THE CEREBRAL BLOOD FLOW IN MALE SUBJECTS AS MEASURED BY THE NITROUS OXIDE TECHNIQUE. NORMAL VALUES FOR BLOOD FLOW, OXYGEN UTILIZATION, GLUCOSE UTILIZATION, AND PERIPHERAL RESISTANCE, WITH OBSERVATIONS ON THE EFFECT OF TILTING AND ANXIETY

Syncope, cerebral perfusion, and oxygenation

During standing, both the position of the cerebral circulation and the reductions in mean arterial pressure (MAP) and cardiac output challenge cerebral autoregulatory (CA) mechanisms. Syncope is most often associated with the upright position and can be provoked by any condition that jeopardizes cerebral blood flow (CBF) and regional cerebral tissue oxygenation (cO(2)Hb). Reflex (vasovagal) responses, cardiac arrhythmias, and autonomic failure are common causes. An important defense against a critical reduction in the central blood volume is that of muscle activity ("the muscle pump"), and if it is not applied even normal humans faint. Continuous tracking of CBF by transcranial Doppler-determined cerebral blood velocity (V(mean)) and near-infrared spectroscopy-determined cO(2)Hb contribute to understanding the cerebrovascular adjustments to postural stress; e.g., MAP does not necessarily reflect the cerebrovascular phenomena associated with (pre)syncope. CA may be interpreted as a frequency-dependent phenomenon with attenuated transfer of oscillations in MAP to V(mean) at low frequencies. The clinical implication is that CA does not respond to rapid changes in MAP; e.g., there is a transient fall in V(mean) on standing up and therefore a feeling of lightheadedness that even healthy humans sometimes experience. In subjects with recurrent vasovagal syncope, dynamic CA seems not different from that of healthy controls even during the last minutes before the syncope. Redistribution of cardiac output may affect cerebral perfusion by increased cerebral vascular resistance, supporting the view that cerebral perfusion depends on arterial inflow pressure provided that there is a sufficient cardiac output.

Pathophysiology of cancer cachexia

Cancer cachexia is a frequent complication observed in patients with malignant tumors. Although several decades have passed since the first focus on the metabolic dysfunction's associated with cancer, few effective therapeutic interventions have been successfully introduced into the medical armamentarium. The present study thoroughly reviews the basic pathophysiology of cancer cachexia and the treatment options already investigated in that field. Experimental and clinical studies were evaluated individually in order to clarify the intricate alterations observed in tumor-bearing patients. The difficulties in introducing sound and effective nutritional support or metabolic manipulation to reverse cancer cachexia are outlined in this review.

Cerebral blood flow and metabolism during exercise

During exercise regional cerebral blood flow (rCBF), as blood velocity in major cerebral arteries and also blood flow in the internal carotid artery increase, suggesting an increase in blood flow to a large part of the brain. Such an increase in CBF is independent of the concomitant increase in blood pressure but is modified by the alteration in arterial carbon dioxide tension (PaCO(2)). Also, the increase in middle cerebral artery mean blood velocity (MCA V(mean)) reported with exercise appears to depend on the ability to increase cardiac output (CO), as demonstrated in response to beta-1 blockade and in patients with cardiac insufficiency or atrial fibrillation.Near-infrared spectroscopy (NIRS) determined cerebral oxygenation supports the alterations in MCA V(mean) during exercise. Equally, the observation that the cerebrovascular CO(2)-reactivity appears to be smaller in the standing than in the sitting and especially in the supine position could relate to the progressively smaller CO. In contrast, during exercise "global" cerebral blood flow (gCBF), as determined by the Kety-Schmidt technique is regarded as being constant. One limitation of the Kety-Schmidt method for measuring CBF is that blood flow in the two internal jugular veins depends on the origin of drainage and it has not been defined which internal jugular venous flow is evaluated. Such a consideration is equally relevant for an evaluation of cerebral metabolism during exercise. While the regional cerebral uptake of oxygen (O(2)) increases during exercise, the global value is regarded as being constant. Yet, during high intensity exercise lactate is taken up by the brain and its O(2) uptake also increases. Furthermore, in the initial minutes of recovery immediately following exercise, brain glucose and O(2) uptake are elevated and lactate uptake remains high.A maintained substrate uptake by the brain after exercise suggests a role for brain glycogen in cerebral activation, but the fate of brain substrate uptake has not yet been determined.

Is cerebral perfusion pressure a major determinant of cerebral blood flow during head elevation in comatose patients with severe intracranial lesions?

OBJECT

Head elevation as a treatment for lower intracranial pressure (ICP) in patients with intracranial hypertension has been challenged in recent years. Therefore, the authors studied the effect of head position on cerebral hemodynamics in patients with severe head injury.

METHODS

The effect of 0 degrees, 15 degrees, 30 degrees, and 45 degrees head elevation on ICP, cerebral blood flow (CBF), systemic arterial (PsaMonro) and jugular bulb (Pj) pressures calibrated to the level of the foramen of Monro, cerebral perfusion pressure (CPP), and the arteriovenous pressure gradient (PsaMonro - Pj) was studied in 37 patients who were comatose due to severe intracranial lesions. The CBF decreased gradually with head elevation from 0 to 45 degrees, from 46.3+/-4.8 to 28.7+/-2.3 ml x min(-1) x 100 g(-1) (mean +/- standard error, p<0.01), and the PsaMonro - Pj from 80+/-3 to 73+/-3 mm Hg (p< 0.01). The CPP remained stable between 0 degrees and 30 degrees of head elevation, at 62+/-3 mm Hg, and decreased from 62+/-3 to 57+/-4 mm Hg between 30 degrees and 45 degrees (p<0.05). A simulation showed that the 38% decrease in CBF between 0 degrees and 45 degrees resulted from PsaMonro - Pj changes for 19% of the decrease, from a diversion of the venous drainage from the internal jugular veins to vertebral venous plexus for 15%, and from CPP changes for 4%.

CONCLUSIONS

During head elevation the arteriovenous pressure gradient is the major determinant of CBF. The influence of CPP on CBF decreases from 0 to 45 degrees of head elevation.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

Cerebral blood flow and oxygen metabolism during cardiopulmonary bypass with moderate hypothermic selective cerebral perfusion

Cerebral blood flow was measured using transcranial doppler during cardiopulmonary bypass in nine patients with selective cerebral perfusion for surgery of arch aorta (group S). For comparison, 11 adult open heart patients (group C) were also measured. The authors' selective cerebral perfusion at 28 degrees C resulted in moderate hypothermia and antegrade perfusion using independent pumps for three branches. Total flow in the three branches was 500 ml/min. A Labodop DP-100 doppler ultrasound velocimeter was used to measure middle cerebral arterial blood flow velocity. Hemoglobin concentration and oxygen saturation were also measured in arterial and jugular venous blood. The arteriovenous oxygen content difference (Ca-vO2) was calculated and multiplied by the middle cerebral arterial blood flow velocity value, which resulted in the cerebral metabolic rate for oxygen (CMRO2). The cerebral perfusion pressure of group S was lower than in group C, and the arterial carbon-dioxide tension (PaCO2) of group S was higher than in group C during cardiopulmonary bypass. Middle cerebral arterial blood flow velocity values of both groups remained constant before, during and after cardiopulmonary bypass. The CMRO2 decreased during cardiopulmonary bypass and showed no difference between the two groups. The changes in PaCO2 might be significant factors in the increase in cerebral blood flow during selective cerebral perfusion. This study supports the conclusion that, compared with our routine open heart surgery procedures, our selective cerebral perfusion procedures had the same cerebral blood flow and oxygen metabolism during cardiopulmonary bypass.

Calculation of the FDG lumped constant by simultaneous measurements of global glucose and FDG metabolism in humans

The lumped constant defined as the conversion factor between the net uptake of fluoro-2-deoxy-D-glucose (FDG) and glucose was calculated from global CMRglc and from positron emission tomography (PET) using FDG as tracer (CMRFDG). Fifteen healthy, normal volunteers (mean age 24 +/- 4 years) were studied. Global CBF and CMRglc were measured with the Kety-Schmidt technique using 133Xe as tracer, and values were corrected for errors from incomplete diffusion equilibrium for inert gas tracer between brain tissue and cerebral venous blood. Measurements of CMRFDG were obtained with PET using the dynamic and single-scan methods and the K1-k3 model. Measurements with the Kety-Schmidt technique and PET-FDG were performed simultaneously. Global CBF was 47.1 +/- 8.0 mL.100 g-1.min-1, and CMRglc was 22.8 +/- 4.1 mumol.100 g-1.min-1. No difference in CMRFDG was found with the two methods (17.8 +/- 1.6 and 18.2 +/- 1.3 mumol .100 g-1.min-1, dynamic and single scan methods, respectively). Accordingly, the lumped constant ranged from 0.80 +/- 0.16 to 0.82 +/- 0.15, with a mean value of 0.81 +/- 0.15. The mean ratio between phosphorylation of FDG and glucose (k3*/k3) was 0.39 +/- 0.25. The discrepancy between the lumped constant determined in this study and previously obtained values can be explained partly by methodologic problems, and we conclude that most of the discrepancy results from previous overestimation of global CBF.

Orthostatic dysregulation in progressive autonomic failure--a transcranial Doppler sonography monitoring

Hemodynamic changes associated with orthostatic hypotension in one patient with pure progressive autonomic failure (PAF) were studied by a passive (70 degrees tilt of the upper part of the body) and an active orthostatic tests. Mean blood pressure (MBP), heart rate (HR) and mean blood flow velocity (MFV) during transcranial Doppler sonography monitoring of the right middle cerebral artery (RMCA) were determined after 10 minutes of rest and after 1, 10 and 60 minutes passive 70 degrees tilt or active standing. Simultaneously, plasma norepinephrine (NE) levels during recumbency and after 1 and 10 min of the orthostatic manoeuvres were measured. Stand-up tilting induced slight decrease in MBP and MFV without changes in HR. Changes of systemic hemodynamics occurred during the first minute of passive standing and they increased within the first hour however the cerebral hemodynamics remained relatively stable. Active standing was accompanied by a severe decrease in the MBP and the MFV in RMCA, and an increase in vascular resistance immediately after the upright position. The hemodynamic changes were not followed by a secondary cardiac acceleration; they increased within the first minute of active standing and evoked a syncope. During squatting as a self-selected preventive mechanism in our patient an increase in MBP and MFV in RMCA occurred. Plasma NE levels in recumbency were lower than the reference values; they decreased with 12.1% after 10 min of passive tilting and with 24.8% after the first minute of active standing. These results showed that PAF is accompanied by a severe orthostatic dysregulation during active standing, associated with a progressive peripheral autonomic deficiency and disturbed mechanisms against gravitational pooling of the blood to the lower extremities. The orthostatic autoregulation of the cerebral hemodynamics seems to be preserved.

The use of hyperventilation and its impact on cerebral ischemia in the treatment of traumatic brain injury

Traumatic brain injury is a common occurrence in the United States, leading to approximately 190,000 deaths or long-term disabilities. Following the primary insult, secondary disturbances in cerebral blood flow (CBF) and metabolism may have deleterious effects on potentially viable neurons. Recent studies evaluating CBF immediately following head injury have revealed flows low enough to produce cerebral ischemia. Hyperventilation is used routinely to lower suspected increased intracranial pressure (ICP). Aggressive hyperventilation produces a marked reduction in CBF, which may give rise to or exacerbate cerebral ischemia, thus enhancing rather than reducing secondary injury. This article reviews the role of hyperventilation in the treatment of increased ICP and its impact on cerebral ischemia following traumatic brain injury.

Role of insulin resistance in the pathogenesis of NIDDM

The above discussion illustrating the multitude of variables which influence insulin sensitivity in normal subjects challenges the prevailing view that insulin sensitivity is genetically determined in patients with NIDDM. The lack of accurate quantitation of all determinants of insulin sensitivity in the cross-sectional studies, and the difficulty in distinguishing between insulin secretion and sensitivity in prospective studies implies that the inherited metabolic abnormality in NIDDM still remains to be defined. The methodological difficulties in assessing the fate of glucose in many insulin-resistant states raise the possibility that defects in glycogen synthesis may not be rate-limiting for insulin action. It seems more likely that defects in glucose transport or phosphorylation are rate-limiting for glucose disposal, and thus represent either the primary regulatory steps or the steps via which distal defects signal their influence on glucose uptake. The above considerations should not be interpreted to suggest that insulin resistance is unimportant in the pathogenesis of NIDDM. It clearly increases the risk of developing NIDDM, and more importantly, its early amelioration by lifestyle modification seems sufficient to prevent NIDDM.

Regional cerebral oxygenation during cardiopulmonary bypass

A significant number of patients suffer transient neuropsychological dysfunction after coronary artery bypass graft (CABG) surgery. Recent studies have implicated reduced levels of O2 supply/demand ratio during the rewarming phase of cardiopulmonary bypass (CPB). Using a near infrared spectroscopy (NIRS) system that permits continuous noninvasive monitoring of regional cerebral O2 saturation (rSO2), we investigated rSO2 during CPB. Following Institution Review Board approval and informed consent, 10 patients undergoing elective CABG were studied. Nonpulsatile CPB with a membrane oxygenator, haemodilution and alpha-state pH management was used. The NIRS system consisting of two low-power laser diode sources (780 nm and 810 nm) with a photodetector placed on the frontal cortex was used to measure continuously haemoglobin, oxyhaemoglobin and cerebral blood volume. Continuous rSO2 measurements were obtained before, during moderate hypothermia, and during the rewarming phase of CPB. A rSO2 < 50% was defined as abnormal. Onset of rSO2 < 50% was at 32 degrees C, and the highest incidence occurred during the late stages of rewarming (35-37 degrees C). On rewarming from hypothermic CPB, 70% of our patients sustained a rSO2 < 50% for an average total duration of 9.3 minutes, primarily at the end of the rewarming period. This suggests a transient global imbalance in cerebral O2 supply and demand in a large percentage of patients during the rewarming phase of CPB. However, it has not been determined how long a rSO2 < 50% can be permitted before neuropsychological dysfunction is induced. Additional studies correlating psychometric testing with rSO2 measurements should prove useful in detecting episodes of impaired cerebral oxygenation, and help define rewarming protocols.

Cerebral blood flow and metabolism in children with severe head injuries. Part 2: Cerebrovascular resistance and its determinants

It has been proposed that in children with severe head injuries the cerebral circulation does not respond appropriately to normal physiological control mechanisms, making children more susceptible than adults to low cerebrovascular resistance, increased cerebral blood flow (cerebral hyperaemia), and raised intracranial pressure. To investigate this issue, 122 serial measurements of cerebrovascular resistance in 17 children with severe head injuries have been performed and related to cerebral perfusion pressure, arterial CO2 (PaCO2), arterial oxygen content (AO2), and the cerebral metabolic rate of oxygen (CMRO2). Cerebrovascular resistance values (mean (SD) 1.54 (0.61) mm Hg.ml-1.100 g.min) were normal or raised in most cases; 71 values (58%) were within the normal range, 39 (32%) above the upper limit, and only 12 (10%) below the lower limit. There was a significant correlation between cerebral perfusion pressure and cerebrovascular resistance (r = 0.32, p = 0.0003), suggesting preservation of pressure autoregulation. This correlation was absent in four of the five children who died or survived with severe handicap. Analysis by multilevel modelling indicated that, as in normal subjects, CMRO2, CPP, AO2, PaCO2, and cerebrovenous pH were important independent determinants of cerebrovascular resistance. The results indicate that normal cerebrovascular reactivity is often preserved in children with severe head injuries but may be impaired in the most severely injured patients.

Cerebral versus systemic hemodynamics during graded orthostatic stress in humans

BACKGROUND

Orthostatic syncope is usually attributed to cerebral hypoperfusion secondary to systemic hemodynamic collapse. Recent research in patients with neurocardiogenic syncope has suggested that cerebral vasoconstriction may occur during orthostatic hypotension, compromising cerebral autoregulation and possibly contributing to the loss of consciousness. However, the regulation of cerebral blood flow (CBF) in such patients may be quite different from that of healthy individuals, particularly when assessed during the rapidly changing hemodynamic conditions associated with neurocardiogenic syncope. To be able to interpret the pathophysiological significance of these observations, a clear understanding of the normal responses of the cerebral circulation to orthostatic stress must be obtained, particularly in the context of the known changes in systemic and regional distributions of blood flow and vascular resistance during orthostasis. Therefore, the specific aim of this study was to examine the changes that occur in the cerebral circulation during graded reductions in central blood volume in the absence of systemic hypotension in healthy humans. We hypothesized that cerebral vasoconstriction would occur and CBF would decrease due to activation of the sympathetic nervous system. We further hypothesized, however, that the magnitude of this change would be small compared with changes in systemic or skeletal muscle vascular resistance in healthy subjects with intact autoregulation and would be unlikely to cause syncope without concomitant hypotension.

METHODS AND RESULTS

To test this hypothesis, we studied 13 healthy men (age, 27 +/- 7 years) during progressive lower body negative pressure (LBNP). We measured systemic flow (Qc is cardiac output; C2H2 rebreathing), regional forearm flow (FBF; venous occlusion plethysmography), and blood pressure (BP; Finapres) and calculated systemic (SVR) and forearm (FVR) vascular resistances. Changes in brain blood flow were estimated from changes in the blood flow velocity in the middle cerebral artery (VMCA) using transcranial Doppler. Pulsatility (systolic minus diastolic/mean velocity) normalized for systemic arterial pressure pulsatility was used as an index of distal cerebral vascular resistance. End-tidal PACO2 was closely monitored during LBNP. From rest to maximal LBNP before the onset of symptoms or systemic hypotension, Qc and FBF decreased by 29.9% and 34.4%, respectively. VMCA decreased less, by 15.5% consistent with a smaller decrease in CBF. Similarly, SVR and FVR increased by 62.8% and 69.8%, respectively, whereas pulsatility increased by 17.2%, suggestive of a mild degree of small-vessel cerebral vasoconstriction. Seven of 13 subjects had presyncope during LBNP, all associated with a sudden drop in BP (29 +/- 9%). By comparison, hyperventilation alone caused greater changes in VMCA (42 +/- 2%) and pulsatility but never caused presyncope. In a separate group of 3 subjects, superimposition of hyperventilation during highlevel LBNP caused a further decrease in VMCA (31 +/- 7%) but no change in BP or level of consciousness.

CONCLUSIONS

We conclude that cerebral vasoconstriction occurs in healthy humans during graded reductions in central blood volume caused by LBNP. However, the magnitude of this response is small compared with changes in SVR or FVR during LBNP or other stimuli known to induce cerebral vasoconstriction (hypocapnia). We speculate that this degree of cerebral vasoconstriction is not by itself sufficient to cause syncope during orthostatic stress. However, it may exacerbate the decrease in CBF associated with hypotension if hemodynamic instability develops.

Effect of head up tilt on cerebral circulation

This study was performed to study the effect of the head up tilt (HUT) on cerebral circulation across the time course (60 degrees HUT for 15 minutes) and across the different angles of HUT (15, 30, 45, 60 degrees HUT for 15 minutes). Cerebral circulation was continuously monitored during 15 minutes of HUT by the carotid Doppler flow meter, the transcranial Doppler flow meter, and the near infra-red spectrophotometer. The results show that the cerebral blood flow decreased during HUT and that the cerebral blood volume decreased initially and then gradually increased. And the magnitude of the effect may have the relationship with the angles of the HUT.

Action of insulin on glucose metabolism in vivo

Insulin plays a key role in the maintenance of normal glucose tolerance by suppressing endogenous glucose production during a meal. Insulin is not, however, involved in the regulation of splanchnic glucose uptake. The latter process appears, based on studies performed in dogs, to be regulated primarily by the arterial-portal glucose gradient and to a smaller extent by glucose mass-action. Regarding peripheral glucose utilization, insulin is not needed to maintain a normal rate of glucose utilization since this can also be achieved by hyperglycaemia and glucose mass-action. Insulin is, however, necessary for the maintenance of normal rates of glucose oxidation and storage in insulin-sensitive tissues, and for the prevention of excessive gluconeogenic substrate production.

Characterization of cellular defects of insulin action in type 2 (non-insulin-dependent) diabetes mellitus

Seven non-insulin-dependent diabetes mellitus (NIDDM) patients participated in three clamp studies performed with [3-3H]- and [U-14C]glucose and indirect calorimetry: study I, euglycemic (5.2 +/- 0.1 mM) insulin (269 +/- 39 pM) clamp; study II, hyperglycemic (14.9 +/- 1.2 mM) insulin (259 +/- 19 pM) clamp; study III, euglycemic (5.5 +/- 0.3 mM) hyperinsulinemic (1650 +/- 529 pM) clamp. Seven control subjects received a euglycemic (5.1 +/- 0.2 mM) insulin (258 +/- 24 pM) clamp. Glycolysis and glucose oxidation were quantitated from the rate of appearance of 3H2O and 14CO2; glycogen synthesis was calculated as the difference between body glucose disposal and glycolysis. In study I, glucose uptake was decreased by 54% in NIDDM vs. controls. Glycolysis, glycogen synthesis, and glucose oxidation were reduced in NIDDM patients (P < 0.05-0.001). Nonoxidative glycolysis and lipid oxidation were higher. In studies II and III, glucose uptake in NIDDM was equal to controls (40.7 +/- 2.1 and 40.7 +/- 1.7 mumol/min.kg fat-free mass, respectively). In study II, glycolysis, but not glucose oxidation, was normal (P < 0.01 vs. controls). Nonoxidative glycolysis remained higher (P < 0.05). Glycogen deposition increased (P < 0.05 vs. study I), and lipid oxidation remained higher (P < 0.01). In study III, hyperinsulinemia normalized glycogen formation, glycolysis, and lipid oxidation but did not normalize the elevated nonoxidative glycolysis or the decreased glucose oxidation. Lipid oxidation and glycolysis (r = -0.65; P < 0.01), and glucose oxidation (r = -0.75; P < 0.01) were inversely correlated. In conclusion, in NIDDM: (a) insulin resistance involves glycolysis, glycogen synthesis, and glucose oxidation; (b) hyperglycemia and hyperinsulinemia can normalize total body glucose uptake; (c) marked hyperinsulinemia normalizes glycogen synthesis and total flux through glycolysis, but does not restore a normal distribution between oxidation and nonoxidative glycolysis; (d) hyperglycemia cannot overcome the defects in glucose oxidation and nonoxidative glycolysis; (e) lipid oxidation is elevated and is suppressed only with hyperinsulinemia.

The cerebral hemodynamics of normotensive hypovolemia during lower-body negative pressure

Although severe hypovolemia can lead to hypotension and neurological decline, many patients with neurosurgical disorders experience a significant hypovolemia while autonomic compensatory mechanisms maintain a normal blood pressure. To assess the effects of normotensive hypovolemia upon cerebral hemodynamics, transcranial Doppler ultrasound monitoring of 13 healthy volunteers was performed during graded lower-body negative pressure of up to -50 mm Hg, an accepted laboratory model for reproducing the physiological effects of hypovolemia. Middle cerebral artery flow velocity declined by 16% +/- 4% (mean +/- standard error of the mean) and the ratio between transcranial Doppler ultrasound pulsatility and systemic pulsatility rose 22% +/- 8%, suggesting cerebral small-vessel vasoconstriction in response to the sympathetic activation unmasked by lower-body negative pressure. This vasoconstriction may interfere with the autoregulatory response to a sudden fall in blood pressure, and may explain the common observation of neurological deficit during hypovolemia even with a normal blood pressure.

Presyncope caused by central hypovolaemia is not preceded by evoked potential alterations

The mechanism(s) responsible for the onset of presyncope during a central hypovolaemic challenge have gone undefined for many years. It has been speculated that a decrease in cerebral blood flow initiates presyncopal responses, which in turn lead to greater decreases in cerebral oxygen delivery and unconsciousness. Somatosensory evoked potentials (SEP) were monitored as a measure of cerebral functioning in ten subjects during presyncopal symptom limiting lower body negative pressure (a central hypovolaemic challenge). SEP latency and amplitudes have been correlated with cerebral oxygen uptake, so SEP activity can serve as an indirect indicator of cerebral homeostasis. SEPs were generated by electrically stimulating the median nerve and recoding the resulting potentials over the contralateral cerebral cortex. While heart rate and mean blood pressure both fell at presyncope, there were no changes noted in either SEP latency or amplitude at any point before (latency = 22.9 +/- 9 ms; amplitude = 2.86 +/- 0.24 microV), during (22.6 +/- 0.9 ms; 2.68 +/- 0.2 microV), or after (22.7 +/- 0.9 ms; 2.37 +/- 0.23 microV) the occurrence of presyncope. We conclude that the onset of presyncope is not associated with a decrease in cerebral function.

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.

Blood-brain transfer of L-phenylalanine declines after peripheral but not central nervous administration of vasopressin

To determine whether a previously reported effect of vasopressin on blood-brain transfer of leucine extends to other large neutral amino acids, we measured the regional blood-brain transfer of L-phenylalanine with the integral technique. Intravenous co-injection of L-phenylalanine and arginine vasopressin (30 nmol to 10 pmol) resulted in a decrease of the permeability-surface area (PaS) product of phenylalanine of between 11 and 39%. In addition, the peptide elicited a decrease of the cerebral blood flow of between 11 and 56% combined with a drastic decrease of the cardiac output (32-64%) and an elevation of the blood pressure to approximately 150% of control values. However, we found no changes of the cardiac output, the blood pressure, or the PaS product of phenylalanine after microdialysis (30 min, 5 microliters min-1) of arginine vasopressin (15 mumol L-1) into the dorsal hippocampus, but cerebral blood flow was decreased. The results support the hypothesis that arginine vasopressin receptors at the blood-brain barrier are involved in the regulation of large neutral amino acid transfer from blood to brain and indicate that these receptors are located at the luminal membrane of the endothelial cells.

Acute and chronic effects of hyperglycaemia on glucose metabolism

In normal man, several hormonal and metabolic adjustments allow the maintenance of the blood glucose concentration within narrow limits. Hyperglycaemia participates in this regulation via stimulation of glucose disposal and inhibition of glucose production. The effects are mediated, in addition to changes in insulin and glucagon secretion, by the mass-action effect of glucose. In both Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetic patients, hyperglycaemia, by mass-action abnormally elevates the basal glucose utilization rate but compensates for reduced postprandial insulin-stimulated glucose disposal. When exposed to chronic hyperglycaemia, the body tissues seem to protect themselves, at least partly, against excessive glucose utilization. These protective mechanisms include both a reduction in insulin stimulated glucose disposal and insulin secretion. Chronic hyperglycaemia may also reduce non-insulin-dependent glucose utilization, at least in rats. In Type 1 diabetic patients with normal peripheral insulin concentrations, chronic hyperglycaemia per se could be a major cause of insulin resistance. In Type 2 diabetic patients, insulin resistance is often already present before the development of overt fasting hyperglycaemia. At the diabetic stage, hyperglycaemia could, however, maintain a self-perpetuating cycle, where the deleterious effects of high glucose concentrations on insulin action and secretion cause further deterioration of glycaemic control. The biochemical basis for hyperglycaemia-induced insulin resistance is still far from clear, but could involve changes in the glucose transporter number and gene expression.

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.

Cerebral anaerobic glycolysis and reduced cerebral oxygen transport in human cerebral malaria

In 12 patients comatose with cerebral malaria, cerebral blood flow was 52.2 (SE 4.0) ml/100 g per min, within the reported range for healthy controls, but cerebral vascular resistance was raised at 1.66 (0.19) mm Hg/ml per 100 g per min. Cerebral oxygen consumption (1.90 [0.23] ml/100 g per min), and cerebral arteriovenous oxygen content difference (3.5 [0.43] ml/dl) were subnormal, while cerebral venous pO2 (5.7 [0.2] kpA) was raised. After recovery of consciousness there were significant decreases in arterial lactate concentration (2.44 [0.45] to 1.19 [0.45] mumol/l) and cerebral lactate production (17.4 [7.9] to 5.6 [1.1] mmol/100 g per minute). These results provide evidence of cerebral anaerobic glycolysis associated with inadequate oxygen delivery to the brain consistent with either inhibition of cerebral oxidative metabolism or the microcirculatory obstruction envisaged in the "mechanical" hypothesis for cerebral malaria.

Changes in cerebral blood flow during anaesthesia and surgery in the sitting position

Serial measurements of global cerebral blood flow (CBF) were made in 15 patients undergoing elective neurosurgical procedures in the sitting position, using a modified intravenous 133Xenon technique. The mean supine CBF rose from 43 (+/-3) ml/100g/min to 62 (+/-6) ml/100g/min in the sitting position and remained elevated at the end of surgery at 62 (+/-5) ml/100g/min. Both increases in CBF were statistically significant with respect to baseline supine values.

Gravity-induced hyperventilation is caused by a reduced brain perfusion

The suggestion that hyperventilation caused by increased gravity is mediated by a decrease in brain perfusion has led us to propose a mathematical model based on: (1) the CO2 balance equation for the respiratory center (RC), and (2) the relationship between RC blood flow (QRC), foot-to-head acceleration (Gz) and PRCCO2, namely, QRC = [1 - a(Gz - 1)](b X PRCCO2 + c), where the coefficients a, b and c can be calculated from data in the literature. QRC is significantly affected by + GZ only at high PaCO2. The model can be used to calculate oxygen pressure in the RC; the numbers so obtained are in good agreement with measurements of jugular vein PO2 obtained by others.

Cerebral glucose consumption following verbal auditory stimulation

We studied the effect of auditory stimulation upon cerebral glucose metabolism in young normals. The stimulus consisted of a non-English discourse which was presented monaurally to 10 normal blindfolded subjects (5 left ear, 5 right); the opposite ear was plugged. Six subjects studied blindfolded and with ears plugged served as controls. Sixteen discrete homologous cortical and subcortical regions of interest were examined. Regional glucose consumption and side-to-side differences in glucose metabolism were analyzed. Monaural stimulation produced significant increases in temporal metabolism contralateral to the side of stimulation. Significant asymmetries in metabolism were found at the temporoparietal junction, inferior parietal region, insula and corpus collosum. The left frontal speech areas remained unaffected. These findings demonstrate that in man the primary auditory pathways retain a contralateral organization. Further, cerebral activation induced by non-meaningful verbal stimulation is widespread within the left temporal and parietal regions but does not impact upon the frontal speech cortices.

Effect of insulin on the distribution and disposition of glucose in man

Understanding the influence of insulin on glucose turnover is the key to interpreting a great number of metabolic situations. Little is known, however, about insulin's effect on the distribution and exchange of glucose in body pools. We developed a physiological compartmental model to describe the kinetics of plasma glucose in normal man in the basal state and under steady-state conditions of euglycemic hyperinsulinemia. A bolus of [3-3H]glucose was rapidly injected into a peripheral vein in six healthy volunteers, and the time-course of plasma radioactivity was monitored at very short time intervals for 150 min. A 1-mU/min kg insulin clamp was then started, thereby raising plasma insulin levels to a high physiological plateau (approximately 100 microU/ml). After 90 min of stable euglycemic hyperinsulinemia, a second bolus of [3-3H]glucose was given, and plasma radioactivity was again sampled frequently for 90 min more while the clamp was continued. Three exponential components were clearly identified in the plasma disappearance curves of tracer glucose of each subject studied, both before and after insulin. Based on stringent statistical criteria, the data in the basal state were fitted to a three-compartment model. The compartment of initial distribution was identical to the plasma pool (40 +/- 3 mg/kg); the other two compartments had similar size (91 +/- 12 and 96 +/- 9 mg/kg), but the former was in rapid exchange with plasma (at an average rate of 1.09 +/- 0.15 min-1), whereas the latter exchanged 10 times more slowly (0.12 +/- 0.01 min-1). The basal rate of glucose turnover averaged 2.15 +/- 0.12 mg/min kg, and the total distribution volume of glucose in the postabsorptive state was 26 +/- 1% of body weight. In view of current physiological information, it was assumed that the more rapidly exchanging pool represented the insulin-independent tissues of the body, while the slowly exchanging pool was assimilated to the insulin-dependent tissues. Insulin-independent glucose uptake was estimated (from published data) at 75% of basal glucose uptake, and was constrained not to change with euglycemic hyperinsulinemia. When the kinetic data obtained during insulin administration were fitted to this model, neither the size nor the exchange rates of the plasma or the rapid pool were appreciably changed. In contrast, the slow pool was markedly expanded (from 96 +/- 9 to 190 +/- 30 mg/kg, P less than 0.02) at the same time as total glucose disposal rose fourfold above basal (to 7.96 +/- 0.85 mg/min kg, P less than 0.001). Furthermore, a significant direct correlation was found to exist between the change in size of the slow pool and the insulin-stimulated rate of total glucose turnover (r=0.92, P<0.01). We conclude that hyperinsulinemia, independent of hyperglycemia, markedly increases the exchangeable mass of glucose in the body, presumably reflecting the accumulation of free, intracellular glucose in insulin-dependent tissues.

Glucose metabolic rate kinetic model parameter determination in humans: the lumped constants and rate constants for [18F]fluorodeoxyglucose and [11C]deoxyglucose

The rate constants and lumped constants (LCs) for [18F]fluorodeoxyglucose ([18F]FDG) and [11C]deoxyglucose ([11C]DG) were determined in humans for the glucose metabolic rate kinetic model used to measure local cerebral glucose consumption. The mean values (+/- SE) of the LCs for [18F]FDG and [11C]DG are 0.52 +/- 0.028 (n = 9) and 0.56 +/- 0.043 (n = 6), respectively. The mean values (+/- SE) of the rate constants k*1, k*2, k*3, and k*4 for [18F]FDG for gray matter are 0.095 +/- 0.005, 0.125 +/- 0.002, 0.069 +/- 0.002, and 0.0055 +/- 0.0003, respectively. The corresponding values for white matter are 0.065 +/- 0.005, 0.126 +/- 0.003, 0.066 +/- 0.002, and 0.0054 +/- 0.0006, respectively. Using these values and previously published values for the rate constants for [11C]DG, the average whole-brain metabolic rates for glucose in normal subjects measured with [18F]FDG and [11C]DG are 5.66 +/- 0.37 (n = 6) and 4.99 +/- 0.23 (n = 6) mg/100 g/min, respectively. These values are not significantly different (t = 1.56, p greater than 0.10) and agree well with reported values in the literature determined by means of the Kety-Schmidt technique.

Local interrelationships of cerebral oxygen consumption and glucose utilization in normal subjects and in ischemic stroke patients: a positron tomography study

With the use of positron emission tomography (PET) and the 15O steady-state-[18F]fluorodeoxyglucose combined method, the local interrelationships between the cerebral metabolic rate for oxygen (CMRO2) and the cerebral metabolic rate for glucose ( CMRGlc ) were investigated in control subjects and in stroke patients. In addition to the classic in vivo autoradiographic approach, a kinetic method was used to measure CMRGlc because it was expected to be more reliable in cerebral ischemia. In control subjects local coupling between CBF, CMRO2, and CMRGlc was confirmed, and acceptable values for the CMRO2/ CMRGlc ratio were found; the latter, however, was lower in white matter than in gray. Uncoupling between CMRO2 and CMRGlc was observed in all stroke patients, suggesting that (1) enhanced anaerobic glycolysis occurred both in reperfused recent infarcts and in chronically ischemic tissue, and (2) substrates other than blood-borne glucose were being oxidized at the borders of recent infarcts. However, methodological uncertainties presently make such observations only tentative. Finally, a coupled depression of CMRO2 and CMRGlc was found in the contralateral cerebellum.

Cardiovascular and haemodynamic responses to tilting and to standing in tetraplegic patients: a review

This paper has reviewed the acute and long-term responses to changes in vertical posture in normal and tetraplegic subjects. It has discussed physiological mechanisms causing orthostatic hypotension in acute cervical spinal cord injured patients, and subsequent factors contributing to its amelioration over time. The long-term adaptive mechanisms are still controversial, probably involving multiple neurological, endocrine, renal, cardiovascular and haemodynamic factors. These factors include inhibition of vagal tone, plasma catecholamine levels, sensitivity of vascular beds to catecholamines, stretch reflexes in blood vessels, spinal BP reflexes, renin-angiotensin system, aldosterone and plasma volume changes. Individual differences may also interact with these various mechanisms, further complicating the issues. Although the fact that most tetraplegics do improve their orthostatic tolerance over time with repeated tilting is manifest, the precise mechanisms allowing this improvement are not. Research is needed to clarify these adaptive mechanisms, as well as to investigate the physiological effects of long-term therapeutic standing in devices such as standing frames.

An evaluation of errors in the determination of blood flow by the indicator fractionation and tissue equilibration (Kety) methods

In this report, the effects of various errors and plasma time courses of indicator concentration on the accurate determination of cerebral blood flow (F) are theoretically analyzed for the tissue equilibration and the indicator fractionation techniques. For the indicator fractionation technique, the impact of sample timing and tissue assaying errors and of indicator backflux were examined; for the tissue equilibration method, errors in the value of the partition coefficient (lambda), sample timing, and tissue assaying were considered. The recommended ways to decrease the effects of errors in the indicator fractionation technique are to administer the indicator by an intravenous bolus and to sample the tissue about 10 s thereafter. Possible errors in the assessment of F by the tissue equilibration technique are diminished by using an indicator infusion schedule which yields a continuous rise in arterial concentration and by selecting a 30-s experiment duration. Surprisingly, the impact of sample timing errors is greater on the determination of F with the tissue equilibration method than with the indicator fractionation technique. For the chosen plasma time courses, there is always a backflux error in an indicator fractionation estimation of F, and this error increases as the flow rate increases. Thus, provided the sample timing and tissue assay errors are small and the value of lambda is known, the tissue equilibration method is the more accurate of the two. If lambda is unknown, then the indicator fractionation technique should be used. In many cases, the indicator fractionation method will provide as accurate an estimate of F as will the tissue equilibration method.

Disturbance of oxidative metabolism of glucose in recent human cerebral infarcts

Eight patients with recent cerebral hemispheric infarction were studied with positron emission tomography and the oxygen-15 steady-state inhalation and [18F]deoxyglucose techniques to obtain values of regional cerebral blood flow, oxygen consumption, and glucose metabolism. The Sokoloff equation, used to calculate glucose metabolism, was simplified to exclude the exponential terms containing the rate constants. A value of the lumped constant quoted for normal brain (0.42) was used for infarcted regions and contralateral hemisphere. Mean regional cerebral blood flow, oxygen consumption, and glucose metabolism were all significantly depressed within the infarcts compared with the mirror regions in the contralateral cerebral hemisphere. The mean fractional extraction of oxygen was low, indicating an adequate supply of oxygen for residual oxidative metabolism. Regional oxygen consumption and glucose metabolism were significantly correlated within the infarcts, but with a relationship of 2 moles of oxygen per mole of glucose--one-third that in the contralateral hemisphere and in normal brain. Although these results suggest that the metabolizing tissue of a recent cerebral infarct utilizes aerobic glycolysis, caution about the validity of this pathophysiological observation is dictated by limitations in current positron emission tomographic tracer methodology.

Prognostic value of cerebral blood flow autoregulation in the long-term prognosis of ischemic cerebrovascular disease

The correlation between long-term prognosis, cerebral blood flow (CBF) and CBF autoregulation was studied in 34 patients with cerebral infarction (mean age, 64 years). CBF was measured by the nitrous oxide method 1-6 months (mean 87 days) after disease onset. CBF autoregulation was evaluated quantitatively from the Dysautoregulation Index (DI) (delta CBF/delta effective MABP). Reductions in effective MABP were induced with a tilt table. No significant correlation was noted among CBF, DI and activities of daily living at the time of measurement. The patients' physical condition was reevaluated by questionnaire 2 years or more (mean 32 months) later. Better functional state at follow-up was related to higher CBF and lower DI values although the differences were not significant. The relationships among CBF, DI and changes in physical condition during the period were evaluated. The mean CBF values in patients with a better prognosis exceeded those of poor prognosis patients. The CBF values in the group who became independent significantly exceeded those in the group that deteriorated (P less than 0.05). The CBF values in the latter showed small but significant decreases during head-up tilting (P less than 0.05). The DI in this group was significantly higher than in the groups with a less severe outcome (P less than 0.01, P less than 0.05, respectively). In conclusion, determinations of CBF autoregulation, together with flow values, in the chronic state may have some value in predicting the long-term prognosis in cerebral infarction.

Cerebral glucose utilization in awake unstressed rats

Regional cerebral glucose utilization (rCMRGlc) was measured in awake unstressed rats by use of [2-14C]glucose autoradiography. Rats prepared by chronic catheterization of the jugular and epigastric veins were placed in specially designed chambers after surgery. The catheters were passed through the top of the chambers. Seven days after surgery, plasma glucose was normal (7.5 mM), plasma ketone bodies were low (0.13 mM), and body weight was maintained. Plasma epinephrine was 31 pg/ml; approximately 3% of the level for stressed (immobilized) rats. Injections and blood sampling through the catheters did not produce changes in heart rate, blood pressure, plasma catecholamines, or blood metabolites. Visual observations did not reveal any signs that the rats were aware of the [2-14C]glucose injections and the subsequent blood sampling. The rCMRGlc in stressed rats was significantly greater than in unstressed rats. This method provides a model for physiological studies in unstressed unanesthetized rats.