The effect of graded hypocapnia and hypercapnia on regional cerebral blood flow and cerebral vessel caliber in the rhesus monkey: study of cerebral hemodynamics following subarachnoid hemorrhage and traumatic internal carotid spasm

Protective Effects of Obstructive Sleep Apnea on Outcomes After Subarachnoid Hemorrhage: A Nationwide Analysis

BACKGROUND

Obstructive sleep apnea (OSA) is typically associated with an increased risk of cardiovascular and cerebrovascular disease. Recent studies, however, have suggested that hypercapnia and chronic intermittent hypoxia may potentially provide protection against ischemic events like stroke.

OBJECTIVE

To evaluate the impact of OSA with presentation, hospital course, and treatment outcomes of patients with subarachnoid hemorrhage (SAH).

METHODS

Data for patients with SAH between the years 2011 and 2015 were extracted from the Nationwide Inpatient Sample (NIS) and stratified based on diagnosis codes for OSA. Univariate analysis was used to assess the prevalence of comorbidities in OSA patients diagnosed with SAH and several covariates, including patient demographics, aneurysmal treatment, in-hospital morality rate, length of stay, and costs. Multivariate logistic regression models analyzed the relationship between several comorbidities, including OSA, tobacco use, and hypertension, and poor outcomes after SAH.

RESULTS

Data from 49 265 SAH patients were used in this study, of which 2408 (4.9%) also had a concomitant OSA diagnosis. Patients with OSA compared to all other SAH patients had a significantly lower in-hospital mortality rate, as well as statistically significant lower odds of vasospasm, stroke, and poor outcomes. Additionally, hypercholesterolemia, obesity, and tobacco use disorder were also associated with more favorable outcomes.

CONCLUSION

SAH patients with OSA are significantly less likely to have a poor outcome when compared to non-OSA patients, despite having an increased risk of several comorbidities.

Snapshot of 1973 and 1974: critical thinkers and contemporary research ideas in neurosurgical anesthesia during the first years of SNACC

The year 2012 marks the 40th anniversary of the Society of Neuroscience in Anesthesiology and Critical Care (SNACC). To celebrate this occasion, we provide a review, speculative synthesis, and commentary addressing research relevant to neurosurgical anesthesiology in 1973 and 1974--the early years of SNACC. We address topics such as effects of anesthetic drugs, neuroprotection, cerebral physiology, and monitoring as they relate to the perioperative care of neurosurgical patients or patients experiencing or at risk for neurological disorders. Our hypothesis is that a review of these publications will identify the foundations of research and practice concepts that persist until today and will also identify concepts that have dwindled or outright disappeared.

Changes in cerebral compartmental compliances during mild hypocapnia in patients with traumatic brain injury

The benefit of induced hyperventilation for intracranial pressure (ICP) control after severe traumatic brain injury (TBI) is controversial. In this study, we investigated the impact of early and sustained hyperventilation on compliances of the cerebral arteries and of the cerebrospinal (CSF) compartment during mild hyperventilation in severe TBI patients. We included 27 severe TBI patients (mean 39.5 ± 3.4 years, 6 women) in whom an increase in ventilation (20% increase in respiratory minute volume) was performed during 50 min as part of a standard clinical CO(2) reactivity test. Using a new mathematical model, cerebral arterial compliance (Ca) and CSF compartment compliance (Ci) were calculated based on the analysis of ICP, arterial blood pressure, and cerebral blood flow velocity waveforms. Hyperventilation initially induced a reduction in ICP (17.5 ± 6.6 vs. 13.9 ± 6.2 mmHg; p < 0.001), which correlated with an increase in Ci (r(2) = 0.213; p = 0.015). Concomitantly, the reduction in cerebral blood flow velocities (CBFV, 74.6 ± 27.0 vs. 62.9 ± 22.9 cm/sec; p < 0.001) marginally correlated with the reduction in Ca (r(2) = 0.209; p = 0.017). During sustained hyperventilation, ICP increased (13.9 ± 6.2 vs. 15.3 ± 6.4 mmHg; p < 0.001), which correlated with a reduction in Ci (r(2) = 0.297; p = 0.003), but no significant changes in Ca were found during that period. The early reduction in Ca persisted irrespective of the duration of hyperventilation, which may contribute to the lack of clinical benefit of hyperventilation after TBI. Further studies are needed to determine whether monitoring of arterial and CSF compartment compliances may detect and prevent an adverse ischemic event during hyperventilation.

Hypercapnia-induced cerebral hyperperfusion: an underrecognized clinical entity

BACKGROUND AND PURPOSE

The incidence of cerebral hyperperfusion and hypoperfusion, respectively, resulting from hypercapnia and hypocapnia in hospitalized patients is unknown but is likely underrecognized by radiologists and clinicians without routine performance of quantitative perfusion imaging. Our purpose was to report the clinical and perfusion imaging findings in a series of patients confirmed to have hypercapnic cerebral hyperperfusion and hypocapnic hypoperfusion.

MATERIALS AND METHODS

Conventional cerebral MR imaging examination was supplemented with arterial spin-labeled (ASL) MR perfusion imaging in 45 patients during a 16-month period at a single institution. Patients presented with an indication of altered mental status, metastasis, or suspected stroke. Images were reviewed and correlated with arterial blood gas (ABG) analysis and clinical history.

RESULTS

Patients ranged in age from 1.5 to 85 years. No significant acute findings were identified on conventional MR imaging. Patients with hypercapnia showed global hyperperfusion on ASL cerebral blood flow (CBF) maps, respiratory acidosis on ABG, and diffuse air-space abnormalities on same-day chest radiographs. Regression analysis revealed a significant positive linear relationship between cerebral perfusion and the partial pressure of carbon dioxide (pCO(2); beta, 4.02; t, 11.03; P < .0005), such that rates of cerebral perfusion changed by 4.0 mL/100 g/min for each 1-mm Hg change in pCO(2).

CONCLUSIONS

With the inception of ASL as a routine perfusion imaging technique, hypercapnic-associated cerebral hyperperfusion will be recognized more frequently and may provide an alternative cause of unexplained neuropsychiatric symptoms in hospitalized patients. In a similar fashion, hypocapnia may account for a subset of patients with normal MR imaging examinations with poor ASL perfusion signal.

Decrease in platelet count as an independent risk factor for symptomatic vasospasm following aneurysmal subarachnoid hemorrhage

Object. Increased platelet consumption is expected in patients with cerebral vasospasm, according to data from clinical and experimental studies. The authors investigated sequential changes in platelet counts in patients with subarachnoid hemorrhage (SAH) and the difference in platelet consumption between patients with and those without symptomatic vasospasm (SV). Variables related to platelet count as well as other clinical and radiological variables were analyzed as independent predictors of SV.

Methods. One hundred consecutive patients who had undergone surgery within 48 hours after SAH onset were entered in the study. Clinical and radiological variables and blood cell counts, including red blood cells, white blood cells, and platelets, after SAH were retrospectively examined. Twenty of these variables were entered into univariate and multivariate analyses to determine predictors for SV.

After SAH, the platelet count decreased to a minimum and then increased rapidly to levels greater than those recorded on admission. This change was specific to SAH, and platelet consumption was more severe in patients with SV than in those without. There were three independent predictors of SV: a ratio of the lowest platelet count and the admission count greater than 0.7 (odds ratio [OR] 0.322, 95% confidence interval [CI] 0.124–0.834, p = 0.0196) and a history of hypertension (OR 0.338, 95% CI 0.126–0.906, p = 0.0311) were negatively significant (that is, decreases the occurrence of SV), and a Fisher Grade 3 (OR 4.42, 95% CI 1.48–13.2, p = 0.0077) was positively significant (that is, increases the occurrence of SV).

Conclusions. The association between a decrease in platelet count and the occurrence of SV indicates the important role of platelets in the pathophysiology of vasospasm following SAH.

Anti-phospholipid antibodies and cerebral vasospasm following subarachnoid haemorrhage

Delayed ischaemic deficits due to cerebral vasospasm contribute to the high morbidity and mortality rates associated with subarachnoid haemorrhage. We evaluated the usefulness of measuring anti-phospholipid antibodies (aPLs) for prediction of the occurrence of symptomatic vasospasm and the outcome after subarachnoid haemorrhage. 32 consecutive patients with subarachnoid haemorrhage due to ruptured cerebral aneurysms were studied. They were admitted and operated on within 72 hours after the onset of subarachnoid haemorrhage. aPLs such as lupus anticoagulants, anti-cardiolipin IgG and anti-cardiolipin IgM were measured repeatedly after admission. Furthermore, platelet count, platelet aggregability and plasma platelet factor 4 were also measured. Eleven among the 32 patients (34.4%) showed positive in the examination for aPLs. Although aPLs could not predict symptomatic vasospasm, once symptomatic vasospasm occurred, patients with aPLs frequently demonstrated cerebral infarction and therefore their outcome was worse. aPLs were associated with a severe initial clinical grade and SAH grade on CT scan. Therefore it may explain the association of aPLs with worse outcome. aPLs were detected between 1 and 7 days. Four of 6 patients (67%) with aPLs became negative between 7 and 13 days after subarachnoid haemorrhage. The mechanism of transient aPLs is unclear but it is more likely to occur in the severer grade patients. The reduction in platelet count, the increased platelet aggregability, and the increased plasma platelet factor 4 concentration were also observed in aPLs-positive patients with symptomatic vasospasm.

Comparative evaluation of acute cerebral vasospasm by the microsphere and the angiography techniques

Experimental subarachnoid haemorrhage (SAH) induces an acute transient cerebral vasospasm. The goal of this study was to compare angiography with iterative measurements of regional cerebral blood flow (rCBF) by the microsphere technique for tracking acute cerebral vasospasm after SAH. Cerebral vasospasm was induced in anaesthetised rabbits by injecting 1 ml of fresh blood in the cisterna magna. In a first experiment, the diameter of the basilar artery was measured by repeated angiograms over 60 min. In a second experiment, rCBF was measured over 60 min by the radioactive microspheres method without and with bilateral ligation of the carotid artery. Without carotid ligation, despite a profound transient vasospasm of the basilar artery, rCBF was unchanged in the cerebellum and cerebrum and was not statistically decreased in the brain stem. However, with bilateral carotid ligation, rCBF dramatically decreased at 5 and 15 min after haemorrhage. At 30 min, despite a persistent 50% decrease in the basilar cross-sectional area, rCBF was no longer different from the control group. Thus in a model of acute vasospasm of the basilar artery, rCBF evaluation by the microsphere technique parallels the cerebral vasospasm evaluated by angiography only when both carotid arteries are ligated.

Platelet-activating factor and antiphospholipid antibodies in subarachnoid haemorrhage

The purpose of this study was to measure plasma platelet-activating factor (PAF) concentration, PAF-acetylhydrolase activities, anti-phospholipid antibody (aPLs) titers, and platelet function in patients with subarachnoid haemorrhage (SAH) and to assess the association of these variables with the development of cerebral vasospasm. Thirty-two patients with SAH due to ruptured cerebral aneurysm were studied. Plasma PAF concentration, PAF-acetylhydrolase activity, platelet count and aggregability, and plasma factor 4 (PF4) concentrations were measured regularly until approximately 2 weeks after SAH. aPLs, including lupus anticoagulant and anti-cardiolipin IgG and IgM were measured within 3 days after SAH. Plasma PAF concentration in patients with SAH showed the highest value on the occasion during 5 to 9 days after SAH. The concentrations were higher in patients with infarction due to vasospasm than in patients without cerebral infarction on any occasions after SAH. Plasma PAF-acetylhydrolase activities did not change in patients, regardless of the presence of cerebral infarction after SAH. Increased platelet consumption and aggregability and higher concentrations of PF 4 were detected in patients with cerebral infarction and not in patients without cerebral infarction. The patients with cerebral infarction due to cerebral vasospasm had aPLs more frequently than the control volunteers. Our findings indicate that increased plasma PAF and aPLs may contribute to the pathogenesis of cerebral vasospasm after SAH.

Reduced cerebral blood flow but intact reactivity to hypercarbia and hypoxia following subarachnoid hemorrhage in rabbits

Subarachnoid hemorrhage (SAH) was produced in rabbits by four subarachnoid injections of blood (n = 7) or saline (n = 6); a control group (n = 6) had no injections. Basilar artery vasospasm was assessed by serial angiograms. Resting CBF (microspheres) and CBF reactivity to hypercapnia (65 and 85 mm Hg) and hypoxia (fractions of inspired oxygen of 0.15 and 0.10) were determined. Basilar artery vasospasm was seen with SAH. Resting CBF was reduced by 31% (SAH 43 +/- 12, saline 65 +/- 17, control 60 +/- 21 ml 100 g-1 min-1), and resting cerebrovascular resistance was increased (SAH 1.84 +/- 0.30, saline 1.31 +/- 0.49, control 1.39 +/- 0.25 mm Hg ml-1 100 g-1 min-1) after SAH. CBF rose to a similar degree in all three groups in response to hypercarbia and hypoxia. We conclude that resting CBF is reduced in this model of SAH, but vascular reactivity remains intact.

Altered cerebrovascular CO2 reactivity following subarachnoid hemorrhage in cats

The authors tested the hypothesis that cerebral blood flow (CBF) reactivity to CO2 was blunted following subarachnoid hemorrhage (SAH). Subarachnoid hemorrhage was produced in five cats by performing four cisterna magna injections of blood in each (SAH Group). A second group of six cats was treated with an antifibrinolytic agent (AF) in addition to four cisterna magna blood injections (SAH+AF Group). Four cats received AF and four cisterna magna injections of saline (Control Group). The presence or absence of basilar artery vasospasm was determined by comparing baseline and follow-up selective angiograms. Cerebral blood flow reactivity was determined by randomly varying the concentration of inspired CO2 to alter PaCO2 from 20 to 75 mm Hg. Regional CBF was measured with radiolabeled microspheres. Basilar artery vasospasm was seen following subarachnoid injection of blood but not of saline. Normocapnic CBF was similar in all three groups in the brain stem (mean +/- standard error of the mean: SAH Group 46 +/- 6, SAH+AF Group 46 +/- 6, and Control Group 44 +/- 9 ml/min/100 gm) and in the supratentorial compartment (SAH Group 53 +/- 8, SAH+AF Group 61 +/- 9, and Control Group 51 +/- 13 ml/min/100 gm). At intermediate levels of hypercarbia (PaCO2 50 +/- 3 mm Hg), CBF increased similarly in all three groups (SAH Group 161% +/- 32%, SAH+AF Group 118% +/- 33%, and Control Group 174% +/- 19% compared to baseline); at higher levels of PaCO2 (60 +/- 3 mm Hg), CBF values were SAH Group 265% +/- 50%, SAH+AF Group 205% +/- 47%, and Control Group 159% +/- 30% of baseline. At the highest level of PaCO2 (75 +/- 6 mm Hg), supratentorial CBF did not increase as much in the SAH+AF Group as in the Control Group (179% +/- 59% vs. 463% +/- 58% of baseline, respectively). The authors conclude that, in this model of SAH, there is no change in normocapnic CBF; however, blood flow reactivity to hypercarbia is blunted. It is possible that this may result from a combination of narrowing of proximal large vessels and globally impaired reactivity of small vessels.

Role of platelet function in symptomatic cerebral vasospasm following aneurysmal subarachnoid hemorrhage

To evaluate the role of platelet function in the pathogenesis of cerebral vasospasm, we compared sequential changes of platelet aggregability and beta-thromboglobulin and thromboxane B2 concentrations in blood samples from the internal jugular and peripheral vein of 13 patients with aneurysmal subarachnoid hemorrhage. Platelet function in blood from the internal jugular vein tended to be enhanced during days 0-1 but recovered to the normal range during days 2-4. After day 5, platelet function showed various patterns depending on the presence of symptomatic vasospasm. In patients without symptomatic vasospasm, sequential changes were relatively minor, with normal or slightly high values. Patients with symptomatic vasospasm already showed high platelet aggregability during the early stage of vasospasm. The concentration of beta-thromboglobulin increased several days after the onset of vasospasm, reaching 80 ng/ml or more in patients with a poor prognosis. Two of the five patients with symptomatic vasospasm showed markedly high concentrations of thromboxane B2 after day 8. These results suggest that vasospasm activates platelets and promotes aggregability and that the resulting increased tendency for thrombus formation may affect the patient's prognosis during the advanced stage.

Cerebrovascular and metabolic changes during the delayed vasospasm following experimental subarachnoid hemorrhage in baboons, and treatment with a calcium antagonist

A model has been designed in baboons for simulating the clinical situation during the late phase of vasospasm in patients with subarachnoid hemorrhage (SAH). A total amount of 14-33 ml autologous blood was injected into the cisternal system on 3 occasions in the course of 4 days. Neurological symptoms were seen, and the mortality rate was 29%. Angiography 3 days after the last injection showed arterial vasoconstriction amounting to 23% in the vertebro-basilar system, and 11% (right) and 18% (left) in the carotid system. Cerebral blood flow (CBF) measured by the intra-arterial 133Xe technique and the cerebral metabolic rate of oxygen (CMRO2) were reduced by 18% and 11%, respectively. The hypercapnic CBF response was significantly impaired, from a mean of 3.90 ml/100 g/min to 1.72 ml/100 g/min of flow increase for each mm Hg elevation of paCO2. Autoregulation, tested by administration of angiotensin II, was also significantly affected as evidenced by a pressure-dependent increment of CBF during hypertension in 5 out of 7 animals tested. The impaired autoregulation was reflected in the autoregulatory index, which in the whole group increased from 0.06 ml/100 g/min for each mm Hg increase in MABP in the pre-SAH animals to 0.29 ml/100 g/min per mm Hg post-SAH. Treatment with the calcium antagonist, nimodipine (0.5 microgram/kg/min i.v. during 45 min), enhanced CBF significantly by 17% before experimental SAH, whereas after SAH the effect was slight and did not reach statistical significance; CMRO2 was not significantly affected in either group. Intravenous nimodipine combined with hypertension resulted in a marked increase in the autoregulatory index to 1.58 ml/100 g/min per mm Hg in pre-SAH animals and a less pronounced increment to 0.58 ml/100 g/min per mm Hg following experimental SAH. The beneficial effect of nimodipine reported in SAH patients is therefore, in view of our findings, more likely due primarily to a protective mechanism at the cellular level than to an influence on the vascular bed.

Deterioration of auditory evoked potentials during cerebellopontine angle manipulations. An interpretation based on an experimental model in dogs

Evoked action potentials from the internal auditory meatus portion of the cochlear nerve (IAM-EAP's) and brain-stem auditory evoked potentials (BAEP's) from the vertex were simultaneously recorded during cerebellopontine angle (CPA) manipulations (retractions of the cerebellar hemisphere and the cochlear nerve) in dogs. The BAEP changes noted in these dogs were the same as those seen in patients. The IAM-EAP's showed graded deterioration related to BAEP changes. The results are as follows: Prolongation of the I-V interpeak latency of BAEP's, the most common finding during CPA manipulations, is the reflection of prolongation of the I-II interpeak latency, which is caused by conduction impairment or block of the nerve impulses between the extracranial portion of the cochlear nerve and the brain stem. The operative manipulations representing stretch or compression injury to the cochlear nerve in the CPA leads to an acute traumatic cranial nerve root lesion--a retrocochlear lesion. The obliteration of all BAEP components including wave I cannot be caused by conduction block. This is caused by occlusion of the internal auditory artery. Wave I of the BAEP's and the P1-N1 complex of the IAM-EAP's are important indicators of cochlear blood flow during surgical intervention. As possible causes of internal auditory artery obstruction, mechanical distortion of the relationship between the anterior inferior cerebellar artery (AICA) and the internal auditory artery at the junctional portion, mechanical vasospasm of the AICA-internal auditory artery complex, and ensuing no-reflow phenomena are discussed. Evoked action potentials are expected to be a useful intraoperative real-time monitor during CPA surgery that can detect rapid changes derived from cochlear artery insufficency. The real-time aspects can overcome some of the disadvantages of BAEP monitoring.

Contribution of the inflow arteries to alterations in total cerebrovascular resistance in the rabbit

The contribution made by the cerebral inflow arteries to total cerebrovascular resistance (CVR) and their importance in producing alterations in cerebral blood flow (CBF) (i.e., changes in CVR) were investigated. The arterial blood pressure at the circle of Willis was measured in 14 anesthetized rabbits via transorbital retrograde cannulation of the ophthalmic artery. CBF was measured in 21 rabbits under identical experimental conditions, using the hydrogen clearance technique. Inflow artery resistance was calculated from the measurements which were made at both normocapnia and hypercapnia throughout hemorrhagic hypotension. Under resting conditions, the inflow arteries made a relatively minor contribution to total CVR (7%). Hypercapnia resulted in a decrease in CVR and an increase in CBF; however, inflow artery resistance remained constant. Autoregulation and reductions in total CVR were observed as PP was reduced to 35 mm Hg. Inflow artery resistance remained constant at pressures greater than 45 mm Hg and increased slightly at PP less than 45 mm Hg. The relative contribution of inflow artery resistance to total CVR increased under the various conditions studied--increasing by a factor of 2 during hypercapnia, by a factor of 3 during hypotension, and by a factor of 4 during hypotension + hypercapnia. We concluded that the large inflow arteries do not participate in the autoregulatory or CO2 responses of the cerebrovasculature of the rabbit.

Cerebrovascular reactivity in patients with ruptured intracranial aneurysms

The cerebral vasomotor reactivity to arterial hypotension and hypocapnia was studied in 34 patients between the 3rd and 13th day after rupture of an intracranial saccular aneurysm. Using the intra-arterial xenon-133 injection method, regional cerebral blood flow (rCBF) and cerebral metabolic rate of oxygen (CMRO2) were measured. The intraventricular pressure and cerebrospinal fluid (CSF) lactate and pH levels were determined. The degree of vasospasm was measured on angiograms taken immediately following the rCBF study. The patients were graded clinically according to the system of Hunt and Hess. Cerebral autoregulation was intact in patients in good clinical condition, but was impaired in patients in poor clinical condition. There was a close correlation between the degree of vasospasm and the degree of autoregulatory impairment, which varied from focal disturbances to global impairment. Intracranial hypertension and CSF lactic acidosis were commonly found in association with vasoparalysis. Cerebrovascular response to hyperventilation was generally preserved, although often reduced. During hyperventilation, the cerebral perfusion pressure became elevated, and increases in CMRO2 were often found, even in patients with severe diffuse spasm and cerebral ischemia. The clinical significance of the results in relation to the treatment of delayed cerebral ischemia and to the use of intraoperative induced hypotension is discussed.

The blood-brain barrier following experimental subarachnoid hemorrhage. Part 1: Response to insult caused by arterial hypertension

In three groups of cats, the authors studied the effect of subarachnoid hemorrhage (SAH) on the permeability of the blood-brain barrier (BBB) to the penetration of Evans blue-protein complex. One group received arterial hypertension alone, one group SAH alone, and one group SAH followed by arterial hypertension. Animals subjected to arterial hypertension alone showed areas of BBB breakdown. However, when cats were rendered hypertensive after SAH, there were no demonstrable BBB lesions. The SAH was produced by intracisternal injection of whole blood and hypertension by the intravenous injection of metaraminol. The preservation of the BBB after SAH is discussed. Vasospasm is considered as a possible hemodynamic variable responsible for the protection of the BBB from hypertensive damage. The need for a new model is proposed to further investigate the state of the BBB after SAH.

Delayed effects of subarachnoid haemorrhage on cerebral metabolism and the cerebrovascular response to hypercapnia in the primate

A technique is described for the production of subarachnoid haemorrhage in baboons and their subsequent recovery for chronic study of cerebrovascular reactivity. The baboons make complete neurological recoveries but the response of their cerebral circulation to hypercapnia is impaired one week later. Baseline values of cerebral blood flow and of cerebral oxygen consumption are unaffected at this time. There is no evidence of hypoxic brain damage.

Cerebral arterial responses to induced hypertension following subarachnoid hemorrhage in the monkey

Regional cerebral blood flow (rCBF), angiographic cerebral arterial caliber, and cerebrospinal fluid (CSF) pressure were measured in rhesus monkeys to determine the effect of experimentally induced subarachnoid hemorrhage (SAH) on cerebral arterial responses to graded increases in blood pressure. These measurements were also performed in a control group of monkeys subjected to a mock SAH by injection of artificial CSF into the cerebral space. Before subarachnoid injection of blood or artificial CSF, graded increases in mean arterial blood pressure (MABP) to a level 40% to 50% above baseline values had no effect on rCBF. The major cerebral arteries constricted and CSF pressure remained unchanged. Similar responses were observed after injections of artificial CSF. When MABP was increased in animals that had been subjected to subarachnoid injection of blood, rCBF increased and was associated with dilatation of the major cerebral arteries and moderate increases in CSF pressure. These results demonstrate that cerebral arterial responses to increases in blood pressure may be abnormal in the presence of subarachnoid blood. The manner in which abnormal cerebral arterial reactivity, changes in blood pressure, and vasospasm combine to determine the level of cerebral perfusion following SAH is postulated.

Hyperemia, CO2 responsiveness, and autoregulation in the white matter following experimental spinal cord injury

The authors present the results of a controlled, randomized study of alterations in spinal cord blood flow, CO2 responsiveness, and autoregulation following experimental spinal cord injury in cats. Permanent paraplegia is shown to be associated with persistent hyperemia, loss of CO2 responsiveness, and impaired autoregulation in the white matter at the injury site. Probable mechanisms underlying these changes in spinal cord vasomotor control are discussed. Marked similarities between vascular responses of injured spinal cord and luxury perfusion of the brain are pointed out.

An automatic bolus injector for use in radiotracer studies of blood flow: design and evaluation

An electromechanical device is described which automatically injects the radiotracer bolus used in the measurement of cerebral blood flow. It consists of two electronically controlled, solenoid operated syringes, one containing the radiotracer solution and the other heparinized saline. Results are presented which show that use of the automatic bolus injector in place of hand injection leads to an improvement in the precision of measured flow values. Additional advantages of the device are discussed.