Arterio-jugular differences of oxygen (AVDO2) for bedside assessment of CO2-reactivity and autoregulation in the acute phase of severe head injury

Cerebral Pressure Autoregulation in Brain Injury and Disorders-A Review on Monitoring, Management, and Future Directions

The role of cerebral pressure autoregulation (CPA) in brain injury and disorders has gained increased interest. The CPA is often disturbed as a consequence of acute brain injury, which contributes to further brain damage and worse outcome. Specifically, in severe traumatic brain injury, CPA disturbances predict worse clinical outcome and targeting an autoregulatory-oriented optimal cerebral perfusion pressure threshold may improve brain energy metabolism and clinical outcome. In aneurysmal subarachnoid hemorrhage, cerebral vasospasm in combination with distal autoregulatory disturbances precipitate delayed cerebral ischemia. The role of optimal cerebral perfusion pressure targets is less clear in aneurysmal subarachnoid hemorrhage, but high cerebral perfusion pressure targets are generally favorable in the vasospasm phase. In acute ischemia, autoregulatory disturbances may occur and autoregulatory-oriented blood pressure (optimal mean arterial pressure) management reduces the risk of hemorrhagic transformation, brain edema, and unfavorable outcome. In chronic occlusive disease such as moyamoya, the gradual reduction of the cerebral circulation leads to compensatory distal vasodilation and the residual CPA capacity predicts the risk for cerebral ischemia. In spontaneous intracerebral hemorrhage, the role of autoregulatory disturbances is less clear, but CPA disturbances correlate with worse clinical outcome. Also, in community-acquired bacterial meningitis, CPA dysfunction is frequent and correlates with worse clinical outcome, but autoregulatory management is yet to be evaluated. In this review, we discuss the role of CPA in different types of brain injury and disease, the strengths and limitations of the monitoring methods, the potentials of autoregulatory management, and future directions in the field.

Blood flow response to orthostatic challenge identifies signatures of the failure of static cerebral autoregulation in patients with cerebrovascular disease

Background

The cortical microvascular cerebral blood flow response (CBF) to different changes in head-of-bed (HOB) position has been shown to be altered in acute ischemic stroke (AIS) by diffuse correlation spectroscopy (DCS) technique. However, the relationship between these relative ΔCBF changes and associated systemic blood pressure changes has not been studied, even though blood pressure is a major driver of cerebral blood flow.

Methods

Transcranial DCS data from four studies measuring bilateral frontal microvascular cerebral blood flow in healthy controls (n = 15), patients with asymptomatic severe internal carotid artery stenosis (ICA, n = 27), and patients with acute ischemic stroke (AIS, n = 72) were aggregated. DCS-measured CBF was measured in response to a short head-of-bed (HOB) position manipulation protocol (supine/elevated/supine, 5 min at each position). In a sub-group (AIS, n = 26; ICA, n = 14; control, n = 15), mean arterial pressure (MAP) was measured dynamically during the protocol.

Results

After elevated positioning, DCS CBF returned to baseline supine values in controls (p = 0.890) but not in patients with AIS (9.6% [6.0,13.3], mean 95% CI, p < 0.001) or ICA stenosis (8.6% [3.1,14.0], p = 0.003)). MAP in AIS patients did not return to baseline values (2.6 mmHg [0.5, 4.7], p = 0.018), but in ICA stenosis patients and controls did. Instead ipsilesional but not contralesional CBF was correlated with MAP (AIS 6.0%/mmHg [− 2.4,14.3], p = 0.038; ICA stenosis 11.0%/mmHg [2.4,19.5], p < 0.001).

Conclusions

The observed associations between ipsilateral CBF and MAP suggest that short HOB position changes may elicit deficits in cerebral autoregulation in cerebrovascular disorders. Additional research is required to further characterize this phenomenon.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02179-8.

Cerebrovascular Response to Propofol, Fentanyl, and Midazolam in Moderate/Severe Traumatic Brain Injury: A Scoping Systematic Review of the Human and Animal Literature

Intravenous propofol, fentanyl, and midazolam are utilized commonly in critical care for metabolic suppression and anesthesia. The impact of propofol, fentanyl, and midazolam on cerebrovasculature and cerebral blood flow (CBF) is unclear in traumatic brain injury (TBI) and may carry important implications, as care is shifting to focus on cerebrovascular reactivity monitoring/directed therapies. The aim of this study was to perform a scoping review of the literature on the cerebrovascular/CBF effects of propofol, fentanyl, and midazolam in human patients with moderate/severe TBI and animal models with TBI. A search of MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and the Cochrane Library from inception to May 2020 was performed. All articles were included pertaining to the administration of propofol, fentanyl, and midazolam, in which the impact on CBF/cerebral vasculature was recorded. We identified 14 studies: 8 that evaluated propofol, 5 that evaluated fentanyl, and 2 that evaluated midazolam. All studies suffered from significant limitations, including: small sample size, and heterogeneous design and measurement techniques. In general, there was no significant change seen in CBF/cerebrovascular response to administration of propofol, fentanyl, or midazolam during experiments where PCO₂ and mean arterial pressure (MAP) were controlled. This review highlights the current knowledge gap surrounding the impact of commonly utilized sedative drugs in TBI care. This work supports the need for dedicated studies, both experimental and human-based, evaluating the impact of these drugs on CBF and cerebrovascular reactivity/response in TBI.

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.

Monitoring of brain and systemic oxygenation in neurocritical care patients

Maintenance of adequate oxygenation is a mainstay of intensive care, however, recommendations on the safety, accuracy, and the potential clinical utility of invasive and non-invasive tools to monitor brain and systemic oxygenation in neurocritical care are lacking. A literature search was conducted for English language articles describing bedside brain and systemic oxygen monitoring in neurocritical care patients from 1980 to August 2013. Imaging techniques e.g., PET are not considered. A total of 281 studies were included, the majority described patients with traumatic brain injury (TBI). All tools for oxygen monitoring are safe. Parenchymal brain oxygen (PbtO2) monitoring is accurate to detect brain hypoxia, and it is recommended to titrate individual targets of cerebral perfusion pressure (CPP), ventilator parameters (PaCO2, PaO2), and transfusion, and to manage intracranial hypertension, in combination with ICP monitoring. SjvO2 is less accurate than PbtO2. Given limited data, NIRS is not recommended at present for adult patients who require neurocritical care. Systemic monitoring of oxygen (PaO2, SaO2, SpO2) and CO2 (PaCO2, end-tidal CO2) is recommended in patients who require neurocritical care.

Early-stage microvascular alterations of a new model of controlled cortical traumatic brain injury: 3D morphological analysis using scanning electron microscopy and corrosion casting

Object

This study was performed to study the microvascular changes that occur during the first 12 hours after traumatic brain injury (TBI) using the corrosion casting technique.

Methods

The authors performed a qualitative and quantitative morphological study of the changes in cerebral vessels at acute (3 hours) and subacute (12 hours) stages after experimental TBI. They used a model of controlled cortical impact (CCI) injury induced by a recently developed electromagnetic device (impactor), focusing their observations mainly on the microvascular alterations responsible for the formation and maintenance of tissue edema and consequent brain swelling during the first hours after TBI. They used corrosion casting, scanning electron microscopy (SEM), light microscopy, and transmission electron microscopy (TEM) to obtain a morphological qualitative map with both 2D and 3D details.

Results

Scanning electron microscopy analysis of vascular casts documented in 3 dimensions the typical injuries occurring after a TBI: subdural, subarachnoid, and intraparenchymal hemorrhages, along with alterations of the morphological characteristics and architecture of both medium-sized and capillary vessels, including ectasia of pial vessels, sphincter constrictions at the origin of the perforating vessels, focal swelling of perforating vessels, widening of intercellular junctions, and some indirect evidence of structural impairment of endothelial cells. All of these vascular alterations were confirmed in 2D analyses using light microscopy and TEM.

Conclusions

The corrosion casting–SEM technique applied to a CCI experimental model proved to be a reliable method for studying the pathophysiology of the vascular alterations occurring at acute and subacute stages after CCI injury. It was also possible to obtain topographical localization of the vascular and cellular events that usually lead to hyperemia, edema, and brain swelling. Moreover, by applying informatic software to anatomical images it was possible to perform quantification and statistical analysis of the observed events.

Intraaortic Balloon Pump Counterpulsation and Cerebral Autoregulation: an observational study

Background

The use of Intra-aortic counterpulsation is a well established supportive therapy for patients in cardiac failure or after cardiac surgery. Blood pressure variations induced by counterpulsation are transmitted to the cerebral arteries, challenging cerebral autoregulatory mechanisms in order to maintain a stable cerebral blood flow. This study aims to assess the effects on cerebral autoregulation and variability of cerebral blood flow due to intra-aortic balloon pump and inflation ratio weaning.

Methods

Cerebral blood flow was measured using transcranial Doppler, in a convenience sample of twenty patients requiring balloon counterpulsation for refractory cardiogenic shock (N = 7) or a single inotrope to maintain mean arterial pressure following an elective placement of an intra-aortic balloon pump for cardiac surgery (N = 13). Simultaneous blood pressure at the aortic root was recorded via the intra-aortic balloon pump. Cerebral blood flow velocities were recorded for six minute intervals at a 1:1 balloon inflation-ratio (augmentation of all cardiac beats) and during progressive reductions of the inflation-ratio to 1:3 (augmentation of one every third cardiac beat). Real time comparisons of peak cerebral blood flow velocities with systolic blood pressure were performed using cross-correlation analysis. The primary endpoint was assessment of cerebral autoregulation using the time delay between the peak signals for cerebral blood flow velocity and systolic blood pressure, according to established criteria. The variability of cerebral blood flow was also assessed using non-linear statistics.

Results

During the 1:1 inflation-ratio, the mean time delay between aortic blood pressure and cerebral blood flow was -0.016 seconds (95% CI: -0.023,-0.011); during 1:3 inflation-ratio mean time delay was significantly longer at -0.010 seconds (95% CI: -0.016, -0.004, P < 0.0001). Finally, upon return to a 1:1 inflation-ratio, time delays recovered to those measured at baseline. During inflation-ratio reduction, cerebral blood flow irregularities reduced over time, whilst cerebral blood flow variability at end-diastole decreased in patients with cardiogenic shock.

Conclusions

Weaning counterpulsation from 1:1 to 1:3 inflation ratio leads to a progressive reduction in time delays between systolic blood pressure and peak cerebral blood flow velocities suggesting that although preserved, there is a significant delay in the establishment of cerebral autoregulatory mechanisms. In addition, cerebral blood flow irregularities (i.e. surrogate of flow adaptability) decrease and a loss of cerebral blood flow chaotic pattern occurs during the end-diastolic phase of each beat in patients with cardiogenic shock.

Continuous monitoring of cerebrovascular pressure reactivity in patients with head injury

Object

Cerebrovascular pressure reactivity is the ability of cerebral vessels to respond to changes in transmural pressure. A cerebrovascular pressure reactivity index (PRx) can be determined as the moving correlation coefficient between mean intracranial pressure (ICP) and mean arterial blood pressure.

Methods

The authors analyzed a database consisting of 398 patients with head injuries who underwent continuous monitoring of cerebrovascular pressure reactivity. In 298 patients, the PRx was compared with a transcranial Doppler ultrasonography assessment of cerebrovascular autoregulation (the mean index [Mx]), in 17 patients with the PET–assessed static rate of autoregulation, and in 22 patients with the cerebral metabolic rate for O2. Patient outcome was assessed 6 months after injury.

Results

There was a positive and significant association between the PRx and Mx (R2 = 0.36, p < 0.001) and with the static rate of autoregulation (R2 = 0.31, p = 0.02). A PRx > 0.35 was associated with a high mortality rate (> 50%). The PRx showed significant deterioration in refractory intracranial hypertension, was correlated with outcome, and was able to differentiate patients with good outcome, moderate disability, severe disability, and death. The graph of PRx compared with cerebral perfusion pressure (CPP) indicated a U–shaped curve, suggesting that too low and too high CPP was associated with a disturbance in pressure reactivity. Such an optimal CPP was confirmed in individual cases and a greater difference between current and optimal CPP was associated with worse outcome (for patients who, on average, were treated below optimal CPP [R2 = 0.53, p < 0.001] and for patients whose mean CPP was above optimal CPP [R2 = −0.40, p < 0.05]). Following decompressive craniectomy, pressure reactivity initially worsened (median −0.03 [interquartile range −0.13 to 0.06] to 0.14 [interquartile range 0.12–0.22]; p < 0.01) and improved in the later postoperative course. After therapeutic hypothermia, in 17 (70.8%) of 24 patients in whom rewarming exceeded the brain temperature threshold of 37°C, ICP remained stable, but the average PRx increased to 0.32 (p < 0.0001), indicating significant derangement in cerebrovascular reactivity.

Conclusions

The PRx is a secondary index derived from changes in ICP and arterial blood pressure and can be used as a surrogate marker of cerebrovascular impairment. In view of an autoregulation–guided CPP therapy, a continuous determination of a PRx is feasible, but its value has to be evaluated in a prospective controlled trial.

Evaluation of a bedside monitor of regional CBF as a measure of CO2 reactivity in neurosurgical intensive care patients

OBJECTIVE

Mild hyperventilation remains a key element in the management of elevated intracranial pressure. However, a harmful effect of hyperventilation on the development or deterioration of ischemic lesions has been shown in patients after severe head trauma. The objective of this study was to investigate the clinical feasibility and reliability of continuous monitoring of regional cerebral blood flow (rCBF) during mild hyperventilation using a thermodiffusion probe. CO2 reactivity was calculated. The measurement of the partial pressure of oxygen (PtiO2) in the cerebral tissue served as a reference parameter.

METHODS

An intraparenchymal intracranial pressure sensor, a multiparameter probe for determining the partial pressure of cerebral gases (pHti, PtiO2, PtiCO2), and a thermodiffusion probe for measuring rCBF were used in 10 intensive care patients. All patients were analgosedated and received pressure-controlled mechanical ventilation. Controlled mild hyperventilation was carried out on 2 consecutive days. CO2 reactivity was determined in relation to both CBF and PtiO2.

RESULTS

Controlled hyperventilation resulted in a rCBF reduction from 30+/-3 mL/100 g/min to 25+/-2.4 mL/100 g/min (-17%; P<0.05) on the first day of examination and 31+/-3.6 mL/100 g/min to 22+/-4.9 mL/100 g/min (-29%; P<0.05) on the second day. Likewise, mild hyperventilation resulted in a reduction of regional cerebral tissue oxygen partial pressure from 20+/-2.9 mm Hg to 15+/-4 (-25%; P<0.05) on the first day and 20+/-3.1 mm Hg to 14+/-1.5 mm Hg (-30%; P<0.05) on the second.

CONCLUSIONS

Continuous monitoring of regional CBF, using an intraparenchymally placed thermodiffusion probe, seems to be a simple and safe bedside technique. The promise of reliably monitoring and interpreting additional parameters such as PtiO2 and PtiCO2 warrants further investigation.

Prehospital HBOC-201 after traumatic brain injury and hemorrhagic shock in swine

BACKGROUND

Data are limited on the actions of hemoglobin based oxygen carriers (HBOCs) after traumatic brain injury (TBI). This study evaluates neurotoxicity, vasoactivity, cardiac toxicity, and inflammatory activity of HBOC-201 (Biopure, Cambridge, Mass.) resuscitation in a TBI model.

METHODS

Swine received TBI and hemorrhage. After 30 minutes, resuscitation was initiated with 10 mL/kg normal saline (NS), followed by either HBOC-201 (6 mL/kg, n = 10) or NS control (n = 10). Supplemental NS was administered to both groups to maintain mean arterial pressure (MAP) >60 mm Hg until 60 minutes, and to maintain cerebral perfusion pressure (CPP) >70 mm Hg from 60 to 300 minutes. The control group received mannitol (1 g/kg) and blood (10 mL/kg) at 90 minutes and half (n = 5) received CPP directed phenylephrine (PE) therapy after 120 minutes. Serum cytokines were measured with ELISA and coagulation was evaluated with thromboelastography. Brains were harvested for neuropathology.

RESULTS

With HBOC administration, MAP, CPP, and brain tissue PO2 were restored within 30 minutes and maintained until 300 minutes. Clot strength and fibrin formation were maintained and 9/10 successfully extubated. In contrast, with control, MAP and brain tissue PO2 did not correct until 120 minutes, after mannitol, transfusion and 40% more crystalloid. Furthermore, without PE, CPP did not reach target and 0/5 could be extubated. Lactate, heart rate, cardiac output, mixed venous oxygenation, muscle oxygenation, serum cytokines, and histology did not differ between groups.

CONCLUSIONS

After TBI, a single HBOC-201 bolus with minimal supplements provided rapid resuscitation, while maintaining CPP and improving brain oxygenation, without causing cardiac dysfunction, coagulopathy, cytokine release, or brain structural changes.

Continuous assessment of cerebrovascular autoregulation after traumatic brain injury using brain tissue oxygen pressure reactivity

OBJECTIVE

To evaluate whether two newly developed indexes of brain tissue oxygen pressure reactivity (ORx and bPtio2) provide information on the status of cerebrovascular autoregulation after traumatic brain injury. This was accomplished by analyzing the relationship between these indexes and an index of cerebrovascular pressure reactivity (PRx). PRx is an established parameter for estimation of cerebrovascular autoregulation.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurosurgical intensive care unit of a university hospital.

PATIENTS

Twenty-seven patients suffering from severe traumatic brain injury.

INTERVENTIONS

Continuous monitoring of mean arterial blood pressure, intracranial pressure, cerebral perfusion pressure, and partial pressure of brain tissue oxygen (Ptio2) was performed for an average of 6.5 days. ORx was calculated as a moving correlation coefficient between values of cerebral perfusion pressure and Ptio2. The bPtio2 was calculated as a moving value of the slope of the linear regression function between cerebral perfusion pressure and Ptio2. PRx was calculated as a moving correlation coefficient between values for intracranial pressure and mean arterial blood pressure. Outcome was assessed at 6 months after traumatic brain injury (Glasgow Outcome Scale).

MEASUREMENTS AND MAIN RESULTS

Both ORx and bPtio2 correlated significantly with PRx (r=.55 for ORx, r=.52 for bPtio2, p<.01). PRx and ORx showed a significantly negative correlation to the monitored Ptio2 values (r=-.42 for PRx, r=-.41 for ORx, p<.05) and outcome (r=-.52 for PRx, r=-.62 for ORx, p<.01), whereas bPtio2 did not.

CONCLUSIONS

ORx and, to a lesser extent, bPtio2 correlated with the autoregulatory marker PRx and provide additional information about the status of cerebrovascular autoregulation after traumatic brain injury. The data also suggested that patients with impaired autoregulation are at increased risk for secondary cerebral hypoxia.

Effects of arginine vasopressin during resuscitation from hemorrhagic hypotension after traumatic brain injury

OBJECTIVE

Two series of experiments were designed to evaluate whether early arginine vasopressin improves acute outcome following resuscitation from traumatic brain injury and severe hemorrhagic hypotension.

DESIGN

Prospective randomized, blinded animal study.

SETTING

University laboratory.

SUBJECTS

Thirty-three swine.

INTERVENTIONS

In series 1 (n = 19), after traumatic brain injury with hemorrhage and 12 mins of shock (mean arterial pressure approximately 20 mm Hg), survivors (n = 16) were initially resuscitated with 10 mL/kg crystalloid. After 30 mins, crystalloid and blood with either 0.1 unit x kg(-1) x hr(-1) arginine vasopressin or placebo was titrated to a mean arterial pressure target >or=60 mm Hg. After 90 mins, all received mannitol and the target was cerebral perfusion pressure >or=60 mm Hg. To test cerebrovascular function, 7.5% inhaled CO2 was administered periodically. In series 2 (n = 14), the identical protocol was followed except the shock period was 20 mins and survivors (n = 10) received a bolus of either arginine vasopressin (0.2 units/kg) or placebo during the initial fluid resuscitation.

MEASUREMENTS AND MAIN RESULTS

In series 1, by 300 mins after traumatic brain injury with arginine vasopressin (n = 8) vs. placebo (n = 8), the fluid and transfusion requirements were reduced (both p < .01), intracranial pressure was improved (11 +/- 1 vs. 23 +/- 2 mmHg; p < .0001), and the CO2-evoked intracranial pressure elevation was reduced (7 +/- 2 vs. 26 +/- 3 mm Hg, p < .001), suggesting improved compliance. In series 2, with arginine vasopressin vs. placebo, cerebral perfusion pressure was more rapidly corrected (p < .05). With arginine vasopressin, five of five animals survived 300 mins, whereas three of five placebo animals died. The survival time with placebo was 54 +/- 4 mins (p < .05 vs. arginine vasopressin).

CONCLUSIONS

Early supplemental arginine vasopressin rapidly corrected cerebral perfusion pressure, improved cerebrovascular compliance, and prevented circulatory collapse during fluid resuscitation of hemorrhagic shock after traumatic brain injury.

Resuscitation with Pressors after Traumatic Brain Injury

BACKGROUND

The purpose of the study was to compare initial resuscitation with arginine vasopressin (AVP), phenylephrine (PE), or isotonic crystalloid fluid alone after traumatic brain injury and vasodilatory shock.

STUDY DESIGN

Anesthetized, ventilated swine (n = 39, 30 +/- 2 kg) underwent fluid percussion traumatic brain injury followed by hemorrhage (30 +/- 2mL/kg) to a mean arterial pressure < 30mmHg, then were randomized to 1 of 5 groups to maintain mean arterial pressure > 60mmHg for 30 to 60minutes, then cerebral perfusion pressure > 60mmHg for 60 to 300minutes, either unlimited crystalloid fluid only (n = 9), arginine vasopressin + fluid (n = 9), phenylephrine + fluid (n = 9), arginine vasopressin only (n = 5), or phenylephrine only (n = 5). Heterologous transfusions were administered if hematocrit was < 13, and mannitol was administered if intracranial pressure was > 20 mmHg. Cerebrovascular reactivity was evaluated with serial CO(2) challenges.

RESULTS

In all groups, physiologic variables were similar at baseline and at the end of shock. On resuscitation, all achieved mean arterial pressure and cerebral perfusion pressure goals. Brain tissue PO(2)s were similar. With fluid only, more blood and mannitol were required, intracranial pressure and peak inspiratory pressure were higher, and cerebrovascular reactivity was decreased (all p < 0.05 versus pressor + fluid). With either pressor + fluid, cardiac output, heart rate, lactate, and mixed venous O(2) saturation were similar to fluid only, but total fluid requirements and urine output were both reduced (p < 0.05). With either pressor only, intracranial pressure remained low, but mixed venous O(2) saturation, cardiac output, and urine output were decreased (all p < 0.05 versus other groups).

CONCLUSIONS

To correct vasodilatory shock after traumatic brain injury, a resuscitation strategy that combined either phenylephrine or arginine vasopressin plus crystalloid was superior to either fluid alone or pressor alone.

Changes in intracranial pressure, coagulation, and neurologic outcome after resuscitation from experimental traumatic brain injury with hetastarch

BACKGROUND

In a model of traumatic brain injury (TBI), 2 protocols compared changes in intracranial pressure (ICP), coagulation, and neurologic outcome after intravenous fluid (IVF) resuscitation with either Hextend (HEX, 6% hetastarch in lactated electrolyte injection) or standard of care, crystalloid plus mannitol (MAN).

METHODS

In the nonsurvivor protocol, swine (n = 28) received a fluid percussion TBI and hemorrhage (27 +/- 3 mL/kg). At 30 minutes, resuscitation began with lactated Ringer's (LR) or HEX. After 60 minutes, MAN (1 g/kg) or placebo was given plus supplemental IVF to maintain cerebral perfusion pressure (CPP) > or = 70 mm Hg for 240 minutes. Swine in the survivor group (n = 15) also underwent TBI and hemorrhage, and resuscitation with HEX was compared to that of normal saline (NS)+MAN. Neurologic outcome and coagulation were evaluated for 72 hours.

RESULTS

In the nonsurvivor protocol, HEX, LR+MAN, and HEX+MAN attenuated the time-related rise of ICP and prevented ICP >20 mm Hg versus LR alone (P < .05). HEX alone maintained CPP (relative to baseline) and decreased total IVF by 50% versus LR +/- MAN (P < .05). MAN had no additive effect with HEX. Coagulation, measured by thromboelastograph reaction time (R), was 11 +/- 1 and 9 +/- 1 minutes at baseline and after TBI (before randomization). At 240 minutes after HEX or LR+MAN, R was 6 +/- 1 or 7 +/- 2 minutes, which indicates a hypercoagulable state, but there was no difference between treatments. In the survivor protocol, ICP and CPP were similar with NS+MAN versus HEX, but IVF requirement was 161 +/- 20 versus 28 +/- 3 mL/kg (P < .05). Motor scores were higher on days 2 and 3 with HEX (P < .05). At 72 hours, R was 28 +/- 14 versus 26 +/- 6 minutes with NS+MAN versus HEX, which indicates a hypocoagulable state, but there was no difference between treatments.

CONCLUSIONS

Hextend as the sole resuscitation fluid after severe TBI reduces fluid requirement, obviates the need for mannitol, improves neurologic outcome, and has no adverse effect on the coagulation profile relative to the crystalloid plus mannitol standard of care.

Assessment of cerebrovascular autoregulation in head-injured patients: a validation study

BACKGROUND AND PURPOSE

Cerebrovascular autoregulation is frequently measured in head-injured patients. We attempted to validate 4 bedside methods used for assessment of autoregulation.

METHODS

PET was performed at a cerebral perfusion pressure (CPP) of 70 and 90 mm Hg in 20 patients. Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo2) were determined at each CPP level. Patients were sedated with propofol and fentanyl. Norepinephrine was used to control CPP. During PET scanning, transcranial Doppler (TCD) flow velocity in the middle cerebral artery was monitored, and the arterio-jugular oxygen content difference (AJDo2) was measured at each CPP. Autoregulation was determined as the static rate of autoregulation based on PET (SROR(PET)) and TCD (SROR(TCD)) data, based on changes in AJDo2, and with 2 indexes based on the relationship between slow waves of CPP and flow velocity (mean velocity index, Mx) and between arterial blood pressure and intracranial pressure (pressure reactivity index, PRx)

RESULTS

We found significant correlations between SROR(PET) and SROR(TCD) (r2=0.32; P<0.01) and between SROR(PET) and PRx (r2=0.31; P<0.05). There were no significant associations between PET data and autoregulation as assessed by changes in AJDo2. Global CMRo2 was significantly lower at the higher CPP (P<0.01).

CONCLUSIONS

Despite some variability, SROR(TCD) and PRx may provide useful approximations of autoregulation in head-injured patients. At least with our methods, CMRo2 changes with the increase in CPP; hence, flow-metabolism coupling may affect the results of autoregulation testing.

Hemodynamic actions of acute ethanol after resuscitation from traumatic brain injury

BACKGROUND

The purposes of this study were to determine how clinically relevant levels of acute ethanol (EtOH) influence cerebral perfusion pressure (CPP), cerebral venous O saturation (Scvo ), and systemic hemodynamics after fluid resuscitation from traumatic brain injury (TBI); and to test the hypothesis that the actions of EtOH on these variables are mediated by adenosine.

METHODS

Anesthetized swine were ventilated (Fio = 0.4) and instrumented. In protocol 1, EtOH (3.5 g/kg, n = 11) or its vehicle (n = 17) was administered orally before TBI + 40% hemorrhage. At 90 minutes post-TBI, resuscitation consisted of shed blood + saline. In protocol 2, either saline (n = 15) or an adenosine-regulating agent (5-amino-4-imidazolecarboxamide riboside) in saline (1 mg/kg bolus + 12 mg/kg/h intravenously [i.v.]) (n = 5), was administered i.v. before TBI + 45% hemorrhage. At 90 minutes post-TBI, resuscitation consisted of saline only (three times shed blood volume). In protocol 3, EtOH was administered i.v. (1 g/kg; 20% vol/vol in saline) followed by either an adenosine receptor antagonist (theophylline, 10 mg/kg) or an adenosine uptake inhibitor (dipyridamole, 0.25 mg/kg).

RESULTS

In protocol 1, with no EtOH, 11 of 17 (65%) survived post-TBI hypotension. Mean arterial blood pressure, cardiac index, and mixed venous oxygen saturation were stable for 1 hour at 40% to 60% below their respective baselines, whereas lactate increased three- to fourfold (all p < 0.05). After fluid resuscitation, most variables rapidly corrected, but intracranial pressure was increased 10 to 15 mm Hg (p < 0.05). With EtOH, 9 of 11 (82%) survived post-TBI hypotension (p = 0.42 vs. no EtOH). After resuscitation from TBI, there were significant effects of EtOH on systemic hemodynamics (mean arterial pressure, cardiac index, mixed venous oxygen saturation), on CPP, on lactate, and on Scvo at normo- and hypercapnia (all p < 0.05). The data from protocol 2 showed that essentially none of these changes were duplicated with an adenosine-regulating agent. In protocol 3, i.v EtOH produced small but significant changes in Scvo, intracranial pressure, and lactate, at normo-, hyper-, and hypocapnia. Dipyridamole and theophylline tended to have opposite, albeit small and not statistically significant, effects on these variables relative to EtOH alone.(2) (2)

CONCLUSION

Acute EtOH (200-300 mg/dL) did not increase mortality after TBI + secondary hypotension, as long as cardiopulmonary support was provided. With EtOH, CPP was maintained and cerebral blood flow appeared to be adequate, if not excessive, with respect to cerebral metabolic demand, as judged by changes in Scvo at normo-, hyper-, and hypocapnia. These changes were probably not mediated, but might have been modulated, by increases in endogenous adenosine.

Continuous cerebral autoregulation monitoring by cross-correlation analysis

In order to validate cross-correlation analysis between spontaneous slow oscillations of arterial blood pressure (aBP) and intracranial pressure (ICP) or flow velocity as a means to assess the status of cerebral autoregulation continuously, we compared its results with different autoregulation bedside tests. The second aim was to check the method's stability over longer time periods. aBP, ICP, and flow velocity in the middle cerebral artery (FV(MCA)) was measured continuously in 13 critically ill comatose patients. Cross-correlation analysis was performed online and offline between aBP and ICP (CC [aBP --> ICP]) and aBP/FV(MCA) (CC [aBP --> FV(MCA)]). Three different autoregulation bedside tests (cuff deflation, transient hyperemic response, orthostatic hypotension) were performed immediately before a 29-min cross-correlation test period. In addition, continuous cross-correlation autoregulation monitoring was performed over multiple hours (in order to analyze for stability and to assess the influence of other factors). Cluster analysis revealed two main clusters. Cluster 1 (indicative for disturbed autoregulation) showed a centroid at t = -0.21 +/- 3.32 sec, r = 0.43 +/- 0.18 for CC [aBP --> ICP], and t = 0 +/- 3.14 sec, r = 0.44 +/- 0.18 for CC [aBP --> FV(MCA)]. Cluster 2 (indicative for normal autoregulation) revealed a centroid at t = 4.94 +/- 3.74 sec, r =- 0.4 +/- 0.16 for CC [aBP --> ICP], and t = 3.38 +/- 4.44 sec, r = -0.38 +/- 0.18 for CC [aBP --> FV(MCA)]. Comparison between the cross-correlation test results and the bedside tests showed a sensitivity of 44-73% for CC [aBP --> FV(MCA)], whereas CC [aBP --> ICP] was more specific (60-80%). Long-term monitoring revealed stable cross-correlation tests in about 45% of the measurement time. It is concluded that cross-correlation between aBP, ICP, and FV(MCA) is a valid means to monitor the autoregulation status continuously, although further improvement of sensitivity and specificity is needed to make it reliable for clinical decision making.

Continuous monitoring of cerebrovascular pressure reactivity allows determination of optimal cerebral perfusion pressure in patients with traumatic brain injury

OBJECTIVES

To define optimal cerebral perfusion pressure (CPPOPT) in individual head-injured patients using continuous monitoring of cerebrovascular pressure reactivity. To test the hypothesis that patients with poor outcome were managed at a cerebral perfusion pressure (CPP) differing more from their CPPOPT than were patients with good outcome.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurosciences critical care unit of a university hospital.

PATIENTS

A total of 114 head-injured patients admitted between January 1997 and August 2000 with continuous monitoring of mean arterial blood pressure (MAP) and intracranial pressure (ICP).

MEASUREMENTS AND MAIN RESULTS

MAP, ICP, and CPP were continuously recorded and a pressure reactivity index (PRx) was calculated online. PRx is the moving correlation coefficient recorded over 4-min periods between averaged values (6-sec periods) of MAP and ICP representing cerebrovascular pressure reactivity. When cerebrovascular reactivity is intact, PRx has negative or zero values, otherwise PRx is positive. Outcome was assessed at 6 months using the Glasgow Outcome Scale. A total of 13,633 hrs of data were recorded. CPPOPT was defined as the CPP where PRx reaches its minimum value when plotted against CPP. Identification of CPPOPT was possible in 68 patients (60%). In 22 patients (27%), CPPOPT was not found because it presumably lay outside the studied range of CPP. Patients' outcome correlated with the difference between CPP and CPPOPT for patients who were managed on average below CPPOPT (r =.53, p <.001) and for patients whose mean CPP was above CPPOPT (r = -.40, p <.05).

CONCLUSIONS

CPPOPT could be identified in a majority of patients. Patients with a mean CPP close to CPPOPT were more likely to have a favorable outcome than those whose mean CPP was more different from CPPOPT. We propose use of the criterion of minimal achievable PRx to guide future trials of CPP oriented treatment in head injured patients.

Resuscitation from severe hemorrhagic shock after traumatic brain injury using saline, shed blood, or a blood substitute

The original purpose of this study was to compare initial resuscitation of hemorrhagic hypotension after traumatic brain injury (TBI) with saline and shed blood. Based on those results, the protocol was modified and saline was compared to a blood substitute, diaspirin cross-linked hemoglobin (DCLHb). Two series of experiments were performed in anesthetized and mechanically ventilated (FiO2 = 0.4) pigs (35-45 kg). In Series 1, fluid percussion TBI (6-8 ATM) was followed by a 30% hemorrhage. At 120 min post-TBI, initial resuscitation consisted of either shed blood (n = 7) or a bolus of 3x shed blood volume as saline (n = 13). Saline supplements were then administered to all pigs to maintain a systolic arterial blood pressure (SAP) of >100 mmHg and a heart rate (HR) of <110 beats/min. In Series 2, TBI (4-5 ATM) was followed by a 35% hemorrhage. At 60 min post-TBI, initial resuscitation consisted of either 500 mL of DCLHb (n = 6) or 500 mL of saline (n = 5). This was followed by saline supplements to all pigs to maintain a SAP of >100 mmHg and a HR of <110 beats/min. In Series 1, most systemic markers of resuscitation (e.g., SAP, HR, cardiac output, filling pressures, lactate, etc.) were normalized, but there were 0/7 vs. 5/13 deaths within 5 h (P = 0.058) with blood vs. saline. At constant arterial O2 saturation (SaO2), mixed venous O2 saturation (SvO2), cerebral perfusion pressure (CPP), and cerebral venous O2 saturation (ScvO2) were all higher, intracranial pressure (ICP) was lower, and CO2 reactivity was preserved with blood vs. saline (all P < 0.05). In Series 2, SAP, ICP, CPP, and lactate were higher with DCLHb vs. saline (all P< 0.05). Cardiac output was lower even though filling pressure was markedly elevated with DCLHb vs. saline (both P< 0.05). Neither SvO2 nor cerebrovascular CO2 reactivity were improved, and ScvO2 was lower with DCLHb vs. saline (P < 0.05). All survived at least 72 h with neuropathologic changes that included sub-arachnoid hemorrhage, midline cerebellar necrosis, and diffuse axonal injury. These changes were similar with DCLHb vs. saline. Thus, whole blood was more effective than saline for resuscitation of TBI, whereas DCLHb was no more, and according to many variables, less effective than saline resuscitation. These experimental results are comparable to those in a recent multicenter trial using DCLHb for the treatment of severe traumatic shock. Further investigations in similar experimental models might provide some plausible explanations why DCLHb unexpectedly increased mortality in patients.

Does an increase in cerebral perfusion pressure always mean a better oxygenated brain? A study in head-injured patients

The adequate management of cerebral perfusion pressure (CPP) continues to be a controversial issue in head-injured patients. The purpose of our study was to test two hypotheses. The first was that in patients with a CPP below 70 mm Hg, oxygen delivery is compromised and that therefore signs of tissue hypoxia would be reflected in low PtiO2 measurements. The second hypothesis was that manipulating mean arterial blood pressure to increase CPP improves oxygen delivery, particularly in patients with a CPP below 70 mm Hg. Twenty-five moderately or severely head-injured patients were included in the study. In all of them PtiO2 was monitored in the non-injured hemisphere using the Licox system (GMS, Kiel-Mielkendorf, Germany). Arterial hypertension was induced with phenylephrine 29 times. To quantify the effect of increasing mean arterial blood pressure (MABP) on oxygen delivery to the brain, the PtiO2-BP index was calculated (PtiO2-BP index = delta PtiO2/delta MABP). In 16 tests (55%) baseline CPP was above or equal to 70 mm Hg and in the remaining 13 (45%) it was below 70 mm Hg. Mean increase in MABP after phenylephrine was 23.7 +/- 10.2 mm Hg. Mean PtiO2 was 29.5 +/- 14.7 mm Hg in patients with a basal CPP of below 70 mm Hg and 28.9 +/- 10.6 mm Hg in patients in the high CPP group. These differences being not statistically significant. The PtiO2-BP index was 0.29 +/- 0.23 in patients with a basal CPP of below 70 mm Hg and in patients with a CPP of above 70 mm Hg this index was 0.16 +/- 0.11 Hg. These differences were not statistically significant (Student's t-test, P = 0.09). In our study a low PtiO2 was not observed in patients with marginally low CPPs (48-70 mm Hg) and readings below 15 mm Hg were observed in cases with both normal or supranormal CPPs. We conclude that episodes of low PtiO2 could not be predicted on the basis of CPP alone. On the other hand, raising CPP did not increase oxygen availability in the majority of cases, even if the CPP was markedly improved.

False autoregulation (pseudoautoregulation) in patients with severe head injury. Its importance in CPP management

False autoregulation has been described as an alteration of autoregulation in which the apparent maintenance of a constant cerebral blood flow (CBF) when increasing cerebral perfusion pressure (CPP) is due to an increase in brain tissue pressure. The objective of our study was to investigate how often false autoregulation occurred in patients with a severe head injury. In forty-six patients with a moderate or severe head injury autoregulation was studied using arteriojugular differences of oxygen (AVDO2) to estimate changes in CBF after inducing arterial hypertension with phenylephrine. Changes in mean arterial blood pressure (MABP), intracranial pressure (ICP), cerebral perfusion pressure (CPP) and AVDO2 were calculated before and after inducing hypertension. Ninety-five episodes of provoked hypertension were studied in 46 patients. In 28 tests (29.5%) a constant or even reduced CBF was detected simultaneously with a median increase in parenchymal ICP of 8.5 mm Hg (false autoregulation). In this group the median of the induced increase in MABP was 20.6 mm Hg with a median increase in CPP of 11.5 mm Hg. From our data we can conclude that false autoregulation is frequently found in patients after a severe head injury. Increasing MABP to obtain a better CPP in these patients is not beneficial because CBF is not modified or may even be reduced.

Monitoring neurologic patients in intensive care

The brain is sensitive to changes in substrate delivery. In neurologically critically ill patients (e.g., those with head injury, subarachnoid hemorrhage, or stroke), interruption of this supply causes ischemic brain damage and thus impairs the outcome. To prevent, detect, and treat these ischemic events as soon as possible, the cerebral blood flow is continuously monitored, its coupling or not with the consumption of oxygen and so forth, and the detected derangements of normal physiology. Intracranial pressure and cerebral perfusion pressure are two parameters that often reflect ischemic events, and thus it is mandatory to continuously measure them. To better assess cerebral hemodynamics, jugular bulb oxymetry and brain pressure tissue oxygen monitoring are two neuromonitoring techniques that allow for a better understanding of the balance between oxygen supply and consumption, and therefore are useful in directing therapy. Transcranial Doppler ultrasonography is a noninvasive technique with the same purpose but with less clinical relevance. The new neuromonitoring technique, microdialysis, is useful for understanding the mechanisms involved in brain ischemia. However, it is clear that the physician who interprets the measurements given by devices and the clinical data (e.g., temperature, glycemia) is still the cornerstone in the management of neurologically critically ill patients.

Limits of intermittent jugular bulb oxygen saturation monitoring in the management of severe head trauma patients

OBJECTIVE

To evaluate, in a prospective, observational study, whether bilateral monitoring of jugular bulb oxyhemoglobin saturation (SjO2), in addition to standard monitoring, results in modification of the management of severe head trauma.

METHODS

The patients underwent bilateral jugular bulb cannulation and observation at 8-hour intervals, during which SjO2 was measured and the neurological condition and physiological variables were assessed. The study group was responsible for evaluating whether the physician's decision-making process was influenced by the detection of SjO2 abnormalities. The SjO2 discrepancy in simultaneous bilateral samples was also evaluated to determine whether it interfered with the interpretation of data and with clinical decision-making. The SjO2-related complications were monitored.

RESULTS

Thirty patients underwent 319 observations. In 96% of patients, SjO2 was normal or high and had no influence on the diagnostic or therapeutic strategies. Treatment decisions were dictated by changes in clinical status and in intracranial and cerebral perfusion pressure. When these parameters were abnormal, treatment was administered, even if SjO2 was normal (101 observations). Conversely, when SjO2 was the only detected abnormality (34 observations), no treatment was administered. Abnormally low SjO2 values, caused by hypovolemia and hypocapnia, were detected in 3.4% of observations and actually modified the management. The discrepancies in simultaneous bilateral samples were substantial and gave rise to relevant interpretation problems. Fifteen percent of jugular catheters showed evidence of bacterial colonization.

CONCLUSION

Intermittent SjO2 monitoring did not substantially influence the management of severe head trauma. Therefore, recommendation for its routine use in all patients seems inadvisable, and indications for this invasive method should no longer be defined on the basis of experts' opinions, but rather on randomized, prospective studies.

Cerebral hemodynamic changes during sustained hypocapnia in severe head injury: can hyperventilation cause cerebral ischemia?

Hyperventilation (HV) is routinely used in the management of increased intracranial pressure (ICP) in severe head injury. However, this treatment continues to be controversial because it has been reported that long-lasting reduced cerebral blood flow (CBF) due to profound sustained hypocapnia may contribute to the development or deterioration of ischemic lesions. Our goal in this study was to analyze the effects of sustained hyperventilation on cerebral hemodynamics (CBF, ICP) and metabolism (arterio jugular differences of lactates = AVDL). CO2-reactivity and CBF was estimated using AVDO2 (arteriojugular differences of oxygen content). Global cerebral ischemia and increased anaerobic metabolism were considered according to AVDO2 and AVDL respectively. Thirty-three patients with severe and moderate head injury and increased ICP were included. Within 72 hours after accident, patients were hyperventilated for a period of 4 hours. During this time jugular oxygen saturation (SjO2), arterial oxygen saturation (SaO2), ICP, mean arterial blood pressure (MABP), AVDO2 and AVDL were recorded. In our study, most patients preserved CO2-reactivity (88.2%). In these cases HV was very effective in lowering ICP. Our findings showed that this reduction was due to a CBF decrease. According to basal AVDO2 twenty-five patients (75.7%) were considered as hyperemic and eight (24.2%) as not hyperemic. Global ischemia and increased anaerobic metabolism were detected in one case in the non-hyperemic group. According to AVDO2 and AVDL, no adverse effects were found during four hours of HV in hyperemic patients. Nevertheless, AVDO2 and AVDL are global measurements and might not detect regional ischemia surrounding focal lesions such as contusions and haematomas. We suggest that monitoring of AVDO2 or other haemometabolic variables should be mandatory when sustained HV is used in the management of head injury patients.

Use of vasopressors to raise cerebral perfusion pressure in head injured patients

Cerebral ischemia due to low cerebral perfusion pressure (CPP) is the most important secondary effect of severe head injury. There is consensus regarding the maintenance of this pressure at levels above 70 mm Hg. One way to elevate CPP is by increasing mean arterial pressure (MAP). In this study, the authors attain this target by using adrenergic vasopressors investigating the effectiveness of dopamine, noradrenaline and methoxamine in 16 severe head injured patients. The results were: a) the increase of MAP effectively increased CPP without changes in intracranial pressure (ICP) and cerebral extraction of oxygen (CEO2); b) noradrenaline at a dose of 0.5 mg to 5 mg/h was effective and safe and might be considered the drug of choice; c) dopamine was not as effective at a high dose of 10 to 42.5 micrograms/kg/min; d) methoxamine given as a bolus was an effective way to control sudden decreases in MAP. It made the patients more responsive to dopamine. No important undesirable reactions occurred during the study.

Effects on intracranial pressure of fentanyl in severe head injured patients

Despite opioids are routinely used for analgesia in head injured patients, the effects of such drugs on ICP and cerebral hemodynamics remain controversial. Cerebrovascular autoregulation (CAR) could be an important factor in the ICP increases reported after opioid administration. In order to describe the effects on intracranial pressure of fentanyl and correlated such effects with autoregulation status, we studied 30 consecutive severe head injury patients who received fentanyl (2 micrograms/kg) intravenously over one minute. Prior to study, CAR was assessed. Monitoring included MAP, HR, SaO2, ETCO2, SjO2 and ICP. Changes in cerebral blood flow (CBF) were estimated from relative changes in AVDO2. Patients mean GCS was 5.7 +/- 1.7 (mean +/- STD) and mean ICP on admission was 23.8 +/- 16.3 mmHg. Fentanyl caused significant increases in ICP and decreases in MAP and CPP, but CBF remained unchanged when estimated by AVDO2. In patients with preserved CAR (34.5%), opioid-induced ICP increase was greater (but not statistically significant) than in those with impaired CAR (65.5%). We conclude than fentanyl moderately increased ICP and decreased MAP and CPP. Our data suggests that in patients with preserved CAR, potent opioids could cause greater increases of ICP, probably due to activation of the vasodilatadory cascade.