Primary structure of the M subunit of the reaction center from Rhodopseudomonas sphaeroides

The reaction center is a membrane-bound bacteriochlorophyll-protein complex that mediates the primary photochemical events in the photosynthetic bacterium Rhodopseudomonas sphaeroides. The previously determined amino-terminal sequences of the three subunits of the reaction center protein were used to design synthetic mixed oligonucleotide probes for the structural genes encoding the subunits. One of these probes was used to isolate and clone a fragment of DNA from R. sphaeroides that contained the gene encoding the M subunit. The nucleotide sequence of this gene was determined by the dideoxy method. In addition, a number of tryptic and chymotryptic peptides from the M protein were isolated and subjected to sequence analysis, and the sequence of the carboxyl terminus was determined. Together with the amino-terminal sequence, the data establish the primary structure of the M protein. The distribution of hydrophobic residues in the amino acid sequence suggests the presence of five membrane-spanning segments. A significant homology was found between the amino acid sequence of the M subunit and a thylakoid membrane protein (M(r) 32,000) from spinach that has been implicated in herbicide and quinone binding.

Modifiable and nonmodifiable risk factors for postoperative delirium after cardiac surgery with cardiopulmonary bypass

OBJECTIVES

Postoperative delirium after cardiac surgery is associated with increased morbidity and mortality as well as prolonged stay in both the intensive care unit and the hospital. The authors sought to identify modifiable risk factors associated with the development of postoperative delirium in elderly patients after elective cardiac surgery in order to be able to design follow-up studies aimed at the prevention of delirium by optimizing perioperative management.

DESIGN

A post hoc analysis of data from patients enrolled in a randomized controlled trial was performed.

SETTING

A single university hospital.

PARTICIPANTS

One hundred thirteen patients aged 65 or older undergoing elective cardiac surgery with cardiopulmonary bypass.

INTERVENTIONS

None. MEASUREMENTS AND MAINS RESULTS: Screening for delirium was performed using the Confusion Assessment Method (CAM) on the first 6 postoperative days. A multivariable logistic regression model was developed to identify significant risk factors and to control for confounders. Delirium developed in 35 of 113 patients (30%). The multivariable model showed the maximum value of C-reactive protein measured postoperatively, the dose of fentanyl per kilogram of body weight administered intraoperatively, and the duration of mechanical ventilation to be independently associated with delirium.

CONCLUSIONS

In this post hoc analysis, larger doses of fentanyl administered intraoperatively and longer duration of mechanical ventilation were associated with postoperative delirium in the elderly after cardiac surgery. Prospective randomized trials should be performed to test the hypotheses that a reduced dose of fentanyl administered intraoperatively, the use of a different opioid, or weaning protocols aimed at early extubation prevent delirium in these patients.

Delirium auf der Intensivstation

Die Prävalenz des Delirs bei Patienten auf der Intensivstation wird mit 20 – 80 % angegeben. Ein Delirium auf der Intensivstation ist äußerst belastend für die betroffenen Patienten und ihre Angehörigen und eine Herausforderung für die betreuenden Pflegenden und Ärzte. Das Delir auf der Intensivstation ist aber auch mit einer höheren Komplikationsrate, längerer Hospitalisation, höheren Kosten, protrahierten kognitiven Beeinträchtigungen und einer höheren Mortalität assoziiert. Dies zeigt, wie wichtig eine gezielte Prävention, das frühzeitige Erkennen und eine angemessene Therapie wären. Verschiedenste Risikofaktoren für die Entwicklung eines Deliriums bei kritisch kranken Patienten sind bekannt, wobei allerdings die wenigsten beeinflussbar sind. Die Pathophysiologie des Deliriums ist weitgehend unklar, vermutlich sind mehrere Mechanismen an der Entstehung beteiligt. Die Diagnose eines Delirs auf der Intensivstation kann insbesondere bei sedierten und intubierten Patienten sehr schwierig sein. Deshalb wurden spezielle diagnostische Instrumente entwickelt, welche an die Besonderheiten dieser Patienten angepasst sind. Für die Therapie des Deliriums auf der Intensivstation stehen neben supportiven Maßnahmen typische und atypische Neuroleptika zur Verfügung.

Bupivacaine concentrations in the lumbar cerebrospinal fluid of patients during spinal anaesthesia

BACKGROUND

Data on bupivacaine concentrations in the cerebral spinal fluid (CSF) during spinal anaesthesia are scarce. The purpose of this study was to determine the concentration of bupivacaine in the lumbar CSF of patients with an adequate level of spinal anaesthesia after injection of plain bupivacaine 0.5%.

METHODS

Sixty patients with an adequate level of spinal block after standardized administration of plain bupivacaine 20 mg in men and of 17.5 mg in women were studied. To measure the CSF bupivacaine concentration, we performed a second lumbar spinal puncture and obtained a CSF sample at a randomized time point 5-45 min after the bupivacaine injection. In addition, we calculated the half-life of bupivacaine in the CSF and tested the hypothesis that the level of spinal block is related to the lumbar CSF bupivacaine concentration.

RESULTS

Men and women had CSF bupivacaine concentrations ranging from 95.4 to 773.0 microg ml(-1) (median 242.4 microg ml(-1)) and from 25.9 to 781.0 microg ml(-1) (median 187.6 microg ml(-1)), respectively. The large variability of bupivacaine concentrations obtained at similar times after subarachnoid administration made calculation of a meaningful half-life of bupivacaine in CSF impossible. There was no association between CSF bupivacaine concentration and spinal block level, and CSF bupivacaine concentrations for the same spinal block level differed between patients by six-fold.

CONCLUSIONS

There is a large variability of CSF bupivacaine concentrations in patients with an adequate level of spinal anaesthesia.

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.

Cerebral perfusion in sepsis-associated delirium

Introduction

The pathophysiology of sepsis-associated delirium is not completely understood and the data on cerebral perfusion in sepsis are conflicting. We tested the hypothesis that cerebral perfusion and selected serum markers of inflammation and delirium differ in septic patients with and without sepsis-associated delirium.

Methods

We investigated 23 adult patients with sepsis, severe sepsis, or septic shock with an extracranial focus of infection and no history of intracranial pathology. Patients were investigated after stabilisation within 48 hours after admission to the intensive care unit. Sepsis-associated delirium was diagnosed using the confusion assessment method for the intensive care unit. Mean arterial pressure (MAP), blood flow velocity (FV) in the middle cerebral artery using transcranial Doppler, and cerebral tissue oxygenation using near-infrared spectroscopy were monitored for 1 hour. An index of cerebrovascular autoregulation was calculated from MAP and FV data. C-reactive protein (CRP), interleukin-6 (IL-6), S-100β, and cortisol were measured during each data acquisition.

Results

Data from 16 patients, of whom 12 had sepsis-associated delirium, were analysed. There were no significant correlations or associations between MAP, cerebral blood FV, or tissue oxygenation and sepsis-associated delirium. However, we found a significant association between sepsis-associated delirium and disturbed autoregulation (P = 0.015). IL-6 did not differ between patients with and without sepsis-associated delirium, but we found a significant association between elevated CRP (P = 0.008), S-100β (P = 0.029), and cortisol (P = 0.011) and sepsis-associated delirium. Elevated CRP was significantly correlated with disturbed autoregulation (Spearman rho = 0.62, P = 0.010).

Conclusion

In this small group of patients, cerebral perfusion assessed with transcranial Doppler and near-infrared spectroscopy did not differ between patients with and without sepsis-associated delirium. However, the state of autoregulation differed between the two groups. This may be due to inflammation impeding cerebrovascular endothelial function. Further investigations defining the role of S-100β and cortisol in the diagnosis of sepsis-associated delirium are warranted.

Trial registration

ClinicalTrials.gov NCT00410111.

Brain dysfunction in sepsis: what can we learn from cerebral perfusion studies?

Investigations on the relationship between sepsis, brain dysfunction, and cerebral perfusion are methodologically very difficult to perform. It is important to interpret the results of such studies in view of our limited ability to diagnose and quantify brain dysfunction and to consider our limited understanding of the mechanisms that lead to or are associated with brain dysfunction in sepsis.

The accuracy of non-invasive carbon dioxide monitoring: a clinical evaluation of two transcutaneous systems

We determined the accuracy of two transcutaneous carbon dioxide monitoring systems (SenTec Digital Monitor with V-Sign Sensor and TOSCA 500 with TOSCA Sensor 92) for the measurement of single values and trends in the arterial partial pressure of carbon dioxide in 122 adult patients during major surgery and in 50 adult patients in the intensive care unit. One or several paired measurements were performed in each patient. The first measurement was used to determine the accuracy of a single value of transcutaneous carbon dioxide; the difference between the first and the last measurements was used to analyse the accuracy and to track trends. We defined a 95% limit of agreement of <or=1 kPa as being clinically useful. There was insufficient agreement between transcutaneous carbon dioxide partial pressure values derived from the two systems and arterial carbon dioxide values for both single values and trends as defined by our suggested limit of agreement. We conclude that these systems cannot replace conventional blood gas analysis in the clinical setting studied.

Effects of simulated hypovolaemia on haemodynamics, left ventricular function, mesenteric blood flow and gastric Pco2

BACKGROUND

Compensated clinically silent hypovolaemia may lead to low cardiac output, hypoperfusion and ischaemia. We investigated the cardiovascular effects of simulated hypovolaemia to determine whether it caused mesenteric ischaemia detectable by gastric tonometry.

METHODS

Thirteen healthy volunteers, aged 21-36 years, were investigated. Lower body negative pressure (LBNP) was used to simulate normotensive hypovolaemia. Cardiovascular parameters were measured using echocardiography. Mesenteric blood flow was investigated using Doppler sonography of the superior mesenteric artery (SMA). Gastric Pco(2) (P(g)co(2)) was measured using gas tonometry. Data were collected at baseline, LBNP and during a recovery period.

RESULTS

Normotensive hypovolaemia was induced successfully in 11 volunteers. There were no significant differences in mean arterial pressure between the three data points (91 +/- 6, 93 +/- 10 and 95 +/- 9 mmHg, respectively). With the induction of LBNP, the heart rate increased from 64 +/- 16 to 73 +/- 16 beats/min (P < 0.001), the cardiac index decreased from 2.7 +/- 1.0 to 1.8 +/- 0.6 l/min/m(2) (P= 0.002) and the systemic vascular resistance increased from 1535 +/- 445 to 2270 +/- 550 dyn s/cm(5) (P < 0.001). The SMA mean flow velocity decreased from 53 +/- 18 to 37 +/- 20 cm/s (69 +/- 20%) (P= 0.007), and increased to 56 +/- 34 cm/s (106 +/- 38%) (P= 0.001) during reperfusion. The SMA resistance increased from 92 +/- 30 to 174 +/- 110 mmHg/l/min (P= 0.004). These changes were reversible after termination of LBNP. By contrast, there were no significant differences in P(g)co(2) between the three data points.

CONCLUSIONS

In these volunteers, the mesenteric vascular bed contributed importantly to the maintenance of arterial pressure during normotensive hypovolaemia. However, this compensated hypovolaemia did not compromise the mesenteric perfusion sufficiently to increase P(g)co(2) and to allow detection by tonometry.

Monitoring the injured brain: ICP and CBF

Raised intracranial pressure (ICP) and low cerebral blood flow (CBF) are associated with ischaemia and poor outcome after brain injury. Therefore, many management protocols target these parameters. This overview summarizes the technical aspects of ICP and CBF monitoring, and their role in the clinical management of brain-injured patients. Furthermore, some applications of these methods in current research are highlighted. ICP is typically measured using probes that are inserted into one of the lateral ventricles or the brain parenchyma. Therapeutic measures used to control ICP have relevant side-effects and continuous monitoring is essential to guide such therapies. ICP is also required to calculate cerebral perfusion pressure which is one of the most important therapeutic targets in brain-injured patients. Several bedside CBF monitoring devices are available. However, most do not measure CBF but rather a parameter that is thought to be proportional to CBF. Frequently used methods include transcranial Doppler which measures blood flow velocity and may be helpful for the diagnosis and monitoring of cerebral vasospasm after subarachnoid haemorrhage or jugular bulb oximetry which gives information on adequacy of CBF in relation to the metabolic demand of the brain. However, there is no clear evidence that incorporating data from CBF monitors into our management strategies improves outcome in brain-injured patients.

Imaging of cerebral blood flow and metabolism in brain injury in the ICU

The heterogeneity of the initial insult and subsequent pathophysiology has made both the study of human head injury and design of randomised controlled trials exceptionally difficult. The combination of multimodality bedside monitoring and functional brain imaging positron emission tomography (PET) and magnetic resonance (MR), incorporated within a Neurosciences Critical Care Unit, provides the resource required to study critically ill patients after brain injury from initial ictus through recovery from coma and rehabilitation to final outcome. Methods to define cerebral ischemia in the context of altered cerebral oxidative metabolism have been developed, traditional therapies for intracranial hypertension re-evaluated and bedside monitors cross-validated. New modelling and analytical approaches have been developed.

Nonlinear Analysis of Cerebral Hemodynamic and Intracranial Pressure Signals for Characterization of Autoregulation

The objective of this study was to determine whether or not the underlying physiological systems that generates spontaneous arterial blood pressure (ABP), cerebral blood flow velocity (CBFV), and intracranial pressure signals could be adequately approximated as a linear stochastic process. Furthermore, a new measure (C) capable of capturing the degree of nonlinear dependency between two ABP and CBFV signals (including a time-varying situation) was proposed for quantifying the degree of cerebral blood flow autoregulation. A surrogate data test of fifteen ABP, CBFV, and intracranial pressure (ICP) segments was conducted for detecting whether there exists a statistically significant deviation from the null hypothesis of linear signals. The extension of the established block computation method of C measure to an adaptive one was achieved. This new algorithm was then applied to study the C evolution using brain injury patients data from a hyperventilation study and two propofol studies. Nonlinearity has not been detected for all the fifteen recordings, neither has nonlinear dependency between CBFV and ABP. However, their presences in some of the signal segments justified the adoption of a nonlinear measure of dependency capable of characterizing both linear and nonlinear correlations for inferring autoregulation status. C measure started to decrease with the introduction of hypocapnia state indicating that hyperventilation may reduce the dependency of CBFV on ABP fluctuations. On the other hand, complex patterns of C measure evolution were observed among 14 cases of propofol data indicating a nontrivial effect of propofol on the dependency of CBFV on ABP.

Impact of Intracranial Pressure and Cerebral Perfusion Pressure on Severe Disability and Mortality After Head Injury

OBJECTIVE

To investigate the relationships between intracranial pressure (ICP), cerebral perfusion pressure (CPP), and outcome after traumatic brain injury.

MATERIAL AND METHODS

A retrospective analysis of prospectively recorded data from 429 patients after head injury requiring intensive treatment on the Neuroscience Intensive Annex and the Neuro Critical Care Unit, Cambridge, UK. ICP, CPP, and arterial blood pressure (ABP) were continuously recorded. Mean values of pressures were compared to outcome assessed at 6 months after injury (using the Glasgow Outcome Scale).

RESULTS

The mortality rate was greater in those having mean ICP greater than 20 mmHg (17% below versus 47% above; p < 0.0001). The mortality rate was dramatically increased for CPP below 55 mmHg (81% below versus 23% above; p < 0.0001). For values of CPP greater than 95 mmHg, favorable outcome was less frequent (50% below versus 28% above; p < 0.033). The rate of severe disability showed the tendency to increase with CPP ( r = 0.87; p = 0.02), suggesting that a higher CPP does not help in achieving favorable outcomes. ICP was greater in those who died in comparison to those who survived (27 +/- 19 mmHg versus 16 +/- 6 mmHg; p < 0.10 - 7), and CPP was lower (68 +/- 21 versus 76 +/- 10 mmHg; p < 0.0002). There was no difference between mean ICP and CPP in good/moderate and severe disability outcome groups.

CONCLUSION

High ICP is strongly associated with fatal outcome. Excessive CPP seems to reduce the probability of achieving a favorable outcome following head trauma.

["Postoperative cognitive dysfunction--do anaesthetics harm memory?"]

Nach Operationen können zwei Formen von kognitiven Störungen auftreten: Das postoperative Delir und die postoperative kognitive Dysfunktion. Letztere äussert sich durch diskrete Störungen des Gedächtnisses, der Aufmerksamkeit und der Sprache, die während Monaten persistieren können. Die wichtigsten Risikofaktoren für die Entwicklung der postoperativen kognitiven Dysfunktion sind ein höheres Alter und die Art des Eingriffs. Bei Patienten, die älter als 60 Jahre alt sind, kann bei rund 25% eine Woche nach einem grösseren Eingriff eine postoperative kognitive Dysfunktion festgestellt werden, drei Monate postoperativ bei 10%. Nach herzchirurgischen Eingriffen ist die Inzidenz noch höher: Bei rund der Hälfte der Patienten kann bei Spitalaustritt eine postoperative kognitive Dysfunktion festgestellt werden, nach sechs Monaten noch bei einem Viertel. Die Pathogenese und die Rolle der Anästhesie bei der Entwicklung der postoperativen kognitiven Dysfunktion sind unklar. Es gibt keine Studien, die den Effekt der Anästhesie unabhängig von einer Operation oder einer Hospitalisation untersuchen. Weder neuere, kurzwirksame Medikamente noch die Wahl der Anästhesietechnik (Regionalanästhesie oder Allgemeinanästhesie) können die postoperative kognitive Dysfunktion verhindern. Eine Prophylaxe oder Therapie ist zum jetzigen Zeitpunkt nicht bekannt.

Physiological thresholds for irreversible tissue damage in contusional regions following traumatic brain injury

Cerebral ischaemia appears to be an important mechanism of secondary neuronal injury in traumatic brain injury (TBI) and is an important predictor of outcome. To date, the thresholds of cerebral blood flow (CBF) and cerebral oxygen utilization (CMRO(2)) for irreversible tissue damage used in TBI studies have been adopted from experimental and clinical ischaemic stroke studies. Identification of irreversibly damaged tissue in the acute phase following TBI could have considerable therapeutic and prognostic implications. However, it is questionable whether stroke thresholds are applicable to TBI. Therefore, the aim of this study was to determine physiological thresholds for the development of irreversible tissue damage in contusional and pericontusional regions in TBI, and to determine the ability of such thresholds to accurately differentiate irreversibly damaged tissue. This study involved 14 patients with structural abnormalities on late-stage MRI, all of whom had been studied with (15)O PET within 72 h of TBI. Lesion regions of interest (ROI) and non-lesion ROIs were constructed on late-stage MRIs and applied to co-registered PET maps of CBF, CMRO(2) and oxygen extraction fraction (OEF). From the entire population of voxels in non-lesion ROIs, we determined thresholds for the development of irreversible tissue damage as the lower limit of the 95% confidence interval for CBF, CMRO(2) and OEF. To test the ability of a physiological variable to differentiate lesion and non-lesion tissue, we constructed probability curves, demonstrating the ability of a physiological variable to predict lesion and non-lesion outcomes. The lower limits of the 95% confidence interval for CBF, CMRO(2) and OEF in non-lesion tissue were 15.0 ml/100 ml/min, 36.7 mumol/100 ml/min and 25.9% respectively. Voxels below these values were significantly more frequent in lesion tissue (all P < 0.005, Mann-Whitney U-test). However, a significant proportion of lesion voxels had values above these thresholds, so that definition of the full extent of irreversible tissue damage would not be possible based upon single physiological thresholds. We conclude that, in TBI, the threshold of CBF below which irreversible tissue damage consistently occurs differs from the classical CBF threshold for stroke (where similar methodology is used to define such thresholds). The CMRO(2) threshold is comparable to that reported in the stroke literature. At a voxel-based level, however (and in common with ischaemic stroke), the extent of irreversible tissue damage cannot be accurately predicted by early abnormalities of any single physiological variable.

Predicting the response of intracranial pressure to moderate hyperventilation

BACKGROUND

Hyperventilation may cause brain ischaemia after traumatic brain injury. However, moderate reductions in PaCO(2) are still an option in the management of raised intracranial pressure (ICP) under some circumstances. Being able to predict the ICP-response to such an intervention would be advantageous. We investigated the ability of pre-hyperventilation ICP and cerebrospinal compensatory reserve to predict the reduction in ICP achievable with moderate hyperventilation in head injured patients.

METHODS

Thirty head injured patients requiring sedation and mechanical ventilation were investigated. ICP was monitored via an intraparenchymal probe and intracranial cerebrospinal compensatory reserve was assessed using an index (R(ap)) based on the relationship between mean ICP and its pulse amplitude. Measurements were made at a constant level of PaCO(2) during a 20-minute baseline period. The patients were then subjected to an acute decrease in PaCO(2) of approximately 1 kPa and, after an equilibration period of 10 minutes, measurements were again made at a constant level of PaCO(2) for a further 20 minutes. A multiple linear regression model, incorporating baseline PaCO(2), ICP, and R(ap) was used to identify the relevant predictors of ICP reduction.

FINDINGS

Baseline ICP and R(ap) were both significant predictors of ICP-reduction (p=0.02 and 0.001 respectively) with R(ap) being the more powerful parameter.

CONCLUSIONS

A model based on cerebrospinal compensatory reserve and ICP can predict the achievable ICP-reduction and may potentially be used to optimise patient selection and intensity of hyperventilation.

Association between outcome, cerebral pressure reactivity and slow ICP waves following head injury

OBJECTIVE

To investigate the relationships between slow vasogenic waves ('B waves') of intracranial pressure (ICP), pressure-reactivity and outcome after traumatic brain injury.

MATERIAL AND METHOD

193 head-injured patients (age 34 +/- 16.7 years; median GCS 6) were monitored from 1997 to 2002. ICP, arterial blood pressure (ABP) were continuously monitored. Pressure-reactivity index (PRx) and magnitude of ICP slow waves were evaluated using the bed-side computers.

RESULTS

Distribution of PRx in different outcome groups indicated that pressure-reactivity was significantly worse in patients with fatal outcome. A magnitude of spontaneous slow waves of ICP was gradually decreasing in poorer outcome grades. Mortality indicated threshold rise from 20% to 70% when averaged PRx increased above 0.3 (p < 0.01). There was no threshold for mortality observed along distribution of magnitude of ICP slow waves. Mortality gradually increased when the magnitude of slow waves decreased (R = -0.26; p < 0.0001).

CONCLUSION

Inadequate pressure-reactivity and low magnitude of slow vasogenic waves of ICP are associated with fatal outcome after head injury. Based on brain monitoring data, differentiation between favourable outcome and severe disability is more problematic than differentiation between survivors and non-survivors.

Effect of cerebral perfusion pressure augmentation on regional oxygenation and metabolism after head injury*

OBJECTIVE

In this study we have used O positron emission tomography, brain tissue oxygen monitoring, and cerebral microdialysis to assess the effects of cerebral perfusion pressure augmentation on regional physiology and metabolism in the setting of traumatic brain injury.

DESIGN

Prospective interventional study.

SETTING

Neurosciences critical care unit of a university hospital.

PATIENTS

Eleven acutely head-injured patients requiring norepinephrine to maintain cerebral perfusion pressure.

INTERVENTIONS

Using positron emission tomography, we have quantified the response to an increase in cerebral perfusion pressure in a region of interest around a brain tissue oxygen sensor (Neurotrend) and microdialysis catheter. Oxygen extraction fraction and cerebral blood flow were measured with positron emission tomography at a cerebral perfusion pressure of approximately 70 mm Hg and approximately 90 mm Hg using norepinephrine to control cerebral perfusion pressure. All other aspects of physiology were kept stable.

MEASUREMENTS AND MAIN RESULTS

Cerebral perfusion pressure augmentation resulted in a significant increase in brain tissue oxygen (17 +/- 8 vs. 22 +/- 8 mm Hg; 2.2 +/- 1.0 vs. 2.9 +/- 1.0 kPa, p < .001) and cerebral blood flow (27.5 +/- 5.1 vs. 29.7 +/- 6.0 mL/100 mL/min, p < .05) and a significant decrease in oxygen extraction fraction (33.4 +/- 5.9 vs. 30.3 +/- 4.6 %, p < .05). There were no significant changes in any of the microdialysis variables (glucose, lactate, pyruvate, lactate/pyruvate ratio, glycerol). There was a significant linear relationship between brain tissue oxygen and oxygen extraction fraction (r = .21, p < .05); the brain tissue oxygen value associated with an oxygen extraction fraction of 40% (the mean value for oxygen extraction fraction in normal controls) was 14 mm Hg (1.8 kPa). The cerebral perfusion pressure intervention resulted in a greater percentage increase in brain tissue oxygen than the percentage decrease in oxygen extraction fraction; this suggests that the oxygen gradients between the vascular and tissue compartments were reduced by the cerebral perfusion pressure intervention.

CONCLUSIONS

Cerebral perfusion pressure augmentation significantly increased levels of brain tissue oxygen and significantly reduced regional oxygen extraction fraction. However, these changes did not translate into predictable changes in regional chemistry. Our results suggest that the ischemic level of brain tissue oxygen may lie at a level below 14 mm Hg (1.8 kPa); however, the data do not allow us to be more specific.

Effects of moderate hyperventilation on cerebrovascular pressure-reactivity after head injury

In volunteers, hyperventilation improves autoregulation. However, in head-injured patients, hyperventilation-induced deterioration and improvement of autoregulation have been reported. We have re-examined this question using an index of pressure reactivity. Thirty patients with severe or moderate head-injury were studied. Arterial blood pressure, cerebral perfusion pressure (CPP), and intracranial pressure (ICP) were recorded over 20 minute epochs separated by ten minutes of equilibration at baseline and during moderate (>3.5 kPa) hyperventilation. End-tidal CO2 was constant during each phase of data acquisition. Pressure reactivity was assessed using an index 'PRx' based on the response of ICP to spontaneous blood pressure changes. Hyperventilation decreased PaCO2 from 5.1 +/- 0.4 to 4.4 +/- 0.4 kPa (p < 0.0001). ICP decreased by 3.7 +/- 2.2 mmHg (p < 0.001). CPP increased by 5.9 +/- 8.2 mmHg (p < 0.001). Overall, PRx did not change significantly with hyperventilation. However, there was a significant negative correlation between baseline PRx and the change in PRx (r = -0.71, p < 0.0001). This suggests that patients with disturbed pressure-reactivity may improve, whereas patients with intact pressure reactivity remain largely unchanged. Our data suggest that the response of pressure reactivity to hyperventilation is heterogeneous. This could be due to hyperventilation-induced changes in cerebral metabolism, or the change in CPP.

Sustained moderate reductions in arterial CO2 after brain trauma Time-course of cerebral blood flow velocity and intracranial pressure

OBJECTIVE

In healthy volunteers cerebral blood flow starts to recover towards baseline within a few minutes of continued hyperventilation due to normalisation of perivascular pH. We investigated the time-course of changes in middle cerebral artery mean flow velocity (FVm) and intracranial pressure (ICP) in head-injured patients during sustained moderate reductions in arterial partial pressure of CO(2) (PaCO(2)).

DESIGN

Observational study.

PATIENTS

Twenty-seven sedated, mechanically ventilated patients with severe head injury.

INTERVENTIONS

Measurements were made during and after routine determination of CO(2)-reactivity: an acute 20% increase in respiratory minute volume was followed by a 10-min stabilisation period and 50 min of continued moderate hyperventilation at a constant PaCO(2) (>3.5 kPa).

MEASUREMENTS AND RESULTS

FVm was monitored with transcranial Doppler, ICP was monitored with intraparenchymal probes. During the 50-min period with stable PaCO(2) FVm increased in 36% of patients. All other patients showed a decline in FVm over the same time period. Overall FVm recovery was -0.03+/-0.14%.min(-1). The time-course of ICP changes was significantly different from that of FVm, with ICP reaching its lowest value earlier than FVm (23+/-12 vs 37+/-20 min; P = 0.001) and returning more rapidly towards baseline than FVm (0.23+/-0.23 vs -0.03+/-0.14%.min(-1); P< 0.0001).

CONCLUSIONS

Head-injured patients may adapt differently to hyperventilation than healthy volunteers. Potentially harmful reductions in cerebral blood flow may persist beyond the duration of useful ICP reduction.

Hyperglycemia and Brain Tissue pH after Traumatic Brain Injury

OBJECTIVE

Hyperglycemia occurring after head injury is associated with poor neurological outcome. We tested the hypothesis that blood glucose levels are associated with brain tissue pH (pH(b)) and that the correction of hyperglycemia would result in an improvement in pH(b).

METHODS

This is a retrospective analysis of a prospectively collected database. Thirty-four patients in a tertiary care neuroscience critical care unit with major traumatic brain injury underwent pH(b) monitoring.

RESULTS

A total of 428 glucose measurements were recorded during pH(b) monitoring. Mean glucose level was 7.1 mmol/L (range, 2.8-21.7 mmol/L) and median (interquartile range) pH(b) was 7.11 mmol/L (7.00-7.19 mmol/L). To account for the correlated, unbalanced nature of the data, a linear generalized estimating equation model was created. This model predicted that for each 1 mmol/L increase in blood glucose, pH(b) changed by -0.011 mmol/L (95% confidence interval, -0.016 to -0.005 mmol/L; P < 0.001). This relationship remained significant in a multivariable model that included cerebral perfusion pressure, brain tissue oxygen and carbon dioxide tension, and brain temperature. Twenty-one episodes of significant hyperglycemia (>or=11.1 mmol/L) treated with intravenous insulin were identified. Insulin therapy significantly reduced blood glucose concentration from a median (interquartile range) of 11.9 mmol/L (range, 11.4-13.6 mmol/L) to 8.8 mmol/L (range, 7.3-9.6 mmol/L; P < 0.001). Baseline pH(b) was not significantly different from pH(b) associated with the subsequent glucose reading of less than 11.1 mmol/L (P = 0.29), but there was a suggestion of improvement if the change in blood glucose was large.

CONCLUSION

Blood glucose is associated with brain tissue acidosis in patients with major head injury. Prospective studies are required to confirm these results and to determine whether treatment of hyperglycemia improves outcome.

Does induced hypertension reduce cerebral ischaemia within the traumatized human brain?

Recent changes in published guidelines for the management of patients with severe head injury are based on data showing that aggressive maintenance of cerebral perfusion pressure (CPP) can worsen outcome due to extracranial complications of therapy. However, it remains unclear whether CPP augmentation could reduce cerebral ischaemia, a finding which might prompt the search for CPP augmentation protocols that avoid these extracranial complications. We studied 10 healthy volunteers and 20 patients within 6 days of closed head injury. All subjects underwent imaging of cerebral blood flow (CBF), blood volume (CBV), oxygen metabolism (CMRO2) and oxygen extraction fraction (OEF) using 15O PET. In addition, for patients, the EEG power ratio index (PRI), burst suppression ratio and somatosensory evoked potentials (SEP) were obtained and CPP was increased from 68 +/- 4 to 90 +/- 4 mmHg using an infusion of norepinephrine and measurements were repeated. Following elevation of CPP, CBF and CBV were increased and CMRO2 and OEF were reduced (P < 0.001 for all comparisons). Regions with a reduction in CMRO2 were associated with the greatest reduction in OEF (r2 = 0.3; P < 0.0001). Although CPP elevation produced a significant fall in the ischaemic brain volume (IBV) (from 15 +/- 16 to 5 +/- 4 ml; P < 0.01) and improved flow metabolism coupling, the IBV was small and clinically insignificant in the majority of these patients. However, the reduction in IBV was directly related to the baseline IBV (r2 = 0.97; P < 0.001) and patients with large baseline IBV showed substantial and clinically significant reductions. CPP augmentation increased the EEG PRI (5.0 +/- 1.5 versus 4.3 +/- 1.4, P < 0.01), implying an overall decrease in neural activity, but these changes did not correlate with the reduction in CMRO2 and there was no change in SEP cortical amplitude (N20-P27). These data provide support for recent changes in recommended CPP levels for head injury management across populations of patients with significant head injury. However, they do not provide guidance on whether the intervention may be more appropriate at earlier stages after injury, or in patients selected because of high baseline IBV. It also remains unclear whether CPP values below 65 mmHg can be safely used in this population. Clarification of the significance of a reduction in CMRO2 and neuronal electrical function will require further study.

Extracellular Brain pH and Outcome following Severe Traumatic Brain Injury

The ability to measure brain tissue chemistry has led to valuable information regarding pathophysiological changes in patients with traumatic brain injury (TBI). Over the last few years, the focus has been on monitoring changes in brain tissue oxygen to determine thresholds of ischemia that affect outcome. However, the variability of this measurement suggests that it may not be a robust method. We have therefore investigated the relationship of brain tissue pH (pH(b)) and outcome in patients with TBI. We retrospectively analyzed prospectively collected data of 38 patients admitted to the Neurosciences Critical Care Unit with TBI between 1998 and 2003, and who had a multiparameter tissue gas sensor inserted into the brain. All patients were managed using an evidence-based protocol targeting CPP > 70 mm Hg. Physiological variables were averaged over 4 min and analyzed using a generalized least squares random effects model to determine the temporal profile of pH(b) and its association with outcome. Median (IQR) minimum pH(b) was 7.00 (6.89, 7.08), median (IQR) maximum pH(b) was 7.25 (7.18, 7.33), and median (IQR) patient averaged pH(b) was 7.13 (7.07, 7.17). pH(b) was significantly lower in those who did not survive their hospital stay compared to those that survived. In addition, those with unfavorable neurological outcome had lower pH(b) values than those with favorable neurological outcome. pH(b) differentiated between survivors and non-survivors. Measurement of pH(b) may be a useful indicator of outcome in patients with TBI.

Direct comparison of cerebrovascular effects of norepinephrine and dopamine in head-injured patients

OBJECTIVE

To directly compare the cerebrovascular effects of norepinephrine and dopamine in patients with acute traumatic brain injury.

DESIGN

Prospective randomized crossover trial.

SETTING

Neurosciences critical care unit of a university hospital.

PATIENTS

Ten acutely head-injured patients requiring vasoactive drugs to maintain a cerebral perfusion pressure of 65 mm Hg.

INTERVENTIONS

Patients were randomized to start the protocol with either norepinephrine or dopamine. Using an infusion of the allocated drug, cerebral perfusion pressure was adjusted to 65 mm Hg. After 20 mins of data collection, cerebral perfusion pressure was increased to 75 mm Hg by increasing the infusion rate of the vasoactive agent. After 20 mins of data collection, cerebral perfusion pressure was increased to 85 mm Hg and again data were collected for 20 mins. Subsequently, the infusion rate of the vasoactive drug was reduced until a cerebral perfusion pressure of 65 mm Hg was reached and the drug was exchanged against the other agent. The protocol was then repeated.

MEASUREMENTS AND MAIN RESULTS

Mean arterial pressure and intracranial pressure were monitored and cerebral blood flow was estimated with transcranial Doppler. Norepinephrine led to predictable and significant increases in flow velocity for each step increase in cerebral perfusion pressure (57.5+/-19.9 cm x sec, 61.3+/-22.3 cm x sec, and 68.4+/-24.8 cm x sec at 65, 75, and 85 mm Hg, respectively; p <.05 for all three comparisons), but changes with dopamine were variable and inconsistent. There were no differences between absolute values of flow velocity or intracranial pressure between the two drugs at any cerebral perfusion pressure level.

CONCLUSIONS

Norepinephrine may be more predictable and efficient to augment cerebral perfusion in patients with traumatic brain injury.

Effect of cerebral perfusion pressure augmentation with dopamine and norepinephrine on global and focal brain oxygenation after traumatic brain injury

OBJECTIVE

To compare the effects of a cerebral perfusion pressure (CPP) intervention achieved with dopamine and norepinephrine after severe head injury.

DESIGN

Prospective, controlled, trial.

SETTING

Neurosciences critical care unit.

PATIENTS

Eleven patients with a head injury, requiring dopamine or norepinephrine infusions to support CPP.

INTERVENTION

Cerebral tissue gas measurements were recorded using a multimodal sensor, and regional chemistry was assessed using microdialysis. Patients received in, randomised order, either dopamine or norepinephrine to achieve and maintain a CPP of 65 mmHg, and then, following a 30-min period of stable haemodynamics, a CPP of 85 mmHg. Data were then acquired using the second agent. Haemodynamic measurements and measurements of cerebral physiology were made during each period.

MEASUREMENTS AND RESULTS

The CPP augmentation with norepinephrine, but not with dopamine, resulted in a significant reduction in arterial-venous oxygen difference (37+/-11 vs 33+/-12 ml/l) and a significant increase in brain tissue oxygen (2.6+/-1.1 vs 3.0+/-1.1 kPa). The CPP intervention did not significantly affect intracranial pressure. There were no significant differences between norepinephrine and dopamine on cerebral oxygenation or metabolism either at baseline or following a CPP intervention; however, the response to a CPP intervention with dopamine seemed to be more variable than the response achieved with norepinephrine.

CONCLUSIONS

If CPP is to be raised to a level higher than 65-70 mmHg, then it is important to recognise that the response to the intervention may be unpredictable and that the vasoactive agent used may be of importance.

Asymmetry of pressure autoregulation after traumatic brain injury

OBJECT

The aim of this study was to assess the asymmetry of autoregulation between the left and right sides of the brain by using bilateral transcranial Doppler ultrasonography in a cohort of patients with head injuries.

METHODS

Ninety-six patients with head injuries comprised the study population. All significant intracranial mass lesions were promptly removed. The patients were given medications to induce sedation and paralysis, and artificial ventilation. Arterial blood pressure (ABP) and intracranial pressure (ICP) were monitored in an invasive manner. A strategy based on the patient's cerebral perfusion pressure (CPP = ABP - ICP) was applied: CPP was maintained at a level higher than 70 mm Hg and ICP at a level lower than 25 mm Hg. The left and right middle cerebral arteries were insonated daily, and bilateral flow velocities (FVs) were recorded. The correlation coefficient between the CPP and FV, termed Mx, was calculated and time-averaged over each recording period on both sides. An Mx close to 1 signified that slow fluctuations in CPP produced synchronized slow changes in FV, indicating a defective autoregulation. An Mx close to 0 indicated preserved autoregulation. Computerized tomography scans in all patients were reviewed; the side on which the major brain lesion was located was noted and the extent of the midline shift was determined. Outcome was measured 6 months after discharge. The left-right difference in the Mx between the hemispheres was significantly higher in patients who died than in those who survived (0.16 +/- 0.04 compared with 0.08 +/- 0.01; p = 0.04). The left-right difference in the Mx was correlated with a midline shift (r = -0.42; p = 0.03). Autoregulation was worse on the side of the brain where the lesion was located (p < 0.035).

CONCLUSIONS

The left-right difference in autoregulation is significantly associated with a fatal outcome. Autoregulation in the brain is worse on the side ipsilateral to the lesion and on the side of expansion in cases in which there is a midline shift.

Intracranial hypertension: what additional information can be derived from ICP waveform after head injury?

OBJECTIVE

Although intracranial hypertension is one of the important prognostic factors after head injury, increased intracranial pressure (ICP) may also be observed in patients with favourable outcome. We have studied whether the value of ICP monitoring can be augmented by indices describing cerebrovascular pressure-reactivity and pressure-volume compensatory reserve derived from ICP and arterial blood pressure (ABP) waveforms.

METHOD

96 patients with intracranial hypertension were studied retrospectively: 57 with fatal outcome and 39 with favourable outcome. ABP and ICP waveforms were recorded. Indices of cerebrovascular reactivity (PRx) and cerebrospinal compensatory reserve (RAP) were calculated as moving correlation coefficients between slow waves of ABP and ICP, and between slow waves of ICP pulse amplitude and mean ICP, respectively. The magnitude of 'slow waves' was derived using ICP low-pass spectral filtration.

RESULTS

The most significant difference was found in the magnitude of slow waves that was persistently higher in patients with a favourable outcome (p<0.00004). In patients who died ICP was significantly higher (p<0.0001) and cerebrovascular pressure-reactivity (described by PRx) was compromised (p<0.024). In the same patients, pressure-volume compensatory reserve showed a gradual deterioration over time with a sudden drop of RAP when ICP started to rise, suggesting an overlapping disruption of the vasomotor response.

CONCLUSION

Indices derived from ICP waveform analysis can be helpful for the interpretation of progressive intracranial hypertension in patients after brain trauma.

Incidence and mechanisms of cerebral ischemia in early clinical head injury

Antemortem demonstration of ischemia has proved elusive in head injury because regional CBF reductions may represent hypoperfusion appropriately coupled to hypometabolism. Fifteen patients underwent positron emission tomography within 24 hours of head injury to map cerebral blood flow (CBF), cerebral oxygen metabolism (CMRO2), and oxygen extraction fraction (OEF). We estimated the volume of ischemic brain (IBV) and used the standard deviation of the OEF distribution to estimate the efficiency of coupling between CBF and CMRO2. The IBV in patients was significantly higher than controls (67 +/- 69 vs. 2 +/- 3 mL; P < 0.01). The coexistence of relative ischemia and hyperemia in some patients implies mismatching of perfusion to oxygen use. Whereas the saturation of jugular bulb blood (SjO2) correlated with the IBV (r = 0.8, P < 0.01), SjO2 values of 50% were only achieved at an IBV of 170 +/- 63 mL (mean +/- 95% CI), which equates to 13 +/- 5% of the brain. Increases in IBV correlated with a poor Glasgow Outcome Score 6 months after injury (rho = -0.6, P < 0.05). These results suggest significant ischemia within the first day after head injury. The ischemic burden represented by this "traumatic penumbra" is poorly detected by bedside clinical monitors and has significant associations with outcome.

Effects of propofol on cerebral oxygenation and metabolism after head injury

BACKGROUND

Flow-metabolism coupling is thought to be deranged after traumatic brain injury, while the effects of propofol on flow-metabolism coupling are controversial. We have used a step increase in target plasma propofol concentration in head injured patients to explore flow-metabolism coupling in these patients.

METHODS

Ten patients with a moderate to severe head injury received a step increase in propofol target controlled infusion of 2 microg x ml(-1). Cerebral tissue gas measurements were recorded using a multimodal sensor, and regional chemistry was assessed using microdialysis. Arterial-jugular venous oxygen differences (AVDO(2)) were measured and all patients had cortical function monitoring (EEG).

RESULTS

The step increase in propofol led to a large increase in EEG burst-suppression ratio (0% (range 0-1.1) to 46.1% (range 0-61.7), P<0.05); however, this did not significantly change tissue gas levels, tissue chemistry, or AVDO(2).

CONCLUSIONS

Flow-metabolism coupling remains intact during a step increase in propofol after traumatic brain injury. The EEG burst-suppression induced by propofol after traumatic brain injury does not appear to be a useful therapeutic tool in reducing the level of regional ischaemic burden.

Pharmacokinetics and pharmacodynamics of dopamine and norepinephrine in critically ill head-injured patients

OBJECTIVE

To explore the pharmacokinetics and pharmacodynamics of dopamine and norepinephrine.

DESIGN

Prospective, controlled, trial.

SETTING

Neurosciences critical care unit.

PATIENTS

Eight patients with a head injury, requiring dopamine or norepinephrine infusions to support cerebral perfusion pressure (CPP).

INTERVENTION

Patients received in randomised order, either dopamine or norepinephrine to achieve and maintain a CPP of 70 mmHg, and then, following a 30-min period of stable haemodynamics, a CPP of 90 mmHg. Data were then acquired using the second agent. Haemodynamic measurements were made during each period and a blood sample was obtained at the end of each study period for analysis of plasma catecholamine concentrations

MEASUREMENTS AND RESULTS

Plasma levels of norepinephrine and dopamine were significantly related to infusion rates but did not have a simple linear relationship to haemodynamic parameters. However, there was a significant quadratic relationship between the infusion rate of dopamine and cardiac index (r2=0.431), and systemic vascular resistance index (r2=0.605), with a breakpoint (at which cardiac index reduced and SVRI increased) at a dopamine plasma level of approximately 50 nM/l (corresponding to an infusion rate of approximately 15 microg.kg(-1).min(-1)).

CONCLUSIONS

Norepinephrine and dopamine have predictable pharmacokinetics; however, those of dopamine do not fit a simple first-order kinetic model. The pharmacodynamic effects of dopamine and norepinephrine show much inter-individual variability and unpredictability. Plasma levels of dopamine appear to relate to variations in adrenergic receptor effects with break points that reflect expectations from infusion-rate related pharmacodynamics.

Predictive value of Glasgow Coma Scale after brain trauma: change in trend over the past ten years

BACKGROUND

Age and the Glasgow Coma Scale (GCS) score on admission are considered important predictors of outcome after traumatic brain injury. We investigated the predictive value of the GCS in a large group of patients whose computerised multimodal bedside monitoring data had been collected over the previous 10 years.

METHODS

Data from 358 subjects with head injury, collected between 1992 and 2001, were analysed retrospectively. Patients were grouped according to year of admission. Glasgow Outcome Scores (GOS) were determined at six months. Spearman's correlation coefficients between GCS and GOS scores were calculated for each year.

RESULTS

On average 34 (SD: 7) patients were monitored every year. We found a significant correlation between the GCS and GOS for the first five years (overall 1992-1996: r = 0.41; p<0.00001; n = 183) and consistent lack of correlations from 1997 onwards (overall 1997-2001: r = 0.091; p = 0.226; n = 175). In contrast, correlations between age and GOS were in both time periods significant and similar (r = -0.24 v r = -0.24; p<0.002).

CONCLUSIONS

The admission GCS lost its predictive value for outcome in this group of patients from 1997 onwards. The predictive value of the GCS should be carefully reconsidered when building prognostic models incorporating multimodality monitoring after head injury.

Responses of posttraumatic pericontusional cerebral blood flow and blood volume to an increase in cerebral perfusion pressure

In and around traumatic contusions, cerebral blood flow (CBF) is often near or below the threshold for ischemia. Increasing cerebral perfusion pressure (CPP) in patients with head injuries may improve CBF in these regions. However, the pericontusional response to this intervention has not been studied. Using positron emission tomography (PET), we have quantified the response to an increase in CPP in and around contusions in 18 contusions in 18 patients. Regional CBF and cerebral blood volume (CBV) were measured with PET at CPPs of 70 and 90 mmHg using norepinephrine to control CPP. Based upon computed tomography, regions of interest (ROIs) were placed as two concentric ellipsoids, each of 1-cm width, around the core of the contusions. Measurements were compared with a control ROI in tissue with normal anatomic appearance. Baseline CBF and CBV increased significantly with increasing distance from the core of the lesion. The increase in CPP led to small increases in CBF in all ROIs except the core. The largest absolute CBF increase was found in the control ROI. Relative CBF increases did not differ between ROIs so that ischemic areas remained ischemic. Pericontusional oedema on computed tomography was associated with lower absolute values of CBF and CBV but did not differ from nonoedematous tissue in the relative response to CPP elevation.

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.

The effects of large-dose propofol on cerebrovascular pressure autoregulation in head-injured patients

In healthy individuals, cerebrovascular pressure autoregulation is preserved or even improved when propofol is infused. We examined the effect of an increase in propofol plasma concentration on pressure autoregulation in 10 head-injured patients. Using target-controlled infusions, the static rate of autoregulation was determined at a moderate (2.3 +/- 0.4 microg/mL) and a large (4.3 +/- 0.04 microg/mL) plasma target concentration of propofol. Using norepinephrine to control cerebral perfusion pressure, transcranial Doppler measurements from the middle cerebral artery were made at a cerebral perfusion pressure of 70 and 85 mm Hg at each propofol concentration. Middle cerebral artery flow velocities at the large propofol concentration were significantly lower than at the moderate concentration, without any concurrent increase in arterio-jugular difference in oxygen content, a finding compatible with maintained flow-metabolism coupling. Despite this, static rate of autoregulation decreased significantly from 54% +/- 36% to 28% +/- 35% (P = 0.029). Our data suggest that after head injury, the cerebrovascular effects of propofol are different from those observed in healthy individuals. We propose that large doses of propofol should be used cautiously in head-injured patients, because there is the potential to increase the injured brain's vulnerability to secondary insults.

IMPLICATIONS

Propofol is used for sedation and control of intracranial pressure in head-injured patients. In contrast to previous data from healthy individuals, we show a deterioration of cerebrovascular pressure autoregulation with fast propofol infusion rates after head injury. Large propofol doses may increase the injured brain's vulnerability to secondary insults.

Cerebrovascular pressure reactivity is related to global cerebral oxygen metabolism after head injury

BACKGROUND

After head injury, impaired cerebrovascular autoregulation has been associated with abnormally high or low cerebral blood flow. The physiological relevance of cerebral blood flow levels is difficult to assess in these patients, whose cerebral metabolic rate for oxygen (CMRO(2)) is known to be abnormal. Investigation of these relations requires quantitative measures of cerebral blood flow and CMRO(2), to allow assessment of oxygen supply and demand relations.

OBJECTIVES

To investigate the relation between dysautoregulation and global cerebral oxygen metabolism following head injury.

METHODS

Using positron emission tomography, global cerebral blood flow, CMRO(2), and oxygen extraction fraction were determined in 22 patients who were investigated in 26 examinations on days 1 to 11 (mean (SD), 3.5 (2.3)) after head injury. Cerebrovascular pressure reactivity was assessed using a pressure reactivity index, calculated as the moving linear correlation coefficient between mean arterial blood pressure and intracranial pressure. Outcome was assessed six months after injury using the Glasgow outcome scale.

RESULTS

Low CMRO(2) was associated with disturbed pressure reactivity (inverse function, R(2) = 0.21, p = 0.018) and there was a correlation between disturbed pressure reactivity and oxygen extraction fraction (quadratic function, R(2) = 0.55, p = 0.0001). There was no significant relation between pressure reactivity and cerebral blood flow. An unfavourable outcome was associated with disturbed pressure reactivity. There was no significant relation between outcome and CMRO(2) or oxygen extraction fraction.

CONCLUSIONS

There is a close relation between dysautoregulation and abnormal cerebral metabolism but not blood flow. Further studies are needed to determine whether metabolic dysfunction is a result of or a cause of disturbed pressure reactivity, and to establish if there is a relation between cerebral oxygen metabolism and outcome.

Cerebral oxygen vasoreactivity and cerebral tissue oxygen reactivity

There has long been an appreciation that cerebral blood flow is modulated to ensure adequate cerebral oxygen delivery in the face of systemic hypoxaemia. There is increasing appreciation of the modulatory role of hyperoxia in the cerebral circulation and a consideration of the effects of such modulation on the maintenance of cerebral tissue oxygen concentration. These newer findings are particularly important in view of the fact that cerebrovascular and tissue oxygen responses to hyperoxia may change in disease. Such alterations provide important insights into pathophysiological mechanisms and may provide novel targets for therapy. However, before the modulatory effects of hyperoxia can be used for diagnosis, to predict prognosis or to direct therapy, a more detailed analysis and understanding of the physiological concepts behind this modulation are required, as are the limitations of the measurement tools used to define the modulation. This overview summarizes the available information in this area and suggests some avenues for further research.

Validation of a tonometric noninvasive arterial blood pressure monitor in the intensive care setting

Intra-arterial measurement is considered the gold standard for continuous, beat-to-beat arterial blood pressure monitoring. However, arterial cannulation can be difficult and may cause complications such as thrombosis and ischaemia. Recently, a tonometric system, the Colin CBM-7000 has been developed for noninvasive beat-to-beat measurement of arterial blood pressure from the radial artery. We assessed the level of agreement between the CBM-7000 and invasive radial artery measurements in 15 patients on a neuro-intensive care unit. Agreement of systolic, diastolic and mean arterial pressure values was limited, with approximately 34% of mean arterial pressures differing by over 10 mmHg. In many cases, this was due to a downward drift of the noninvasive measurements over time. Furthermore, there was a tendency to underestimate low pressures and overestimate high pressures. In our opinion, the Colin CBM-7000 cannot be recommended for continuous blood pressure monitoring in the intensive care setting.

Hyperoxia and the cerebral hemodynamic responses to moderate hyperventilation

BACKGROUND

A reduction in the arterial partial pressure of CO2 (PaCO2) leads to a rapid reduction in cerebral blood flow (CBF). However, despite continuing hypocapnia there is secondary recovery of CBF over time as a result of increases in lactic acid production. Hyperoxia is thought to modulate the production of lactic acid. This study examined the kinetics of middle cerebral artery flow velocity (MCA FV) reduction during hyperventilation, and its modulation by hyperoxia.

METHODS

Cerebral blood flow was assessed using transcranial Doppler ultrasound in nine healthy, awake human volunteers. Subjects were ventilated, via a mouthpiece, to achieve a stable end-tidal CO2 (PETCO2). After a 20-min baseline period the minute volume on the ventilator was passively increased by approximately 20% to reduce PETCO2 by 0.75-1 kPa. After a 10-min stabilization period the new PETCO2 level was maintained at a constant level for 20 min, and MCA FV recovery was measured during this 20-min period. Subjects undertook the protocol breathing air and breathing 100% oxygen.

RESULTS

The PETCO2 level was (mean +/- SD) 4.9 +/- 0.4 kPa (normoxia baseline), 4.0 +/- 0.3 kPa (normoxia hyperventilation), 4.6 +/- 0.4 kPa (hyperoxia baseline) and 3.9 +/- 0.4 kPa (hyperoxia hyperventilation). CO2 reactivity was significantly lower with normoxia than hyperoxia (16.5 +/- 3.8 vs. 21.2 +/- 4.6 % kPa-1; P< 0.05). Middle cerebral artery FV recovery was significantly more rapid with normoxia than hyperoxia (0.23 +/- 0.17 vs. 0.08 +/- 0.1 % baseline min-1; P< 0.01).

CONCLUSIONS

Our results suggest that cerebral hemodynamic responses to moderate hyperventilation are different in normoxic and hyperoxic conditions. Clinical assessment of CO2 reactivity and CBF recovery during hyperventilation should take the degree of arterial oxygenation into account.

Assessment of the Ventrix parenchymal intracranial pressure monitoring probe (NL950-P) and Monitor (NL950-100) in a 3 Tesla magnetic resonance scanner

Magnetic resonance (MR) imaging and spectroscopy provide important information in patients with acute head injury. However, optimal patient management requires intracranial pressure (ICP) monitoring. There are few reports on the use of ICP sensors in an MR environment. We tested the Ventrix parenchymal intracranial pressure monitoring probe and monitor (Integra Neurosciences, USA), modified by the use of a fibre-optic extension cable, within a 3 Tesla MR system. The device performed well in the MR environment, but one element within the fibre-optic extension was significantly ferromagnetic. The ICP probe produced a small susceptibility artefact on spin echo images, and a larger artefact on gradient echo images. The MR safety of the integrated system is probably acceptable, but could be easily improved with minor modifications. Although the system is MR compatible and produces generally acceptable imaging even at 3 Tesla, there is significant degradation of image quality during gradient echo sequences.

Continuous assessment of cerebral autoregulation: clinical and laboratory experience

The method for the continuous assessment of cerebral autoregulation using slow waves of MCA blood flow velocity (FV) and cerebral perfusion pressure (CPP) or arterial pressure (ABP) has been introduced seven years ago. We intend to review its clinical applications in various scenarios. Moving correlation coefficient (3-6 min window), named Mx, is calculated between low-pass filtered (0.05 Hz) signals of FV and CPP or ABP (when ICP is not measured directly). Data from ventilated 243 head injuries and 15 patients after poor grade subarachnoid haemorrhage, 38 patients with Carotid Artery stenosis, 35 patients with hydrocephalus and fourteen healthy volunteers is presented. Good agreement between the leg-cuff test and Mx has been confirmed in healthy volunteers (r = 0.81). Mx also correlated significantly with the static rate of autoregulation and transient hyperaemic response test. Autoregulation was disturbed (p < 0.021) by vasospasm after SAH and worse in patients with hydrocephalus in whom CSF circulation was normal (p < 0.02). In head injury, Mx indicated disturbed autoregulation with low CPP (< 55 mmHg) and too high CPP (> 95 mmHg). Mx strongly discriminated between patients with favourable and unfavourable outcome (p < 0.00002). This method can be used in many clinical scenarios for continuous monitoring of cerebral autoregulation, predicting outcome and optimising treatment strategies.

Should we measure cerebral blood flow in head-injured patients?

Inadequate cerebral blood flow (CBF) after head injury is an important cause of secondary ischaemic damage. Rapid identification of episodes of hypo- or hyperperfusion would allow timely intervention and would possibly improve outcome. Despite a large number of methods to estimate CBF, this concept is only marginally implemented in clinical practice. The methods to detect such episodes are limited for technical reasons, but also because the thresholds of ischaemia and hyperaemia are variable after head injury. Furthermore, we are not always able to manipulate CBF in a controlled manner. Accordingly, it is not surprising that attempts to compare a CBF-targeted strategy with another management option have failed to demonstrate a clear benefit. Methods need to be developed that allow either identification of thresholds for critically low or high CBF in individual patients, allow monitoring oxygen extraction fraction, representing circulatory reserve, or alternatively provide a measure of the volume of ischaemic or hyperaemic brain.

Assessment of the Caradyne WhisperFlow for administration of continuous positive airway pressure in a 3 Tesla magnetic resonance scanner

Demand for magnetic resonance investigations in critically ill patients is increasing. While these patients frequently need ventilatory support, not all of them require controlled ventilation and many may be treated with continuous positive airway pressure. Controlled ventilation, with the concurrent need for sedation, may be inappropriate when airway physiology is being studied and may retard weaning. No commercially available ventilator designed for the magnetic resonance environment can deliver high flow continuous positive airway pressure. We tested the Caradyne Whisperflow flow generator and five Whisperflow valves (2.5-15 cmH2O airway pressure) within a 3 Tesla environment for safety and possible dysfunction. All components had minimal ferromagnetic properties and tests showed no clinically relevant change in flow delivery or oxygen concentration in the magnetic field. In addition, the airway pressure generated by the valves was not affected by the magnetic field. We conclude that the tested system can be safely used in a 3 Tesla magnetic resonance environment.

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.