Cerebrospinal fluid pulse pressure and the pulsatile variation in cerebral blood volume: an experimental study in dogs.

The cerebrospinal fluid pulse pressure (CSFPP) has found application as a measure of intracranial elastance. However, CSFPP is also dependent on the magnitude of the pulsatile variation in cerebral blood volume (delta Vb). The purpose of the present study was to assess the effect on delta Vb of changes in systemic arterial pressure (SAP) and arterial carbon dioxide tension (PaCO2) as well as elevation of intracranial pressure (ICP). Therefore, delta Vb was computed from the electromagnetically measured flow profile in the vertebral artery of the dog on the assumption of a nonpulsatile cerebral venous outflow. During arterial hypotension, delta Vb was increased due to a shift of flow from diastole to systole, whereas mean flow was not affected. The reverse phenomenon was observed when SAP was raised. Changes in PaCO2 had little effect on pulsatile blood flow. The changes in total blood flow that occurred were evenly distributed over the cardiac cycle. Consequently, delta Vb was not significantly affected, although CSFPP was considerably changed. When ICP was raised, a breakpoint pressure was observed above which cerebral blood flow (CBF) decreased and CSFPP and delta Vb increased. This contradiction was explained by the finding of a decrease in diastolic flow, causing the fall in CBF, whereas systolic flow relative to mean flow was increased, resulting in an increased delta Vb. The underlying mechanisms of the pulsatile flow changes are extensively discussed. It is argued that the arterial inflow profile is largely determined by the compliance of the inflow section of the cerebral vascular bed. Vascular compliance is significantly altered by changes in SAP and ICP because they affect the transmural pressure of the vessels, whereas this is not the case during changes in PaCO2.

Stimulus processing during apparent unconsciousness in anesthetized volunteers.

Return of motor-responses upon request as an indicator of stimulus processing during apparent unconsciousness in general anesthesia was studied in 8 healthy, male volunteers during prolonged inhalation of nitrous oxide. First the minimal effective concentration of nitrous oxide was established for each volunteer, based upon continued absence of motor-responses to repeated verbal commands. One week later this concentration of nitrous oxide was administered for a 3-hr. period; return of motor-responses after at least 30 min. of absence was considered a sign of so-called unconscious perception. Four volunteers showed return of motor-response within the 3 hr. of exposure, but two of these had been rather restless throughout the session. Results indicate that unexpected processing of information by patients may occur during presumed unconsciousness after a prolonged inhalation of nitrous oxide in general anesthesia.

The role of the pulsatile pressure variations in intracranial pressure monitoring.

The magnitude of the pulsatile intracranial pressure variations (CSF pulse pressure) is determined by the elastance of the craniospinal system and by the magnitude of the pulsatile variations in cerebral blood volume (CBV). The pulsatile change in CBV is, among other factors, determined by the compliance of the cerebral vascular bed which, in its turn, is dependent on the cerebral vasomotor tone. This concept has led the authors to devise a method for the assessment of both the elastance and the state of the cerebral vasomotor tone based on the relationship between CSF pulse pressure and intracranial pressure. This relationship was found to be of a linear nature both in clinical patients and in experimental animals. A significant, positive correlation was found between the slope of this relationship and the value of the craniospinal volume-pressure relationship: the elastance coefficient. During elevation of the intracranial pressure a breakpoint was observed in the relationship between CSF pulse pressure and the intracranial pressure above which the pulse pressure increased more rapidly. The elastance remained constant above this breakpoint. The same phenomenon was observed during plateau waves in clinical patients. Induced changes in systemic arterial pressure produced opposite effects on CSF pulse pressure and elastance coefficient. In these cases the discrepancy between pulse pressure and elastance was attributed to the pulsatile changes in CBV and this could be verified by means of electromagnetic flowmetry. The advantage of this method is that all the information is contained within the intracranial pressure signal itself, from which it can be extracted by simple means without the use of invasive tests.

Clinical observations on the relationship between cerebrospinal fluid pulse pressure and intracranial pressure.

A method is described for monitoring the relationship between CSF pulse pressure and ICP in clinical patients. Highly significant linear relationships were found during 65 continuous ICP recordings in 58 patients. The slope value of this relationship showed a positive correlation with the elastance coefficient, a volume-pressure parameter assessed by bolus injection. However, the correlation was too weak to allow for a confident prediction of the elastance coefficient on the basis of CSF pulse pressure in the individual patient. This was attributed to the variable magnitude of the volume change underlying the CSF pulse pressure: the pulsatile variation in cerebral blood volume. This quantity was calculated on the basis of a mathematical model from the slope value and the elastance coefficient and was found to vary between 0.36 and 4.38 ml. During plateau waves a disproportionate increase in pulse pressure with the ICP was observed in contrast with a relative decrease in intracranial elastance. This phenomenon was ascribed to an increase in the pulsatile variation in cerebral blood volume. It is concluded that, under certain conditions, the intracranial volume-pressure relationship can be continuously monitored by means of CSF pulse pressure analysis. The findings during plateau waves suggest that the pulse pressure also reflects the state of the cerebral vasomotor tone.

Tolerance to nitrous oxide in volunteers.

Nociception and loss of awareness during exposure to anaesthetic concentration of nitrous oxide (N2O) were studied in eight male medical students. The cold water nociception test, where a hand is immersed in 0 degree C stirred water, was used for measurement of nociception. At irregular intervals an auditory command was given to oppose two fingers, and this served to monitor consciousness. The selected inspiratory concentration of N2O used per individual was sufficient to induce a loss of consciousness for more than 2.5 min, within 10 min of exposure to N2O. This concentration of N2O varied from 60% to 80%. The experimental exposure to N2O lasted 3 h. In all volunteers significant antinociception was observed within 2 min of exposure to N2O. The maximal analgesic effect was observed between 20 and 30 min of exposure to N2O. The analgesic effect of N2O gradually decreased and was absent in all eight volunteers within 150 min. Two volunteers regained consciousness at 77 and 91 min of exposure, whilst still breathing 60 and 80% N2O. These results show that tolerance to antinociceptive effects of N2O in man rapidly develops and that awareness may occur in some volunteers during prolonged exposure to N2O.

A method for two dimensional multi-segmental kinematic and kinetic analysis of normal and pathological human gait.

The application of an integrated gait analysis system in clinical research is described. With this system it is possible to analyse the walking pattern of various groups of patients with regard to foot-floor contact, kinematics and kinetics, using a Selspot system. In order to study various groups of patients an extensive body-segment model adapted to the variety in patient groups is dealt with and is related to data from the literature. Special attention has been paid to the analysis or errors in the quantities used in kinematics and kinetics. The various sources of errors leading to inaccuracy of the data describing the walking pattern are discussed.

Visual evoked potentials, auditory evoked potentials and EEG in shunted hydrocephalic children.

Visual evoked potentials (VEP) and auditory evoked potentials (AEP) were studied together with the EEG, in 15 hydrocephalic children who had been shunted previously, and in a control group of 10 normal children. From the control group normal VEP's, AEP's and EEG's were obtained. In all 15 hydrocephalic children the EEG was abnormal. AEP's were normal in 9 and abnormal in 6 cases. VEP's were normal in 7 and abnormal in 8 cases. Only 4 patients showed both abnormal VEP's and AEP's. No relation could be demonstrated between the severity of EEG disturbances and evoked response abnormalities.

Effects of hypercapnia and arterial hypotension and hypertension on cerebrospinal fluid pulse pressure and intracranial volume-pressure relationships.

In twelve anaesthetised, ventilated dogs the effects of hypercapnia and pharmacologically induced arterial hypotension and hypertension on the interrelation between volume-pressure response (VPR) and cerebro-spinal fluid (CSF) pulse pressure were studied during continuous inflation of a supratentorial extradural balloon. Hypercapnia did not significantly affect the intracranial volume-pressure relationships, but did cause a significant increase in gradient of the relationship between CSF pulse pressure and intracranial pressure (ICP). Alteration of the arterial blood pressure showed opposite effects on VPR and CSF pulse pressure. A decrease in VPR and an increase in pulse pressure were observed during arterial hypotension; the reverse was found during arterial hypertension. The discrepancy between the effects on VPR and CSF pulse pressure of the variables under study was explained by changes in the transient increase in cerebral blood volume per cardiac cycle. On the basis of the results of this study it will be possible, during clinical ICP monitoring, to interpret changes in the CSF pulse pressure to ICP ratio in terms of changes in intracranial volume-pressure relationships.

Cerebrospinal fluid pulse pressure and intracranial volume-pressure relationships.

In six anaesthetised and ventilated dogs the CSF pulse pressure was compared with the volume-pressure response (VPR) during continuous inflation of an extradural balloon. Both pulse pressure and VPR increased linearly with the ventricular fluid pressure (VFP) up to a mean VFP of 60 mmHg. At this pressure a breakpoint occurred above which the CSF pulse pressure showed a steeper linear increase, while the VPR remained constant. It is suggested that the breakpoint is related to failure of autoregulation, and that in non-autoregulating patients the CSF pulse pressure is a better parameter of the clinical state than the VPR.