Autoregulation and CO2 responses of cerebral blood flow in patients with acute severe head injury

Acute alterations of endothelin-1 and iNOS expression and control of the brain microcirculation after head trauma

The biosynthetic equilibrium between endothelin-1 (ET-1, a vasoconstricting agent) and nitric oxide (NO, a gas with vasodilating effects) is thought to play a role in the autoregulation of microvessel contractility and maintenance of adequate perfusion after traumatic brain injury. ET-1 is a constitutively expressed peptide, while the gene that encodes for the inducible nitric oxide synthase (iNOS, an enzyme responsible for the synthesis of excessive and toxic amounts of NO) is solely activated after brain injury. We employed the Marmarou acceleration impact model of brain injury (400 g from 2 m) to study the effect of closed head trauma on the rat brain microcirculation. Following head trauma we analyzed changes of cerebral cortex perfusion using laser Doppler flowmetry and ultrastructural alterations of endothelial cells. We temporally correlated these changes with the expression of ET-1 (immunocytochemistry) and iNOS (in situ hybridization) to assess the role of these vasoactive agents in vascular contractility and cortical perfusion. Cortical perfusion was reduced by approximately 50% during the second hour as compared to values during preceding time points after TBI, reached a peak minutes before 3 h, and subsequently showed a trend towards normalization. A significant reduction in the lumen of microvessels and severe distortion of their shape were observed after the fourth hour post-trauma. At the same time period ET-1 expression in endothelial cells was stronger than in microvessels of control animals. ET-1 expression was further increased at 24 h after TBI. iNOS mRNA synthesis was strongly upregulated in the same cells at 4 h but was undetectable at 24 h post trauma. Our combined functional, cellular and molecular approach supports the notion that ET-1 and iNOS are expressed differentially in time within individual endothelial cells of cortical microvessels for the control of cortical blood flow following closed head trauma. This differential expression further indicates a reciprocal interaction in the synthesis of these two molecules which may underlie the control of microvascular autoregulation.

Cerebrovascular reactivity following focal brain ischemia in the rat: a functional magnetic resonance imaging study

An essential goal of stroke research is to identify potentially salvageable regions of brain that may respond to therapy. However, current imaging methods are inadequate for this purpose. We therefore used dynamic magnetic resonance imaging of vascular reactivity following focal occlusion in the rat to determine whether measurement of perfusion reserve would help resolve this problem. We used the increase in blood-oxygen-level-dependent (BOLD) signal that occurs in normal brain following a CO2 challenge, to map vascular reactivity over the brain at 30-min intervals for 3.5 h after complete (CO) or partial (PO) focal ischemia. We assessed the regional correspondence between reactivity changes and areas of lowered apparent diffusion coefficient (ADC) and initial perfusion deficit. The area of lowered ADC was significantly smaller in the PO group compared to the CO group despite similar areas of perfusion deficit (P < 0.05). We identified four distinct areas within hypoperfused brain: a core area with low/absent reactivity and low ADC; borderzone areas with normal reactivity and either reduced ADC (CO group) or normal ADC (PO group); and an area with normal ADC and reduced/absent reactivity. In all ischemic regions, the BOLD peak arrival time in the brain was delayed or absent. There was a negative correlation between BOLD peak latency time and ADC (r = -0.42, P < 0.001), although latency alone did not differentiate individual ischemic regions. In conclusion, combining perfusion, ADC, and vascular reactivity mapping of the ischemic brain enables improved discrimination of core and borderzone regions.

Cerebral hemodynamics during arterial and CO(2) pressure changes: in vivo prediction by a mathematical model

The aim of this work was to analyze changes in cerebral hemodynamics and intracranial pressure (ICP) evoked by mean systemic arterial pressure (SAP) and arterial CO(2) pressure (Pa(CO(2))) challenges in patients with acute brain damage. The study was performed by means of a new simple mathematical model of intracranial hemodynamics, particularly aimed at routine clinical investigation. The model was validated by comparing its results with data from transcranial Doppler velocity in the middle cerebral artery (V(MCA)) and ICP measured in 44 tracings on 13 different patients during mean SAP and Pa(CO(2)) challenges. The validation consisted of individual identification of 6 parameters in all 44 tracings by means of a best fitting algorithm. The parameters chosen for the identification summarize the main aspects of intracranial dynamics, i.e., cerebrospinal fluid circulation, intracranial elastance, and cerebrovascular control. The results suggest that the model is able to reproduce the measured time patterns of V(MCA) and ICP in all 44 tracings by using values for the parameters that lie within the ranges reported in the pathophysiological literature. The meaning of parameter estimates is discussed, and comments on the main virtues and limitations of the present approach are offered.

[Cerebral hemometabolism: variability in the acute phase of traumatic coma]

OBJECTIVE

to evaluate the interrelationships between cerebral and systemic hemometabolic alterations in patients with severe traumatic brain injury managed according to a standardized therapeutic protocol.

DESIGN

prospective, interventional study in patients with traumatic coma.

SETTING

a general Intensive Care Unit in a teaching hospital.

PATIENTS AND METHODS

twenty-seven patients (21M e 6F), aging 14 - 58 years, with severe acute brain trauma, presenting with three to eight points on the Glasgow Coma Scale, were prospectively evaluated according to a cumulative protocol for the management of acute intracranial hypertension, where intracranial pressure (ICP) and cerebral extraction of oxygen (CEO2) were routinely measured. Hemometabolic interrelationships involving mean arterial pressure (MAP), ICP, arterial carbon dioxide tension (PaCO2), CEO2, cerebral perfusion pressure (CPP) and systemic extraction of oxygen (SEO2) were analyzed.

INTERVENTIONS

routine therapeutic procedures.

RESULTS

no correlation was found between CEO2 and CPP (r = -0.07; p = 0.41). There was a significant negative correlation between PaCO2 and CEO2 (r = -0.24; p = 0.005) and a positive correlation between SEO2 and CEO2 (r = 0.24; p = 0.01). The mortality rate in this group of patients was 25.9% (7/27).

CONCLUSION

1) CPP and CEO2 are unrelated; 2) CEO2 and PaCO2 are closely related; 3) during optimized hyperventilation, CEO2 and SEO2 are coupled.

L-arginine partially restores the diminished CO2 reactivity after mild controlled cortical impact injury in the adult rat

Using an open cranial window technique, the authors investigated the mechanisms associated with the suppressed CO2 reactivity after mild controlled cortical impact (CCI) injury in rats. The dilation of arterioles (n = 7) to hypercapnia before injury was 38 +/- 12%, which was significantly reduced both at 1 hour (23 +/- 15% dilation) and at 2 hours after injury (11 +/- 19% dilation). In the presence of L-arginine (10 mmol/L topical suffusion, 300 mg/kg intravenous infusion), the dilation of pial arterioles (n = 6) to hypercapnia was partially restored to 30 +/- 6% at 2 hours after injury. In the presence of the nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP) (10(-8) mol/L topical suffusion), the dilation of pial arterioles (n = 5) to hypercapnia remained diminished (5 +/- 7%) at 2 hours after injury. The dilation of pial arterioles (n = 4) to hypercapnia also remained suppressed (5 +/- 6%) with topical suffusion of the free radical scavengers, polyethylene glycol-superoxide dismutase (60 units/mL) and polyethylene glycol-catalase (40 units/mL). The authors have shown that L-arginine at least partially restores the diminished response to hypercapnia after mild CCI injury. Furthermore, these data suggest that the beneficial effects of L-arginine are mediated by a combination of providing substrate for NO synthase and scavenging free radicals.

Is cerebral perfusion pressure a major determinant of cerebral blood flow during head elevation in comatose patients with severe intracranial lesions?

OBJECT

Head elevation as a treatment for lower intracranial pressure (ICP) in patients with intracranial hypertension has been challenged in recent years. Therefore, the authors studied the effect of head position on cerebral hemodynamics in patients with severe head injury.

METHODS

The effect of 0 degrees, 15 degrees, 30 degrees, and 45 degrees head elevation on ICP, cerebral blood flow (CBF), systemic arterial (PsaMonro) and jugular bulb (Pj) pressures calibrated to the level of the foramen of Monro, cerebral perfusion pressure (CPP), and the arteriovenous pressure gradient (PsaMonro - Pj) was studied in 37 patients who were comatose due to severe intracranial lesions. The CBF decreased gradually with head elevation from 0 to 45 degrees, from 46.3+/-4.8 to 28.7+/-2.3 ml x min(-1) x 100 g(-1) (mean +/- standard error, p<0.01), and the PsaMonro - Pj from 80+/-3 to 73+/-3 mm Hg (p< 0.01). The CPP remained stable between 0 degrees and 30 degrees of head elevation, at 62+/-3 mm Hg, and decreased from 62+/-3 to 57+/-4 mm Hg between 30 degrees and 45 degrees (p<0.05). A simulation showed that the 38% decrease in CBF between 0 degrees and 45 degrees resulted from PsaMonro - Pj changes for 19% of the decrease, from a diversion of the venous drainage from the internal jugular veins to vertebral venous plexus for 15%, and from CPP changes for 4%.

CONCLUSIONS

During head elevation the arteriovenous pressure gradient is the major determinant of CBF. The influence of CPP on CBF decreases from 0 to 45 degrees of head elevation.

Regional cerebral blood flow after cortical impact injury complicated by a secondary insult in rats

BACKGROUND AND PURPOSE

Traumatic injury makes the brain susceptible to secondary insults. An uncomplicated mild lateral cortical impact injury (3 m/s, 2.5-mm deformation) that causes little or no permanent sequelae results in a large contusion at the impact site when the traumatic injury is complicated by a secondary insult, such as 40 minutes of bilateral carotid occlusion.

METHODS

To determine whether the increased sensitivity to secondary insults in this model is caused by a vascular mechanism, cerebral blood flow (CBF) was measured with (14)C-isopropyliodoamphetamine quantitative autoradiography, and brain tissue PO(2) (PbtO(2)) was measured at the impact site and in the contralateral parietal cortex.

RESULTS

In animals that underwent bilateral carotid occlusion 1 hour after the impact injury, CBF and PbtO(2) were lower at the impact site than they were in animals that had either the impact injury or the carotid occlusion alone. In the immediate area of the impact, CBF was 14+/-6 mL. 100 g(-1). min(-1) in the animals with the impact injury followed by carotid occlusion compared with 53+/-24 mL. 100 g(-1). min(-1) in the animals with the impact injury alone and 74+/-14 mL. 100 g(-1). min(-1) in the animals with the carotid occlusion alone (P<0.001). At the time of this very low CBF value in the animals with the carotid occlusion after the impact injury, PbtO(2) at the impact site averaged 1.3+/-1.6 mm Hg and was <3 mm Hg in 5 of the 6 animals. In contrast, PbtO(2) in the animals with the impact injury alone averaged 9.3+/-2.9 mm Hg, and none of the animals had a PbtO(2) of <3 mm Hg (P=0.008).

CONCLUSIONS

The CBF and PbtO(2) findings in this model suggest that the reduced CBF after traumatic injury predisposes the brain to secondary insults and results in ischemia when confronted with a reduction in cerebral perfusion pressure.

Cerebrovenous blood temperature-influence of cerebral perfusion pressure changes and hyperventilation: evaluation in a porcine study and in man

The objective of the first part of this study was to use an animal model to investigate the relationship between temperature in the cerebrovenous compartment and cerebral perfusion pressure. In the second part of the study, the objective was to examine the influence of hyperventilation and hypothermia on jugular bulb temperature and body temperature in patients undergoing elective neurosurgery. Intracranial pressure was increased artificially by inflating an infratentorial supracerebellar placed balloon catheter in nine pigs under general anesthesia. Temperature was monitored by thermocouples inserted in the sagittal sinus, white matter of the left lobe and abdominal aorta during the ensuing decrease in cerebral profusion pressure (CPP). Cerebrovenous blood temperature (jugular bulb) and body temperature (urinary bladder) were simultaneously monitored in 24 patients undergoing craniotomy. Moderate hyperventilation was performed in all patients. Cerebrovenous blood and core body temperature were recorded and differences between these two temperatures calculated at the beginning and the end of hyperventilation. At the beginning of the intracranial pressure (ICP), increase mean temperatures of cerebrovenous blood and cerebral tissue (left lobe) were lower than core body temperature. During CPP reduction the difference between core body temperature and cerebrovenous blood temperature increased significantly from 0.86+/-0.44 degrees C prior to ICP rise to 1.19+/-0.58 degrees C at maximum ICP. Before hyperventilation, cerebrovenous blood temperature was higher in 19 patients (+/- difference: 0.34 degrees C +/- 0.27) and equal or lower in five patients (difference: -0.08 degrees C +/- 0.11), than core body temperature. At the end of hyperventilation, the difference between cerebrovenous blood temperature and core body temperature increased (+0.42 degrees C +/- 0.24) in those 19 patients who had started with a higher cerebrovenous blood temperature and decreased (-0.10 degrees C +/- 0. 18) in the other five patients. Both studies demonstrated that the temperature of cerebrovenous blood is influenced by maneuvers which are supposed to decrease cerebral blood flow.

Traumatic brain injury reduces myogenic responses in pressurized rodent middle cerebral arteries

Traumatic brain injury (TBI) reduces cerebral vascular pressure autoregulation in experimental animals and in patients. In order to understand better the mechanisms of impaired autoregulation, we measured myogenic responses to changes in intraluminal pressure in vitro in pressurized, rodent middle cerebral arteries (MCAs) harvested after TBI. In an approved study, male Sprague-Dawley rats (275-400 g) were anesthetized, intubated, ventilated with 2.0% isoflurane in O2/air, and prepared for fluid percussion TBI. The isoflurane concentration was reduced to 1.5%, and rats (n = 6 per group) were randomly assigned to receive sham TBI followed by decapitation 5 or 30 min later or moderate TBI (2.0 atm) followed by decapitation 5 or 30 min later. After decapitation, MCA segments were removed, mounted on an arteriograph, and pressurized. MCA diameters were measured as transmural pressure was sequentially reduced. MCA diameters remained constant or increased in the sham groups as intraluminal pressure was reduced from 100 to 40 mm Hg. In both TBI groups, diameter decreased with each reduction in pressure. In summary, MCAs removed from uninjured, isoflurane-anesthetized rats had normal vasodilatory responses to decreased intraluminal pressure. In contrast, after TBI, myogenic vasodilatory responses were significantly reduced within 5 min of TBI and the impaired myogenic responses persisted for at least 30 min after TBI.

Prevention of secondary ischemic insults after severe head injury

OBJECTIVE

The purpose of this study was to compare the effects of two acute-care management strategies on the frequency of jugular venous desaturation and refractory intracranial hypertension and on long-term neurologic outcome in patients with severe head injury.

DESIGN

Randomized clinical trial.

SETTING

Level I trauma hospital.

PATIENTS

One hundred eighty-nine adults admitted in coma because of severe head injury.

INTERVENTIONS

Patients were assigned to either cerebral blood flow (CBF)-targeted or intracranial pressure (ICP)-targeted management protocols during randomly assigned time blocks. In the CBF-targeted protocol, cerebral perfusion pressure was kept at >70 mm Hg and PaCO2 was kept at approximately 35 torr (4.67 kPa). In the ICP-targeted protocol, cerebral perfusion pressure was kept at >50 mm Hg and hyperventilation to a PaCO2 of 25-30 torr (3.33-4.00 kPa) was used to treat intracranial hypertension.

MEASUREMENTS AND MAIN RESULTS

The CBF-targeted protocol reduced the frequency of jugular desaturation from 50.6% to 30% (p = .006). Even when the frequency of jugular desaturation was adjusted for all confounding factors that were significant, the risk of cerebral ischemia was 2.4-fold greater with the ICP-targeted protocol. Despite the reduction in secondary ischemic insults, there was no difference in neurologic outcome. Failure to alter long-term neurologic outcome was probably attributable to two major factors. A low jugular venous oxygen saturation was treated in both groups, minimizing the injury that occurred in the ICP-targeted group. The beneficial effects of the CBF-targeted protocol may have been offset by a five-fold increase in the frequency of adult respiratory distress syndrome.

CONCLUSIONS

Secondary ischemic insults caused by systemic factors after severe head injury can be prevented with a targeted management protocol. However, potential adverse effects of this management strategy may offset these beneficial effects.

Cerebrovascular reactivity to CO(2) and hypotension after mild cortical impact injury

Cerebrovascular reactivity to CO(2) or hypotension was studied in vivo and in vitro [pressurized arteries ( approximately 82 micrometer) and arterioles ( approximately 30 micrometer)] at 1 h after mild controlled cortical impact (CCI) injury in rats. The cortical perfusion response [assessed using laser-Doppler flowmetry (LDF)] to altered CO(2) was diminished (up to 81%) after mild CCI injury. The responses to CO(2) alterations in arteries and arterioles isolated from the injured cortex were similar to responses in vessels isolated from sham-injured animals. After mild CCI injury, the autoregulatory response to hypotension (measured using LDF) was maintained or even enhanced, depending on the method used to measure the response. Vessels isolated from the injury site showed a response to changes in pressure similar to that in vessels isolated from sham-injured rats. We conclude that mild CCI injury produces complicated alterations in cerebrovascular control. Whereas the autoregulatory response to hypotension was maintained or even enhanced, the in vivo vascular response to CO(2) was severely compromised. The altered response to CO(2) was not caused by an intrinsic vascular perturbation but rather an altered milieu after mild CCI injury.

The consequences of traumatic brain injury on cerebral blood flow and autoregulation: a review

In this decade, the brain argueably stands as one of the most exciting and challenging organs to study. Exciting in as far as that it remains an area of research vastly unknown and challenging due to the very nature of its anatomical design: the skull provides a formidable barrier and direct observations of intraparenchymal function in vivo are impractical. Moreover, traumatic brain injury (TBI) brings with it added complexities and nuances. The development of irreversible damage following TBI involves a plethora of biochemical events, including impairment of the cerebral vasculature, which render the brain at risk to secondary insults such as ischemia and intracranial hypertension. The present review will focus on alterations in the cerebrovasculature following TBI, and more specifically on changes in cerebral blood flow (CBF), mediators of CBF including local chemical mediators such as K+, pH and adenosine, endothelial mediators such as nitric oxide and neurogenic mediators such as catecholamines, as well as pressure autoregulation. It is emphasized that further research into these mechanisms may help attenuate the prevalence of secondary insults and therefore improve outcome following TBI.

Temporal effect of severe controlled cortical impact injury in the rat on the myogenic response of the middle cerebral artery

The present study examined the effect of severe traumatic brain injury (TBI) on the myogenic response in the rat middle cerebral artery (MCA). Rats were subjected to severe controlled cortical impact (CCI; 5 m/sec, 130-msec duration, 3-mm deformation) injury over the right parietal cortex. At 2, 24, and 120 h postinjury, ipsilateral and contralateral segments of MCAs were isolated, mounted in a vessel chamber, and pressurized. After equilibration, the myogenic tone, the difference in vessel diameter in the presence and absence of calcium for a given pressure, and the myogenic response (the active contractile response elicited by a vessel to increasing pressure), were measured. At 24 h postsurgery, there was a significant interaction between myogenic tone and pressure in the ipsilateral and contralateral MCAs when TBI was compared to shams. However, this was not apparent, at the 2- and 120-h time points. At 2- and 24-h postsurgery, there was a significant interaction between myogenic response and pressure in the ipsilateral MCAs when TBI was compared to shams. While the response of the vascular smooth muscle was altered following injury, it was still functioning, suggesting that these vessels compensate, perhaps through alternate mechanisms or by relying on those remaining intact mechanisms.

Continuous monitoring of cerebrovascular pressure-reactivity in head injury

OBJECTIVE

Cerebrovascular vasomotor reactivity reflects changes in smooth muscle tone in the arterial wall in response to changes in transmural pressure or concentration of carbon dioxide in blood. We have investigated whether slow waves in ABP and ICP may be used to derive an index which reflects reactivity of vessels to changes in arterial blood pressure.

METHODS

A method for the continuous monitoring of the association between slow spontaneous waves in ICP and AP has been adopted in a group of 98 head injured patients. ABP, ICP and transcranial Doppler blood flow velocity (FV) in the middle cerebral artery was recorded daily (20 to 120 minutes time periods). A Pressure-Reactivity Index (PRx) was calculated as a moving correlation coefficient between 40 consecutive samples of values for ICP and ABP averaged over 5 seconds. A moving correlation coefficient between spontaneous fluctuations of mean FV and CPP (Mx), which was previously reported to describe cerebral blood flow autoregulation, was also calculated. In an additional 25 patients, PRx was calculated and recorded continuously along with mean ICP, ABP and parameters describing ICP waveform.

RESULTS

A positive PRx correlated with high ICP (r = 0.366; p < 0.001), low admission GCS (r = 0.29; p < 0.01), and poor outcome at 6 months after injury (r = 0.48; p < 0.00001). During the first two days following injury, PRx was positive (p < 0.05) in patients with unfavourable outcome. The correlation between PRx and Mx (r = 0.63) was highly significant (p < 0.000001). Continuous recordings demonstrated that PRx was able to indicate individual thresholds of vascular reactivity for CPP, ICP, and ventilation parameters.

CONCLUSION

Computer analysis of slow waves in ABP and ICP is able to provide a continuous index of cerebrovascular reactivity to changes in arterial pressure, which is of prognostic significance.

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

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

Evaluation of cerebrovascular CO2-reactivity and autoregulation in patients with post-traumatic diffuse brain swelling (diffuse injury III)

The present study was undertaken to elucidate the status of autoregulation and CO2-reactivity soon after injury in patients with a post-traumatic diffuse bilateral brain swelling. A prospective study was carried out in 31 consecutively admitted patients with a severe head injury and a Diffuse Brain Injury type III, following the definition stated by the Traumatic Coma Data Bank classification. To evaluate CO2-reactivity, AVDO2 was measured before and after ventilator manipulations. Assuming a constant CMRO2 during the test, changes in 1/AVDO2 reflect changes in CBF. Patients with changes in estimated CBF below or equal to 1% were included in the impaired/abolished CO2-reactivity group. To test autoregulation, hypertension was induced using phenylephrine. Arterial and jugular blood samples were taken to calculate AVDO2 before and after a steady state of MABP was obtained. Cerebrovascular response to CO2 was globally preserved in all but two cases (6.5%). In contrast, autoregulation was globally preserved in 10 (32.3%) and impaired/abolished in 21 cases (67.7%). Our data do not support the premise that increasing cerebral perfusion pressure by inducing arterial hypertension is beneficial in those patients with a diffuse brain swelling in whom autoregulation is impaired or abolished. Clinical implications for treatment are discussed.

[The internal environment and intracranial hypertension]

Intracranial pressure depends on cerebral tissue volume, cerebrospinal fluid volume (CSFV) and cerebral blood volume (CBV). Physiologically, their sum is constant (Monro-Kelly equation) and ICP remains stable. When the blood brain barrier (BBB) is intact, the volume of cerebral tissue depends on the osmotic pressure gradient. When it is injured, water movements across the BBB depend on the hydrostatic pressure gradient. CBV depends essentially on cerebral blood flow (CBF), which is strongly regulated by cerebral vascular resistances. In experimental studies, a decrease in oncotic pressure does not increase cerebral oedema and intracranial hypertension (ICHT). On the other hand, plasma hypoosmolarity increases cerebral water content and therefore ICP, if the BBB is intact. If it is injured, neither hypoosmolarity nor hypooncotic pressure modify cerebral oedema. Therefore, all hypotonic solutes may aggravate cerebral oedema and are contra-indicated in case of ICHT. On the other hand, hypooncotic solutes do not modify ICP. The osmotic therapy is one of the most important therapeutic tools for acute ICHT. Mannitol remains the treatment of choice. It acts very quickly. An i.v. perfusion of 0.25 g.kg-1 is administered over 20 minutes when ICP increases. Hypertonic saline solutes act in the same way, however they are not more efficient than mannitol. CO2 is the strongest modulating factor of CBF. Hypocapnia, by inducing cerebral vasoconstriction, decreases CBF and CBV. Hyperventilation is an efficient and rapid means for decreasing ICP. However, it cannot be used systematically without an adapted monitoring, as hypocapnia may aggravate cerebral ischaemia. Hyperthermia is an aggravating factor for ICHT, whereas moderate hypothermia seems to be beneficial both for ICP and cerebral metabolism. Hyperglycaemia has no direct effect on cerebral volume, but it may aggravate ICHT by inducing cerebral lactic acidosis and cytotoxic oedemia. Therefore, infusion of glucose solutes is contra-indicated in the first 24 hours following head trauma and blood glucose concentration must be closely monitored and controlled during ICHT episodes.

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.

Changes in cerebral blood flow during PaCO2 variations in patients with severe closed head injury: comparison between the Fick and transcranial Doppler methods

OBJECT

The aim of this study was to reassess whether middle cerebral artery blood flow velocity (MCAv) variations measured by transcranial Doppler ultrasonography during acute PaCO2 manipulation adequately reflect cerebral blood flow (CBF) changes in patients with severe closed head injury.

METHODS

The study was performed by comparing MCAv variations to changes in CBF as assessed by measurements of the difference in the arteriovenous content in oxygen (AVDO2). The authors initiated 35 CO2 challenges in 12 patients with severe closed head injury during the acute stage. By simultaneous recording of systemic and cerebral hemodynamic parameters, 105 AVDO2 measurements were obtained. Patients were stratified into two groups, "high" and "low," with respect to whether their resting values of MCAv were greater than 100 cm/second during moderate hyperventilation. Four patients displayed an elevated MCAv, which was related to vasospasm in three cases and to hyperemia in one case. The PaCO2 and intracranial pressure levels were not different between the two groups. The slope of the regression line between 1 divided by the change in (delta)AVDO2 and deltaMCAv was not different from identity in the low group (1/deltaAVDO2 = 1.08 x deltaMCAv - 0.07, r = 0.93, p < 0.001) and significantly differed (p < 0.05) from the slope of the high group (1/deltaAVDO2 = 1.46 x deltaMCAv - 0.4, r = 0.83, p < 0.001).

CONCLUSIONS

In patients with severe closed head injury, MCAv variations adequately reflect CBF changes as assessed by AVDO2 measurements in the absence of a baseline increase in MCAv. These observations indicate that both moderate variations in PaCO2 and variations in cerebral perfusion pressure do not act noticeably on the diameter of the MCA. The divergence from the expected relationship in the high group seems to be due to the heterogeneity of CO2-induced changes in cerebrovascular resistance between differing arterial territories.

Interaction among autoregulation, CO2 reactivity, and intracranial pressure: a mathematical model

The relationships among cerebral blood flow, cerebral blood volume, intracranial pressure (ICP), and the action of cerebrovascular regulatory mechanisms (autoregulation and CO2 reactivity) were investigated by means of a mathematical model. The model incorporates the cerebrospinal fluid (CSF) circulation, the intracranial pressure-volume relationship, and cerebral hemodynamics. The latter is based on the following main assumptions: the middle cerebral arteries behave passively following transmural pressure changes; the pial arterial circulation includes two segments (large and small pial arteries) subject to different autoregulation mechanisms; and the venous cerebrovascular bed behaves as a Starling resistor. A new aspect of the model exists in the description of CO2 reactivity in the pial arterial circulation and in the analysis of its nonlinear interaction with autoregulation. Simulation results, obtained at constant ICP using various combinations of mean arterial pressure and CO2 pressure, substantially support data on cerebral blood flow and velocity reported in the physiological literature concerning both the separate effects of CO2 and autoregulation and their nonlinear interaction. Simulations performed in dynamic conditions with varying ICP underline the existence of a significant correlation between ICP dynamics and cerebral hemodynamics in response to CO2 changes. This correlation may significantly increase in pathological subjects with poor intracranial compliance and reduced CSF outflow. In perspective, the model can be used to study ICP and blood velocity time patterns in neurosurgical patients in order to gain a deeper insight into the pathophysiological mechanisms leading to intracranial hypertension and secondary brain damage.

Modeling cerebral autoregulation and CO2 reactivity in patients with severe head injury

The mathematical model presented in a previous work is used to simulate the time pattern of intracranial pressure (ICP) and of blood velocity in the middle cerebral artery (VMCA) in response to maneuvers simultaneously affecting mean systemic arterial pressure (SAP) and end-tidal CO2 pressure. In the first stage of this study, a sensitivity analysis was performed to clarify the role of some important model parameters [cerebrospinal fluid (CSF) outflow resistance, intracranial elastance coefficient, autoregulation gain, and the position of the regulation curve] during CO2 alteration maneuvers performed at different SAP levels. The results suggest that the dynamic "ICP-VMCA" relationship obtained during changes in CO2 pressure may contain important information on the main factors affecting intracranial dynamics. In the second stage, the model was applied to the reproduction of real ICP and velocity tracings in neurosurgical patients. Ten distinct tracings, taken from six patients during CO2 changes at different mean SAP levels, were reproduced. Best fitting between model and clinical curves was achieved by minimizing a least-squares criterion function and adjusting certain parameters that characterize CSF circulation, intracranial compliance, and the strength of the regulation mechanisms. A satisfactory reproduction was achieved in all cases, with parameter numerical values in the ranges reported in clinical literature. It is concluded that the model may be used to give reliable estimations of the main factors affecting intracranial dynamics in individual patients, starting from routine measurements performed in neurosurgical intensive care units.

Cerebral perfusion pressure in head-injured patients: a noninvasive assessment using transcranial Doppler ultrasonography

OBJECT

The authors studied the reliability of a new method for noninvasive assessment of cerebral perfusion pressure (CPP) in head-injured patients in which mean arterial blood pressure (ABP) and transcranial Doppler middle cerebral artery mean and diastolic flow velocities are measured.

METHODS

Cerebral perfusion pressure was estimated (eCPP) over periods of continuous monitoring (20 minutes-2 hours, 421 daily examinations) in 96 head-injured patients (Glasgow Coma Scale score < 13) who were admitted to the intensive care unit. All patients were sedated, paralyzed, and ventilated. The eCPP and the measured CPP (ABP minus intracranial pressure, measured using an intraparenchymal microsensor) were compared. The correlation between eCPP and measured CPP was r=0.73; p < 10(-6). In 71% of the examinations, the estimation error was less than 10 mm Hg and in 84% of the examinations, the error was less than 15 mm Hg. The method had a high positive predictive power (94%) for detecting low CPP (< 60 mm Hg). The eCPP also accurately reflected changes in measured CPP over time (r > 0.8; p < 0.001) in situations such as plateau and B waves of intracranial pressure, arterial hypotension, and refractory intracranial hypertension. A good correlation was found between the average measured CPP and eCPP when day-by-day variability was assessed in a group of 41 patients (r=0.71).

CONCLUSIONS

Noninvasive estimation of CPP by using transcranial Doppler ultrasonography may be of value in situations in which monitoring relative changes in CPP is required without invasive measurement of intracranial pressure.

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.

Contribution of mathematical modelling to the interpretation of bedside tests of cerebrovascular autoregulation

OBJECTIVES

Cerebral haemodynamic responses to short and longlasting episodes of decreased cerebral perfusion pressure contain information about the state of autoregulation of cerebral blood flow. Mathematical simulation may help to elucidate which of the indices, that can be derived using transcranial Doppler ultrasonography and trends of intracranial pressure and blood pressure, are useful in clinical tests of autoregulatory reserve.

METHODS

Time dependent interactions between pressure, flow, and volume of cerebral blood and CSF were modelled using a set of non-linear differential equations. The model simulates changes in arterial blood inflow and storage, arteriolar and capillary blood flow controlled by cerebral autoregulation, venous blood storage and venous outflow modulated by changes in ICP, and CSF storage and reabsorption. The model was used to simulate patterns of blood flow during either short or longlasting decreases in cerebral perfusion pressure. These simulations can be considered as clinically equivalent to a short compression of the common carotid artery, systemic hypotension, and intracranial hypertension. Simulations were performed in autoregulating and non-autoregulating systems and compared with recordings obtained in patients.

RESULTS

After brief compression of the common carotid artery, a subsequent transient hyperaemia can be interpreted as evidence of intact autoregulation. During longlasting sustained hypoperfusion, a gradual increase in the systolic value of the blood flow velocity waveform along with a decrease in the diastolic value is specific for an autoregulating cerebrovascular system.

CONCLUSION

Modelling studies help to interpret both clinical and experimental cerebral haemodynamic phenomena and their dependence on the state of autoregulation.

Evaluation of hemodynamic responses in head injury patients with transcranial Doppler monitoring

Transcranial Doppler (TCD) can monitor middle cerebral artery (MCA) velocity which can be recorded simultaneously with other physiologic parameters such as end tidal (Et) CO2, arterial blood pressure and intracranial pressure (ICP), in head injured patients. Relative changes in MCA velocity can be used to reflect relative MCA blood flow changes during ICP waves, and also to evaluate cerebral autoregulation, CO2 reactivity and hemodynamic responses to mannitol and barbiturates. The utility and practicality of short intervals of TCD monitoring to evaluate hemodynamic responses, was evaluated in a group of 22 head injured patients (average Glasgow coma score 6). During ICP A waves, MCA velocity always decreased during the peak of the wave, and during ICP B waves, fluctuated synchronously with the ICP. Dynamic cerebral autoregulation, and reactivity to CO2, were reduced within 48 hours of admission. Impaired cerebral autoregulation within 48 hours of admission did not correlate with outcome at 1 month. Mannitol infusion caused an increase in MCA velocity (15.4 +/- 7.9%) which was significantly correlated to the impairment of dynamic autoregulation (r = 0.54, p < 0.0001). The MCA velocity response to a test dose of barbiturates was significantly correlated to the ICP (r = 0.61, p < 0.01) response as well as to the CO2 reactivity (r = 0.37, p < 0.05). Continuous MCA velocity monitoring using TCD may be useful in evaluating a variety of hemodynamic responses in head injury patients and may replace more cumbersome cerebral blood flow techniques which have been used in the past for these purposes.

Effects of dihydroergotamine and sumatriptan on isolated human cerebral and peripheral arteries and veins

BACKGROUND

Pharmacological cerebral vasoconstriction has recently been suggested as treatment for patients with increased intracranial pressure (ICP) after severe traumatic brain lesions. Hypothetically, a moderate constriction of precapillary resistance vessels might be advantageous since it decreases intracapillary blood pressure, and a contraction of cerebral veins might effectively reduce intracranial blood volume and ICP. This report examines the in vitro effects of two vasoconstrictors, dihydroergotamine (DHE) and sumatriptan, which may be considered for treatment of increased ICP.

METHODS

The reactivity of isolated small human cerebral subcutaneous and omental arteries and veins were studied during exposure to different concentrations of DHE and sumatriptan.

RESULTS

Both sumatriptan and DHE induced concentration-dependent contractions in human cerebral arteries and veins and 50% of maximum contractions were obtained at significantly lower concentrations of DHE than of sumatriptan. The maximum contraction of cerebral arteries was significantly higher with sumatriptan than with DHE. Both drugs caused contractions of subcutaneous arteries at concentrations of 10(-7)-10(-6)M, which is within the therapeutic concentration range of sumatriptan, while no effect was obtained in omental vessels.

CONCLUSIONS

Both DHE and sumatriptan cause contraction of isolated human cortical arteries and veins at very low concentrations. The differences observed between the two drugs may be explained by the fact that DHE is an alpha-adrenergic as well as a 5-HT agonist while sumatriptan acts specifically on 5-HT receptors. The study supports the hypothesis underlying the use of DHE for the treatment of increased ICP in patients with severe traumatic brain lesions.

A computing system for the clinical and experimental investigation of cerebrovascular reactivity

We present a computing system for the recording and on-line analysis of analogue signals derived from bedside cerebrovascular monitors in different pathophysiological conditions. These include arterial blood pressure and oxygen saturation, end-tidal carbon dioxide concentration, cerebral blood flow velocities using transcranial Doppler ultrasonography, and concentration changes in cerebral oxy- and deoxyhaemoglobin from near infrared spectroscopy. Configuration and analysis adopts arithmetic expressions of different signal processing functions, various statistical properties for each signal, frequency spectrum analysis using fast Fourier transformation, and correlation/cross-correlation. The software offers off-line analysis of non-invasive tests of cerebrovascular reactivity. Several examples of clinical assessment of cerebrovascular reactivity are presented, including cerebral haemodynamic stress tests which employ carbon dioxide, acetazolamide, the breath holding test, leg cuff inflation and deflation, and transient carotid artery compression. Application within the experimental setting with induced haemorrhagic hypotension can also be used.

Continuous assessment of the cerebral vasomotor reactivity in head injury

OBJECTIVE

Cerebrovascular vasomotor reactivity reflects changes in smooth muscle tone in the arterial wall in response to changes in transmural pressure or the concentration of carbon dioxide in blood. We investigated whether slow waves in arterial blood pressure (ABP) and intracranial pressure (ICP) may be used to derive an index that reflects the reactivity of vessels to changes in ABP.

METHODS

A method for the continuous monitoring of the association between slow spontaneous waves in ICP and arterial pressure was adopted in a group of 82 patients with head injuries. ABP, ICP, and transcranial doppler blood flow velocity in the middle cerebral artery was recorded daily (20- to 120-min time periods). A Pressure-Reactivity Index (PRx) was calculated as a moving correlation coefficient between 40 consecutive samples of values for ICP and ABP averaged for a period of 5 seconds. A moving correlation coefficient (Mean Index) between spontaneous fluctuations of mean flow velocity and cerebral perfusion pressure, which was previously reported to describe cerebral blood flow autoregulation, was also calculated.

RESULTS

A positive PRx correlated with high ICP (r = 0.366; P < 0.001), low admission Glasgow Coma Scale score (r = 0.29; P < 0.01), and poor outcome at 6 months after injury (r = 0.48; P < 0.00001). During the first 2 days after injury, PRx was positive (P < 0.05), although only in patients with unfavorable outcomes. The correlation between PRx and Mean index (r = 0.63) was highly significant (P < 0.000001).

CONCLUSION

Computer analysis of slow waves in ABP and ICP is able to provide a continuous index of cerebrovascular reactivity to changes in arterial pressure, which is of prognostic significance.

Evaluation of the transient hyperemic response test in head-injured patients

The transient hyperemic response test has been shown to provide an index of cerebral autoregulation in healthy individuals and in patients who have suffered a subarachnoid hemorrhage. In this study, the test was applied to patients who had received a severe head injury, and the value of the test was assessed by comparing its result with the individual's clinical condition (Glasgow Coma Scale [GCS] score), cerebral perfusion pressure (CPP), transcranial Doppler wave form-derived index for cerebral autoregulation (relationship between the CPP and the middle cerebral artery flow velocity), and outcome (Glasgow Outcome Scale [GOS] score). Forty-seven patients, aged 16 to 63 years, with head injuries were included in the study. Signals of intracranial pressure, arterial blood pressure, flow velocity, and cortical microcirculatory flux were digitized and recorded for a period of 30 minutes using special computer software. Two carotid compressions were performed at the beginning of each recording. The transient hyperemic response ratio (THRR: the ratio of the hyperemic flow velocity recorded after carotid release and the precompression baseline flow velocity) was calculated, as was the correlation coefficient Sx used to describe the relationship between slow fluctuations in the systolic flow velocity and CPP throughout the period of recording. No significant changes in CPP were found during compression. There was a significant correlation between the THRR and the Sx (r = 0.49, p < 0.0001). The hyperemic response proved to be lower in patients who exhibited a poor clinical grade at presentation (GCS scores < 6, p = 0.01) and lower in patients achieving a poor outcome (GOS scores of 3, 4, and 5, p = 0.003). Loss of postcompression hyperemia occurred when the CPP fell below 50 mm Hg. The carotid compression test provides a simple index of cerebral autoregulation that is relevant to the clinical condition and outcome of the severely head injured patient.

Rapid sequence intubation in adults with elevated intracranial pressure: a survey of emergency medicine residency programs

A questionnaire entitled "Survey of Protocols for Rapid Sequence Intubation in Previously Healthy Adults with Elevated Intracranial Pressure" was distributed to the program directors of all 100 emergency medicine residency programs listed in the Directory of Graduate Medical Education Programs in February 1995. The medical literature on rapid sequence intubation in patients with suspected intracranial pressure elevations was reviewed. The findings of the review were compared with the survey responses. Sixty-seven program directors responded to the survey. Sixty-five programs performed rapid sequence intubation in their institution. Five programs performed 0 to 10 procedures annually. Six performed 10 to 30 annually, 19 performed 30 to 50, 17 performed 50 to 100, and 18 performed more than 100. Succinylcholine and vecuronium were the most frequently used neuromuscular blockers. Midazolam and thiopental were the most frequently used sedative induction agents. Most programs use a defasciculating agent prior to succinylcholine administration. The majority of programs do not use a priming agent before the use of a nondepolarizing neuromuscular blocking agent. Intravenous lidocaine was routinely administered prior to neuromuscular blockade. Fentanyl was the most frequently used other pretreatment medication. Rapid sequence intubation is used to facilitate definitive, emergent airway management in patients with suspected intracranial pressure elevations in almost all of the emergency medicine residency programs that responded to the survey. Most of these programs follow the guidelines recommended in the medical literature. The majority of these guidelines, however, are based on statistical data performed in the laboratory or nonemergency environments. Further clinical studies in an emergency medicine environment must be performed to determine the optimal drug regimen for rapid sequence intubation in patients with elevated intracranial pressure.

Cerebral autoregulation following minor head injury

The purpose of this study was to determine whether patients with minor head injury experience impairments in cerebral autoregulation. Twenty-nine patients with minor head injuries defined by Glasgow Coma Scale (GCS) scores of 13 to 15 underwent testing of dynamic cerebral autoregulation within 48 hours of their injury using continuous transcranial Doppler velocity recordings and blood pressure recordings. Twenty-nine age-matched normal volunteers underwent autoregulation testing in the same manner to establish comparison values. The function of the autoregulatory response was assessed by the cerebral blood flow velocity response to induced rapid brief changes in arterial blood pressure and measured as the autoregulation index (ARI). Eight (28%) of the 29 patients with minor head injury demonstrated poorly functioning or absent cerebral autoregulation versus none of the controls, and this difference was highly significant (p = 0.008). A significant correlation between lower blood pressure and worse autoregulation was found by regression analysis in head-injured patients (r = 0.6, p < 0.001); however, lower blood pressure did not account for the autoregulatory impairment in all patients. Within this group of head-injured patients there was no correlation between ARI and initial GCS or 1-month Glasgow Outcome Scale scores. This study indicates that a significant number of patients with minor head injury may have impaired cerebral autoregulation and may be at increased risk for secondary ischemic neuronal damage.

Impaired cerebral autoregulation after mild brain injury

BACKGROUND

Severe head injury may impair cerebral autoregulation, which can increase the risk of secondary neuronal injury. The likelihood of impairment in autoregulation is assumed to be low with mild head injury. We report here the absence of cerebral autoregulation in a patient who suffered a concussion from an automobile accident 6 days earlier.

METHODS

The patient participated in a clinical study approved by the institutional human subjects review committee, investigating the dose-effect relationship of anesthetics on cerebral autoregulation. The patient was scheduled to undergo repair of a knee injury suffered during a motor vehicle accident, during which she had a concussion. The screening evaluation revealed no evidence of neurologic disease. The test was to be performed three times in each patient: baseline autoregulation measurements during stable fentanyl-nitrous oxide anesthesia, second and third measurements during low dose and high dose of the anesthetic to which the patient was assigned. Autoregulation was tested by increasing the mean systemic blood pressure from 80 mm Hg-100 mm Hg using a phenylephrine infusion while simultaneously recording flow velocity from a middle cerebral artery using transcranial Doppler ultrasonography.

RESULTS

Static autoregulation testing during baseline testing demonstrated complete absence of this homeostatic mechanism and the study was canceled. Repeated testing in the recovery unit after the patient awoke showed identical results.

CONCLUSIONS

Trivial mild head injury may result in loss of cerebral autoregulation. A clinical study of a larger series to document the incidence is warranted.

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.

Impact of traumatic subarachnoid hemorrhage on outcome in nonpenetrating head injury. Part II: Relationship to clinical course and outcome variables during acute hospitalization

Patients with a nonpenetrating head injury and traumatic subarachnoid hemorrhage (tSAH) on admission head computed tomography scan (n = 240) were compared with patients without tSAH matched in terms of admission postresuscitation Glasgow Coma Scale (GCS) values, age, sex, and the presence of one or more types of intracranial mass lesions. Admission Injury Severity Score was higher only in tSAH patients with admission GCS scores between 13 and 15; GCS values at 6, 24, and 48 hours were lower for tSAH patients. Patients with tSAH underwent fewer craniotomies, but more than twice as many tSAH patients had high intracranial pressure at the time of ventriculostomy placement and 6 hours after admission. tSAH patients underwent more chest procedures and their incidence of hypoxia and hypotension was greater. tSAH patients spent more days in intensive care unit, more total days hospitalized, and had worse Glasgow Outcome Scale scores at acute hospital discharge. Fewer tSAH patients were discharged home, and almost 1.5 times as many tSAH patients died during hospitalization. Given a similar overall degree of injury at admission, patients with tSAH associated with a nonpenetrating head injury had a worse outcome than similar patients without tSAH.

Monitoring of cerebral autoregulation in head-injured patients

BACKGROUND AND PURPOSE

Disturbed cerebral autoregulation has been reported to correlate with an unfavorable outcome after head injury. Using transcranial Doppler ultrasonography, we investigated whether hemodynamic responses to spontaneous variations of cerebral perfusion pressure (CPP) provide reliable information on cerebral autoregulatory reserve.

METHODS

We studied 82 patients with head injury daily. Waveforms of intracranial pressure (ICP), arterial pressure, and transcranial Doppler flow velocity (FV) were captured during 2-hour periods. Time-averaged mean FV (FVm) and the FV during cardiac systole (FVs) were resolved. The correlation coefficient indices between FVm and CPP (Mx) and between FVs and CPP (Sx) during spontaneous fluctuations of CPP were calculated during 3-minute epochs and averaged for each investigation.

RESULTS

Mx and Sx correlated with CPP (r = -.34, P = < .002; r = -.2, P = NS. respectively), with ICP (r = .46, P < .0001; r = .34, P < .003, respectively), with admission Glasgow Coma Scale score (r = -.34, P < .0025; r = -.38, P < .0008, respectively), and with outcome after head injury (r = .41, P < .0002; r = .48, P < .00009, respectively). In patients who died, cerebral autoregulation was severely disturbed during the first 2 days after injury.

CONCLUSIONS

Indices derived from spontaneous fluctuations of FV waveform and CPP describe cerebral vascular pressure reactivity. They correlate with outcome after head injury and therefore may be used to guide autoregulation-oriented intensive therapy.

Cerebral autoregulation is impaired in patients resuscitated after cardiac arrest

BACKGROUND

Cerebral autoregulation is important to maintain a constant perfusion in the face of changes in blood pressure. It is reported to be impaired in pathologic states, including hypertension, cerebral infarction, and head injury. However, it is not clear whether cerebral autoregulation is impaired in resuscitated patients after cardiac arrest.

METHODS

Cerebral autoregulation in comatose patients after cardiac arrest was assessed by using an indirect method of cerebral blood flow (CBF). Eight patients who had cardiac arrest outside of the hospital and were successfully resuscitated in the emergency room were included in this study. A catheter was inserted percutaneously into the right internal jugular vein and positioned so that the tip was in the jugular bulb. Mean arterial pressure (MAP) was changed to a value of 30% lower or higher than baseline MAP by infusing trimethaphan or methoxamine, respectively. At each MAP level, arterial and jugular bulb venous blood gases were measured, and arterial-jugular bulb venous oxygen content difference (AVDO2) was calculated.

RESULTS

The 1/AVDO2 (CBFI) and oxygen saturation of jugular bulb venous blood (SjvO2) significantly decreased at the lower MAP level, and significantly increased at the higher MAP level. The ratio of the CBFI at the lower MAP level to the CBFI at baseline (CBFI-L/CBFI-B) inversely correlated with the SjvO2 at baseline.

CONCLUSIONS

Assuming that the cerebral metabolic rate of oxygen does not change during the interventions in MAP, the changes of CBFI and SjvO2 seen after the decrease or increase in MAP indicate that cerebral autoregulation was impaired in these resuscitated patients. The degree of the impairment of cerebral autoregulation may be secondary to the degree of brain injury caused by the cerebral ischemia accompanying cardiac arrest.

Severe head injury in the United Kingdom and Ireland: a survey of practice and implications for management

OBJECTIVE

To study the current intensive care management of patients with severe head injury (defined as a Glasgow Coma Scale score of < or = 8) in neurosurgical referral centers in the United Kingdom (UK) and ireland.

DATA COLLECTION

A questionnaire was sent to the directors of the 44 neurosurgical referral units identified from the UK Medical Directory. After 4 wks, a copy of the questionnaire was sent to all nonresponders, with a cover letter urging them to respond. The aim was to collect data regarding the characteristics of the intensive care units (ICU), sedation, monitoring modalities used, the treatment of intracranial hypertension, and general care of severely head-injured patients.

DATA EXTRACTION

Forty completed questionnaires were returned. Only 35 (88%) centers provided care for the severely head-injured as defined in the questionnaire. Patients were managed in specialized neurosurgical ICUs in 66% of centers and in general ICUs in the remainder of the centers. The ICUs were coordinated by an anesthesiologist in 66% of instances and by a neurosurgeon in 23%. The mean number of beds per units was 7.9 (range 4 to 16), with 1:1 nurse/bed ratio and 5.5 nurses per bed (total number of nursing staff per bed) (range 2.75 to 8). Annual caseload varied between units with the majority of units (49%) receiving between 25 and 50 patients with severe head injury, 23% receiving between 50 and 100 patients with severe head injury, and 29% receiving > 100 patients with severe head injury. There was considerable variability in both the nature of monitoring and therapy between centers. Although blood and central venous pressures were invasively monitored in > 50% of the patients in 94% and 77% of the centers, respectively, intracranial pressure was only monitored routinely in 57% of the centers. Jugular venous bulb oximetry, transcranial Doppler ultrasonography, electroencephalography, and near-infrared spectroscopy were rarely used. Nearly all centers used propofol and midazolam for sedation, with morphine, fentanyl, and alfentanil as the main analgesics. Muscle relaxation was commonly used, with 40% of the centers employing it in 100% of their patients. Atracurium and vecuronium were the most commonly used agents. Only 68% of the centers had a protocol for the treatment of intracranial hypertension. Although hyperventilation to a Paco2 of 26 to 30 torr (3.5 to 4.0 kPa) was the norm in the majority of centers (56%), two centers aimed for Paco2 values < 26 torr (< 3.5 kPa). A quarter of the units did not aim for a cerebral perfusion pressure of > 60 mm Hg. Mild hypothermia was rarely used and 14% of the centers continued to use corticosteroids for the treatment of intracranial hypertension as a result of head trauma.

CONCLUSION

We conclude that there are wide variations in the management of the severely head-injured patient in the UK and Ireland. Some of the therapies employed are not supported by available research findings. Rationalization (using rational management, i.e., based on good evidence) of the intensive care management of severe head injury with the development of widely accepted guidelines may result in an improvement in the quality of care of the head-injured patient.

Detection of impaired cerebral autoregulation using spectral analysis of intracranial pressure waves

Successful resuscitation following severe traumatic brain injury (TBI) requires rapid evaluation of intracranial pressure (ICP), cerebrovascular reactivity (autoregulation), and cerebral metabolism. During impaired autoregulation, inadequate cerebral blood flow (CBF) can lead to ischemia while excessive CBF can result in elevated ICP. Without information regarding the state of autoregulation, treatment of either situation may ameliorate one problem but exacerbate the other. It has been hypothesized that fast Fourier transform (FFT) analysis of arterial blood pressure (BP) and ICP waves can differentiate states of intact and impaired autoregulation. BP and ICP waves were recorded in canines before and after ischemic injury during arterial normotension, hypertension, and hypotension induced with dopamine or nitroprusside infusion. Transfer functions (TFn) were calculated from FFT spectra as ratios of ICP and BP harmonic peak amplitudes to distinguish states of vasoreactivity. During normotension and hypertension, autoregulation was intact and TF1 averaged 0.05. During hypotension, TF1 averaged 0.22 (8 x baseline, p < 0.010). During impaired autoregulation following ischemic injury, TF1 averaged 0.50 (18 x baseline, p < 0.010; 2 x nitroprusside levels, p < 0.01). This large difference in TF relative to baseline extended over a large range of BP (60 < BP < 180 mm Hg). Based on these data and previous results, it was estimated that TF can differentiate impaired autoregulation from effects solely related to elevated ICP or active vasodilation for ICP < 30-40 mm Hg. This suggests that for specific, but widely applicable physiologic conditions, spectral analysis can identify states of impaired autoregulation and, as an adjunct to traditional monitoring techniques, aid in acute resuscitation and prevention of secondary injury in TBI.

Secondary insults increase injury after controlled cortical impact in rats

Secondary ischemic insults are common after severe head injury and contribute to poor neurological outcome. To study the increased vulnerability of the traumatized brain to secondary insults, bilateral carotid occlusion was produced after a controlled cortical impact injury in rats. The injury produced by either the impact injury or the bilateral carotid occlusion was mild to moderate when studied individually. The 1 and 3 m/sec impact injuries alone caused no detectable contusion at the impact side and minimal neuronal loss in the hippocampus. The 5 m/sec impact injury alone resulted in a small contusion with a median volume of 5.4 mm3. The 40-min period of bilateral carotid occlusion alone caused no cortical injury and no neuronal loss in the CA1 region of the hippocampus. When the 40 min of bilateral carotid occlusion was produced 1 h after the impact injury, there was an increase in the damage produced. The contusion volume was significantly larger after the 3 and 5 m/sec impact injuries and the hippocampal neuronal loss was significantly greater after the 1 and 3 m/sec impact injuries. When varying durations of bilateral carotid occlusion were produced 1 h after a 3 m/sec impact injury, contusion volume was significantly larger after bilateral carotid occlusion duration of 40 min, and CA1 neuronal loss was significantly greater after bilateral carotid occlusion durations of 30 and 40 min. When 40 min of bilateral carotid occlusion was produced at different time intervals after a 3 m/sec injury, the increased contusion volume was maximal when bilateral carotid occlusion occurred at 4 h after the impact injury, and the increased neuronal loss in the CA3 region of the hippocampus was maximal when bilateral carotid occlusion occurred at 1 h after the impact injury. By 24 h after the impact injury, 40 min of bilateral carotid occlusion had minimal consequences, similar to the effect in sham-injured animals. These results mimic the clinical situation where secondary insults of a severity that would not cause permanent neurological damage in a normal person are associated with a marked worsening of neurological outcome after head injury and where the injured brain is most susceptible to secondary insults in the first few hours after injury.

Effect of transient moderate hyperventilation on dynamic cerebral autoregulation after severe head injury

OBJECTIVE

This study was undertaken to evaluate the effect of acute moderate hyperventilation on cerebral autoregulation in head-injured patients.

METHODS

Dynamic cerebral autoregulation was analyzed by use of transcranial doppler ultrasonography before and after hyperventilation in 10 patients with severe head injury. All of the patients were artificially ventilated and underwent continuous monitoring of arterial blood pressure, intracranial pressure, and end-tidal carbon dioxide. To test autoregulation, rapid transient decreases in systemic blood pressure were achieved by quickly releasing large blood pressure cuffs that were inflated around both thighs. This resulted in a drop of 24 +/- 6 mm Hg in mean systemic blood pressure, which lasted an average of 49 +/- 24 seconds. Cerebral blood flow velocity was monitored continuously in both middle cerebral arteries by use of transcranial doppler ultrasonography. The percentage change in middle cerebral artery velocity was used as an index of the change in cerebral blood flow during the autoregulatory response. The change in estimated cerebrovascular resistance, immediately after the blood pressure drop, or the rate of regulation was used to analyze the effectiveness of the cerebral autoregulation. This value was calculated by determining the rate of increase in middle cerebral artery velocity during the 1st 5 seconds after a blood pressure drop, relative to the rate of increase of the cerebral perfusion pressure.

RESULTS

The average rate of regulation during normocapnia at pCO2 of 37 mm Hg was 11.4 +/- 5% per second. After reduction of the pCO2 to 28 mm Hg, the average rate of regulation improved significantly (P < 0.001) to 17.7 +/- 6% per second. Autoregulation improved, despite no significant change in the cerebral perfusion pressure during hyperventilation. The degree of improvement in autoregulation was significantly correlated with the CO2 reactivity (r = 0.45, P < 0.05) but did not correlate (r = -0.23, P = 0.33) with the change in arterial pH value after hyperventilation.

CONCLUSION

These results confirm the finding that dynamic autoregulation is disturbed in severe head injury and that moderate transient hyperventilation can temporarily improve the efficiency of the autoregulatory response, probably as a result of a transient increase in vascular tone.

Evaluation of impaired cerebral autoregulation by the Valsalva maneuver

BACKGROUND AND PURPOSE

Transcranial Doppler sonography has recently been used to describe cerebral hemodynamics during the Valsalva maneuver in normal human subjects. Since some changes in flow velocity during the Valsalva maneuver seem to reflect the brain's autoregulatory response to a decrease in cerebral perfusion pressure during the strain, we hypothesized that this method could identify vascular territories with impaired autoregulatory capacity.

METHODS

Eight patients with unilateral (n=7) or bilateral (n=1) severe obstruction of the internal carotid artery and impaired vascular responses to the CO2 reactivity test and to dynamic autoregulation testing were studied. We compared changes in flow velocities and blood pressures during defined phases of the Valsalva maneuver in the patients with the results in a group of 17 normal volunteers. We defined two indices to evaluate autoregulatory capacity based on the response to the Valsalva maneuver.

RESULTS

During the Valsalva maneuver, changes in flow velocity in the middle cerebral arteries ipsilateral to the lesions showed characteristic abnormalities compared with the normal pattern. Autoregulatory indices of these vessels as defined by the Valsalva maneuver were significantly different from those with normal vascular reactivity to CO2 (P<.0001). There were good correlations between an index based on the changes in flow velocity and blood pressure in phase II and the results of the CO2 test (r=.78; P<.0001) or those of dynamic autoregulatory testing (r=.6; P<.0001).

CONCLUSIONS

Vascular territories with severely impaired vasomotor reactivity due to carotid obstruction can be identified by transcranial Doppler sonography by their pattern of flow velocity changes if their autoregulatory capacity is challenged during the Valsalva maneuver.

Brain tissue pO2 in relation to cerebral perfusion pressure, TCD findings and TCD-CO2-reactivity after severe head injury

As a reliable continuous monitoring of cerebral blood flow and/or cerebral oxygen metabolism is necessary to prevent secondary ischaemic events after severe head injury (SHI) the authors introduced brain tissue pO2 (ptiO2) monitoring and compared this new parameter with TCD-findings, cerebral perfusion pressure (CPP) and CO2-reactivity over time on 17 patients with a SHI. PtiO2 reflects the balance between the oxygen offered by the cerebral blood flow and the oxygen consumption by the brain tissue. According to TCD-CO2-reactivity PtiO2-CO2-reactivity was introduced. After initially (day 0) low mean values (ptiO2 7.7 +/- 2.6 mmHg, TCD 60.5 +/- 32.0 cm/sec and CPP 64.5 +/- 16.0 mmHg/, ptiO2 increased together with an increase in blood flow velocity of the middle cerebral artery and CPP. The relative hyperaemic phase on days 3 and 4 was followed by a decrease of all three parameters. Although TCD-CO2-reactivity was except for day 0 (1.4 +/- 1.5%), sufficient, ptiO2-CO2-reactivity sometimes showed so-called paradox reactions from day 0 till day 3, meaning an increase of ptiO2 on hyperventilation. Thereafter ptiO2-CO2-reactivity increased, increasing the risk of inducing ischaemia by hyperventilation. The authors concluded that ptiO2-monitoring might become an important tool in our treatment regime for patients requiring haemodynamic monitoring.