Brain tissue oxygenation during hemorrhagic shock, resuscitation, and alterations in ventilation

Icariin mitigates anxiety-like behaviors induced by hemorrhagic shock and resuscitation via inhibiting of astrocytic activation

BACKGROUND

Abnormal activation of astrocytes in the amygdala contributes to anxiety after hemorrhagic shock and resuscitation (HSR). Nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB)-associated epigenetic reprogramming of astrocytic activation is crucial to anxiety. A bioactive monomer derived from Epimedium icariin (ICA) has been reported to modulate NF-κB signaling and astrocytic activation.

PURPOSE

The present study aimed to investigate the effects of ICA on post-HSR anxiety disorders and its potential mechanism of action.

METHODS

We first induced HSR in mice through a bleeding and re-transfusion model and selectively inhibited and activated astrocytes in the amygdala using chemogenetics. Then, ICA (40 mg/kg) was administered by oral gavage once daily for 21 days. Behavioral, electrophysiological, and pathological changes were assessed after HSR using the light-dark transition test, elevated plus maze, recording of local field potential (LFP), and immunofluorescence assays.

RESULTS

Exposure to HSR reduced the duration of the light chamber and attenuated open-arm entries. Moreover, HSR exposure increased the theta oscillation power in the amygdala and upregulated NF-κB p65, H3K27ac, and H3K4me3 expression. Contrarily, chemogenetic inhibition of astrocytes significantly reversed these changes. Chemogenetic inhibition in astrocytes was simulated by ICA, but chemogenetic activation of astrocytes blocked the neuroprotective effects of ICA.

CONCLUSION

ICA mitigated anxiety-like behaviors induced by HSR in mice via inhibiting astrocytic activation, which is possibly associated with NF-κB-induced epigenetic reprogramming.

Peaks and valleys: oscillatory cerebral blood flow at high altitude protects cerebral tissue oxygenation

Introduction.Oscillatory patterns in arterial pressure and blood flow (at ∼0.1 Hz) may protect tissue oxygenation during conditions of reduced cerebral perfusion and/or hypoxia. We hypothesized that inducing oscillations in arterial pressure and cerebral blood flow at 0.1 Hz would protect cerebral blood flow and cerebral tissue oxygen saturation during exposure to a combination of simulated hemorrhage and sustained hypobaric hypoxia.Methods.Eight healthy human subjects (4 male, 4 female; 30.1 ± 7.6 year) participated in two experiments at high altitude (White Mountain, California, USA; altitude, 3800 m) following rapid ascent and 5-7 d of acclimatization: (1) static lower body negative pressure (LBNP, control condition) was used to induce central hypovolemia by reducing chamber pressure to -60 mmHg for 10 min(0 Hz), and; (2) oscillatory LBNP where chamber pressure was reduced to -60 mmHg, then oscillated every 5 s between -30 mmHg and -90 mmHg for 10 min(0.1 Hz). Measurements included arterial pressure, internal carotid artery (ICA) blood flow, middle cerebral artery velocity (MCAv), and cerebral tissue oxygen saturation (ScO2).Results.Forced 0.1 Hz oscillations in mean arterial pressure and mean MCAv were accompanied by a protection of ScO2(0.1 Hz: -0.67% ± 1.0%; 0 Hz: -4.07% ± 2.0%;P = 0.01). However, the 0.1 Hz profile did not protect against reductions in ICA blood flow (0.1 Hz: -32.5% ± 4.5%; 0 Hz: -19.9% ± 8.9%;P = 0.24) or mean MCAv (0.1 Hz: -18.5% ± 3.4%; 0 Hz: -15.3% ± 5.4%;P = 0.16).Conclusions.Induced oscillatory arterial pressure and cerebral blood flow led to protection of ScO2during combined simulated hemorrhage and sustained hypoxia. This protection was not associated with the preservation of cerebral blood flow suggesting preservation of ScO2may be due to mechanisms occurring within the microvasculature.

Effects of hypothermia and hypothermia combined with hypocapnia on cerebral tissue oxygenation in piglets

BACKGROUND

Hypothermia and its combination with hypocapnia are frequently associated with anesthesia.

AIMS

The goal was to investigate the effects of hypothermia and hypothermia combined with hypocapnia (hypothermia-hypocapnia) on cerebral tissue oxygenation in anesthetized piglets.

METHODS

Twenty anesthetized piglets were randomly allocated to hypothermia (n = 10) or hypothermia-hypocapnia (n = 10). Cerebral monitoring comprised a tissue oxygen partial pressure (PtO₂ ), a laser Doppler probe, and a near-infrared spectroscopy sensor, measuring regional oxygen saturation (rSO₂ ). After baseline recordings, hypothermia (35.5-36.0°C) with or without hypocapnia (target PaCO₂ : 28-30 mm Hg) was induced. Once treatment goals were achieved (Tr0), they were maintained for 30 minutes (Tr30).

RESULTS

No changes in PtO₂ but a significant increase in rSO₂ (Tr0 (mean difference 8.9[95% CI for difference3.99 to 13.81], P < .001); Tr30 (10.8[6.20 to 15.40], P < .001)) were detected during hypothermia. With hypothermia-hypocapnia, a decrease in PtO₂ (Tr0 (-3.2[-6.01 to -0.39], P = .021; Tr30 (-3.3[-5.8 to -0.80], P = .006)) and no significant changes in rSO₂ occurred. Cerebral blood flow decreased significantly from baseline to Tr0 independently of treatment (-0.89[-0.18 to -0.002], P = .042), but this was more consistently observed with hypothermia-hypocapnia.

CONCLUSIONS

The hypothermia-induced reduction in oxygen delivery was compensated by lowered metabolic demand. However, hypothermia was not able to compensate for an additional reduction in oxygen delivery caused by simultaneous hypocapnia. This resulted in a PtO₂ drop, which was not reflected by a downshift in rSO₂ .

Brain Tissue Oxygen Response as Indicator for Cerebral Lactate Levels in Aneurysmal Subarachnoid Hemorrhage Patients

BACKGROUND

Early detection of cerebral ischemia and metabolic crisis is crucial in critically ill subarachnoid hemorrhage (SAH) patients. Variable increases in brain tissue oxygen tension (PbtO2) are observed when the fraction of inspired oxygen (FiO2) is increased to 1.0. The aim of this prospective study was to evaluate whether a 3-minute hyperoxic challenge can identify patients at risk for cerebral ischemia detected by cerebral microdialysis.

METHODS

Twenty consecutive severe SAH patients undergoing continuous cerebral PbtO2 and microdialysis monitoring were included. FiO2 was increased to 1.0 for 3 minutes (the FiO2 challenge) twice a day and PbtO2 responses during the FiO2 challenges were related to cerebral microdialysis-measures, ie, lactate, the lactate-pyruvate ratio, and glycerol. Multivariable linear and logistic regression models were created for each outcome parameter.

RESULTS

After predefined exclusions, 274 of 400 FiO2 challenges were included in the analysis. Lower absolute increases in PbtO2 ([INCREMENT]PbtO2) during FiO2 challenges were significantly associated with higher cerebral lactate concentration (P<0.001), and patients were at higher risk for ischemic lactate levels >4 mmol/L (odds ratio 0.947; P=0.04). Median (interquartile range) [INCREMENT]PbtO2 was 7.1 (4.6 to 12.17) mm Hg when cerebral lactate was >4 mmol/L and 10.2 (15.76 to 14.24) mm Hg at normal lactate values (≤4 mmol/L). Median [INCREMENT]PbtO2 was significantly lower during hypoxic than during hyperglycolytic lactate elevations (4.6 vs. 10.6 mm Hg, respectively; P<0.001). Lactate-pyruvate ratio and glycerol levels were mainly determined by baseline characteristics.

CONCLUSIONS

A 3-minute FiO2 challenge is an easy to perform and feasible bedside diagnostic tool in SAH patients. The absolute increase in PbtO2 during the FiO2 challenge might be a useful surrogate marker to estimate cerebral lactate concentrations and might be used to identify patients at risk for impending ischemia.

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

BACKGROUND

Hypocapnia is a common alteration during anesthesia in neonates.

AIM

To investigate the effects of hypocapnia and hypocapnia combined with hypotension (HCT) on cerebral perfusion and tissue oxygenation in anesthetized piglets.

METHOD

Thirty anesthetized piglets were randomly allocated to groups: moderate hypocapnia (mHC), severe hypocapnia (sHC), and HCT. Cerebral monitoring comprised a tissue oxygen partial pressure and a laser Doppler probe inserted into the brain tissue as well as a near-infrared spectroscopy (NIRS) sensor placed on the skin, measuring regional oxygen saturation. Hypocapnia was induced by hyperventilation (target PaCO₂ mHC: 3.7-4; sHC: 3.1-3.3 kPa) and hypotension by blood withdrawal and nitroprusside infusion (mean blood pressure: 35-38 mm Hg). Data were analyzed at baseline, during (Tr20, Tr40, Tr60) and after (Post20, Post40, Post60) treatment.

RESULTS

Compared to baseline, tissue oxygen partial pressure decreased significantly and equally during all treatments (mean [SD] at baseline: mHC 35.7 [32.45]; sHC: 28.1 [20.24]; HCT 25.4 [10.3] and at Tr60: mHC: 29.9 [27.36]; sHC: 22.2 [18.37]; HCT: 18.4 [9.5] mm Hg). Decreased laser Doppler flow was detected with all treatments at Tr20 (mHC: 0.9 [0.18]; sHC: 0.88 [0.15]; HCT: 0.97 [0.13] proportion from baseline). Independently of group, regional oxygen saturation varied only after reverting and not during treatment. Blood lactate, pH, HCO₃^- , and PaO₂ increased during treatment with no differences between groups.

CONCLUSION

This animal model revealed reduced cerebral blood flow and brain tissue oxygenation during hypocapnia without detectable changes in regional oxygen saturation as measured by NIRS. Changes occurred as early as during moderate hypocapnia.

A Swine Model of Severe Propranolol Toxicity Permitting Direct Measurement of Brain Tissue Oxygenation

INTRODUCTION

High-dose insulin (HDI) therapy has been used successfully for beta-blocker toxicity, but needs further study when hypotension persists despite HDI. The objective was to develop a model of propranolol toxicity with persistent hypotension despite HDI and to develop means to measure cerebral oxygen tension (PbrO₂).

METHODS

Eight anesthetized Yorkshire pigs were instrumented with a tracheostomy, Swan-Ganz catheter, arterial catheter, and intra-cerebral pressure and oxygen monitor. Intravenous propranolol was given until the initial point of toxicity (POT); 25% reduction from baseline mean arterial pressure (MAP) × heart rate (HR). At the initial POT, normal saline (NS) bolus and infusion along with HDI infusion were started. The propranolol infusion was titrated up slowly to induce hypotension. Group 2 pigs received a norepinephrine (NE) infusion after a secondary POT defined as a MAP < 50 mmHg. NE was titrated to maintain subsequent MAPs > 50 mmHg. Cardiac output, HR, MAP, PbrO₂, and intracranial pressure were then recorded every 5 min until death or 4 h. Systemic vascular resistance, potassium, and glucose were also measured. Surviving pigs were euthanized.

RESULTS

Two pigs received unique doses for protocol development. One pig developed a tachyarrhythmia prior to protocol, one failed to reach secondary POT, leaving 2 pigs in each group reaching secondary POT. The range of PbrO₂ recordings for group 1 was 12.7-48.5 mmHg and 9.2-26.2 mmHg for group 2.

CONCLUSION

We report a pilot study swine model of propranolol toxicity with hypotension despite HDI, in which physiologic measures including PbrO₂ are achieved. Our toxicity model can be used in the future, and the hemodynamic and brain monitoring model may prove important for subsequent research in various contexts.

Hemorrhagic Shock and the Microvasculature

The microvasculature plays a central role in the pathophysiology of hemorrhagic shock and is also involved in arguably all therapeutic attempts to reverse or minimize the adverse consequences of shock. Microvascular studies specific to hemorrhagic shock were reviewed and broadly grouped depending on whether data were obtained on animal or human subjects. Dedicated sections were assigned to microcirculatory changes in specific organs, and major categories of pathophysiological alterations and mechanisms such as oxygen distribution, ischemia, inflammation, glycocalyx changes, vasomotion, endothelial dysfunction, and coagulopathy as well as biomarkers and some therapeutic strategies. Innovative experimental methods were also reviewed for quantitative microcirculatory assessment as it pertains to changes during hemorrhagic shock. The text and figures include representative quantitative microvascular data obtained in various organs and tissues such as skin, muscle, lung, liver, brain, heart, kidney, pancreas, intestines, and mesentery from various species including mice, rats, hamsters, sheep, swine, bats, and humans. Based on reviewed findings, a new integrative conceptual model is presented that includes about 100 systemic and local factors linked to microvessels in hemorrhagic shock. The combination of systemic measures with the understanding of these processes at the microvascular level is fundamental to further develop targeted and personalized interventions that will reduce tissue injury, organ dysfunction, and ultimately mortality due to hemorrhagic shock. Published 2018. Compr Physiol 8:61-101, 2018.

Physiological complexity of acute traumatic brain injury in patients treated with a brain oxygen protocol: utility of symbolic regression in predictive modeling of a dynamical system

Predictive modeling of emergent behavior, inherent to complex physiological systems, requires the analysis of large complex clinical data streams currently being generated in the intensive care unit. Brain tissue oxygen protocols have yielded outcome benefits in traumatic brain injury (TBI), but the critical physiological thresholds for low brain oxygen have not been established for a dynamical patho-physiological system. High frequency, multi-modal clinical data sets from 29 patients with severe TBI who underwent multi-modality neuro-clinical care monitoring and treatment with a brain oxygen protocol were analyzed. The inter-relationship between acute physiological parameters was determined using symbolic regression (SR) as the computational framework. The mean patient age was 44.4±15 with a mean admission GCS of 6.6±3.9. Sixty-three percent sustained motor vehicle accidents and the most common pathology was intra-cerebral hemorrhage (50%). Hospital discharge mortality was 21%, poor outcome occurred in 24% of patients, and good outcome occurred in 56% of patients. Criticality for low brain oxygen was intracranial pressure (ICP) ≥22.8 mm Hg, for mortality at ICP≥37.1 mm Hg. The upper therapeutic threshold for cerebral perfusion pressure (CPP) was 75 mm Hg. Eubaric hyperoxia significantly impacted partial pressure of oxygen in brain tissue (PbtO2) at all ICP levels. Optimal brain temperature (Tbr) was 34-35°C, with an adverse effect when Tbr≥38°C. Survivors clustered at [Formula: see text] Hg vs. non-survivors [Formula: see text] 18 mm Hg. There were two mortality clusters for ICP: High ICP/low PbtO2 and low ICP/low PbtO2. Survivors maintained PbtO2 at all ranges of mean arterial pressure in contrast to non-survivors. The final SR equation for cerebral oxygenation is: [Formula: see text]. The SR-model of acute TBI advances new physiological thresholds or boundary conditions for acute TBI management: PbtO2≥25 mmHg; ICP≤22 mmHg; CPP≈60-75 mmHg; and Tbr≈34-37°C. SR is congruous with the emerging field of complexity science in the modeling of dynamical physiological systems, especially during pathophysiological states. The SR model of TBI is generalizable to known physical laws. This increase in entropy reduces uncertainty and improves predictive capacity. SR is an appropriate computational framework to enable future smart monitoring devices.

Association of out-of-hospital advanced airway management with outcomes after traumatic brain injury and hemorrhagic shock in the ROC hypertonic saline trial

OBJECTIVE

Prior studies suggest adverse associations between out-of-hospital advanced airway management (AAM) and patient outcomes after major trauma. This secondary analysis of data from the Resuscitation Outcomes Consortium Hypertonic Saline Trial evaluated associations between out-of-hospital AAM and outcomes in patients suffering isolated severe traumatic brain injury (TBI) or haemorrhagic shock.

METHODS

This multicentre study included adults with severe TBI (GCS ≤8) or haemorrhagic shock (SBP ≤70 mm Hg, or (SBP 71-90 mm Hg and heart rate ≥108 bpm)). We compared patients receiving out-of-hospital AAM with those receiving emergency department AAM. We evaluated the associations between airway strategy and patient outcomes (28-day mortality, and 6-month poor neurologic or functional outcome) and airway strategy, adjusting for confounders. Analysis was stratified by (1) patients with isolated severe TBI and (2) patients with haemorrhagic shock with or without severe TBI.

RESULTS

Of 2135 patients, we studied 1116 TBI and 528 shock; excluding 491 who died in the field, did not receive AAM or had missing data. In the shock cohort, out-of-hospital AAM was associated with increased 28-day mortality (adjusted OR 5.14; 95% CI 2.42 to 10.90). In TBI, out-of-hospital AAM showed a tendency towards increased 28-day mortality (adjusted OR 1.57; 95% CI 0.93 to 2.64) and 6-month poor functional outcome (1.63; 1.00 to 2.68), but these differences were not statistically significant. Out-of-hospital AAM was associated with poorer 6-month TBI neurologic outcome (1.80; 1.09 to 2.96).

CONCLUSIONS

Out-of-hospital AAM was associated with increased mortality after haemorrhagic shock. The adverse association between out-of-hospital AAM and injury outcome is most pronounced in patients with haemorrhagic shock.

Brain tissue oxygenation and cerebral perfusion pressure thresholds of ischemia in a standardized pig brain death model

BACKGROUND

Neurointensive care of traumatic brain injury (TBI) patients is currently based on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) targeted protocols. Monitoring brain tissue oxygenation (BtipO2) is of considerable clinical interest, but the exact threshold level of ischemia has been difficult to establish due to the complexity of the clinical situation. The objective of this study was to use the Neurovent-PTO (NV) probe, and to define critical cerebral oxygenation- and CPP threshold levels of cerebral ischemia in a standardized brain death model caused by increasing the ICP in pig. Ischemia was defined by a severe increase of cerebral microdialysis (MD) lactate/pyruvate ratio (L/P ratio > 30).

METHODS

BtipO2, L/P ratio, Glucose, Glutamate, Glycerol and CPP were recorded using NV and MD probes during gradual increase of ICP by inflation of an epidural balloon catheter with saline until brain death was achieved.

RESULTS

Baseline level of BtipO2 was 22.9 ± 6.2 mmHg, the L/P ratio 17.7 ± 6.1 and CPP 73 ± 17 mmHg. BtipO2 and CPP decreased when intracranial volume was added. The L/P ratio increased above its ischemic levels, (>30)when CPP decreased below 30 mmHg and BtipO2 to <10 mmHg.

CONCLUSIONS

A severe increase of ICP leading to CPP below 30 mmHg and BtipO2 below 10 mmHg is associated with an increase of the L/P ratio, thus seems to be critical thresholds for cerebral ischemia under these conditions.

Sympathetic innervation does not contribute to glycerol release in ischemic flaps

BACKGROUND

Extracellular glycerol as detected by microdialysis has been used as a surrogate marker for (ischemic) tissue damage and cellular membrane breakdown in the monitoring of free microvascular musculocutaneous flaps. One confounding factor for glycerol as a marker of ischemic cell damage is the effect of lipolysis and associated glycerol release as induced by sympathetic signalling alone. We hypothesized that extracellular glycerol concentrations in a microvascular flap with sympathetic innervation would be confounded by intact innervation per se as compared to denervated flap. Clinical relevance is related to the use of both free and pedicled flaps in reconstructive surgery. We tested the hypothesis in an experimental model of microvascular musculocutaneal flaps.

METHODS

Twelve pigs were anesthetized and mechanically ventilated. Two identical rectus abdominis musculocutaneal flaps were raised for the investigation. In the A-flaps the adventitia of the artery and accompanying innervation was carefully stripped, while in the B-flaps it was left untouched. Flap ischemia was induced by clamping both vessels for 60 minutes. The ischemia was confirmed by measuring tissue oxygen pressure, while extracellular lactate to pyruvate ratio indicated the accompanying anaerobic metabolism locally.

RESULTS

Intramuscular and subcutaneal extracellular glycerol concentrations were measured by microdialysate analyzer. Contrary to our hypothesis, glycerol concentrations were comparable between the two ischemia groups at 60 minutes (p = 0.089, T-test).

CONCLUSIONS

In this experimental model of vascular flap ischemia, intact innervation of the flap did not confound ischemia detection by glycerol. Extrapolation of the results to clinical setting warrants further studies.

Comparison between cerebral tissue oxygen tension and energy metabolism in experimental subdural hematoma

BACKGROUND

An experimental swine model (n = 7) simulating an acute subdural hematoma (ASDH) was employed (1) to explore the relation between the brain tissue oxygenation (PbtO(2)) and the regional cerebral energy metabolism as obtained by microdialysis, and (2) to define the lowest level of PbtO(2) compatible with intact energy metabolism.

METHODS

ASDH was produced by infusion of 7 ml of autologous blood (infusion rate 0.5 ml/min) by a catheter placed subdurally. PbtO(2) and microdialysis probes were placed symmetrically in the injured ("bad-side") and non-injured ("good-side") hemispheres. Intracranial pressure (ICP) was monitored in the "good-side."

RESULTS

ICP, cerebral perfusion pressure (CPP), PbtO(2), glucose, lactate, pyruvate, lactate-pyruvate ratio (LP ratio), glutamate, and glycerol were recorded at baseline (60 min) and post trauma (360 min). After the creation of the ASDH, PbtO(2) decreased significantly in both the hemispheres (P < 0.001). No significant difference was found between the sides post trauma. The LP ratio, glutamate, and glycerol in the "bad-side" increased significantly over the "good-side" where the values remained within the normal limits. A PbtO(2) value below approximately 25 mmHg was found to be associated with disturbed energy metabolism in the "bad-side" but not in the "good-side." No correlation was found between the LP ratio and PbtO(2) in either hemisphere.

CONCLUSIONS

PbtO(2) monitoring accurately describes tissue oxygenation but does not disclose whether the oxygen delivery is sufficient for maintaining cerebral energy metabolism. Accordingly, it may not be possible to define a threshold level for PbtO(2) below which energy failure and permanent tissue damage occurs.

Reviewing the reliability, effectiveness and applications of Licox in traumatic brain injury

AIMS AND OBJECTIVES

To review the pathophysiology, accuracy, effectiveness and use of Licox for brain tissue oxygen monitoring in traumatic brain injury (TBI).

BACKGROUND

The Licox monitoring system allows continuous monitoring of partial pressure of brain tissue oxygen (PbO(2)), brain tissue temperature and intracranial pressure (ICP). The application and effectiveness of the use of Licox in TBI is not clearly explored within the literature.

INCLUSION CRITERIA

A date limit of 1995-2009, English language, all animal and human studies and the following terms were searched: Licox, brain tissue oxygenation, cerebral oxygenation and TBI. MEDLINE database was the primary data source.

EXCLUSION CRITERIA

All paediatric papers were excluded from the search. Studies not related to pathophysiology and management of TBI and brain tissue oximetry in adults were excluded. Data relevant to the subject under consideration were extracted by three independent clinicians to form a narrative report. Studies were critically evaluated using the NHS Public Health Resource Unit's checklist for each study analysed.

CONCLUSIONS

Licox offers new insights into cerebral pathology and physiology. The continuous bedside monitoring provides real-time data that can be used to improve patient management and prognosis in specialist units by trained and experienced staff. More research is required to understand the limitations of this technology and why it is not in widespread use. RELEVENCE TO CLINICAL PRACTICE: A clinical tool that could be utilized more often in the right setting to improve care to patients suffering from TBI by disseminating more information on this unique tool.

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

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

Inappropriate prehospital ventilation in severe traumatic brain injury increases in-hospital mortality

In the setting of acute brainstem herniation in traumatic brain injury (TBI), the use of hyperventilation to reduce intracranial pressure may be life-saving. However, undue use of hyperventilation is thought to increase the incidence of secondary brain injury through direct reduction of cerebral blood flow. This is a retrospective review determining the effect of prehospital hyperventilation on in-hospital mortality following severe TBI. All trauma patients admitted directly to a single level 1 trauma center from January 2000 to January 2007 with an initial Glasgow Coma Scale (GCS) score <or=8 were included in the study (n = 77). Patients without documented or with late (>20 min) arterial blood gas at presentation (n = 12) were excluded from the study. The remaining population (n = 65) was sorted into three groups based on the initial partial pressure of carbon dioxide: hypocarbic (Pco(2) < 35 mm Hg), normocarbic (Pco(2) 35-45 mm Hg), and hypercarbic (Pco(2) > 45 mm Hg). Outcome was based on mortality during hospital admission. Survival was found to be related to admission Pco(2) in head trauma patients requiring intubation (p = 0.045). Patients with normocarbia on presenting arterial blood gas testing had in-hospital mortality of 15%, significantly improved over patients presenting with hypocarbia (in-hospital mortality 77%) or hypercarbia (in-hospital mortality 61%). Although there are many reports of the negative impact of prophylactic hyperventilation following severe TBI, this modality is frequently utilized in the prehospital setting. Our results suggest that abnormal Pco(2) on presentation after severe head trauma is correlated with increased in-hospital mortality. We advocate normoventilation in the prehospital setting.

Correlation of brain tissue oxygen tension with cerebral near-infrared spectroscopy and mixed venous oxygen saturation during extracorporeal membrane oxygenation

The aim of this prospective, animal study was to compare brain tissue oxygen tension (PbtO(2)) with cerebral near infrared spectroscopy (NIRS) and mixed venous oxygen saturation (SVO(2)) during venoarterial extracorporeal membrane oxygenation (VA ECMO) in a porcine model. This was accomplished using twelve immature piglets with surgically implanted catheters placed in the superficial cerebral cortex to measure brain PbtO(2) and microdialysis metabolites. The NIRS sensor was placed overlying the forehead to measure cerebral regional saturation index (rSO(2)i) while SVO(2) was measured directly from the ECMO circuit. Animals were placed on VA ECMO followed by an initial period of stabilization, after which they were subjected to graded hypoxia and recovery. Our results revealed that rSO(2)i and SVO(2) correlated only marginally with PbtO(2) (R(2)=0.32 and R(2)=0.26, respectively) while the correlation between rSO(2)i and SVO( 2) was significantly stronger (R(2)=0.59). Cerebral metabolites and rSO(2)i were significantly altered during attenuation of PbtO( 2), p<0.05). A subset of animals, following exposure to hypoxia, experienced markedly delayed recovery of both rSO(2)i and PbtO( 2) despite rapid normalization of SVO(2). Upon further analysis, these animals had significantly lower blood pressure (p=0.001), lower serum pH (p=0.01), and higher serum lactate (p=0.02). Additionally, in this subgroup, rSO(2)i correlated better with PbtO(2) (R(2)=0.76). These findings suggest that, in our ECMO model, rSO(2)i and SVO( 2) correlate reasonably well with each other, but not necessarily with brain PbtO(2) and that NIRS-derived rSO(2)i may more accurately reflect cerebral tissue hypoxia in sicker animals.

Sitting can cause ischaemia in the subcutaneous tissue of the buttocks, which implicates multilayer tissue damage in the development of pressure ulcers

A better understanding of how pressure ulcers develop in the buttocks will improve prophylactic measures. Our aim was to investigate signs of reduced perfusion and ischaemia in the subcutaneous fat in the buttocks during sitting. A microelectrode was used to quantify oxygen (pO(2)). Metabolites that indicate aerobic or anaerobic metabolism (glucose, lactate, pyruvate, and glycerol) were quantified using microdialysis. Sixteen healthy people were studied while they sat on a wheel chair cushion, and a hard surface. Sitting pressures were mapped, and the thickness of the subcutaneous fatty layer was measured. The results showed that pO(2) and glucose were significantly reduced during sitting, and for pO(2) the effect is significantly more profound during sitting on a hard surface. After loading, both glucose and pO(2) increased significantly. We conclude that the subcutaneous adipose tissue covering the ischial tuberosities becomes ischaemic during sitting. This finding supports the theory that not only is the skin involved in early development of pressure ulcers, but also the deeper tissues.

Brain tissue oxygen pressure monitoring in awake patients during functional neurosurgery: the assessment of normal values

Local brain tissue oxygen (ptiO2) monitoring is frequently applied in patients at risk for cerebral ischemia. To identify ischemic thresholds, the normal range of local brain tissue oxygen pressure (ptiO2) values needs to be established. Ideally, such normal values are determined in healthy and awake subjects, so as to eliminate the possible influences of anesthetics on cerebral physiology or ptiO2. Thus far, however, such measurements have not been conducted, and to fill this void, we determined the ptiO2 values in normal white matter of awake patients undergoing functional stereotactic brain surgery. In 25 otherwise healthy patients, who underwent functional neurosurgery for treatment of a refractory movement disorder under local anesthesia, the ptiO2 of white matter was recorded continuously using a polarographic Clark type electrode monitoring system. Preoperative screening ruled out cognitive dysfunction or structural cerebral lesions. Reliable intraoperative ptiO2 values were obtained in 22 patients. After an adaptation period of 118+/-35 min (range, 47-171 min), we found an average normal ptiO2 of 22.6+/-7.2 mm Hg in the frontal white matter. In 11 patients, ptiO2 measurements were continued postoperatively for 24 h. During this period, a similar normal ptiO2 value of 23.1+/-6.6 mm Hg was found. No iatrogenic complications occurred. In conclusion, the normal ptiO2 of cerebral white matter is most likely lower than previously assumed. Further, the long adaptation time renders this widely applied monitoring instrument unreliable in detecting ischemia early after insertion and limits its usefulness for intraoperative monitoring.

Management of hypertensive emergencies in acute brain disease: evaluation of the treatment effects of intravenous nicardipine on cerebral oxygenation

Object

Inappropriate sudden blood pressure (BP) reductions may adversely affect cerebral perfusion. This study explores the effect of nicardipine on regional brain tissue O2 (PbtO2) during treatment of acute hypertensive emergencies.

Methods

A prospective case–control study was performed in 30 patients with neurological conditions and clinically elevated BP. All patients had a parenchymal PbtO2 and intracranial pressure bolt inserted following resuscitation. Using a critical care guide, PbtO2 was optimized. Intravenous nicardipine (5–15 mg/hour) was titrated to systolic BP < 160 mm Hg, diastolic BP < 90 mm Hg, mean arterial BP (MABP) 90–110 mm Hg, and PbtO2 > 20 mm Hg. Physiological parameters—intracranial pressure, PbtO2, central venous pressure, systolic BP, diastolic BP, MABP, fraction of inspired O2, and cerebral perfusion pressure (CPP)—were compared before infusion, at 4 hours, and at 8 hours using a t-test.

Results

Sixty episodes of hypertension were reported in 30 patients (traumatic brain injury in 13 patients; aneurysmal subarachnoid hemorrhage in 11; intracerebral and intraventricular hemorrhage in 3 and 1, respectively; arteriovenous malformation in 1; and hypoxic brain injury in 1). Nicardipine was effective in 87% of the patients (with intravenous β blockers in 4 patients), with a 19.7% reduction in mean 4-hour MABP (115.3 ± 13.1 mm Hg preinfusion vs 92.9 ± 11.40 mm Hg after 4 hours of therapy, p < 0.001). No deleterious effect on mean PbtO2 was recorded (26.74 ± 15.42 mm Hg preinfusion vs 27.68 ± 12.51 mm Hg after 4 hours of therapy, p = 0.883) despite significant reduction in CPP. Less dependence on normobaric hyperoxia was achieved at 8 hours (0.72 ± 0.289 mm Hg preinfusion vs 0.626 ± 0.286 mm Hg after 8 hours of therapy, p < 0.01). Subgroup analysis revealed that 12 patients had low pretreatment PbtO2 (10.30 ± 6.49 mm Hg), with higher CPP (p < 0.001) requiring hyperoxia (p = 0.02). In this group, intravenous nicardipine resulted in an 83% improvement in 4- and 8-hour PbtO2 levels (18.1 ± 11.33 and 19.59 ± 23.68 mm Hg, respectively; p < 0.01) despite significant reductions in both mean MABP (120.6 ± 16.65 vs 95.8 ± 8.3 mm Hg, p < 0.001) and CPP (105.00 ± 20.7 vs 81.2 ± 15.4 mm Hg, p < 0.001).

Conclusions

Intravenous nicardipine is effective for the treatment of hypertensive neurological emergencies and has no adverse effect on PbtO2.

Brain tissue oxygen tension is more indicative of oxygen diffusion than oxygen delivery and metabolism in patients with traumatic brain injury

OBJECTIVES

Despite the growing clinical use of brain tissue oxygen monitoring, the specific determinants of low brain tissue oxygen tension (P(bt)O2) following severe traumatic brain injury (TBI) remain poorly defined. The objective of this study was to evaluate whether P(bt)O2 more closely reflects variables related to cerebral oxygen diffusion or reflects cerebral oxygen delivery and metabolism.

DESIGN

Prospective observational study.

SETTING

Level I trauma center.

PATIENTS

Fourteen TBI patients with advanced neuromonitoring underwent an oxygen challenge (increase in FiO2 to 1.0) to assess tissue oxygen reactivity, pressure challenge (increase in mean arterial pressure) to assess autoregulation, and CO2 challenge (hyperventilation) to assess cerebral vasoreactivity.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

P(bt)O2 was measured directly with a parenchymal probe in the least-injured hemisphere. Local cerebral blood flow (CBF) was measured with a parenchymal thermal diffusion probe. Cerebral venous blood gases were drawn from a jugular bulb venous catheter. We performed 119 measurements of PaO2, arterial oxygen content (CaO2), jugular bulb venous oxygen tension (PVO2), venous oxygen content (CVO2), arteriovenous oxygen content difference (AVDO2), and local cerebral metabolic rate of oxygen (locCMRO2). In multivariable analysis adjusting for various variables of cerebral oxygen delivery and metabolism, the only statistically significant relationship was that between P(bt)O2 and the product of CBF and cerebral arteriovenous oxygen tension difference (AVTO2), suggesting a strong association between brain tissue oxygen tension and diffusion of dissolved plasma oxygen across the blood-brain barrier.

CONCLUSIONS

Measurements of P(bt)O2 represent the product of CBF and the cerebral AVTO2 rather than a direct measurement of total oxygen delivery or cerebral oxygen metabolism. This improved understanding of the cerebral physiology of P(bt)O2 should enhance the clinical utility of brain tissue oxygen monitoring in patients with TBI.

In vitro comparison of two generations of Licox and Neurotrend catheters

BACKGROUND

Clinical reports on brain tissue oxygen tension differ in suggested threshold values for defining cerebral ischemia using the Licox and Neurotrend/Paratrend system. We evaluated in vitro performance of both first and second generation devices.

MATERIALS AND METHODS

Response rate and accuracy in solutions with oxygen tensions from 0 to 150 mm Hg were measured.

FINDINGS

Ninety-five percent Response times were 102 +/- 13 seconds for first generation Licox probes and 135 +/- 24 s for Paratrend (n = 6, each probe), with second generation probes at 134 +/- 4 and 116 +/- 16 s respectively. At pO2 150 mmHg Licox and Paratrend probes were accurate with 2.2% and 2.1% error, respectively and 2.6% and 4.1% for later generation. At pO2 18 mmHg, Paratrend overestimated by 16.5% (absolute error range 2.18 to 4.18 mmHg), 7.4% for Neurotrend, Licox underestimated by 1.8% (absolute error range 0.08 to 0.52 mmHg) with 3.6% for the second generation probe.

CONCLUSIONS

Differences between the first generation probe types, while statistically significant (p < 0.001), may not be clinically relevant. Overestimation of pO2 by Neurotrend and small underestimation by Licox partially explain differences in published thresholds for cerebral ischemia. The Neurotrend was slightly more accurate and faster than the Paratrend system.

Deep sedation with dexmedetomidine in a porcine model does not compromise the viability of free microvascular flap as depicted by microdialysis and tissue oxygen tension

BACKGROUND

Deep sedation is often necessary after major reconstructive plastic surgery in the face and neck regions to prevent sudden spontaneous movements capable of inflicting mechanical injury to the transplanted musculocutaneous flap(s). An adequate positioning may help to optimize oxygenation and perfusion of the transplanted tissues. We hypothesized that dexmedetomidine, a central alpha2-agonist and otherwise potentially ideal postoperative sedative drug, may induce vasoconstriction in denervated flaps, and thus increase the risk of tissue deterioration.

METHODS

Two symmetrical myocutaneous flaps were raised on each side of the upper abdomen in 12 anesthetized pigs. The sympathetic nerve fibers were stripped from the arteries in one of the flaps (denervated flap), while nerve fibers were kept untouched in the other (innervated flap). After simulation of ischemia and reperfusion periods, the animals were randomized to deep postoperative sedation with either propofol (n = 6) or dexmedetomidine (n = 6). Flap tissue metabolism was monitored by microdialysis and tissue-oxygen partial pressure. Glucose, lactate, and pyruvate concentrations were analyzed from the dialysate every 30 min for 4 h.

RESULTS

Mean arterial blood pressure was higher in the dexmedetomidine group (P = 0.036). Flap tissue metabolism remained stable throughout the experiment as measured by lactate-pyruvate and lactate-glucose ratios (median ranges 14.3-24.5 for lactate-pyruvate and 0.3-0.6 for lactate-glucose) and by tissue-oxygen partial pressure, and no differences were found between groups.

CONCLUSIONS

Our data suggest that dexmedetomidine, even if used for deep sedation, does not have deleterious effects on local perfusion or tissue metabolism in denervated musculocutaneous flaps.

Cerebral oxygen delivery by liposome-encapsulated hemoglobin: a positron-emission tomographic evaluation in a rat model of hemorrhagic shock

Liposome-encapsulated Hb (LEH) is being developed as an artificially assembled, low-toxicity, and spatially isolated Hb-based oxygen carrier (HBOC). Standard methods of evaluating oxygen carriers are based on surrogate indicators of physiology in animal models of shock. Assessment of actual delivery of oxygen by HBOCs and resultant improvement in oxygen metabolism at the tissue level has been a technical challenge. In this work, we report our findings from 15O-positron emission tomographic (15O-PET) evaluation of LEH in a rat model of 40% hypovolemic shock. In vitro studies showed that PEGylated LEH formulation containing approximately 7.5% Hb and consisting of neutral lipids (distearoylphosphatidylcholine:cholesterol:alpha-tocopherol, 51.4:46.4:2.2) efficiently picks up 15O-labeled oxygen gas. The final preparation of LEH contained 5% human serum albumin to provide oncotic pressure. Cerebral PET images of anesthetized rats inhaling 15O-labeled O2 gas showed efficient oxygen-carrying and delivery capacity of LEH formulation. From the PET images, we determined cerebral metabolic rate of oxygen (CMR(O2)) as a direct indicator of oxygen-carrying capacity of LEH as well as oxygen delivery and metabolism in rat brain. Compared with control fluids [saline and 5% human serum albumin (HSA)], LEH significantly improved CMR(O2) to approximately 80% of baseline level. Saline and HSA resuscitation could not improve hypovolemia-induced decrease in CMR(O2). On the other hand, resuscitation of shed blood was the most efficient in restoring oxygen metabolism. The results suggest that 15O-PET technology can be successfully employed to evaluate potential oxygen carriers and blood substitutes and that LEH resuscitation in hemorrhage enhances oxygen delivery to the cerebral tissue and improves oxygen metabolism in brain.

The impact of prehospital ventilation on outcome after severe traumatic brain injury

BACKGROUND

Prehospital intubation has been challenged on the grounds that it predisposes to hyperventilation, which is detrimental after traumatic brain injury (TBI), and impairs venous return in patients with hypovolemia. We sought to determine the incidence of hyperventilation among a cohort of trauma patients undergoing prehospital intubation and the impact of ventilation on outcome after severe TBI.

METHODS

Data were prospectively collected for all intubated trauma patients transported directly from the field for a period of 14 months (n = 574). An arrival Pco2 <30 mm Hg was termed severe hypocapnea and considered a marker of hyperventilation. Patients with a Pco2 >45 mm Hg were considered severely hypercapneic. Targeted ventilation was defined as a Pco2 between 30 and 35 mm Hg based on the Brain Trauma Foundation guidelines.

RESULTS

The rate of severe hypocapnea was 18% and women were more likely to be hyperventilated (p < 0.05). Patients with severe hypercapnia had higher Injury Severity Scores and were more likely hypotensive, hypoxic, and acidodic (p < 0.05). Patients in the targeted ventilation range were less likely to die than were those outside the range even after excluding the severe hypercapnea group (odds ratio, 0.57; 95% confidence interval, 0.33-0.99). This effect was even greater among patients with isolated TBI (odds ratio, 0.31; 95% confidence interval, 0.10-0.96).

CONCLUSION

Targeted prehospital ventilation is associated with lower mortality after severe TBI.

Severe hemodilutional anemia increases cerebral tissue injury following acute neurotrauma

Anemia may worsen neurological outcomes following traumatic brain injury (TBI) by undefined mechanisms. We hypothesized that hemodilutional anemia accentuates hypoxic cerebral injury following TBI. Anesthetized rats underwent unilateral TBI or sham injury (n > or = 7). Target hemoglobin concentrations between 50 and 70 g/l were achieved by exchanging 40-50% of the blood volume (1:1) with pentastarch. The effect of TBI, anemia, and TBI-anemia was assessed by measuring brain tissue oxygen tension (Pbr(O(2))), regional cerebral blood flow (rCBF), jugular venous oxygen saturation (Sjv(O(2))), cerebral contusion area, and nuclear staining for programmed cell death. Baseline postinjury Pbr(O(2)) values in the TBI and TBI-anemia groups (9.3 +/- 1.3 and 11.3 +/- 4.1 Torr, respectively) were lower than the uninjured controls (18.2 +/- 5.2 Torr, P < 0.05 for both). Hemodilution caused a further reduction in Pbr(O(2)) in the TBI-anemia group relative to the TBI group without anemia (7.8 +/- 2.7 vs. 14.8 +/- 3.9 Torr, P < 0.05). The rCBF remained stable after TBI and increased comparably after hemodilution in both anemia and TBI-anemia groups. The Sjv(O(2)) was elevated after TBI (87.4 +/- 8.9%, P < 0.05) and increased further following hemodilution (95.0 +/- 1.6%, P < 0.05). Cerebral contusion area and nuclear counts for programmed cell death were increased following TBI-anemia (4.1 +/- 3.0 mm(2) and 686 +/- 192, respectively) relative to TBI alone (1.3 +/- 0.3 mm(2) and 404 +/- 133, respectively, P < 0.05 for both). Hemodilutional anemia reduced cerebral Pbr(O(2)) and oxygen extraction and increased cell death following TBI. These results support our hypothesis that acute anemia accentuated hypoxic cerebral injury after neurotrauma.

The life-saving properties of blood: mitigating cerebral insult after traumatic brain injury

Transfusion of packed red blood cells in critically injured patients has been a lifesaving (although not completely benign) intervention for decades. The traumatically injured brain has been thought to be particularly susceptible to injury from anemia, due to the well-documented association of worsening mortality and functional outcome in the presence of hypotension and hypoxia, as well as the known vulnerability of many neuronal populations to ischemia. Red blood cell transfusion has been used in traumatic brain injury (TBI) to prevent cerebral ischemia by maximizing the oxygencarrying capacity of blood that is otherwise decreased by blood loss and dilution with crystalloid fluid replacement during resuscitation. Although many practitioners have commonly utilized hemoglobin (Hgb) or hematocrit thresholds for transfusion in these patients, the rationale for this practice has largely been centered on older studies in general critical care populations and animal evidence. Furthermore, in addition to an ideal " target " Hgb, many other questions remain about this clinical practice, such as the optimal duration of maintaining a specific Hgb level, and the ultimate effects of transfusion on neurological and functional outcome.

The role of tissue oxygen monitoring in patients with acute brain injury

Cerebral ischaemia is implicated in poor outcome after brain injury, and is a very common post-mortem finding. The inability of the brain to store metabolic substrates, in the face of high oxygen and glucose requirements, makes it very susceptible to ischaemic damage. The clinical challenge, however, remains the reliable antemortem detection and treatment of ischaemic episodes in the intensive care unit. Outcomes have improved in the traumatic brain injury setting after the introduction of progressive protocol-driven therapy, based, primarily, on the monitoring and control of intracranial pressure, and the maintenance of an adequate cerebral perfusion pressure through manipulation of the mean arterial pressure. With the increasing use of multi-modal monitoring, the complex pathophysiology of the injured brain is slowly being unravelled, emphasizing the heterogeneity of the condition, and the requirement for individualization of therapy to prevent secondary adverse hypoxic cerebral events. Brain tissue oxygen partial pressure (Pb(O2) monitoring is emerging as a clinically useful modality, and this review examines its role in the management of brain injury.

Monitoring of cerebral oxygenation with near infrared spectroscopy and tissue oxygen partial pressure during cardiopulmonary resuscitation in pigs

BACKGROUND AND OBJECTIVE

The present study was designed to compare cerebral oxygenation measured with near infrared spectroscopy and local brain tissue oxygen partial pressure, respectively, in pigs during cardiopulmonary resuscitation. Since tissue overlying the brain may have an impact on near infrared spectroscopy readings, we tested whether optode placement on intact skin or on the skull yielded comparable results.

METHODS

Twelve healthy pigs were anaesthetized and subjected to continuous haemodynamic, near infrared spectroscopy and brain tissue oxygen partial pressure monitoring. After 4 min of untreated ventricular fibrillation, cardiopulmonary resuscitation was started and arginine vasopressin was administered repeatedly three times. Near infrared spectroscopy values recorded were both the tissue oxygenation index and the tissue haemoglobin index as well as relative changes of chromophores (haemoglobin and cytochrome oxidase). Four animals served as control and were measured with both near infrared spectroscopy optodes mounted on the intact skin of the forehead, while in the remaining eight animals, one near infrared spectroscopy optode was implanted directly on the skull.

RESULTS

Near infrared spectroscopy readings at the skin or at the skull differed consistently throughout the study period. After arginine vasopressin administration, near infrared spectroscopy values at the different locations showed a transient dissociation. In contrast to near infrared spectroscopy measured on intact skin, near infrared spectroscopy readings obtained from skull showed a significant correlation to brain tissue oxygen partial pressure values (r = 0.67, P < 0.001).

CONCLUSION

Near infrared spectroscopy readings obtained from skin and skull differed largely after vasopressor administration. Near infrared spectroscopy optode placement therefore may have an important influence on the tissue region investigated.

Cerebral oxygenation in major pediatric trauma: its relevance to trauma severity and outcome

INTRODUCTION

Trauma is the commonest cause of death in the pediatric population, which is prone to diffuse primary brain injury aggravated by secondary insults (eg, hypoxia, hypotension). Standard monitoring involves intracranial pressure (ICP) and cerebral perfusion pressure, which do not reflect true cerebral oxygenation (oxygen delivery [Do(2)]). We explore the merits of a brain tissue oxygen-directed critical care guide.

METHODS

Sixteen patients with major trauma (Injury Severity Score, >16/Pediatric Trauma Score [PTS], <7) had partial pressure of brain tissue oxygen (Pbto(2)) monitor (Licox; Integra Neurosciences, Plainsboro, NJ) placed under local anesthesia using twist-drill craniostomy and definitive management of associated injuries. Pbto(2) levels directed therapy intensity level (ventilator management, inotrops, blood transfusion, and others). Patient demographics, short-term physiological parameters, Pbto(2), ICP, Glasgow Coma Score, trauma scores, and outcomes were analyzed to identify the patients at risk for low Do(2).

RESULTS

There were 10 males and 6 females (mean age, 14 years) sustaining motor vehicle accident (14), falls (1), and assault (1), with a mean Injury Severity Score of 36 (16-59); PTS, 3 (0-7); and Revised Trauma Score, 5.5 (4-11). Eleven patients (70%) had low Do(2) (Pbto(2), <20 mm Hg) on admission despite undergoing standard resuscitation affected by fraction of inspired oxygen, Pao(2), and cerebral perfusion pressure (P = .001). Eubaric hyperoxia improved cerebral oxygenation in the low-Do(2) group (P = .044). The Revised Trauma Score (r = 0.65) showed moderate correlation with Pbto(2) and was a significant predictor for low Do(2) (P = .001). In patients with Pbto(2) of less than 20 mm Hg, PTS correlated with cerebral oxygenation (r = 0.671, P = .033). The mean 2-hour Pbto(2) and the final Pbto(2) in survivors were significantly higher than deaths (21.6 vs 7.2 mm Hg [P = .009] and 25 vs 11 mm Hg [P = .01]). Although 4 of 6 deaths were from uncontrolled high ICP, PTS and 2-hour low Do(2) were significant for roots for mortality.

CONCLUSIONS

Pbto(2) monitoring allows for early recognition of low-Do(2) situations, enabling appropriate therapeutic intervention.

Hyperventilation in head injury: a review

The aim of this review was to consider the effects of induced hypocapnia both on systemic physiology and on the physiology of the intracranial system. Hyperventilation lowers intracranial pressure (ICP) by the induction of cerebral vasoconstriction with a subsequent decrease in cerebral blood volume. The downside of hyperventilation, however, is that cerebral vasoconstriction may decrease cerebral blood flow to ischemic levels. Considering the risk-benefit relation, it would appear to be clear that hyperventilation should only be considered in patients with raised ICP, in a tailored way and under specific monitoring. Controversy exists, for instance, on specific indications, timing, depth of hypocapnia, and duration. This review has specific reference to traumatic brain injury, and is based on an extensive evaluation of the literature and on expert opinion.

Packed red blood cell transfusion increases local cerebral oxygenation

OBJECTIVE

To determine a) whether packed red blood cell transfusion (RBCT) increases local brain tissue oxygen partial pressure (Pbto2) in a neurocritical care population; and b) what (if any) demographic, clinical, or physiologic variables mediate the assumed change.

DESIGN

Prospective observational study.

SETTING

A neurosurgical intensive care unit at a university-based level I trauma center and tertiary care hospital.

PATIENTS

Thirty-five consecutive volume-resuscitated patients with subarachnoid hemorrhage or traumatic brain injury, without cardiac disease, requiring Pbto2 monitoring and receiving RBCT were studied between October 2001 and December 2003.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The following physiologic variables were measured and compared 1 hr before and after RBCT: Pbto2, intracranial pressure, cerebral perfusion pressure, hemoglobin oxygen saturation (Sao2), Fio2, hemoglobin, and hematocrit. An increase in Pbto2 was observed in 26 of the 35 patients (74%). In nine patients, Pbto2 decreased after RBCT. The mean (+/-sd) increase in Pbto2 for all patients was 3.2 +/- 8.8 mm Hg (p = .02), a 15% change from baseline (1 hr before RCBT). This Pbto2 increase was associated with a significant mean increase in hemoglobin and hematocrit after RBCT (1.4 +/- 1.1 g/dL and 4.2% +/- 3.3%, respectively; both p < .001). Cerebral perfusion pressure, Sao2, and Fio2 were similar before and after RBCT. Among the 26 patients whose Pbto2 increased, the mean increase in Pbto2 was 5.1 +/- 9.4 mm Hg or a 49% mean increase (p < .01).

CONCLUSIONS

RBCT is associated with an increase in Pbto2 in most patients with subarachnoid hemorrhage or traumatic brain injury. This mean increase appears to be independent of cerebral perfusion pressure, Sao2, and Fio2. Further study is required to determine why Pbto2 decreases in some patients after RBCT.

Influence of moderate and profound hyperventilation on cerebral blood flow, oxygenation and metabolism

OBJECTIVE

The aim of the present study was to examine the impact of moderate and profound hyperventilation on regional cerebral blood flow (rCBF), oxygenation and metabolism.

MATERIALS AND METHODS

Twelve anesthetized pigs were subjected to moderate (mHV) and profound (pHV) hyperventilation (target arterial pO(2): 30 and 20 mmHg, respectively) for 30 min each, after baseline normoventilation (BL) for 1 h. Local cerebral extracellular fluid (ECF) concentrations of glucose, lactate, pyruvate and glutamate as well as brain tissue oxygenation (p(ti)O(2)) were monitored using microdialysis and a Licox oxygen sensor, respectively. In nine pigs, regional cerebral blood flow (rCBF) was also continuously measured via a thermal diffusion system.

RESULTS

Both moderate and profound hyperventilation resulted in a significant decrease in rCBF (BL: 37.9+/-4.3 ml/100 g/min; mHV: 29.4+/-3.6 ml/100 g/min; pHV: 23.6+/-4.7 ml/100 g/min; p<0.05) and p(ti)O(2) (BL: 22.7+/-4.1 mmHg; mHV: 18.9+/-4.9 mmHg; pHV: 13.0+/-2.2 mmHg; p<0.05). A p(ti)O(2) decrease below the critical threshold of 10 mmHg was induced in three animals by moderate hyperventilation and in five animals by profound hyperventilation. Furthermore, significant increases in lactate (BL: 1.06+/-0.18 mmol/l; mHV: 1.36+/-0.20 mmol/l; pHV: 1.67+/-0.17 mmol/l; p<0.005), pyruvate (BL: 46.4+/-7.8 micromol/l; mHV: 58.0+/-10.3 micromol/l; pHV: 66.1+/-12.7 micromol/l; p<0.05), and lactate/glucose ratio were observed during hyperventilation. (Data are presented as mean+/-S.E.M.)

CONCLUSIONS

Both moderate and profound hyperventilation may result in insufficient regional oxygen supply and anaerobic metabolism, even in the uninjured brain. Therefore, the use of hyperventilation cannot be considered as a safe procedure and should either be avoided or used with extreme caution.

NEURON-SPECIFIC-ENOLASE IS INCREASED IN PLASMA AFTER HEMORRHAGIC SHOCK AND AFTER BILATERAL FEMUR FRACTURE WITHOUT TRAUMATIC BRAIN INJURY IN THE RAT

Neuron-specific enolase (NSE) is an acknowledged marker of traumatic brain injury. Several markers originally considered reliable in the setting of traumatic brain injury have been challenged after having been studied more extensively. The aim of our experimental study was to determine whether NSE is a reliable marker of traumatic brain injury early after trauma. Hemorrhagic shock was achieved by bleeding anesthetized rats to a mean arterial pressure (MAP) of 30-35 mmHg through a femoral catheter until incipient decompensation. MAP was maintained at 30-35 mmHg until 40% of shed blood had been administered as Ringer's solution and was then increased and maintained at 40-45 mmHg for 40 min by further administration of Ringer's solution, mimicking the phase of inadequate preclinical resuscitation. Blood samples were drawn at the end of the 40-min period of inadequate resuscitation. Femur fracture was achieved in anesthetized rats by bilateral application of forceps. Blood samples were drawn 30 and 60 min after fracture. Hemorrhagic shock caused NSE increase versus laboratory controls at the end of inadequate resuscitation (P < 0.01). Bilateral femur fracture caused NSE increase versus laboratory controls 30 min after fracture, which was significant 60 min after fracture (P < 0.01). During femur fracture, MAP remained at a level that is not associated with shock in rats. Our findings show for the first time that NSE increases after hemorrhagic shock as well as after femur fracture without hemorrhagic shock in rats. From a clinical point of view, these findings indicate that NSE cannot be considered a reliable marker of traumatic brain injury early after trauma in cases associated with hemorrhagic shock and/or femur fracture.

The Use of Quantitative End-Tidal Capnometry to Avoid Inadvertent Severe Hyperventilation in Patients With Head Injury After Paramedic Rapid Sequence Intubation

BACKGROUND

This study aimed to determine whether field end-tidal carbon dioxide CO2 (ETCO2) monitoring decreases inadvertent severe hyperventilation after paramedic rapid sequence intubation.

METHODS

Data were collected prospectively as part of the San Diego Paramedic Rapid Sequence Intubation Trial, which enrolled adults with severe head injuries (Glasgow Coma Score, 3-8) that could not be intubated without neuromuscular blockade. After preoxygenation, the patients underwent rapid sequence intubation using midazolam and succinylcholine. A maximum of three intubation attempts were allowed before Combitube insertion was mandated. Tube confirmation was accomplished by physical examination, qualitative capnometry, pulse oximetry, and syringe aspiration. Standard ventilation parameters (tidal volume, 800 mL; 12 breaths/minute) were taught. One agency used portable ETCO2 monitors, with ventilation modified to target ETCO2 values of 30 to 35 mm Hg. Trial patients transported by aeromedical crews also underwent ETCO2 monitoring. The primary outcome measure was the incidence of inadvertent severe hyperventilation, defined as arterial blood gas partial pressure of CO2 (pCO2) of less than 25 mm Hg at arrival, for patients with and those without ETCO2 monitoring. These groups also were compared in terms of age, gender, clinical presentation, Abbreviated Injury Score, Injury Severity Score, arrival arterial blood gas data, and survival.

RESULTS

The study enrolled 426 patients and administered neuromuscular blocking agents to 418 patients. Endotracheal intubation was successful for 355 of these patients (85.2%). Another 58 patients (13.6%) underwent Combitube insertion. For 291 successfully intubated patients, arrival pCO2 values were documented, with continuous ETCO2 monitoring performed for 144 of these patients (49.4%). Patients with ETCO2 monitoring had a lower incidence of inadvertent severe hyperventilation than those without ETCO2 monitoring (5.6% vs. 13.4%; odds ratio, 2.64; 95% confidence interval, 1.12-6.20; p = 0.035). There were no significant differences in terms of age, gender, clinical presentation, Abbreviated Injury Score, Injury Severity Score, arrival partial pressure of oxygen (PO2) and pH, or survival. The patients in both groups with severe hyperventilation had a significantly higher mortality rate than the patients without hyperventilation (56 vs. 30%; odds ratio, 2.9; 95% confidence interval, 1.3-6.6; p = 0.016), which could not be explained solely on the basis of their injuries.

CONCLUSIONS

The use of ETCO2 monitoring is associated with a decrease in inadvertent severe hyperventilation.

Trans-sodium crocetinate increases oxygen delivery to brain parenchyma in rats on oxygen supplementation

Trans-sodium crocetinate (TSC) is a vitamin A-analog that increases diffusivity of oxygen in aqueous solutions, including plasma. The current study is the initial investigation of the effects of TSC on oxygen delivery to brain. Adult male rats were intubated and ventilated with 21%, 60%, or 100% oxygen. A craniotomy was performed and a Licox rat brain tissue PO(2) probe inserted into parietal cortex. Rats were then administered intravenous infusions of either TSC or saline and brain tissue PO(2) values were recorded. TSC significantly increased brain tissue oxygen delivery. This effect was minimal in rats ventilated with normal air and substantial in rats on oxygen supplementation. Arterial blood gas parameters did not differ within groups. These results provide clear indication to study the utility of TSC in ameliorating hypoxic/ischemic insults in neurological disorders.

Hemodilutional anemia is associated with increased cerebral neuronal nitric oxide synthase gene expression

Severe hemodilutional anemia may reduce cerebral oxygen delivery, resulting in cerebral tissue hypoxia. Increased nitric oxide synthase (NOS) expression has been identified following cerebral hypoxia and may contribute to the compensatory increase in cerebral blood flow (CBF) observed after hypoxia and anemia. However, changes in cerebral NOS gene expression have not been reported after acute anemia. This study tests the hypothesis that acute hemodilutional anemia causes cerebral tissue hypoxia, triggering changes in cerebral NOS gene expression. Anesthetized rats underwent hemodilution when 30 ml/kg of blood were exchanged with pentastarch, resulting in a final hemoglobin concentration of 51.0 +/- 1.2 g/l (n = 7 rats). Caudate tissue oxygen tension (Pbr(O(2))) decreased transiently from 17.3 +/- 4.1 to 14.4 +/- 4.1 Torr (P < 0.05), before returning to baseline after approximately 20 min. An increase in CBF may have contributed to restoring Pbr(O(2)) by improving cerebral tissue oxygen delivery. An increase in neuronal NOS (nNOS) mRNA was detected by RT-PCR in the cerebral cortex of anemic rats after 3 h (P < 0.05, n = 5). A similar response was observed after exposure to hypoxia. By contrast, no increases in mRNA for endothelial NOS or interleukin-1beta were observed after anemia or hypoxia. Hemodilutional anemia caused an acute reduction in Pbr(O(2)) and an increase in cerebral cortical nNOS mRNA, supporting a role for nNOS in the physiological response to acute anemia.

Hemorrhagic shock induces an S 100 B increase associated with shock severity

S 100 B is a glial marker of cerebral Injury. In a previous clinical study, we found an S 100 B increase within the first 24 h in patients with multiple trauma and hemorrhagic shock but without cerebral trauma. The aim of our current experimental study was to determine whether this posttraumatic S 100 B increase is caused by extracerebral soft tissue injury or by hemorrhagic shock and whether it is associated with the severity of hemorrhagic shock. Hemorrhagic shock was achieved by bleeding anesthetized rats to a mean arterial pressure (MAP) of 30-35 mmHg through a femoral catheter and maintaining this MAP until incipient decompensation. At incipient decompensation, MAP was either increased immediately to 40-45 mmHg (moderate shock) or was maintained until 40% of shed blood had been returned (severe shock), and then increased to 40-45 mmHg. Resuscitation was provided after 40-45 mmHg MAP had been maintained for 40 min. Soft tissue injury was achieved by midline laparotomy performed at the onset of hemorrhagic shock or without shock and was maintained for 30 min. Hemorrhagic shock caused an early S 100 B increase at the onset of decompensation. S 100 B remained increased for 24 h and was significantly higher after severe than after moderate shock. In contrast, soft tissue injury without hemorrhagic shock caused no S 100 B increase. The data presented demonstrate for the first time that the S 100 B increase is induced by hemorrhagic shock and is associated with the severity of shock.

Tissue oxygen monitoring during hemorrhagic shock and resuscitation: a comparison of lactated Ringer's solution, hypertonic saline dextran, and HBOC-201

BACKGROUND

The ideal resuscitation fluid for the trauma patient would be readily available to prehospital personnel, universally compatible, effective when given in small volumes, and capable of reversing tissue hypoxia in critical organ beds. Recently developed hemoglobin-based oxygen-carrying solutions possess many of these properties, but their ability to restore tissue oxygen after hemorrhagic shock has not been established. We postulated that a small-volume resuscitation with HBOC-201 (Biopure) would be more effective than either lactated Ringer's (LR) solution or hypertonic saline dextran (HSD) in restoring baseline tissue oxygen tension levels in selected tissue beds after hemorrhagic shock. We further hypothesized that changes in tissue oxygen tension measurements in the deltoid muscle would reflect the changes seen in the liver and could thus be used as a monitor of splanchnic resuscitation.

METHODS

This study was a prospective, blinded, randomized resuscitation protocol using anesthetized swine (n = 30), and was modeled to approximate an urban prehospital clinical time course. After instrumentation and splenectomy, polarographic tissue oxygen probes were placed into the liver (liver PO2) and deltoid muscle (muscle PO2) for continuous tissue oxygen monitoring. Swine were hemorrhaged to a mean arterial pressure (MAP) of 40 mm Hg over 20 minutes, shock was maintained for another 20 minutes, and then 100% oxygen was administered. Animals were then randomized to receive one of three solutions: LR (12 mL/kg), HSD (4 mL/kg), or HBOC-201 (6 mL/kg). Physiologic variables were monitored continuously during all phases of the experiment and for 2 hours postresuscitation.

RESULTS

At a MAP of 40 mm Hg, tissue PO2 was 20 mm Hg or less in both the liver and muscle beds. There were no significant differences in measured liver or muscle PO2 values after resuscitation with any of the three solutions in this model of hemorrhagic shock. When comparing the hemodynamic effects of resuscitation, the cardiac output was increased from shock values in all three animal groups with resuscitation, but was significantly higher in the animals resuscitated with HSD. Similarly, MAP was increased by all solutions during resuscitation, but remained significantly below baseline except in the group of animals receiving HBOC-201 (p < 0.01). HBOC-201 was most effective in both restoring and sustaining MAP and systolic blood pressure. There was excellent correlation between liver and deltoid muscle tissue oxygen values (r = 0.8, p < 0.0001).

CONCLUSION

HBOC-201 can be administered safely in small doses and compared favorably to resuscitation with HSD and LR solution in this prehospital model of hemorrhagic shock. HBOC-201 is significantly more effective than HSD and LR solution in restoring MAP and systolic blood pressure to normal values. Deltoid muscle PO2 reflects liver PO2 and thus may serve as an index of the adequacy of resuscitation in critical tissue beds.

Small-Volume Resuscitation with the Hemoglobin Substitute HBOC-201: Effect on Brain Tissue Oxygenation

OBJECTIVES

To investigate the effects of small-volume resuscitation with a hemoglobin based oxygen carrier on brain tissue oxygen tension (PbrO2) in hemorrhaged swine.

METHODS

Clark-type electrodes were inserted into the brain tissue of 6 swine to measure PbrO2 directly. Swine were hemorrhaged to a MAP of 40 mm Hg for 20 minutes. Resuscitation was performed with a bolus infusion of HBOC-201 (6 cc/kg; Biopure Corp.) and high-flow oxygen (100%). Swine were observed for an additional 2 hours.

RESULTS

PbrO2 prior to hemorrhage was 48.7 +/- 4.7 mm Hg with 100% inspired oxygen. PbrO2 rapidly declined to 7.6 +/- 5.3 mm Hg in response to hemorrhage. Small-volume resuscitation with HBOC-201 and high-flow oxygen resulted in a significant increase (p < 0.001) in PbrO2 to 44.6 +/- 8.1 mm Hg. MAP was also significantly increased to 84% of baseline. These elevations were sustained during the observation period.

CONCLUSIONS

Resuscitation with HBOC-201 can restore and sustain cerebral oxygenation and MAP. These results suggest that a small-volume bolus of HBOC-201 may provide adequate oxygen and pressure support during the initial management of hemorrhage.

On the origin of respiratory artifacts in BOLD-EPI of the human brain

BOLD-based functional MRI (fMRI) can be used to explicitly measure hemodynamic aspects and functions of human neuro-physiology. As fMRI measures changes in regional cerebral blood flow and volume as well as blood oxygenation, rather than neuronal brain activity directly, other processes that may change the above parameters have to be examined closely to assess sensitivity and specificity of fMRI results. Physiological processes that can cause artifacts include cardiac action, breathing and vasomotion. Although there has been substantial research on physiological artifacts and appropriate compensation methods, controversy still remains on the mechanisms that cause the fMRI signal fluctuations. Respiratory-correlated fluctuations may either be induced by changes of the magnetic field homogeneity due to moving organs, intra-thoracic pressure differences, respiration-dependent vasodilation or oxygenation differences. The aim of this study was to characterize the impact of different breathing patterns by varying respiration frequency and/or tidal volume on EPI time courses of the resting human brain. The amount of respiration-related oscillations during three respiration patterns was quantified, and statistically significant differences were obtained in white matter only: p < 0.03 between 6 vs. 12 ml/kg body weight end tidal volume at a respiration frequency of 15/min, p < 0.03 between 12 vs. 6 ml/kg body weight and 15 vs. 10 respiration cycles/min. There was no significant difference between 15 vs. 10 respiration cycles/min at an end tidal volume of 6 ml/kg body weight (p = 0.917). In addition, the respiration-affected brain regions were very similar with EPI readout in the a-p and l-r direction. Based on our results and published literature we hypothesize that venous oxygenation oscillations due to changing intra-thoracic pressure represent a major factor for respiration-related signal fluctuations and increase significantly with increasing end tidal volume in white matter only.

Advanced monitoring in the intensive care unit: brain tissue oxygen tension

Cerebral monitoring of patients with acute intracranial disorders generally focuses on intracranial pressure and cerebral perfusion pressure monitoring. Over the past few years, several new techniques have become available for more detailed routine monitoring of cerebral oxygenation and metabolism. Brain tissue oxygen pressure measurement is increasingly being used for evaluation of cerebral oxygenation. This article discusses brain tissue oxygen pressure measurement in regards to its technical aspects, safety, reliability, and value relative to other techniques for evaluation of cerebral oxygenation. Published experimental and clinical data are considered, and the current status of the clinical use and indications of the technique are summarized. Monitoring may be performed in relatively undamaged parts of the brain or, preferably, in the penumbra region of an intracerebral lesion. Pathophysiologic evidence warrants targeting therapy for patients with traumatic brain injury and subarachnoid hemorrhage toward improvement of cerebral oxygenation guided by continuous monitoring of brain tissue oxygen tension.

Hyperventilation in traumatic brain injury patients: inconsistency between consensus guidelines and clinical practice

BACKGROUND

This study assessed patients with traumatic brain injury (TBI) to determine whether prehospital and community hospital providers employed hyperventilation therapy inconsistent with consensus recommendation against its routine use.

METHODS

This prospective analysis of 37 intubated TBI patients without herniation, undergoing helicopter transport to an urban Level I center, entailed flight crews' noting of assisted ventilation rate (AVR) and end-tidal carbon dioxide (ETCO2) upon their arrival at trauma scenes or community hospitals. A priori-set levels of AVR and ETCO2 were used to assess frequency of guideline-inconsistent hyperventilation, and Fisher's exact and Kruskal-Wallis tests assessed association between guideline-inconsistent hyperventilation and manual vs. mechanical ventilation mode.

RESULTS

Inappropriately high AVR and low ETCO2 were seen in 60% and 70% of patients, respectively. Manual ventilation was associated with guideline-inconsistent hyperventilation assessed by AVR (p = 0.038) and ETCO2 (p = 0.022).

CONCLUSION

Prehospital and community hospital hyperventilation practices are not consistent with consensus recommendations for limitation of hyperventilation therapy.