Infusion mechanisms in brain white matter and its dependence of microstructure: an experimental study of hydraulic permeability

Objective: Hydraulic permeability is a topic of deep interest in biological materials because of its important role in a range of drug delivery-based therapies. The strong dependence of permeability on the geometry and topology of pore structure and the lack of detailed knowledge of these parameters in the case of brain tissue makes the study more challenging. Although theoretical models have been developed for hydraulic permeability, there is limited consensus on the validity of existing experimental evidence to complement these models. In the present study, we measure the permeability of white matter (WM) of fresh ovine brain tissue considering the localised heterogeneities in the medium using an infusion based experimental set up, iPerfusion. We measure the flow across different parts of the WM in response to applied pressures for a sample of specific dimensions and calculate the permeability from directly measured parameters. Furthermore, we directly probe the effect of anisotropy of the tissue on permeability by considering the directionality of tissue on the obtained values. Additionally, we investigate whether WM hydraulic permeability changes with post-mortem time. To our knowledge, this is the first report of experimental measurements of the localised WM permeability, showing the effect of axon directionality on permeability. This work provides a significant contribution to the successful development of intra-tumoural infusion-based technologies, such as convection-enhanced delivery (CED), which are based on the delivery of drugs directly by injection under positive pressure into the brain

On Brain Oedema

Traumatic brain injury and bacterial meningitis may at a glance appear as two different disease entities. The host reaction, however, to both trauma and infection involves a strong inflammatory response, with the classical symptomology of rubor, tumor, calor, dolor et functio laesa. Tumor, swelling, will increase the volume within the closed cranial vault, and may thereby raise the intracranial pressure to critical levels, affecting cerebral blood flow and oxygenation. In this thesis, brain oedema and its origins are investigated in experimental models of brain trauma and bacterial meningitis, and in patients suffering severe head injury. The studies are focused on fluid therapy, blood-brain barrier permeability changes, and their effects on oedema formation. In study I and II, it was shown that plasma volume expansion with crystalloid compared to colloid fluids resulted in increased cortical brain oedema after brain trauma in rats, and increased intracranial pressure in experimental meningitis in cats. Evidence is given that increased permeability of the blood-brain barrier is a key determinant of tissue water content after the insults. In study III, it was shown that post treatment with prostacyclin in experimental meningitis reduces systemic plasma volume loss, and may diminish the rise in intracranial pressure. In study IV, the effect of statin treatment after brain trauma in rats was investigated. No effect on brain oedema, cortical blood flow, or the transfer constant for a small molecule was detected, but capillary patency was better preserved in the statin group. Statin treatment was associated with increased plasma levels of nitric oxide, and decreased levels of prostacyclin. In study V, post-traumatic permeability changes of the blood-brain barrier in 17 brain trauma patients were quantified using contrast enhanced computerized tomography with multiple scanning during 25 minutes. It was shown that blood to brain transfer for iohexol was increased up to 20-fold in traumatized tissue. The relevance of these results regarding our understanding of the pathophysiology of post-traumatic brain oedema, and the implications for the clinical management of brain trauma patients are discussed

Should chloride-rich crystalloids remain the mainstay of fluid resuscitation to prevent ‘pre-renal’ acute kidney injury?: con

The high chloride content of 0.9% saline leads to adverse pathophysiological effects in both animals and healthy human volunteers, changes not seen after balanced crystalloids. Small randomized trials confirm that the hyperchloremic acidosis induced by saline also occurs in patients, but no clinical outcome benefit was demonstrable when compared with balanced crystalloids, perhaps due to a type II error. A strong signal is emerging from recent large propensity-matched and cohort studies for the adverse effects that 0.9% saline has on the clinical outcome in surgical and critically ill patients when compared with balanced crystalloids. Major complications are the increased incidence of acute kidney injury and the need for renal replacement therapy, and that pathological hyperchloremia may increase postoperative mortality. However, there are no large-scale randomized trials comparing 0.9% saline with balanced crystalloids. Some balanced crystalloids are hypo-osmolar and may not be suitable for neurosurgical patients because of their propensity to cause brain edema. Saline may be the solution of choice used for the resuscitation of patients with alkalosis and hypochloremia. Nevertheless, there is evidence to suggest that balanced crystalloids cause less detriment to renal function than 0.9% saline, with perhaps better clinical outcome. Hence, we argue that chloride-rich crystalloids such as 0.9% saline should be replaced with balanced crystalloids as the mainstay of fluid resuscitation to prevent ‘pre-renal’ acute kidney injury

The glymphatic hypothesis: the theory and the evidence.

The glymphatic hypothesis proposes a mechanism for extravascular transport into and out of the brain of hydrophilic solutes unable to cross the blood-brain barrier. It suggests that there is a circulation of fluid carrying solutes inwards via periarterial routes, through the interstitium and outwards via perivenous routes. This review critically analyses the evidence surrounding the mechanisms involved in each of these stages. There is good evidence that both influx and efflux of solutes occur along periarterial routes but no evidence that the principal route of outflow is perivenous. Furthermore, periarterial inflow of fluid is unlikely to be adequate to provide the outflow that would be needed to account for solute efflux. A tenet of the hypothesis is that flow sweeps solutes through the parenchyma. However, the velocity of any possible circulatory flow within the interstitium is too small compared to diffusion to provide effective solute movement. By comparison the earlier classical hypothesis describing extravascular transport proposed fluid entry into the parenchyma across the blood-brain barrier, solute movements within the parenchyma by diffusion, and solute efflux partly by diffusion near brain surfaces and partly carried by flow along "preferred routes" including perivascular spaces, white matter tracts and subependymal spaces. It did not suggest fluid entry via periarterial routes. Evidence is still incomplete concerning the routes and fate of solutes leaving the brain. A large proportion of the solutes eliminated from the parenchyma go to lymph nodes before reaching blood but the proportions delivered directly to lymph or indirectly via CSF which then enters lymph are as yet unclear. In addition, still not understood is why and how the absence of AQP4 which is normally highly expressed on glial endfeet lining periarterial and perivenous routes reduces rates of solute elimination from the parenchyma and of solute delivery to it from remote sites of injection. Neither the glymphatic hypothesis nor the earlier classical hypothesis adequately explain how solutes and fluid move into, through and out of the brain parenchyma. Features of a more complete description are discussed. All aspects of extravascular transport require further study

New diagnostic approaches to monitor irrigating fluid absorption

New diagnostic approaches to monitor irrigating fluid absorption Rinsing the endoscopic operating field with an irrigating solution entails the risk of absorption of the fluid. The physiological consequences of such absorption are explored and two new methods for monitoring the amount of absorption are proposed. Methods: Paper 1: 25 anesthetized pigs were randomized to control or continuous infusion of 100 ml/kg over 90 min of either glycine 1.5%, mannitol 3% or mannitol 5%. Several invasive measurements and calculations were performed to describe the pathophysiological processes. Paper 2: Exhaled air nitrous oxide (N2O) concentrations were measured in 12 volunteers receiving intravenous infusions, containing dissolved nitrous oxide and simulating fluid absorption. Paper 3: Comparison of N2O and ethanol for detecting absorption in 86 patients, at 2 centres, undergoing transurethral resection of the prostate (TURP) in spinal anesthesia. Paper 4: A 3-part evaluation of glucose as a tracer in fluid absorption detection. Part 1 was a clinical study in 250 patients undergoing TURP with and without a glucose-containing irrigant. Part 2 investigated the glucose kinetics in 10 volunteers receiving 20 ml/kg of acetated Ringer's solution with 1% glucose over 30 min. In part 3, data was used for computer simulations of various absorption patterns. Results: (Paper 1): Infusions rendered a hypokinetic hypotensive state. Intracellular volume expansion, intracranial pressure elevation and myocardial damage were greater for glycine 1.5%. (Paper 2): N2O is a useful tracer for noninvasive fluid absorption monitoring. It identifies the pattern and the volume of absorption with a 95% predicition interval of ± 200 ml. (Paper 3): The N2O method is feasible in a clinical setting. Agreement with the ethanol method was volume dependent and about twice that of N2O versus known volume. (Paper 4): Sodium and glucose showed a strong inverse linear relation for all patients including diabetics. The glucose levels almost doubled after the experimental infusions, which volume diluted the plasma by 17.7%. Simulations showed that the infused volume correlated with the rise in glucose where an increase by more than 1.4 mmol/L could detect absorption with 95% confidence. Conclusion: The pathophysiological process and treatment rationale of massive nonelectrolytic irrigating fluid absorption was outlined. The N2O method allows noninvasive online monitoring of irrigating fluid absorption with better resolution and similar or better prediction of absorbed volume compared to the ethanol method. Glucose can be used as a tracer for retrospective evaluation of irrigating fluid absorption

A Model for Studying Vasogenic Brain Edema

Convection-enhanced delivery (CED) is a proven method for targeted drug delivery to the brain that circumvents the blood-brain barrier (BBB). Little study has been conducted in understanding CED in pathological brain states. This is of importance when dealing with chemotherapeutic agent delivery to brain tumors, where vasogenic edema (VE) exists. The current study aims to characterize a model of VE suitable for studying CED.VE was produced in the right hemisphere of the rat brain using multiple infusions of hyperosmotic mannitol (0.25mL/kg/s over 30 seconds) delivered through the right internal carotid artery. Magnetic resonance imaging (MRI) revealed consistent edema formation and high water levels in the ipsilateral gray and white matter within an hour of the first infusion. Evan\u27s Blue (EB) staining verified that VE has formed. However, apparent diffusion coefficient (ADC) and histological examination revealed also that some possible cytotoxic edema formed.This model provides a reproducible technique for generating a large area of edema for CED study. Further studies with lower doses of mannitol, while titrating to changes in ADC and values for fractional water content, may modify this model with a greater component of VE and less cerebral toxicity

modeling pure vasogenic edema in the rat brain

Targeted drug delivery to the brain is difficult to achieve using conventional techniques, largely due to the blood-brain barrier’s (BBB) impediment to drug diffusion into the brain parenchyma. In response, development of convection-enhanced delivery (CED) offers the ability to circumvent the BBB and target specific areas of the brain. Predictability of infusate movement in pathological brain states during CED will maximize the effectiveness of this treatment, and therefore modeling of infusate movement must be characterized. Previous work from our lab effectively modeled CED in rats using the middle carotid artery occlusion model of cytotoxic edema. However, previous models examined for vasogenic edema study did not show pure vasogenic edema. The purpose of this study was to develop a model of pure vasogenic edema in the rat brain. In this study, we show that stereotactic 9 µL infusion of 1.0 mM DCA over 45 minutes into the rat corpus callosum reproducibly creates pure vasogenic edema, as observed in the peritumoral white matter surrounding gliomas

Image-based predictive modelling frameworks for personalised drug delivery in cancer therapy

Peer reviewe

Space medicine research publications: 1983-1984

A list of publications supported by the Space Medicine Program, Office of Space Science and Applications is given. Included are publications entered into the Life Sciences Bibliographic Database by The George Washington University as of October 1, 1984