Effect of hypertonic urea on cerebrospinal fluid pressure and brain volume

Analytic Reviews : Fluid Resuscitation in Head Injury

Head injury, either alone or in combination with hy povolemic shock, is the leading cause of traumatic death in this country. Factors contributing to mortality in clude the primary impact injury as well as subsequent ischemia and hypoperfusion. Intravenous fluid therapy is required in all of these patients. However, fluid ther apy may increase brain swelling and cerebral edema formation which could lead to an increase in intracra nial pressure and a reduction in cerebral perfusion pres sure. The use of standard fluid therapy has been ques tioned, and novel therapies involving hyperosmolar and hypertonic solutions are now being investigated. This review covers recent advances in the understanding of the effects of fluid resuscitation on the brain. It also includes a brief summary of the determinants of trans- capillary fluid exchange and a review of relevant cere bral circulatory physiology and the physiological aberra tions produced by brain injury.

Simultaneous multiagent hyperpolarized (13)C perfusion imaging

PURPOSE

To demonstrate simultaneous hyperpolarization and imaging of three (13)C-labeled perfusion MRI contrast agents with dissimilar molecular structures ([(13)C]urea, [(13)C]hydroxymethyl cyclopropane, and [(13)C]t-butanol) and correspondingly variable chemical shifts and physiological characteristics, and to exploit their varying diffusibility for simultaneous measurement of vascular permeability and perfusion in initial preclinical studies.

METHODS

Rapid and efficient dynamic multislice imaging was enabled by a novel pulse sequence incorporating balanced steady state free precession excitation and spectral-spatial readout by multiband frequency encoding, designed for the wide, regular spectral separation of these compounds. We exploited the varying bilayer permeability of these tracers to quantify vascular permeability and perfusion parameters simultaneously, using perfusion modeling methods that were investigated in simulations. "Tripolarized" perfusion MRI methods were applied to initial preclinical studies with differential conditions of vascular permeability, in normal mouse tissues and advanced transgenic mouse prostate tumors.

RESULTS

Dynamic imaging revealed clear differences among the individual tracer distributions. Computed permeability maps demonstrated differential permeability of brain tissue among the tracers, and tumor perfusion and permeability were both elevated over values expected for normal tissues.

CONCLUSION

Tripolarized perfusion MRI provides new molecular imaging measures for specifically monitoring permeability, perfusion, and transport simultaneously in vivo.

How well can in vitro brain microcapillary endothelial cell models predict rodent in vivo blood-brain barrier permeability?

The object of the study was to improve the blood-brain barrier (BBB) permeability in vitro-invivo correlations (IVIVC) between in vitro brain microcapillary endothelial cell (BMEC) models and the well-tested rodent in situ brain perfusion technique. Porcine, bovine, rat, mouse, and human in vitro BMEC apparent permeability values, P(e), (14 studies from several laboratories: 229 P(e), 60 compounds) were analyzed by a novel biophysical model encoded in a weighted nonlinear regression procedure to determine the aqueous boundary layer (ABL) thickness and the paracellular parameters: porosity-pathlength (dual-pore model), pore radius, and water channel electrostatic potential. The refined parameters were then used to transform the P(e) values into the transendothelial permeability (P(c)) values. Porcine BMEC mono-culture models showed tight junctions comparable to those reported in several Caco-2 studies. Bovine cultures were somewhat leakier. In the human primary cultured cell and the hCMEC/D3 cell line data, IVIVC based on P(e) values has r(2) = 0.14. With transformed permeability values, r(2) = 0.58. Comparable improvements were found in the other species data. By using the in vitro transendothelial P(c) values in place of the apparent P(e) values, IVIVC can be dramatically improved.

METHOD FOR COMBINED ULTRASTRUCTURAL AND BIOCHEMICAL ANALYSIS OF NEURAL TISSUE

No significant change was found in the electrolytes and lipids of the brain analyzed after glutaraldehyde fixation by perfusion of laboratory animals; such fixation also satisfactorily preserves neural tissues for electron microscopy. The brains of normal and tumor-bearing C3H mice, Wistar rats, and New Zealand rabbits were studied. Little difference was found in the dry weight and the content of sodium, potassium, total lipid and lipid fractions, and in the sulfate space (S³⁵O₄) between specimens from unperfused and perfused animals, whether normal or tumor-bearing. The results suggest the possibility of using selected regions of the nervous system, dissected after fixation, for chemical study and at the same time characterizing similar regions morphologically with the electron microscope.

Osmotherapy. Basic concepts and controversies

Osmotherapy with compounds such as mannitol has become a mainstay of neurologic and neurosurgical intensive care. Elevated intracranial pressure is the most common indication. A substantive debate remains as to the appropriate timing of administration and the optimal fluid management protocol, and experts disagree about the clinically relevant mechanisms of action of osmotic diuretics. This article briefly summarizes the basic literature on the physical actions of mannitol, addresses commonly asked questions, and highlights some of the controversies that arise at the bedside.

Brain swelling after dialysis: old urea or new osmoles?

The pathogenesis of brain swelling and neurological deterioration after rapid hemodialysis (dialysis disequilibrium syndrome) is controversial. The "reverse urea hypothesis" suggests that hemodialysis removes urea more slowly from the brain than from the plasma, creating an osmotic gradient that results in cerebral edema. The "idiogenic osmole hypothesis" proposes that an osmotic gradient between brain and plasma develops during rapid dialysis because of newly formed brain osmoles. In this review, the experimental basis for the two hypotheses are critically examined. Based on what is known about the physiology of urea and water diffusion across the blood-brain barrier, and empiric observations of brain solute composition after experimental hemodialysis, we conclude that the "reverse urea hypothesis" remains a viable explanation for dialysis disequilibrium and that rapid reduction of a high urea level in and of itself predisposes to this condition.

Management of raised intracranial pressure

This review has been written at an unfortunate time. Novel questions are being asked of the old therapies and there is an abundance of new strategies both to lower ICP and protect the brain against cerebral ischaemia. In the United Kingdom, the problem is to ensure that appropriate patients continue to be referred to centres where clinical trials of high quality can be undertaken. One of the success stories of the past decade has been the decline in the number of road accidents as a result of seat belt legislation, improvements in car design and the drink/driving laws. Hence, fortunately there are fewer patients with head injuries to treat and it is even more important that patients are appropriately referred if studies to assess efficacy of the new strategies are not to be thwarted. The nihilistic concept that intensive investigation with ICP monitoring for patients with diffuse head injury or brain swelling following evacuation of a haematoma or a contusion has no proven beneficial effect on outcome, requires revision. A cocktail of therapies may be required that can be created only when patients are monitored in sufficient detail to reveal the mechanisms underlying their individual ICP problem. Ethical problems may arise over how aggressively therapy for intracranial hypertension should be pursued and for how long. There has always been the concern that cranial decompression or prolonged barbiturate coma may preserve patients but with unacceptably severe disability. Some patients may be salvaged from herniating with massive cerebral infarction with the use of osmotherapy but is the outcome acceptable? Similar considerations apply to some children with metabolic encephalopathies. Where such considerations have been scrutinised in patients with severe head injury, the whole spectrum of outcomes appears to be shifted so that the number of severe disabilities and persistent vegetative states are not increased. However, it is important to be sensitive to such issues based on experience of the particular cause of raised intracranial pressure in a given age group.

Germanium-68 as a possible marker for silicon transport in rat brain

Silicon (Si) is an essential trace element normally present in brain and cerebrospinal fluid, although the mechanism by which it enters and distributes in brain is largely unknown. Due to the short radioactive half-life of 31Si (156 min) we have investigated the use of 68Germanium (68-Ge, half-life 282 days) as a possible marker for Si transport in rat brain over longer periods than are possible with 31Si. Adult male anaesthetised rats were given a bolus of 68Ge I.V. and arterial blood samples taken during experiments that lasted between 5 min and 3 days. At termination, the brain was removed and analysed for radioactivity as were the plasma samples. Data were analyzed by Graphical Analysis which showed that the blood-brain barrier permeability to 68Ge (Kin approximately 10(-4) ml/min/g) is similar to that for many non protein-bound electrolytes in plasma and that 68Ge fluxes across cerebral capillaries are bidirectional. The autoradiographic distribution of 68Ge in brain was homogenous. Our results are in agreement with those of previous studies using 31Si or 68Ge, which suggest 68Ge may be a useful marker for Si when investigating the role of this element in conditions such as neurodegenerative diseases.

65Zn uptake from blood into brain in the rat

Zinc is essential for the normal development and function of the CNS, although little is known about brain zinc homeostasis. Therefore, in this investigation we have studied 65Zn uptake by brain from blood and have measured the blood-brain barrier permeability to 65Zn in the anaesthetised rat in vivo. Adult male Wistar rats within the weight range 500-600 g were used. 65ZnCl2 and 125I-albumin, the latter serving as a vascular marker, were injected intravenously in a bolus of normal saline. Sequential arterial blood samples were taken during experiments that lasted between 5 min and 5 h, after which the whole brain was removed, dissected, and analysed for radioisotope activity. Data have been analysed by graphical analysis, which suggests that after 30 min of circulation, 65Zn uptake by brain from blood is unidirectional with an influx rate constant, Kin, of approximately 5 X 10(-4) ml/min/g. At circulation times of less than 30 min, 65Zn fluxes between blood and brain are bidirectional, where influx has a K value of greater than 5 X 10(-4) ml/min/g. In addition to the blood space, the brain appears to contain a rapidly exchanging compartment(s) for 65Zn of approximately 4 ml/100 g, which is not CSF.

65zinc uptake from blood into brain and other tissues in the rat

Zinc is essential for normal growth, development and brain function although little is known about brain zinc homeostasis. Therefore, in this investigation we have studied 65Zn uptake from blood into brain and other tissues and have measured the blood-brain barrier permeability to 65Zn in the anaesthetized rat in vivo. Adult male Wistar rats within the weight range 500-600 g were used. 65ZnCl2 and [125I]albumin, the latter serving as a vascular marker, were injected in a bolus of normal saline I.V. Sequential arterial blood samples were taken during experiments that lasted between 5 min and 5 hr. At termination, samples from the liver, spleen, pancreas, lung, heart, muscle, kidney, bone, testis, ileum, blood cells, csf, and whole brain were taken and analysed for radio-isotope activity. Data have been analysed by Graphical Analysis which suggests 65Zn uptake from blood by all tissues sampled was unidirectional during this experimental period except brain, where at circulation times less than 30 min, 65Zn fluxes were bidirectional. In addition to the blood space, the brain appears to contain a rapidly exchanging compartment(s) for 65Zn of about 4 ml/100g which is not csf.

Effect on intracranial pressure of furosemide combined with varying doses and administration rates of mannitol

The first part of this study investigated the combined use of furosemide and mannitol in the treatment of elevated intracranial pressure (ICP). Two groups of dogs were studied to determine if renal excretion of mannitol was altered in the presence of furosemide. No significant difference in excretion was noted between the two groups. Fifteen animals were used in other studies to identify the most advantageous sequence of administration of furosemide and mannitol. Infusion of mannitol followed by furosemide 15 minutes later resulted in the most profound and sustained ICP reduction. The effect on ICP reduction of varying the mannitol dose was observed in studies using single doses of 0.5 gm/kg, 0.75 gm/kg, and 1 gm/kg. The larger mannitol dose, resulting in a greater blood-brain osmotic gradient, proved to be the most efficacious in ICP reduction. A further 15 animals were used in investigations to determine whether changing the rate of delivery of the most effective mannitol dose (1 gm/kg) influenced resultant ICP reduction. The results indicated that rapid administration (2 ml/kg/min) produced higher peak serum concentrations of mannitol and more profound lowering of ICP than the same dose delivered at slower rates.

Carrier-mediated transport of vasopressin across the blood-brain barrier of the mouse

A brain to blood carrier-mediated transport system for arginine vasopressin (AVP) was investigated in mice after intraventricular injection of iodinated AVP and varying amounts of unlabeled material or candidate inhibitors. Residual activity in the brain detected after decapitation was used as the main determinant of transport activity. The half-time disappearance of iodinated AVP from the brain was 12.4 min, the Vmax was 1.41 nmol/g-min, and the apparent Km was 28.7 nmol/g. A 30-nmol dose of AVP, mesotocin, arginine vasotocin, pressinoic amide, pressinoic acid, tocinoic acid, and lysine vasotocin, but not oxytocin, lysine vasopressin, AVP free acid, tocinoic amide, Tyr-MIF-1, or cyclo Leu-Gly, significantly (P less than 0.05) inhibited the transport of iodinated AVP out of the brain. The 30 nmol dose of AVP had no effect on the transport of iodide or iodotyrosine out of the brain. High-performance liquid chromatography showed that 59.2% of the radioactivity found in the blood 2 min after an i.c.v. injection of labeled AVP eluted at the same position as labeled AVP compared with 68.8% of radioactivity eluting at that position after material was infused i.v. for 2 min. This indicates that intact peptide is transported across the blood-brain barrier and that most of the degradation of AVP occurs during circulation in the blood. Calculations based on the appearance of radioactivity in the periphery showed that 56.2% of the material injected centrally would have been transported into the periphery by 10 min. This appearance of material in the periphery was inhibited by the simultaneous injection of an excess of unlabeled peptide. Water loading significantly decreased the brain to blood transport rate of AVP by 40%. It is concluded that a saturable system exists for brain to blood transport of AVP and some structurally similar peptides.

Cerebrospinal fluid pressure of anaesthetized rats during intracerebroventricular infusion

Increases in cerebrospinal fluid pressure (CSFP) were measured in the lateral ventricle in barbiturate anaesthetized male Sprague Dawley rats during intracerebroventricular (IVT) infusions into the contralateral ventricle. IVT infusions of isotonic artificial CSF (art-CSF) solutions at 10 and 38 microliters/hr increased mean CSFP from control preinfusion level of 3.6 cm H2O to 4.6 cm H20 (n.s.) and 5.2 cm H2O (p less than 0.01) respectively with CSFP appearing to attain equilibrium after 30-60 min of infusion. IVT infusion of hyperosmolar art CSF solutions (saccharide and salt solutions of approximate 1000 mOsm/kg) at 38 microliters/hr resulted in a larger increase of CSFP which equilibrated at 8.5 cm H2O (p less than 0.001) after 90 min of infusion. It is suggested that on the basis of CSFP measurements in these and other experiments cited that IVT infusions be run at infusion rates of less than 40 microliters/hr to ensure minimal physiological change.

Effects of rapid mannitol infusion on cerebral blood volume. A positron emission tomographic study in dogs and man

Positron emission tomography was used to study the effect of a rapid infusion of mannitol on cerebral blood volume (CBV) in five dogs and in three human subjects. The ability of mannitol to reduce intracranial pressure (ICP) has always been attributed to its osmotic dehydrating effect. The effects of mannitol infusion include increased osmolality, total blood volume, central venous pressure (CVP), and cerebral blood flow, and decreased hematocrit, hemoglobin concentration, serum sodium level, and viscosity. Mannitol's ability to dilate the cerebral vasculature, either directly or indirectly, and thus to transiently increase ICP, is a subject of controversy. By in vivo labeling of red cells with carbon-11, the authors were able to demonstrate an early increase in CBV in dogs of 20%, 27%, and 23% (mean increase, p less than 0.05) at 1, 2, and 3 minutes, respectively, after an infusion of 20% mannitol (2 gm/kg over a 3-minute period). The animals' muscle blood volume increased by 27% (mean increase, p less than 0.05) 2 minutes after infusion. In the human subjects, lower doses and a longer duration of infusion (1 gm/kg over 4 minutes) resulted in an increase in CBV of 8%, 14% (p less than 0.05), and 10% at 1, 2, and 3 minutes, respectively, after infusion. In dogs, ICP increased by 4 mm Hg (mean increase, p less than 0.05) 1 minute after the infusion, before decreasing sharply. The ICP was not measured in the human subjects. Hematocrit, hemoglobin, sodium, potassium, osmolality, heart rate, mean arterial pressure (MAP), and CVP were measured serially. Results of these measurements, as well as the significant decrease in MAP that occurred after mannitol infusion, are discussed. This study demonstrated that rapid mannitol infusion increases CBV and ICP. The increase in muscle blood volume, in the presence of a decreased MAP and an adequate CVP, suggests that mannitol may have caused vasodilation in these experiments.

Effect of mannitol on ICP and CBF and correlation with pressure autoregulation in severely head-injured patients

In a previous paper, the authors showed that mannitol causes cerebral vasoconstriction in response to blood viscosity decreases in cats. The present paper describes the changes in intracranial pressure (ICP) and cerebral blood flow (CBF) after mannitol administration in a group of severely head-injured patients with intact or defective autoregulation. The xenon-133 inhalation method was used to measure CBF. Autoregulation was tested by slowly increasing or decreasing the blood pressure by 30% and measuring CBF again. Mannitol was administered intravenously in a dose of 0.66 gm/kg; 25 minutes later, CBF and ICP were measured once again. In the group with intact autoregulation, mannitol had decreased ICP by 27.2%, but CBF remained unchanged. In the group with defective autoregulation, ICP had decreased by only 4.7%, but CBF increased 17.9%. One of the possible explanations for these findings is based on strong indications that autoregulation is mediated through alterations in the level of adenosine in response to oxygen availability changes in cerebral tissue. The decrease in blood viscosity after mannitol administration leads to an improved oxygen transport to the brain. When autoregulation is intact, more oxygen leads to decreased adenosine levels, resulting in vasoconstriction. The decrease in resistance to flow from the decreased blood viscosity is balanced by increased resistance from vasoconstriction, so that CBF remains the same. This might be called blood viscosity autoregulation of CBF, analogous to pressure autoregulation. Vasoconstriction also reduces cerebral blood volume, which enhances the effect of mannitol on ICP through dehydration of the brain. When autoregulation is not intact there is no vasoconstriction in response to increased oxygen availability; thus, CBF increases with decreased viscosity. With the lack of vasoconstriction, the effect on ICP through dehydration is not enhanced, so that the resulting decrease in ICP is much smaller. Such a mechanism explains why osmotic agents do not change CBF but decrease ICP in normal animals or patients with intact vasoconstriction, but do (temporarily) increase CBF in the absence of major ICP changes after stroke.

Mannitol causes compensatory cerebral vasoconstriction and vasodilation in response to blood viscosity changes

There is no proof that osmotic agents such as mannitol lower intracranial pressure (ICP) by decreasing brain water content. An alternative mechanism might be a reduction in cerebral blood volume through vasoconstriction. Mannitol, by decreasing blood viscosity, would tend to enhance cerebral blood flow (CBF), but the cerebral vessels would constrict to keep CBF relatively constant, analogous to pressure autoregulation. The cranial window technique was used in this study to measure the pial arteriolar diameter in cats, together with blood viscosity and ICP changes after an intravenous bolus of 1 gm/kg of mannitol. Blood viscosity decreased immediately; the greatest decrease (23%) occurred at 10 minutes, and at 75 minutes there was a "rebound" increase of 10%. Vessel diameters decreased concomitantly, the largest decrease being 12% at 10 minutes, which is exactly the same as the 12% decrease in diameter associated with pronounced hyperventilation (PaCO2 30 to 19 mm Hg) in the same vessels; at 75 minutes vessel diameter increased by 12%. With hyperventilation, ICP was decreased by 26%; 10 minutes after mannitol was given, ICP decreased by 28%, and at 75 minutes it showed a rebound increase of 40%. The correlation between blood viscosity and vessel diameter and between vessel diameter and ICP was very high. An alternative explanation is offered for the effect of mannitol on ICP, the time course of ICP changes, "rebound effect," and the absence of influence on CBF, all with one mechanism.

Acute dimethyl sulphoxide therapy in experimental brain oedema, effect of dose and concentration on brain water and electrolyte content

Albino rabbits with experimental brain oedema produced by a combined left hemisphere cryogenic injury and a metabolic insult by a 6-aminonicotinamide (6-ANA) were administered intravenous dimethyl sulphoxide (DMSO) in varying concentrations and doses in the following manner: Subgroup A (concentration response) received 1.0 g/kg bolus of a 10, 20, 30, or 40% solution. Subgroup B (dose response) received as a 20% solution a 1.0 g/kg bolus, 1.5 g/kg bolus, or 2.0 g/kg infusion. One hour following administration of the agent, the animals were killed, their brains rapidly removed by craniectomy and brain water, sodium and potassium measured. Significant decreases in brain sodium and water content in the right hemisphere were noted in both subgroups A (p less than 0.05) and B (p less than 0.005) and in the left hemisphere in subgroup B only (p less than 0.005). There is an apparent effect on brain oedema by a DMSO mediated sodium dependent water mobilization.

Methodology for the control of intracranial pressure with hypertonic mannitol

The response to intravenous bolus administrations and continuous infusions of hypertonic mannitol to control elevated (greater than 25 torr) intracranial pressure (ICP) is presented. Sixty patients received 120 bolus infusions of mannitol (0.18-2.5 g/kg/dose) with a prompt peak reduction at 44 minutes (range 18 to 120 minutes). There was no relationship between dosage and rapidity of peak response. All administrations of 1.0g/kg/dose, or higher, consistently reduced ICP 10% or more from control values, but dosages below 1 gm/kg/dose did not always reduce ICP. Return to control ICP following mannitol was unpredictable, and was related to the initial ICP and the volume of fluid replacement. A continuous infusion of mannitol was administered to maintain ICP below 25 torr in 18 patients. This infusion ranged from 6 to 100 hours (X 28.8+/-28.9 hours) and required a total dosage of 2-20 mg/kg, and was successful in 16 of the 18 patients. Emphasis is placed on close observation of the patient's serum osmolality and electrolytes during therapy, as well as quality and magnitude of replacement. No set rules are given for control of ICP, but a guideline is made to meet the individual patient's requirements.

Treatment of experimental cerebral infarction in rats with levodopa or with glycerol

Administration of large amounts of levodopa did not improve survival rates of rats after acute cerebral infarction induced by injection of carbon microspheres. However, when 10% glycerol was used, the number of rats that survived after cerebral infarction was significantly greater than in the control or in the levodopa-treated rats. Combination of levodopa and glycerol therapy also significantly improved the survival rate of beneficial effect. Pathological findings (gross or microscopic) indicate striking changes in brain tissue after embolization. Development of brain edema of the infarcted left hemisphere corresponded to the type of treatment and to the length of animal survival. Brain-tissue histology indicates that glycerol-treated animals developed less severe edema and had less tissue disruption than control animals. The results suggest that treatment of edema should be one of the primary steps in therapy after acute cerebral infarction.

Uptake of [14C]urea by the in vivo choroid plexus--cerebrospinal fluid--brain system: identification of sites of molecular sieving

1. The time course of the uptake of [(14)C]urea by the lateral ventricular choroid plexus of the adult rat in vivo was analysed to delineate further the permeability characteristics of the epithelial membrane of this secretory tissue.2. Eight hours after I.P. injection, [(14)C]urea attained a steady-state distribution in 78% of the tissue water of lateral ventricular choroid plexus; similarly, approximately 8 hr was required for radiourea to reach a steady-state concentration in both the cerebral cortex and cerebrospinal fluid (c.s.f.).3. Results obtained for compartment analysis were used to calculate the concentration of [(14)C]urea in the epithelium of the lateral ventricular plexus during the approach to and at steady-state distribution. Even after 1 hr of distribution, the [(14)C]urea concentration in choroid cell water was less than 15% of that in plasma water.4. Although the concentration of radiourea in choroid cell water continually increased after 3 hr, it remained in equilibrium with the concentration of [(14)C]urea in c.s.f. water. At the steady state (i.e., 8 hr), the distribution of [(14)C]urea between the water of plasma and that of the choroidal epithelium was considerably away from equilibrium (i.e., by 25-30%).5. An analysis of the concentration gradients for [(14)C]urea across both the apical (c.s.f.-facing) and basolateral (plasma-facing) membranes of the epithelium of the lateral ventricular plexus suggests that the movement of urea is hindered to a greater extent by the basolateral membrane than by the apical membrane.6. Only a single half-time component (1.3 hr) can be resolved from analysis of the curve describing the time course of uptake of radiourea by the choroid epithelial cell compartment.7. The concentration gradient data suggest that urea penetrates from blood to c.s.f. via the choroid plexus by a transcellular pathway; however, it is not possible to rule out a paracellular pathway for urea movement.8. At the steady state, radiourea distributes into 88% of the water of cerebral cortex. This observation, together with the finding of a steady-state concentration gradient for [(14)C]urea from cortical tissue to c.s.f., constitutes evidence that urea movement is hindered at the blood-brain barrier as well as at the blood-c.s.f. barrier.