CEREBRAL BLOOD FLOW DETERMINED BY SATURATION AND DESATURATION WITH KRYPTON-85: AN EVALUATION OF THE VALIDITY OF THE INERT GAS; METHOD OF KETY AND SCHMIDT

Global cerebral blood flow and metabolism during acute hyperketonemia in the awake and anesthetized rat

In the human setting, it has been shown that acute increase in the concentration of ketone bodies by infusion of beta-hydroxybutyrate increased the cerebral blood flow (CBF) without affecting the overall cerebral metabolic activity. The mechanism by which this effect of ketone bodies was mediated is not known. Alterations in several parameters may possibly explain the increase in CBF and the resetting of the relation between CBF and cerebral metabolism. To study this phenomenon further, we measured global CBF and global cerebral metabolism with the Kety-Schmidt technique in the wakeful rat before and during infusion of ketone bodies. During acute hyperketonemia (average concentration of beta-hydroxybutyrate: 6 mmol/L), global CBF increased 65% from 108 to 178 mL/100 g min and the cerebral metabolic rates for both oxygen and glucose remained constant. This resetting of the relation between CBF and cerebral metabolism could not be explained by alterations in blood pH or arterial CO2 tension. By measuring cerebral intracellular pH by 31P nuclear magnetic resonance spectroscopy, it could further be concluded that the brain pH was unchanged during acute hyperketonemia. These observations indicate that the mechanism responsible for the increase in CBF is rather a direct effect on the cerebral endothelium than via some metabolic interactions.

Perfusion-diffusion compartmental models describe cerebral helium kinetics at high and low cerebral blood flows in sheep

This study evaluated the relative importance of perfusion and diffusion mechanisms in compartmental models of blood:tissue helium exchange in the brain. Helium has different physiochemical properties from previously studied gases, and is a common diluent gas in underwater diving where decompression schedules are based on theoretical models of inert gas kinetics. Helium kinetics across the cerebrum were determined during and after 15 min of helium inhalation, at separate low and high steady states of cerebral blood flow in seven sheep under isoflurane anaesthesia. Helium concentrations in arterial and sagittal sinus venous blood were determined using gas chromatographic analysis, and sagittal sinus blood flow was monitored continuously. Parameters and model selection criteria of various perfusion-limited or perfusion-diffusion compartmental models of the brain were estimated by simultaneous fitting of the models to the sagittal sinus helium concentrations for both blood flow states. Purely perfusion-limited models fitted the data poorly. Models that allowed a diffusion-limited exchange of helium between a perfusion-limited tissue compartment and an unperfused deep compartment provided better overall fit of the data and credible parameter estimates. Fit to the data was also improved by allowing countercurrent diffusion shunt of helium between arterial and venous blood. These results suggest a role of diffusion in blood:tissue helium equilibration in brain.

The physiological kinetics of nitrogen and the prevention of decompression sickness

Decompression sickness (DCS) is a potentially crippling disease caused by intracorporeal bubble formation during or after decompression from a compressed gas underwater dive. Bubbles most commonly evolve from dissolved inert gas accumulated during the exposure to increased ambient pressure. Most diving is performed breathing air, and the inert gas of interest is nitrogen. Divers use algorithms based on nitrogen kinetic models to plan the duration and degree of exposure to increased ambient pressure and to control their ascent rate. However, even correct execution of dives planned using such algorithms often results in bubble formation and may result in DCS. This reflects the importance of idiosyncratic host factors that are difficult to model, and deficiencies in current nitrogen kinetic models. Models describing the exchange of nitrogen between tissues and blood may be based on distributed capillary units or lumped compartments, either of which may be perfusion- or diffusion-limited. However, such simplistic models are usually poor predictors of experimental nitrogen kinetics at the organ or tissue level, probably because they fail to account for factors such as heterogeneity in both tissue composition and blood perfusion and non-capillary exchange mechanisms. The modelling of safe decompression procedures is further complicated by incomplete understanding of the processes that determine bubble formation. Moreover, any formation of bubbles during decompression alters subsequent nitrogen kinetics. Although these factors mandate complex resolutions to account for the interaction between dissolved nitrogen kinetics and bubble formation and growth, most decompression schedules are based on relatively simple perfusion-limited lumped compartment models of blood: tissue nitrogen exchange. Not surprisingly, all models inevitably require empirical adjustment based on outcomes in the field. Improvements in the predictive power of decompression calculations are being achieved using probabilistic bubble models, but divers will always be subject to the possibility of developing DCS despite adherence to prescribed limits.

The Kety-Schmidt technique for repeated measurements of global cerebral blood flow and metabolism in the conscious rat

Cerebral activation will increase cerebral blood flow (CBF) and cerebral glucose uptake (CMRglc) more than it increases cerebral uptake of oxygen (CMR(O2)). To study this phenomenon, we present an application of the Kety-Schmidt technique that enables repetitive simultaneous determination of CBF, CMR(O2), CMRglc and CMRlac on awake, non-stressed animals. After constant intravenous infusion with 133Xenon, tracer infusion is terminated, and systemic arterial blood and cerebral venous blood are continuously withdrawn for 9 min. In this paper, we evaluate if the assumptions applied with the Kety-Schmidt technique are fulfilled with our application of the method. When measured twice in the same animal, the intra-individual variation for CBF, CMR(O2), and CMRglc were 10% (SD: 25%), 8% (SD: 25%), and 9% (SD: 28%), respectively. In the awake rat the values obtained for CBF, CMR(O2) and CMRglc were 106 mL [100 g](-1) min(-1), 374 micromole [100 g](-1) min(-1) and 66 micromole [100 g](-1) min(-1), respectively. The glucose taken up by the brain during wakefulness was fully accounted for by oxidation and cerebral lactate efflux. Anaesthesia with pentobarbital induced a uniform reduction of cerebral blood flow and metabolism by approximately 40%. During halothane anaesthesia CBF and CMRglc increased by approximately 50%, while CMR(O2) was unchanged.

Diffusion-limited, but not perfusion-limited, compartmental models describe cerebral nitrous oxide kinetics at high and low cerebral blood flows

This study aimed to evaluate the relative importance of diffusion-limited vs. perfusion-limited mechanisms in compartmental models of blood-tissue inert gas exchange in the brain. Nitrous oxide concentrations in arterial and brain efferent blood were determined using gas chromatographic analysis during and after 15 min of nitrous oxide inhalation, at separate low and high steady states of cerebral blood flow (CBF) in five sheep under halothane anesthesia. Parameters and model selection criteria of various perfusion- or diffusion-limited structural models of the brain were estimated by simultaneous fitting of the models to the mean observed brain effluent nitrous oxide concentration for both blood flow states. Perfusion-limited models returned precise, credible estimates of apparent brain volume but fit the low CBF data poorly. Diffusion-limited models provided better overall fit of the data, which was best described by exchange of nitrous oxide between a perfusion-limited brain compartment and an unperfused compartment. In individual animals, during the low CBF state, nitrous oxide kinetics displayed either fast, perfusion-limited behavior or slow, diffusion-limited behavior. This variability was exemplified in the different parameter estimates of the diffusion limited models fitted to the individual animal data sets. Results suggest that a diffusion limitation contributes to cerebral nitrous oxide kinetics.

Cerebral blood flow and metabolism in patients with chronic liver disease undergoing orthotopic liver transplantation

Changes in cerebral hemodynamics and metabolism associated with anesthesia and liver transplantation may present particular hazards for patients with cirrhosis. Fifteen patients undergoing liver transplantation were studied, 7 of whom had encephalopathy. Cerebral blood flow (CBF) was measured at the start of surgery, during veno-venous bypass and post reperfusion, using a method based on the Kety-Schmidt method. Cerebral metabolism was assessed by measuring the cerebral metabolic rate for oxygen (CMRO2) and the lactate oxygen index (LOI). The cerebral vascular reactivity to carbon dioxide (CO2) was studied during the preanhepatic and post reperfusion phases. During the preanhepatic period, the median CBF was 44 mL/100 g/min at an arterial carbon dioxide tension (PaCO2) of 3.8 kPa. After reperfusion the CBF increased (P < .02) to 102 mL/100 g/min, the arterial hydrogen ion concentration increased from 39 nmol/L to 53 nmol/L (P < .02) and the jugular venous oxygen saturation from 74% to 89% (P < .02). CBF was similar in patients with and without encephalopathy. The cerebral vascular reactivity to CO2 remained intact, although after reperfusion, the CBF for a given PaCO2 was greater, and the slope of the CBF/CO2 response curve diminished. The CMRO2 was normal in patients without encephalopathy. In the encephalopathic patients, the CMRO2 was low during all stages of transplantation (0.54, 0.86, 1.24 mL/100 g/min, respectively). Patients with encephalopathy may be at increased risk of hypoxemic brain injury during transplantation. To minimize this possibility, more detailed neurological monitoring may be useful.

Estimation of cerebral blood flow at bedside by continuous jugular thermodilution

The Kety-Schmidt technique can be regarded as the reference method for the measurement of cerebral blood flow (CBF). However, the method is somewhat cumbersome for routine use in the intensive care unit (ICU) at the beside. The continuous thermodilution technique developed many years ago for the measurement of coronary sinus blood flow can be applied for the measurement of jugular blood flow (JBF). However, the measurement of JBF by thermodilution has never been validated using the Kety-Schmidt reference method. We first validate the continuous thermodilution in vitro by comparison with a volumetric flow. The thermodilution method is accurate for flows between 50 and 900 ml min-1 with a mean difference volumetric-thermodilution flow of -1 +/- 18 ml min-1 (mean +/- SD), and precise with a coefficient of variability ranging between 1.21% and 2.50%. In vivo accuracy was assessed by comparing in 15 comatose patients CBF measured using the Kety-Schmidt (CBFKS) method and estimated from JBF measured by thermodilution (CBFTH) at four levels of arterial PaCO2 (25, 30, 35, and 40 mm Hg). The mean difference CBFKS-CBFTH is -0.9 +/- 3.6 ml min-1 100 g-1. In vivo precision of the method was good, with a coefficient of variability of 4.1% in mean. We conclude that jugular continuous thermodilution technique is a reliable method for estimating CBF at the bedside. This technique allows repeated measurements jugular bulb blood sampling for brain metabolic studies.

Correlation between cerebral blood flow values obtained by Xenon/CT and Kety-Schmidt (N2O) methods

The means of the cerebral blood flow (CBF) values obtained by the stable xenon enhanced CT (Xe/CT) method using two different CT scanners were compared with global CBF value obtained by the Kety-Schmidt (N2O) method as a reference. Xe/CT CBF values were obtained using a GE CT9200 (31 patients, 2 flow maps, 120 kV, absorption constant of 0.040) as well as a GE ProSeed Accell (38 patients, 4 flow maps, 80 kV, absorption constant of 0.028). The protocol of inhalation in the Xe/XT method consisted of 4 min wash-in and 4-min wash-out of 35% stable xenon. In the Kety-Schmidt method, 15% N2O gas was inhaled for 10 min. The N2O content of blood samples was measured using a van Slyke-Neill blood gas analyzer. We corrected all obtained CBF values for a PaCO2 of 34 mmHG (CBF34). The global CBF34 values obtained by the Kety-Schmidt method were linearly correlated with the CBF34 values obtained using the CT9200 and with those obtained using the ProSeed Accell, and the regression line equations were, respectively, Y = 0.64X + 13.7 (X: CT9200, Y: Kety-Schmidt, r = 0.666, p < 0.01) and Y = 0.99X + 11.2 (X: ProSeed Accell, Y: Kety Schmidt, r = 0.756, p < 0.01). Since the CBF values obtained by the Xe/CT method using different CT scanners are not always the same as the global CBF values obtained by the Kety-Schmidt method, CBF values obtained by the Xe/CT method should be corrected referring to the regression line obtained by applying both methods for each patient.

Persistent resetting of the cerebral oxygen/glucose uptake ratio by brain activation: evidence obtained with the Kety-Schmidt technique

Global cerebral blood flow (CBF), global cerebral metabolic rates for oxygen (CMRO2), and for glucose (CMRglc), and lactate efflux were measured during rest and during cerebral activation induced by the Wisconsin card sorting test. Measurements were performed in healthy volunteers using the Kety-Schmidt technique. Global CMRO2 was unchanged during cerebral activation, whereas global CBF and global CMRglc both increased by 12%, reducing the molar ratio of oxygen to glucose consumption from 6.0 during baseline conditions to 5.4 during activation. Data obtained in the period following cerebral activation showed that the activation-induced resetting of the relation between CMRglc and CMRO2 persisted virtually unaltered for > or = 40 min after the mental activation task was terminated. The activation-induced increase in cerebral lactate efflux measured over the same time period accounted for only a small fraction of the activation-induced excess glucose uptake. These data confirm earlier reports that brain activation can induce resetting of the cerebral oxygen/glucose consumption ratio, and indicate that the resetting persists for a long period after cerebral activation has been terminated and physiologic stress indicators returned to baseline values. Activation-induced resetting of the cerebral oxygen/glucose uptake ratio is not necessarily accounted for by increased lactate production from nonoxidative glucose metabolism.

Cerebral blood flow during cardiac operations: comparison of Kety-Schmidt and xenon-133 clearance methods

This study simultaneously compared the standard Kety-Schmidt and the modified xenon-133 (133Xe) clearance techniques for measuring cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2) during cardiac operations. The validity of the CBF method is important because our management of the patient during cardiopulmonary bypass (CPB) is based, in part, on our understanding of the cerebral hemodynamics during CPB. In 20 patients undergoing coronary artery bypass grafting, CBF and CMRO2 were determined by both methods. Measurements were made before onset of CPB and once during CPB. Ten patients underwent CPB with systemic normothermia (37 degrees C) and 10 with systemic hypothermia (27 degrees C). Anesthesia consisted of fentanyl and midazolam. CPB pump flows were kept at 2.2 to 2.4 L.min-1.m-2 and alpha-stat pH management was used. Xenon-133 clearance significantly underestimated CBF and CMRO2 relative to the Kety-Schmidt technique before CPB and at both bypass temperatures. Values obtained by 133Xe clearance were approximately 50% of that measured by the Kety-Schmidt method. The modified 133Xe technique as typically used during cardiac operations does not appear to measure CBF accurately; this leads to corresponding errors in CMRO2 calculations. Determination of CMRO2 and cerebral autoregulatory function during cardiac operations appears to be more appropriate if based on the more direct Kety-Schmidt technique. Accordingly, our management of CPB with respect to cerebral perfusion as it has been determined by the modified 133Xe clearance method may require reassessment.

Representation and identity - convergence of brain research and mind-brain philosophy

The localization or representation of mental abilities in the brain have always been considered as key questions for understanding the organization of the human nervous system. Particularly with the advent of modern electrophysiological and imaging techniques that provide maps of electromagnetic fields and metabolic processes on the living central nervous system, the representation theory is experiencing a scientific renaissance in neurology, but is only one theory, however, in the succession of a long philosophical tradition dealing with the possible identification of mental phenomena and brain processes. This dichotomy was formulated at the latest in the Cartesian dualism of res cogitans and res extensa of the mind-body problem. Nowadays philosophical discussion, on the contrary, is dominated by monistic concepts that attempt to explain the mental realm on an organic foundation in order not to succumb to the problem of a psychophysical dualism. Of these, the identity theory offers a philosophically plausible concept postulating that the identity of brain conditions and mental phenomena is based on organic foundations. In this theory, the efforts of brain research converge on the representations of mental phenomena in the human nervous system. In a comprehensive approach, both concepts could complement each other.

Average blood flow and oxygen uptake in the human brain during resting wakefulness: a critical appraisal of the Kety-Schmidt technique

The Kety-Schmidt technique can be regarded as the reference method for measurement of global average cerebral blood flow (average CBF) and global average cerebral metabolic rate of oxygen (average CMRO2). However, in the practical application of the method, diffusion equilibrium for inert gas tracer between the brain and its venous blood is not reached. As a consequence, normal values for CBF and CMRO2 of 54 ml 100 g-1 min-1 and 3.5 ml 100 g-1 min-1 obtained with the Kety-Schmidt technique are an overestimation of the true values. Using the Kety-Schmidt technique we have performed 57 measurements of CBF and CMRO2 during EEG-verified wakeful rest in young normal adults. In order to estimate the equilibrium values for CBF and CMRO2, a simple computer-based simulation model was employed to quantitate the systematic overestimation caused by incomplete tracer equilibrium. When correcting the measured data, we find that the true average values for CBF and CMRO2 in the healthy young adult are approximately 46 ml 100 g-1 min-1 and approximately 3.0 ml 100 g-1 min-1. Previous studies have suggested that some of the variation in CMRO2 values could be ascribed to differences in cerebral venous anatomy. However in the present study, no correlation between CMRO2 and cerebral venous anatomy as imaged by magnetic resonance angiography could be established. Our data show that the interindividual variation of CMRO2 is 11% (coefficient of variation).

A practical method of serial bedside measurement of cerebral blood flow and metabolism during neurointensive care

Acute encephalopathy is a major cause of death and neurological handicap in children. The principle aims of treatment are to provide adequate cerebral blood flow for the brain's metabolic needs and to prevent intracranial pressure rising above the level at which brain herniation occurs. Rational management requires an understanding of the pathophysiological changes in cerebral blood flow and metabolism which occur. The paucity of data on this subject reflects the perceived difficulty of measuring cerebral blood flow and cerebral metabolism in children. A modification of the Kety Schmidt technique of measuring cerebral blood flow and cerebral metabolism is described. This modification makes it possible to perform serial bedside measurements in children receiving intensive care. This method was used to perform 348 measurements in 58 children. The method was reproducible and no significant complications were encountered. The results indicated that appreciable changes in cerebral blood flow and metabolism could occur in individual patients over time, emphasising the importance of serial measurements. This technique may provide a practical means of monitoring cerebral blood flow and metabolism in very sick children receiving neurointensive care and evaluating the efficacy of treatment.

Cerebral oxygen metabolism and cerebral blood flow in man during light sleep (stage 2)

We measured cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) during light sleep (stage 2) in 8 young healthy volunteers using the Kety-Schmidt technique with 133Xe as the inert gas. Measurements were performed during wakefulness and light sleep as verified by standard polysomnography. Unlike our previous study in man showing a highly significant 25% decrease in CMRO2 during deep sleep (stage 3-4) we found a modest but statistically significant decrease of 5% in CMRO2 during stage 2 sleep. Deep and light sleep are both characterized by an almost complete lack of mental activity. They differ in respect of arousal threshold as a stronger stimulus is required to awaken a subject from deep sleep as compared to light sleep. Our results suggest that during non-rapid eye movement sleep cerebral metabolism and thereby cerebral synaptic activity is correlated to cerebral readiness rather than to mental activity.

Predominant abnormality in cerebral glucose utilization in late-onset dementia of the Alzheimer type: a cross-sectional comparison against advanced late-onset and incipient early-onset cases

Global cerebral blood flow and the cerebral metabolic rates of oxygen, CO2, glucose and lactate were studied in 11 patients aged 61-78 years who had been clinically diagnosed as suffering from incipient late-onset dementia of the Alzheimer type (DAT), and in 7 patients aged 66-83 years, in whom advanced late-onset DAT had been diagnosed, using the Kety-Schmidt technique. In incipient late-onset DAT, the predominant abnormality was a 45% reduction in cerebral glucose utilization, whereas cerebral blood flow and the cerebral metabolic rate of oxygen were diminished by only 17% and 18%, respectively. A severe imbalance between oxygen utilization and glucose utilization thus became obvious. In contrast, in advanced stages of late-onset DAT, this imbalance between oxygen and glucose utilization rates in the brain became smaller and smaller, and cerebral blood flow diminished markedly; these biological brain parameters finally all settled down at between 55% and 65% of the corresponding control values. The predominant abnormality in brain glucose utilization in incipient late-onset DAT may be associated with an impairment of its control mechanism(s), which are assumed to be either an influence of brain insulin action, or brain insulin receptor function, or both.

Reference values for resting blood flow to organs of man

The lack of a reliable quantitative description of blood flow in man has hampered the development of accurate biokinetic models of essential elements, drugs, imaging agents, and carcinogens. In this paper we review and analyse data on blood flow and identify representative percentages of cardiac output and absolute blood flow rates to organs and tissues of man for use as reference values for biokinetic models. To keep the review and analysis to a manageable size we have limited attention to the resting state and have suggested reference values for absolute and relative flow rates only for adult males and females.

Glucose metabolism as the site of the primary abnormality in early-onset dementia of Alzheimer type?

Global cerebral blood flow, oxidative brain metabolism, and the cerebral arteriovenous differences of amino acids and ammonia were studied in 20 clinically diagnosed patients with early-onset dementia of Alzheimer type (DAT). Eleven healthy age-matched subjects and 15 healthy young volunteers served as controls. The most prominent abnormality in patients with early-onset DAT was a 44% reduction in the cerebral metabolic rate of glucose and a fourfold increase of lactate production, whereas cerebral blood flow and the cerebral metabolic rate of oxygen were found not to be altered. The cerebral amino-N balance substantially changed in patients with early-onset DAT, showing a massive loss of amino acids and ammonia from the brain, which was indicative of excess protein catabolism due to cell degeneration in the acutely diseased brain. The abnormality found in glucose metabolism may suggest a perturbed control of glycolytic breakdown of glucose and its first oxidation step at the pyruvate dehydrogenase complex level, this thus being of pivotal significance in early-onset DAT.

Minimum cerebral blood flow and metabolism during craniotomy. Effect of thiopental loading

Cerebral blood flow and metabolism were measured repeatedly during surgery for cerebral tumours by a modification of the classical Kety & Schmidt method using 133Xe infusion intravenously. Our standard procedure for neuroanaesthesia (pentobarbitone-fentanyl induction, halothane-nitrous oxide maintenance) reduced blood flow from 47.1 to 24.2 ml X 100 g X ml-1 and metabolism from 3.30 to 1.83 ml O2 X 100 g-1 X ml-1. PaCO2 was reduced by hyperventilation from 5.3 to 3.6 kPa. Additional thiopental loading and maintenance using 4 + 4 mg X kg-1 (n = 5) or 8 + 8 mg X kg-1 (n = 5) reduced cerebral metabolism by an additional 15% (P less than 0.01) and blood flow by 16.5% (P less than 0.01), while mean arterial blood pressure fell from 11.0 to 9.9 kPa (P less than 0.05). PaCO2 remained unchanged. This additional reduction in cerebral metabolism and blood flow is small, but we nevertheless conclude that it may well be of clinical interest to the problem of protecting the brain in case of episodes of focal cerebral ischaemia which may arise peroperatively during intracranial surgery.

A new model of subarachnoid hemorrhage in experimental animals with the purpose to examine cerebral vasospasm

Using 20 rabbits, we tried to establish a new model of experimental subarachnoid hemorrhage (SAH) for examining both acute and chronic cerebral vasospasm. A cranial opening was drilled, and a puncture made on the posterior branch of the middle cerebral artery. A second puncture was made in the superior sagittal sinus for additional withdrawal of subarachnoid blood. The bleeding thus induced resulted in arterial spasm which was studied by using serial electrocorticograms, cerebral blood flow measurement with 133Xe, and videomicroscopy of the small pial vessels at various intervals. After death of the animals, the brains were observed to identify the extention of the bleeding. It was indeed obvious that large amounts of subarachnoid blood clots had accumulated. This investigation showed that the rabbit can be used as a new experimental model of SAH. With a two-puncture method, it is possible to simulate the clinical phenomenon of a ruptured aneurysm, that seems to produce acute and chronic cerebral vasospasm. For the latter, the accumulation of blood clots in the basal surfaces plays an important role. The three methods of observation, videomicroscopy, cerebral blood flow measurements, and electrocorticography appeared to provide useful information in the study of biphasic vasospasm in the rabbit.

Methods for assessment of the effect of drugs on cerebral blood flow in man

The cerebral circulation is not an unimportant vascular bed but it is difficult one to investigate. Methodological difficulties are the main reasons for the paucity of clinical pharmacology studies to date. Of the methods currently available those of the McHenry (1964) and Wyper et al. (1976) appear to be the most useful to the clinical investigator. Increasing use of multiple detectors even with inhalation techniques may give evidence of regional drug effects upon the cerebral circulation. Improvements in methodology in the last few years, particularly the development of atraumatic methods are likely to act as an important spur to research in this field.

Cerebral blood flow autoregulation in the rat

Cerebral blood flow autoregulation (CBFA) to changes in perfusion pressure has not been previously reported in the rat. A modification of the Kety and Schmidt technique employing 133Xenon was used to measure cerebral blood flow (CBF) in paralyzed adult Sprague Dawley rats passively ventilated with 70% nitrous oxide and 30% oxygen. At a mean arterial blood pressure (MABP) of 121 +/- 19 mm Hg, and a mean arterial PCO2 of 36.2 +/- 2.9 mm Hg, mean CBF was 103 +/- 22 ml/min/100 gm of brain. CBF responses to hypercarbia were 4.9 ml/min/100 gm per mm Hg change in arterial PCO2. CBF was measured during steady state levels of hypo- and hypertension induced by phlebotomy, or by intravenous metaraminol, over the MABP range of 48-205 mm Hg. From a MABP of 80 to 160 mm Hg. CBF remained nearly constant, indicating the presence of CBFA. However, when MABP exceeded 160 mm Hg, CBF became pressure dependent, indicating a "breakthrough" of autoregulation in acute severe hypertension.

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.

Measurement of regional myocardial blood flow in man: description and critique of the method using xenon-133 and a scintillation camera

A method has been devised to measure regional myocardial blood flow in man. The approach consists of selective injection of xenon-133 into a coronary artery and the external monitoring of radioisotope washout curves from multiple areas of the myocardium with a multiple crystal scintillation camera. Rate constants of isotope washout are calculated using a monoexponential model, and the capillary blood flow rates in multiple regions of the heart are calculated by the Kety formula. The pattern of perfusion rates is related to the coronary arteriogram obtained in the same study. Myocardial perfusion patterns obtained in patients with and without coronary artery disease and during atrial pacing are given, as well as examples of results obtained in myocardial aneurysms, in regions of ischemic myocardium supplied by collateral vessels and after tracer injection into an aortocoronary bypass graft. Advantages of the method are discussed along with limitations related to the tracer, the scintillation camera, the form of mathematical analysis and the three dimensional nature of the heart.

Measurement of regional cerebral blood flow with 133xenon and a multiple-crystal scintillation camera

A method was devised to measure gray and white matter cerebral blood flow simultaneously in multiple regions of the brain using 133 Xenon and a multiple-crystal scintillation camera. Following magnification cerebral arteriography, 133 Xe was injected into the internal carotid artery and the washout of tracer was monitored with a scintillation camera which consists of 294 individually collimated NaI (TI) crystals. These data, obtained from each crystal overlying the brain, were processed by a weighted least-squares nonlinear regression technique. The blood flow rates of the rapid and slow compartments were calculated by the Kety-Schmidt formula along with 95% confidence limits for each measurement.

In four patients, local increases or decreases in regional cerebral flow were correlated with areas of pathology. In one patient with a cerebral arteriovenous malformation, regions of local shunting of tracer were identified. Application of a three-compartment analysis to these curves permitted estimation of the magnitude of shunting along with gray and white matter flow in the lesion. The increased discrimination provided by the multiple-crystal camera and the estimates of measurement accuracy obtained by this mathematical analysis may facilitate more precise localization of regional blood flow abnormalities in intracranial disease.

Cerebrovascular regulation in preganglionic and postganglionic autonomic insufficiency

Cerebral blood flow was measured in four subjects with chronic idiopathic autonomic insufficiency using 85 Kr inhalation, cerebral A-V samples for scintillation counting, and O 2 content determination. Blood pressure was varied by up or down tilting and L-norepinephrine infusion. In three patients with typical Shy-Drager syndrome and preganglionic denervation, CBF regulation to pressure and Pa CO CO 2 change was intact. In a fourth patient with postganglionic denervation, CBF autoregulation was absent to changes in blood pressure but was preserved normally to increases or decreases in Pa CO CO 2 CBF and CMRO 2 , at rest were normal (52.6 cc and 3.1 cc). Hyperventilation to Pa CO CO 2 34.3 mm failed to restore autoregulation to increased blood pressure. The results imply a functional role for postganglionic autonomic fibers in CBF autoregulation.

Cerebral blood flow following induced subarachnoid hemorrhage in the monkey

✓ The acute effects of experimental subarachnoid hemorrhage on cerebral blood flow were investigated in 14 adult rhesus monkeys injected with fresh autogenous blood through a needle positioned within the subfrontal subarachnoid space. Cerebral blood flow was measured by the xenon133 tissue clearance method before hemorrhage, and afterward at 30-minute intervals for a 3-hour period. Post-anesthetic neurological status was graded according to Botterell's classification. Twelve monkeys showed a significant decrease in cerebral perfusion, eight displayed focal neurological deficits, and four were moribund. There was a correlation between the degree of impaired circulation and the severity of neurological deficit. Four additional monkeys subjected to subarachnoid acidic saline injection showed no reduction in cerebral blood flow. In three animals cerebral perfusion was increased during the first hour after injection. It is suggested that measurement of cerebral blood flow may be a more valuable prognostic indication of cerebral function and survival than the angiographic demonstration of arterial vasospasm.

Regional cerebral blood flow in man determined by the initial slope of the clearance of intra-arterially injected 133Xe

The regional cerebral blood flow can be calculated from the initial slope of the logarithmically displayed clearance curve following intra-arterial injection of 133 Xe (rCBF initial ). The relationship between this parameter and the values resulting from stochastic (height over area) and compartmental analyses is extensively discussed. Experimental results demonstrate the theoretically expected close relationship between rCBF initial and flow of gray substance (rCBF initial 20% to 30% lower than F g ). It is shown how the cerebral clearance curve (normally biexponential) with low flow values becomes gradually monoexponential. Thus only flow of gray substance changes, whereas flow of white substance is independent of CBF ∞ . CBF 10 was shown to overestimate CBF ∞ with about 15% independent of the flow level. Correlation between CBF initial and CBF 10 was linear (r=0.98) at CBF 10 values above 20 ml/100 gm/min.

The CBF initial normal value is found to be 64±9 ml/100 gm/min, and the interchannel coefficient of variation is 8.2%. A correction for remaining radioactivity from previous measurements is described. Using this, no significant difference was found between repeated resting state measurements.

The CBF initial -Paco 2 relationship was found to be best described as exponential. In a group of patients with various intracranial diseases, 1 mm Hg change in Paco 2 resulted in 4% change of CBF initial quite independent of the CBF initial level.

The comparative analysis of isotope clearance curves in normal and ischemic brain

The regional cerebral blood flow has been measured by intracarotid 133 Xenon in ten normal subjects, ten with miscellaneous cerebral ischemic lesions, eight with transient ischemic attacks and seven with completed strokes. The data have been examined by the stochastic and two-compartmental methods of analysis and by reference to the slope of the initial two minutes of the clearance curves. In normal unanesthetized normocapnic subjects the correlation between the results obtained by each of these methods is high. In ischemic lesions the correlation is less god. All three methods, however, are liable to miss, or to misrepresent the significance of, disturbances which can be demonstrated when the proportion and rate of perfusion of gray and white matter are separately estimated.

Visual assessment of regional myocardial perfusion utilizing radioactive xenon and scintillation photography

A method was devised to visualize the areas of left ventricle supplied by various coronary arterial branches utilizing scintillation photography and an inert radioactive gas which distributes instantaneously between coronary blood and perfused myocardial cells. From 1 to 5 millicuries of 133 Xe dissolved in saline solution was injected through a catheter into the anterior descending or posterior circumflex branch of the left coronary artery of 15 dogs. Scintillation images produced during arrival and washout of 133 Xe in the various regions of the heart were recorded by an image intensifier scintillation camera and high speed television monitor and were reproduced on Polaroid film during replay of the videotape on an oscilloscope. The resulting scintiphotographs defined the region of the left ventricle supplied by the coronary arterial branch. In nine of the dogs acute myocardial infarctions were produced by occluding one or the other branch of the left coronary artery. When 133 Xe was injected again into the coronary artery proximal to the occlusion, that area of the left ventricle that was deprived of nutrient blood flow was no longer visualized on the gamma-ray scintiphotograph. The results indicate that regional myocardial perfusion may be dynamically visualized in the intact animal.

Estimation of coronary blood flow by washout of diffusible indicators

In 13 canine hearts, 158 disappearance curves for ¹³³Xe and antipyrine-¹²⁵I, given by intra-arterial slug injection, were recorded at a wide range of perfusion rates. Flow rates (ml/100 g/min) calculated from these curves by a variety of methods were compared with measured flow rates (F_a) per weight of perfused tissue. Perfusion of isolated, supported hearts and of anterior descending coronary arteries in open-chest dogs provided similar data. The semilogarithmic slope of curves from apex or whole heart decreased with time, particularly at high flow rates. There was a small, consistent difference in shape between antipyrine and xenon curves, suggesting that radioactivity in fat contributed somewhat to this tailing. Estimation of flow rate from the steepest semilog slope yielded an average value of 1.1F_a for all rates; estimation from slope at 30% of peak radioactivity gave 0.9F_a. The curves were closely described by a two-exponential equation which gave flow estimates of 0.95F_a when collimation limited the observations to the heart apex, and lower values when the whole heart was observed. Peak height/area methods gave values of approximately 0.75F_a in spite of various compensations for the impossibility of recording the curve until radioactivity = 0.

Limitation of the use of inert gases in the measurement of small gut mucosal blood flow

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Human cerebral blood flow measured by two inert gas techniques. Comparison of the Kety-Schmidt method and the intra-arterial injection method

Two inert gas methods for measurement of tissue blood flow were compared. The two methods are (1) Kety and Schmidt's inert gas saturation method, in which the tissue saturation is followed by detecting the concentration of the tracer in the venous blood leaving the organ and (2) the intraarterial radioactive inert gas injection method where the tissue washout is measured by detecting the residue inside the tissue by an external scintillation detector.

Theoretical considerations show a close relation between the two methods when carried to the time of full saturation or desaturation of the tissue or when stopped before full saturation.

Experimental studies of cerebral blood flow in nine human subjects using Kr 85 revealed a close correlation between the two methods without any systematic difference between the two sets of data.

The Kety-Schmidt outflow detection method is the method of choice for studying over-all tissue perfusion and metabolism. The residue detection method is best suited for measuring flow through circumscribed regions inside an organ and for resolving the flow curve to yield estimates of flow and relative weight of various tissue components.

Cerebral blood flow in man at high altitude. Role of cerebrospinal fluid pH in normalization of flow in chronic hypocapnia

Cerebral blood flow was determined by an N 2 O method in 7 normal men at sea level and after 6 to 12 hr and 3 to 5 days at 3810 m altitude. An infrared N 2 O analyzer was used both to measure end-tidal PN 2 O so that it could be kept constant for 15 min and to determine blood N 2 O, for which a simple gas extraction method was devised. In addition, acute changes in cerebral blood flow were estimated from cerebral A-V O 2 differences. Control cerebral blood flow was 43 ml per 100 g per min; it increased 24% at 6 to 12 hours and 13% at 3 to 5 days at altitude. After 3 to 5 days, pH of cerebrospinal fluid was normal (7.31) in four subjects while arterial blood pH was alkaline (7.47); arterial blood Pco 2 had fallen from 41 to 30 mm Hg. Acute correction of hypoxia restored cerebral blood flow to control while mean Pco 2 was still 31 mm Hg. Addition of O 2 and CO 2 to inspired air raised cerebral blood flow 34% above control at Pao 2 = 170, Paco 2 = 35 mm Hg. Values obtained by extrapolation suggest that if arterial Pco 2 was raised to control (41 mm Hg), cerebral blood flow would have been 60% above control. Cerebral blood flow thus appears to return to normal at the prevailing Paco 2 , probably because the pH of cerebrospinal fluid and of the extracellular fluid of cerebral vascular smooth muscle is kept normal by active transport across the ‘blood-brain’ barrier. It is postulated that an ion-impermeable barrier separates the blood stream from extracellular fluid of the smooth muscle of cerebral arterioles.

Regional cerebral blood flow in man determined by intra-arterial injection of radioactive inert gas

The intra-arterial injection of a radioactive inert gas is described for measurement of regional cerebral blood flow in man from externally recorded clearance curves.

The stochastic analysis for calculating the average cerebral blood flow in ml/100 g/min is compared to an analysis of the curve into two compartments with fast and slow clearance rates representing flow in grey and white cerebral matter respectively. An estimate of the relative weight of the grey matter in the cerebral region counted from brain is also obtained.

A detailed description is given of the experimental procedure using krypton 85 or xenon 133 as the tracer gas. A brief injection time and short time constant in the recording ratemeter are important for obtaining a correct estimate of the maximal height of the curve. Mono-exponential extrapolation beyond the time of observation (usually 15 minutes) is employed. The practical procedure for calculating the results according to the stochastic and compartmental models are described with particular emphasis on the simple "10 minutes-average-flow-value, " CBF (10) which corresponds directly to the results of the classical Kety-Schmidt inert gas inhalation method for measurement of the average blood flow of the whole brain.