Continuous monitoring of cortical perfusion by laser Doppler flowmetry in ventilated patients with head injury

A method for monitoring cortical perfusion by laser Doppler flowmetry (LDF) in the neurointensive care unit is described. Out of 22 patients with head injuries, reliable and long term recordings were obtained in 16. Laser Doppler flowmetry registered changes in cortical microcirculatory flow in response to spontaneous waves of raised intracranial pressure, and to therapeutic manoeuvres that altered the cerebral perfusion pressure. Comparisons of variations in flux signal with cerebral perfusion pressure provided an indication of the autoregulatory state of the cortical microcirculation, and analysis of raw LDF data demonstrated an autoregulatory breakpoint of cerebral perfusion pressure of 58 mm Hg, below which cortical perfusion failed. Although middle cerebral artery flow velocities were generally tightly coupled with LDF signal changes, episodes of uncoupling were seen. The potential uses and limitations of LDF in the neurointensive care setting are discussed.

Computer supported multimodal bed-side monitoring for neuro intensive care

Multimodal monitoring in neuro-intensive care requires computer supported data analysis and archiving. This paper describes a computer system that is able to integrate the results of analysis of intracranial pressure, cerebral perfusion pressure, transcranial Doppler blood flow velocity, jugular bulb oxygen saturation, laser Doppler blood flow, near infrared spectroscopy. A research-oriented software installed in a standard IBM PC equipped with a low-cost analog-to-digital converter allows a broad spectrum of waveform analyses, from calculation of simple mean values to a selective waveform detection and cross-correlation analysis. The architecture for the signal processing algorithm, the principles of system interfacing and data presentation are discussed briefly. Results from two years of multimodal monitoring in neuro-intensive care show that, apart from simple recording of time trends of monitored variables, certain calculated parameters are particularly useful in the continuous assessment of cerebral haemodynamic and compensatory reserves. Such parameters include: transcranial Doppler pulsatility indices, the dependence between blood flow velocity or laser Doppler flux and cerebral perfusion pressure and the correlation coefficient between pulse amplitude and mean intracranial pressure.

Assessment of cerebral autoregulation with ultrasound and laser Doppler wave forms--an experimental study in anesthetized rabbits

The aim of the study was to correlate changes in transcranial Doppler blood flow velocity wave form in the basilar artery with cortical red blood cell flux measured with a laser Doppler flowmeter during hemorrhage-induced hypotension in anesthetized and ventilated New Zealand rabbits. Although systolic flow velocity and flux exhibited an autoregulatory threshold at 45 mm Hg, diastolic flow velocity started to fall when mean arterial blood pressure fell below 65 mm Hg. The difference between the mean arterial blood pressure at which diastolic blood flow velocity decreases and the pressure at which mean flux decreases is the difference between systolic and diastolic blood pressure. The increasing divergence between systolic and diastolic flow velocities was reflected in an increase in the amplitude of blood flow velocity pulsations and pulsatility indices. An increase in flux pulsatile wave form was noted as cerebral resistive vessels dilated with hypotension.

Monitoring of cerebrospinal dynamics using continuous analysis of intracranial pressure and cerebral perfusion pressure in head injury

Cerebrospinal dynamics has been investigated by statistical analysis of results of computerised monitoring of 80 head injured patients admitted to the Intensive Care Unit at Pinderfields General Hospital. One minute average values of intracranial pressure (ICP), systemic arterial pressure (ABP), cerebral perfusion pressure (CPP), amplitude of the fundamental component of the intracranial pressure pulse wave and the short-term moving correlation coefficient between that amplitude and mean ICP (RAP) were recorded. It was found that reduction of CPP down to 40 mmHg was more often caused by decrease in ABP than increase in ICP. Further falls in CPP below 40 mmHg were caused by substantial increases in ICP above 25 mmHg. The relationship between the ICP pulse wave amplitude and CPP showed a significant gradual increase in amplitude with CPP decreasing from 75 to 30 mmHg. For CPP below 30 mmHg there is a sharp decrease in amplitude followed by a change in the coefficient RAP from positive to negative values. This was interpreted as a sign of critical disturbance in cerebral circulation.

Shunt audit: a computerized method for testing the performance characteristics of CSF shunts in vitro

Studies of long-term shunt performance in vivo demonstrate that some 81% fail within 12 years. Such failure is multifactorial in origin: the patient, the surgical technique and the shunt may all prove fallible. Recent studies have shown that there is considerable variability of the performance characteristics of individual shunts when tested for short time periods in relatively simple rigs, and that they do not always behave according to the manufacturers' specifications. We have developed a computerized shunt rig for the long-term evaluation of a valve's performance in vitro using both pressure-flow studies (where flow through the shunt is evaluated for controlled differential pressure across the shunt) and flow-pressure studies (where the differential pressure across the shunt is evaluated for controlled flow rates through the shunt). This rig consists of a pressure transducer, electronic balance, computer-controlled infusion pump and blood pressure systems calibrator that stimulates different wave form patterns. An IBM PC controls all the devices and evaluates the performances characteristics according to various test protocols. Our initial observations with this rig confirm that progressive changes in shunt function occur over long periods of time (weeks).

[Evaluation of intracranial volume compensation analyzing changes of harmonic components of intracranial pressure pulse wave]

Intracranial pressure (ICP) pulse wave was analysed during infusion tests performed in hospital (hydrocephalus) and in cats. Pulse wave changes were evaluated calculating the high frequency centroid index (HFC) in the HFC, found to be a very valuable parameter in intracranial volume-pressure situation evaluation. The HFC decrease occurred much earlier than the rise of ICP in both clinical and experimental studies. Thus the HCF index should be very usefull in clinical evaluation of intracranial volume-pressure relation disturbances.

Frequency-dependent properties of cerebral blood transport--an experimental study in anaesthetized rabbits

Carotid arterial blood pressure and blood flow velocity in the basilar artery were studied in nine New Zealand rabbits under general anaesthesia. The cerebrovascular impedances for mean and maximal flow velocity were defined as the ratios of the respiratory and pulse waveforms of arterial blood pressure and blood flow velocities. The absolute values of these impedances were expressed and analysed as functions of frequency. Both functions decreased as frequency increased, demonstrating that a pulsatile blood movement need a lower energy expenditure than a continuous one. A minimum absolute value of impedance for flow velocity mean was found near 2.5-4 Hz in each rabbit. The impedance for mean flow velocity could be modelled by a second-order circuit, whereas the distribution of impedance for the maximal velocity had a more uniform character and could be approximated by a first-order model. A method of assessment of changes in cerebrovascular resistance using frequency properties of calculated impedance was proposed.

Testing of cerebral autoregulation in head injury by waveform analysis of blood flow velocity and cerebral perfusion pressure

Thirty five head injured patients were subjected to day-by-day monitoring of intracranial pressure (ICP), arterial blood pressure (ABP) and blood flow velocity (FV) in middle cerebral artery. Parameters describing cerebral autoregulation: flow velocity time average, systolic, diastolic values, pulsatility index (PI), estimate of cerebrovascular resistance (CVR = CPP/FV) etc. were analyzed as functions of CPP (ABP-ICP). The results show that for CPP below 55 mmHg the evidence of exhausted autoregulation can be observed in FV, CVR, PI. The method is helpful in assessment ot the optimal level of CPP in order to reduce the probability of ischaemic brain insults.

CO2 cerebrovascular reactivity as a function of perfusion pressure--a modelling study

A mathematical model is described that demonstrated the properties of cerebral vascular resistance and compliance expressed as a function of cerebral perfusion pressure (CPP) and arterial CO2 partial pressure (PaCO2). The hypercapnic induced shift of the lower limit of autoregulation to a higher range of CPP, as shown by this model, is a useful characteristic that facilitates the differentiation between normal and impaired autoregulation described previously in experimental studies. Dynamic properties of cerebrovascular circulation derived from the relationship between pulse wave of CBF waveform and CPP have been analysed at different levels of PaCO2-phenomenon, being often described as dependence of blood flow velocity pulsatility index on the autoregulatory reserve. The model was also used to interpret interhemispheric asymmetry of CBF reactivity to changes in arterial concentration of CO2 in patients with carotid artery stenosis.

Management of raised intracranial pressure

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

Cerebrospinal compensation in hydrocephalic children

One hundred and fifteen cases of hydrocephalus in children were analysed. Cerebrospinal compensatory reserve was assessed by a computerized, constant rate, lumbar infusion test. Head circumference and ventricular size were measured and a psychometric examination carried out. A classification of hydrocephalus based on resting cerebrospinal fluid pressure (CSFP) and resistance to cerebrospinal fluid outflow (RCSF) was introduced. Parameters of compensatory reserve were compared in atrophy (low CSFP, low RCSF), normal-pressure hydrocephalus (low CSFP, increased RCSF), non-communicating hydrocephalus (high CSFP, low RCSF) and acute hydrocephalus (high CSFP and increased RCSF). Significant differences were found between the factors describing compensatory ability in these groups. Sixty-two patients could be classified on the basis of resting CSFP and RCSF. Differentiation between the types of hydrocephalus was shown to be more accurate when all variables measured during the pressure-volume test were considered. The patterns of the time courses of CSFP during rate infusion tests in the different types of hydrocephalus are presented.

Experimental aspects of cerebrospinal hemodynamics: the relationship between blood flow velocity waveform and cerebral autoregulation

The dynamic relationships among mean flow velocity, its pulsatile amplitude (FVa), cortical cerebral blood flow (CBF), and cerebral perfusion pressure (CPP) were studied in normal rabbits and rabbits with subarachnoid hemorrhage using 8-MHz pulsed transcranial Doppler ultrasound and hydrogen clearance under conditions of systemic hypotension and intracranial hypertension. A two-slope relationship was observed between FVa and CPP with a break point that correlated closely with the lower limit of CBF autoregulation in each animal. Below this CPP break point, FVa varied directly with CPP, and above the break point FVa varied inversely with CPP. In this experimental model, an inverse correlation between FVa and CPP indicates intact CBF autoregulation, whereas loss of that correlation implies exhaustion of autoregulatory reserve. Simultaneous recording and computation of FVa, CPP, and the correlation coefficient between FVa and CPP may be a means of monitoring CBF autoregulation in clinical practice.

The hyperaemic response to a transient reduction in cerebral perfusion pressure. A modelling study

A mathematical model of cerebral blood flow and the cerebrospinal fluid circulation is described which permits the study of phenomena caused by dynamic changes in cerebrovascular autoregulatory or cerebrospinal fluid compensatory reserves. A transient decrease in cerebral perfusion pressure was produced by carotid artery compression. Comparison of the computer simulations with clinical and experimental data, reported elsewhere, suggests that the transient hyperaemic response (THR) is proportional to the strength of the autoregulatory response. The relationships between the magnitude and time course of the THR, and the period and level of reduction in CPP were studied. This model suggests that simple clinical tests based on the examination of THR using transcranial Doppler velocity measurements are of potential value for the non-invasive assessment of the autoregulatory reserve.

Computerized infusion test compared to steady pressure constant infusion test in measurement of resistance to CSF outflow

Resistance to cerebro-spinal fluid outflow is together with intracranial pressure the most important parameter in the investigation of patients with disturbances of CSF dynamics. The methods for determination of resistance are either unreliable or too time-consuming for routine clinical use, which has limited the popularity of this kind of measurement. In this paper a method for computerized acquisition and processing of an infusion test is described. A good correlation to a standard technique is documented.

A computer system for the identification of the cerebrospinal compensatory model

A computer system, based on IBM PC, was designed for the cerebrospinal compensatory model identification. The intracranial pressure (ICP) signal, registered during the lumbo-lumbar infusion test is analyzed by means of the spectral analysis algorithm in order to measure precisely the pulse wave amplitude. The amplitude and the mean ICP level, calculated repetetively within the period of about 8 seconds, are stored on the disk and form the basis for further model identification. Three different methods of identification were applied. They enable one to estimate the fundamental model parameters, such as: resistance to the cerebrospinal fluid resorption, pressure-volume index, baseline pressure, rate of formation of the cerebrospinal fluid. Statistical evaluation of the results of the infusion test analysis obtained by means of the system described in two groups of hydrocephalic patients (children and adults) is presented.

[Characteristics of intracranial volume-pressure relationship in hydrocephalic children with reference to age]

The purpose of the study was establishing of the effect of child's age, and thus the biomechanical properties of cranial coverings, on the parameters characterizing the intracranial space in the infusion test. The results of the infusion tests performed as a supplementary diagnostic examination in 59 cases of infantile hydrocephalus were subjected to statistical analysis. The studied material and the methods used for its processing no significant differences were found in the mean values of the biomechanical parameters of intracranial space were found between the group of younger children (aged up to 2 years) and older children (over 2 years). This finding may be important in the interpretation of the results of the infusion test, since it was not found that the biomechanical properties of the cranial coverings had any significant effect on the results of this test.

Comparison between classic-differential and automatic shunt functioning on the basis of infusion tests

Infusion tests were performed in order to examine cerebrospinal compensatory ability in two groups of patients with impaired compensation, subjected to shunt implantation. The functioning of the classic differential shunts was compared to the automatically (Orbis-Sigma) functioning shunts. The reference group, with intact compensatory parameters is also presented. The influence of different types of shunts on the intracranial compensation ability was compared. Automatic shunts produce less decrease in the resorption resistance of CSF in the post-shunted examination than the classic differential shunts. Classic differential shunts disturb the mono-exponential character of the pressure-volume relationship in a higher degree than automatic shunts. Shunt functioning models were proposed as well as the method of detection of eventual recovery to the normal resorption reserve. This method can be applied only to patients with automatic shunts.

Early pathomorphological changes and intracranial volume-pressure: relations following the experimental sagittal sinus occlusion

Saggital sinus occlusion was produced in cats. The intracranial volume-pressure relations were studied using the lumbar infusion tests. Right after occlusion ICP rised from 7.1 +/- 1.8 to 12.4 +/- 4.1 mmHg. The values of outflow resistance and volume-pressure response increased also and remained significantly unchanged with only slight decrease of the volume-pressure response. The morphometric study of the surface veins showed the dilatation of the veins draining to the occluded saggital sinus for about 10-20%. No signs of the blood-brain barrier disruption were observed.

The role of cerebrospinal compensatory parameters in the estimation of functioning of implanted shunt system in patients with communicating hydrocephalus (preliminary report)

Twelve patients presenting with communicating hydrocephalus were studied. In 9 cases where the mean ICP level remained below 10 mmHg the symptoms of normal pressure hydrocephalus were observed. All of the patients were treated by shunt system implantation. The clinical and psychological state, cranial computerized tomography and parameters of the cerebrospinal compensatory mechanisms, evaluated using the constant rate infusion test were compared before and after treatment. In most of the patients (11) the pathologically enlarged ventricles persisted. Only in three cases no clinical improvement was noticed. In this group the resistance to the cerebrospinal fluid absorption and the fluid formation rate were estimated as normal. In the group with improvement the normalization of the resistance (4), decrease in the cerebrospinal fluid formation rate (4) and decrease in the cerebrospinal system elasticity (1) can be pointed out as factors responsible for improvement manifested after shunting. Therefore the resorption resistance and the formation of cerebrospinal fluid should be considered as predictive factors in the shunt implantation in hydrocephalic patients, and play an important role in the diagnosis of this entity.

Analysis of intracranial pressure waveform during infusion test

An analysis of intracranial pressure (ICP), based on an examination of the temporary correlation between the changes in amplitude of the pulse wave and the mean ICP level, is presented. The paper contains a discussion of the preliminary results of the method when applied to the analysis of ICP as monitored during infusion tests in a group of 24 children. Infusion of a certain volume of CSF is a good example of an uncompensated volume process, introduced externally into the intracranial space. Results allow an interpretation of the short term correlation coefficient RAP (correlation coefficient between ICP and variations of the amplitude of fundamental component of the pulse wave AMP), as a steady state index. According to this interpretation, the presented analysis enables the observation of a loss of equilibrium during the test. Other phenomena can also be observed, for instance a recovery to equilibrium after the test, nonlinearities of amplitude-pressure relationship, vasomotor reflexes etc.