CHANGES IN BRAIN VOLUME AND BLOOD CONTENT AFTER EXPERIMENTAL CONCUSSION

The effect of continuous positive airway pressure on total cerebral blood flow in healthy awake volunteers

PURPOSE

Continuous positive airway pressure (CPAP) is the gold standard treatment for obstructive sleep apnea. However, the physiologic impact of CPAP on cerebral blood flow (CBF) is not well established. Ultrasound can be used to estimate CBF, but there is no widespread accepted protocol. We studied the physiologic influence of CPAP on CBF using a method integrating arterial diameter and flow velocity (FV) measurements obtained for each vessel supplying blood to the brain.

METHODS

FV and lumen diameter of the left and right internal carotid, vertebral, and middle cerebral arteries were measured using duplex Doppler ultrasound with and without CPAP at 15 cm H(2)O, applied in a random order. Transcutaneous carbon dioxide (PtcCO(2)), heart rate (HR), blood pressure (BP), and oxygen saturation were monitored. Results were compared with a theoretical prediction of CBF change based on the effect of partial pressure of carbon dioxide on CBF.

RESULTS

Data were obtained from 23 healthy volunteers (mean ± SD; 12 male, age 25.1 ± 2.6 years, body mass index 21.8 ± 2.0 kg/m(2)). The mean experimental and theoretical CBF decrease under CPAP was 12.5 % (p < 0.001) and 11.9 % (p < 0.001), respectively. The difference between experimental and theoretical CBF reduction was not statistically significant (3.84 ± 79 ml/min, p = 0.40). There was a significant reduction in PtcCO(2) with CPAP (p = <0.001) and a significant increase in mean BP (p = 0.0017). No significant change was observed in SaO(2) (p = 0.21) and HR (p = 0.62).

CONCLUSION

Duplex Doppler ultrasound measurements of arterial diameter and FV allow for a noninvasive bedside estimation of CBF. CPAP at 15 cm H(2)O significantly decreased CBF in healthy awake volunteers. This effect appeared to be mediated predominately through the hypocapnic vasoconstriction coinciding with PCO(2) level reduction. The results suggest that CPAP should be used cautiously in patients with unstable cerebral hemodynamics.

Quantification of regional cerebral blood flow in rats using an arteriovenous shunt and micro-PET

INTRODUCTION

Measurement of regional cerebral blood flow (rCBF) in rodents can provide knowledge of pathophysiology of the cerebral circulation, but generally requires blood sampling for analysis during positron emission tomography (PET). We therefore tested the feasibility of using an arteriovenous (AV) shunt in rats for less invasive blood analysis.

METHODS

Six anesthetized rats received [15O]H2O and [15O]CO PET scans with their femoral artery and vein connected by an AV shunt, the activity within which was measured with a germanium ortho-oxysilicate scintillation detector. The [15O]H2O was intravenously injected either at a faster or slower injection rate, while animals were placed either with their head or heart centered in the gantry. The time-activity curve (TAC) from the AV shunt was compared with that from the cardiac ventricle in PET image. The rCBF values were calculated by a nonlinear least-square method using the dispersion-corrected AV-shunt TAC as an input.

RESULTS

The AV-shunt TAC had higher signal-to-noise ratio, but also had delay and dispersion compared with the image-derived TAC. The delay time between the AV-shunt TAC and image-based TAC ranged from 11 to 21 s, while the dispersion was estimated to be ∼5 s as a time constant of the dispersion model of exponential function, and both were properly corrected. In a steady-state condition of [15O]CO PET, the blood activity concentration by AV-shunt TAC was also comparable in height with the image-based TAC corrected for partial volume. Whole-brain CBF values measured by [15O]H2O were 0.37±0.04 (mean±S.D.) ml/g/min, partition coefficient was 0.73±0.04 ml/g, and the CBF varied in a linear relationship with partial pressure of carbon dioxide during each scan.

CONCLUSIONS

The AV-shunt technique allows less invasive, quantitative and reproducible measurement of rCBF in [15O]H2O PET studies in rats than direct blood sampling and radioassay.

Neurosurgeons and ideas before their time

As a complementary inquiry to previous studies on citation analysis in neurosurgery, a group of ideas before their time has been arbitrarily selected by the author from 3792 first-authored papers written between 1897 and 1980 by 50 of the first American neurosurgeons. There were eight neurosurgeons who proposed 12 original theories or procedures whose importance was not recognized at the time of publication. Although the value of these ideas was not initially judged to be significant, these concepts became a part of the intellectual consensus of neurosurgery or related disciplines after varying periods of time. Technical constraints constituted the most important reason for delayed recognition of these innovative and creative ideas. Other factors were an unsuitable medium of communication, fixed scientific attitudes, prestige of previous authors, incomplete literature review by later authors, and a failure to verify observations by other investigators. The question is raised as to whether these factors might continue to affect present and future clinical investigation and laboratory research in neurosurgery.

The effects of graded experimental trauma on cerebral blood flow and responsiveness to CO2

The effects of graded mechanical cerebral trauma on cerebrovascular reactivity to CO2 was studied in 26 cats. A fluid-wave percussion model was employed which delivered an epidural trauma of fixed duration and variable amplitude. The animals were maintained at arterial normoxia, with constant monitoring of intracranial and systemic arterial pressures, electroencephalograms, and end-tidal CO2. Following trauma, cerebral blood flow was measured using the H2 ion clearance technique at PaCO2 levels ranging sequentially from 20 to 60 mm Hg. Cerebrovascular reactivity for control animals (uninjured) was 2.7%. In the group with mild trauma (0.76 to 1.90 atm) reactivity was impaired (1.7%), and it was abolished in the severely injured group (2.90 to 4.60 atm). Mild injuries did not alter resting blood flows, while severe trauma resulted in a significant decrease in cerebrovascular resistance. Intracranial and systemic arterial pressures were altered proportionately to the level of cerebral injury. The authors propose that trauma to the brain-stem vasoregulatory centers accounts for these findings.

Experimental head injury in the rat. Part 1: Mechanics, pathophysiology, and morphology in an impact acceleration trauma model

Impact acceleration was used to elicit cerebral concussion in the albino rat. The pathophysiological response and morphological damage from the concussion were studied in groups of animals. The animals were grouped according to impact velocity (6-11 m/sec), and the threshold and different degrees of the concussive response were established. The concept of concussion as primarily a neuronal, functional disturbance was confirmed. However, this reaction was readily influenced by respiratory and circulatory changes, and morphological damage. Defined reactions could be elicited in groups of animals according to impact velocity. Thus, the model appeared well standardized for further studies of concussion pathophysiology, expressed in terms of cerebral energy metabolism and blood flow.

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

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

Response of cereborspinal fluid pressure to hyperbaric oxygenation

The response of cerebrospinal fluid pressure (CSFP) to hyperbaric oxygenation (OHP) was investigated in 13 patients with acute cerebral damage and in dogs with or without experimentally produced cerebral damage. To elucidate the mechanism of the CSFP response, continuous measurements of carotid blood flow, arterial blood pressure, central venous pressure, and superior sagittal sinus pressure and CSFP were made before, during and after OHP. There was considerable variation in the response of CSFP to OHP in the patients, but three main patterns emerged; type I (nine cases), CSFP decreased at the beginning and rose again at the end of OHP, type II (two cases), CSFP fell with OHP and remained significantly lower than pretreatment level after it, and type III (two cases), CSFP showed little change with OHP. An animal without cerebral damage commonly showed a type I response of CSFP to OHP; the changes of CSFP at the beginning and end of OHP are mainly due to the changes of the cerebral blood flow. There may be two different actions of OHP on cerebral oedema, one decreasing cerebral oedema and another (mainly affecting the normal brain) producing cerebral oedema. Information obtained from the response of CSFP to OHP may be useful in judging the severity and pathophysiological state of cerebral damage.

Trauma to the nervous system

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The treatment of craniocerebral injuries and prevention of anoxia

Even mild head injuries may cause cerebral swelling and vascular alterations, including vasoparalysis and increased vascular permeability. The severer the injury, the more pronounced these changes. They may cause death. Maintenance of adequate oxygen supply to the brain is the most effectual means of preventing or reducing the severity of this secondary effect of cerebral trauma, and the preservation of a good respiratory exchange is therefore essential in a patient who is comatose as a result of a head injury. The commonly employed measures such as an oral airway, suction and oxygen therapy may be tried first, but if the patient continues to have respiratory distress or cyanosis, an intratracheal tube should be employed for 24 hours; and then if there is no improvement, tracheotomy should be carried out. If the injury is severe, tracheotomy from the beginning may be a life-saving measure.

DISCUSSION ON CEREBRAL ŒDEMA

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