Regional cerebral blood flow and intraventricular pressure in acute head injuries

Hyperventilation in Adult TBI Patients: How to Approach It?

Hyperventilation is a commonly used therapy to treat intracranial hypertension (ICTH) in traumatic brain injury patients (TBI). Hyperventilation promotes hypocapnia, which causes vasoconstriction in the cerebral arterioles and thus reduces cerebral blood flow and, to a lesser extent, cerebral blood volume effectively, decreasing temporarily intracranial pressure. However, hyperventilation can have serious systemic and cerebral deleterious effects, such as ventilator-induced lung injury or cerebral ischemia. The routine use of this therapy is therefore not recommended. Conversely, in specific conditions, such as refractory ICHT and imminent brain herniation, it can be an effective life-saving rescue therapy. The aim of this review is to describe the impact of hyperventilation on extra-cerebral organs and cerebral hemodynamics or metabolism, as well as to discuss the side effects and how to implement it to manage TBI patients.

Overview of Monitoring of Cerebral Blood Flow and Metabolism after Severe Head Injury

The relationships between cerebral blood flow (CBF), cerebral metabolism (cerebral metabolic rate of oxygen, CMRO2) and cerebral oxygen extraction (arteriovenous difference of oxygen, AVDO2) are discussed, using the formula CMRO2 = CBF × AVDO2. Metabolic autoregulation, pressure autoregulation and viscosity autoregulation can all be explained by the strong tendency of the brain to keep AVDO2 constant. Monitoring of CBF, CMRO2 or AVDO2 very early after injury is impractical, but the available data indicate that cerebral ischemia plays a considerable role at this stage. It can best be avoided by not "treating" arterial hypertension and not using too much hyperventilation, while generous use of mannitol is probably beneficial. Once in the ICU, treatment can most practically be guided by monitoring of jugular bulb venous oxygen saturation. If saturation drops below 50%, the reason for this must be found (high intracranial pressure, blood pressure not high enough, too vigorous hyperventilation, arterial hypoxia, anemia) and must be treated accordingly.

Temporal profile of intracranial pressure and cerebrovascular reactivity in severe traumatic brain injury and association with fatal outcome: An observational study

BACKGROUND

Both intracranial pressure (ICP) and the cerebrovascular pressure reactivity represent the dysregulation of pathways directly involved in traumatic brain injury (TBI) pathogenesis and have been used to inform clinical management. However, how these parameters evolve over time following injury and whether this evolution has any prognostic importance have not been studied.

METHODS AND FINDINGS

We analysed the temporal profile of ICP and pressure reactivity index (PRx), examined their relation to TBI-specific mortality, and determined if the prognostic relevance of these parameters was affected by their temporal profile using mixed models for repeated measures of ICP and PRx for the first 240 hours from the time of injury. A total of 601 adults with TBI, admitted between September 2002 to January 2016, and with high-resolution continuous monitoring from a single centre, were studied. At 6 months postinjury, 133 (19%) patients had a fatal outcome; of those, 88 (78%) died from nonsurvivable TBI or brain death. The difference in mean ICP between those with a fatal outcome and functional survivors was only significant for the first 168 hours after injury (all p < 0.05). For PRx, those patients with a fatal outcome also had a higher (more impaired) PRx throughout the first 120 hours after injury (all p < 0.05). The separation of ICP and PRx was greatest in the first 72 hours after injury. Mixed models demonstrated that the explanatory power of the PRx decreases over time; therefore, the prognostic weight assigned to PRx should similarly decrease. However, the ability of ICP to predict a fatal outcome remained relatively stable over time. As control of ICP is the central purpose of TBI management, it is likely that some of the information that is reflected in the natural history of ICP changes is no longer apparent because of therapeutic intervention.

CONCLUSIONS

We demonstrated the temporal evolution of ICP and PRx and their relationship with fatal outcome, indicating a potential early prognostic and therapeutic window. The combination of dynamic monitoring variables and their time profile improved prediction of outcome. Therefore, time-driven dynamic modelling of outcome in patients with severe TBI may allow for more accurate and clinically useful prediction models. Further research is needed to confirm and expand on these findings.

Cerebral hemodynamic changes of mild traumatic brain injury at the acute stage

Mild traumatic brain injury (mTBI) is a significant public health care burden in the United States. However, we lack a detailed understanding of the pathophysiology following mTBI and its relation to symptoms and recovery. With advanced magnetic resonance imaging (MRI), we can investigate brain perfusion and oxygenation in regions known to be implicated in symptoms, including cortical gray matter and subcortical structures. In this study, we assessed 14 mTBI patients and 18 controls with susceptibility weighted imaging and mapping (SWIM) for blood oxygenation quantification. In addition to SWIM, 7 patients and 12 controls had cerebral perfusion measured with arterial spin labeling (ASL). We found increases in regional cerebral blood flow (CBF) in the left striatum, and in frontal and occipital lobes in patients as compared to controls (p = 0.01, 0.03, 0.03 respectively). We also found decreases in venous susceptibility, indicating increases in venous oxygenation, in the left thalamostriate vein and right basal vein of Rosenthal (p = 0.04 in both). mTBI patients had significantly lower delayed recall scores on the standardized assessment of concussion, but neither susceptibility nor CBF measures were found to correlate with symptoms as assessed by neuropsychological testing. The increased CBF combined with increased venous oxygenation suggests an increase in cerebral blood flow that exceeds the oxygen demand of the tissue, in contrast to the regional hypoxia seen in more severe TBI. This may represent a neuroprotective response following mTBI, which warrants further investigation.

Quantitative lobar cerebral blood flow for outcome prediction after traumatic brain injury

The aim of this study was to examine cortical cerebral blood flow (CBF) in patients with traumatic brain injury (TBI) and determine whether lobar cortical CBF is a better predictor of long-term neurological outcome assessed by the Glasgow Outcome Scale (GOS) than global cortical CBF. Ninety-eight patients with TBI had a stable xenon computed tomography scan (Xe/CT-CBF study) performed at various time points after their initial injury. Spearman's correlation coefficients and Kruskall-Wallis' test were used to examine the relationship between patient age, emergency room Glasgow Coma Scale (GCS), Injury Severity Score, prehospital hypotension, prehospital hypoxia, mechanism of injury, type of injury, side of injury, global average CBF, lobar CBF, number of lobes with CBF below normal, and GOS (discharge, 3 and 6 months). Univariate ordinal regression was performed using these same variables and in combination with principle component analysis (PCA) to determine independent variables for multi-variate ordinal regression. Significant correlation between age, GCS, prehospital hypotension, type of injury, global average CBF, lobar CBF, number of lobes below normal CBF, and GOS was found. Individual lobar CBF was highly correlated with global CBF and the number of lobes below normal CBF. PCA found one principle component among these three CBF variables; therefore, average global CBF and number of lobes with CBF below normal were each chosen as independent variables for multiple ordinal regression, which found age, GCS, and prehospital hypotension, global average CBF, and number of lobes below normal CBF significantly associated with GOS. This study found global average CBF and lobar CBF significantly correlated with GOS at follow-up. There was, however, no individual cerebral lobe that was more predictive than any other, which puts into question the value of calculating lobar CBF versus global CBF in predicting GOS.

Evaluation of the impact of implementing the emergency medical services traumatic brain injury guidelines in Arizona: the Excellence in Prehospital Injury Care (EPIC) study methodology

Traumatic brain injury (TBI) exacts a great toll on society. Fortunately, there is growing evidence that the management of TBI in the early minutes after injury may significantly reduce morbidity and mortality. In response, evidence-based prehospital and in-hospital TBI treatment guidelines have been established by authoritative bodies. However, no large studies have yet evaluated the effectiveness of implementing these guidelines in the prehospital setting. This article describes the background, design, implementation, emergency medical services (EMS) treatment protocols, and statistical analysis of a prospective, controlled (before/after), statewide study designed to evaluate the effect of implementing the EMS TBI guidelines-the Excellence in Prehospital Injury Care (EPIC) study (NIH/NINDS R01NS071049, "EPIC"; and 3R01NS071049-S1, "EPIC4Kids"). The specific aim of the study is to test the hypothesis that statewide implementation of the international adult and pediatric EMS TBI guidelines will significantly reduce mortality and improve nonmortality outcomes in patients with moderate or severe TBI. Furthermore, it will specifically evaluate the effect of guideline implementation on outcomes in the subgroup of patients who are intubated in the field. Over the course of the entire study (~9 years), it is estimated that approximately 25,000 patients will be enrolled.

Evidence of "regional hyperemia" in patients with severe closed head injury using single-photon emission computed tomography

Regional cerebral blood flow (rCBF) was assessed in 32 patients with acute/subacute (n = 18) or chronic (n = 14) head injury using single-photon emission computed tomography (SPECT) and (99m)Tc-hexamethylpropylenenamine oxime (HMPAO) (n = 20) or (123)I-IMP (n = 12). Twelve of the 18 patients with acute/subacute head injury were studied with a high-resolution three-head camera and (99m)Tc-HMPAO. Twenty-eight SPECT studies showed regional abnormalities of tracer uptake. In all cases in which computed tomography (CT) and/or magnetic resonance imaging (MRI) (n = 30) were available for comparison, SPECT showed similar or more extensive abnormalities with high agreement on the laterality of the lesions. Particularly in the 18 patients with acute/subacute head injury, SPECT revealed more and larger lesions than the anatomical scans. In 9 of the 12 patients, at least one region with increase in HMPAO uptake reflecting "hyperemia" was detected (mean study time after injury, 6.3 ± 2.7 days). Follow-up SPECT scans in two patients showed decreased tracer uptake in the previous hyperemic regions and encephalomalacia in the anatomical scan. Thus, hyperemia may reflect "luxury perfusion" in early phases of head injury. In three patients with diffuse axonal injury, a generalized reduction in cortical tracer activity, especially in occipital/parietal regions, was observed. The typical finding in chronic head injury was observation of lesions of similar sizes and high agreement in focal abnormalities between SPECT and CT and/or MRI. However, in nine patients with behavioral disturbances, a decrease in the cortical/basal ganglia was detected. Thus, our study confirms the presence of hyperemia in acute/subacute head injury and demonstrates the value of SPECT for assessing functional impairment in these patients.

Cerebral blood flow and autoregulation after pediatric traumatic brain injury

Traumatic brain injury is a global health concern and is the leading cause of traumatic morbidity and mortality in children. Despite a lower overall mortality than in adult traumatic brain injury, the cost to society from the sequelae of pediatric traumatic brain injury is very high. Predictors of poor outcome after traumatic brain injury include altered systemic and cerebral physiology, including altered cerebral hemodynamics. Cerebral autoregulation is often impaired after traumatic brain injury and may adversely impact the outcome. Although altered cerebrovascular hemodynamics early after traumatic brain injury may contribute to disability in children, there is little information regarding changes in cerebral blood flow and cerebral autoregulation after pediatric traumatic brain injury. This review addresses normal pediatric cerebral physiology and cerebrovascular pathophysiology after pediatric traumatic brain injury.

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.

Does age play a role in recovery from sports-related concussion? A comparison of high school and collegiate athletes

OBJECTIVE

To evaluate symptoms and neurocognitive recovery patterns after sports-related concussion in high school and college athletes.

STUDY DESIGN

College athletes (n = 371) and high school athletes (n = 183) underwent baseline neuropsychological evaluation between 1997 and 2000. Individuals who received a concussion during athletic competition (n = 54) underwent serial neuropsychologic evaluation after injury and were compared with a noninjured within-sample control group (n = 38). Main outcome measures included structured interview, four memory measures, and Concussion Symptom Scale ratings. Baseline to postinjury change scores and multiple analyses of variance were used to compare recovery curves within and between groups.

RESULTS

High school athletes with concussion had prolonged memory dysfunction compared with college athletes with concussion. High school athletes performed significantly worse than age-matched control subjects at 7 days after injury (F = 2.90; P <.005). College athletes, despite having more severe in-season concussions, displayed commensurate performance with matched control subjects by day 3 after concussion. Self-report of postconcussion symptoms by student athletes was not predictive of poor performance on neuropsychologic testing.

CONCLUSIONS

Caution and systematic evaluation should be undertaken before returning athletes with concussion to competition. Sole reliance on the self-report of the athlete may be inadequate. Preliminary data may suggest a more protracted recovery from concussion in high school athletes.

Efficacy of hyperventilation, blood pressure elevation, and metabolic suppression therapy in controlling intracranial pressure after head injury

OBJECT

Hyperventilation therapy, blood pressure augmentation, and metabolic suppression therapy are often used to reduce intracranial pressure (ICP) and improve cerebral perfusion pressure (CPP) in intubated head-injured patients. In this study, as part of routine vasoreactivity testing, these three therapies were assessed in their effectiveness in reducing ICP.

METHODS

Thirty-three patients with a mean age of 33 +/- 13 years and a median Glasgow Coma Scale (GCS) score of 7 underwent a total of 70 vasoreactivity testing sessions from postinjury Days 0 to 13. After an initial 133Xe cerebral blood flow (CBF) assessment, transcranial Doppler ultrasonography recordings of the middle cerebral arteries were obtained to assess blood flow velocity changes resulting from transient hyperventilation (57 studies in 27 patients), phenylephrine-induced hypertension (55 studies in 26 patients), and propofol-induced metabolic suppression (43 studies in 21 patients). Changes in ICP, mean arterial blood pressure (MABP), CPP, PaCO2, and jugular venous oxygen saturation (SjvO2) were recorded. With hyperventilation therapy, patients experienced a mean decrease in PaCO2 from 35 +/- 5 to 27 +/- 5 mm Hg and in ICP from 20 +/- 11 to 13 +/- 8 mm Hg (p < 0.001). In no patient who underwent hyperventilation therapy did SjvO2 fall below 55%. With induced hypertension, MABP in patients increased by 14 +/- 5 mm Hg and ICP increased from 16 +/- 9 to 19 +/- 9 mm Hg (p = 0.001). With the aid of metabolic suppression, MABP remained stable and ICP decreased from 20 +/- 10 to 16 +/- 11 mm Hg (p < 0.001). A decrease in ICP of more than 20% below the baseline value was observed in 77.2, 5.5, and 48.8% of hyperventilation, induced-hypertension, and metabolic suppression tests, respectively (p < 0.001 for all comparisons). Predictors of an effective reduction in ICP included a high PaCO2 for hyperventilation, a high study GCS score for induced hypertension, and a high PaCO2 and a high CBF for metabolic suppression.

CONCLUSIONS

Of the three modalities tested to reduce ICP, hyperventilation therapy was the most consistently effective, metabolic suppression therapy was variably effective, and induced hypertension was generally ineffective and in some instances significantly raised ICP. The results of this study suggest that hyperventilation may be used more aggressively to control ICP in head-injured patients, provided it is performed in conjunction with monitoring of SjvO2.

Effects of hyperbaric oxygenation

OBJECT

Hyperbaric oxygenation (HBO) therapy has been shown to reduce mortality by 50% in a prospective randomized trial of severely brain injured patients conducted at the authors' institution. The purpose of the present study was to determine the effects of HBO on cerebral blood flow (CBF), cerebral metabolism, and intracranial pressure (ICP), and to determine the optimal HBO treatment paradigm.

METHODS

Oxygen (100% O2, 1.5 atm absolute) was delivered to 37 patients in a hyperbaric chamber for 60 minutes every 24 hours (maximum of seven treatments/patient). Cerebral blood flow, arteriovenous oxygen difference (AVDO2), cerebral metabolic rate of oxygen (CMRO2), ventricular cerebrospinal fluid (CSF) lactate, and ICP values were obtained 1 hour before and 1 hour and 6 hours after a session in an HBO chamber. Patients were assigned to one of three categories according to whether they had reduced, normal, or raised CBF before HBO. In patients in whom CBF levels were reduced before HBO sessions, both CBF and CMRO2 levels were raised 1 hour and 6 hours after HBO (p < 0.05). In patients in whom CBF levels were normal before HBO sessions, both CBF and CMRO2 levels were increased at 1 hour (p < 0.05), but were decreased by 6 hours after HBO. Cerebral blood flow was reduced 1 hour and 6 hours after HBO (p < 0.05), but CMRO2 was unchanged in patients who had exhibited a raised CBF before an HBO session. In all patients AVDO2 remained constant both before and after HBO. Levels of CSF lactate were consistently decreased 1 hour and 6 hours after HBO, regardless of the patient's CBF category before undergoing HBO (p < 0.05). Intracranial pressure values higher than 15 mm Hg before HBO were decreased 1 hour and 6 hours after HBO (p < 0.05). The effects of each HBO treatment did not last until the next session in the hyperbaric chamber.

CONCLUSIONS

The increased CMRO2 and decreased CSF lactate levels after treatment indicate that HBO may improve aerobic metabolism in severely brain injured patients. This is the first study to demonstrate a prolonged effect of HBO treatment on CBF and cerebral metabolism. On the basis of their data the authors assert that shorter, more frequent exposure to HBO may optimize treatment.

Carbon dioxide reactivity, pressure autoregulation, and metabolic suppression reactivity after head injury: a transcranial Doppler study

OBJECT

Contemporary management of head-injured patients is based on assumptions about CO2 reactivity, pressure autoregulation (PA), and vascular reactivity to pharmacological metabolic suppression. In this study, serial assessments of vasoreactivity of the middle cerebral artery (MCA) were performed using bilateral transcranial Doppler (TCD) ultrasonography.

METHODS

Twenty-eight patients (mean age 33 +/- 13 years, median Glasgow Coma Scale score of 7) underwent a total of 61 testing sessions during postinjury Days 0 to 13. The CO2 reactivity (58 studies in 28 patients), PA (51 studies in 23 patients), and metabolic suppression reactivity (35 studies in 16 patients) were quantified for each cerebral hemisphere by measuring changes in MCA velocity in response to transient hyperventilation, arterial blood pressure elevation, or propofol-induced burst suppression, respectively. One or both hemispheres registered below normal vasoreactivity scores in 40%, 69%, and 97% of study sessions for CO2 reactivity, PA, and metabolic suppression reactivity (p < 0.0001), respectively. Intracranial hypertension, classified as intracranial pressure (ICP) greater than 20 mm Hg at the time of testing, was associated with global impairment of CO2 reactivity, PA, and metabolic suppression reactivity (p < 0.05). A low baseline cerebral perfusion pressure (CPP) was also predictive of impaired CO2 reactivity and PA (p < 0.01). Early postinjury hypotension or hypoxia was also associated with impaired CO2 reactivity (p < 0.05), and hemorrhagic brain lesions in or overlying the MCA territory were predictive of impaired metabolic suppression reactivity (p < 0.01). The 6-month Glasgow Outcome Scale score correlated with the overall degree of impaired vasoreactivity (p < 0.05).

CONCLUSIONS

During the first 2 weeks after moderate or severe head injury, CO2 reactivity remains relatively intact, PA is variably impaired, and metabolic suppression reactivity remains severely impaired. Elevated ICP appears to affect all three components of vasoreactivity that were tested, whereas other clinical factors such as CPP, hypotensive and hypoxic insults, and hemorrhagic brain lesions have distinctly different impacts on the state of vasoreactivity. Incorporation of TCD ultrasonography-derived vasoreactivity data may facilitate more injury- and time-specific therapies for head-injured patients.

Effects of hyperbaric oxygenation therapy on cerebral metabolism and intracranial pressure in severely brain injured patients

OBJECT

Hyperbaric oxygenation (HBO) therapy has been shown to reduce mortality by 50% in a prospective randomized trial of severely brain injured patients conducted at the authors' institution. The purpose of the present study was to determine the effects of HBO on cerebral blood flow (CBF), cerebral metabolism, and intracranial pressure (ICP), and to determine the optimal HBO treatment paradigm.

METHODS

Oxygen (100% O2, 1.5 atm absolute) was delivered to 37 patients in a hyperbaric chamber for 60 minutes every 24 hours (maximum of seven treatments/patient). Cerebral blood flow, arteriovenous oxygen difference (AVDO2), cerebral metabolic rate of oxygen (CMRO2), ventricular cerebrospinal fluid (CSF) lactate, and ICP values were obtained 1 hour before and 1 hour and 6 hours after a session in an HBO chamber. Patients were assigned to one of three categories according to whether they had reduced, normal, or raised CBF before HBO. In patients in whom CBF levels were reduced before HBO sessions, both CBF and CMRO2 levels were raised 1 hour and 6 hours after HBO (p < 0.05). In patients in whom CBF levels were normal before HBO sessions, both CBF and CMRO2 levels were increased at 1 hour (p < 0.05), but were decreased by 6 hours after HBO. Cerebral blood flow was reduced 1 hour and 6 hours after HBO (p < 0.05), but CMRO2 was unchanged in patients who had exhibited a raised CBF before an HBO session. In all patients AVDO2 remained constant both before and after HBO. Levels of CSF lactate were consistently decreased 1 hour and 6 hours after HBO, regardless of the patient's CBF category before undergoing HBO (p < 0.05). Intracranial pressure values higher than 15 mm Hg before HBO were decreased 1 hour and 6 hours after HBO (p < 0.05). The effects of each HBO treatment did not last until the next session in the hyperbaric chamber.

CONCLUSIONS

The increased CMRO2 and decreased CSF lactate levels after treatment indicate that HBO may improve aerobic metabolism in severely brain injured patients. This is the first study to demonstrate a prolonged effect of HBO treatment on CBF and cerebral metabolism. On the basis of their data the authors assert that shorter, more frequent exposure to HBO may optimize treatment.

[Uses and abuses of hyperventilation in severe traumatic brain injury]

A critical evaluation was done about the guidelines and effects of the hyperventilation maneuver on prevention and treatment of increased intracranial pressure (ICP) that follows severe traumatic brain injury (TBI). The prophylactic use of hyperventilation should be avoided after severe TBI acute phase, unless high venous O2 values are recorded at jugular bulb blood (SjO2), or to allow time when there are evidences of neurologic deterioration with posturing. The lack of cerebrovascular response to hyperventilation to low the ICP means that the blood brain barrier (BBB) function is extensively impaired. Then, hyperventilation may be used as a screening therapeutic test in acute severe TBI, since BBB impairment is the pointer that other available clinical procedures for high ICP control (sedation, paralysis and osmotic diuretics) are not workable. A new pathogenetic hypothesis about traumatic brain edema and its therapeutic approach is presented.

Impaired autoregulation of cerebral blood flow in an experimental model of traumatic brain injury

In order to study the pathophysiology and the intracranial hemodynamics of traumatic brain injury, we have developed a modified closed-head injury model of impact-acceleration that expresses several features of severe head injury in humans, including acute and long-lasting intracranial hypertension, diffuse axonal injury, neuronal necrosis, bleeding, and edema. In view of the clinical relevance of impaired autoregulation of cerebral blood flow after traumatic brain injury, and aiming at further characterization of the model, we investigated the autoregulation efficiency 24 h after experimental closed-head injury. Cortical blood flow was continuously monitored with a laser-Doppler flowmeter, and the mean arterial blood pressure was progressively decreased by controlled hemorrhage. Relative laser-Doppler flow was plotted against the corresponding mean arterial blood pressure, and a two-line segmented model was applied to determine the break point and slopes of the autoregulation curves. The slope of the curve at the right hand of the break point was significantly increased in the closed head injury group (0.751 +/- 0.966%/mm Hg versus -0.104 +/- 0.425%/mm Hg,p = 0.028). The break point tended towards higher values in the closed head injury group (62.2 +/- 20.8 mm Hg versus 46.9 +/- 12.7 mm Hg; mean +/- SD, p = 0.198). It is concluded that cerebral autoregulation in this modified closed head injury model is impaired 24 h after traumatic brain injury. This finding, in addition to other characteristic features of severe head injury established earlier in this model, significantly contributes to its clinical relevance.

The Brain Trauma Foundation. The American Association of Neurological Surgeons. The Joint Section on Neurotrauma and Critical Care. Hyperventilation

Chronic prophylactic hyperventilation therapy should be avoided during the first 5 days after severe TBI and particularly during the first 24 h. CBF measurements in patients with severe TBI demonstrate that blood flow early after injury is low and strongly suggest that in the first few hours after injury the absolute values approach those consistent with ischemia. These findings are corroborated by AVdO2 and SjO2 and brain tissue O2 measurements. Hyperventilation will reduce CBF values even further, but will not consistently cause a reduction of ICP and may cause loss of autoregulation. The cerebral vascular response to hypocapnia is reduced in those with the most severe injuries (subdural hematomas and diffuse contusions), and there is substantial local variability in perfusion. While the CBF level at which irreversible ischemia occurs has not been clearly established, ischemic cell change has been demonstrated in 90% of those who die following TBI, and there is PET evidence that such damage is likely to occur when CBF drops below 15-20 cc/100 g/min. A prospective randomized clinical trial has determined that outcomes are worse when TBI patients are treated with chronic prophylactic hyperventilation therapy. Within the standard, guideline, and options, specific paCO2 thresholds have been described that are different for each of the three parameters. These individual thresholds were selected based on the preponderance of literature supporting those thresholds in the contexts of the statements which included them. With the exception of the threshold included for the standard in this guideline, it is emphasized that the paCO2 threshold is not as important as the general concept of hyperventilation. The preponderance of the physiologic literature concludes that hyperventilation during the first few days following severe traumatic brain injury, whatever the threshold, is potentially deleterious in that it can promote cerebral ischemia.

Neurotrauma care in the new millennium

The next millennium will see an explosion of neuromonitoring technology that will provide a more detailed understanding of brain-injured patients. This understanding will allow an individualized and intelligent application of the wide range of therapies that will become available. The measure of success for all of these endeavors will be individual patients and physicians' ability to return them to their normal lives.

Posttraumatic cerebral ischemia after fluid percussion brain injury: an autoradiographic and histopathological study in rats

OBJECTIVES

Mild-to-moderate reductions in local cerebral blood flow (ICBF) have been reported to occur in rats after moderate (1.7-2.2 atm) fluid percussion brain injury. The purpose of this study was to determine whether evidence for severe ischemia (i.e., mean ICBF < 0.25 ml/g/min) could be demonstrated after severe brain injury. In addition, patterns of indium-labeled platelet accumulation and histopathological outcome were correlated with the hemodynamic alterations.

METHODS

Sprague-Dawley rats (n = 23), anesthetized with halothane and maintained on a 70:30 mixture of nitrous oxide:oxygen and 0.5% halothane, underwent normothermic (37 degrees C) parasagittal fluid percussion brain injury (2.4-2.6 atm). Indium-111-tropolone-labeled platelets were injected 30 minutes before traumatic brain injury (TBI), while 14C-iodoantipyrine was infused 30 minutes after trauma for ICBF determination. Sham-operated animals (n = 8) underwent similar surgical procedures but were not injured. For histopathological analysis, traumatized rats (n = 5) were perfusion-fixed 3 days after TBI.

RESULTS

In autoradiographic images of indium-labeled platelets, abnormal platelet accumulation that was most pronounced overlying the pial surface was commonly associated with severe reductions in ICBF within underlying cortical regions 30 minutes after TBI. For example, within the lateral parietal cortex, ICBF was significantly reduced from 1.67 +/- 0.11 ml/g per minute (mean +/- standard error of the mean) in sham-operated animals to 0.23 +/- 0.03 ml/g per minute within the traumatized group. In addition to focal severe ischemia, moderate reductions in ICBF were detected throughout the traumatized hemisphere, including the frontal and occipital cortices, hippocampus, thalamus, and striatum. Mild decreases in ICBF were also observed throughout the contralateral cerebral cortex. At 3 days after severe TBI, histopathology demonstrated intracerebral and subarachnoid hemorrhage associated with cerebral contusion and selective neuronal necrosis.

CONCLUSION

These data indicate that multiple cerebrovascular abnormalities, including subarachnoid hemorrhage, focal platelet accumulation, and severe ischemia, are important early events in the pathogenesis of cortical contusion formation after TBI. Injury severity is expected to be a critical factor in determining what therapeutic strategies are attempted in the clinical setting.

Assessment of cerebral pressure autoregulation in humans--a review of measurement methods

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

Cerebral hemodynamic response to CO2 after severe head injury: clinical and prognostic implications

OBJECTIVE

To study the cerebrovascular reactivity to CO2 after severe head injury to establish the clinical and prognostic relevance of CO2 reactivity.

METHODS

Cerebrovascular reactivity to CO2 was studied in 20 patients with severe head injuries at 3.0+/-1.8 days after trauma onset. Two cerebral blood flow studies were performed to measure CO2 reactivity: the first study in a condition of normocapnia and the second study in a condition of relative hypocapnia.

RESULTS

Global reactivity was superimposable to that found in awake, normocapnic subjects and did not correlate with age and Glasgow Coma Scale score but was dependent on the type of brain lesion. Moreover, reactivity correlated with outcome in patients studied after the first 3 days after trauma.

CONCLUSIONS

Our data suggest that cerebrovascular reactivity is (a) almost preserved after a severe head injury; (b) significantly influenced by type of brain lesion; (c) prognostically relevant only in patients studied after the first 3 days after trauma.

On the effect of calcium antagonists on cerebral blood flow in rats. A comparison of nimodipine and flunarizine

To assess the influence of nimodipine treatment in brain tissue at different levels of blood pressure, we estimated the cerebral blood flow using hydrogen clearance. Rats were treated with nimodipine (n = 8), its placebo (n = 10), flunarizine (n = 11) and its placebo (n = 10), and a group of controls (n = 10). Cerebral blood flow was estimated during arterial normo-, hyper- and hypotension. The lowest cerebral blood flow estimates calculated for nimodipine were 43.8 +/- 7.8, 90.9 +/- 13.3, and 33.6 +/- 6.1 ml/min/100 g for normo-, hyper- and hypotension, respectively. Cerebral blood flow in the nimodipine placebo group was 84.1 +/- 10.3, 139.9 +/- 19.9, and 55.2 +/- 10.5 ml/min/100 g. In the flunarizine group, the blood flow was 77.3 +/- 15.2, 144.7 +/- 15.0, and 43.8 +/- 5.9 ml/min/100 g. In the control group, cerebral blood flow was 90.0 +/- 29.1, 143.0 +/- 42.1, and 75.5 +/- 29.8 ml/min/100 g. The low blood flow in the nimodipine group might have been a consequence of brain edema caused by extravasates. Thus impaired blood flow reduces the usefulness of nimodipine in the prevention of vasospasm. Flunarizine is a potential alternative treatment of vasospasm treatment as well as for cerebral blood flow improvement, as shown in our experimental study.

Characterization of cerebral hemodynamic phases following severe head trauma: hypoperfusion, hyperemia, and vasospasm

The extent and timing of posttraumatic cerebral hemodynamic disturbances have significant implications for the monitoring and treatment of patients with head injury. This prospective study of cerebral blood flow (CBF) (measured using 133Xe clearance) and transcranial Doppler (TCD) measurements in 125 patients with severe head trauma has defined three distinct hemodynamic phases during the first 2 weeks after injury. The phases are further characterized by measurements of cerebral arteriovenous oxygen difference (AVDO[2]) and cerebral metabolic rate of oxygen (CMRO[2]). Phase I (hypoperfusion phase) occurs on the day of injury (Day 0) and is defined by a low CBF calculated from cerebral clearance curves integrated to 15 minutes (mean CBF 32.3 +/- 2 ml/100 g/minute), normal middle cerebral artery (MCA) velocity (mean V[MCA] 56.7 +/- 2.9 cm/second), normal hemispheric index ([HI], mean HI 1.67 +/- 0.11), and normal AVDO(2) (mean AVDO[2] 5.4 +/- 0.5 vol%). The CMRO, is approximately 50% of normal (mean CMRO(2) 1.77 +/- 0.18 ml/100 g/minute) during this phase and remains depressed during the second and third phases. In Phase II (hyperemia phase, Days 1-3), CBF increases (46.8 +/- 3 ml/100 g/minute), AVDO(2) falls (3.8 +/- 0.1 vol%), V(MCA) rises (86 +/- 3.7 cm/second), and the HI remains less than 3 (2.41 +/- 0.1). In Phase III (vasospasm phase, Days 4-15), there is a fall in CBF (35.7 +/- 3.8 ml/100 g/minute), a further increase in V(MCA) (96.7 +/- 6.3 cm/second), and a pronounced rise in the HI (2.87 +/- 0.22). This is the first study in which CBF, metabolic, and TCD measurements are combined to define the characteristics and time courses of, and to suggest etiological factors for, the distinct cerebral hemodynamic phases that occur after severe craniocerebral trauma. This research is consistent with and builds on the findings of previous investigations and may provide a useful temporal framework for the organization of existing knowledge regarding posttraumatic cerebrovascular and metabolic pathophysiology.

Characterization of cerebral hemodynamic phases following severe head trauma: hypoperfusion, hyperemia, and vasospasm

The extent and timing of posttraumatic cerebral hemodynamic disturbances have significant implications for the monitoring and treatment of patients with head injury. This prospective study of cerebral blood flow (CBF) (measured using 133Xe clearance) and transcranial Doppler (TCD) measurements in 125 patients with severe head trauma has defined three distinct hemodynamic phases during the first 2 weeks after injury. The phases are further characterized by measurements of cerebral arteriovenous oxygen difference (AVDO2) and cerebral metabolic rate of oxygen (CMRO2). Phase I (hypoperfusion phase) occurs on the day of injury (Day 0) and is defined by a low CBF15 calculated from cerebral clearance curves integrated to 15 minutes (mean CBF15 32.3 ± 2 ml/100 g/minute), normal middle cerebral artery (MCA) velocity (mean VMCA 56.7 ± 2.9 cm/second), normal hemispheric index (mean HI 1.67 ± 0.11), and normal AVDO2 (mean AVDO2 5.4 ± 0.5 vol%). The CMRO2 is approximately 50% of normal (mean CMRO2 1.77 ± 0.18 ml/100 g/minute) during this phase and remains depressed during the second and third phases. In Phase II (hyperemia phase, Days 1-3), CBF increases (46.8 ± 3 ml/100 g/minute), AVDO2 falls (3.8 ± 0.1 vol%), VMCA velocity rises (86 ± 3.7 cm/second), and the HI remains less than 3 (2.41 ± 0.1). In Phase III (vasospasm phase, Days 4-15), there is a fall in CBF (35.7 ± 3.8 ml/100 g/minute), a further increase in VMCA (96.7 ± 6.3 cm/second), and a pronounced rise in the HI (2.87 ± 0.22).

This is the first study in which CBF, metabolic, and TCD measurements are combined to define the characteristics and time courses of, and to suggest etiological factors for, the distinct cerebral hemodynamic phases that occur after severe craniocerebral trauma. This research is consistent with and builds on the findings of previous investigations and may provide a useful temporal framework for the organization of existing knowledge regarding posttraumatic cerebrovascular and metabolic pathophysiology.

Cerebral hemodynamic and metabolic changes caused by brain retraction after aneurysmal subarachnoid hemorrhage

OBJECTIVE

The cerebral hemodynamic and metabolic effects of aneurysmal subarachnoid hemorrhage are complex. To investigate the impact of surgical retraction, we analyzed position emission tomography (PET) studies that measured the regional cerebral metabolic rate for oxygen, regional oxygen extraction fraction, and regional cerebral blood flow in four patients before and after right frontotemporal craniotomies for clipping of ruptured anterior circulation aneurysms.

METHODS

Preoperative studies were conducted 1 day before surgery and postoperative studies 6 to 17 days after surgery. No patient had hydrocephalus or intracerebral hematoma. At the time of the second PET study, none of the patients had signs of clinical vasospasm. Regional measurements were obtained from the right ventrolateral frontal and anterior temporal regions corresponding to the area of retraction and compared to the same regions in the opposite hemisphere. To establish a quantitative means to differentiate between hemodynamic and metabolic changes related to arterial vasospasm and those caused by brain retraction, we studied a second group of preoperative patients, who had undergone PET during angiographic and clinical vasospasm.

RESULTS

There was a 45% reduction in regional cerebral metabolic rate for oxygen (1.87 +/- 0.22 to 1.04 +/- 0.28 ml 100 g-1 min-1) and 32% reduction in regional oxygen extraction fraction (0.41 +/- 0.04 to 0.28 +/- 0.03) in the region of retraction but no change in the opposite hemisphere (paired t test; P = 0.042 and 0.003, respectively). There was no change in regional cerebral blood flow in any region. Brain retraction produced a focal area of tissue injury at the site of retractor blade placement, as compared to more diffuse vascular territory changes produced by vasospasm.

CONCLUSION

This reduction in the cerebral metabolic rate of oxygen and the oxygen extraction fraction indicates a primary reduction in metabolism and uncoupling of flow and metabolism (luxury perfusion). Similar findings of luxury perfusion have been reported after ischemic stroke and traumatic brain injury. Further studies will be necessary to fully understand the clinical and pathophysiological significance of these observations.

The use of hyperventilation and its impact on cerebral ischemia in the treatment of traumatic brain injury

Traumatic brain injury is a common occurrence in the United States, leading to approximately 190,000 deaths or long-term disabilities. Following the primary insult, secondary disturbances in cerebral blood flow (CBF) and metabolism may have deleterious effects on potentially viable neurons. Recent studies evaluating CBF immediately following head injury have revealed flows low enough to produce cerebral ischemia. Hyperventilation is used routinely to lower suspected increased intracranial pressure (ICP). Aggressive hyperventilation produces a marked reduction in CBF, which may give rise to or exacerbate cerebral ischemia, thus enhancing rather than reducing secondary injury. This article reviews the role of hyperventilation in the treatment of increased ICP and its impact on cerebral ischemia following traumatic brain injury.

Estimation of cerebral blood flow at bedside by continuous jugular thermodilution

The Kety-Schmidt technique can be regarded as the reference method for the measurement of cerebral blood flow (CBF). However, the method is somewhat cumbersome for routine use in the intensive care unit (ICU) at the beside. The continuous thermodilution technique developed many years ago for the measurement of coronary sinus blood flow can be applied for the measurement of jugular blood flow (JBF). However, the measurement of JBF by thermodilution has never been validated using the Kety-Schmidt reference method. We first validate the continuous thermodilution in vitro by comparison with a volumetric flow. The thermodilution method is accurate for flows between 50 and 900 ml min-1 with a mean difference volumetric-thermodilution flow of -1 +/- 18 ml min-1 (mean +/- SD), and precise with a coefficient of variability ranging between 1.21% and 2.50%. In vivo accuracy was assessed by comparing in 15 comatose patients CBF measured using the Kety-Schmidt (CBFKS) method and estimated from JBF measured by thermodilution (CBFTH) at four levels of arterial PaCO2 (25, 30, 35, and 40 mm Hg). The mean difference CBFKS-CBFTH is -0.9 +/- 3.6 ml min-1 100 g-1. In vivo precision of the method was good, with a coefficient of variability of 4.1% in mean. We conclude that jugular continuous thermodilution technique is a reliable method for estimating CBF at the bedside. This technique allows repeated measurements jugular bulb blood sampling for brain metabolic studies.

Hyperemia following traumatic brain injury: relationship to intracranial hypertension and outcome

The role of posttraumatic hyperemia in the development of raised intracranial pressure (ICP) has important pathophysiological and therapeutic implications. To determine the relationship between hyperemia (cerebral blood flow (CBF) > 55 ml/100 g/minute), intracranial hypertension (ICP > 20 mm Hg), and neurological outcome, 193 simultaneous measurements of ICP and CBF (xenon-133 method) were obtained in 59 patients with moderate and severe head injury. Hyperemia was associated with an increased incidence of simultaneous intracranial hypertension compared to nonhyperemic CBF measurements (32.2% vs. 21.6%, respectively; p < 0.059). However, in 78% of blood flow studies in which ICP was greater than 20 mm Hg, CBF was less than or equal to 55 ml/100 g/minute. At least one episode of hyperemia was documented in 34% of patients, all of whom had a Glasgow Coma Scale (GCS) score of 9 or below. In 12 individuals with hyperemia without simultaneous intracranial hypertension, ICP was greater than 20 mm Hg for an average of 11 +/- 16 hours and favorable outcomes were seen in 75% of patients. In contrast, in eight individuals with hyperemia and at least one episode of hyperemia-associated intracranial hypertension, ICP was greater than 20 mm Hg for an average of 148 +/- 84 hours (p < 0.001), and a favorable outcome was seen in only one patient (p < 0.001). Compared to the remainder of the cohort, patients with hyperemia-associated intracranial hypertension were distinctive in being the youngest, exhibiting the lowest GCS scores (all < or = 6), and having the highest incidence of effaced basilar cisterns and intractable intracranial hypertension. In the majority of individuals with hyperemia-associated intracranial hypertension, their clinical profile suggests the occurrence of a severe initial insult with resultant gross impairment of metabolic vasoreactivity and pressure autoregulation. In a minority of these patients, however, high CBF may be coupled to a hypermetabolic state, given their responsiveness to metabolic suppressive therapy. In patients with hyperemia but without intracranial hypertension, elevated CBF is also likely to be a manifestation of appropriate coupling to increased metabolic demand consistent with a generally favorable outcome. This study supports the concept that there are multiple etiologies of both elevated blood flow and intracranial hypertension after head injury.

Cerebral blood flow and vasoresponsivity within and around cerebral contusions

There is increasing evidence that regional ischemia plays a major role in secondary brain injury. Although the cortex underlying subdural hematomas seems particularly vulnerable to ischemia, little is known about the adequacy of cerebral blood flow (CBF) or the vasoresponsivity within the vascular bed of contusions. The authors used the xenon-enhanced computerized tomography (CT) CBF technique to define the CBF and vasoresponsivity of contusions, pericontusional parenchyma, and the remainder of the brain 24 to 48 hours after severe closed head injury in 10 patients: six patients with one contusion and four with two contusions, defined as mixed or high-density lesions on CT scanning. The CBF within the contusions (29.3 +/- 16.4 ml/100 g/minute, mean +/- standard deviation) was significantly lower than both that found in the adjacent 1-cm perimeter of normal-appearing tissue (42.5 +/- 15.8 ml/100 g/minute) and the mean global CBF (52.5 +/- 17.5 ml/100 g/minute) (p < 0.004, repeated-measures analysis of variance). A subset of seven patients (10 contusions) also underwent a second Xe-CT CBF study during mild hyperventilation (a PaCO2 of 24-32 mm Hg). In only two of these 10 contusions was vasoresponsivity less than 1% (range 0%-7.6%); in the rim of normal-appearing pericontusional tissue, it was 0.4% to 9.1%. The authors conclude that CBF within intracerebral contusions is highly variable and is often above 18 ml/100 g/minute, the reported threshold for irreversible ischemia. Intracontusional CBF is significantly reduced relative to surrounding brain parenchyma, and CO2 vasoresponsivity is usually present. In the contusion and the surrounding parenchyma, vasoresponsivity may be nearly three times normal, suggesting hypersensitivity to hyperventilation therapy. Given this possible hypersensitivity and relative hypoperfusion within and around cerebral contusions, these lesions are particularly vulnerable to secondary injury such as that which may be caused by hypotension or aggressive hyperventilation.

Widespread hemodynamic depression and focal platelet accumulation after fluid percussion brain injury: a double-label autoradiographic study in rats

Cerebrovascular damage leading to subsequent reductions in local cerebral blood flow (lCBF) may represent an important secondary injury mechanism following traumatic brain injury (TBI). We determined whether patterns of 111-indium-labeled platelet accumulation were spatially related to alterations in lCBF determined autoradiographically 30 min after TBI. Sprague-Dawley rats (n = 8), anesthetized with halothane and maintained on a 70:30 (vol/vol) mixture of nitrous oxide/oxygen and 0.5% halothane, underwent parasagittal fluid percussion brain injury (1.7-2.2 atm). 111-Indium-tropolone-labeled platelets were injected 30 min prior to TBI while [14C]-iodoantipyrine was infused 30 min after trauma. Sham-operated animals (n = 7) underwent similar surgical procedures but were not injured. In autoradiographic images of the indium-labeled platelets, focal sites of platelet accumulation within the traumatized hemisphere were restricted to the pial surface (five of eight rats), the external capsule underlying the lateral parietal cortex (five of eight rats), and within cerebrospinal fluid (CSF) compartments (six of eight rats). In contrast, mild-to-moderate reductions in lCBF, not restricted to sites of platelet accumulation, were seen throughout the traumatized hemisphere. Flow reductions were most severe in coronal sections underlying the impact site. For example, within the lateral parietal cortex and hippocampus, lCBF was significantly reduced [p <0.01; analysis of variance (ANOVA)] from 1.71 +/- 0.34 (mean +/- SD) and 0.78 +/- 0.12 ml/g/min, respectively, versus 0.72 +/- 0.17 and 0.41 +/- 0.06 ml/g/min within the traumatized hemisphere. Significant flow reductions were also seen in remote cortical and subcortical areas, including the right frontal cortex and striatum. These results indicate that focal platelet accumulation and widespread hemodynamic depression are both early consequences of TBI. Therapeutic strategies directed at these early microvascular consequences of TBI may be neuroprotective by attenuating secondary ischemic processes.

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

Autoregulation and CO2-reactivity can be impaired independently of each other in many brain insults, the so-called 'dissociated vasoparalysis'. The theoretical combination of preserved CO2-reactivity and impaired or abolished autoregulation can have many clinical implications in the daily management of brain injured patients. To optimize their treatment, a bedside assessment of autoregulation and CO2-reactivity is desirable. When cerebral metabolic rate of oxygen is constant, changes in arterio-jugular differences of oxygen (AVDO2) reflect changes in CBF. In these situations relative changes in AVDO2 can be viewed as inverse changes in CBF and used as an evaluation method of CO2-reactivity and autoregulation. In 39 consecutive severe head injury patients with a mean age of 28 +/- 17 years and a diffuse brain injury, cerebrovascular response to changes in pCO2 was tested in the acute phase after injury (18 +/- 8 hours). In 28 of those cases autoregulation was also assessed. A relative CBF value (1/AVDO2) was calculated from baseline AVDO2 and was expressed as 100%. Changes in 1/AVDO2 after inducing pCO2 changes give a good estimate of changes in global CBF. Two different indexes were calculated for CO2-reactivity: 1) absolute CO2-reactivity (CO2RABS) and 2) percentage reactivity (CO2R%). CO2R% was used to separate patients with impaired/abolished CO2-reactivity from those with preserved CO2-reactivity. Patients with CO2R% above 1% were considered in the intact CO2-reactivity group and patients in whom CO2R% was below or equal to 1% were included in the impaired/abolished CO2-reactivity group. Only five cases (12.8%) presented an impaired/abolished CO2-reactivity. AVDO2 response to induced hypertension was studied in a subset of 28 patients. Phenylephrine was used to increase MABP about 25%. All AVDO2 values were corrected for changes in pCO2. Patients with changes in 1/AVDO2 less than or equal to 20% were included in the intact autoregulation group. Patients with estimated CBF changes above 20% were classified as having an impaired autoregulation (impaired/abolished). In 12 patients (43%) autoregulation was intact. In the remaining 16 patients (57%) autoregulation was imparied. Of the 28 cases, CO2-reactivity was impaired in only five cases. All patients with an impaired CO2-reactivity also had an impaired autoregulation. Monitoring relative changes in AVDO2 permits a reliable study of CO2-reactivity and autoregulation at the bedside. Introducing these variables into the day-to-day management should be considered in treatment protocols.

Effect of hyperventilation, mannitol, and ventriculostomy drainage on cerebral blood flow after head injury

Therapies to lower intracranial pressure (ICP) after traumatic brain injury (TBI) include hyperventilation (HV), intravenous mannitol (IM), and cerebrospinal fluid drainage from a ventriculostomy (DV). To determine the effects of these therapies on cerebral blood flow (CBF), fiberoptic oximetry was used to measure jugular venous O2 saturation (SjvO2) as an index of the CBF to cerebral metabolic rate for O2 (CMRO2) ratio after IM (25 g IV for more than 5 min), DV (3 min), or HV (increase respiratory rate by 4) therapy for elevated ICP. Assuming CMRO2 is constant, changes in SjvO2 reflect changes in CBF. Continuous measurements of SjvO2, ICP, blood pressure, arterial O2 saturation, and end-tidal CO2 were obtained in 22 patients with a Glasgow Coma Scale score of 5.3 +/- 0.4 (mean +/- SD) in the first 5 days after TBI. Therapy was initiated a total of 196 times when ICP was > 15 mm Hg for > 5 minutes, and measurements made at 20 minutes after treatment were compared with those made just before. After DV, ICP fell in 90% of the observations by 8.6 +/- 0.7 mm Hg (mean +/- SEM, n = 119); after IM, ICP fell in 90% of the observations by 7.4 +/- 0.7 mm Hg (n = 43); and after HV, ICP fell in 88% of the observations by 6.3 +/- 1.2 mm Hg (n = 14). In patients where ICP fell, SjvO2 increased by 2.49 +/- 0.7% saturation (from 68.0 +/- 1.3%) with IM, but only by 0.39 +/- 0.4% saturation (from 67.2 +/- 0.9%) with DV.(ABSTRACT TRUNCATED AT 250 WORDS)

Focal cerebral hyperemia after focal head injury in humans: a benign phenomenon?

To assess the relationship between posttraumatic cerebral hyperemia and focal cerebral damage, the authors performed cerebral blood flow mapping studies by single-photon emission computerized tomography (SPECT) in 53 patients within 3 weeks of brain injury. Focal zones of hyperemia were present in 38% of patients. Hyperemia was correlated with clinical features and early computerized tomography (CT) and magnetic resonance (MR) imaging performed within 48 hours of the SPECT study and late CT and MR studies at 3 months. The hyperemia was observed primarily in structurally normal brain tissue (both gray and white matter), as revealed by CT and MR imaging, immediately adjacent to intraparenchymal or extracerebral focal lesions; it persisted for up to 10 days, but was never seen within the edematous pericontusional zones. The percentage of patients in the hyperemic group having brief (< 30 minutes) or no loss of consciousness was significantly higher than in the nonhyperemic group (twice as high, p < 0.05). Other clinical parameters were not significantly more common in the hyperemic group. The mortality of patients with focal hyperemia was lower than that of individuals without it, and the outcome of survivors with hyperemia was slightly better than patients without hyperemia. These results differ from the literature, which suggests that global post-traumatic hyperemia is primarily an acute, malignant phenomenon associated with increased intracranial pressure, profound unconsciousness, and poor outcome. The current results agree with more recent studies which show that posttraumatic hyperemia may occur across a wide spectrum of head injury severity and may be associated with favorable outcome.

Continuous regional cerebral cortical blood flow monitoring in head-injured patients

Continuous regional cerebral cortical blood flow (rCoBF) was monitored with thermal diffusion flowmetry in 56 severely head-injured patients. Adequate, reliable data were accumulated from 37 patients (21 acute subdural hematomas, 10 cerebral contusions, 4 epidural hematomas, and 2 intracerebral hematomas). The thermal sensor was placed at the time of either craniotomy or burr hole placement. In 15 patients, monitoring was initiated within 8 hours of injury. One-third of the comatose patients monitored within 8 hours had rCoBF measurements of 18 ml per 100 g per minute or less, consistent with previous reports of significant ischemia in the early postinjury period. Initial rCoBF measurements were similar in the patients with Glasgow Coma Scale scores of 3 to 7 and in those with scores of 8 or greater. In patients with poor outcomes, rCoBF measurements did not change significantly from initial measurements; however, in those patients who had better outcomes, final rCoBF measurements were higher than initial rCoBF measurements. The patients who had better outcomes experienced normalization of rCoBF during the period of monitoring, and patients with poor outcomes had markedly reduced final rCoBF. These changes were statistically significant. When management was based strictly upon the intracranial pressure, examples of inappropriate treatment were found. For example, hyperemia and increased intracranial pressure treated with mannitol caused further rCoBF increase, and elevated intracranial pressure with low cerebral blood flow treated with hyperventilation increased the severity of ischemia. In 3 (5%) of 56 patients, wound infections developed. Continuous rCoBF monitoring in head-injured patients offers new therapeutic and prognostic insights into their management.