Textbook of Neuroanaesthesia and Critical Care - part 2 ppsx

Effects of sevoflurane on intracranial pressure, cerebral blood flow and cerebral metabolism... Effect of head elevation on intracranial pressure, cerebral perfusion pressure and cerebra

74 Rampil IJ, Lockhart SH, Eger El, Weiskopf RB Human EEG dose response to desflurane Anesthesiology 1990; 73: A1218.

75 Tonner PH, Scholz J, Krause T, Paris A, Von Knobelsdroff G, Schulte an Esch J Administration of sufentanil and nitrous oxide blunts cardiovascular responses to desflurane but does not prevent an increase in middle cerebral artery flow velocity Eur J Anaesthesiol 1997; 14: 389–396

76 Bundgaard H, Von Oettingen G, Larsen KM et al Effects of sevoflurane on intracranial pressure, cerebral blood flow and cerebral metabolism A dose-response

Page 32 study in patients subjected to craniotomy for cerebral tumours Acta Anaesthesiol Scand 1998; 42: 621–627.

77 Summors A, Gupta A, Matta BF Dynamic cerebral autoregulation during sevoflurane anaesthesia: a comparison with isoflurane Anesth Analg 1999; 88: 341–345

78 Gupta S, Heath K, Matta BF The effect of incremental doses of sevoflurane on cerebral pressure autoregulation in humans: a transcranial Doppler study Br J Anaesth 1997; 79: 469–472

79 Matta BF, Mayberg TS, Lam AM Direct cerebrovasodilatory effects of halothane, isoflurane and desflurane during induced isoelectric encephalogram in humans Anesthesiology 1995; 83: 980–985

propofol-80 Heath K, Gupta S, Matta BF Direct cerebral vasodilatory effect of sevoflurane Anesthesiology 1997; 87: A177

81 Heath KJ, Gupta S, Matta BF The effects of sevoflurane on cerebral hemodynamics during propofol anesthesia Anesth Analg 1997; 85: 1284–1287

82 Gupta S, Heath K, Matta BF Effect of incremental doses of sevoflurane on cerebral pressure autoregulation in humans Br J Anaesth 1997; 79: 469–472

83 Kassel NF, Hitchon PW, Gerk MK, Sokoll MD, Hill TR Alterations in cerebral blood flow, oxygen metabolism, and electrical activity produced by high dose sodium thiopental Neurosurgery 1980; 7: 598–603

84 Milde LN, Milde JH, Michenfelder JD Cerebral functional, metabolic, and hemodynamic effects of etomidate in dogs

90 Keykhah MM, Smith DS, Carlsson C, Safo Y, Englebach I, Harp JR Influence of sufentanil on cerebral metabolism and

circulation in the rat Anesthesiology 1985; 63: 274–280

91 McPherson RW, Krempasanka E, Eimerl D, Traystman RJ Effects of alfentanil on cerebral vascular reactivity in dogs Br J Anaesth 1985; 57: 1232–1238

92 Murkin JM, Ferrar JK, Tweed WA, McKenzie FN, Guiraudon G Cerebral autoregulation and flow/metabolism coupling during cardiopulmonary bypass: the influence of PaCO2 Anesth Analg 1987; 66: 825–832

93 Sperry RJ, Bailey PL, Reichman MV, Peterson PB, Pace NL Fentanyl and sufentanil increase intracranial pressure in head trauma patients Anesthesiology 1992; 77: 416–420

94 Åkeson J, Björkman S, Messeter K, Rosén I, Helfer M Cerebral pharmacodynamics of anaesthetic and subanaesthetic doses of ketamine in the normoventilated pig Acta Anaesthesiol Scand 1993; 37: 211–218

95 Menon DK, Burdett NG, Carpenter TA, Hall LD Functional MRI of ketamine-induced changes in rCBF: an effect at the NMDA receptor? (abstract) Br J Anaesth 1993, 71: 767P

96 Mayberg TS, Lam AM, Matta BF, Domino K, Winn HR Ketamine does not increase cerebral blood flow velocity or intracranial pressure during isoflurane-nitrous oxide anesthesia in patients undergoing craniotomy Anesth Analg 1995; 81: 84–89

97 Forster A, Juge O, Morel D Effects of midazolam on cerebral blood flow in human volunteers Anesthesiology 1982; 56: 453–455

98 Fleischer JE, Milde JH, Moyer TP, Michenfelder JD Cerebral effects of high-dose midazolam and subsequent reversal with RO 15–1788 in dogs Anesthesiology 1988; 68: 234–242

Page 33 oxygen difference, and outcome in head injured patients J Neurol Neurosurg Psychiatry 1992; 55: 594–603.

106 Bouma GJ, Muizelaar JP, Stringer WA, Choi SC, Fatouros P, Young HF Ultra early evaluation of regional cerebral blood flow

in severely head-injured patients using xenon-enhanced computerized tomography J Neurosurg 1992; 77: 360–369

107 Martin NA, Patwardhan RV, Alexander MJ et al Characterization of cerebral hemodynamic phases following severe head trauma: hypoperfusion, hyperaemia and vasospasm J Neurosurg 1997; 87: 9–19

108 Martin NA, Doberstein C, Zane C, Caron MJ, Thomas K, Becker DP Posttraumatic cerebral arterial spasm: transcranial Doppler ultrasound, cerebral blood flow and angiographic findings J Neurosurg 1992; 77: 575–583

109 Marion DW, Darby J, Yonas H Acute regional cerebral blood flow changes caused by severe head injuries J Neurosurg 1991; 74: 407–414

110 McLaughlin MR, Marion DW Cerebral blood flow within and around cerebral contusions J Neurosurg 1996; 85: 871–876

111 Menon DK, Minhas P, Herrod NJ et al Cerebral ischaemia associated with hyperventilation: a PET study Anesthesiology 1997; 87: A176

112 Chan KH, Dearden NM, Miller JD, Andrews PJ, Midgley S Multimodality monitoring as a guide to treatment of intracranial hypertension after severe brain injury Neurosurgery 1993; 32: 547–552

113 Feldman Z, Kanter MJ, Robertson CS et al Effect of head elevation on intracranial pressure, cerebral perfusion pressure and cerebral blood flow in head injured patients J Neurosurg 1992; 76: 207–211

114 Lassen NA, Agnoli A The upper limit of autoregulation of cerebral blood flow on the pathogenesis of hypertensive

encephalopathy Scand J Clin Lab Invest 1973; 30: 113–116

115 Voldby B, Enevoldsen EM, Jensen FT Cerebrovascular reactivity in patients with ruptured intracranial aneurysm J Neurosurg 1985; 62: 59–67

116 Pickard JD, Matheson M, Patterson J, Wyper D Prediction of late ischemic complications after cerebral aneurysm surgery by the intraoperative measurement of cerebral blood flow J Neurosurg 1980; 53: 305–308

117 Kassell NF, Peerless SJ, Durward QJ, Beck DW, Drake CG, Adams HP Treatment of ischaemic deficits from vasospasm with intravascular volume expansion and induced arterial hypertension Neurosurgery 1982; 11: 337–343

118 Yamaura I, Tani E, Maeda Y, Minami N, Shindo H Endothelin-1 of canine basilar artery in vasospasm J Neurosurg 1992; 76: 99–105

119 Clozel M, Watanabe H BQ-123, a peptidic endothelin ETA receptor antagonist, prevents the early cerebral vasospasm

following subarachnoid hemorrhage after intracisternal but not intravenous injection Life Sci 1993; 52: 825–834

120 Jakobsen M, Skj3/4dt T, Enevoldsen E Cerebral blood flow and metabolism following subarachnoid hemorrhage: effect of subarachnoid blood Acta Neurol Scand 1991; 8: 226–233

121 Pickard JD, Murray GD, Illingworth R et al Effect of oral nimodipine on cerebral infarction and outcome after subarachnoid hemorrhage: British Aneurysm Nimodipine Trial BMJ 1989; 298: 636–642

122 Origitano TC, Wascher TM, Reichman OH, Anderson DE Sustained increased cerebral blood flow with prophylactic

hypervolemic haemodilution ('Triple – H' Therapy) after subarachnoid haemorrhage Neurosurgery 1990; 27: 729–738

123 Darby JM, Yonas H, Marks EC, Durham S, Snyder RW, Nemoto EM Acute cerebral blood flow response to dopamine-induced hypertension after subarachnoid hemorrhage J Neurosurg 1994; 80: 857–864

124 Zakhary R, Gaine SP, Dinennan JL et al Heme oxygenase 2: endothelial and neuronal localisation and role in dependent relaxation Proc Natl Acad Sci USA 1996; 93: 795–798

endothelial-125 Iadecola C Neurogenic control of the cerebral microcirculation: is dopamine mining the store? Nature Neurosci 1998; 1: 363–364

3—

Mechanisms of Injury and Cerebral Protection

Patrick W Doyle & Arun K Gupta

Page 37Mechanisms of neural injury, cerebral protection and cerebral resuscitation have been an area of intensive research over the last 20 years and the pathophysiological and biochemical processes responsible for the development and propagation of neural injury are becoming clearer Despite this explosion of research, current approaches to reducing permanent injury have remained largely

unchanged over the past few decades The pathophysiological processes that lead to neuronal cell death and loss of function are similar whether the CNS insult is the consequence of intraoperative injury, stroke or trauma In its correct terminology, cerebral protection refers to interventions aimed at reducing neural injury that are instituted before a possible ischaemic event, while cerebral resuscitation refers to interventions that occur after such an event.1 In practice, many of the mechanisms involved in both phases of the process are identical and the following discussion will deal with both forms of intervention as one

Mechanisms of Injury

All brain injury can be thought of as being constituted of basic primary, secondary and molecular and biochemical processes

Whatever the primary insult, there will always be secondary and molecular damage, not only in the core of the lesion but also in the penumbral region Central to all mechanisms of injury are cerebral ischaemia and hypoxia Global ischaemia refers to events which result in complete hypoperfusion of the entire organ or where no potential for recruitment of collateral flow exists Focal ischaemia refers to the occlusion of an artery distal to the circle of Willis, which permits some collateral flow, thus resulting in a dense

ischaemic core with a partially perfused surrounding penumbral zone.1 Tissues in the penumbral zone may be more salvageable and hence provide a realistic target for neuroprotection

Basic Mechanisms of Injury

All the principal types of brain damage that occur clinically can now be reproduced in experimental models.2,3 These can be

classified as traumatic, ischaemic or hypoxic Traumatic brain injury may be due to the trauma associated with accidents or personal violence Alternatively, the trauma may be iatrogenic and accompany a variety of operative procedures, including retraction, shear forces, direct tissue destruction, haemorrhage and vessel disruption with subsequent infarction These injuries are typically followed

by brain swelling, leading to an increase in intracranial volume and intracranial pressure and a consequent reduction in cerebral blood flow In addition to direct tissue injury, acceleration-deceleration forces may result in shearing of nerve fibres and microvascular

structures in the process termed diffuse axonal injury.4,5 While this was originally thought to occur at the time of injury, there is accumulating evidence that the event of axonal shearing is the culmination of processes that mature over hours

Secondary insults are initiated as a consequence of the primary injury but may not be apparent for an interval following the injury Intracranial haemorrhage is the most common local structural cause of clinical deterioration and death in patients who have

experienced a lucid interval after traumatic injury.6,7,8 The pathophysiology associated with this process may reflect simple

physiological consequences of ischaemia arising from pressure effects to underlying and distant brain regions, shift of vital structures and axonal disruption, reductions in cerebral blood flow and metabolism, hydrocephalus and herniation However, metabolic

processes may cause more subtle changes and ischaemia may not just be due to local microcirculatory compression but also the consequence of vasoactive substances released from the haematoma In addition, glucose utilization has also been found to be markedly increased in pericontusional and perihaemorrhagic regions, possibly due to activation of excitatory neuronal systems.2,9 In addition, extravasated subarachnoid blood can cause vasospasm both locally and at distant sites with aggravation of ischaemia

Systemic physiological insults may occur as a consequence of the primary lesion but can contribute to worsening neural injury These include hypoxia, hypotension, hypercarbia, hyperthermia, anaemia and electrolyte disturbances Hypoxia may be the result of airway obstruction, aspiration, thoracic injury, primary hypoventilation or pulmonary shunting.3 Hypotension has been found to occur in 32–35% of patients in emergency departments, which may be due to systemic causes.7 This causes a decrease in cerebral perfusion pressure, which may be aggravated by a high ICP, disruption of cerebrovascular autoregulation, vasospasm and change in cerebral blood flow patterns Hyperthermia may be due to infection, thrombophlebitis, drug reactions or a defect in the thermoregulatory system This results in excessive excitotoxic neurotransmitter release, altered protein kinase C activity and augmented

pathophysiological effects of ischaemia Hypercarbia causes vasodilatation of cerebral blood vessels, with increased ICP, and exacerbation of any mass or oedema effect It may also be associated with cerebral metabolic acidosis

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injury.19 While the general consensus is that preischaemic hyperglycaemia is deleterious, in some studies it has been shown to delay the onset of ischaemic Ca++ influx from the ECF and potentiate reextrusion of Ca++ following recirculation These findings may reflect the modulatory effects of the type of ischaemia (focal versus global), its duration and extent and the completeness of the ischaemic insult.10,20 It is thought that acidosis enhances production of reactive free radicals, causes oedema, aggravated tissue damage and delayed seizures and prevents recovery of mitochondrial metabolism.14,19 However, it still remains to be fully

established whether exaggerated intra-ischaemic acidosis enhances postischaemic production of reactive oxygen species (ROS)

Ionic Pump Failure

A variety of membrane ionic pumps, including Na+K+, Na+–Ca++, and Ca++–H+, as well as Cl– and HCO3– leakage fluxes, maintain

are energy-consuming processes, ischaemia-induced decreases in ATP production result in loss of ionic pump function, with changes in

pump failure but in the 'milieu' of ischaemic tissue, local depolarization leads to activation of ionic conductances The increased energy demands of

Siësjo and Siësjo describe three major cascades of reactions:19

1 sustained perturbation of cell Ca++metabolism;

2 persistent depression of protein synthesis;

3 programmed cell death

Calcium

Ca ions play an important role in normal membrane excitation and cellular processes.21 Normally extracellular concentrations are maintained at a higher concentration than free cytosolic concentrations by an ionic ATP-dependent pump Failure of ATP energy metabolism will have a deleterious effect on this homoeostasis.23 It is postulated that the primary defect in cells mortally injured by a transient period of ischaemia is an inability to regulate Ca++.24 The slow gradual rise in Ca++ is caused by the release of glutamate from the presynaptic nerve endings, primarily via activation of receptors of the NMDA type This leads to excessively high cytosolic

Ca++ levels Activation of the AMPA receptors also results in a Na+– dependent depolarization which causes further Ca++ influx via voltage-gated channels The secondary loss of cell Ca++ homoeostasis may also affect the relationship between Ca++ leaks and Ca++extrusion across membranes of the sarcoplasmic reticulum Exposure of mitochondria to excess Ca++ causes them to swell and release intramitochondrial components This reflects a sudden increase in the permeability of the mitochondrial inner membrane which allows the release of H+, Ca++, Mg++, and other low molecular weight components There is strong evidence that mitochondrial dysfunction is an early recirculation event following long periods of ischaemia or ischaemia complicated by hyperglycaemia,

qualifying as a direct cause of bioenergetic failure.19 The effects of Ca++are summarized in Figure 3.1

Depression of Protein Synthesis

Normal protein synthesis is an early casualty of the ischaemic cascade Normally the glutamate-induced Ca influx would result in

transcription and translation of the immediate early genes (IEGs) c-fos and c-jun These IEGs regulate the transcription of genes that

code for proteins of repair.19,23 These include the heat shock protein family, nerve growth factors, brain-derived neurotrophic factor, neurotropin-3 and enhanced expression of genes for glucose transports.25 A block in translation due to focal or global ischaemia may thus affect the production of these stress proteins, trophic factors or enzymes and enhance ischaemic damage (Fig 3.2)

Programmed Cell Death (PCD)

PCD, or apoptosis when it occurs during development, is a process that weeds out approximately half of all neurones produced during neurogenesis, leaving only those that make useful functional connections to other neurones and end-organs.25 It is a cell death characterized by membrane blebbing, cell shrinkage, nuclear condensation and fragmentation There are considerable data that indicate that the mechanisms leading to apoptotic and necrotic forms of cell injury are very similar.19 In apoptosis, cells and nuclei shrink, condense and fragment and are rapidly phagocytosed by macrophages There is no leakage of cellular contents and thus no reactive response During cell injury, cells swell, burst and necrose The rupture of

Figure 3.1

to the central nervous system Williams and Wilkins, Baltimore, 1996, pp 7–19)

intracellular contents into the ECF space provides a stimulus for a reactive response.26 PCD is an active process which requires protein synthesis and is executed by the activation of 'death genes',27 probably triggered by stimuli such as free radicals, Ca++

accumulation, excitatory amino acids (glutamate), cytokines, antigens, hormones and apoptotic receptor signalling.16,19The apoptotic process also involves changes in cell surface chemistry to enable recognition by macrophages Much of the delayed neuronal necrosis that accounts for cell death hours or days subsequent to reperfusion after ischaemic injury appears to be caused by PCD,25 and signs

of apoptosis are often encountered in the penumbral zone of a focal ischaemic area.19

Lipid Peroxidation and Free Radical Formation

Free radicals are reactive chemical species that damage DNA, denature structural and functional proteins and result in peroxidation

of membrane lipids Free radicals are formed as a consequence of several processes including phospholipase activation by cytosolic

Ca++, transitional metal reactions which involve free iron, arachidonate metabolism and oxidant production by inflammatory cells These processes result in the formation of superoxide radicals, which are protonated in the ischaemic environment of the ischaemic brain to produce highly reactive hydroxyl radicals Normally aerobic cells produce free radicals, which are then consumed by free radical scavengers, e.g a-tocopherol and ascorbic acid, or appropriate enzymes, e.g superoxide dismutase In states where enzymatic processes are disrupted (ischaemia) or hyperoxia occurs (reperfusion), there may be excessive production of oxidants, in particular superoxide, hydrogen peroxide and the hydroxyl radical These highly reactive oxidant species cause peroxidation of membrane phospholipids, oxidation of cellular proteins and nucleic acids and can attack both neuronal membranes as well as cerebral

vasculature.10 It appears that free radicals target cerebral microvasculature and that with other inflammatory mediators, e.g activating factor, cause microvascular dysfunction and blood–brain–barrier disruption.19 The brain is particularly vulnerable to oxidant attack due to intrinsically low levels of tissue antioxidant activity

platelet-Endothelial nitric oxide (NO) is normally associated with relaxation of vascular endothelium and in this setting may aid recovery from acute ischaemic insults However, generation of neuronal NO, often triggered by EAAs, may result in cellular injury One of the mechanisms of such injury involves the combination of NO with hydroxyl radicals to generate the highly reactive peroxynitrite species, which can result in molecular oxidant injury

Adhesion Molecule Expression

Acute brain injury is known to be associated with an inflammatory response29 and there is evidence that leucocytes are involved in the production of brain swelling up to 10 days postinjury Gupta et al have demonstrated that normal brain endothelial cells express low levels of leucocyte cell adhesion molecules (CAMs), and that these molecules are upregu-

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Figure 3.2 (1) Ischaemia causes axon terminals to release excitoxic glutamate, which opens

(2) Excess calcium, sodium and other indicators of ischaemia activate protein kinases which (3) phosphorylate immediate early gene (IEG) transcription factors (4) These travel from the cytoplasm into the nucleus where they induce the transcription of IEG

DNA (e.g c-fos and c-jun), making IEG mRNA (5) IEG mRNA leaves the nucleus

and is translated at ribosomes into IEG protein (e.g Fos and Jun families) (6) These gene-specific IEG products travel from the cytoplasm into the nucleus where they initiate transcription of DNA that codes for proteins of repair or the endonucleases that cause programmed cell death (PCD) (7) Repair or PCD mRNA then goes out

to ribosomes in the cytoplasm where it is translated into proteins of repair (e.g

heat shock proteins) or PCD endonucleases (8) Neurones that are distant from the ischaemic area are signalled to induce IEG transcription and translation

lated in a time-dependent manner following head injury in humans.30 Activation of these CAMs recruits neutrophils to the damaged area, thereby occluding capillaries and enhancing free radical production This has important implications for the potential strategies using antibodies that have been found experimentally.31,32

Brain Oedema

Two types of oedema occur: cytotoxic and vasogenic Cytotoxic oedema is due to failure of ionic pumps with resultant ionic and fluid shifts Vasogenic oedema is due to the release of mediators that damage endothelial cells, basement membrane matrix and/or glial cells, resulting in blood–brain barrier breakdown Specific mediators that have been involved in this process include arachidonic acid metabolites, free radicals, bradykinin and platelet-activating factor The resulting oedema can cause increases in intracranial pressure, with reduction in cerebral perfusion pressure (and cerebral ischaemia) and herniation of brain structures

Cerebral Protection

Cerebral protection implies interventions designed to prevent pathophysiolgical processes from occurring, whilst cerebral

resuscitation refers to intervention instituted after onset of the ischaemic insult, in order

to interrupt the process.1 It goes without saying that any form of cerebral or neuroprotection begins with the fundamentals of any resuscitative treatment, i.e basic airway, respiratory and cardiovascular support Unless normoxia and normotension are maintained, the application of drugs that antagonize the processes listed above is bound to be ineffective These basics of clinical management are dealt with in appropriate sections elsewhere.

While this discussion will focus on agents that achieve neuroprotection by reversing one or more of the secondary injury processes listed above, other therapeutic interventions can reduce neuronal injury by optimizing cerebrovascular physiology Drugs such as mannitol and dexamethasone can reduce posttraumatic and peritumoral oedema respectively and thus augment cerebral perfusion pressure and oxygen delivery Similarly, the use of haemodynamic augmentation with hypervolaemia and hypertension can enhance cerebral blood flow in the setting of intracranial hypertension or cerebral vasospasm

Anaesthetic Agents

Barbiturates

As early as 1966 it was recognized that barbiturates had a neuroprotective effect and they have served as the prototype for anaesthetic protection against cerebral ischaemia The primary CNS protective mechanism of the barbiturates is attributed to their ability to decrease the cerebral metabolic rate, thus improving the ratio of oxygen supply to oxygen demand.33 More specifically, these agents appear to selectively reduce the energy expenditure required for synaptic transmission, whilst maintaining the energy required for basic cellular functions.34 Mechanisms by which these effects may be exerted are listed in Box 3.1

Maximal metabolic suppression by anaesthetic agents can reduce oxygen demands to approximately 50% of baseline values, since the remaining oxygen utilization is required to support cellular integrity rather than suppressible electrical activity.36 Barbiturates appear to be particularly protective in conditions of focal ischaemia as even though blood flow may be reduced, some synaptic transmission continues and its suppression can improve oxygen demand/supply relationships.25 Such electrical activity is absent during global ischaemia and studies to date have failed to demonstrate any improved clinical outcome with anaesthetic

neuroprotection following cardiac arrest.37,38 There is currently considerable caution in assigning neuroprotective properties to agents based on studies conducted before the confounding effects of mild hypothermia were well documented

1 Reduction in synaptic transmission

2 Reduction in calcium influx

3 Ability to block sodium channels and membrane stabilization

4 Improvement in distribution of regional cerebral blood flow

5 Suppression of cortical EEG activity

6 Reduction in cerebral oedema

7 Free radical scavenging

8 Potentiate GABA-ergic activity

9 Alteration of fatty acid metabolism

10 Suppression of catecholamine-induced hyperactivity

11 Reduction in CSF secretion

12 Anaesthesia, deafferentation, and immobilization

13 Uptake of glutamate in synapses

Box 3.1 Mechanisms by which anaesthetic agents may exert their neuroprotective

suppression These animals maintained better cerebral perfusion, ECF biochemical and electrolyte levels and aerobic metabolism when compared with controls While it is likely that propofol produces at least some of these neuroprotective effects via metabolic suppression, it has been documented that the agent is a potent free radical scavenger.44

Etomidate

Etomidate has been reported to possess similar cerebral metabolic protective effects to the barbiturates, but is disadvantaged by its adrenal suppressant effects and ability to cause myoclonic movements.45–48As is the case with barbiturates, no further reduction of

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CMR occurs when additional drug is administered beyond a dose sufficient to produce isoelectricity.49 Again there appears to be no benefit in complete global ischaemic states.50

Opioids, Ketamine and Benzodiazepines

Opioids are not thought generally to have neuroprotective properties but they do blunt stress-induced responses Ketamine is an NMDA antagonist and has been shown to be protective in animal models of ischaemia.51 While the benzodiazepines decrease cerebral blood flow and cerebral metabolic rate, these effects are less impressive than with the intravenous anaesthetic agents Despite occasional reports of neuroprotective benefit,52these drugs are not generally thought to be useful neuroprotective agents

Inhalation Anaesthetic Agents

The primary mechanisms by which the inhalation agents, like the barbiturates, exert their cerebral protective effects may be their ability to suppress cortical electrical activity, and thus reduce the oxygen demands associated with synaptic transmission.53,54,55 The reality of these effects is that they may be far more complex than once believed.54,55 Halothane, not usually regarded as a cerebral protectant, provides a similar degree of protection to sevoflurane although it results in less metabolic suppression.56,57 It appears that

as inhalation agents only suppress cortical activity and not membrane/organelle function, the degree of suppression would only translate into a very short time of additional preserved organelle homoeostasis and would not provide a major clinical benefit.55However, the idea that inhalation agents provide cerebral protection is well established Mechanisms by which these may occur are given in Box 3.1 Nitrous oxide is still used as part of a balanced technique for many procedures, without obvious adverse effect However, it is unique amongst inhalation agents in that if any effect on neuronal protection, that effect may be detrimental to

Calcium channel antagonists have been successful in treating patients with subarachnoid haemorrhage and though they were thought

to produce their effects by ameliorating vasospasm,63 it now appears that direct cytoprotective effects may be important However, despite some initial enthusiasm,62 studies in traumatic brain injury and stroke have generally shown no clinical benefit One possible explanation for these failures is that the calcium channel antagonists are only effective in blocking L-type channels, leaving T and N channels functional.33 However, magnesium and cobalt are non-selective antagonists at all types of voltage-sensitive and NMDA-activated channels that are involved in calcium influx into neurones and this may account for their documented neuroprotective effect.61 Other calcium antagonists have been reported to ameliorate ischaemic lesions, namely isradipine, Semopamil and RS-87476 (a Na+/Ca+ channel modulator), but their neuroprotective efficacy and mechanisms of action are as yet not fully extablished.28

Sodium channel blockers have also been used as neuroprotective agents Lignocaine-induced anaesthesia involves the selective blockade of Na channels in neuronal membranes, with resultant decrease in neuronal transmission This reduces the CMRO2 by that component of cellular metabolism responsible for synaptic transmission In addition, it also reduces ionic leaks, i.e Na+ influx and

K+ efflux, and this reduces Na+–K+–ATPase pump energy requirements While experimental studies are encouraging, human trials are awaited.1,64 Other Na+ channel blockers are the local anaesthetic agents QX-314 and QX-222 which have shown good in vitro results

but again, human studies are awaited.33 Enadoline is a new opioid with Na+channel-blocking properties under investigation

The only ion channel blocker currently recommended for clinical use by the National Stroke Association in the USA is nimodipine in the setting of subarachnoid haemorrhage but recent papers have challenged even this use.33,65 While nicardipine is reputed to cause less systemic hypotension than nimodipine and is marketed in the USA for similar clinical indications, no other channel blockers are currently available in the UK

Excitatory Amino Acid Antagonists

To date, approximately 19 agents that block EAA receptors have been shown to be effective in a variety of experimental brain injury models.60 Non-

competitive NMDA channel antagonists such as dizolcipine (MK-801) have a theoretical advantage over competitive agents such as CGS 19755, in that competitive antagonism may be overcome by the pathologically high concentrations of glutamate associated with cerebral ischaemia.66 These two agents have proved particularly encouraging as neuroprotectants.67–71 Again, as with so many neuroprotective agents, these seem to be more effective in focal ischaemia72,73 than global ischaemia, although this may be open to debate.74 One possible explanation for this is the occurrence of spontaneous depolarizations and repolarizations in the penumbral tissues of an infarct These processes produce a heavy metabolic demand on the tissues and it may be here that NMDA antagonists act.75 In global ischaemia no such processes occur and thus the antagonists may have no target on which to act Unfortunately the non-selective blockers have been associated with the development of neuronal vacuolation in the posterior cingulate region in experimental models,29 and have hence not been rapidly brought into clinical use

Other NMDA antagonists have been shown to have experimental neuroprotective properties,76 including agents which have been used in man Ketamine has been shown to improve cognitive function77,78,79 and dextromethorphan has been shown to improve neurologic motor function and decrease regional oedema formation80,81,82 in experimental models The degree of physiological blockade of the NMDA receptor by Mg2+ ions may also be important and administration has been reported to be protective against cerebral ischaemia.83,84

AMPA receptor antagonists may well be more effective for both global and focal ischaemia,85,86 and they appear not to have the same psychomimetic effects as the NMDA agents Other agents that have been used in experimental neuroprotective research include felbamate (acting at glycine sites) and nitroso compounds, such as nitroprusside and glyceryl trinitrate, that act at redox modulator

sites and prevent EAA-induced neuronal death in in vitro models.66 Riluzole, a novel compound that inhibits presynaptic release of glutamate, has neuroprotective effects in rodent models.87

Free Radical Scavengers

The efficacy of the administration of protective enzymes or free radical scavengers in ameliorating neurologic injury after cerebral ischaemia is the subject of much investigation.10 The beneficial effect depends on the involvement of free radicals in the pathological process, the biologic compatibility of the scavengers, appropriate dose selection and the ability to deliver the agent to the cellular site where the free radical is active Pretreatment with α-tocopherol has been found to have beneficial effects in cerebral ischaemia,88,89subarachnoid haemorrhage,90 spinal cord injury91 and CNS trauma.92,93 Other agents that have been tested include the iron chelator deferoxamine,94 superoxide dismutase,95,96 dimethyl superoxide,97 superoxide dismutase conjugated to polyethylene glycol98,99,100and tirilazad mesylate.101 Although all these agents have been shown to exhibit neuroprotective efficacy in animal models, there have been no successful clinical trials to date Indeed, initial optimism regarding pegorgotein (PEG conjugated superoxide dismutase) and tirilazad mesylate has recently been proven to be unfounded.29

Free Fatty Acids and Prostaglandin Inhibitors

Calcium-induced phospholipase activation during ischaemia releases free fatty acids from membrane phospholipids These FFAs can uncouple oxidative phosphorylation in mitochondria and cause efflux of Ca2+ and K+ into the cytosol and increases in levels of arachidonic acid, which is the rate-limiting substrate for prostanoid synthesis Increase of arachidonic acid (the commonest FFA), during cerebral insults, results in increased concentrations of the endoperoxides PGG2 and PGH2, which are the precursors of

prostacyclin (PC/PGI2), and thromboxane A2 made in vascular endothelial cells and platelets respectively This results in inactivation

of prostacyclin synthetase and relative overproduction of thromboxane A2.102,103This relative imbalance between vasoconstrictor and vasodilator prostaglandins may contribute to postischaemic hypoperfusion Arachidonic acid is also converted to leukotrienes which act as inflammatory mediators and may be associated with further free radical generation.104 It is debatable at this stage whether inhibitors of the arachidonic cascade might be effective in ischaemia as although these compounds (indomethacin, ibuprofen) have been found to show variable neuroprotective efficacy in some studies of global ischaemia,105,106 there were inconsistent effects on hypoperfusion and neurologic outcome.107,108

Hypothermia

Hypothermia treatment (mechanical cooling) was first described in 1943 and there have been sporadic attempts over the last 50 years

to use it as a treatment modality.60 Recent trials have suggested that it may be useful in patients with head injury.109,110 The most recent trial 109 concludes that treatment with moderate

prevention of a cerebral hyperthermic response to ischaemia.54,57 In studies where brain temperature has been increased compared with those with hypothermia, infarct size is increased This highlights the importance of meticulous monitoring and control of cerebral temperature in studies of pharmacological neuroprotection.

Clinical Practice 29

The success of experimental neuroprotection is undeniable and new publications continue to explore novel and exciting therapeutic targets However, the major challenge facing clinical neuroscientists is the general failure to translate these successes into positive results from outcome trials, possible reasons for which are listed in Box 3.3

1 Reduction of rate of energy use for electrophysiological cortical activity and the

homoeostatic functions required to maintain cellular integrity

2 Reduction of extracellular concentrations of excitatory amino acids

3 Suppressing the posttraumatic inflammatory response

4 Attenuating free radical production

5 Maintenance of high energy phosphate

• Experimental demonstration of neuroprotection incomplete (functional endpoints?)

• Inappropriate agent: mechanism of action not relevant in humans

• Inappropriate dose of agent (plasma levels suboptimal either globally or in

subgroups)

• Poor brain penetration by agent

• Efficacy limited by side effects that worsen outcome (e.g hypotension)

• Inappropriate timing: mechanism of action not active at time of administration

• Inappropriate or inadequate duration of therapy

• Study population too sick to benefit

• Study population too heterogeneous: efficacy only in an unidentifiable subgroup

• Study cohort too small to remove effect of confounding factors

• Failure of randomization to evenly distribute confounding factors

• Insensitive, inadequate or poorly implemented outcome measures

Box 3.3 Possible causes of failure of trials of clinical neuroprotection 29

Two radically different approaches have been suggested to overcoming the problems inherent in patient heterogeneity and lack of sensitivity of outcome measures The first of these is to accept that these problems are unavoidable and mount larger outcome trials

of 10–20,000 patients which will address benefits of a magnitude less than the 10% improvement in outcome that most drug trials are designed to detect The alternative strategy is to mount smaller but much more detailed studies in homogeneous subgroups of patients whose physiology is characterized by modern monitoring and imaging techniques Repeated application of these techniques during the course of a trial can provide evidence of reversal of pathophysiology and hence mechanistic efficacy Such surrogate endpoints could then be used to select drugs or combinations of drugs for larger outcome trials It is likely that both approaches will find a place, depending on the setting

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The skull is a rigid, closed box and contains the brain, cerebrospinal fluid (CSF), arterial blood and venous blood Brain function depends on the maintenance of the cerebral circulation within that closed space and arterial pressure forces blood into the skull with each heartbeat CSF is being formed and absorbed and the result of these forces is a distinct pressure, the intracranial pressure (ICP) The difference between the mean arterial pressure (MAP) and the mean ICP is the pressure forcing blood through the brain, the cerebral perfusion pressure (CPP)

ICP is normal up to about 15 mmHg but it is not a static pressure and varies with arterial pulsation, with breathing and during coughing and straining Each of the intracranial constituents occupies a certain volume and, being essentially liquid, is

incompressible In the closed box of the skull, if one of the intracranial constituents increases in size, then either one of the other constituents must decrease in size or the ICP will rise Two of the constituents, CSF and venous blood, are contained in systems that connect to low-pressure spaces outside the skull, so displacement of these two constituents from the intracranial to the extracranial space may occur This mechanism, then, compensates for a volume increase affecting any one of the intracranial constituents The displacement of CSF is an important compensatory mechanism and is illustrated in the CT scan in Figure 4.1 where in response to the generalized development of cerebral oedema following head injury, the ventricles have been so compressed by the brain swelling that they are visible only as a slit CSF absorption may increase as ICP rises and the CSF volume will be reduced

foramen magnum, where compression of the vital centres is associated with bradycardia, hypertension and respiratory irregularity followed by apnoea.2

The symptoms and signs produced by a supratentorial tumour depend on its rate of growth and whether it is

Figure 4.2

CT scan of a patient showing an extradural haematoma with considerable shift of the midline

developing in a relatively silent area of the brain or in one of the eloquent areas, such as the motor cortex A tumour developing in a silent area can achieve large size before presenting with symptoms and signs of raised ICP (Fig 4.3) In this situation a major disruption of ICP dynamics may be present, with significant brain shift A tumour may present rapidly if it is in an eloquent area, if it

is a fast-growing tumour or if it causes CSF obstruction Chapter 1 describes some of the common syndromes associated with tumour development

Haematomas are usually fairly rapidly growing lesions and although they set in train the compensating mechanisms for intracranial space occupation, they will produce signs of raised ICP at an earlier stage.3

Space occupation in the posterior fossa has some characteristic features The posterior fossa is a much smaller space than the anterior and middle cranial fossae and as tumours developing in the posterior fossa are growing in a more confined space, they tend not to grow to large size The relatively small volume of the posterior fossa means that tumours tend to produce a rise in ICP early and this

is accentuated by the fact that they frequently produce CSF obstruction Distortion of the mid brain and compression of the lower cranial nerves may also be produced by posterior fossa tumours

The bulk of the brain can also be increased by the development of cerebral oedema and frequently cerebral oedema is seen in association with a tumour (Fig 4.4) The degree of space occupation produced by the oedema can be so great as to turn a relatively minor degree of space occupation from a small tumour into a major problem requiring urgent treatment Klatzo4,5 provided a simple classification of cerebral oedema into two types: vasogenic and cytotoxic In vasogenic brain oedema (VBO), the development of oedema results from damage to the blood–brain barrier, so that there is an increase in permeability of the cerebral capillaries and serum proteins leak into the brain parenchyma The hydrostatic forces generated by the Starling balance at the capillary provide the impetus for the oedema fluid to spread through the brain; white matter, which has a less dense structure than grey, tends to offer less resistance VBO may develop around neoplasms, haematomas and cerebral abcesses and in traumatized areas of the brain

intravascular pressure accelerates the rate of oedema spread Eventually the fluid reaches the ependymal surface of the ventricles, where it passes into the CSF to be transported and absorbed by the mechanisms that regulate CSF outflow.7 The production and maintenance of a low sagittal sinus venous pressure is important in allowing the resolution of cerebral oedema

Cytotoxic brain oedema occurs after hypoxic or ischaemic episodes The reduced state of oxygen delivery results in failure of the intracellular ATP-dependent sodium pump and therefore intracellular sodium accumulates followed by rapid increases in