intracranial pressure: current status in monitoring and management

Intracranial Pressure: Current Status in Monitoring and Management

Thomas G. Luerssen

One of the most frequently occurring questions in the neurological critical care of children involves the indications for measurement of intracranial pressure (ICP) and the appropriate therapies for abnormally elevated ICP. Advances in monitoring technology have improved the safety and accuracy of ICP measurement. Clinical and basic research into the mechanisms of brain swelling and the efficacy of various therapies, especially in the realm of traumatic brain injury, has allowed the development of rational and specific treatment strategies for elevated ICP. For several diseases, the ability to measure and manage ICP has resulted in marked improvements in outcomes. This article reviews the indications for, and recommended methods of, measuring ICP in children and discusses the status of therapies commonly used to control elevated ICP, Copyright �9 1997 by W.B. Saunders Company

T HE AGE OF INTRACRANIAL pressure (ICP) monitoring in humans began almost 50 years

ago with the pioneering work of Gillaume and Janny in France and Lundberg in Sweden. 1,2 The modem age of ICP management probably began with the development and widespread application of computed tomographic (CT) scanning of the brain in the early 1970s. Since that time, further advances in brain imaging and physiologic monitoring have resulted in improved understanding of the causes and effects of brain swelling. In certain diseases, most notably traumatic brain injury, therapeutic interventions aimed directly at controlling abnormally elevated ICP have clearly resulted in improved survival and neurologic outcomes. In other brain insults, for instance hypoxic and ischemic injuries, control of ICE if even possible, has not improved outcomes. This article briefly reviews the current status of ICP monitoring, the two most common means of measuring ICP and their attendant complications, and the therapies that have been shown to be useful in treating elevated ICE

INTRACRANIAL PRESSURE

The Monro-Kellie Doctrine

There are several recent comprehensive reviews of the physiology, mathematics, and significance of ICP. 3-6 The development of the science related to

From the Pediatric Neurosurgery Service, Department of Neurological Surgery, Indiana University Medical Center, James Whitcomb Riley Hospital for Children, Indianapolis, IN.

Address reprint requests to Thomas G. Luerssen, MD, Pediat- ric Neurosurgery Service, James Whitcomb Riley Hospital for Children, One Children's Square, Room 2510, Indianapolis, IN 46202-5200.

Copyright �9 1997 by W.B. Saunders Company 1071-9091/97/0403-0001 $5.00/0

ICP stems from the century old Monro-Kellie doctrine. 7,8 In this theory, one must assume that the calvarium is a rigid container of four relatively noncompressible elements. Three elements are the normally occurring brain substance, the cerebrospinal fluid (CSF), and the intravascular blood. The fourth element occurs only in pathological situations that result in additional intracranial mass, such as hemorrhage, neoplasm, foreign body, or cysts. The global ICP represents the sum of the partial pressures of each of these compartments. In the rigid container and because each component is considered to be noncompressible, the pressures are directly related to the volumes of the compartments. Any increase in the volume of one compartment must be completely compensated for by a decrease in volume in the remaining compartments or global ICP will rise. CSF volumes may be shifted into the spinal subarachnoid space. Venous blood volume decreases as the dural venous sinuses become compressed. This ability to compensate for changes in the volumes of the intracranial contents has been termed compliance. As compliance dimin- ishes, additional volume in any compartment will cause elevated ICE At the extreme, arterial blood flow is compromised and regional or global ischemia occurs. In fact, the deleterious effects of elevated ICP are thought to be directly related to the development of brain ischemia. When compliance is low, very small changes in the volume of a compartment can result in marked changes in ICP.

The Monro-Kellie doctrine does not apply in children who still have open fontanels and expand- able cranial sutures. Instead, chronic increases in intracranial volume that can occur, for instance, in hydrocephalus or brain tumors, may be compensated by virtue of an increase in the size of the

146 Seminars in Pediatric Neurology, Vol 4, No 3 (September), 1997: pp 146-155

ICP MONITORING AND MANAGEMENT 147

cranial vault. However, (and despite a common assumption that the fontanel and open cranial sutures can protect a young child from elevated ICP) fontanels and sutures provide little, if any, protection for acute, rapid increases in intracranial volumes or pressures.

Furthermore, to be a normal container, the child's skull needs to grow normally. Disorders such as some of the craniosynostoses could result in elevated ICE in the absence of any additional "intracranial" abnormality, because the skull itself is unable to accommodate normal cerebral growth.

Finally, the Monro-Kellie doctrine, although useful as a conceptual model, has been found to be too simplistic to explain all of the pathophysiology of ICE In some situations, the compensation within an individual compartment and between the compartments does not occur uniformly. CSF pathways can be occluded at any point in the normal system. This may result in disparate pressures, and differing compensatory mechanisms, on either side of the blockage. Furthermore, focal swelling or compression of brain tissue can occur. Recent laboratory studies of this phenomenon indicate that, in the presence of certain expanding supratentorial mass lesions, gradients occur within the brain substance itself. These findings suggest that the brain may be more compressible than previously thought and that local pressure increases may not be transmitted equally to all brain regions. 9,1~ This means that an ICP measured from a single region may not be an accurate reflection of tissue pressures in all brain regions.

Normal Values

The definition of "normal" ICP varies with the age of the patient. In the newborn, ICP averages approximately 45 mm H20, or 6.1 mm Hg, and rises slowly thereafter throughout childhood.ll At times, the ICP may actually be subatmospheric in young children with large anterior fontanels. Nor- mal ICP in adults is approximately 10 to 15 mm Hg. ~2 In adult patients suffering traumatic brain injury, an ICP of 20 mm Hg is considered to be abnormally elevated and should be treated. Al- though much less information regarding the threshold for instituting therapy to reduce ICP exists for children, it is likely that the treatment threshold is also age related, and lower than that recommended for adults.

Derived Values

With the advent of ICP monitoring, it quickly became apparent to clinical scientists that single isolated measurements of ICP are of limited value for understanding the state of the intracranial dynamics. As mentioned above, some indication of the compliance of the system can be derived by measuring the volume that must be added or removed to cause a change in the ICP. 4'5 Knowing about the compliance of the system should be as valuable as knowing the pressure, because changes in compliance should predate any changes in ICP. This information could be used to institute or change therapy before changes in the absolute ICPJ 3J4

Indirect information about the overall compliance in the system can also be derived from either inspection or mathematical analysis of the ICP wave form. In general, an increasing pulse pressure in the ICP wave is associated with decreasing intracranial compliance. 5 More sophisticated analyses of the ICP wave forms have been undertaken using real-time computer analysis of the amplitude and shape of the ICP waves recorded by ICP monitors with a very high frequency response. 15-18 Although these studies provide interesting and potentially important information, systems using these techniques have not yet been widely applied in neurologic intensive care.

As indicated previously, the major aim of measuring and managing elevated ICP is the prevention of cerebral ischemia. Knowledge of ICP and systemic blood pressure allows one to estimate the perfusion state of the brain, a value that has been termed cerebral perfusion pressure (CPP). CPP has been defined as the mean arterial pressure (MAP) minus the ICR It is important to remember that CPP is only an indirect indicator of cerebral blood flow (CBF), 192~ and that the two terms should not be used interchangeably.

The critical CPP for maintenance of normal CBF in adults is approximately 52 mm Hg. 21 However, it is now thought that intracranial pathology of any type raises this critical CPP threshold to 70 to 90 mm Hg. 22 It is also clear that the maintenance of a minimum CPP is extremely important when managing brain insults associated with elevated ICP. Some authorities use the maintenance of CPP as the major focus of therapy, including the early and preferential use of vasopressors to substantially

148 THOMAS G. LUERSSEN

increase systemic blood pressure. 23,24 The results of this therapy in adult patients have been encouraging. However, there is also evidence that elevated ICE in and of itself, is an important determinant of outcome, and that the major factors contributing to harmful reductions in CPP are actually systemic hypotension occurring in the face of elevated ICE 25,26 Whether one chooses to focus primarily on ICP or CPP to guide therapy, there is universal agreement that any episodes of hypotension, even mild in degree and brief in duration, must be assiduously avoided. 27

Attention to CPP in children is probably as important as in adults. However, as with ICE one could surmise that normal CPP should be lower in children by virtue of their lower normal MAP and ICP, and that this relationship should be age related. Preliminary studies appear to confirm this. A recent report indicates that the critical CPP threshold is indeed lower in children, and that the fundamental cerebrovaScular physiology may be different between children and adults. 28 Considering this, and until more information is available, it is not recommended that "CPP therapy," especially using the physiologic parameters established for adult patients, be undertaken in children.

INTRACRANIAL PRESSURE MONITORING

Types and Selection of ICP Monitors

Before the early 1980s, most ICP monitors were fluid coupled devices of variable utility and accuracy. 29,3~ Of all of the fluid coupled devices, the ventricular catheter is, by far, the most accurate. It is easy to understand and troubleshoot and has the significant added advantage of allowing the therapeutic drainage of CSF to lower ICE However, this device also requires the full penetration of the brain, some skill in placement, especially in the case of small or shifted ventricles, and carries a small but definite risk of becoming infected.

The subsequent development and now widespread use of implantable fiberoptic or strain gauge microtransducers has added substantially to our ability to safely and accurately measure ICE They can be easily placed in almost any clinical circum- stance because they can provide pressure measurements from the cortex, the ventricles, the subdural space, or the subarachnoid space. 31 The complication rate for these monitors is extremely low, mostly because of the small diameter and the lack of fluid coupling. 3z33 However, these monitors are

complex electronic devices and are therefore expen- sive. Breakage or malfunction generally requires complete replacement of the intracranial probe.

Other devices that have been used to measure ICP include the subarachnoid bolts, subdural catheters, and a variety of extradural transducers. All of these devices are less desirable because of complex- ity, inaccuracy, difficulty in placement, or problems in maintenance. 34

Attempts have been made at measuring ICP noninvasively in infants by the application of a variety of fontanel "tonometers." These devices require a certain minimal size of the anterior fontanel, a lack of scalp swelling, normal skin turgor, and a high degree of maintenance to obtain accurate readings. 35 These concerns, as well as the limited number of disorders requiring continuous ICP measurements in infants, have limited the interest in and widespread use of fontanel tonom- etry.

The selection of the type of ICP monitor should be guided by the clinical presentation and the therapeutic strategy that is chosen for each child. Thus, children who present with elevated ICP due to hydrocephalus who cannot or should not undergo ventricular shunting, are monitored (and treated) by placement of a ventricular catheter. This group might include children with ventricular ob- structions due to infectious processes, acute hemorrhages, and preoperative and postoperative patients with neoplasms that are causing obstructive hydrocephalus. Children undergoing therapy for shunt malfunction or infection who are critically ill are also candidates for this type of monitor. In most children with these presentations, the ventricular system is quite enlarged, and placement of the catheter is easy and safe.

Children presenting with diseases that cause diffuse brain swelling, or whose ventricles are small, shifted, or otherwise inaccessible, and those who would not benefit by therapeutic drainage of ventricular CSF, are better suited to a parenchymal monitoring system. Examples of disorders in this category are metabolic encephalopathies, postoperative patients with normally patent CSF pathways, and children undergoing study for the diagnosis of slit ventricle syndrome or shunt malfunction with small ventricles.

Occasionally, a disease process may require placement of two "monitors." This happens not uncommonly when considering the optimal manage-


ment of traumatic brain injury. In some children with severe traumatic brain injury, therapeutic CSF drainage is a valuable adjunct for controlling elevated ICE However, it may not be possible to place a ventricular catheter at the time that therapy needs to begin. Instead, one can insert a parenchy- mat monitor and begin aggressive medical management. This may allow some expansion of the ventficular system such that safe and successful cannulation of the ventricles can occur. Subse- quently, the parenchymal monitor remains to guide therapy, whereas the ventricular catheter becomes an avenue of that therapy.

Clinical Utility of ICP Monitoring

Of all the clinical indications for it use, ICP monitoring has had the most positive impact on the management of severe traumatic brain injury. The use of ICP monitoring is clearly justified in this disorder because abnormal elevation of ICP occurs commonly and numerous studies show that control of ICP improves outcome. 36-4~ All children in post-traumatic coma, that is with Glasgow Coma Scale (GCS) 41 scores of 8 or less after systemic resuscitation, should undergo management of their brain injury using ICP monitoring. Exceptions to this recommendation include children who are clearly postictal after an early post-traumatic sei- zure, who also have a normal CT scan, and perhaps, infants who have severe injuries from nonacciden- tal mechanisms whose CT scans show massive infarction. Children with higher GCS scores who exhibit evidence of brain swelling, shift, cisternal compression, or intracerebral hemorrhages are at risk for deterioration and will also benefit from ICP monitoring. 42

The management of some other disorders may be aided by ICP monitoring. ICP monitoring has been reported to be a useful adjunct in the management of some children with cranial or craniofacial dysos- tosis. 43'44 With the current effort to avoid permanent ventricular shunting in children with brain tumors, ventricular catheters can provide temporary therapeutic ventricular decompression before and after resection of the tumor, and provide a rational means of testing the re-establishment of normal CSF pathways in the early postoperative period. Children who have undergone prolonged or complicated surgical procedures who may require a period of time to recover from the effects of surgery or anesthesia, or who are at risk for deterioration due

to swelling or hemorrhage after surgery, may benefit from placement of an ICP monitor in the postoperative period. As with traumatic brain injury, elevations of ICP should predate the occurrence of neurological signs and allow early investi- gation and intervention. Finally, ICP monitoring has been used in the management of shunted hydrocephalus or the "slit-ventricle" syndrome. 45 The information obtained from ICP monitoring in these children may be the only objective means of determining the cause and appropriate management of their symptoms.

For many other disorders associated with brain swelling, the role of ICP monitoring is, at best, unclear. A good example is in the management of hypoxic injuries, such as those that occur in "near drowning." Although these injuries are commonly associated with elevated ICP, most centers with a large experience have abandoned ICP monitoring because efforts to control ICP in these patients did not improve outcome. 46 Similar analyses can be made for a number of other neurological disorders, including ischemic injuries, infections, and poison- ings.

The clinical use of ICP monitoring in the management of metabolic coma is controversial. Eleva- tions of ICP are common in these disorders. However, the effect of directly managing ICP appears to be variable. The impact of measuring and managing ICP in the now rare encephalopathy associated with Reye's syndrome appeared to be beneficial, 47 although randomized, controlled trials were not completed. ICP monitoring has also been used in management of patients with fulminant hepatic failure, 48 but the complication rate of ICP monitoring is extremely high in this group of patients, especially in the pediatric age group. 49

Complications of ICP Monitoring

The complications of ICP monitoring are dependent on the type of monitor and the presence of systemic disturbances. Parenchymal monitoring is associated with a very low complication rate, even in relatively high-risk patients. Although placement of these monitors requires the penetration of dura and cortex, it appears that clinically important intracranial hemorrhage occurs rarely, and almost exclusively in patients with coagulopathy.

Parenchymal monitoring is also unlikely to be complicated by infection. In two large series involving both adults and children, the overall infection


rate was reported at well under 1%. 32,33 All of the infections in these series were local scalp infections at the monitoring site. The occurrence of infection was not related either to the maintenance of the monitor or to the duration of monitoring.

The use of ventricular catheters for ICP monitoring carries a somewhat higher complication rate, although the benefits derived from the use of these monitors frequently outweigh the additional risk. Ventricular catheters are larger than the parenchymal probes, and proper placement requires complete penetration of the brain and accurate position- ing in the ventricles. These properties alone increase the risk of intracerebral or intraventricular hemorrhage, as well as the risk of malfunction. Despite this, the reported risk of hemorrhagic complications appears to be less than 2% 50 and is also likely to be related to the presence of coagulopathy. Mechanical complications may be unavoidable in some patients, although centers with a large experience in the use of ventricular catheters are likely to have fewer complications related to placement and maintenance.

The major complication associated with the use of ventricular catheters is infection. Most recent series have reported infection rates under 10%. 51-54

Unlike the parenchymal monitors, the likelihood of infection with ventricular catheters increases with the duration of monitoring. This risk is not reduced by prophylactically changing the catheter at speci- fied intervals. 52,53,55 Therefore, ventricular catheters should be used for as brief a period as necessary and then removed. If and when therapeutic CSF drainage is no longer needed, a parenchymal monitor can be placed to allow a longer period of pressure measurement.

MEDICAL THERAPY OF ELEVATED ICP

The goal of any therapy directed toward elevated ICP is to reduce the pressure enough to ensure an adequate supply of well-oxygenated blood to all regions of the brain. Therefore, the initial priority in management is the establishment and maintenance of normal systemic blood pressure and oxygenation. Although this seems obvious, recent studies of head-injured patients using continuous computerized recordings of a number of systemic parameters indicated that many critically ill patients experience unrecognized episodes of hypotension or hypoxia during their critical care management. 56 This is an important concern, especially

when one considers that many of the side effects of the common therapies for elevated ICP result in decreased cerebral blood flow or systemic hypotension.

As might be expected, no one method of treatment, or series of therapies, or combination of therapies is appropriate management for all causes of elevated ICP. Even in individual patients, the therapeutic strategies may change several times during the course of treatment. The key to successful management of elevated ICP lies in the attention to detail. This includes not only frequent assessment of changes in ICP, but also the development of an understanding of the intracranial dynamics in each patient, including the ongoing responses to therapy. The treatments for ICP are systemic therapies that have secondary, perhaps undesirable, effects that must be taken into account. Local or systemic complications, such as fever, infection, sepsis, renal failure, electrolyte imbalances, or anemia, can obviate an otherwise successful management plan and must be avoided if at all possible.

Control of Airway and Ventilation

Early assessment and control of the airway and respiration have profound beneficial effects in patients with elevated ICE Both hypoxia and hypercarbia result in cerebral vasodilatation, which can substantially aggravate increased ICE There- fore, respiratory parameters should be monitored in all patients who might have increased ICE This includes assessment of systemic oxygenation by continuous pulse oximetry and frequent assessment of respiratory rates and patterns. If there is any indication that the patient is developing respiratory insufficiency, elective endotracheal intubation and mechanical ventilation should be undertaken. The intubation should be performed by a person experi- enced in pediatric intubations, using the "rapid sequence induction" technique. This involves the sequential administration of a short-acting sedative/ hypnotic agent followed by a rapidly acting neuromuscular blocker and cricoid pressure to prevent regurgitation.

The beneficial impact of mechanical ventilation on the intracranial dynamics can be considerable. Oxygenation is improved and can be precisely controlled. Variations in Paco2 are minimized. Both of these conditions will reduce fluctuations in cerebral blood volume and can stabilize the ICE However, sufficient analgesics and sedation must


be administered to the intubated patient to alleviate coughing and straining and the attendant increases in intrathoracic and central venous pressure that accompany these responses. In many circumstances, neuromuscular blockade is necessary to minimize movement and allow control of respiration. 57 Obviously, the neurological examination is obscured by these treatments and, dependent on the disease process, ICP monitoring should be considered for patients who require mechanical ventilation.

Fluid Management

Until recently, and despite limited scientific justification, it has been dogmatically recommended that fluid administration be restricted in patients with elevated ICE 58 It is now clear that any beneficial effects on serum osmolality caused by fluid restriction are outweighed by the attendant hypovolemia that results from this "therapy." Most authorities now recommend that somewhat hyper- tonic crystalloid solutions should be administered to patients in volumes sufficient to maintain a normal or even slightly increased intravascular votume. 59 Accordingly, one needs an accurate knowledge of the patient's fluid balances as well as frequent monitoring of blood chemistries.

Head Position

One of the simplest methods for reducing elevated ICP is facilitating normal cerebral venous drainage. Rotation of the head or flexion of the neck impedes jugular venous flow and raises ICP. 6~ Therefore, careful attention should be taken in placing and maintaining the head in a truly neutral position. In infants, this may require some elevation of the trunk to allow the head to fall back into a truly neutral position. Even the tape securing an endotracheal tube can be constricting and impede jugular venous drainage enough to affect the ICP.

Elevation of the head of the bed by 15 to 30 degrees can reduce the ICP by reducing the venous outflow pressure. However, this maneuver may actually be harmful in patients who are hypovole- mic or hypotensive, because the salutary effect on ICP is offset by a net reduction in cerebral perfusion. 61 Keeping this proviso in mind, recent studies in head-injured patients suggest that when patients are kept euvolemic, head elevation is an effective means of reducing ICP without compromising CPP or CBE 62

Corticosteroids

The use of glucocorticosteroids may be beneficial in the management of some patients with elevated ICE These agents have been valuable adjuncts in the management of patients with brain tumors for more than 30 years. 63 Clearly, the symptoms that are relieved by steroid administration appear to be due to elevated ICP. How this is accomplished is not known with certainty, because dexamethasone has been shown to have little, if any, direct effect on brain water content or ICE. 64,65

In most other situations involving elevated ICP, for instance, cerebral infarction, hemorrhage, trauma, and hypoxic encephalopathies, the routine use of steroids has not been shown to be beneficial and may, in fact, be harmful. The use of steroids has been well studied in adult patients with head injuries, and these agents did not improve outcome or lower intracranial pressure in most of the reported clinical trials. 66

CSF Drainage

Ventricular drainage reduces ICP by directly reducing the intracranial CSF volume. Since the original description by Lundberg, 2 it has been clear that the use of external ventricular drainage can provide an extremely effective, nonpharmacologi- cal, therapy of elevated ICE For pediatric patients, especially those with diseases that result in diffuse brain swelling, the major limiting factor in instituting this therapy is compression or obliteration of the ventricular system that prevents safe cannulation of the ventricle. A recent report has described the use of controlled lumbar CSF drainage in pediatric patients with severe acute traumatic brain injury and elevated ICP refractory to other therapies. 67 These authors were careful to exclude patients with intracraniat masses or shifts and used specific CT scan evidence that the subarachnoid spaces were patent before beginning this therapy. All of the children also had functioning ventricular catheters. Although the initial results in this small population of children are encouraging, this therapy should probably not be used in isolation and only in the circumstances that the authors describe.

Osmotic Diuretics

The use of osmotic diuretics has a become a well-established therapy for elevated ICP. Al- though a solution of urea was initially used for this


purpose, mannitol has now become the agent of choice for treating elevated ICE 68 Mannitol lowers ICP, improves CBF, and initially expands plasma volume by drawing water from the body tissues into the vascular space. 69 Mannitol reduces ICP by at least two mechanisms of action. One is a dehydrating action that reduces brain volume. 64 However, the immediate effects of mannitol administration on ICP appear to be caused by the agent's changes of the rheological characteristics of the blood. These changes, which include increased plasma volume, decreased hematocrit, and decreased viscosity, result in a vasoconstrictive response that decreases the cerebral blood volume (CBV) . 70,71

The administration of mannitol can also have a variety of adverse effects. Urine output increases dramatically after the administration of mannitol, and all patients receiving this drug should have a bladder catheter. The chronic administration of mannitol; especially if given as a continuous infusion, can result in the accumulation of the agent in the brain, which can actually increase brain swelling. 72 If the serum osmolality is allowed to increase to over 320 mOsm/kg, acute renal failure can result. This tendency is increased if the patient is receiving other nephrotoxic drugs. Finally, unless specific steps are taken to prevent it, the administration of mannitol will cause a progressively worsen- ing hypovolemia, which can deleteriously affect the CPP.

To avoid these complications, it has been recommended that mannitol be given in intermittent intravenous boluses of 250 to 1,000 mg/kg, with the dosage titrated directly to its effect on the ICE No more mannitol than that necessary to obtain a therapeutic effect should be given. 68 Frequent assessment of serum osmolality and electrolytes should be obtained, and the serum osmolality should not exceed about 310 mOsm/kg. Maintenance of normal intravascular volume is paramount. Urine losses should be replaced with crystalloid or plasma expanders. The use of central venous pressure monitoring, in addition to serum electrolyte and osmolality data, can be very helpful for guiding fluid replacement therapy.

Hyperventilation Historically, the use of moderate or intense

hyperventilation has been a routine therapy for patients with elevated ICE Hyperventilation re-

duces ICP by causing a pH-dependent cerebral arteriolar vasoconstriction, which decreases both CBF and CBV. 73 The therapeutic response to hyperventilation is rapid, but it is also short lived presumably owing to ongoing buffering of the pH in the interstitial space. 74 Therefore, to maintain a therapeutic effect, the intensity of the hyperventilation must be continuously increased.

This therapy has been commonly applied to patients with brain insults as a "prophylactic" treatment of brain swelling even in the absence of objective clinical signs of elevated ICP or direct measurements of pressure. However, recent studies indicate that the routine and prolonged use of hyperventilation may not be beneficial and may even be harmful to some patients by causing or exacerbating ischemic injuries to the brain. 75 There- fore, in the absence of objective evidence of elevated ICE the routine use of intense, unmoni- tored, hyperventilation is no longer recommended.

However, when used in appropriate situations and aided by physiologic monitoring, hyperventilation is a powerful and effective means of treating elevated ICE Substantial beneficial effects with minimal risk of ischemic complications can be obtained by maintaining the Paco2 between 30 and 35 mm Hg. When elevated ICP is refractory to other therapies, more intense hyperventilation can be undertaken. This therapy can be administered more safely by using some indicator of the status of the cerebral circulation, such as jugular venous oxygen saturation. 76 Profound hyperventilation therapy should be used only when necessary and for as short a period as necessary. The withdrawal of hyperventilation should occur during the period of ICP monitoring and should be gradual because of the tendency of treated patients to exhibit rebound cerebral vasodilatation. 77

Barbiturates High-dose barbiturate therapy effectively re-

duces ICP in a variety of disorders that are associated with brain swelling. 78 These drugs appear to exert their effect on ICP by suppressing brain metabolism. The associated reduction of coupled CBF, and therefore CBV, reduces the ICe. 79,80

These drugs also have a variety of harmful systemic effects including hemodynamic instability sl,82 and immunosuppression. 83,84 Accordingly, barbiturates should not be used indiscriminately for the management of increased ICE As with hyper-


ventilation, the major clinical studies of high-dose barbiturate therapy for head injury do not support the use of these agents as a routine or prophylactic treatment of brain injury or increased ICP. 85 How- ever, in patients with ICP elevations that are resistant to standard therapies, high-dose barbiturate therapy is a reasonable therapeutic option. 37

When barbiturate therapy for ICP is undertaken, continuous monitoring of all the relev _ant physiological parameters is necessary. This includes direct monitoring of arterial blood pressure and intravascular volume as well as ICE Occasionally, it may be necessary to place a Swan-Ganz catheter to measure cardiac output and peripheral vascular resistance. This therapy can be extremely complicated, and everyone involved with the patient should be familiar with the physiological changes and complications that accompany the use of high-dose barbiturate therapy.

Targeted Therapy Elevated ICP, brain swelling, and cerebral edema

continue to present a major challenge in neurologic critical care. Two decades of very active clinical and basic research have resulted in an improved understanding of the mechanisms resulting in ele-

vated ICP and the responses to various therapies. The availability of elegant neuroimaging and physiologic monitoring technology has resulted in safer and more effective treatments for patients with elevated ICE Remarkably, all of the progress to date has occurred in the notable absence of any new, unique, or specific drug therapies for brain swelling. Instead, the advances that we have seen are the direct result of a more rational and scientifi- cally justified application of "standard" therapies. Much of our current understanding of the treatment of ICP and brain injury is a result of the research and teaching of the late Professor J. Douglas Miller. One of the many concepts that we owe to this master scientist and clinician, and his colleagues in Edinburgh, Scotland, is the idea of "targeting" the therapy of ICP to the specific cause. 86 For instance, some patients appear to respond better to osmotic therapy than others. Some patients respond better to hypnotic therapy than osmotic therapy. The problem lies in correctly identifying very specific pathological processes in the midst of what is usually a rapidly changing and multifactorial disease. These complex issues are a major part of the current focus of research into ICP and its management.

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intracranial pressure: current status in monitoring and management

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