lower limit of cerebral blood flow autoregulation is the critical cerebral perfusion pressure at which cerebral blood flow begins to fall. It is important that cerebral perfusion pressure be maintained above this level to ensure adequate cerebral blood flow, especially in patients with high intracranial pressure. However, the critical cerebral perfusion pressure of 50 mm Hg, obtained by decreasing mean arterial pressure, differs from the value of 30 mm Hg, obtained by increasing intracranial pressure, which we previously showed was due to microvascular shunt flow maintenance of a falsely high cerebral blood flow. The present study shows that the critical cerebral perfusion pressure, measured by increasing intracranial pressure to decrease cerebral perfusion pressure, is inaccurate but accurately determined by dopamine-induced dynamic intracranial pressure reactivity and cerebrovascular reactivity.

DESIGN

Cerebral perfusion pressure was decreased either by increasing intracranial pressure or decreasing mean arterial pressure and the critical cerebral perfusion pressure by both methods compared. Cortical Doppler flux, intracranial pressure, and mean arterial pressure were monitored throughout the study. At each cerebral perfusion pressure, we measured microvascular RBC flow velocity, blood-brain barrier integrity (transcapillary dye extravasation), and tissue oxygenation (reduced nicotinamide adenine dinucleotide) in the cerebral cortex of rats using in vivo two-photon laser scanning microscopy.

SETTING

University laboratory.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

At each cerebral perfusion pressure, dopamine-induced arterial pressure transients (~10 mm Hg, ~45 s duration) were used to measure induced intracranial pressure reactivity (Δ intracranial pressure/Δ mean arterial pressure) and induced cerebrovascular reactivity (Δ cerebral blood flow/Δ mean arterial pressure).

MEASUREMENTS AND MAIN RESULTS

At a normal cerebral perfusion pressure of 70 mm Hg, 10 mm Hg mean arterial pressure pulses had no effect on intracranial pressure or cerebral blood flow (induced intracranial pressure reactivity = -0.03 ± 0.07 and induced cerebrovascular reactivity = -0.02 ± 0.09), reflecting intact autoregulation. Decreasing cerebral perfusion pressure to 50 mm Hg by increasing intracranial pressure increased induced intracranial pressure reactivity and induced cerebrovascular reactivity to 0.24 ± 0.09 and 0.31 ± 0.13, respectively, reflecting impaired autoregulation (p < 0.05). By static cerebral blood flow, the first significant decrease in cerebral blood flow occurred at a cerebral perfusion pressure of 30 mm Hg (0.71 ± 0.08, p < 0.05).

CONCLUSIONS

Critical cerebral perfusion pressure of 50 mm Hg was accurately determined by induced intracranial pressure reactivity and induced cerebrovascular reactivity, whereas the static method failed.

A new model-free index of dynamic cerebral blood flow autoregulation

The classic dynamic autoregulatory index (ARI), proposed by Aaslid and Tiecks, is one of the most widely used methods to assess the efficiency of dynamic cerebral autoregulation. Although this index is often used in clinical research and is also included in some commercial equipment, it exhibits considerable intra-subject variability, and has the tendency to produce false positive results in clinical applications. An alternative index of dynamic cerebral autoregulation is proposed, which overcomes most of the limitations of the classic method and also has the advantage of being model-free. This new index uses two parameters that are obtained directly from the response signal of the cerebral blood flow velocity to a transient decrease in arterial blood pressure provoked by the sudden release of bilateral thigh cuffs, and a third parameter measuring the difference in slope of this response and the change in arterial blood pressure achieved. With the values of these parameters, a corresponding classic autoregulatory index value could be calculated by using a linear regression model built from theoretical curves generated with the Aaslid-Tiecks model. In 16 healthy subjects who underwent repeated thigh-cuff manoeuvres, the model-free approach exhibited significantly lower intra-subject variability, as measured by the unbiased coefficient of variation, than the classic autoregulatory index (p = 0.032) and the Rate of Return (p<0.001), another measure of cerebral autoregulation used for this type of systemic pressure stimulus, from 39.23%±41.91% and 55.31%±31.27%, respectively, to 15.98%±7.75%.

Variants of the endothelial nitric oxide gene and cerebral blood flow after severe traumatic brain injury

Experimental studies suggest that nitric oxide produced by endothelial nitric oxide synthase (NOS3) plays a role in maintaining cerebral blood flow (CBF) after traumatic brain injury (TBI). The purpose of this study was to determine if common variants of the NOS3 gene contribute to hypoperfusion after severe TBI. Fifty-one patients with severe TBI were studied. Cerebral hemodynamics, including global CBF by the stable xenon computed tomography (CT) technique, internal carotid artery flow volume (ICA-FVol), and flow velocity in intracranial vessels, were measured within 12 h of injury, and at 48 h after injury. A blood sample was collected for DNA analysis, and genotyping of the following variants of the NOS3 gene was performed: -786T>C, 894G>T, and 27bp VNTR. Cerebral hemodynamics were most closely related to the-786T>C genotype. CBF averaged 57.7±3.0 mL/100 g/min with the normal T/T genotype, 47.0±2.5 mL/100 g/min with the T/C, and 37.3±8.8 mL/100 g/min with the C/C genotype (p=0.0146). Cerebrovascular resistance followed an inverse pattern with the highest values occurring with the C/C genotype (p=0.0027). The lowest ICA-FVol of 124±43 mL/min was found at 12 h post-injury in the more injured hemisphere of the patients with the C/C genotype (p=0.0085). The mortality rate was 20% in patients with the T/T genotype and 17% with the T/C genotype. In contrast, both of the patients with the C/C genotype were dead at 6 months post-injury (p=0.022). The findings in this study support the importance of NO produced by NOS3 activity in maintaining CBF after TBI, since lower CBF values were found in patients having the -786C allele. The study suggests that a patient's individual genetic makeup may contribute to the brain's response to injury and determine the patient's chances of surviving the injury. The results here will need to be studied in a larger number of patients, but could explain some of the variability in outcome that occurs following severe TBI.

Cerebral hemodynamic effects of acute hyperoxia and hyperventilation after severe traumatic brain injury

The purpose of this study was to examine the effects of hyperventilation or hyperoxia on cerebral hemodynamic parameters over time in patients with severe traumatic brain injury (TBI). We prospectively studied 186 patients with severe TBI. CO₂ and O₂ reactivity tests were conducted twice a day on days 1-5 and once daily on days 6-10 after injury. During hyperventilation there was a significant decrease in intracranial pressure (ICP), mean arterial pressure (MAP), jugular venous oxygen saturation (Sjvo₂), brain tissue Po₂ (Pbto₂), and flow velocity (FV). During hyperoxia there was an increase in Sjvo₂ and Pbto₂, and a small but consistent decrease in ICP, end-tidal carbon dioxide (etco₂), partial arterial carbon dioxide pressure (Paco₂), and FV. Brain tissue oxygen reactivity during the first 12 h after injury averaged 19.7 ± 3.0%, and slowly decreased over the next 7 days. The autoregulatory index (ARI; normal = 5.3 ± 1.3) averaged 2.2 ± 1.5 on day 1 post-injury, and gradually improved over the 10 days of monitoring. The ARI significantly improved during hyperoxia, by an average of 0.4 ± 1.8 on the left, and by 0.5 ± 1.8 on the right. However, the change in ARI with hyperoxia was much smaller than that observed with hyperventilation. Hyperventilation increased ARI by an average of 1.3 ± 1.9 on the left, and 1.5 ± 2.0 on the right. Pressure autoregulation, as assessed by dynamic testing, was impaired in these head-injured patients. Acute hyperoxia significantly improved pressure autoregulation, although the effect was smaller than that induced by hyperventilation. The very small change in Paco₂ induced by hyperoxia does not appear to explain this finding. Rather, the vasoconstriction induced by acute hyperoxia may allow the cerebral vessels to respond better to transient hypotension. Further studies are needed to define the clinical significance of these observations.

L-arginine reactivity in cerebral vessels after severe traumatic brain injury

OBJECTIVES

Traumatic brain injury (TBI) causes an early reduction of cerebral blood flow (CBF). The purpose was to study cerebrovascular endothelial function by examining the reactivity of cerebral vessels to L-arginine.

METHODS

Fifty-one patients with severe TBI were prospectively studied by measuring cerebral hemodynamics before and after the administration of L-arginine, 300 mg/kg at 12 hours and at 48 hours after injury. These hemodynamic measurements, using transcranial Doppler techniques, included internal carotid flow volume as an estimate of hemispheric CBF, flow velocity in intracranial vessels, CO(2) reactivity, and dynamic pressure autoregulation using thigh cuff deflation and carotid compression methods. Changes in the hemodynamics with L-arginine administration were analyzed using a general linear mixed model.

RESULTS

L-arginine produced no change in mean arterial pressure, intracranial pressure, or brain oxygenation. Overall, L-arginine induced an 11.3% increase in internal carotid artery flow volume (P=0.0190). This increase was larger at 48 hours than at 12 hours (P=0.0045), and tended to be larger in the less injured hemisphere at both time periods. The response of flow velocity in the intracranial vessels was similar, but smaller differences with administration of L-arginine were observed. There was a significant improvement in CO(2) reactivity with L-arginine, but no change in dynamic pressure autoregulation.

DISCUSSION

The low response of the cerebral vessels to L-arginine at 12 hours post-injury with improvement at 48 hours suggests that dysfunction of cerebrovascular endothelium plays a role in the reduced CBF observed after TBI.

Practical aspects of bedside cerebral hemodynamics monitoring in pediatric TBI

INTRODUCTION

Disturbances in cerebral hemodynamics may have a profound influence on secondary injury after traumatic brain injury (TBI), and many therapies in the neurocritical care unit may adversely affect cerebral blood flow. However, the clinician is often unaware of this when it occurs because practical methods for monitoring cerebral hemodynamics by the bedside have been lacking. Current imaging studies only provide a snapshot of the brain at one point in time, giving limited information about a dynamic condition.

DISCUSSION

This review will focus on key pathophysiological concepts required to understand changes in cerebral hemodynamics after TBI and the principles, potential benefits, and limitations of currently available bedside monitoring techniques, including transcranial Doppler, autoregulation, and local/regional cerebral blood flow.

Quantification of dynamic blood flow autoregulation in optic nerve head of rhesus monkeys

Autoregulation capacity has been classically assessed with a 'two-point' measurement or static autoregulation (sAR). In such an approach, stabilized hemodynamic parameters are determined before and after a perfusion pressure challenge. Analysis of dynamic autoregulation (dAR), an early phase of blood flow response to a sudden perfusion pressure change is emerging as a preferred approach to assess the capacity of autoregulation in many non-ocular tissues and has developed rapidly in the last decade. The purpose of this study was to develop a method to quantify dAR in the optic nerve head (ONH). In six pentobarbital (6-9 mg/kg/h, IV) anesthetized rhesus monkeys, dAR was elicited by increasing intraocular pressure (IOP) from 10 to 30 or 40 mmHg (IOP(10-30)/IOP(10-40)) manometrically via switch between reservoirs connected to the anterior chamber. Relative blood flow changes during dAR in the ONH, estimated with a laser speckle flowgraph (LSFG), were continuously measured for 1 min. Time-domain parameters of dAR response, including: BF(Deltamax) (maximal blood flow decrease, %), K(r) (descending slope of blood flow from baseline to BF(Deltamax)) and T(r) (descending time of blood flow from baseline to BF(Deltamax)) were extracted and analyzed offline. For each monkey, same procedure was repeated three times during three different visits. The test-retest repeatability and inter-ocular difference of the parameters was statistically evaluated. During IOP(10-30) and IOP(10-40), the mean arterial BP was 89 +/- 7 and 85 +/- 6 mmHg, respectively. Immediately after the reservoir was switched, the blood flow started to decline and reached maximal in approximately 4 s. The blood flow then returned back toward baseline despite continuous IOP increase, which took 8-11 s to reach the level of the raised reservoir. The general pattern of blood flow responses was similar between IOP(10-30) and IOP(10-40) and there was no statistically significant difference for T(r) (P > 0.05). However, IOP(10-40) caused greater BF(Deltamax) and deeper K(r) than IOP(10-30) (P < 0.0001 and P < 0.05, respectively). The blood flow during steady state, 5 min after IOP elevation, showed no statistically significant difference from baseline (P > 0.05). All dAR parameters (T(r), K(r) and BF(Deltamax)) showed no significant difference across the 3 visits (Repeat measures ANOVA, P = 0.7, 0.2 and 0.2, respectively); the corresponding coefficients of variance were 24%, 43% and 34% during IOP(10-30) and 11.8%, 30.3% and 19.0% during IOP(10-40). The mean dAR parameters between the eyes showed no statistically differences (P = 0.6) during both IOP(10-30) and IOP(10-40). The current study showed that a rapid ocular perfusion pressure decrease induced by a sudden IOP step increase evoked a transient and reproducible dAR response in the ONH of non-human primates measured with LSFG. Quantitative analysis of dAR may provide a direct view of vasomotorial activity in the resistant vessels and thus a new approach to assess the autoregulatory capacity in the ONH.

Monitoring of cerebrovascular autoregulation: facts, myths, and missing links

The methods for continuous assessment of cerebral autoregulation using correlation, phase shift, or transmission (either in time- or frequency-domain) were introduced a decade ago. They express dynamic relationships between slow waves of transcranial Doppler (TCD), blood flow velocity (FV) and cerebral perfusion pressure (CPP), or arterial pressure (ABP). We review a methodology and clinical application of indices useful for monitoring cerebral autoregulation and pressure-reactivity in various scenarios of neuro-critical care.

FACTS

Poor autoregulation and loss of pressure-reactivity are independent predictors of fatal outcome following head injury. Autoregulation is impaired by too low or too high CPP when compared to autoregulation with normal CPP (usually between 60 and 85 mmHg; and these limits are highly individual). Hemispheric asymmetry of the bi-laterally assessed autoregulation has been associated with asymmetry of CT scan findings: autoregulation was found to be worse ipsilateral to contusion or lateralized edema causing midline shift. The pressure-reactivity (PRx index) correlated with a state of low CBF and CMRO2 revealed using PET studies. The PRx is easier to monitor over prolonged periods of time than the TCD-based indices as it does not require fixation of external probes. Continuous monitoring with the PRx can be used to direct CPP-oriented therapy by determining the optimal CPP for pressure-reactivity. Autoregulation indices are able to reflect transient changes of autoregulation, as seen during plateau waves of ICP. However, minute-to-minute assessment of autoregulation has a poor signal-to-noise ratio. Averaging across time (30 min) or by combining with other relevant parameters improves the accuracy. MYTHS: It is debatable whether the TCD-based indices in head injured patients can be calculated using ABP instead of CPP. Thresholds for functional and disturbed autoregulation dramatically depends on arterial tension of CO2--therefore, comparison between patients cannot be performed without comparing their PaCO2. The TCD pulsatility index cannot accurately detect the lower limit of autoregulation. MISSING LINKS: We still do not know whether autoregulation-oriented therapy can be understood as a consensus between CPP-directed protocols and the Lund-concept. What are the links between endothelial function and autoregulation indices? Can autoregulation after head injury be improved with statins or EPO, as in subarachnoid hemorrhage? In conclusion, monitoring cerebral autoregulation can be used in a variety of clinical scenarios and may be helpful in delineating optimal therapeutic strategies.

Dynamic cerebral autoregulation: should intracranial pressure be taken into account?

BACKGROUND

Although the inclusion of cerebral perfusion pressure (CPP) is a standard feature in static testing of autoregulation after head injury, controversy surrounds the use of CPP versus arterial blood pressure (ABP) in dynamic tests. The aim of our project was to assess the discrepancies between methods of dynamic autoregulation testing based on CPP or ABP, and study possible differences in their prognostic value.

METHOD

Intermittent recordings of intracranial pressure (ICP), ABP and middle cerebral artery blood flow velocity (FV) waveforms were made in 151 anaesthetised and ventilated adult head injured patients as part of their required care. Indices of dynamic autoregulation were calculated as a moving correlation coefficient of 60 samples (total time 3 min) of 6 s mean values of FV and ABP (Mxa) or FV and CPP (Mx). Values of Mx and Mxa were averaged over multiple recordings in each patient and correlated with outcome at 6 months post injury.

FINDINGS

Association between Mx and Mxa was moderately strong (r(2) = 0.73). However, limit of 95% accordance between both indices was +/-0.32. Mxa was significantly greater than Mx (0.22 +/- 0.22 versus 0.062 +/- 0.28; p < 0.000001). The difference between Mx and Mxa decreased with impairment of autoregulation (r = -0.39; p < 0.000001). Mean value of Mx showed a significant difference between dichotomized outcome groups (better autoregulation in patients with favourable than unfavourable outcome), while Mxa did not.

CONCLUSIONS

Although relatively similar in a large group of patients, the differences between these two methods of assessment of dynamic autoregulation may be considerable in individual cases. When ICP is monitored, CPP rather than ABP should be included in the calculation of the autoregulatory index.