Multimodal monitoring in neurointensive care

Observational study of intracranial pressure instability in patients with pseudotumour cerebri syndrome

Introduction

A fixed CSF pressure (CSFp) of 25 cmH2O (18 mmHg) has been utilised to date to define and classify pseudotumour cerebri syndrome (PTCS). Furthermore, ICP monitoring, and CSF infusion tests have not been frequently performed in this group of patients.

Research question

We aimed to report typical, unusual and unstable patterns of ICP in patients with PTCS.

Material and methods

We reviewed the recordings of CSF infusion tests and overnight ICP monitoring of patients with suspected or confirmed IIH between January 2003-December 2020.We excluded all patients with a shunt in situ and selected recordings that represented unstable patterns of ICP changes in PTCS.

Results

463 CSF infusion tests and 26 ICP monitorings of PTCS patients had been performed in this timeframe. We divided results of observed pattern into two group: those with known venous sinus measurements (Group A) and those without (Group B). Observed recordings formed a total of 5 and 4 different patterns respectively, based on the behaviour of ICP and slow waves at rest, overnight, and during infusion as well as in relationship to the clinical presentation of each patient.

Discussion and conclusion

Accurate monitoring of ICP in PTCS is quintessential. Full understanding of each element of its pathophysiology and their interaction would be essential and include quantification of the CSF pressure not only as a number, but also with consideration of its dynamic contents. Cerebral venous pressure measurements and/or monitoring may be useful. Consideration of the presence or absence of papilloedema in the context of disturbed CSF dynamics could reveal further diagnostic and therapeutic insights.

Consensus Clinical Guidance for Diagnosis and Management of Adult COVID-19 Encephalopathy Patients

Encephalopathy, a common condition among patients hospitalized with COVID-19, can be a challenge to manage and negatively affect prognosis. While encephalopathy may present clinically as delirium, subsyndromal delirium, or coma and may be a result of systemic causes such as hypoxia, COVID-19 has also been associated with more prolonged encephalopathy due to less common but nevertheless severe complications, such as inflammation of the brain parenchyma (with or without cerebrovascular involvement), demyelination, or seizures, which may be disproportionate to COVID-19 severity and require specific management. Given the large number of patients hospitalized with severe acute respiratory syndrome coronavirus-2 infection, even these relatively unlikely complications are increasingly recognized and are particularly important because they require specific management. Therefore, the aim of this review is to provide pragmatic guidance on the management of COVID-19 encephalopathy through consensus agreement of the Global COVID-19 Neuro Research Coalition. A systematic literature search of MEDLINE, medRxiv, and bioRxiv was conducted between January 1, 2020, and June 21, 2021, with additional review of references cited within the identified bibliographies. A modified Delphi approach was then undertaken to develop recommendations, along with a parallel approach to score the strength of both the recommendations and the supporting evidence. This review presents analysis of contemporaneous evidence for the definition, epidemiology, and pathophysiology of COVID-19 encephalopathy and practical guidance for clinical assessment, investigation, and both acute and long-term management.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Objective cardiovascular assessment in the neonatal intensive care unit

Traditionally, cardiovascular well-being was essentially based on whether the mean blood pressure was above or below a certain value. However, this singular crude method of assessment provides limited insight into overall cardiovascular well-being. Echocardiography has become increasingly used and incorporated into clinical care. New objective modality assessments of cardiovascular status continue to evolve and are being evaluated and incorporated into clinical care. In this review article, we will discuss some of the recent advances in objective assessment of cardiovascular well-being, including the concept of multimodal monitoring. Sophisticated haemodynamic monitoring systems are being developed, including mechanisms of data acquisition and analysis. Their incorporation into clinical care represents an exciting next stage in the management of the infant with cardiovascular compromise.

BrainSignals Revisited: Simplifying a Computational Model of Cerebral Physiology

Multimodal monitoring of brain state is important both for the investigation of healthy cerebral physiology and to inform clinical decision making in conditions of injury and disease. Near-infrared spectroscopy is an instrument modality that allows non-invasive measurement of several physiological variables of clinical interest, notably haemoglobin oxygenation and the redox state of the metabolic enzyme cytochrome c oxidase. Interpreting such measurements requires the integration of multiple signals from different sources to try to understand the physiological states giving rise to them. We have previously published several computational models to assist with such interpretation. Like many models in the realm of Systems Biology, these are complex and dependent on many parameters that can be difficult or impossible to measure precisely. Taking one such model, BrainSignals, as a starting point, we have developed several variant models in which specific regions of complexity are substituted with much simpler linear approximations. We demonstrate that model behaviour can be maintained whilst achieving a significant reduction in complexity, provided that the linearity assumptions hold. The simplified models have been tested for applicability with simulated data and experimental data from healthy adults undergoing a hypercapnia challenge, but relevance to different physiological and pathophysiological conditions will require specific testing. In conditions where the simplified models are applicable, their greater efficiency has potential to allow their use at the bedside to help interpret clinical data in near real-time.

Model-based indices describing cerebrovascular dynamics

Understanding the dynamic relationship between cerebral blood flow (CBF) and the circulation of cerebrospinal fluid (CSF) can facilitate management of cerebral pathologies. For this reason, various hydrodynamic models have been introduced in order to simulate the phenomena governing the interaction between CBF and CSF. The identification of hydrodynamic models requires an array of signals as input, with the most common of them being arterial blood pressure, intracranial pressure, and cerebral blood flow velocity; monitoring all of them is considered as a standard practice in neurointensive care. Based on these signals, physiological parameters like cerebrovascular resistance, compliances of cerebrovascular bed, and CSF space could then be estimated. Various secondary model-based indices describing cerebrovascular dynamics have been introduced, like the cerebral arterial time constant or critical closing pressure. This review presents model-derived indices that describe cerebrovascular phenomena, the nature of which is both physiological (carbon dioxide reactivity and arterial hypotension) and pathological (cerebral artery stenosis, intracranial hypertension, and cerebral vasospasm). In a neurointensive environment, real-time monitoring of a patient with these indices may be able to provide a detection of the onset of a cerebrovascular phenomenon, which could have otherwise been missed. This potentially "early warning" indicator may then prove to be important for the therapeutic management of the patient.

Continuous monitoring of the Monro-Kellie doctrine: is it possible?

The Monro-Kellie doctrine describes the principle of homeostatic intracerebral volume regulation, which stipulates that the total volume of the parenchyma, cerebrospinal fluid, and blood remains constant. Hypothetically, a slow shift (e.g., brain edema development) in the irregular vasomotion-driven exchanges of these compartmental volumes may lead to increased intracranial hypertension. To evaluate this paradigm in a clinical setting and measure the processes involved in the regulation of systemic intracranial volume, we quantified cerebral blood flow velocity (CBFv) in the middle cerebral artery, arterial blood pressure (ABP), and intracranial pressure (ICP), in 238 brain-injured subjects. Relative changes in compartmental compliances C(a) (arterial) and C(i) (combined venous and CSF compartments) were mathematically estimated using these raw signals through time series analysis; C(a) and C(i) were used to compute an index of cerebral compliance (ICC) as a moving correlation coefficient between C(a) and C(i). Conceptually, a negative ICC would represent a functional Monro-Kellie doctrine by illustrating volumetric compensations between C(a) and C(i). Clinical observations show that Lundberg A-waves and arterial hypertension were associated with negative ICC, whereas in refractory intracranial hypertension, a positive ICC was observed. In subjects who died, ICC was significantly greater than in survivors (0.46 ± 0.027 versus 0.22 ± 0.017; p<0.01) over the first 5 days of intensive care. The mortality rate is 5% when ICC is less than 0, and 43% when above 0.7. ICC above 0.7 was associated with terminally elevated ICP (chi-square p=0.026). We propose that the Monro-Kellie doctrine can be monitored in real time to illustrate the state of intracranial volume regulation.

Outcome in neurocritical care: Advances in monitoring and treatment and effect of a specialized neurocritical care team

OBJECTIVE

To review current advances in the treatment of critically ill neurologic patients, including specialized care by neurointensivists.

DESIGN

Review article.

MAIN DISCUSSION AND CONCLUSIONS

Significant developments in the fields of neurology and neurosurgery have led to improved treatments for the critically ill neurologic patient. The major areas reviewed include neuromonitoring, disease-specific treatments, and specialized neurocritical care units and team. The current trend is for the application of the so-called multimodality neuromonitoring, which includes the use of several monitoring techniques, including intracranial pressure, brain electrophysiology, brain metabolism and oxygenation, and cerebral blood flow, among others. Many new therapies that have been introduced are discussed, including thrombolytic therapy for acute ischemic stroke, induced hypothermia for comatose survivors of cardiac arrest, and endovascular coiling for ruptured cerebral aneurysms. Lastly, the introduction of neurointensivists and neurocritical care units has been associated with reduced hospital mortality and resource utilization without changes in readmission rates or long-term mortality rates.

ICM+: software for on-line analysis of bedside monitoring data after severe head trauma

ICM software was developed in 1986 in Warsaw, Poland and has been in use at the University of Cambridge Neurocritical Care Unit for 10 years collecting data from bed-side monitors in nearly 600 severely head injured patients and calculating secondary indices describing cerebral autoregulation and pressure-volume compensation. The new software ICM+ includes a much extended calculation engine that allows easy configuration and on-line trending of complex parameters. The program records raw signals, and calculates time trends of summary parameters. Configuration and analysis utilises arithmetic expressions of signal processing functions to calculate various statistical properties for each signal, frequency spectrums and derivatives, as well as correlations/cross-correlations between signals. The software allows configuration of several levels of analysis before calculating the final time trends. The final data are displayed in a variety of ways including simple time trends, as well as time window based histograms, cross histograms, correlations etc. All this allows complex information coming off the bed-side monitors to be summarized in a concise fashion and presented to medical and nursing staff in a simple way that alerts them to the development of various pathological processes. The system provides a universal tool for clinical and academic purposes. Its flexibility and advanced signal processing is specialized for the needs of multidisciplinary brain monitoring.

Use of ICM+ software for on-line analysis of intracranial and arterial pressures in head-injured patients

OBJECTIVE

To summarize our experience from the first 2 years of use of the ICM+ software in our Neurocritical Care Unit (NCCU).

MATERIALS AND METHODS

Ninety-five head-injured patients (74 males, 21 females), average age 36 years, were managed in the NCCU. Intracranial pressure (ICP) was monitored using Codman intraparenchymal probes and arterial blood pressure (ABP) was measured from the radial artery. Signals were monitored by ICM+ software calculating mean values of ICP, ABP, cerebral perfusion pressure (CPP) and various indices describing pressure reactivity, compensation and vascular waveforms of ICP (pulse amplitude, respiratory, and slow waves), etc.

RESULTS

Mean ICP was 17 mmHg, mean CPP was 73 mmHg. Seven patients showed permanent disturbance of cerebral autoregulation (mean pressure reactivity index above 0.3). Pressure reactivity index demonstrated significant U-shape relationship with CPP, suggesting loss of pressure reactivity at too low (CPP < 55 mmHg) and too high CPPs (CPP > 95 mmHg). Mean ICP was inversely correlated with respiratory rate (R = 0.46; p < 0.0001; reciprocal model).

CONCLUSION

The new version of ICM+ software proved to be useful clinically in the NCCU. It allows continuous monitoring of pressure reactivity and exploratory analysis of factors implicating intracranial hypertension.

Multimodal Monitoring in the Neurological Intensive Care Unit

BACKGROUND

Neurocritical care is a specialty that focuses on the critical care management of patients with catastrophic neurologic diseases. Brain ischemia and hypoxia are often central causes of brain damage in these patients. Until recently, the only methods widely accepted for monitoring in the neurological intensive care unit have been intracranial pressure and cerebral perfusion pressure monitoring. Recent developments in technology have resulted in several new monitoring techniques that can provide the neurointensivist with information, at the cellular level, that can help guide management.

REVIEW SUMMARY

The brain requires a continuous blood-borne supply of oxygen and glucose for normal metabolism. Ischemia occurs when supply is insufficient to meet the metabolic demand. Cerebral blood flow can now be directly monitored using laser Doppler or thermal diffusion techniques. Transcranial cerebral oximetry can estimate regional cerebral oxygen saturation, although the reliability is questionable. Jugular bulb oximetry can provide a global assessment of oxygen delivery, and consumption and brain tissue oxygen tension monitoring can provide a focal measurement of cerebral oxygenation. Intracerebral microdialysis can provide information about glucose metabolism and the overall health of the neuron.

CONCLUSIONS

New monitoring techniques can provide the neurointensivist with crucial information about brain physiology and metabolism. Combining these techniques ("multimodal monitoring") can produce a more accurate overall picture. This approach, along with new computer systems for integrating data at the bedside, may change the way patients with brain injury are monitored and treated in the future.

Multi-modal monitoring of acute brain injury

OBJECTIVE

To review the scientific basis for and utility of the traditional cerebral monitors used currently in neurointensive care, together with research techniques that are soon likely to become used in managing severe head injury and subarachnoid haemorrhage.

DESIGN AND CONTENT

Firstly, the pathophysiology of acute brain injury including cerebral haemodynamics, oxygen and metabolism and the role of secondary insults are discussed. Secondly, the importance of assessment of cerebrovascular autoregulation and reactivity is reviewed together with methods for its continuous non-invasive measurement using transcranial Doppler and intracranial pressure/arterial pressure recordings. Thirdly, the respective roles of jugular venous oxygen and brain tissue oxygen monitoring are analysed. Fourthly, the use of cerebral microdialysis is described, together with an overview of its utility.

CONCLUSION

Cerebral multimodal monitoring can be helpful for the optimal management of acute brain injury and essential for future exploratory trials of neuroprotective drugs.

How can we measure substrate, metabolite and neurotransmitter concentrations in the human brain?

Cerebral injury and disease is associated with fundamental derangements in metabolism, with changes in the concentration of important substrates (e.g. glucose), metabolites (e.g. lactate) and neurotransmitters (e.g. glutamate and y-aminobutyric acid) in addition to changes in oxygen utilization. The ability to measure these substances in the human brain is increasing our understanding of the pathophysiology of trauma, stroke, epilepsy and tumours. There are several techniques in clinical practice already in use and new methods are under evaluation. Such techniques include the use of cerebral probes (e.g. microdialysis. voltammetry and spectrophotometry) and functional imaging (e.g. positron emission tomography and magnetic resonance spectroscopy). This review describes these techniques in terms of their principles and clinical applications.

On-line monitoring of substrate delivery and brain metabolism in head injury

Head injury is associated with complex pathophysiological changes in metabolism. The objective of the study was to investigate these changes by applying on-line bedside monitoring of cerebral metabolism using microdialysis. Following approval by the Local Ethics Committee and consent from the next of kin, a microdialysis catheter was inserted into the frontal cortex of patients with severe head injury. Twenty-one patients were studied for 102.3 +/- 26.9 hours (mean +/- 95% confidence interval; total 89.4 patient monitoring days). The overall cerebral glucose (mean of means) was 1.63 +/- 0.31 mM with periods of undetectable glucose recorded. The cerebral lactate and lactate/pyruvate ratio were 4.69 +/- 0.61 mM and 29.9 +/- 3.73 respectively. Patients who died (n = 4) or who were severely disabled (not proceeding to rehabilitation, n = 5) had a tendency towards lower glucose (1.39 +/- 0.35 mM), higher lactate (5.10 +/- 1.02 mM) and higher lactate/pyruvate ratios (35.5 +/- 7.67) compared to patients with good outcome (home or proceeding to rehabilitation, n = 12, glucose 1.80 +/- 0.49 mM, lactate 4.38 +/- 0.85 mM, lactate/pyruvate ratio 27.9 +/- 4.33). Trends in these metabolic parameters relating to outcome were identifiable. In the majority of patients, cerebral glutamate levels (overall mean of means 9.47 +/- 4.59 microM) were initially high and then declined to stable levels. Patients in whom the glutamate level remained elevated or in whom secondary rises in glutamate were seen had a poor outcome. The application of bedside analysis of microdialysis enables the progress of the patient to be monitored on-line. In addition to establishing trends of improving and deteriorating metabolism, the technique has the potential to monitor the effects of therapeutic manoeuvres on the biochemistry.

Monitoring neurologic patients in intensive care

The brain is sensitive to changes in substrate delivery. In neurologically critically ill patients (e.g., those with head injury, subarachnoid hemorrhage, or stroke), interruption of this supply causes ischemic brain damage and thus impairs the outcome. To prevent, detect, and treat these ischemic events as soon as possible, the cerebral blood flow is continuously monitored, its coupling or not with the consumption of oxygen and so forth, and the detected derangements of normal physiology. Intracranial pressure and cerebral perfusion pressure are two parameters that often reflect ischemic events, and thus it is mandatory to continuously measure them. To better assess cerebral hemodynamics, jugular bulb oxymetry and brain pressure tissue oxygen monitoring are two neuromonitoring techniques that allow for a better understanding of the balance between oxygen supply and consumption, and therefore are useful in directing therapy. Transcranial Doppler ultrasonography is a noninvasive technique with the same purpose but with less clinical relevance. The new neuromonitoring technique, microdialysis, is useful for understanding the mechanisms involved in brain ischemia. However, it is clear that the physician who interprets the measurements given by devices and the clinical data (e.g., temperature, glycemia) is still the cornerstone in the management of neurologically critically ill patients.

Limits of intermittent jugular bulb oxygen saturation monitoring in the management of severe head trauma patients

OBJECTIVE

To evaluate, in a prospective, observational study, whether bilateral monitoring of jugular bulb oxyhemoglobin saturation (SjO2), in addition to standard monitoring, results in modification of the management of severe head trauma.

METHODS

The patients underwent bilateral jugular bulb cannulation and observation at 8-hour intervals, during which SjO2 was measured and the neurological condition and physiological variables were assessed. The study group was responsible for evaluating whether the physician's decision-making process was influenced by the detection of SjO2 abnormalities. The SjO2 discrepancy in simultaneous bilateral samples was also evaluated to determine whether it interfered with the interpretation of data and with clinical decision-making. The SjO2-related complications were monitored.

RESULTS

Thirty patients underwent 319 observations. In 96% of patients, SjO2 was normal or high and had no influence on the diagnostic or therapeutic strategies. Treatment decisions were dictated by changes in clinical status and in intracranial and cerebral perfusion pressure. When these parameters were abnormal, treatment was administered, even if SjO2 was normal (101 observations). Conversely, when SjO2 was the only detected abnormality (34 observations), no treatment was administered. Abnormally low SjO2 values, caused by hypovolemia and hypocapnia, were detected in 3.4% of observations and actually modified the management. The discrepancies in simultaneous bilateral samples were substantial and gave rise to relevant interpretation problems. Fifteen percent of jugular catheters showed evidence of bacterial colonization.

CONCLUSION

Intermittent SjO2 monitoring did not substantially influence the management of severe head trauma. Therefore, recommendation for its routine use in all patients seems inadvisable, and indications for this invasive method should no longer be defined on the basis of experts' opinions, but rather on randomized, prospective studies.

Snapshot view of emergency neurosurgical head injury care in Great Britain and Ireland

OBJECTIVES

To study the availability of neurosurgical intensive care for the traumatically brain injured in all 36 neurosurgical centres in the United Kingdom and Ireland receiving head injuries, the response times to referral, and the advice given to the referring hospitals.

METHODS

Telephone survey of receiving neurosurgeons regarding their bed status and their advice on three hypothetical case scenarios. Outcome measures included response times for an acute head injury to be accepted to a neurosurgical centre; the intensive care bed status; variations in advice given to the referring hospitals with regard to ventilation, use of mannitol, steroids, anticonvulsants, and antibiotics.

RESULTS

There were 43 neurosurgical intensive care beds available for an overall estimated population of 63.6 million. There were 1.8 beds available/million of the population for non-ventilated patients, 0.64 beds available/million for ventilated patients, and 0.55 beds available/million for ventilated paediatric patients. London had a shortage of beds with 0.19 adult beds for ventilation/million north of the Thames and 0.14 adult beds for ventilation/million south of the Thames. The median response time for a patient with an extradural haematoma to be accepted for transfer was 6 minutes and 89% of such a referral was accepted within 30 minutes. Clinically significant delays in receiving referrals (over 30 minutes) occurred in four units. Practices regarding the use of hyperventilation, mannitol, anticonvulsants, and antibiotics showed little conformity and in some cases were against the available evidence and advice given by published guidelines.

CONCLUSIONS

There is a severe shortage of available emergency neurosurgical beds especially in the south east of England. The lack of immediately available neurosurgical intensive care beds results in delays of transfer that could adversely affect the outcome of surgery for traumatic intracranial haematoma. Advice given to the referring units by the receiving doctors is very variable.

[Evaluation of cerebral metabolism]

The brain requires high amounts of energy for cellular homeostasis and neurological functions. This energy is mainly supplied by the oxidation of glucose, although other substrates may be used in critical situations. The capacity of the brain to conserve energy is illustrated by flow metabolism coupling, and by the separation between functional and basal activity. The evaluation of cerebral metabolism is defined as the assessment of energy substrates availability and utilization. This evaluation may be performed in physiological or pathological conditions, at a regional or at the global level, at rest or during activation processes. The main techniques discussed in this review include monitoring of venous blood in the jugular bulb, cerebral microdialysis, near infrared spectroscopy, cerebral oximetry using microprobe electrodes, positron emission tomography, magnetic resonance and the magnetoencephalography.

Importance of textual data in multimodality monitoring

OBJECTIVES

The use of multimodality monitoring of patients in the intensive care unit (ICU) and the subsequent collection and analysis of such data are increasing. The aim of this work was to assess the importance of recording complementary textual data referring to patient care maneuvers, calibrations, and other incidents, in addition to the raw numerical values.

DESIGN

A retrospective analysis of multimodality monitoring data, which included comments entered concurrently at the bedside, collected from head-injured patients admitted to an ICU.

PATIENTS

One hundred forty-seven patients with a postresuscitation Glasgow Coma Scale score of < or = 12 were monitored for a total of nearly 1 million minutes on up to eight commonly used channels.

MEASUREMENTS AND MAIN RESULTS

Approximately 13,000 comments were added to the raw data at the time of collection. The data were subsequently validated using these comments as indicators of artifactual values. The comments were classified into a surprisingly small number of important categories, with the most frequent referring to monitor calibrations and regular ICU care maneuvers. The difference between validated and unvalidated data on the quantity of secondary insult observed was in some cases nearly 50%.

CONCLUSIONS

This work demonstrates that such textual information should be recorded concurrently with the raw monitoring values to ensure proper interpretation of the data in any retrospective analysis. Furthermore, it also suggests that a small number of prespecified categories could be used in the on-line validation of such data.

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

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

Continuous assessment of the cerebral vasomotor reactivity in head injury

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Use of near-infrared spectroscopy in the adult

Adult near-infrared spectroscopy is a potential method for observing changes in cerebral oxygenation non-invasively. Access of light to the adult brain requires requires penetration through extracranial tissues; hence the detection of changes in cerebral chromophore concentration can only be achieved by using near-infrared spectroscopy in the reflectance-mode thereby adding variables which are difficult to control. These include the effects of variable anatomy, different intra-optode distances and the presence of an extra- to intracranial collateral blood supply. Although movements of oxygenated haemoglobin concentration following specific cerebral stimuli can be demonstrated, the challenge of separating changes which occur within the extracranial compartment from those occurring in the intracranial compartments remains. Our experience with near-infrared spectroscopy in the three adult clinical scenarios of carotid endarterectomy, head injury and carbon dioxide stress testing will be presented. The influence of extracranial contamination is demonstrated, as are the methods we have developed to help control for extracranial contamination. Provisional experience with spatially resolved spectroscopy technology will also be presented.

Monitoring of cerebral autoregulation in head-injured patients

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Early effects of mannitol in patients with head injuries assessed using bedside multimodality monitoring

OBJECTIVE

We have employed bedside multimodality methods to assess the influence of a slow (20 min) bolus of hypertonic mannitol on cerebral hemodynamics in comatose patients with head injuries.

METHODS

Middle cerebral artery flow velocities (FV) and cortical microcirculatory flows were measured in comatose patients with head injuries after the administration of 200 ml of 20% mannitol. A comparison was made with the effects of an identical bolus of isotonic saline. Fourteen patients with diffuse head injuries and with raised intracranial pressure were selected, and mannitol infusion studies were conducted when clinically indicated (n = 23). Using transcranial doppler and laser doppler flowmetry (LDF), indices of estimated cerebrovascular resistance (eCVR) were calculated for the macro- (eCVR-FV) and micro- (eCVR-LDF) circulation.

RESULTS

During mannitol infusion, a significant rise in cerebral perfusion pressure was detected (+10%, P = 0.03) as a result of a fall in intracranial pressure (-21%, P = 0.001). Increases in both FV (+13%, P < 0.001) and LDF (+14%, P = 0.002) occurred only after the administration of mannitol and persisted beyond completion of infusion. The effect on FV and LDF decayed exponentially, with a time constant of 34.0 and 38.0 minutes, respectively, and was independent of the pressure autoregulatory status. There was a tendency for eCVR-FV and eCVR-LDF to decrease. No significant effects resulted from the administration of saline.

CONCLUSION

Bedside multimodality monitoring may provide a useful means for assessing the effects of therapy in the comatose patient. The mechanisms by which mannitol reduces intracranial pressure in patients with head injuries are discussed.