Editorial: On the critical lower level of cerebral blood flow in man with particular reference to carotid surgery

Influence of blood pressure on internal carotid artery blood flow during combined propofol-remifentanil and thoracic epidural anesthesia

Background and Aims

Anesthesia often reduces mean arterial pressure (MAP) to a level that may compromise cerebral blood flow. We evaluated whether phenylephrine treatment of anesthesia-induced hypotension affects internal carotid artery (ICA) blood flow and whether anesthesia affects ICA flow and CO2 reactivity.

Material and Methods

The study included twenty-seven patients (65 ± 11 years; mean ± SD) undergoing esophageal resection (n = 14), stomach resection (n = 12), or a gastroentero anastomosis (n = 1) during combined propofol-remifentanil and thoracic epidural anesthesia. Duplex ultrasound evaluated ICA blood flow. Evaluations were before and after induction of anesthesia, before and after the administration of phenylephrine as part of standard care to treat anesthesia-induced hypotension at a MAP below 60 mmHg, and the hypocapnic reactivity of ICA flow was determined before and during anesthesia.

Results

Induction of anesthesia reduced MAP from 108 ± 12 to 66 ± 16 mmHg (P < 0.0001) and ICA flow from 340 ± 92 to 196 ± 52 mL/min (P < 0.0001). Phenylephrine was administered to 24 patients (0.1-0.2 mg) and elevated MAP from 53 ± 8 to 73 ± 8 mmHg (P = 0.0001) and ICA flow from 191 ± 43 to 218 ± 50 mL/min (P = 0.0276). Furthermore, anesthesia reduced the hypocapnic reactivity of ICA flow from 23 (18-33) to 14%/kPa (10-22; P = 0.0068).

Conclusion

Combined propofol-remifentanil and thoracic epidural anesthesia affect ICA flow and CO2 reactivity. Phenylephrine partly restored ICA flow indicating that anesthesia-induced hypotension contributes to the reduction in ICA flow.

Motor evoked potential monitoring can evaluate ischemic tolerance to carotid artery occlusion during surgery

Balloon test occlusion (BTO) is a useful examination for evaluating ischemic tolerance to internal carotid artery (ICA) occlusion. The aim of this study was to investigate the relationships between intraoperative motor evoked potential (MEP) monitoring and the results of preoperative BTO. Between 2013 and 2017, 32 patients undergoing surgery under general anesthesia with intraoperative MEP monitoring, in whom preoperative BTO was performed, were identified. A receiver operator characteristic (ROC) analysis was performed to determine the appropriate cutoff value of MEP amplitude for BTO-positive. Furthermore, the accuracy of MEP monitoring for BTO-positive was compared with electroencephalogram (EEG) and somatosensory evoked potential (SEP) monitoring. Four of 32 (12.5%) patients were BTO-positive. The cutoff value of MEP amplitude for BTO-positive was a > 80% reduction from the baseline level, which showed sensitivity of 100% and specificity of 100%. Thus, the sensitivity and specificity for BTO-positive were significantly higher for MEP than for EEG (100% and 72.0%, p = 0.02) in 28 patients, but they were not significantly different compared with SEP (33.3% and 100%, p = 0.48) in 21 patients. MEP monitoring might be one of the alternatives for evaluating ischemic tolerance to ICA occlusion during surgery. The cutoff value of MEP amplitude was a > 80% reduction.

Diagnostic accuracy of somatosensory evoked potential and electroencephalography during carotid endarterectomy

BACKGROUND AND PURPOSE

Perioperative stroke risk following carotid endarterectomy (CEA) is reported to be approximately 2-3%. The diagnostic accuracies of intraoperative EEG and SSEP monitoring during CEA have been studied separately. However, to date, the effectiveness of simultaneous EEG and SSEP monitoring during CEA has only been evaluated in small study populations. This study examined the diagnostic accuracy of combined EEG and SSEP monitoring in a large (N = 1165) patient population.

METHODS

This study included 1165 patients who underwent CEA from 2000 to 2012 at the University of Pittsburgh Medical Center. The sensitivities, specificities, and diagnostic odds ratio of EEG and SSEP monitoring methods were examined separately and together. Receiver operating characteristic curves were plotted to assess sensitivity and specificity of single and combined Intraoperative monitoring (IONM) methods.

RESULTS

Maximum sensitivity was obtained with multimodality monitoring with an IONM change in either EEG or SSEP of 50.00 (95% CI, 30.66-69.34). The specificity of simultaneous EEG and SSEP changes was 93.95 (95% CI, 92.28-95.35%). Maximum area under ROC curve obtained for IONM change in either EEG or SSEP was 0.660 (95% CI, 0.547-0.773, p-value 0.004).

CONCLUSION

The diagnostic accuracy of multimodality IONM during CEA is higher than an approach using single modality IONM. Simultaneous EEG and SSEP monitoring improves the likelihood of detecting periprocedural strokes after CEA. Neuro protective therapies to prevent periprocedural strokes can be based on changes in SSEP and EEG during CEA.

Correlation of Continuous EEG Monitoring with [O-15]H2O Positron Emission Tomography Determination of Cerebral Blood Flow during Balloon Test Occlusion of the Internal Carotid Artery

The purpose of this paper was to evaluate the utility of continuous electroencephalography (EEG) during balloon test occlusion (BTO) of the internal carotid artery (ICA). Continuous EEG monitoring and [O-15] H2O PET cerebral blood flow (CBF) studies were completed in 34 patients undergoing BTO of the ICA. CBF determinations were obtained as a baseline without carotid occlusion, and following balloon occlusion, with continuous EEG monitoring. Patients were divided into three groups based on clinical and CBF response to BTO.

Group I had no clinical signs or symptoms and had a CBF decrease less than 10 ml/100 g/min ipsilateral to the occlusion. Group II had no symptoms but CBF fell to 35 to 25 ml/100 g/min on the occluded side. Group III were clinically unable to tolerate occlusion or CBF fell to less than 25 ml/100 g/min on the occluded side.

The results of continuous 21 channel EEG monitoring were assessed at the time of the examination and retrospectively reviewed for changes in the EEG pattern indicative of ischaemia. On the basis of PET CBF, eighteen patients were classified as Group I, four as Group II, and twelve as Group III. EEG evidence of ischaemia was seen in three patients, all members of Group III. Of the three patients, only one patient had clinical signs or symptoms of ischaemia. All four patients in Group II had PET quantitated CBF levels indicating carotid sacrifice should be done with caution or following a presacrifice by-pass procedure, and nine patients in Group III with PET quantitated CBFs below eligibility for carotid sacrifice, were not identified by EEG monitoring.

Even when CBF falls below 25 ml/100 g/minute continuous EEG monitoring is insensitive to reduction in perfusion. Reliance upon EEG for detection of cerebral hypoperfusion in interventionl neuroradiological procedures will significantly underestimate ischaemic risk.

Continuous neuromonitoring using transcranial Doppler reflects blood flow during carbon dioxide challenge in primates with global cerebral ischemia

OBJECTIVE

At present, there is no consensus on the optimal monitoring method for cerebral blood flow (CBF) in neurointensive care patients. The aim of the present study was to investigate whether continuous transcranial Doppler (TCD) monitoring with modulation of partial pressure of CO2 reflects CBF changes. This hypothesis was tested in 2 pathological settings in which cerebral ischemia can be imminent: after an episode of cerebral ischemia and during vasospasm after subarachnoid hemorrhage.

METHODS

Sixteen cynomolgus monkeys were divided into 3 groups: 1) chemoregulation in control animals to assess the physiological range of CBF regulation, 2) chemoregulation during vasospasm after subarachnoid hemorrhage, and 3) chemoregulation after transient cerebral ischemia. We surgically placed a thermal CBF probe over the cortex perfused by the right middle cerebral artery. Corresponding TCD values were acquired simultaneously while partial pressure of CO2 was changed within a range of 25 to 65 mm Hg (chemoregulation). A correlation coefficient of CBF with TCD values of greater than r equals 0.8 was considered clinically relevant.

RESULTS

CBF and CBF velocity correlated strongly after cerebral ischemia (r = 0.83, P < 0.001). Correlations were poor in chemoregulation controls (r = 0.2) and in the vasospasm group (r = 0.55).

CONCLUSION

The present study provides experimental support that, in clearly defined conditions, continuous TCD monitoring combined with chemoregulation testing may provide an estimate of CBF in the early postischemic period.

Intracarotid delivery of drugs: the potential and the pitfalls

The major efforts to selectively deliver drugs to the brain in the past decade have relied on smart molecular techniques to penetrate the blood-brain barrier, whereas intraarterial drug delivery has drawn relatively little attention. Meanwhile, rapid progress has been made in the field of endovascular surgery. Modern endovascular procedures can permit highly targeted drug delivery by the intracarotid route. Intracarotid drug delivery can be the primary route of drug delivery or it could be used to facilitate the delivery of smart neuropharmaceuticals. There have been few attempts to systematically understand the kinetics of intracarotid drugs. Anecdotal data suggest that intracarotid drug delivery is effective in the treatment of cerebral vasospasm, thromboembolic strokes, and neoplasms. Neuroanesthesiologists are frequently involved in the care of such high-risk patients. Therefore, it is necessary to understand the applications of intracarotid drug delivery and the unusual kinetics of intracarotid drugs.

Electroencephalography (EEG) and somatosensory evoked potentials (SEP) to prevent cerebral ischaemia in the operating room

We review the principal aspects of EEG and SEP to detect and prevent cerebral ischaemia in the operating room during interventions at risk. EEG and SEP are variables that indirectly reflect cerebral blood flow (CBF) provided that anaesthetic regimen, body temperature, and arterial blood pressure of the patient are stable. When CBF decreases and reaches the functional threshold, slowing and/or attenuation of EEG occurs while the amplitude and the latency of cortical SEP are, respectively decreased and lengthened. Based on these changes, numerous criteria corresponding to critical thresholds have been defined. A decrease in EEG amplitude greater than 30% or EEG changes lasting more than 30 s have been considered as significant by clinicians. The main criteria resulting from computerized EEG analysis were a reduction in total power and/or in spectral edge frequency. Regarding SEP, a more than 50% decrease in N20 amplitude and/or a more than 1 ms increase in central conduction time were the most frequently used criteria. According to the bulk of literature, it may be concluded that processed EEG analysis is more sensitive than visual EEG analysis to detect cerebral ischaemia, and that SEP are not less sensitive than conventional EEG. Moreover, literature shows that SEP are as specific as computerized EEG analysis to disclose ischaemia during carotid endarterectomy.

Somatosensory Evoked Potentials versus Locoregional Anaesthesia in the Monitoring of Cerebral Function During Carotid Artery Surgery: Preliminary Results of a Prospective Study

OBJECTIVE

to relate changes in somatosensory-evoked potentials (SEPs) with onset of neurological deficits in patients having carotid endarterectomy (CEA) under locoregional anaesthesia.

METHODS

a prospective study of 50 consecutive patients.

RESULTS

SEPs yielded an accuracy of 98%, specificity 100%, and sensitivity 89%. In all concordant cases the onset of a neurological deficit in awake patients corresponded to a 30--40% reduction in amplitude of N20-P25 waveforms. After shunting, the N20-P25 took 2--3 min to return to normal.

CONCLUSIONS

SEPs are associated with a 2% false negative rate. Their threshold for detecting cerebral ischaemia is lower than the currently reported value for patients under general anaesthesia. The time needed for evoked potentials (2--3 min) to return to normal after shunting limits their usefulness in verifying effective shunting.

General anaesthesia versus cervical block and perioperative complications in carotid artery surgery

PURPOSE

To compare the influence of anaesthetic technique on perioperative complications in patients undergoing carotid endarterectomy.

MATERIAL AND METHODS

In a retrospective study of 1020 consecutive patients who underwent carotid artery surgery over 10 years, perioperative neurologic and cardiologic complications and the use of an internal carotid artery shunt were compared in 337 patients (33%) treated under general anaesthesia and 683 (67%) under cervical block. The two groups had similar characteristics. The most frequent surgical indication was symptomatic carotid artery disease (91.5%). The remaining patients had asymptomatic severe internal carotid lesions (> 70%).

RESULTS

The overall perioperative stroke rate was 1.9%, the death-stroke rate 0.7% and the cardiac complication rate 0.8%. The perioperative stroke rate was higher in the general anaesthesia group than in the cervical block group (3.2% vs 1.3%, p = 0.01). Cardiac complication rates were similar in the two groups. A carotid artery shunt was used in 75 patients (22%) receiving general anaesthesia and in 92 patients (13%) receiving cervical block (p = 0.0004). The causes of stroke in the cervical block group were intraoperative embolism (4 cases, 26%), perioperative thromboembolism (7 cases, 58%) and clamping ischaemia (1 case, 16%). Mechanisms causing stroke in the general anaesthesia group remained unidentified or uncertain.

CONCLUSIONS

Cervical block anaesthesia yields better perioperative results than general anaesthesia probably because it allows more reliable cerebral monitoring, reducing or even eliminating perioperative strokes related to clamping ischaemia. It facilitates detection of the mechanism underlying intraoperative stroke allowing surgical techniques and intraoperative management to be modified accordingly. Cervical block anaesthesia significantly reduces the need for internal carotid artery shunting.

Vertebrobasilar thrombosis: diagnosis, management, and the use of intra-arterial thrombolytics

OBJECTIVES

To review the diagnosis and management of vertebrobasilar thrombosis and to discuss the use of thrombolytics in the treatment of this disease.

DATA SOURCES

Selected references discussing epidemiology, anatomy, pathophysiology, diagnosis, therapy, and rehabilitation of vertebrobasilar occlusive disease.

STUDY SELECTION

Studies addressing acute intervention and outcome in the therapy of vertebrobasilar thrombosis were reviewed.

DATA EXTRACTION

Only those studies with angiographic documentation of arterial thrombosis and, in the case of thrombolysis, recanalization, were considered valid.

DATA SYNTHESIS

Thrombosis of the vertebrobasilar system is a highly fatal disease and should be treated as a neurologic emergency. The key to effective management depends on early recognition of the symptom complex and a thorough understanding of the anatomy and pathophysiology of the disease process.

CONCLUSIONS

A timely, integrated, multidisciplinary approach to the patient with vertebrobasilar thrombosis can improve outcome. The use of thrombolytics in the treatment of vertebrobasilar occlusion holds promise but the benefits have not yet been proven in a controlled, randomized study.

Regional cerebral blood flow measured with N-isopropyl-p-[123I]iodoamphetamine and its redistribution in ischemic cerebrovascular disease

BACKGROUND AND PURPOSE

The relation between the redistribution phenomenon and regional cerebral blood flow and its clinical significance were investigated in stroke patients.

METHODS

Single-photon emission computed tomography studies using N-isopropyl-p-[123I]iodoamphetamine were performed on 16 patients (26 to 77 years old) with chronic infarction and 10 age-matched normal control subjects. Regional cerebral blood flow was quantitatively measured by a microsphere model, and the redistribution on delayed images was analyzed in ischemic lesions.

RESULTS

Supratentorial mean cerebral blood flow and the ratio of gray matter to white matter in normal subjects were 52.7 +/- 5.0 mL/100 g per minute and 2.34, respectively. Low-activity areas of ischemic lesions on early images were classified into two abnormal zones, an infarct area and a peri-infarct area. These regions were characterized by regional blood flow averaging 9 to 20 mL/100 g per minute and 22 to 41 mL/100 g per minute, respectively. Redistribution, which was minimally present in the infarct area, was markedly enhanced in the peri-infarct area. After bypass surgery, we observed a significant increase of blood flow (+22%) in the peri-infarct area.

CONCLUSIONS

The data indicate that the redistribution phenomenon depends on the maintenance of a minimal blood flow that would sustain cellular function and that this phenomenon is useful to evaluate bypass surgery in patients with chronic infarction.

Cerebral haemodynamics during carotid cross-clamping

Carotid artery cross-clamping ischaemia during carotid endarterectomy (CEA) causes 5-30% of perioperative neurological deficits. This study was performed to identify possible clinical situations at higher risk for carotid cross-clamping ischaemia. 606 consecutive patients underwent CEA and were retrospectively studied; they were grouped according to risk factors, presence of associated vascular diseases, clinical pattern, angiographic and CT scan findings. Stump pressure measurement was provided in all patients, perioperative monitoring during CEA was performed by electroencephalogram (EEG) in 469 (77%) and somatosensorial evoked potentials (SEP) in 137 (23%). Local anaesthesia was used in 88 (14.5%) patients. Ischaemic changes during carotid cross-clamping were registered in 118 patients (19.5%). The incidence of cross-clamping ischaemia was then related to different factors; it affected 5.6% of asymptomatics, 25.4% of patients with fixed stroke and 38.5% of those with stenosis and contralateral occlusion. Angiographic and clinical correlation showed that patients with more severe lesions are mostly affected by clamping ischaemia (up to 55% in those with stroke and stenosis with contralateral occlusion). Age, hypertension and diabetes do not significantly affect incidence of ischaemic changes. Positive CT scan increased this risk; statistical relevance was found in regard to patients with unilateral or bilateral stenosis and in those with transient ischaemic attacks. A higher risk can be expected for subjects with more severe clinical and instrumental findings, even if no patients can be considered completely at risk or risk free. Perioperative monitoring is always mandatory and is of great importance in detecting ischaemic changes and preventing cerebral damage using a temporary intraluminal shunt.

Perioperative stroke. Part I: General surgery, carotid artery disease, and carotid endarterectomy

Although stroke, defined as a focal neurological deficit lasting more than 24 hr, is uncommon in the perioperative period, its associated mortality and long-term disability are high. No large-scale data are available to identify the importance of recognized risk factors for stroke in the perioperative period. A review of the literature shows that the incidence and mechanism of its occurrence are influenced by the presence of cardiovascular disease and the type of surgery. The most common cause of perioperative stroke is embolism. In non-cardiac surgery, the incidence of perioperative stroke is higher among the elderly. Properly administered, controlled hypotension is associated with minimal risk of stroke. Cerebral vasospasm may be the cause of focal cerebral ischaemia in eclamptic patients, and the aggressive treatment of hypertension may exacerbate the neurological damage. The risk of stroke associated with carotid endarterectomy is closely related to the preoperative neurological presentation, and the experience of the surgical/anaesthetic team. Symptomatic cerebrovascular disease, acute stroke, asymptomatic carotid lesions, preoperative assessment of risk, local and general anaesthesia, cerebral protection and monitoring during carotid endarterectomy are discussed with reference to reducing the risk of perioperative stroke. Adequate monitoring and protection have minimized the risk of ischaemia from carotid clamping, and the major mechanism of stroke is embolization.

Caffeine and human cerebral blood flow: a positron emission tomography study

Positron emission tomography (PET) was used to quantify the effect of caffeine on whole brain and regional cerebral blood flow (CBF) in humans. A mean dose of 250 mg of caffeine produced approximately a 30% decrease in whole brain CBF; regional differences in caffeine effect were not observed. Pre-caffeine CBF strongly influenced the magnitude of the caffeine-induced decrease. Caffeine decreased paCO2 and increased systolic blood pressure significantly; the change in paCO2 did not account for the change in CBF. Smaller increases in diastolic blood pressure, heart rate, plasma epinephrine and norepinephrine, and subjectively reported anxiety were also observed.

Local fibrinolytic therapy in ischemic carotid pathology

Sixteen patients underwent local fibrinolytic therapy for thrombosis or embolism of the main trunk or intracranial branches of the internal carotid artery. There were eight cases of complete thrombosis of the internal carotid artery, five proximal stenoses of the internal carotid artery with extensive thrombus, one thrombus of the carotid siphon and two middle cerebral artery emboli. Indications for treatment included transient ischemic attacks in 11 cases, cerebral ischemia after carotid arteriography in two cases, and after surgery for atheromatous lesions of the carotid bifurcation in three cases. The fibrinolytic therapy was initiated during carotid surgery in three cases, where extended thrombosis of the internal carotid artery was discovered, which was inaccessible to a Fogarty catheter. The other 13 cases were treated during arteriography procedures. Lysis of the clot was always obtained. One patient died of hematoma of the frontal lobe. All other patients survived and showed neurologic improvement. The neurologic outcome was dependent on the duration and the degree of initial ischemia. Fibrinolytic therapy appears to be beneficial therapy for certain cases of cerebral ischemia.

The effect of Fluosol-DA on oxygen availability in focal cerebral ischemia

The effects of Fluosol-DA 35% (15 ml/Kg, IV) on cortical oxygen availability (O2a), a relative measurement of cortical oxygen tension, were examined in 16 cats subjected to temporary middle cerebral artery (MCA) occlusion. The cats were divided equally into control and treatment groups and half of each group underwent MCA occlusion at room air ventilation and the other half at 100% O2 ventilation. Prior to occlusion, Fluosol had no effect on arterial PO2 or cortical O2a at normoxia, however, there was a significant increase in both arterial PO2 and O2a following 100% O2 ventilation (pre vs post Fluosol). In an additional 5 cats, Fluosol resulted in a transient decrease in blood oxygen content due to hemodilution. Concurrent to this decrease, cerebral blood flow increased substantially resulting in a net increase in oxygen delivery. Within ipsilateral hemispheres, MCA occlusion resulted in decreased O2a to levels below the pre-occlusion normoxic values in all animals except those treated with Fluosol and ventilated with 100% O2. During reperfusion, O2a immediately recovered to hyperoxic levels in the Fluosol 100% O2 animals, whereas in the other three groups, O2a returned more gradually towards the pre-occlusion value. Since neither 100% O2 alone nor Fluosol plus room air ventilation significantly improved O2a in the ischemic cortex, we conclude that increased delivery of plasma plus Fluosol bound oxygen was responsible for the observed improvements in O2a following MCA occlusion.

Hemodynamically significant stenosis of the internal carotid artery treated with endarterectomy. Case report

Repeated cerebral blood flow (CBF) measurements with xenon-133 inhalation and single photon emission tomography were performed in a patient suffering a minor stroke with subsequent orthostatic-provoked transient ischemic attacks (TIA's). Angiography revealed a thread-like internal carotid artery and an occluded external carotid artery on the side of the ischemic neurological symptoms. Computerized tomography and technetium-99m-pertechnetate brain scintigraphy 2 weeks after stroke were both normal. Before reconstructive vascular surgery, resting CBF showed a hypoperfused area corresponding to the clinical symptoms. Diamox (acetazolamide, 1 gm) increased CBF by 24% in the unaffected hemisphere, whereas even a slight decrease in flow ("steal") was seen in the maximally affected region. In contrast, theophylline (220 mg) reduced CBF in the unaffected hemisphere and caused a slight increase in the previously maximally hypoperfused area ("inverse steal"). After surgery, the flow pattern practically normalized and the TIA's disappeared. The CBF measurements before surgery and also after the injection of the vasoactive drugs indicated that focal hemodynamic insufficiency elicited the TIA's, and pointed at a low mean arterial blood pressure of about 35 mm Hg in the affected hemisphere. The perioperative finding of a mean blood pressure in the internal carotid artery of 31 mm Hg on the symptomatic side confirmed that the brain tissue had a severely reduced perfusion pressure. On clamping the artery, a stump pressure of 22 mm Hg and electroencephalogram flattening was noted, so a temporary internal shunt was inserted. The findings demonstrate that preoperative CBF measurements, including studies of the regional vasoreactivity, may identify patients with hemodynamic TIA's. These patients are at particular risk of developing cerebral ischemia during carotid endarterectomy, as any further compromise of the inflow may precipitate frank ischemia.

Neuropsychological performance one week after carotid endarterectomy reflects intra-operative ischemia

Fourteen patients with severe bilateral carotid artery stenosis underwent carotid endarterectomy. Intra-operative ischemia was monitored by somatosensory evoked potentials (SSEP) bilaterally. Neuropsychological evaluations were completed within two days before operation and 4-9 days after operation. Complete loss of N1-P1 or P1-N2 components of the SSEP (seen in 4 patients) was associated with a worsening of neuropsychological abilities (p less than .01). Two of these patients subsequently had strokes (7 and 35 days after operation). No other patients in the series have had strokes. Patients whose N1-P1 or P1-N2 amplitudes decreased by 50% or more performed worse after operation than patients with less severe reductions in these amplitudes (p less than .02). Time since first ischemic symptoms, age, education, clamp time, pre-operative stroke, and interval from surgery to assessment were not statistically related to changes in neuropsychological abilities. Patients with ischemic events in the week prior to surgery tended to improve in neuropsychological abilities 4-9 days after operation (p less than .05). Recentness of ischemic episode, however, was not related to intra-operative SSEP change. Results suggest the potential utility of intra-operative SSEP monitoring and early post-operative neuropsychological assessments both for clinical and research purposes.

Cerebral perfusion and cerebral protection during carotid endarterectomy

The relative safety of carotid endarterectomy depends upon surgical expertise combined with appropriate monitoring of cerebral perfusion and the ability to intervene either surgically or pharmacologically to match cerebral perfusion to cerebral oxygen requirement. Methods of monitoring adequacy of cerebral perfusion are reviewed and include regional cerebral blood flow measurements, electroencephalographic monitoring, carotid stump pressure measurements, jugular venous oxygen partial pressure, neurological assessment and plethysmographic techniques. When cerebral perfusion is inadequate, such procedures as insertion of a temporary bypass shunt and maintenance of normocarbia may improve cerebral perfusion. Anticoagulants along with pharmacologic intervention to either increase cerebral perfusion pressure or reduce cerebral oxygen requirement may be used to preserve cerebral function.

Collateral cerebral vascular resistance in patients with significant carotid stenosis

This study tests, the hypothesis that asymptomatic patients with hemodynamically significant internal carotid artery stenosis have a lower ipsilateral collateral cerebral vascular resistance and hence greater blood flow than their symptomatic cohorts. We measured internal carotid artery blood flow and cerebral perfusion pressures intraoperatively prior to and after carotid endarterectomy in 35 symptomatic and 10 asymptomatic patients with hemodynamically significant internal carotid artery stenosis. When the stenosis produced 30% or greater reduction in blood flow the calculated nondimensional normalized ratio of collateral cerebral vascular resistance of ipsilateral hemisphere cerebral vascular resistance was 1.15 +/- 0.83 (mean +/- SD) for the 10 asymptomatic patients and 2.98 +/- 1.89 for the 35 symptomatic patients (p = 0.0044). For the subgroup of 22 patients with 50% or greater reduction in internal carotid artery blood flow the resistance ratios were 0.782 +/- 0.541 for the 5 asymptomatic patients and 3.21 +/- 2.26 for the 17 symptomatic patients (p = 0.029). These results suggest that asymptomatic patients with hemodynamically significant internal carotid artery stenoses have a lower collateral cerebral vascular resistance than their symptomatic cohorts. The low collateral resistance may provide an adequate collateral cerebral blood flow to prevent ischemia and symptoms.

Reversal of focal "misery-perfusion syndrome" by extra-intracranial arterial bypass in hemodynamic cerebral ischemia. A case study with 15O positron emission tomography

Tomographic images of cerebral blood flow (CBF) and oxygen extraction fraction (OEF) using the 15O continuous inhalation technique, and positron emission tomography, were obtained from a patient with cerebral ischemia distal to an occluded left internal carotid artery. There was a focal mismatch between CBF and oxygen metabolism in the brain supplied by the middle cerebral artery where CBF was decreased and OEF increased ("misery-perfusion syndrome" as opposed to "luxury-perfusion syndrome"). These abnormalities were most marked in the parieto-occipital watershed area. After left superficial temporal to middle cerebral artery anastomosis, the clinical attacks ceased and a repeat study did not demonstrate the previous CBF and OEF abnormalities. This suggests that this pattern of abnormalities indicates potential viable tissue. The concept of "misery-perfusion" may be of some importance in the pathophysiological mechanisms of hemodynamic cerebral ischemia and serve as a rational basis for revascularization procedures.

Correlation between rCBF and histological changes following temporary middle cerebral artery occlusion

Correlations between changes in regional, cortical, cerebral blood flow (rCBF) and histological changes in the corresponding brain regions were examined following middle cerebral arterial occlusion in 24 cats. In all animals, the duration of arterial occlusion was 2 hours followed by 2 hours of recirculation. The animals were divided into 2 groups according to the severity of the observed histological damage. Severe cortical damage was observed in 8 cats (Group A), and, in the remaining 16 cats, little or no cortical damage was seen (Group B). There was a statistically significant difference between these 2 groups in the average rCBF values during ischemia. During recirculation, there was a prompt and uniform recovery of rCBF in animals in group B but a marked diversity; of rCBF ranging from hyperemia to oligemia in animals in group A. This diversity of rCBF reflects inhomogenous blood flow. This study indicates potential hazards for surgical revascularization in the aute stage of stroke when brain damage has progressed beyond a certain level.

Protection of the brain from hypoxia: a review

A functional classification of hypoxia of the brain has been presented and some of its significant aspects have been discussed. Mechanisms of protection from hypoxia of the brain were reviewed under the headings of prevention, hyperventilation, hypothermia and protection by barbiturates. In prevention of hypoxia of the brain, avoidance of factors producing a fall in cerebral perfusing pressure was emphasized. Hyperventilation is not advised unless one can readily measure regional cerebral blood flow. In the operating room, normocarbia or slight hypocarbia is recommended. Animal studies indicate a protective role of barbiturates in ischaemic hypoxia of the brain. However, it should be emphasized that, at present, hypothermia is the only established means of protection against hypoxia of the brain in man, when it is induced prior to the hypoxic insult. The evidence for protection by barbiturates has been found only in experimental animals. If one can extrapolate the results of studies in animals to man, then potential benefits would be expected in clinical stroke, cardiac arrest, in operations on the carotid artery and in head injury.

Autoregulation of cerebral blood flow in hypertensive patients. The modifying influence of prolonged antihypertensive treatment on the tolerance to acute, drug-induced hypotension

Autoregulation of cerebral blood flow (CBF) was studied by the arteriovenous oxygen difference method in 13 patients with untreated or ineffectively treated severe hypertension, nine patients with effectively treated, formerly severe hypertension, and ten normotensive controls. Resting mean blood pressure in these three groups was 145 +/- 17 (1 SD) mm Hg, 116 +/- 18 mm Hg, and 98 +/- 10 mm Hg, respectively. Blood pressure was decreased by trimethaphan infusion combined with head-up tilt. The lower limit of CBF autoregulation in the three groups was 113 +/- 17 mm Hg, 96 +/- 17 mm Hg, and 73 +/- 9 mm Hg, and the lowest tolerated blood pressure where mild symptoms of brain hypoperfusion were encountered was 65 +/- 10 mm Hg, 53 +/- 18 mm Hg, and 43 +/- 8 mm Hg. These pressures were all significantly higher (P less than 0.01) in the group of untreated or ineffectively treated hypertensive patients than in the normotensive group demonstrating a shift of CBF autoregulation in the former. The observations in effectively treated hypertensive patients strongly suggested a readaptation of CBF autoregulation toward normal in some cases. In four hypertensive patients studied twice it was found that 8-12 months of antihypertensive treatment on average did not influence the lower limit of CBF autoregulation.

XIII. Cerebral circulation and metabolism in stroke. Cerebral circulation and metabolism in stroke study group

An understanding of the cerebral circulation is so fundamental to comprehension of the pathogenesis of stroke that cerebral blood flow and metabolism merit review in this series of reports. The authors recognize that the research described here is very technical in nature and may appear to have little practical application to clinical medicine. Nevertheless, these matters are basic to the development of precise methods for the measurement of regional cerebral blood flow in man which could be used to monitor the therapy of stroke with greater success than is possible at present.