The sequence of alterations in the vital signs during acute experimental increased intracranial pressure

Correlation of cerebral microvascular circulation with vital signs in cerebral compression and the validity of three concepts: vasodilation, autoregulation, and terminal rise in arterial pressure

Background

Vasodilation, autoregulation, and rising arterial pressure are three common concepts in cerebral compression, believed to improve cerebral blood flow to maintain the brain's nutrition. However, these concepts are unclear, unproven, and based on assumptions. This study aimed to correlate cerebral circulation with alterations of vital signs and to evaluate the above concepts based on physics and hemodynamics.

Methods

Without new animal experiments, a large amount of data: recording of vital signs, long movies of cerebral circulation, and numerous photos of histological examination and microvessels obstruction in cerebral compression in cats was studied, and only partial and preliminary results were reported in 1970. The experiments were supported by an NIH grant for head injury, done before the 1985 Institutional Animal Care and Use Committee requirement. The advent of digital technology facilitated digitizing and stepwise correlating them and evaluating the validity of the above concepts.

Results

As cerebral compression increased intracranial pressure (ICP), veins dilated, not arteries, and arterial microvessels obstructed, diminished, and stopped cerebral circulation. Simultaneously, vital signs deteriorated, and pupils became fixed and dilated. There was no evidence for what is believed as autoregulation.

Conclusion

In cerebral compression, rising ICP obstructs cerebral arterial microvessels while simultaneously deteriorating vital signs. There is no evidence for dilatation of the arteries; only veins dilate, best-called venodilation. There is no evidence of autoregulation; what occurs is a cerebral compartmental syndrome. The terminal rise of arterial pressure is the hemodynamic result of cerebral circulation cessation, overloading the aorta. None of the concepts benefit the brain's nutrition.

Cerebral microvessel perfusion and pathologic alteration of the brain during drowsiness and coma caused by brain tumor: a laboratory study on rats

BACKGROUND

In cerebral compression, deterioration of consciousness and coma are traditionally thought to be caused by compression, shift, hemorrhage, or herniation of the brain stem. The objective of this study was to evaluate vascular perfusion and pathologic alteration in the entire brain during drowsiness and coma.

METHODS

Brain tumors were developed in 3 newborn rat litters by inoculation of KSV (a murine erythroblastosis virus) into their brain. Within several weeks, brain tumors developed. When the animals became drowsy or comatose, their brains were perfused with microbarium, India ink, or paraformaldehyde solution. In 2 animals, the brain vasculature was casted with plastic materials. The brains were either fixed for magnification radiography or prepared for histologic examination.

RESULTS

The brains of control animals showed an abundance of microvessels and penetrating capillaries located perpendicular to the cortex and deep within the brain. The latter entities cannot be detected even in the best routine cerebral angiography in human beings. Microvessels were obstructed, in a patchy and dispersed fashion, during drowsiness, especially in the ipsilateral hemisphere. Obstruction of microvessels was present not only in the brain stem but also in the rest of the brain and in the cerebellum of comatose animals; larger vessels appeared to be markedly narrowed. The study also revealed evidence of diffuse infarcts, cellular ischemia, swelling, and periventricular damage throughout the brain.

CONCLUSIONS

During drowsiness and coma caused by cerebral compression, cerebral capillaries progressively obstruct not only in the brain stem but also throughout the brain, considerably in a more severe pattern during coma than during drowsiness. These likely cause the diffuse neurologic disabilities and behavioral changes often seen after recovery from coma caused by cerebral compression.

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.

Elevation of extracellular glutamate in the final, ischemic stage of progressive epidural mass lesion in cats

Epidural mass lesions may cause ischemia due to progressive intracranial hypertension. In order to investigate the impact of intracranial pressure on accumulation of neuroactive substances, we gradually raised intracranial pressure in five halothane anesthetized cats by inflation of an epidural balloon. We evaluated in the parietal cortex contralateral to the site of balloon inflation, alterations of extracellular glutamate and purine catabolites and of the lactate/pyruvate ratio in relation to changes of intracranial, cerebral perfusion and mean arterial blood pressure. In a complementary experiment, regional cerebral blood flow was assessed by sequential positron emission tomography. In this simplified mass lesion model, extracellular glutamate increased in all cats at a late, critical stage after tentorial herniation, when intracranial pressure had increased to more than 90 mm Hg, cerebral perfusion pressure had decreased below 40-50 mm Hg. Positron emission tomography assessments revealed that the ischemic threshold for glutamate accumulation was in the range of 15-20 mL/100 g/min. Purine catabolites and the lactate/pyruvate ratio increased somewhat earlier than glutamate, but also after reaching the critical, terminal stage. We conclude that in this model of progressive epidural compression, glutamate-mediated excitotoxic processes at sites remote from the initial focal lesion depend on processes such as delayed ischemia in combination with tentorial herniation and systemic hypotension. These processes seem to be initiated by a decrease of cerebral perfusion pressure below a threshold of 40-50 mm Hg.

Interrelations of laser doppler flowmetry and brain tissue oxygen pressure during ischemia and reperfusion induced by an experimental mass lesion

The objective of this study was to assess interrelations between bilateral changes of cortical laser doppler flowmetry and intraparenchymal, subcortical partial tissue oxygen tension in the course of an experimental trauma. Ten animals served as a sham group, 8 Sprague-Dawley rats received an unilateral, focal parietal mass lesion. The bilateral course of cortical blood flow measured by laser doppler flowmetry (LDF) was correlated with subcortical, intraparenchymal partial tissue oxygen tension [p(ti)O2]. In the sham-operated group, laser doppler mean flow values drifted between 9.0% and 9.5% and showed no significant changes over time neither between the hemispheres nor within each hemisphere. Absolute mean p(ti)O2 in sham-operated animals was 32.4 mm Hg in the left and 30.5 mm Hg in the right hemisphere. In the trauma group, mean laser doppler flow values during maximum brain compression decreased ipsilateral to 20.3% and contralateral to 34.4% of the baseline values. P(ti)O2 decreased ipsilateral from 25.9 to 6.6 mm Hg (25.4%) and contralateral from 22.6 to 9.8 mm Hg (43.6%). After balloon deflation, cortical LDF was restored much faster compared to p(ti)O2, but did not reach baseline values [ipsilateral 61.6% (p < 0.05); contralateral 75.8% of baseline values]. The p(ti)O2 values reached 25.2 mm Hg (97%) ipsilateral and 23.7 mm Hg (105%) contralateral. A temporary phase of reactive hyperemia occurred sporadically shortly after decompression. Both parameters showed a significant but rather weak correlation (r = 0.56; p < 0.001). Based upon these findings, we conclude that intraparenchymal, subcortical p(ti)O2 measurements supplemented on-line cortical CBF monitoring and score out discontinuous alternative measurement techniques in detecting hemodynamically relevant events. The small spatial resolution of LDF and p(ti)O2 probes, however, which in the small animal model may be of negligible influence, does raise the question whether the values obtained represent the microcirculatory situation of the human brain.

[Intracranial hypertension in the infant: from its physiopathology to its therapeutic management]

The pathophysiology of elevated intracranial pressure (ICP) is assessed from a three cerebral compartment model and from brain compliance. The mechanisms leading to elevated ICP (expanding process, cerebral edema, brain swelling, hydrocephalus) and their consequences (brain herniation, ischemia-anoxia phenomenon, Cushing reaction and neurogenic pulmonary edema) are overviewed. The causes of elevated ICP in children are reported with emphasis on traumatology. Diagnostic procedures include clinical assessment, fundoscopy, cerebral computerized tomography scan and specific problems of cerebrospinal fluid investigation. Methods and results of intracranial pressure monitoring are reported. The treatment of elevated ICP is based upon clinical follow-up and monitoring of ICP. General therapeutic rules consist of adequate position, suppression of any neck, skull and abdominal compression, stimuli limitation and fluid restriction. Specific treatments include mechanical ventilation, sedation and analgesia, barbiturates, anticonvulsant drugs, mannitol, corticosteroids, hypothermia, enteral nutrition, and antibiotics.

Acute increase of myocardial workload, hemodynamic instability, and myocardial histological changes induced by brain death in the cat

Brain death-related hemodynamic instability may preclude donor heart procurement. The relationships between the initial changes of myocardial workload, hemodynamic deterioration, and myocardial histological changes caused by acute induction of brain death are unclear. Cats (n = 15) were submitted to brain death by rapid inflation of an intracranial balloon. A further 12 cats served as a sham-operated control group. The changes in heart rate, mean arterial blood pressure, systolic and diastolic arterial blood pressure, left ventricular developed pressure, LV dP/dtmax, rate-pressure product (RPP), and circulating noradrenaline and adrenaline were studied during 240 min after the induction of brain death. Central venous pressure was kept constant. The hearts were histologically examined afterward. Electrocerebral activity disappeared within 30 sec after balloon inflation. At 3 min, noradrenaline and adrenaline levels had increased 75- and 40-fold, respectively, compared to pre-induction levels. The hemodynamic response was characterized by an early and rapid increase of hemodynamic variables at 2.9 +/- 0.2 min. This was followed by a second phase of normalization or deterioration. Two distinct subgroups (n = 9) became hemodynamically unstable (HDU), characterized by a systolic arterial blood pressure < 90 mm Hg, at 108 +/- 29 min, and progressively deteriorated to 67 +/- 8 mm Hg at 240 min after inflation of the balloon. The hemodynamic variables of the other, hemodynamically stable (HDS), subgroup (n = 6) normalized at 60 min after inflation. Hemodynamic deterioration of the HDU subgroup compared to the HDS subgroup was significant at 10 min after induction of brain death. The maximum values of RPP were similar in the two subgroups. Respiratory and metabolic variables at the end of the experiment were not different in both subgroups. Histological evidence of myocardial damage was present in 73% (11/15) of the brain dead cats and absent in the control group. The histological changes were identified both in hearts of HDU (6/9) and HDS (5/6) cats. In the cat, no relationships were demonstrated between the acute increase of myocardial workload, the occurrence of hemodynamic deterioration, and myocardial histological changes after rapid induction of brain death. These results may contribute to the discussion whether hemodynamic instability of the donor is an appropriate exclusion criterion for heart transplantation.

Relationship of new-onset systemic hypertension and normal pressure hydrocephalus

Communicating normal pressure hydrocephalus (NPH) is an important remote complication of traumatic brain injury (TBI). The diagnosis of this hydrocephalus depends largely on clinical signs and symptoms, including cognitive deterioration, gait changes and incontinence. However, many of these signs are also seen during post-traumatic amnesia, making early recognition of this syndrome difficult. A case study of one man post-TBI, who presented with new-onset hypertension as a sign of NPH, prompted a retrospective chart review of all patients admitted over a 2-year period with a diagnosis of NPH. Ninety per cent of patients had one or more of the classic triad of NPH and 25% of patients had symptoms suggestive of raised intracranial pressure (unexplained nausea, headache and visual disturbance). Mean systolic and diastolic blood pressures among the 20 subjects for six consecutive days pre-operatively compared with those for days 8-14 and 15-21 post-operatively showed no significant differences; a subgroup of five patients (25%), however, demonstrated a significant change in blood pressure temporally related to shunting. We suggest that demonstration of new-onset systemic hypertension may also be a clinical sign suggestive of NPH useful in the evaluation of the TBI patient.

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.

Spectral analysis of EEG during carotid endarterectomy

Spectral analysis of the electroencephalogram (EEG) was monitored during 105 carotid endarterectomies. Seventy-eight percent of the patients showed no significant change in EEG spectral power as a result of clamping of the internal carotid artery. Two patterns of change were observed in the remaining 22% of patients: partial reduction (significant decrease of power in one or two of three frequency bands) and global reduction (significant decrease of power in all three frequency bands). High frequencies (over 10.5 Hz) changed more frequently with clamping than did low frequencies (less than 6 Hz), but reduction of high frequencies alone was tolerated with no postoperative deficits. The only non-shunted patient demonstrating global EEG reduction for the duration of carotid clamping suffered a transient hemiparesis.

Regional cerebral blood flow and CSF pressures during the Cushing response induced by an infratentorial expanding mass

An experimental study was carried out in eight dogs to investigate whether the Cushing response (CR) during intracranial hypertension is due to pressure per se, tissue distortion, or ischemia in the brain stem. To minimize the effects of rostrocaudal displacement, intracranial pressure was raised by an expanding mass lesion located in the posterior fossa. Regional cerebral blood flow (rCBF) was measured with radioactive microspheres and compartmental cerebrospinal fluid (CSF) pressures were recorded during the CR which was induced by the continuous inflation at a constant rate of an infratentorial epidural rubber balloon in two groups of four dogs. In one group (A) rCBF was measured at the onset of the CR and in the other group (B) at the peak of the systemic blood pressure rise. In the animals of group A blood flow in the mesencephalon, pons and upper medulla oblongata was reduced from control values by 32%, 57% and 85% respectively. In group B blood flow in the same areas did not differ significantly from pre-inflation values. In contrast, the recorded balloon volume, which was assumed to be an index of mechanical distortion of the brain stem, varied considerably at the beginning of the blood pressure rise (from 2.5 to 4.7% of the calculated intracranial space). Similarly, CSF pressure in the posterior fossa at the onset of the CR also varied considerably (from 52 to 117 mmHg). Thus, the large quantitative variations meant that both absolute pressure and tissue distortion were poor predictors of the onset of the CR. The findings suggest that ischemia, rather than brain stem distortion per se or pressure by itself, is responsible for the initiation of the CR. The rise in blood pressure elicited during the CR seems capable of restoring blood flow in the brain stem back to control values.

Regional cerebral blood flow and CSF pressures during Cushing response induced by a supratentorial expanding mass

In order to delineate the critical blood flow pattern during the Cushing response in intracranial hypertension, regional cerebral blood flow was measured with radioactive microspheres in 12 anesthetized dogs at respiratory arrest caused either by expansion of an epidural supratentorial balloon or by cisternal infusion. Regional cerebrospinal fluid pressures were recorded and the local cerebral perfusion pressure calculated in various cerebrospinal compartments. In the 8 dogs of the balloon expansion group, the systemic arterial pressure was unmanipulated in 4, while it was kept at a constant low level (48 and 70 mm Hg) in 2 dogs and, in another 2 dogs, at a constant high level (150 and 160 mm Hg) induced by infusion of Aramine. At respiratory arrest, regional cerebral blood flow had a stereotyped pattern and was largely independent of the blood pressure level. In contrast, concomitant pressure gradients between the various cerebrospinal compartments varied markedly in the 3 animal groups, increasing with higher arterial pressure. Flow decreased by 85-100% supratentorially and by 70-100% in the upper brain stem down to the level of the upper pons, while changes in the lower brain stem were minor, on the average 25%. When intracranial pressure was raised by cisternal infusion in 4 dogs, the supratentorial blood flow pattern at respiratory arrest was approximately similar to the flow pattern in the balloon inflation group. However, blood flow decreased markedly (74-85%) also in the lower brain stem. The results constitute another argument in favour of the Cushing response in supratentorial expansion being caused by ischemia in the brain stem. The critical ischemic region seems to be located rostrally to the oblongate medulla, probably in the pons.

Efferent discharges of sympathetic and parasympathetic nerve fibers during increased intracranial pressure in anesthetized cats in the absence and presence of pressor response

Efferent discharges of the cervical sympathetic cardiovascular and vagal type 1 fibers in response to increased intracranial pressure (ICP) were simultaneously recorded in cats anesthetized with pentobarbitone and ventilated artificially. Sympathetic outflow of renal nerve fibers was also recorded in some animals. The type 1 fibers were assumed to be cardiac vagal fibers, from the response behavior such a pulse-synchronicity to respiratory and heart rhythm, reflex activation from arterial baroreceptors and reciprocal relationship of the activity to sympathetic ones during slower fluctuations of hemodynamic changes, and which occur spontaneously during Mayer waves. The vagal type 1 discharges increased to various amplitudes with increase in ICP and in the absence and the presence of pressor response. Efferent outflow of the renal and cervical sympathetic fibers frequently decreased with a moderate increase in ICP. There was a slight decrease or no apparent change in the blood pressure, and a higher elevation of ICP ensued. Heart rates decreased with increase in ICP, while the rate frequently increased with levels of ICP over about 120 mm Hg. Changes in the vagal and sympathetic discharges always began at a time before the initiation of cardiovascular response to the elevated ICP. However, when ICP was repeatedly increased, the increase in vagal discharges progressively decayed and was accompanied by vigorous sympathetic firings and a marked pressor response. The sympathetic outflow also decayed following the decrease in vagal activities. The present findings of changes in the vagal type 1 discharges demonstrate clear participation of parasympathetic as well as sympathetic nerve activity in the occurrence of cardiovascular responses to increased ICP. Changes in both these autonomic nerve responses may explain the initial fall in arterial blood pressure and pressor responses associated with bradycardia or tachycardia, at different levels of elevated ICP.

Systems analysis of intracranial pressure. Comparison with volume-pressure test and CSF-pulse amplitude analysis

Systems analysis is explored as a method of evaluating intracranial pressure (ICP). The intracranial cavity is characterized by a transfer function that is evaluated by the blood pressure pulse acting as the system input and the ICP pulse acting as the output. A comparison is made of the ability of systems analysis, volume-pressure test (VPT), and cerebrospinal fluid-pulse amplitude analysis (CSFPAA) to distinguish between an epidural balloon inflation (EBI) and an intraventricular infusion (IVI) at various steady state levels of ICP. The VPT could not distinguish between EBI and IVI at any level of ICP, and above 30 mm Hg the volume-pressure response decreased. Spectral analysis was able to distinguish EBI from IVI above 30 mm Hg, and CSFPAA was demonstrated to be a simplified spectral analysis. Changes in ICP waveform generated during each cardiac cycle appear to be related to changes in vasomotor reactivity and may have value in the clinical monitoring of ICP.

Systemic vascular responses to increased intracranial pressure. 1. Effects of progressive epidural ballon expansion on intracranial pressure: and systemic circulation

This paper details the results of experimental studies, on 16 dogs with artificially-induced intracranial space-occupying lesions, of the systemic vascular responses and the intracranial pressure changes (both in the supratentorial and infratentorial compartments) induced by increasing intracranial pressure. The changes produced were divided into two phases such that phase 1 detailed the alterations observed from the start of the balloon inflation up to the initiation of the systemic pressor response. Phase 2 recorded those alterations which occurred during, and immediately after, the period of systemic hypertension (see Fitch et al., 1977). The changes observed during phase 1, and presented in this communication, were those of increasing intracranial pressures and decreasing mean arterial pressure and heart rate. These alterations were associated with decreases in supratentorial perfusion pressure and increases in transtentorial pressure gradient and arrhythmia index.

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.

The effects of mass-induced intracranial pressures on arterial hypertension and survival in awake cats

✓ Intracranial pressures above and below the tentorium, arterial blood pressure, heart rate, and respiratory rate were recorded continuously before, during, and after expansion of a supratentorial mass in awake unsedated cats. In general, as the mass enlarged, the intracranial pressure rose; however, considerable variation was observed among animals with respect to specific mass size and associated intracranial pressures. There was considerable variation in the relationship of supratentorial pressure to infratentorial pressure. No animal survived that had sustained a mass-induced pressure exceeding 1100 mm H2O, and survival was shorter with greater pressures. Systemic hypertension occurred always and only when the infratentorial pressure exceeded 600 mm H2O, regardless of the magnitude of the associated supratentorial intracranial pressure. The methodological limitations of previous studies of mass-induced intracranial hypertension appear to have been substantially reduced by the technique described.

Intracranial pressure gradients associated with experimental cerebral embolism

In a series of 20 cats unilateral cerebral oil embolism was followed by the development of clinical signs of increased intracranial pressure. Simultaneous bilateral epidural pressure measurements revealed marked pressure gradients between both cerebral hemispheres. Such local (tissue) pressure gradients possibly influence local cerebral blood flow (mainly in the diseased areas) by means of alterations in local tissue perfusion pressure.