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

The cerebrovascular response to norepinephrine: A scoping systematic review of the animal and human literature

Intravenous norepinephrine (NE) is utilized commonly in critical care for cardiovascular support. NE's impact on cerebrovasculature is unclear and may carry important implications during states of critical neurological illness. The aim of the study was to perform a scoping review of the literature on the cerebrovascular/cerebral blood flow (CBF) effects of NE. A search of MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and Cochrane Library from inception to December 2019 was performed. All manuscripts pertaining to the administration of NE, in which the impact on CBF/cerebral vasculature was recorded, were included. We identified 62 animal studies and 26 human studies. Overall, there was a trend to a direct vasoconstriction effect of NE on the cerebral vasculature, with conflicting studies having demonstrated both increases and decreases in regional CBF (rCBF) or global CBF. Healthy animals and those undergoing cardiopulmonary resuscitation demonstrated a dose-dependent increase in CBF with NE administration. However, animal models and human patients with acquired brain injury had varied responses in CBF to NE administration. The animal models indicate an increase in cerebral vasoconstriction with NE administration through the alpha receptors in vessels. Global and rCBF during the injection of NE displays a wide variation depending on treatment and model/patient.

Pre-Clinical Traumatic Brain Injury Common Data Elements: Toward a Common Language Across Laboratories

Traumatic brain injury (TBI) is a major public health issue exacting a substantial personal and economic burden globally. With the advent of "big data" approaches to understanding complex systems, there is the potential to greatly accelerate knowledge about mechanisms of injury and how to detect and modify them to improve patient outcomes. High quality, well-defined data are critical to the success of bioinformatics platforms, and a data dictionary of "common data elements" (CDEs), as well as "unique data elements" has been created for clinical TBI research. There is no data dictionary, however, for preclinical TBI research despite similar opportunities to accelerate knowledge. To address this gap, a committee of experts was tasked with creating a defined set of data elements to further collaboration across laboratories and enable the merging of data for meta-analysis. The CDEs were subdivided into a Core module for data elements relevant to most, if not all, studies, and Injury-Model-Specific modules for non-generalizable data elements. The purpose of this article is to provide both an overview of TBI models and the CDEs pertinent to these models to facilitate a common language for preclinical TBI research.

Gender influences the initial impact of subarachnoid hemorrhage: an experimental investigation

Aneurysmal subarachnoid hemorrhage (SAH) carries high early patient mortality. More women than men suffer from SAH and the average age of female SAH survivors is greater than that of male survivors; however, the overall mortality and neurological outcomes are not better in males despite their younger age. This pattern suggests the possibility of gender differences in the severity of initial impact and/or in subsequent pathophysiology. We explored gender differences in survival and pathophysiology following subarachnoid hemorrhage induced in age-matched male and female rats by endovascular puncture. Intracranial pressure (ICP), cerebral blood flow (CBF), blood pressure (BP) and cerebral perfusion pressure (CPP) were recorded at and after induction of SAH. Animals were sacrificed 3 hours after lesion and studied for subarachnoid hematoma size, vascular pathology (collagen and endothelium immunostaining), inflammation (platelet and neutrophil immunostaining), and cell death (TUNEL assay). In a second cohort, 24-hour survival was determined. Subarachnoid hematoma, post-hemorrhage ICP peak, BP elevation, reduction in CPP, intraluminal platelet aggregation and neutrophil accumulation, loss of vascular collagen, and neuronal and non-neuronal cell death were greater in male than in female rats. Hematoma size did not correlate with the number of apoptotic cells, platelet aggregates or neutrophil. The ICP peak correlated with hematoma size and with number of apoptotic cells but not with platelet aggregates and neutrophil number. This suggests that the intensity of ICP rise at SAH influences the severity of apoptosis but not of inflammation. Mortality was markedly greater in males than females. Our data demonstrate that in rats gender influences the initial impact of SAH causing greater bleed and early injury in males as compared to females.

Traumatic brain injury: clinical and pathological parameters in an experimental weightdrop model

PURPOSE

To investigate the function of an experimental cranium trauma model in rats.

METHODS

The equipment, already described in the literature and under discreet adaptations, is composed by a platform that produces closed head impact controlled by weight drop with pre-defined and known energy. 25 Wistar male rats (Rattus norvegicus albinus) were divided into five equal groups that received different quantities of cranial impact energy: G1, G2, G3 and G4 with 0,234J, 0,5J, 0,762J and 1J respectively and G5 (Sham). Under intense analgesia, each group was evaluated clinically in a sequence of intervals and had their encephalon removed for pathologic analysis.

RESULTS

Important clinical alterations (convulsions, bradycardia, bradypnea and abnormal postures) and focal pathologic (hematomas and hemorrhages) kept proportion with the intensity of the impact. No fracture was observed and the group 4 had 80% mortality rate.

CONCLUSION

The experimental cranium trauma animal model by weight drop is an alternative of low cost and easy reproduction that allows evaluating clinical and pathological alterations in accordance with studies in experimental surgery aims for new traumatic brain injury approach in rats.

Intraoperative jugular bulb desaturation during acute aneurysmal rupture

PURPOSE

To describe an episode of acute jugular venous desaturation during intraoperative rupture of a cerebral aneurysm.

CLINICAL FEATURES

A 57-yr-old patient was scheduled for clipping of a large unruptured basilar tip aneurysm. Abrupt bulging of the brain was observed after bone flap removal, but before dura was opened. This was associated with concurrent development of systemic hypertension to 200/120 mmHg and jugular venous bulb (S(jv)O(2)) desaturation to 13%. Rupture of aneurysm was confirmed by frank blood in cerebrospinal fluid drainage from the lumbar subarachnoid catheter.

CONCLUSIONS

Abrupt S(jv)O(2) desaturation prior to dural opening may suggest an acute increase in intracranial pressure, which in our case followed aneurysmal rupture; the systemic response to increased intracranial pressure (Cushing's response) may be ineffective in maintaining cerebral perfusion.

Effects of elevated ICP on brain function: can the multiparametric monitoring system detect the 'Cushing Response'?

The 'Cushing Response' is a significant phenomenon associated with elevated ICP. The purpose of our study was to examine the effects of the intracranial hypertension level and duration on the cerebral tissue physiology, using a Multiprobe assembly (MPA). The parameters monitored simultaneously included ICP, CBF, mitochondrial NADH redox state, extracellular K+ and H+ levels, DC potential and ECoG, calculated CPP and blood pressure. Two groups of rats were used. In one group, ICP was elevated to 50-60 mmHg for 13-15 min and, in the second group, ICP was elevated to 20 mmHg for 30 min. The results show that ICP of 50-60 mmHg led to CPP reduction below the lower limits of autoregulation. However, ICP of 20 mmHg, even for a prolonged period of time is completely tolerated. Additionally, we found that the 'Cushing Response', developed in the moderate treatment (ICP = 20 mmHg) is beneficial, assuring high CBF levels under intracranial hypertension. Furthermore, CBF and CPP monitoring, apparently, are not sufficient for autoregulation assessment; more parameters are needed.

A new reproducible model of an epidural mass lesion in rodents. Part I: Characterization by neurophysiological monitoring, magnetic resonance imaging, and histopathological analysis

OBJECT

The goal of this study was to characterize a new model of an epidural mass lesion in rodents by means of neurophysiological monitoring, magnetic resonance imaging, and histopathological analysis.

METHODS

Changes in intracranial pressure (ICP), cerebral perfusion pressure (CPP), and laser Doppler flowmetry (LDF) values, intraparenchymal tissue partial oxygen pressure (PtiO2), and electroencephalography (EEG) activity were evaluated in the rat during controlled, epidural expansion of a latex balloon up to a maximum ICP of 60 mm Hg. The initial balloon inflation was followed by periods of sustained inflation (30 +/- 1 minute) and reperfusion (180 +/- 5 minutes). Histopathological analysis and magnetic resonance (MR) imaging were performed to characterize the lesion. The time to maximum balloon expansion and the average balloon volume were highly reproducible. Alterations in EEG activity during inflation first appeared when the CPP decreased to 57 mm Hg, the LDF value to 66% of baseline values. and the PtiO2 to 12 mm Hg. During maximum compression, the CPP was reduced to 34 mm Hg, the LDF value to 40% of baseline, and the PtiO2 to 4 to 5 mm Hg. The EEG tracing was isoelectric during prolonged inflation and the values of LDF and PtiO2 decreased due to accompanying hypotonia. After reperfusion, the CPP was significantly decreased (p < 0.05) due to the elevation of ICP. Both the LDF value and EEG activity displayed incomplete restoration, whereas the value of PtiO2 returned to normal. Histological analysis and MR imaging revealed brain swelling with a midline shift and a combined cortical-subcortical ischemic lesion beyond the site of balloon compression.

CONCLUSIONS

This novel model of an epidural mass lesion in rodents closely resembles the process observed in humans. Evaluation of pathophysiological and morphological changes was feasible by using neurophysiological monitoring and MR imaging.

Multiparametric monitoring of brain under elevated intracranial pressure in a rat model

Intracranial hypertension may develop in most patients exposed to traumatic head injury. In many cases, patients enduring elevated intracranial pressure (ICP) will incur morbidity or mortality. Several methods are used in animal models to investigate the influence of ICP elevation on physiological parameters. In this study, we developed a cisterna magna model by adding a mechanism for warming the mock cerebrospinal fluid (CSF) entering the cisterna space to a temperature of 37 degrees C and combined this method for ICP elevation with the multiparametric monitoring system (Multiprobe Assembly [MPA]). Using the MPA, we monitored, for the first time, mitochondrial NADH redox state as well as ionic homeostasis under elevated ICP in a rat model. In addition, we monitored cerebral blood flow (CBF) by laser Doppler flowmetry, ECoG (bipolar electrodes), and surface temperature. Blood pressure was measured in the cannulated femoral artery. The ICP (monitored by Camino probe) was elevated to 50-60 mm Hg for 13-15 min, followed by 2 h of recovery. The results show that CBF was decreased by 90%, while NADH was elevated by 80% as compared to the normoxic levels. Complete depolarization occurred as evidence by the decrease in extracellular Ca2+ and a significant increase in K+. All parameters recovered 10 min after reopening the cannula to the cisterna magna to air pressure. We conclude that ICP elevation through the cisterna magna infusion method, used simultaneously with multiparametric monitoring, supplies reliable information on the brain tissue metabolic state with intracranial hypertension in a rat model.

Cerebral blood flow changes in response to elevated intracranial pressure in rabbits and bluefish: a comparative study

In mammals, the cerebrovascular response to increases in intracranial pressure may take the form of the Cushing response, which includes increased mean systemic arterial pressure, bradycardia and diminished respirations. The mechanism, effect and value of these responses are debated. Using laser-Doppler flowmetry to measure cerebral blood flow, we analyzed the cardiovascular responses to intracranial pressure raised by epidural infusion of mock cerebrospinal fluid in the bluefish and in the rabbit, and compare the results. A decline in cerebral blood flow preceding a rise in mean systemic arterial pressure was observed in both species. Unlike bluefish, rabbits exhibit a threshold of intracranial pressure below which cerebral blood flow was maintained and no cardiovascular changes were observed. The difference in response between the two species was due to the presence of an active autoregulatory system in the cerebral tissue of rabbits and its absence in bluefish. For both species studied, the stimulus for the Cushing response seems to be a decrement in cerebral blood flow. The resulting increase in the mean systemic arterial pressure restores cerebral blood flow to levels approaching controls.

The effect of resuscitative moderate hypothermia following epidural brain compression on cerebral damage in a canine outcome model

A canine model of temporary epidural cerebral compression and standardized intensive care was developed to evaluate the effect of resuscitative (postinsult) moderate systemic hypothermia. A balloon was inflated over the temporal region to maintain contralateral intraventricular pressure (IVP) at 62 mm Hg for 90 minutes. For a 66-hour period after initiation of brain compression, the intubated dogs received controlled ventilation and standard intensive care. From 66 to 90 hours postinjury, the extubated dogs were evaluated as to functional outcome. Morphological brain damage was evaluated at 90 hours or earlier if brain death occurred. Eight dogs in a control group were maintained at a body of temperature of 38 degrees C. Eight treated dogs had core body temperature reduced by surface cooling starting 15 minutes after balloon inflation, first to 31 degrees C for 5 hours and then to 35 degrees C from 5 to 62 hours after insult. Intraventricular pressure increased to 20 mm Hg or greater in the control group at a mean of 2.9 hours (range 2 to 4 hours) following the insult, and in the hypothermic group at a mean of 14.8 hours (range 5 to 30 hours)--that is, during the time period when the body temperature was 35 degrees C, not 31 degrees C (p = 0.01). There was no difference in peak pressures between the two groups. Brain death occurred in four of the eight dogs in the normothermic group at 18, 24, 24, and 48 hours (mean +/- standard deviation 28 +/- 13 hours) and in three of the eight in the hypothermic group at 27, 42, and 45 hours (mean 38 +/- 10 hours) (not significant). The animals surviving 90 hours (four in the normothermic and five in the hypothermic group) were neurologically near normal. The total mean macroscopically damaged brain volume was 2584 +/- 1890 cu mm in the normothermic versus 765 +/- 611 cu mm in the hypothermic group (p = 0.03). The mean necrotic volume was 741 +/- 599 cu mm in the normothermic versus 263 +/- 346 cu mm in the hypothermic group (p = 0.07). Microscopically, the damaged regions consisted of ischemic neurons, reactive glia, edema, vascular endothelial hypertrophy, and erythrocyte extravasation. It is concluded that, in this model, immediate postinsult hypothermia of 31 degrees C (not 35 degrees C) for 5 hours prevents a rise in IVP and significantly decreases cerebral tissue damage, but does not prevent brain herniation during rewarming.

Studies on supratentorial subdural bleeding using a porcine model

A porcine model for an acute lethal arterial subdural bleeding in man is presented. Blood from the abdominal aorta was led via an electronic drop recorder into a collapsed intracranial subdural rubber balloon. Systemic arterial pressure (SAP), two intracranial pressures and 6 other vital parameters were monitored continuously in spontaneously breathing (n = 4) and mechanically ventilated (n = 4) pigs. In both animal groups bleeding caused an immediate rise in intracranial pressures (ICP) with transtentorial pressure gradients developing. As a result the cerebral perfusion pressures (CPP) decreased progressively, leading to an isoelectric EEG. In spontaneously breathing animals, the pressure changes resulted in apnoea within 2-4 minutes, irregularities in heart rhythm and in a marked rise in SAP (the Cushing reaction). A final collapse of all pressures occurred after 222 +/- 68 sec at a mean bleeding volume of 10.3 +/- 1.9 ml. In contrast, in mechanically ventilated animals, the course of bleeding was less dramatic. No change in cardiac rhythm or rise in SAP appeared despite a larger mean bleeding volume (12.0 +/- 1.6 ml). Instead, SAP slowly fell, reaching a level of approximately 40 mm Hg within 1 hour, while CPP concomitantly decreased from 120 mm Hg to 15 mm Hg. The findings in this and in a parallel study are explained in terms of the intracranial volume tolerance concept (Zwetnow et al. 1986). The beneficial effect of assisted ventilation on the course of subdural bleeding is multifactorial, involving both metabolic and mechanical mechanisms.

Changes in CSF pressures during experimental acute arterial subdural bleeding in pig

The effects of acute arterial subdural bleeding on cerebrospinal fluid (CSF) pressure and 12 other vital parameters were studied in spontaneously breathing pigs (group 1, n = 9) and in mechanically ventilated pigs (group 2, n = 18) to analyze quantitatively the bleeding course and the lethal mechanism. Spontaneously breathing animals all succumbed after a mean bleeding volume of 45.6 +/- 8.9 ml, corresponding to about 50 per cent of the intracranial volume, and a mean bleeding duration of 11.0 +/- 2.6 min. Rapid rise in CSF pressures, marked transtentorial pressure gradients, and progressive reductions of cerebral perfusion pressure leading to a permanently iso-electric EEG, apnoea and to a terminal rise in arterial pressure (Cushing response), was the rule in these animals. The mechanically ventilated animals had smaller bleeding volumes (34.3 +/- 8.1 ml), but longer bleeding durations (13.8 +/- 5.8 min). In this group 7 animals survived. They had no pressure gradients, and only moderate changes in arterial pressure and EEG. The 11 animals that succumbed had marked transtentorial pressure gradients, but smaller increments in arterial pressure than the spontaneously breathing animals. At autopsy, subdurally located blood was found throughout the intracranial and spinal subdural compartments and along the spinal nerve roots in both groups. The results of this study suggest that survival after acute subdural haematoma is influenced by the presence of transtentorial pressure gradients and by the spinal sac acting as a space for expansion. The beneficial effect of artificial ventilation is discussed.

Cerebral blood flow and intracranial pressure during experimental subarachnoid haemorrhage

The relationships of intracranial pressure (ICP), systemic blood pressure (SBP) and cerebral blood flow (CBF) during experimental subarachnoid haemorrhage were investigated in cats. Continuous monitoring of regional cerebral blood flow (rCBF) was done by a thermal diffusion method using a Peltier stack. During haemorrhage ICP rose within 5.4 +/- 0.97 minutes from 10.5 +/- 4.9 to 176.1 +/- 27.8 mmHg. This strong increase of ICP resulted in a temporary arrest of cerebral circulation. The Cushing response during the haemorrhage could not improve the cerebral circulation, but in contrast caused a further increase of ICP. After the haemorrhage the cerebral blood flow normalised within minutes. It is concluded, that the Cushing response during a subarachnoid haemorrhage should be regarded as a deleterious rather than a beneficial mechanism.

Systemic arterial hypertension in head trauma

The importance of maintaining adequate cerebral perfusion pressure to prevent cerebral ischemia is a well accepted concept in the management of patients with head injury. The potentially deleterious effects of too great a perfusion pressure, however, are generally less well appreciated. The occurrence of a hyperadrenergic state after head injury, and the effects of elevated blood pressure on the injured brain are reviewed, with emphasis placed on the pathophysiologic implications of a disturbance of the blood-brain barrier and of autoregulation in promoting brain swelling and formation of edema.

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 pressure: a review of clinical problems, measurement techniques and monitoring methods

Intracranial hypertension is a dangerous condition and is common in patients suffering from a severe head injury or from a variety of pathological problems. Measurement of intracranial pressure (ICP) is considered by many to be a valuable aid in the management of such patients. Despite the invasive nature of the more widely-used measurement techniques, and hence their associated risks, results from many centres have convincingly shown that ICP measurement enables management of intracranial hypertension to be rationally approached, and a direct measure of the progress and outcome of treatment to be obtained. This paper begins with a description of normal cerebrospinal fluid (c.s.f.) pressure. This is followed by an account of the pressure-volume relationship of the intracranial system, and the control mechanisms that regulate the ICP during limited increases in mass volume within the cranium. The consequences of these for patient management are discussed. Reasons why ICP is monitored, and the benefits arising, are described. The paper then concentrates upon the methods of measuring ICP and discusses their relative merits and limitations. A selection of typical pressure sensors is described. Finally, methods that have been devised to monitor ICP and to anticipate intracranial hypertension are reviewed, and the direction of work in this area is assessed.

Influence of blood pressure on tolerance to an intracranial expanding mass

In 3 groups of 4 dogs with normotensive, induced-hypotensive and induced-hypertensive blood pressure respectively, continuous expansion of an extradural supratentorial balloon led to respiratory arrest at inflation volumes which increased with increasing blood pressure. This positive correlation between the volume tolerance to an expanding lesion and blood pressure was also found in similar experiments on 4 hypotensive and 4 hypertensive cats. Monitoring cerebrospinal fluid pressures in the cerebral lateral ventricles, in the posterior fossa and in the spinal subarachnoid space showed that absolute pressures in the various compartments as well as the intercompartmental pressure gradients at the moment of respiratory arrest were increased in proportion to the level of the systemic arterial pressure in each case. These observations do not support current concepts that brain-stem distortion alone or that stimulation of baroreceptors in the posterior fossa are responsible for eliciting the Cushing response. The fact that the supratentorial perfusion pressure was the only parameter which did not differ significantly under the different experimental conditions suggests that the mechanism responsible for the respiratory arrest is local brain tissue ischemia, probably near the tentorial incisure. The magnitude of gain in volume tolerance, when mean arterial pressure was varied from 60 mmHg to 190 mmHg, was 87% suggesting that the blood pressure may have a critical role in an intracranial lesion. These findings have clinical implications.

Evaluation of I.V. labetalol for treatment of posttraumatic hyperdynamic state

A hyperdynamic state, characterized by an elevated blood pressure and tachycardia is frequently seen during the first few days following severe multiple trauma. We examined the cardiovascular effects of the alpha and beta adrenoceptor blocking agent labetalol in patients presenting a hyperdynamic cardiovascular state some days after major trauma. Ten patients with a heart rate-systolic blood pressure product (RPP) of more than 2000 during 6 consecutive hours, despite normovolaemia, adequate ventilation, analgesia and sedation were investigated. After a mean dose of 2.1 +/- 1.2 mg X kg-1 (mean +/- SD) of labetalol injected intravenously over a 10-min period, heart rate decreased from 117 +/- 28 to 102 +/- 19 beats X min-1, systolic arterial pressure from 25 +/- 3.5 to 18.5 +/- 2.7 kPa, diastolic pressure from 11 +/- 1.7 to 9.5 +/- 1.7 kPa, mean arterial blood pressure from 15.5 +/- 2.1 to 12.4 +/- 2.1 kPa, and the RPP from 2880 +/- 867 to 1853 +/- 373. The beneficial effect of this dose lasted 24 h in 8 of 10 patients without additional administration. No important side effects such as cardiac arrhythmias, hypotension, or bronchospasm were noted. We conclude that labetalol used in fractional intravenous doses permits an adequate treatment of a "hypertension-tachycardia syndrome" in severely injured patients.

Acute intracranial hypertension and brain-stem blood flow. An experimental study

This study has been carried out to evaluate the effect of supratentorial mass lesions on the local cerebral blood flow (CBF) of the brain stem. Local CBF of the thalamus, inferior colliculus, and medulla oblongata, and supra- and infratentorial pressure were serially measured in 52 cats with intracranial hypertension produced by supratentorial balloon expansion. The mean control local CBF's in the thalamus, inferior colliculus, and medulla oblongata were 37.5, 42.1, and 30.7 ml/100 gm/min, respectively. At 20 to 30 mm Hg of supratentorial pressure, the local CBF of the thalamus started to decrease, and at 20 mm Hg of infratentorial pressure, the local CBF of the inferior colliculus began to decrease. Finally, at 40 to 60 mm Hg of infratentorial pressure, the local CBF of the medulla oblongata was affected. At the beginning of uncal herniation, indicated by anisocoria, the mean local CBF of the inferior colliculus abruptly decreased from 33.7 to 19.6 ml/100 gm/min in 16 cats. The Cushing response was evoked at a mean supratentorial pressure of 93.4 mm Hg and infratentorial pressure of 49.9 mm Hg in 16 cats. When the systemic arterial pressure was increased to the highest level in 13 cats, the mean local CBF of the medulla oblongata did not show significant change (a decrease from 22.8 to 20.9 ml/100 gm/min). The results suggest that at the beginning of uncal herniation, the local CBF of the upper brain stem markedly decreased. During the Cushing response, the local CBF of the medulla oblongata did not change significantly.

Cerebral autoregulation in unconscious patients with brain injury

In 18 unconscious patients with traumatic brain injury, the cerebral autoregulation was tested during the first 2-3 weeks after the acute trauma. Regional cerebral blood flow (rCBF) was measured by the intra-arterial 133xenon washout method before and after an increase of about 20% in the mean arterial blood pressure (MABP) by angiotensin. The difference between MABP and intraventricular pressure (IVP) was used as cerebral perfusion pressure (PP). Simultaneously, ventricular fluid pH, lactate and pyruvate were measured. Regional loss of autoregulation indicated by a 20% flow increase was observed in 29 out of 35 studies (83%), while hemispheric loss of autoregulation was observed in only one study. The results of the autoregulation tests were unrelated to the clinical outcome, the presence of brain-stem lesion, and the ventricular fluid pH, lactate and lactate/pyruvate ratio. In repeated studies, a gradual normalization of the autoregulation was observed about 5 days after the acute trauma.

Effect of systemic arterial blood pressure on cerebral blood flow in intracranial hypertension

In five baboons and 11 cats cerebral ischaemia was produced either by inflating an epidural balloon and or by ligating major arteries supplying the brain. Fifteen of the animals developed intracranial hypertension after cerebral ischaemia. If ICP were high, but still significantly lower than MABP, elevation of MABP by noradrenaline infusions was accompanied by a proportional increase of ICP. However, the increase of ICP was lower than that of MABP so that CPP was raised. CBF measured by the 133Xenon clearance technique was significantly increased by arterial hypertension in eight cases. The proportional increase of CPP and CBF by elevation of arterial blood pressure was substantially greater, the lower ICP was immediately after ischaemia. There was no effect of MABP in cases in which ICP equalled MABP.

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).