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

Autonomic Nervous System Activity during Refractory Rise in Intracranial Pressure

Refractory intracranial hypertension (RIH) is a dramatic increase in intracranial pressure (ICP) that cannot be controlled by treatment. Recent reports suggest that the autonomic nervous system (ANS) activity may be altered during changes in ICP. Our study aimed to assess ANS activity during RIH and the causal relationship between rising in ICP and autonomic activity. We reviewed retrospectively 24 multicenter (Cambridge, Tromso, Berlin) patients in whom RIH developed as a pre-terminal event after acute brain injury (ABI). They were monitored with ICP, arterial blood pressure (ABP), and electrocardiography (ECG) using ICM+ software. Parameters reflecting autonomic activity were computed in time and frequency domain through the measurement of heart rate variability (HRV) and baroreflex sensitivity (BRS). Our results demonstrated that a rise in ICP was associated to a significant rise in HRV and BRS with a higher significance level in the high-frequency HRV (p < 0.001). This increase was followed by a significant decrease in HRV and BRS above the upper-breakpoint of ICP where ICP pulse-amplitude starts to decrease whereas the mean ICP continues to rise. Temporality measured with a Granger test suggests a causal relationship from ICP to ANS. The above results suggest that a rise in ICP interacts with ANS activity, mainly interfacing with the parasympathetic-system. The ANS seems to react to the rise in ICP with a response possibly focused on maintaining the cerebrovascular homeostasis. This happens until the critical threshold of ICP is reached above which the ANS variables collapse, probably because of low perfusion of the brain and the central autonomic network.

Observations on the Cerebral Effects of Refractory Intracranial Hypertension After Severe Traumatic Brain Injury

BACKGROUND

Raised intracranial pressure (ICP) is a prominent cause of morbidity and mortality after severe traumatic brain injury (TBI). However, in the clinical setting, little is known about the cerebral physiological response to severe and prolonged increases in ICP.

METHODS

Thirty-three severe TBI patients from a single center who developed severe refractory intracranial hypertension (ICP > 40 mm Hg for longer than 1 h) with ICP, arterial blood pressure, and brain tissue oxygenation (PBTO2) monitoring (subcohort, n = 9) were selected for retrospective review. Secondary parameters reflecting autoregulation (including pressure reactivity index-PRx, which was available in 24 cases), cerebrospinal compensatory reserve (RAP), and ICP pulse amplitude were calculated.

RESULTS

PRx deteriorated from 0.06 ± 0.26 a.u. at baseline levels of ICP to 0.57 ± 0.24 a.u. (p < 0.0001) at high levels of ICP (> 50 mm Hg). In 4 cases, PRx was impaired (> 0.25 a.u.) before ICP was raised above 25 mm Hg. Concurrently, PBTO2 decreased from 27.3 ± 7.32 mm Hg at baseline ICP to 12.68 ± 7.09 mm Hg at high levels of ICP (p < 0.001). The pulse amplitude of the ICP waveform increased with increasing ICP but showed an 'upper breakpoint'-whereby further increases in ICP lead to decreases in pulse amplitude-in 6 out of the 33 patients.

DISCUSSION

Severe intracranial hypertension after TBI leads to decreased brain oxygenation, impaired pressure reactivity, and changes in the pulse amplitude of ICP. Impaired pressure reactivity may denote increased risk of developing refractory intracranial hypertension in some patients.

Intracranial Pressure Is a Determinant of Sympathetic Activity

Intracranial pressure (ICP) is the pressure within the cranium. ICP rise compresses brain vessels and reduces cerebral blood delivery. Massive ICP rise leads to cerebral ischemia, but it is also known to produce hypertension, bradycardia and respiratory irregularities due to a sympatho-adrenal mechanism termed Cushing response. One still unresolved question is whether the Cushing response is a non-synaptic acute brainstem ischemic mechanism or part of a larger physiological reflex for arterial blood pressure control and homeostasis regulation. We hypothesize that changes in ICP modulates sympathetic activity. Thus, modest ICP increase and decrease were achieved in mice and patients with respectively intra-ventricular and lumbar fluid infusion. Sympathetic activity was gauged directly by microneurography, recording renal sympathetic nerve activity in mice and muscle sympathetic nerve activity in patients, and gauged indirectly in both species by heart-rate variability analysis. In mice (n = 15), renal sympathetic activity increased from 29.9 ± 4.0 bursts.s⁻¹ (baseline ICP 6.6 ± 0.7 mmHg) to 45.7 ± 6.4 bursts.s⁻¹ (plateau ICP 38.6 ± 1.0 mmHg) and decreased to 34.8 ± 5.6 bursts.s⁻¹ (post-infusion ICP 9.1 ± 0.8 mmHg). In patients (n = 10), muscle sympathetic activity increased from 51.2 ± 2.5 bursts.min⁻¹ (baseline ICP 8.3 ± 1.0 mmHg) to 66.7 ± 2.9 bursts.min⁻¹ (plateau ICP 25 ± 0.3 mmHg) and decreased to 58.8 ± 2.6 bursts.min⁻¹ (post-infusion ICP 14.8 ± 0.9 mmHg). In patients 7 mmHg ICP rise significantly increases sympathetic activity by 17%. Heart-rate variability analysis demonstrated a significant vagal withdrawal during the ICP rise, in accordance with the microneurography findings. Mice and human results are alike. We demonstrate in animal and human that ICP is a reversible determinant of efferent sympathetic outflow, even at relatively low ICP levels. ICP is a biophysical stress related to the forces within the brain. But ICP has also to be considered as a physiological stressor, driving sympathetic activity. The results suggest a novel physiological ICP-mediated sympathetic modulation circuit and the existence of a possible intracranial (i.e., central) baroreflex. Modest ICP rise might participate to the pathophysiology of cardio-metabolic homeostasis imbalance with sympathetic over-activity, and to the pathogenesis of sympathetically-driven diseases.

Brainstem Monitoring in the Neurocritical Care Unit: A Rationale for Real-Time, Automated Neurophysiological Monitoring

Patients with severe traumatic brain injury or large intracranial space-occupying lesions (spontaneous cerebral hemorrhage, infarction, or tumor) commonly present to the neurocritical care unit with an altered mental status. Many experience progressive stupor and coma from mass effects and transtentorial brain herniation compromising the ascending arousal (reticular activating) system. Yet, little progress has been made in the practicality of bedside, noninvasive, real-time, automated, neurophysiological brainstem, or cerebral hemispheric monitoring. In this critical review, we discuss the ascending arousal system, brain herniation, and shortcomings of our current management including the neurological exam, intracranial pressure monitoring, and neuroimaging. We present a rationale for the development of nurse-friendly-continuous, automated, and alarmed-evoked potential monitoring, based upon the clinical and experimental literature, advances in the prognostication of cerebral anoxia, and intraoperative neurophysiological monitoring.

Changes in Intracranial Morphology, Regional Cerebral Water Content and Vital Physiological Variables during Epidural Bleeding

Epidural bleeding was produced in 8 anaesthetised and heparinised dogs by an artificial system. Changes in vital physiological variables were related to intracranial shifts and tissue water content assessed with MR imaging. Six animals survived while 2 succumbed. In the surviving animals intracranial shifts and compressions remained unchanged from an early stage. The cerebral perfusion pressure was reduced from between 80 and 110 mm Hg to between 40 and 60 mm Hg. Some increase in supratentorial white matter tissue water was observed. In the lethal experiments cerebral perfusion pressure fell to less than 40 mm Hg. Moreover, secondary delayed anatomical changes were seen including hydrocephalus. Increase in cerebral tissue water was more intense and widespread than in the survivors. These findings indicate that the outcome of epidural bleeding is related to cerebral perfusion pressure with secondary deterioration resulting from additional volume loading from increased tissue water and hydrocephalus.

Spinal cord intramedullary pressure in thoracic scoliotic deformity: a cadaveric study

STUDY DESIGN

In vitro cadaveric study of thoracic spinal cord intramedullary pressure (IMP) in scoliotic deformity.

OBJECTIVE

To define the relationship between thoracic scoliotic deformity and spinal cord IMP.

SUMMARY OF BACKGROUND DATA

Clinical studies of patients with thoracic scoliosis without other spinal pathology (spinal stenosis, etc.) have rarely reported an associated thoracic myelopathy. Previous clinical and cadaveric studies of kyphosis have reported associated myelopathy and increased spinal cord IMP. We sought to determine if IMP changes in response to main thoracic scoliotic deformity.

METHODS

In 6 fresh-frozen cadavers, a progressive main thoracic scoliotic deformity was created. Cadavers were positioned sitting with physiological spinal alignment, head stabilized using a skull clamp and spine segmentally instrumented from occiput to L3. The T3-T4 ligamentum flavum was removed, dura opened, and 3 pressure sensors were advanced caudally to T4-T5, T7-T8, and T10-T11 within the cord parenchyma. A step-wise main thoracic scoliotic deformity was then induced by sequentially releasing and retightening the skull clamp while coronally bending, concavity compressing, and convexity distracting posterior segmental instrumentation, allowing closure of lateral segmental osteotomies. After each step, fluoroscopic images and pressure measurements were obtained; the T4-T11 coronal Cobb angle was measured.

RESULTS

Induction of main thoracic scoliosis did not significantly increase IMP. The mean main thoracic maximal scoliotic deformity created was 77° ± 2° (range: 71°-84°). At maximal deformity, the mean ΔIMP at T4-T5, T7-T8, T10-T11 was 2.2 ± 1.9 mm Hg, 1.0 ± 0.7 mm Hg, and 1.0 ± 0.8 mm Hg, respectively.

CONCLUSION

In this cadaveric study, main thoracic scoliotic deformity did not significantly increase thoracic IMP. This correlates with clinical presentation such that clinical studies of patients with thoracic scoliosis without other spinal pathology have rarely reported an associated thoracic myelopathy with the thoracic scoliosis. This study helps explain the relative absence of myelopathy in isolated main thoracic coronal plane deformity.

LEVEL OF EVIDENCE

5.

Modified brain stem auditory evoked potentials in patients with intracranial mass lesions

The authors report their experience utilizing a recently described rapid rate, binaural click and 1000-Hz tone burst modification of the brain stem auditory evoked potentials (BAEP), modified (MBP), in 27 symptomatic patients with non-brain stem compressive space-taking cerebral lesions (22), hydrocephalus (4), and pseudotumor cerebri (1).  Many presented with clinical signs suggestive of increased intracranial pressure (ICP) and focal neurological deficits. The cerebral lesions, mostly large tumors with edema, had very substantial radiological signs of mass effect. Fourteen patients were also studied following surgical decompression. A number of significant changes in the wave V and Vn latency/intensity and less so amplitude/intensity function was found in the 27 patients, compared to normal volunteers, as well as those studied pre- and postoperatively. Similar MBP changes had been noted in normal volunteers placed in a dependent head position. Possible mechanisms to explain these findings are discussed.  The MBP methodology shows promise and further development could make neuro-intensive care unit monitoring practical.

Spinal cord intramedullary pressure in thoracic kyphotic deformity: a cadaveric study

STUDY DESIGN

In vitro cadaveric study of thoracic spinal cord intramedullary pressure (IMP) in kyphotic deformity.

OBJECTIVE

To define the relationship between thoracic spinal kyphotic deformity and spinal cord IMP.

SUMMARY OF BACKGROUND DATA

Previous studies of asymptomatic volunteers have revealed that there is wide variation in regional sagittal neutral upright thoracic spinal alignment with "normal" thoracic T4-T12 kyphosis ranging up to approximately +69° for 98.5% of the asymptomatic adult population. We sought to determine whether IMP changes in response to increasing thoracic kyphosis.

METHODS

In 8 fresh-frozen cadavers, a progressive kyphotic deformity was created. Cadavers were positioned sitting with physiological thoracic kyphosis, head stabilized using a skull clamp, and spine segmentally instrumented from occiput to L2. The T3-T4 ligamentum flavum was removed, dura opened, and 3 pressure sensors were advanced caudally to T4-T5, T7-T8, and T11-T12 within the cord parenchyma. A stepwise thoracic kyphotic deformity was then induced by sequentially releasing and retightening the skull clamp while distracting posterior short segment rods and closing anterior segmental osteotomies. After each step, fluoroscopic images and pressure measurements were obtained; the T4-T12 Cobb angle was measured.

RESULTS

Minor IMP increases of 2 to 5 mm Hg were observed at 1 or more spinal cord levels in 1 of 8 cadavers when the Cobb angle was less than +51° and in 4 of 8 cadavers when the angle was more than +51° and less than +63°. For Cobb angles more than +51° and less than +63°, a statistically significant, minor increase in IMP was detected at the T7-T8 level only (P = 0.02). At Cobb angles exceeding +63°, ΔIMP progressively increased at 1 or more spinal cord levels in 8 of 8 cadavers. Cobb angles ranging from +63° to +149° resulted in statistically significant increases in IMP ranging to more than 50 mm Hg. ΔIMP did not correlate with segmental spinal canal diameter (stenosis).

CONCLUSION

Thoracic kyphosis less than +51° resulted in no meaningful increase in IMP, whereas kyphosis measuring +51° to +63° resulted in minor increases in IMP. After the thoracic kyphosis exceeded +63°, IMP increased significantly. ΔIMP with spinal alignment may help explain the wide range of "normal" thoracic neutral upright sagittal alignment in studies of asymptomatic adult individuals and may help further define thoracic kyphotic deformity.

Individual value of brain tissue oxygen pressure, microvascular oxygen saturation, cytochrome redox level, and energy metabolites in detecting critically reduced cerebral energy state during acute changes in global cerebral perfusion

The authors assessed the diagnostic value of brain tissue oxygen tension (PbrO2), microvascular oxygen saturation (SmvO2), cytochrome oxidase redox level (Cyt a+a3 oxidation), and cerebral energy metabolite concentrations in detecting acute critical impairment of cerebral energy homeostasis. Each single parameter as well as derived multimodal indices (arteriovenous difference in oxygen content [AVDO2], cerebral metabolic rate for oxygen [CMRO2], fractional microvascular oxygen extraction [OEF]) were investigated during controlled variation of global cerebral perfusion using a cisternal infusion technique in 16 rabbits. The objective of this study was to determine whether acute changes between normal, moderately, and critically reduced cerebral perfusion as well as frank ischemia defined by local cortical blood flow (lcoBF), brain electrical activity (BEA), and brain stem vasomotor control can be reliably identified by SmvO2, PbrO2, Cyt a+a3 oxidation, or energy metabolites (glutamate, lactate/pyruvate ratio). PbrO2, SmvO2, and Cyt a+a3 oxidation, but not cerebral perfusion pressure, were closely linked to lcoBF and BEA and allowed discrimination between normal, moderately reduced, and critically reduced cerebral perfusion (P < 0.01). Glutamate concentrations and the lactate/pyruvate ratio varied significantly only between moderately reduced cerebral perfusion and frank ischemia (complete loss of BEA and brain stem vasomotor control). Therefore, PbrO2, SmvO2, and Cyt a+a3 oxidation, but not glutamate and the lactate/pyruvate ratio, reliably predict the transition from moderately to critically reduced cerebral perfusion with impending energy failure.

Treatment of transtentorial herniation unresponsive to hyperventilation using hypertonic saline in dogs: effect on cerebral blood flow and metabolism

We tested the hypothesis that transtentorial herniation (TTH) represents a state of cerebral ischemia that can be reversed by hypertonic saline. Because of the high mortality associated with TTH, new therapeutic strategies need to be developed for rapid and effective reversal of this process. We produced TTH (defined by acute dilatation of one or both pupils) by creating supratentorial intracerebral hemorrhage with autologous blood injection in seven mongrel dogs anesthetized using intravenous pentobarbital and fentanyl. We measured serial rCBF (regional cerebral blood flow) using radiolabeled microspheres in regions around and distant to the hematoma. Cerebral oxygen extraction and oxygen consumption (CMRO2) were measured by serial sampling of cerebral venous blood from the sagittal sinus. Mean arterial pressure (MAP) and intracranial pressure (ICP) were continuously monitored. TTH was successfully reversed over a mean period of 25.7 +/- 4.9 minutes after intravenous administration of 23.4% sodium chloride (1.4 mL/kg) in all animals. All measurements were recorded 15, 30, 60, and 90 minutes after administration of 23.4% sodium chloride. Compared to prehematoma ICP (14.1 +/- 1.7 mm Hg, mean +/- SE), elevation in ICP was observed during TTH (36.2 +/- 7.2 mm Hg) with no change in cerebral perfusion pressure (CPP) (80.4 +/- 4.7 vs. 76.7 +/- 10.1 mm Hg) because of concomitant elevation in mean arterial pressure. Compared to baseline values, there was a reduction in rCBF (mL/100 gm/min +/- SE) in brainstem (12.1 +/- 2.0 vs. 21.4 +/- 1.4), gray matter (18.2 +/- 2.1 vs. 31.4 +/- 1.8), and white matter (8.6 +/- 1.7 vs.18.7 +/- 0.9) in the hemisphere contralateral to the hematoma; and gray matter (12.9 +/- 2.9 vs. 27.9 +/- 2.2) and white matter (8.3 +/- 2.0 vs.19.9 +/- 1.0) in the ipsilateral hemisphere distant from the hematoma. Administration of 23.4% sodium chloride resulted in reduced ICP at 15 minutes (12.7 +/- 1.4) and 30 minutes (15.6 +/- 3.1) after administration. RCBF values were restored in all regions studied after administration of 23.4% sodium chloride with an increase in CMRO2 (1.8 +/- 0.4 vs. 3.9 +/- 0.7 mL O2 /100 gm/min). Compared with baseline values, rCBF increased in the ipsilateral (31.7 +/- 2.5 vs. 63.4 +/- 11.7) and contralateral (28.7 +/- 1.9 vs. 45.5 +/- 5.7) thalamus at 15 minutes after administration of 23.4% sodium chloride. TTH represented a state of ischemia in brainstem and supratentorial gray and white matter in the presence of adequate CPP, suggesting mechanical compression of vessels at the level of tentorium. Hypertonic saline reversed TTH, and restored both rCBF and CMRO2, although hyperemia was observed immediately after reversal of TTH. Administration of hypertonic saline may preserve neurologic function during the interim period between TTH and surgical intervention.

Brain stem blood flow, pupillary response, and outcome in patients with severe head injuries

OBJECTIVE

Acute pupillary dilation in a head-injured patient is a neurological emergency. Pupil dilation is thought to be the result of uncal herniation causing mechanical compression of the IIIrd cranial nerve and subsequent brain stem compromise. However, not all patients with herniation have fixed and dilated pupils, and not all patients with nonreactive, enlarged pupils have uncal herniation. Therefore, we have tested an alternative hypothesis that a decrease in brain stem blood flow (BBF) is a more frequent cause of mydriasis and brain stem symptomatology after severe head injury. We determined the relation of BBF to outcome and pupillary response in patients with severe head injuries.

METHODS

One hundred sixty-two patients with a Glasgow Coma Scale score of 8 or less underwent stable xenon computed tomographic blood flow determination at the level of the superior colliculus, and this blood flow was correlated with pupillary features, intracranial pressure, computed tomographic scan pathology, and outcome.

RESULTS

A BBF of less than 40 ml/100 g/min was significantly associated with poor outcome (P < 0.009). In patients with bilaterally nonreactive pupils, the BBF was 30.5+/-16.8 ml/100 g/min, and in those with normally reactive pupils, the BBF was 43.8+/-18.7 ml/100 g/min (P < 0.001). Intracranial pressure and the presence of a brain stem lesion observed on the computed tomographic scan did not correlate with BBF, pupillary size, or reactivity. Unfavorable outcome at 12 months was directly related to age (P = 0.062) and inversely related to pupillary responsiveness (P = 0.0006), pupil size (P = 0.005), and BBF of less than 40 ml/100 g/min (P = 0.009).

CONCLUSION

These findings suggest that pupillary dilation is associated with decreased BBF and that ischemia, rather than mechanical compression of the IIIrd cranial nerve, is an important causal factor. More important, pupil dilation may be an indicator of ischemia of the brain stem. If cerebral blood flow and cerebral perfusion pressure can be rapidly restored in the patient with severe head injury who has dilated pupils, the prognosis may be good.

Transcranial Doppler and cortical microcirculation at increased intracranial pressure and during the Cushing response: an experimental study on rabbits

The effect of increased intracranial pressure on the flow velocity of the basilar artery was measured with transcranial ultrasonic Doppler in New Zealand White rabbits under alpha-chloralose anesthesia and artificial respiration. Laser Doppler flowmetry served to study changes of the cortical microcirculation. The results confirm a high inverse correlation of the diastolic flow velocity, the pulsatility index, and the resistance index with the cerebral perfusion pressure (CPP). During acute intracranial hypertension, however, these parameters do not show a good correlation with the local cortical blood flow. The absence of a correlation was evident over a wide CPP range down to values of 35 mm Hg. Only at CPP values below this critical threshold is the microcirculation impaired. The threshold is reached at pulsatility index values of more than 2.0 and at resistance index values of more than 0.8. Therefore, transcranial Doppler indices permit the detection of critical reductions of microcirculatory blood flow. The Cushing reaction occurred with a constant time lag of 5.5 +/- 0.7 seconds after the loss of CPP. The Cushing reaction did not establish systolic blood flow, which remained below the functional threshold, as concluded from the temporary loss of somatosensory evoked potentials.

Cerebral vasodilation capacity: acute intracranial hypertension and supra- and infra-tentorial artery velocity recording

This experiment is the first to compare cerebral vasomotor reactivity in the supra- and infra-tentorial regions in baseline conditions and during progressive acute intracranial hypertension. The increase in intracranial pressure was performed using liquid pressure transmission in two groups of 16 rabbits by elevating a saline infusion bottle connected to the subdural space. Cerebral microvessel dilation capacity was studied using acetazolamide arterial infusion during three stages of 20 min: at baseline conditions, with an intracranial pressure value equal to half the diastolic arterial pressure and with an intracranial pressure equal to the diastolic arterial pressure. The effects of acetazolamide in the basilar artery and in the carotid siphon were simultaneously monitored by transcranial Doppler sonography during all the experiments. The changes in cerebral vasomotor reactivity occurred with the same intensity and latency in both vascular compartments in baseline conditions. The maximum amplitude of changes happened 30 s later in the basilar artery than in the carotid siphon. When intracranial pressure was above half the diastolic arterial pressure, the vasomotor tone began to decrease in the carotid siphon which supplies a small region of the rabbit brain, whereas it was maintained in the basilar artery. This effect could be explained by brain tissue hypertension. Vasomotor reactivity had nearly disappeared in all the cerebral arteries investigated when intracranial hypertension was equal to the diastolic arterial pressure. These results show evidence of a direct and late effect of acute elevation of intracranial pressure on cerebral microvascular tone. This begins in the supra-tentorial region but there is an early local effect on the carotid siphon due to the brain tissue pressure.

Concurrent changes in intracranial pressure, cerebral blood flow velocity, and brain energy metabolism in rabbits with acute intracranial hypertension

The relationship between intracranial pressure or cerebral perfusion pressure (CPP), cerebral blood flow, and brain energy failure is unpredictable throughout the development of acute intracranial hypertension. The purpose of the present study was to correlate intracranial pressure with cerebral blood flow velocities and brain energy metabolism in adult rabbits. The acute intracranial hypertension was achieved by pressure transmission. Transcranial Doppler wave-forms were obtained from the basilar artery for monitoring cerebral blood flow velocities. 31P-Magnetic resonance spectroscopy was used to assess brain energy metabolism. The diastolic blood flow velocity began to decrease significantly (34.5%) when the intracranial pressure was equal to half the diastolic arterial pressure for a CPP of 36 +/- 18 mmHg. Circulatory cerebral resistances increased significantly (55%) for the same value of CPP. Diastolic frequency was near zero when intracranial pressure approached diastolic arterial pressure (51 +/- 12 mmHg), corresponding to a CPP of 30 +/- 15 mmHg. At the same time, only a tendency for brain energy metabolism to decrease was observed. Consequently, transcranial Doppler sonography could be proposed for the follow-up of intracranial hypertension. Magnetic resonance spectroscopy could help to monitor these patients and could be especially proposed in case of high intracranial pressure (near diastolic arterial pressure). The joint use of these two methods would help in making appropriate therapeutic decision in humans.

Cerebral blood flow autoregulation in acute intracranial hypertension

The present series of experiments was carried out to investigate CBF autoregulation during fixed levels of acute increased intracranial pressure (ICP). Three groups of six rats each, one with normal ICP (8 mmHg), one with moderately increased ICP (30 mmHg), and one with severely increased ICP (50 mmHg), were investigated. ICP was maintained by continuous infusion of lactated Ringer solution into the cisterna magna. Cerebral perfusion pressure (CPP), calculated as mean arterial blood pressure--ICP, was increased by intravenously infused norepinephrine and decreased by controlled bleeding. In all groups the corresponding autoregulation curve included a plateau where CBF was independent of changes in CPP, demonstrating intact autoregulation. However, a significant shift of the lower limit of autoregulation (LL) toward lower CPP levels during severe intracranial hypertension was observed (p < 0.006). In the controls the LL was found at CPP = 73 +/- 6 mmHg, in moderately increased ICP the LL was 59 +/- 4 mmHg, and in severely increased ICP the LL was 51 +/- 4 mmHg. These results indicate that an acute elevation of ICP activates a reserve capacity of cerebral resistance vessels that dilate further below the normal physiological LL to maintain CBF at low levels of CPP.

Regional blood flow in brain and peripheral tissues during acute experimental arterial subdural bleeding

The effects of a large intracranial arterial subdural bleeding on regional blood flow in the brain (rCBF) and in other body organs were studied, using a porcine model. The bleeding was produced by leading blood through a catheter from the abdominal aorta via an electronic drop recorder into the subdural compartment (SDC) over the left cerebral hemisphere. Pressures in the right lateral cerebral ventricle and in the cisterna magna were recorded along with 15 other vital parameters. Measurements of rCBF were carried out using radioactive microspheres 1) before the start of bleeding, 2) during the early bleeding phase, and 3) during the late bleeding phase. When the bleeding was initiated, the intracranial pressures rose within one minute to a level approximately 40 mmHg below the systemic arterial pressure, whilst the latter usually decreased 30-40 mmHg. In the subsequent early bleeding phase the cerebral perfusion pressure and the bleeding pressure fluctuated at a level of approximately 40 mmHg for several minutes. In the late bleeding phase, the perfusion pressure decreased maximally, even when a Cushing reaction was activated. During the early bleeding phase the changes in rCBF varied between the cerebral regions. However, the mean flow remained largely constant in the presence of a decreasing cerebrovascular resistance, indicating that autoregulation of CBF was intact. Concomitantly, cardiac output and heart rate decreased, whilst regional blood flow in extracerebral organs tended to increase, possibly due to an intracranial effect on the autonomic nervous system. In the late bleeding phase, rCBF was critically reduced in all regions, in spite of a marked rise in systemic arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation by angiography of the lateral dominance of the drainage of the dural venous sinuses

Venous drainage dominance of the dural venous sinuses may be defined as the drainage only or mainly into one of the transverse sinuses, as shown by bilateral carotid angiography. The aim of this study was to evaluate the venous drainage dominance in bilateral carotid angiograms of 189 cases retrospectively. Among these cases 41.3% showed drainage mainly to the right side, 37.6% showed equal drainage to each side, 18.5% showed drainage mainly to the left side, 2.1% showed drainage only to the right side and 0.53% showed drainage only to the left side. Cerebral venous drainage dominance is of great importance and should be considered before operations on patients for radical neck dissection, removal of tumors in the neck that invade the internal jugular vein or tumors of the glomus jugulare which may require ligation of the internal jugular vein.

Acute intracranial hypertension and basilar artery blood flow velocity recorded by transcranial Doppler sonography: an experimental study in rabbits

The relationship between intracranial hypertension and basilar artery blood flow is not well known, and it is not yet definite that the reduction of cerebral flow depends on cerebral perfusion pressure rather than microvessel compression. The purpose of the study described here was to investigate the effect of acute intracranial pressure on the basilar flow velocity, the cerebral perfusion pressure, and the systemic arterial pressure. The basilar Doppler signal was recorded continuously in 24 New Zealand rabbits by transcranial pulsed Doppler method. The acute intracranial hypertension was induced by the progressive raising, in steps of 5 mmHg, of a saline infusion bottle connected to an epidural sensor. The intracranial hypertension induced a decrease in diastolic and mean flow velocities in the basilar artery, and an increase in the resistance index. Cerebral perfusion pressure was significantly correlated with flow parameters. The basilar diastolic flow began to decrease significantly from a 35-40 mmHg intracranial pressure and for a 37 mmHg + 20 SD cerebral perfusion pressure, without significant variation of arterial pressure. Diastolic flow dropped to zero for a 53 mmHg intracranial pressure and a 30 mmHg + 15 SD cerebral perfusion pressure. These results show that high intracranial pressure values are necessary for significantly reducing basilar artery blood flow. This effect, and the increase of circulatory cerebral resistance, occurred before significant changes in systemic arterial pressure.

Haemodynamic and cerebral blood flow alterations after reduction of increased cerebrospinal fluid pressure in dogs

To clarify some of the mechanisms for the hypotension that may occur after cranial decompression, the authors examined alterations in cerebral blood flow (CBF) and systemic and pulmonary haemodynamic variables when cerebrospinal fluid (CSF) pressure was increased and then suddenly reduced in eight anaesthetized dogs. After CSF pressure was elevated to 50-85 mmHg for two hours, CBF decreased from 46.3 +/- 4.4 to 31.6 +/- 8.5 ml.100 g-1.min-1 (mean +/- SD, P less than 0.01). Mean systemic arterial pressure (MAP), mean pulmonary artery pressure (MPAP), pulmonary artery wedge pressure (PAWP), and systemic vascular resistance index (SVRI) increased by 20 +/- 11 mmHg, 3.9 +/- 2.5 mmHg, 5.2 +/- 3.3 mmHg, and 1448 +/- 1377 dynes.sec.cm-5.m2 from baseline values, respectively (P less than 0.01). Rapid reduction of increased CSF pressure caused CBF to increase to 61.5 +/- 19.1 ml.100 g-1.min-1, whereas MAP, MPAP, PAWP, and SVRI decreased by 22 +/- 11 mmHg, 2.4 +/- 0.9 mmHg, 2.3 +/- 2.0 mmHg, and 1289 +/- 1237 dynes.sec.cm-5.m2 from previous values (P less than 0.01) at 30 min following the decompression. However, cardiac index and pulmonary vascular resistance index remained unchanged during the study period. The present animal data indicate that the decrease in MAP after decompression is mainly a result of a reduction in systemic vascular resistance.

Cerebral blood flow during experimental epidural bleeding in swine

Regional cerebral blood flow (rCBF) was studied during an aggressive epidural bleed, using a ventilated swine model. rCBF, regional organ blood flow and cardiac output were measured using the radioactive microsphere technique. Blood flows were measured prior to the start of bleeding (Stage 1), when intracranial pressures had reached a plateau and supratentorial perfusion pressure was reduced by about 50% (Stage 2), and at isoelectric EEG (Stage 3). Supratentorial rCBF did not change significantly between stages 1 and 2 while rCVR decreased, implying autoregulatory activity. Cerebral ischaemia developed between stages 2 and 3 when rCBF values fell to levels between 20 and 50% of control values. Infratentorial rCBF changes were similar but less marked, so that adequate brain stem perfusion was maintained below the upper mesencephalon. The left temporal and left parietal cortex and upper mesencephalon suffered a greater reduction in rCBF than other regions, due to proximity to the haematoma and tentorial herniation. The supratentorial perfusion pressure at stage 2 was 60 mm Hg associated with a haematoma volume of 6% of the intracranial volume (ICV). The infratentorial perfusion pressure never fell below 60 mm Hg. The Cushing response was absent when the EEG became isoelectric. This is tentatively ascribed to the absence of hypoxia, because mechanical ventilation was used. Instead systemic arterial hypotension accompanied bleeding in this ventilated model. This hypotension was due to falling cardiac output and peripheral vasodilation.

The effects of an extradural expanding lesion on regional intracranial pressure, blood flow, somatosensory conduction and brain herniation: an experimental study in baboons

Intracranial pressure (ICP) differences, change of local blood flow (CBF) using the hydrogen clearance technique, change in the somatosensory evoked potential (SEP) to median nerve stimulation and pupillary size were investigated during progressive elevation of the ICP (using an extradural balloon) in 6 anaesthetized baboons. CBF was measured in the frontal cortex, somatosensory cortex, thalamus (nucleus ventralis posterior lateralis-VPL), medial lemniscus (ML), lateral lemniscus (LL) and caudate nucleus (CN). Conduction along the somatosensory pathway between C2 at the neck and VPL was compared with conduction between VPL and primary somatosensory cortex. The amplitude of the cortical SEP was also studied. ICP gradients between hemispheres developed as the pressure was increased to in excess of 50 mm Hg. CBF was significantly reduced from control in the cortex and VPL on the side ipsilateral to the balloon at 50 mm Hg ICP. A significant decrease in ML flow occurred bilaterally at 70 mm Hg ICP. Conduction time was increased significantly between the right VPL and cortex at a pressure of 50 mm Hg. The amplitude of the cortical response was significantly reduced at 30 mm Hg on the right side and 50 mm Hg on the left. Aniscoria occurred at 50 mm Hg ICP and the pupils became dilated at 70 mm Hg. The SEP was possibly more sensitive than the pupillary reactions as an indication of tentorial herniation in these experiments.

Responses of intracranial pressure (ICP) produced by stimulating the pressor area in the brainstem at various levels of blood pressure and ICP in cats

An increase in intracranial pressure (ICP) was produced by stimulating brainstem pressor sites in cats anesthetized with alpha-chloralose. The ICP responses were augmented by lowering prestimulus BP and reduced by elevating prestimulus BP. In contrast, stimulus-induced pressor response of BP showed no consistent correlation to prestimulus BP. When the mean amplitude of stimulus-induced ICP responses at the control prestimulus ICP (within 18 mmHg) was plotted against the mean of the prestimulus BP levels for each site examined, the sites were classified into 2 groups by the regression line; sites generating a marked ICP response above the line and those generating a small ICP response on and under the line. The former sites were located in the paramedian region of the reticular formation including nuclei parvocellularis and gigantocellularis. The latter sites scattered throughout the brainstem pressor area. The ICP response at the former sites was markedly increased at an elevated prestimulus ICP. The peak ICP response at 30-50 mmHg of prestimulus ICP was 70-100 mmHg, similar to plateau waves. The ratio of ICP response size to BP response size was negatively correlated to prestimulus BP and the regression line was 2-5 times steeper at an elevated prestimulus ICP (18-60 mmHg) than at the control ICP. On the other hand, the negative relation between the response ratio and the BP for the latter sites produced no such change at the increased prestimulus ICP. These findings suggest that the ICP response is produced primarily by neurogenic intracranial vasodilation, which works most effectively at moderately decreased cerebral perfusion pressure. This mechanism may be involved in a series of events that results in plateau waves.

Meningitis presenting as hypertension

A 21 month old girl who presented with what seemed to be hypertensive encephalopathy is described. Although her encephalopathy resolved with antihypertensive treatment, subsequent investigations revealed haemophilus meningitis.

Activity of peripheral sympathetic efferent nerves in experimental subarachnoid haemorrhage. Part I: Observations at the time of intracranial hypertension

The origin and pathomechanism of vegetative disturbances in patients suffering from subarachnoid haemorrhage are not completely clarified. Since some of these alterations in vegetative functions may well be attributed to acute changes in sympathetic activity, we initiated a study to investigate this modality in experimentally induced subarachnoid haemorrhage. Experiments were performed on 51 cats, anaesthetized with alpha-chloralose and urethane, immobilized and artificially ventilated. Compound electrical discharges of the left vertebral, cardiac and renal sympathetic nerves, ECG, EEG, end-tidal CO2, systemic arterial blood pressure and intracranial pressure were recorded on a polygraph. Subarachnoid haemorrhage was simulated by the injection of 1-5 ml of fresh, autologous blood into the cisterna magna. Mock cerebrospinal fluid was also injected as a control. Our results showed that in induced subarachnoid haemorrhage, not the blood itself but the intracranial pressure elevation might be responsible for the strong increase in sympathetic efferent activity. With the direct recording of the electrical activity of the three sympathetic nerves, we were able to verify the sympathetic overactivity underlying the cardiovascular disturbances during intracranial pressure elevation. Regarding the mechanism of the overactivity, most probably not the ischaemia or hypoxia, but the mechanical distortion of the medulla could be the adequate stimulus of the sympathetic overactivity and the Cushing response during intracranial pressure elevation.

Pediatric applications of serial auditory brainstem and middle-latency evoked response recordings

Serial auditory brainstem (ABR) and middle-latency (AMR) response recordings were made for 12 children (8 male, 4 female) ranging in age from 2 weeks to 10 years. A total of 40 ABR and 32 AMR assessments were carried out at bedside in varied hospital environments, including a pediatric intensive care unit (ICU), a neonatal ICU and an operating room. Clinical entities were distributed as follows: acute, severe head injury (5), hydrocephalus (2), meningomyelocele (2), hyperbilirubinemia (1), ototoxic drug overdose (1), severe developmental delay (1). Auditory evoked responses were applied in monitoring peripheral and central auditory system status, and contributed to medical, surgical and audiologic management. Abnormalities of the ABR were reversed in some children, such as those with hydrocephalus, with medical or surgical therapy. In other cases, such as a hyperbilirubinemic child, a marked ABR abnormality apparently reversed spontaneously. We present five cases to illustrate diverse applications of serial auditory evoked response measures in children.

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.

Cerebral blood flow autoregulation and hypertension

Hypertension and antihypertensive therapy have clinically important effects on cerebral blood flow. The autoregulatory changes that occur with chronic arterial hypertension should influence the clinician's choice of antihypertensive agents and the rapidity with which the blood pressure is lowered in order to avoid symptoms of focal or global cerebral hypoperfusion.