Cerebral haemodynamics during experimental intracranial hypertension

Effects of increased intracranial pressure on cerebrospinal fluid influx, cerebral vascular hemodynamic indexes, and cerebrospinal fluid lymphatic efflux

The glymphatic-lymphatic fluid transport system (GLFTS) consists of glymphatic pathway and cerebrospinal fluid (CSF) lymphatic outflow routes, allowing biological liquids from the brain parenchyma to access the CSF along with perivascular space and to be cleaned out of the skull through lymphatic vessels. It is known that increased local pressure due to physical compression of tissue improves lymphatic transport in peripheral organs, but little is known about the exact relationship between increased intracranial pressure (IICP) and GLFTS. In this study, we verify our hypothesis that IICP significantly impacts GLFTS, and this effect depends on severity of the IICP. Using a previously developed inflating balloon model to induce IICP and inject fluorescent tracers into the cisterna magna, we found significant impairment of the glymphatic circulation after IICP. We further found that cerebrovascular occlusion occurred, and cerebrovascular pulsation decreased after IICP. IICP also interrupted the drainage of deep cervical lymph nodes and dorsal meningeal lymphatic function, enhancing spinal lymphatic outflow to the sacral lymph nodes. Notably, these effects were associated with the severity of IICP. Thus, our findings proved that the intensity of IICP significantly impacts GLFTS. This may have translational applications for preventing and treating related neurological disorders.

Autonomic Nervous System Activity during Refractory Rise in Intracranial Pressure

Refractory intracranial hypertension (RIH) is a dramatic increase in intracranial pressure (ICP) that cannot be controlled by treatment. Recent reports suggest that the autonomic nervous system (ANS) activity may be altered during changes in ICP. Our study aimed to assess ANS activity during RIH and the causal relationship between rising in ICP and autonomic activity. We reviewed retrospectively 24 multicenter (Cambridge, Tromso, Berlin) patients in whom RIH developed as a pre-terminal event after acute brain injury (ABI). They were monitored with ICP, arterial blood pressure (ABP), and electrocardiography (ECG) using ICM+ software. Parameters reflecting autonomic activity were computed in time and frequency domain through the measurement of heart rate variability (HRV) and baroreflex sensitivity (BRS). Our results demonstrated that a rise in ICP was associated to a significant rise in HRV and BRS with a higher significance level in the high-frequency HRV (p < 0.001). This increase was followed by a significant decrease in HRV and BRS above the upper-breakpoint of ICP where ICP pulse-amplitude starts to decrease whereas the mean ICP continues to rise. Temporality measured with a Granger test suggests a causal relationship from ICP to ANS. The above results suggest that a rise in ICP interacts with ANS activity, mainly interfacing with the parasympathetic-system. The ANS seems to react to the rise in ICP with a response possibly focused on maintaining the cerebrovascular homeostasis. This happens until the critical threshold of ICP is reached above which the ANS variables collapse, probably because of low perfusion of the brain and the central autonomic network.

CSF Dynamics for Shunt Prognostication and Revision in Normal Pressure Hydrocephalus

BACKGROUND

Despite the quantitative information derived from testing of the CSF circulation, there is still no consensus on what the best approach could be in defining criteria for shunting and predicting response to CSF diversion in normal pressure hydrocephalus (NPH).

OBJECTIVE

We aimed to review the lessons learned from assessment of CSF dynamics in our center and summarize our findings to date. We have focused on reporting the objective perspective of CSF dynamics testing, without further inferences to individual patient management.

DISCUSSION

No single parameter from the CSF infusion study has so far been able to serve as an unquestionable outcome predictor. Resistance to CSF outflow (Rout) is an important biological marker of CSF circulation. It should not, however, be used as a single predictor for improvement after shunting. Testing of CSF dynamics provides information on hydrodynamic properties of the cerebrospinal compartment: the system which is being modified by a shunt. Our experience of nearly 30 years of studying CSF dynamics in patients requiring shunting and/or shunt revision, combined with all the recent progress made in producing evidence on the clinical utility of CSF dynamics, has led to reconsidering the relationship between CSF circulation testing and clinical improvement.

CONCLUSIONS

Despite many open questions and limitations, testing of CSF dynamics provides unique perspectives for the clinician. We have found value in understanding shunt function and potentially shunt response through shunt testing in vivo. In the absence of infusion tests, further methods that provide a clear description of the pre and post-shunting CSF circulation, and potentially cerebral blood flow, should be developed and adapted to the bed-space.

Visualization of Intracranial Pressure Insults After Severe Traumatic Brain Injury: Influence of Individualized Limits of Reactivity

Cerebral perfusion pressure (CPP) lower limits of reactivity can be determined almost continuously after severe traumatic brain injury (TBI), and deviation below the lower limit carries important prognostic information. In this study, we used a recently derived coloured contour method for visualizing intracranial pressure (ICP) insults to describe the influence of having a CPP above the CPP lower limits of reactivity after severe TBI. In a cohort of 729 patients, we examined the relationship between ICP insults and the 6-month Glasgow Outcome Scale score, using colour-coded plots, as described previously. We then assessed this relationship when ICP insults were above or below the CPP lower limit of reactivity. We found a curvilinear relationship whereby even prolonged durations of low-intensity ICP insults were not associated with poor outcomes but short durations of high-intensity insults were. When only ICP insults with a CPP below the CPP lower limit of reactivity were considered, a much lower intensity of ICP insults could be tolerated. A CPP above the lower limits of reactivity exerts a protective effect, whereas a CPP below the lower reactivity limits renders the patient vulnerable to increased morbidity from intracranial hypertension.

The Limited Impact of Current Therapeutic Interventions on Cerebrovascular Reactivity in Traumatic Brain Injury: A Narrative Overview

Current intensive care unit (ICU) treatment strategies for traumatic brain injury (TBI) care focus on intracranial pressure (ICP)- and cerebral perfusion pressure (CPP)-directed therapeutics, dictated by guidelines. Impaired cerebrovascular reactivity in moderate/severe TBI is emerging as a major associate with poor outcome and appears to dominate the landscape of physiologic derangement over the course of a patient's ICU stay. Within this article, we review the literature on the known drivers of impaired cerebrovascular reactivity in adult TBI, highlight the current knowledge surrounding the impact of guideline treatment strategies on continuously monitored cerebrovascular reactivity, and discuss current treatment paradigms for impaired reactivity. Finally, we touch on the areas of future research, as we strive to develop specific therapeutics for impaired cerebrovascular reactivity in TBI. There exists limited literature to suggest advanced age, intracranial injury patterns of diffuse injury, and sustained ICP elevations may drive impaired cerebrovascular reactivity. To date, the literature suggests there is a limited impact of such ICP/CPP guideline-based therapies on cerebrovascular reactivity, with large portions of a given patients ICU period spent with impaired cerebrovascular reactivity. Emerging treatment paradigms focus on the targeting individualized CPP and ICP thresholds based on cerebrovascular reactivity, without directly targeting the pathways involved in its dysfunction. Further work involved in uncovering the molecular pathways involved in impaired cerebrovascular reactivity is required, so that we can develop therapeutics directed at its prevention and treatment.

Observations on the Cerebral Effects of Refractory Intracranial Hypertension After Severe Traumatic Brain Injury

BACKGROUND

Raised intracranial pressure (ICP) is a prominent cause of morbidity and mortality after severe traumatic brain injury (TBI). However, in the clinical setting, little is known about the cerebral physiological response to severe and prolonged increases in ICP.

METHODS

Thirty-three severe TBI patients from a single center who developed severe refractory intracranial hypertension (ICP > 40 mm Hg for longer than 1 h) with ICP, arterial blood pressure, and brain tissue oxygenation (PBTO2) monitoring (subcohort, n = 9) were selected for retrospective review. Secondary parameters reflecting autoregulation (including pressure reactivity index-PRx, which was available in 24 cases), cerebrospinal compensatory reserve (RAP), and ICP pulse amplitude were calculated.

RESULTS

PRx deteriorated from 0.06 ± 0.26 a.u. at baseline levels of ICP to 0.57 ± 0.24 a.u. (p < 0.0001) at high levels of ICP (> 50 mm Hg). In 4 cases, PRx was impaired (> 0.25 a.u.) before ICP was raised above 25 mm Hg. Concurrently, PBTO2 decreased from 27.3 ± 7.32 mm Hg at baseline ICP to 12.68 ± 7.09 mm Hg at high levels of ICP (p < 0.001). The pulse amplitude of the ICP waveform increased with increasing ICP but showed an 'upper breakpoint'-whereby further increases in ICP lead to decreases in pulse amplitude-in 6 out of the 33 patients.

DISCUSSION

Severe intracranial hypertension after TBI leads to decreased brain oxygenation, impaired pressure reactivity, and changes in the pulse amplitude of ICP. Impaired pressure reactivity may denote increased risk of developing refractory intracranial hypertension in some patients.

Cerebral Blood Flow in Low Intracranial Pressure Headaches-What is Known?

Headaches attributed to low cerebrospinal fluid (CSF) pressure are described as orthostatic headaches caused by spontaneous or secondary low CSF pressure or CSF leakages. Regardless of the cause, CFS leaks may lead to intracranial hypotension (IH) and influence cerebral blood flow (CBF). When CSF volume decreases, a compensative increase in intracranial blood volume and cerebral vasodilatation occurs. Sinking of the brain and traction on pain-sensitive structures are thought to be the causes of orthostatic headaches. Although there are many studies concerning CBF during intracranial hypertension, little is known about CBF characteristics during low intracranial pressure. The aim of this review is to examine the relationship between CBF, CSF, and intracranial pressure in headaches assigned to low CSF pressure.

Using computerized tomography perfusion to measure cerebral hemodynamics following treatment of traumatic brain injury in rabbits

The present study aimed to investigate the use of computerized tomography (CT) perfusion for evaluating cerebral hemodynamics following traumatic brain injury (TBI) in rabbits. The animals were randomly assigned into four groups (n=10 animals/group): i) Control, ii) TBI, iii) TBI + common decompression and iv) TBI + controlled decompression groups. A TBI model was established in rabbits using epidural balloon inflation. In the groups receiving intervention, animals were provided common decompression or controlled decompression treatments. Conventional CT and CT perfusion scanning were performed, with cerebral hemodynamic indices, including regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV) and mean transit time (MTT) being measured. Blood-brain barrier (BBB) permeability was evaluated using Evans blue staining. Compared with those in the control group, rCBF and rCBV values of the bilateral temporal lobes and basal ganglion in the TBI, TBI + common decompression and TBI + controlled decompression groups were significantly lower, whereas the MTT values were markedly prolonged and Evans blue dye content was greatly increased (P<0.01). Controlled decompression was demonstrated to be more potent than common decompression for preventing TBI-induced decline in rCBF and rCBV values in the bilateral temporal lobes and basal ganglion, as well as reversing TBI-induced extension of MTT in the bilateral temporal lobes (P<0.01 vs. TBI group). However, neither common nor controlled decompression could reduce TBI-induced increase in BBB permeability. In conclusion, these findings indicate that CT perfusion may be used to monitor cerebral hemodynamics following TBI in rabbits. Controlled decompression was deduced to be more potent than common decompression for preventing abnormalities in cerebral hemodynamics after TBI.

Use of a Doppler-Based Pulsatility Index to Evaluate Cerebral Hemodynamics in Neurocritical Patients After Hemicraniectomy

OBJECTIVES

As a noninvasive method for evaluation of cerebral hemodynamics, the correct interpretation of transcranial Doppler or transcranial imaging (TCI) data remains a major challenge. We explored how to interpret the pulsatility index (PI) derived via TCI during evaluations of cerebral hemodynamics in posthemicraniectomy patients.

METHODS

We included patients who underwent invasive arterial pressure and intracranial pressure (ICP) monitoring and simultaneous TCI examinations after hemicraniectomy. We classified the PI of the middle cerebral artery (MCA) into ipsilateral (craniectomy side) and contralateral (opposite side) and analyzed both data sets. The statistical analysis was performed by the Bland-Altman approach, by calculating intraclass correlation coefficients and Spearman correlations, and by drawing receiver operating characteristic curves. Pulsatility index probability charts were created for ICPs exceeding 20, 25, and 30 mm Hg and cerebral perfusion pressures (CPPs) lower than 70, 60, and 50 mm Hg; we thus explored defined ICP and CPP values.

RESULTS

The ipsilateral and contralateral MCA PI data differed. Only the ipsilateral MCA PI showed a weak correlation with ICP (r = 0.378; P < .001). The receiver operating characteristic curve analysis revealed limited diagnostic utility of bilateral MCA PIs for ICP and CPP assessments. An extremely elevated MCA PI indicated that patients were at high risk of a dangerous ICP elevation or CPP reduction. However, MCA PI values within the normal range did not effectively rule out an ICP of 20 mm Hg or higher but effectively eliminated a CPP lower than 50 mm Hg.

CONCLUSIONS

In posthemicraniectomy patients, the Doppler-based MCA PI value was ineffectively for quantitative ICP and CPP evaluations but a useful index for assessment of cerebral hemodynamics in terms of the probability of an ICP elevation or a CPP reduction.

Impaired cerebral compensatory reserve is associated with admission imaging characteristics of diffuse insult in traumatic brain injury

Background

Continuous assessment of cerebral compensatory reserve is possible using the moving correlation between pulse amplitude of intra-cranial pressure (AMP) and intra-cranial pressure (ICP), called RAP. Little is known about the behavior and associations of this index in adult traumatic brain injury (TBI). The goal of this study is to evaluate the association between admission cerebral imaging findings and RAP over the course of the acute intensive care unit stay.

Methods

We retrospectively reviewed 358 adult TBI patients admitted to the Addenbrooke’s Hospital, University of Cambridge, from March 2005 to December 2016. Only non-craniectomy patients were studied. Using archived high frequency physiologic signals, RAP was derived and analyzed over the first 48 h and first 10 days of recording in each patient, using grand mean, percentage of time above various thresholds, and integrated area under the curve (AUC) of RAP over time. Associations between these values and admission computed tomography (CT) injury characteristics were evaluated.

Results

The integrated AUC, based on various thresholds of RAP, was statistically associated with admission CT markers of diffuse TBI and cerebral edema. Admission CT findings of cortical gyral effacement, lateral ventricle compression, diffuse cortical subarachnoid hemorrhage (SAH), thickness of cortical SAH, presence of bilateral contusions, and subcortical diffuse axonal injury (DAI) were all associated with AUC of RAP over time. Joncheere-Terpstra testing indicated a statistically significant increase in mean RAP AUC across ordinal categories of the abovementioned associated CT findings.

Conclusions

RAP is associated with cerebral CT injury patterns of diffuse injury and edema, providing some confirmation of its potential measurement of cerebral compensatory reserve in TBI.

Electronic supplementary material

The online version of this article (10.1007/s00701-018-3681-y) contains supplementary material, which is available to authorized users.

Twenty-Five Years of Intracranial Pressure Monitoring After Severe Traumatic Brain Injury: A Retrospective, Single-Center Analysis

BACKGROUND

Intracranial pressure (ICP) is a clinically important variable after severe traumatic brain injury (TBI) and has been monitored, along with clinical outcome, for over 25 yr in Addenbrooke's hospital, Cambridge, United Kingdom. This time period has also seen changes in management strategies with the implementation of protocolled specialist neurocritical care, expansion of neuromonitoring techniques, and adjustments of clinical treatment targets.

OBJECTIVE

To describe the changes in intracranial monitoring variables over the past 25 yr.

METHODS

Data from 1146 TBI patients requiring ICP monitoring were analyzed. Monitored variables included ICP, cerebral perfusion pressure (CPP), and the cerebral pressure reactivity index (PRx). Data were stratified into 5-yr epochs spanning the 25 yr from 1992 to 2017.

RESULTS

CPP increased sharply with specialist neurocritical care management (P &lt; 0.0001) (introduction of a specific TBI management algorithm) before stabilizing from 2000 onwards. ICP decreased significantly over the 25 yr of monitoring from an average of 19 to 12 mmHg (P &lt; 0.0001) but PRx remained unchanged. The mean number of ICP plateau waves and the number of patients developing refractory intracranial hypertension both decreased significantly. Mortality did not significantly change in the cohort (22%).

CONCLUSION

We demonstrate the evolving trends in neurophysiological monitoring over the past 25 yr from a single, academic neurocritical care unit. ICP and CPP were responsive to the introduction of an ICP/CPP protocol while PRx has remained unchanged.

Validation of Pressure Reactivity and Pulse Amplitude Indices against the Lower Limit of Autoregulation, Part I: Experimental Intracranial Hypertension

The purpose of this study was to provide validation of intracranial pressure (ICP) derived continuous indices of cerebrovascular reactivity against the lower limit of autoregulation (LLA). Utilizing an intracranial hypertension model within white New Zealand rabbits, ICP, transcranial Doppler (TCD), laser Doppler flowmetry (LDF), and arterial blood pressure were recorded. Data were retrospectively analyzed in a cohort of 12 rabbits with adequate signals for interrogating the LLA. We derived continuous indices of cerebrovascular reactivity: PRx (correlation between ICP and mean arterial pressure [MAP]), PAx (correlation between pulse amplitude of ICP [AMP] and MAP), and Lx (correlation between LDF-based cerebral blood flow [CBF] and cerebral perfusion pressure [CPP]). LLA was derived via piecewise linear regression of CPP versus LDF or CPP versus systolic flow velocity (FVs) plots. We then produced error bar plots for PRx, PAx, and Lx against 2.5 mm Hg bins of CPP, to display the relationship between these indices and the LLA. We compared the CPP values at clinically relevant thresholds of PRx and PAx, to the CPP defined at the LLA. Receiver operating curve (ROC) analysis was performed for each index across the LLA using 2.5 mm Hg bins for CPP. The mean LLA was 51.5 ± 8.2 mm Hg. PRx and PAx error bar plots demonstrate that each index correlates with the LLA, becoming progressively more positive below the LLA. Similarly, CPP values at clinically relevant thresholds of PRx and PAx were not statistically different from the CPP derived at the LLA. Finally, ROC analysis indicated that PRx and PAx predicted the LAA, with areas under the curve (AUCs) of 0.795 (95% confidence interval [CI]: 0.731-0.857, p < 0.0001) and 0.703 (95% CI: 0.631-0.775, p < 0.0001), respectively. Both PRx and PAx generally agree with LLA within this experimental model of intracranial hypertension. Further analysis of clinically used indices of autoregulation across the LLA within pure arterial hypotension models is required.

Relationship Between Brain Pulsatility and Cerebral Perfusion Pressure: Replicated Validation Using Different Drivers of CPP Change

Background

Determination of relationships between transcranial Doppler (TCD)-based spectral pulsatility index (sPI) and pulse amplitude (AMP) of intracranial pressure (ICP) in 2 groups of severe traumatic brain injury (TBI) patients (a) displaying plateau waves and (b) with unstable mean arterial pressure (MAP).

Methods

We retrospectively reviewed patients with severe TBI and continuous TCD monitoring displaying either plateau waves or unstable MAP from 1992 to 1998. We utilized linear and nonlinear regression techniques to describe both cohorts: cerebral perfusion pressure (CPP) versus AMP, CPP versus sPI, mean ICP versus ICP AMP, mean ICP versus sPI, and AMP versus sPI.

Results

Nonlinear regression techniques were employed to analyze the relationships with CPP. In plateau wave and unstable MAP patients, CPP versus sPI displayed an inverse nonlinear relationship (R² = 0.820 vs. R² = 0.610, respectively), with the CPP versus sPI relationship best modeled by the following function in both cases: PI = a + (b/CPP). Similarly, in both groups, CPP versus AMP displayed an inverse nonlinear relationship (R² = 0.610 vs. R² = 0.360, respectively). Positive linear correlations were displayed in both the plateau wave and unstable MAP cohorts between: ICP versus AMP, ICP versus sPI, AMP versus sPI.

Conclusions

There is an inverse relationship through nonlinear regression between CPP versus AMP and CPP versus sPI display. This provides evidence to support a previously-proposed model of TCD pulsatility index. ICP shows a positive linear correlation with AMP and sPI, which is also established between AMP and sPI.

Electronic supplementary material

The online version of this article (doi:10.1007/s12028-017-0404-9) contains supplementary material, which is available to authorized users.

Increased ICP and Its Cerebral Haemodynamic Sequelae

OBJECTIVES

Increased intracranial pressure (ICP) is a pathological feature of many neurological diseases; however, the local and systemic sequelae of raised ICP are incompletely understood. Using an experimental paradigm, we aimed to describe the cerebrovascular consequences of acute increases in ICP.

MATERIALS AND METHODS

We assessed cerebral haemodynamics [mean arterial blood pressure (MAP), ICP, laser Doppler flowmetry (LDF), basilar artery Doppler flow velocity (Fv) and estimated vascular wall tension (WT)] in 27 basilar artery-dependent rabbits during experimental (artificial lumbar CSF infusion) intracranial hypertension. WT was estimated as the difference between critical closing pressure and ICP.

RESULTS

From baseline (~9 mmHg) to moderate increases in ICP (~41 mmHg), cortical LDF decreased (from 100 to 39.1%, p < 0.001), while mean global Fv was unchanged (from 47 to 45 cm/s, p = 0.38). In addition, MAP increased (from 88.8 to 94.2 mmHg, p < 0.01 and WT decreased (from 19.3 to 9.8 mmHg, p < 0.001). From moderate to high ICP (~75 mmHg), both global Fv and cortical LDF decreased (Fv, from 45 to 31.3 cm/s, p < 0.001; LDF, from 39.1 to 13.3%, p < 0.001) while MAP increased further (94.2 to 114.5 mmHg, p < 0.001) and estimated WT was unchanged (from 9.7 to 9.6 mmHg, p = 0.35).

CONCLUSION

In this analysis, we demonstrate a cortical vulnerability to increases in ICP and two ICP-dependent cerebro-protective mechanisms: with moderate increases in ICP, WT decreases and MAP increases to buffer cerebral perfusion, while with severe increases of ICP, an increased MAP predominates.

Intracranial Pressure Is a Determinant of Sympathetic Activity

Intracranial pressure (ICP) is the pressure within the cranium. ICP rise compresses brain vessels and reduces cerebral blood delivery. Massive ICP rise leads to cerebral ischemia, but it is also known to produce hypertension, bradycardia and respiratory irregularities due to a sympatho-adrenal mechanism termed Cushing response. One still unresolved question is whether the Cushing response is a non-synaptic acute brainstem ischemic mechanism or part of a larger physiological reflex for arterial blood pressure control and homeostasis regulation. We hypothesize that changes in ICP modulates sympathetic activity. Thus, modest ICP increase and decrease were achieved in mice and patients with respectively intra-ventricular and lumbar fluid infusion. Sympathetic activity was gauged directly by microneurography, recording renal sympathetic nerve activity in mice and muscle sympathetic nerve activity in patients, and gauged indirectly in both species by heart-rate variability analysis. In mice (n = 15), renal sympathetic activity increased from 29.9 ± 4.0 bursts.s⁻¹ (baseline ICP 6.6 ± 0.7 mmHg) to 45.7 ± 6.4 bursts.s⁻¹ (plateau ICP 38.6 ± 1.0 mmHg) and decreased to 34.8 ± 5.6 bursts.s⁻¹ (post-infusion ICP 9.1 ± 0.8 mmHg). In patients (n = 10), muscle sympathetic activity increased from 51.2 ± 2.5 bursts.min⁻¹ (baseline ICP 8.3 ± 1.0 mmHg) to 66.7 ± 2.9 bursts.min⁻¹ (plateau ICP 25 ± 0.3 mmHg) and decreased to 58.8 ± 2.6 bursts.min⁻¹ (post-infusion ICP 14.8 ± 0.9 mmHg). In patients 7 mmHg ICP rise significantly increases sympathetic activity by 17%. Heart-rate variability analysis demonstrated a significant vagal withdrawal during the ICP rise, in accordance with the microneurography findings. Mice and human results are alike. We demonstrate in animal and human that ICP is a reversible determinant of efferent sympathetic outflow, even at relatively low ICP levels. ICP is a biophysical stress related to the forces within the brain. But ICP has also to be considered as a physiological stressor, driving sympathetic activity. The results suggest a novel physiological ICP-mediated sympathetic modulation circuit and the existence of a possible intracranial (i.e., central) baroreflex. Modest ICP rise might participate to the pathophysiology of cardio-metabolic homeostasis imbalance with sympathetic over-activity, and to the pathogenesis of sympathetically-driven diseases.

Individualizing Thresholds of Cerebral Perfusion Pressure Using Estimated Limits of Autoregulation

OBJECTIVES

In severe traumatic brain injury, cerebral perfusion pressure management based on cerebrovascular pressure reactivity index has the potential to provide a personalized treatment target to improve patient outcomes. So far, the methods have focused on identifying "one" autoregulation-guided cerebral perfusion pressure target-called "cerebral perfusion pressure optimal". We investigated whether a cerebral perfusion pressure autoregulation range-which uses a continuous estimation of the "lower" and "upper" cerebral perfusion pressure limits of cerebrovascular pressure autoregulation (assessed with pressure reactivity index)-has prognostic value.

DESIGN

Single-center retrospective analysis of prospectively collected data.

SETTING

The neurocritical care unit at a tertiary academic medical center.

PATIENTS

Data from 729 severe traumatic brain injury patients admitted between 1996 and 2016 were used. Treatment was guided by controlling intracranial pressure and cerebral perfusion pressure according to a local protocol.

INTERVENTIONS

None.

METHODS AND MAIN RESULTS

Cerebral perfusion pressure-pressure reactivity index curves were fitted automatically using a previously published curve-fitting heuristic from the relationship between pressure reactivity index and cerebral perfusion pressure. The cerebral perfusion pressure values at which this "U-shaped curve" crossed the fixed threshold from intact to impaired pressure reactivity (pressure reactivity index = 0.3) were denoted automatically the "lower" and "upper" cerebral perfusion pressure limits of reactivity, respectively. The percentage of time with cerebral perfusion pressure below (%cerebral perfusion pressure < lower limit of reactivity), above (%cerebral perfusion pressure > upper limit of reactivity), or within these reactivity limits (%cerebral perfusion pressure within limits of reactivity) was calculated for each patient and compared across dichotomized Glasgow Outcome Scores. After adjusting for age, initial Glasgow Coma Scale, and mean intracranial pressure, percentage of time with cerebral perfusion pressure less than lower limit of reactivity was associated with unfavorable outcome (odds ratio %cerebral perfusion pressure < lower limit of reactivity, 1.04; 95% CI, 1.02-1.06; p < 0.001) and mortality (odds ratio, 1.06; 95% CI, 1.04-1.08; p < 0.001).

CONCLUSIONS

Individualized autoregulation-guided cerebral perfusion pressure management may be a plausible alternative to fixed cerebral perfusion pressure threshold management in severe traumatic brain injury patients. Prospective randomized research will help define which autoregulation-guided method is beneficial, safe, and most practical.