The effect of increased intracranial pressure on cerebrovascular hemodynamics

Advanced Pediatric Neurosonography Techniques: Contrast-Enhanced Ultrasonography, Elastography, and Beyond

Recent technical advances in neurosonography continue broadening the diagnostic utility, sensitivity, and specificity of ultrasound for detecting intracranial abnormalities bed side. The clinical and functional applications of neurosonography have significantly expanded since the 1980s when transcranial Doppler sonography first allowed anatomic and hemodynamic delineation of the intracranial vessels through the thin temporal skull. In the past few years, contrast-enhanced ultrasonography, elastography, 3D/4D reconstruction tools, and high-resolution microvessel imaging techniques have further enhanced the diagnostic significance of neurosonography. Given these advances, a thorough familiarity with these new techniques and devices is crucial for a successful clinical application allowing improved patient care. It is essential that future neurosonography studies compare these advanced techniques against the current "gold standard" computed tomography and magnetic resonance imaging to assure the accuracy of their diagnostic potential. This review will provide a comprehensive update on currently available advanced neurosonography techniques.

Effects of alterations in arterial CO2 tension on cerebral blood flow during acute intracranial hypertension in rats

Cerebrovascular reactivity to CO2 in clinical and experimental studies has been found to be impaired during increased intracranial pressure (ICP). However, from previous study results it has not been possible to estimate whether the impairment was caused by elevated ICP, or caused by decreased cerebral perfusion pressure (CPP). The current study was carried out in a group of unmanipulated control rats and in six investigation groups of six rats each: two groups with elevated ICP (30 and 50 mm Hg) and spontaneous arterial blood pressure (MABP), two groups with spontaneous ICP and arterial hypotension (77 and 64 mm Hg), and two groups with elevated ICP (30 and 50 mm Hg) and arterial hypertension (124 mm Hg). Intracranial hypertension was induced by continuous infusion of lactated Ringer's solution into the cisterna magna, arterial hypotension by controlled bleeding, and arterial hypertension by continuous administration of norepinephrine intravenously. Cerebral blood flow (CBF) was measured repetitively by the intraarterial 133Xe method at different levels of arterial PCO2. In each individual animal, CO2 reactivity was calculated from an exponential regression line obtained from the corresponding CBF/PaCO2 values. By plotting each individual value of CO2 reactivity against the corresponding CPP value from the seven investigation groups, CPP was significantly and directly related to CO2 reactivity of CBF (P < .001). No correlation was found by plotting CO2 reactivity values against the corresponding MABP values or the corresponding ICP values. Thus, the results show that CO2 reactivity is at least partially determined by CPP and that the impaired CO2 reactivity observed at intracranial hypertension and arterial hypotension may be caused by reduced CPP.

Initial small-volume hypertonic resuscitation of shock and brain injury: short- and long-term effects

BACKGROUND

Initial small-volume hypertonic saline resuscitation of a combined hemorrhagic shock and head injury model was studied.

METHODS

Twenty-three sheep underwent hemorrhage (20 mL/kg) and parietal freeze injury followed by initial bolus resuscitation with lactated Ringer's solution (40 mL/kg) or 7.5% hypertonic saline (HS) (4 mL/kg). Cardiac index was maintained with lactated Ringer's solution for either 2 or 24 hours. Parietal lobe water content, blood volume, and blood flow were determined. Intracranial pressure (millimeters of mercury) was followed.

RESULTS

Overall fluid requirements (milliliters per kilogram) were less at 2 and 24 hours with HS resuscitation. Early intracranial pressure was less with HS resuscitation. Brain water contents were similar between groups. Blood flow in injured and blood volume in uninjured parietal lobe were less for HS at 2 hours, although not different at 24 hours.

CONCLUSIONS

Less fluid was needed in the short- and long-term with HS resuscitation. Early intracranial pressure was higher with lactated Ringer's solution resuscitation, possibly in part owing to increased blood volume.

Traumatic brain edema: an overview

This article provides a brief summary of concepts describing the formation and resolution of traumatic brain edema. Recent laboratory and clinical data are reviewed targeted toward resolving the contribution of edema to the swelling process. These data, indicate that blood volume is reduced in areas of ischemia following traumatic injury and edema volume is increased. Thus, edema is the major contributor to the swelling process in diffuse injury. As clinical MRI studies have not revealed barrier compromise in the presence of swelling, it is considered that other forms of edema, primarily ischemic and neurotoxic, make a substantial contribution to the edema volume.

Physiologic parameters of the Cushing reflex

The effects of increased intracranial pressure and blood gas tensions on systemic blood pressure were examined in this study. Intracranial pressure was raised hydrostatically and blood gas tensions, blood pressure, and respiration were monitored in anesthetized dogs. Small gradual increments in intracranial pressure resulted in increased cerebral venous carbon dioxide tension, followed by increased respiration, a gradual rise in blood pressure, and finally an increase in heart rate. The results of this study indicate that blood pressure changes appear to be determined by alterations in carbon dioxide tension following increases in intracranial pressure; small increases in intracranial pressure elicit a cluster of physiologic responses, all directed toward stabilization of local cerebral carbon dioxide tension.

Intracranial blood velocity in head injury. A transcranial ultrasound Doppler study

We report our experience monitoring head-injured patients by means of transcranial Doppler sonography. Cerebral velocity measurements and waveforms change in a consistent pattern with the existence of intracranial pressure, and it is possible to discriminate low versus high flow states. The technique, in contrast to cerebral blood flow measurement, is reliable, atraumatic, and repeatable so that diagnostic assessments can be made and the patient can be followed for therapeutic efficacy.

Hypertonicity of intestinal smooth muscle as a factor of intestinal ischemia in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is thought to be secondary to mucosal ischemia. Because blood flow to the submucosal plexus is derived from vessels traversing three separate layers of visceral smooth muscle (longitudinal, circular, and muscularis mucosa), we investigated whether an increase in their tone might elicit mucosal ischemia. The intestinal intraluminal pressure (IIP) and the superior mesenteric artery (SMA) blood flow were evaluated in 23 dogs before and after either ligation of the SMA or neostigmine infusion into the SMA. Changes in vascularity were assessed by silicone rubber casting, India ink, or arteriography. Ten minutes after ligation of the SMA, there was a considerable increase in peristalsis, IIP, and inability to fill the intestinal microcirculation by the three methods described. Mucosal necrosis was present three hours later. In the neostigmine infusion group after a transient increase in mesenteric flow, the IIP rose 750%, while the mesenteric flow fell by 40%, mucosal necrosis occurred in one hour. When myotomy was performed on the antimesenteric border, mucosal necrosis was prevented. In a third group, neostigmine infused (femoral artery) in the hind limb demonstrated vasodilating effects. The data indicate that an increase in the myogenic tone and frequency of contraction of intestinal smooth muscle can produce mucosal ischemia, thus, intestinal hypertonicity may be an important factor in the pathogenesis of intestinal ischemia and possibly NEC. The effects of neostigmine in these experiments raise questions regarding its use during anesthesia in neonates with intestinal low flow states.

Biomathematics of intracranial CSF and haemodynamics. Simulation and analysis with the aid of a mathematical model

A mathematical model of the isolated intracranial system including autoregulation of cerebral blood flow with the aid of a variable cerebrovascular resistance is described. The rate of formation of cerebrospinal fluid is assumed to depend on the regional blood flow through the choroid plexuses. This model is extended by cardiovascular components including the left ventricle of the heart, the aorta and the peripheral resistance. Additionally the model contains control circuits to simulate the short-time behaviour of the blood pressure regulation with the aid of the baroreceptor reflex. Disturbances of central regulation of blood pressure are simulated depending on changes of the regional blood flow through the brain stem. The application of the model is demonstrated by the analysis of the influence of arterial blood pressure upon the intracranial pulse pressure relationship (PPR) and upon the pressure response to a volume pressure test. Theoretical considerations and simulations reveal an opposite effect of arterial blood pressure (ABP) and its amplitude upon PPR. The ICP amplitude rises with decreasing ABP or increasing ABP amplitude. Breakpoints and other deviations from a linear PPR over the whole ICP range are studied by the analysis of the transfer function. The application of the model concerning parameter estimation methods is demonstrated and discussed. Simulations of rhythmic phenomena with the aid of the extended model point out possible approaches to quantitative descriptions of disturbances of central regulation.

Decerebrate rigidity and vegetative signs in the acute midbrain syndrome with special regard to motor activity and intracranial pressure

Decerebrate rigidity, intracranial pressure and vegetative signs were studied in 25 patients. Advanced statistical techniques were used to analyse the interrelationships between muscle activity (IEMG), blood pressure (ABP), intracranial pressure (ICP), pulse rate (HR), respiratory rate (RR), and central venous pressure (CVP) occurring during paroxysms of decerebration. The pattern of reaction is influenced by compression or stress imposed on the brain stem at the tentorial incisure and is related to the degree of cisternal obstruction. Significant differences in reaction were disclosed between provoked and spontaneous decerebration posturing.

Biochemical changes in neurosurgical patients under critical care treated with mannitol

A Study was carried out on ten patients undergoing operations for brain tumors, who were treated with mannitol solutions. This caused a significant depletion of serum sodium ion and in an increase of discriminate osmolality (to 60 mosM/kg H2O). A hypothesis about the particular biochemical mechanism, involving the electrolyte and water distribution, is presented.

Treatment of experimental brain oedema following sudden decompression, surgical wound, and cold lesion with vasoprotective drugs and the proteinase inhibitor "Trasylol"

The study was performed on 81 cats with three models of experimental brain oedema: sudden decompression, surgical wound, and cold injury. During the experiments blood pressure, central venous pressure, and intracranial pressure were recorded. The blood-brain-barrier was tested with Evans blue solution. The gray and white matter tissue was sampled at the end of the experiment, and the water content and sodium and potassium concentrations were determined. The animals with the same experimental model were divided into three groups: untreated, treated with the vasoprotective agents, and treated with the protease inhibitor Trasylol. In the sudden decompression model after balloon deflation, white matter haemorrhages and oedema development were found in gray matter and basal nuclei. In animals treated with the vasoprotective drugs, haemorrhages were not observed, and oedematous changes were less pronounced. The Trasylol effect on oedema development was not significant in this model. In the surgical wound model, oedematous changes were observed after 24 hours following the lesion. Oedema occurred in the white matter, as in the animals with cold lesions. In both models--surgical wound and cold lesion--the beneficial effect of Trasylol was shown, while the effect of Aescorin was less evident. The results obtained seemed to testify to the usefulness of both Trasylol and vasoprotective drugs in the prevention and treatment of brain oedema in neurosurgical patients.

The limitation of pulsatile flow through the aqueduct of Sylvius as a cause of hydrocephalus

The concept is advanced that hydrocephalus results from limitation in the pulsatile flow of CSF downwards through the aqueduct of Sylvius during systole which is necessary to accommodate for the pulsatile pressure and volume increase that accompanies the propagation of the arterial pulse through the brain. Evidence is given to show that flow through the fixed human aqueduct is disturbed and not laminar. Further, with the pressures availalbe, the aqueduct is only just large enough to pass the quantity of fluid which must be vented extracranially during systole. Should the capacity of this systolic venting mechanism be exceeded, physical strain will cause cellular damage in the periventricular and periaqueductal regions which, if prolonged, will lead to tissue destruction and hydrocephalus. There appear to be two main causes for hydrocephalus resulting from this mechanism. Firstly, structural lesions, restricting the lumina of the CSF-venting pathways, especially the aqueduct, will reduce the volume of CSF that can flow through these pathways during systole. The hydrocephalic process will then be continuous and only limited when tissue destruction reduces the systolic volume expansion of the brain such that it can be accomodated by the restricted CSF venting pathways. Secondly, conditions which may increase the amount of the systolic volume expansion of the brain beyond the capacity of the CSF venting pathways. Raised mean intracranial pressure is the most important of these conditions. In such cases the hydrocephalus will be limited by the duration of the causal process and possibly also by the enlargement of the venting pathways, as a result of tissue destruction. This hypothesis also accounts for hydrocephalus resulting from obliteration of the cortical subarachnoid space, obstruction to the cranial venous drainage, deformities in the region of the foramen magnum and arterial encroachment upon the ventricular system.

Cerebral hemodynamic changes during plateau waves in brain-tumor patients

The plateau wave, one of the wave forms observed in patients with increased intracranial pressure, has previously been extensively investigated, but its pathophysiological aspect is as yet unclear. The authors undertook a study of cerebral hemodynamic changes while the plateau waves were observed in five brain-tumor patients. Although the number of cases studied was small, a remarkable decrease in cerebrovascular resistance was seen in all patients during the plateau waves. It is suggested that the plateau waves are caused by a marked cerebral vasodilatation. The present results support the thesis that cerebral blood volume is increased during the plateau waves. The plateau waves are closely related to the intrinsic vasomotor control of cerebral circulation, and can occur as long as cerebral vasodilating ability is maintained, irrespective of the existence of cerebral autoregulation.

Disturbances in the blood-brain barrier and cerebral blood flow after rapid brain decompression in the cat

The effect of sudden decompression of the brain on the blood-brain barrier and cerebral blood flow was studied in cats. The decompression experiments were performed on 9 cats which had been subjected to two hours of compression with an epidural balloon and on 10 cats subjected to four hours of balloon compression. In both experimental groups one could observe haemorrhages and disturbances in the blood-brain barrier in the cortex of the previously compressed hemisphere and in the basal nuclei. The extent of these changes depended on the duration of the epidural compression. At the same time, a decrease in cerebral blood flow, mainly in the brain cortex, was observed in both experimental groups.

Absence of autoregulation in peripheral nerve blood flow

Blood flow was measured in the sciatic nerve of cats utilizing the method of hydrogen polarography. The mean baseline blood flow for all animals was found to be 47.1 ml/100 g/min +/- 14.9 SD. The flow changes produced by lowering the blood pressure by exsanguination and elevation by the use of angiotensin were then evaluated. The highest (normal) levels of blood flow were observed between the mean blood pressures of 80-110 mm Hg. At mean systemic arterial pressures of less than 85, there was a marked decrease in peripheral nerve blood flow with no detectable flow being measured below mean systemic pressures of 50 mm Hg. Above 105 mm Hg mean arterial pressure, there was a very gradual and progressive decline in blood flow to the levels measured at 200 mm Hg. These findings indicate a complete absence of vascular autoregulation in the peripheral nerve trunks.

In vivo determination of cerebral blood volume with radioactive oxygen-15 in the monkey

A method for the in vivo determination of cerebral blood volume was tested in 15 adult rhesus monkeys. The technique utilized external residue detection and required the serial measurement of two mean transit times, namely, that of an intravascular tracer, C15O-hemoglobin, and that of a diffusible tracer, H215O. In computing the mean transit time for the intravascular tracer, the conventional Hamilton extrapolation of the downslope of the recording obtained for the washout of the tracer from the brain subsequent to an intracarotid bolus injection was found to be inadequate, yielding a mean transit time that systematically underestimated that parameter. Alternatively, the use of a power law extrapolation, as proposed by Huang, allowed a more accurate prediction of the vascular mean transit time. The preliminary studies testing the method predicted that the relationship between cerebral blood volume (CBV) and cerebral blood flow (CBF) was adequately represented by the equation CBV = 0.80CBF0.38, with a correlation coefficient of r = 0.90 for the cerebral blood flow range of 16 to 134 ml/100 g min-1 with a normocapnic cerebral blood volume of 3.5 ml/100 g perfused brain tissue (arterial Pco2 = 37 torr, CBF = 50 ml/100 g min-1).

Chronic cerebral arterial spasm. The role of intracranial pressure

The author used isolated rabbit common carotid and femoral arteries perfused at a constant pressure of 90 mm Hg to examine the variation of flow (F) with transmural pressure (TMP). When the TMP was reduced below 50 to 60 mm Hg in arteries with normal smooth muscle tone, arterial resistance increased significantly causing a reduction in flow. It is suggested that the diffuse arterial narrowing that occurs in patients with severe intracranial hypertension may be the result of a similar reduction in TMP. In the presence of active vasoconstriction, any increase in extraluminal (intracranial) pressure (ICP) resulted in a substantial increase in arterial resistance and subsequent reduction of flow. This F-TMP relationship depended only on the initial degree of constriction and was independent of the vasoconstrictor used to achieve this constriction and of the artery in which this constriction was produced. A review of the literature suggests that human cerebral arteries normally exhibit only mild constrictions in response to subarachnoid blood during the chronic phase of spasm. In the present study, a mild constriction in the absence of increased ICP or a moderate increase in ICP (45 mm Hg) in the absence of constriction produced minor reductions in arterial diameter and an average flow reduction of only 5% to 10%. However, when ICP was increased to 45 mm Hg in the presence of a mild constriction, severe arterial narrowing resulted and flow was reduced by 50%. Therefore, it is suggested that chronic arterial spasm is the result of a mild constriction which is amplified by the simultaneous occurrence of increased ICP. Phenoxybenzamine was found to be effective in reversing and preventing these contractions. The improvement in flow produced by phenoxybenzamine decreased as the TMP was reduced below 60 mm Hg. The effects of both diffuse and local spasm on cerebral blood flow are discussed.

Effects of increased intracranial pressure on cerebral blood volume, blood flow, and oxygen utilization in monkeys

The relationship of cerebral blood volume (CBV) to cerebral perfusion pressure (CPP), cerebral blood flow (CBF), and the cerebral metabolic rate for oxygen (CMRO2) was examined in rhesus monkeys. In vivo tracer methods employing radioactive oxygen-15 were used to measure CBV, CBF, and CMRO2. Cerebral perfusion pressure was decreased by raising the intracranial pressure (ICP) by infusion of artificial cerebrospinal fluid (CSF) into the cisterna magna. The production of progressive intracranial hypertension to an ICP of 70 torr (CPP of 40 torr) caused a rise in CBV accompanied by a steady CBF. With a further increase in ICP to 94 torr, CBV remained elevated without change while CBF declined significantly. Cerebral metabolic rate for oxygen did not change significantly during intracranial hypertension. For comparison, CPP was lowered by reducing mean arterial blood pressure in a second group of monkeys. Only CBF was measured in this group. In this second group of animals, the lower limit of CBF autoregulation was reached at a higher CPP (CPP approximately to 80 torr) than when an increase in ICP was employed (CPP approximately to 30 torr).

The effects of changes in PaCO2 on cerebral blood volume, blood flow, and vascular mean transit time

The relationships between cerebral blood volume (CBV), cerebral blood flow (CBF), and the cerebral vascular mean transit time (t v ) during acute changes in the Pa CO 2 over a range of 15 to 76 torr were investigated in vivo in rhesus monkeys by serially determining the mean transit time of a vascular tracer, 15 O-labeled carboxyhemoglobin, and the mean transit time of a diffusible tracer, 15 O-labeled water. Over this range of Pa CO 2 , a significant linear relationship of CBV = 0.041 Pa CO 2 + 2.0 was found. For each one torr change in Pa CO 2 , there is a change in CBV of 0.041 ml/100 gm of perfused tissue. At a normocarbic value of Pa CO 2 (37 torr), an average value of 3.5 ml/100 gm was found. A nonlinear relationship of CBV and CBF was found. This relationship is expressed in the equation, CBV = 0.80 CBF 0.38 . A significant linear relationship was found between CBF and Pa CO 2 . This was described by the equation, CBF = 1.8 Pa CO 2 - 16.75. For each one torr change in the PaCO 2 , there is a 1.8 ml/100 gm per minute change in the CBF. At a normocarbic value of Pa CO 2 (37 torr), an average value of CBF of 50 ml/100 gm per minute was found. The relationship of CBV and t v was nonlinear and was expressed in the equation, t C15O = 41 CBF -0.62 .

Interrelationship between blood pressure and regional cerebral blood flow in experimental intracranial hypertension

✓ The interrelationship between systemic blood pressure (BP), regional cerebral blood flow (rCBF), and intracranial pressure (ICP) was investigated in two experimental models of intracranial hypertension in cats. In one group, ICP was raised by the inflation of an extradural balloon; in the other, brain swelling was produced. The effects of raised blood pressure on rCBF and ICP in the two groups differed considerably. In the “brain-swelling” group, elevated BP had no beneficial effects on rCBF. When ICP approached diastolic BP, an increase in BP was followed by a marked increase in ICP and a decrease in rCBF. Therefore, the elevated BP often observed in extreme intracranial hypertension (Cushing response) cannot be regarded as a beneficial, compensatory defense mechanism, but rather as a deleterious phenomenon.

Experience with an intracranial pressure transducer readjustable in vivo. Technical note

✓ A new implantable miniature intracranial pressure transducer is described whose main advantage is the possibility of zero point calibration in vivo. Comparative studies verify that epidural pressure corresponds well with ventricular fluid pressure. During long-term monitoring of 30 patients the transducer proved both safe and reliable.

The effects of increased intracranial pressure on flow through major cerebral arteries in vitro

The effect of transmural pressure (TMP) (intraluminal minus extraluminal pressure) on flow was measured for 26 isolated major cerebral arteries from human autopsies. The maximum flow rate through an artery was determined by the perfusion pressure (PP), the maximum vessel caliber, and the presence and magnitude of external resistances. Assuming a diastolic PP of 85 mm Hg, intracranial pressure increases above 33±2 SEM mm Hg (45±3 cm H 2 O) resulted in a reduction of flow through these arteries. For atherosclerotic arteries, flow was reduced by 50% at a TMP of 12±1 mm Hg; for nonatherosclerotic arteries, the critical TMP for 50% flow reduction was 20±1 mm Hg. Flow ceased at a TMP of +3 mm Hg for small (cerebellar) arteries to -7 mm Hg for large (posterior cerebral) arteries. For grossly atherosclerotic arteries, this closing pressure was as low as -15 mm Hg. When two arteries were cannulated and perfused in parallel, preferential flow reductions of up to 50% were noted in one of the arteries with no flow change in the other at the same TMP. This preferential narrowing depended on the relative sizes and degree of atherosclerosis of the two arteries.

Wall thickness to lumen diameter ratios were obtained for all arteries and their relevance to the possibility of active closure was discussed.

Cerebral blood flow regulation during experimental brain compression

✓ The effect of brain compression on cerebral blood flow was measured in 13 anesthetized, ventilated dogs by inflation of extradural balloons. The effects of the raised intracranial pressure, so produced, were correlated with the presence or absence of autoregulation of cerebral blood flow to induced changes of arterial pressure, which was tested immediately prior to each episode of inflation of the balloon. Cerebral blood flow was measured by a venous outflow method and monitored continuously, together with arterial and supratentorial intracranial pressure; arterial pCO2 and body temperature were held constant. Three stages were identified. When autoregulation to a change of arterial pressure was intact, initial inflation of the balloon did not reduce cerebral blood flow until the difference between arterial and intracranial pressure (which was taken to represent cerebral perfusion pressure) was less than 40 mm Hg. When autoregulation was impaired, which occurred after the first inflation of the balloon or was due to preceding arterial hypotension, raised intracranial pressure caused an immediate reduction of cerebral blood flow. At this stage of impaired autoregulation there was a tendency for hyperemia to develop on deflation of the balloon. Finally, after repeated inflation and deflation of the balloon, when brain swelling supervened, cerebral blood flow decreased steadily and failed to increase despite induced increases of arterial pressure and cerebral perfusion pressure.

Raised intracranial pressure and cerebral blood flow. 2. Supratentorial and infratentorial mass lesions in primates

Changes in cerebral blood flow with increasing intracranial pressure were studied in anaesthetized baboons during expansion of a subdural balloon in one of two different sites. With an infratentorial balloon, cerebral blood flow bore no clear relation to intracranial pressure, but was linearly related to cerebral perfusion pressure. Apart from an initial change in some animals, cerebrovascular resistance remained constant with increasing intracranial pressure, and autoregulation appeared to be lost from the outset. With a supratentorial balloon, cerebral blood flow remained constant as intracranial pressure was increased to levels around 60 mm Hg, corresponding to a cerebral perfusion pressure range of approximately 100 to 40 mmHg. Cerebrovascular resistance fell progressively, and autoregulation appeared to be effective during this phase. At higher intracranial pressure levels (lower cerebral perfusion pressure levels), autoregulation was lost and cerebral blood flow became directly dependent on cerebral perfusion pressure. The importance of the cause of the increase in intracranial pressure on the response of the cerebral circulation and the relevance of these findings to the clinical situation are discussed.

The effect of mannitol on cerebral blood flow. An experimental study

✓ The effect of mannitol on cerebral blood flow was studied in anesthetized baboons, both at normal and raised intracranial pressure. At normal intracranial pressure, rapid intravenous infusion of mannitol (1.5 gm/kg in 10 min) led to a sharp transient rise in cerebral blood flow during and immediately after the period of infusion. This was associated with a reduction in cerebrovascular resistance and a variable change in cerebral metabolic rate (CMRO2). Other parameters measured did not change significantly. A similar response was seen during hypercapnia. Under conditions of raised intracranial pressure (supratentorial subdural balloon) mannitol infusion did not alter cerebral blood flow in three of four animals. In the remaining animal, however, a marked increase in blood flow occurred without any concomitant change in cerebral perfusion pressure. When a further infusion of mannitol was subsequently given to these animals while the intracranial pressure was artificially maintained, there was very little change in cerebral blood flow. The possible causes of the increase in cerebral blood flow at normal intracranial pressure and the clinical implications of these findings are discussed.

Hydrostatically raised intracranial pressure

✓ Hydrostatic pressure with artificial cerebrospinal fluid (CSF) was applied through a needle inserted into the cisterna magna of rabbits breathing spontaneously. Blood pressure, confluens sinuum pressure and oxygen tension, respiratory rate and volume, and acid-base balance were recorded until respiratory arrest. Blood pressure and confluens sinuum pressure and respiratory volume rose; confluens sinuum oxygen and arterial carbon dioxide tension dropped. The significant similarities and differences in changes in the same parameters following local cold injury to the brain are discussed. Comparisons between different experimental models for raised intracranial pressure must take into consideration the differing reactions of the brain.