Control of brain temperature during experimental global ischemia in rats

In vivo brain temperature mapping using polymer optical fiber Bragg grating sensors

Variation of the brain temperature is strongly affected by blood flow, oxygen supply, and neural cell metabolism. Localized monitoring of the brain temperature is one of the most effective ways to correlate brain functions and diseases such as stroke, epilepsy, and mood disorders. While polymer optical fibers (POFs) are considered ideal candidates for temperature sensing in the brain, they have never been used so far in vivo. Here, we developed for the first, to the best of our knowledge, time an implantable probe based on a microstructured polymer optical fiber Bragg grating (FBG) sensor for intracranial brain temperature mapping. The temperature at different depths of the brain (starting from the cerebral cortex) and the correlation between the brain and body core temperature of a rat were recorded with a sensitivity of 33 pm/°C and accuracy <0.2°C. Our in vivo experimental results suggest that the proposed device can achieve real-time and high-resolution local temperature measurement in the brain, as well as being integrated with existing neural interfaces.

Neuroprotective Effects of Nasopharyngeal Perfluorochemical Cooling in a Rat Model of Subarachnoid Hemorrhage

OBJECTIVE

Subarachnoid hemorrhage (SAH) frequently results in severe morbidity, even mortality. Hypothermia is known to have a neuroprotective effect in ischemic injuries. The aim of this study was to determine whether nasopharyngeal (NP) perfluorochemical (PFC) cooling could be used in a rat model of SAH model for neuroprotection.

METHODS

SAH was induced in 16 male Sprague-Dawley rats by cisterna magna injection of 0.3 mL autologous blood. Vital signs, temperatures, cerebral blood flow (CBF), and brain histology were assessed. Brain cooling was performed on the treatment group using the NP-PFC method starting from 20 minutes after SAH.

RESULTS

No SAH-related deaths were observed in either group. SAH caused an immediate decrease in mean arterial pressure (17.0% ± 4.90% below baseline values). SAH induction caused a significant and rapid decrease in CBF from baseline (approximately -65%, ranging from -32% to -85%) in both hemispheres. In the left hemisphere, cooling facilitated the return of CBF to baseline values within 20 minutes of treatment with further increase in CBF that stabilized by the 2 hours after injury time point. Quantitative immunohistochemistry showed that there were significantly more NeuN-positive cells in the cortex and significantly fewer IBA-1-positive microglia and glial fibrillary acidic protein-positive astrocytes cells in both cortex and hippocampus in the animals that received NP-PFC cooling compared with no treatment, reflecting preserved neuronal integrity and reduced inflammation.

CONCLUSIONS

The data from this study indicate that local hypothermia by NP-PFC cooling supports return of CBF and neuronal integrity and suppresses the inflammatory response in SAH, suggestive of a promising neuroprotective approach in management of SAH.

Temperature Control in Rodent Neuroprotection Studies: Methods and Challenges

Extensive animal research facilitated the clinical translation of therapeutic hypothermia for cardiac arrest in adults and hypoxic-ischemic injury in infants. Similarly, clinical interest in hypothermia for other brain injuries, such as stroke, has been greatly supported by positive findings in preclinical work. The reliability, validity, and utility of animal models, among many research practices (blinding, randomization, etc.), are key to successful clinical translation. Here, we review methods used to induce and maintain hypothermia in animal models. These include physical and pharmacological methods. We emphasize the advantages and limitations of each approach, and the importance of using clinically relevant cooling protocols and appropriate monitoring and reporting approaches. Moreover, we performed a literature survey of ischemic stroke studies published in 2015 to highlight the continuing risk of temperature confounds in neuroprotection studies. For example, many still do not accurately monitor and report temperature during surgery (23.5%), even though almost half of these studies (46.0%) use pharmaceutical agents that likely influence temperature. We hope this review stimulates awareness and discussion of the importance of temperature in neuroprotective studies.

Changes in Energy Levels by Dexamethasone in Ischemic Hearts and Brains in Male Mice

BACKGROUND

Glucocorticoids have been shown to alleviate ischemia-induced myocardial injury, while aggravating neuronal damage caused by ischemia. As energy failure is a predominant factor in cellular viability, we examined the effects of glucocorticoids on energy utilization in the mouse heart and brain.

METHODS

Seventy-two male ddY mice were assigned to 1 of 3 groups: saline (S), dexamethasone (a glucocorticoid without mineralocorticoid activity, 5 mg/kg) (D), and metyrapone (a potent inhibitor of the synthesis of glucocorticoids, 100 mg/kg) (M) groups (n=24 in each). Three hours after intraperitoneal administration, all animals were decapitated, and the heads were frozen in liquid nitrogen after 0, 0.5, 1, or 2 minutes (n=6 in each). The hearts were immediately removed and frozen in liquid nitrogen after 0, 5, 10, or 20 minutes of incubation at 37°C (n=6 in each). The concentrations of adenylates and monoamines were determined by high-performance liquid chromatography.

RESULTS

In the heart, the adenosine 5'-triphosphate (ATP) concentration did not differ among the 3 groups at 0 minute of ischemia (3 h of S, D, or M treatment). Ischemia for 5 minutes decreased the ATP content to 21% of the basal level in the S group. The ATP decrease was suppressed by either the D or M treatment, such that after 5 minutes ATP levels were 63% and 64% of each basal level, respectively. In the brain, the ATP level in the M group was 62% of that in the S group at 0 minute of ischemia, and the 5'-monophosphate (AMP) level was 276% of that in the S group. Brain dopamine metabolism was facilitated by dexamethasone, and suppressed by metyrapone.

CONCLUSIONS

The relationship between effects of glucocorticoids on ischemia-induced changes in energy levels and cellular viability was not clearly elucidated.

Electromagnetic treatment to old Alzheimer's mice reverses β-amyloid deposition, modifies cerebral blood flow, and provides selected cognitive benefit

Few studies have investigated physiologic and cognitive effects of "long-term" electromagnetic field (EMF) exposure in humans or animals. Our recent studies have provided initial insight into the long-term impact of adulthood EMF exposure (GSM, pulsed/modulated, 918 MHz, 0.25-1.05 W/kg) by showing 6+ months of daily EMF treatment protects against or reverses cognitive impairment in Alzheimer's transgenic (Tg) mice, while even having cognitive benefit to normal mice. Mechanistically, EMF-induced cognitive benefits involve suppression of brain β-amyloid (Aβ) aggregation/deposition in Tg mice and brain mitochondrial enhancement in both Tg and normal mice. The present study extends this work by showing that daily EMF treatment given to very old (21-27 month) Tg mice over a 2-month period reverses their very advanced brain Aβ aggregation/deposition. These very old Tg mice and their normal littermates together showed an increase in general memory function in the Y-maze task, although not in more complex tasks. Measurement of both body and brain temperature at intervals during the 2-month EMF treatment, as well as in a separate group of Tg mice during a 12-day treatment period, revealed no appreciable increases in brain temperature (and no/slight increases in body temperature) during EMF "ON" periods. Thus, the neuropathologic/cognitive benefits of EMF treatment occur without brain hyperthermia. Finally, regional cerebral blood flow in cerebral cortex was determined to be reduced in both Tg and normal mice after 2 months of EMF treatment, most probably through cerebrovascular constriction induced by freed/disaggregated Aβ (Tg mice) and slight body hyperthermia during "ON" periods. These results demonstrate that long-term EMF treatment can provide general cognitive benefit to very old Alzheimer's Tg mice and normal mice, as well as reversal of advanced Aβ neuropathology in Tg mice without brain heating. Results further underscore the potential for EMF treatment against AD.

Long-term electromagnetic field treatment enhances brain mitochondrial function of both Alzheimer's transgenic mice and normal mice: a mechanism for electromagnetic field-induced cognitive benefit?

We have recently reported that long-term exposure to high frequency electromagnetic field (EMF) treatment not only prevents or reverses cognitive impairment in Alzheimer's transgenic (Tg) mice, but also improves memory in normal mice. To elucidate the possible mechanism(s) for these EMF-induced cognitive benefits, brain mitochondrial function was evaluated in aged Tg mice and non-transgenic (NT) littermates following 1 month of daily EMF exposure. In Tg mice, EMF treatment enhanced brain mitochondrial function by 50-150% across six established measures, being greatest in cognitively-important brain areas (e.g. cerebral cortex and hippocampus). EMF treatment also increased brain mitochondrial function in normal aged mice, although the enhancement was not as robust and less widespread compared to that of Tg mice. The EMF-induced enhancement of brain mitochondrial function in Tg mice was accompanied by 5-10 fold increases in soluble Aβ1-40 within the same mitochondrial preparations. These increases in mitochondrial soluble amyloid-β peptide (Aβ) were apparently due to the ability of EMF treatment to disaggregate Aβ oligomers, which are believed to be the form of Aβ causative to mitochondrial dysfunction in Alzheimer's disease (AD). Finally, the EMF-induced mitochondrial enhancement in both Tg and normal mice occurred through non-thermal effects because brain temperatures were either stable or decreased during/after EMF treatment. These results collectively suggest that brain mitochondrial enhancement may be a primary mechanism through which EMF treatment provides cognitive benefit to both Tg and NT mice. Especially in the context that mitochondrial dysfunction is an early and prominent characteristic of Alzheimer's pathogenesis, EMF treatment could have profound value in the disease's prevention and treatment through intervention at the mitochondrial level.

Genomic response of the rat brain to global ischemia and reperfusion

To identify genes that are involved in ischemia response of the brain, we have evaluated changes of gene expression in rat cerebrum after 15 min complete global ischemia, followed by reperfusion for 1 h, 6 h or 24 h. The expression profiles of approximately 30,000 transcripts from three subjects in each group (including sham-operated controls) were monitored employing oligonucleotide microarrays. About 20,000 transcripts were detectable in rat brains. The levels of 576 transcripts (approximately 2.9%) were significantly altered in response to experimental ischemia. 419 transcripts were up- and 157 downregulated; 39 transcripts changed after 1 h reperfusion, 174 after 6 h and 462 after 24 h. Results from quantitative real-time reverse transcription PCR of 18 selected genes showed excellent agreement with the microarray data. There is surprisingly little overlap between gene regulation patterns at different reperfusion times (only seven genes displayed significant changes in transcript levels at all reperfusion times. Several genes that were previously unknown to be involved in ischemia-response have been identified. Analyses of gene ontology patterns and the most strongly regulated transcripts showed that the immediate response to an ischemia/reperfusion is mediated by the induction of specific transcription factors and stress genes. Delayed gene expression response is characterised by inflammation and immune-related genes. These results support the hypothesis that the brain's response to ischemia is an active, specific and coordinated process.

Prolonged deep hypothermic circulatory arrest in rats can be achieved without cognitive deficits

Cardiopulmonary bypass (CPB) and deep hypothermic circulatory arrest (DHCA) enable surgical repair of cardiovascular defects. However, neurological complications can result after both CPB and DHCA. We sought to investigate if 75 min of CPB or DHCA caused motor, cognitive or histological deficits in rats. Three groups were studied: DHCA, CPB, and sham. Rats in the DHCA group were subjected to 75 min DHCA at 15 degrees C, with a total CPB duration of 75 min. Rats in the CPB group were subjected to 75 min of normothermic CPB. Shams received the same anesthesia, cannulations and infusions. Motor function was assessed using beam testing on days 3-13. Cognitive performance was evaluated using Morris water maze tasks on days 7-13. Overall Performance Category (OPC) and Neurologic Deficit Score (NDS) were assessed daily. Histological Damage Score (HDS) was assessed in survivors on day 14. Sustained deficits on beam testing were seen only in the CPB group. Rats in the CPB and DHCA groups exhibited similar cognitive performance vs. sham. There were no differences in OPC or NDS between groups. Neuronal degeneration was present only in small foci in rats after DHCA (n=4/7). However, HDS was not different in individual brain regions or viscera between DHCA or CPB vs. sham. Surprisingly, CPB, but not DHCA was associated with motor deficits vs. sham, and no cognitive deficits were seen in either group vs. sham. Future studies with longer DHCA duration will be necessary to provide targets to assess novel preservation strategies.

An early bolus of hypertonic saline hydroxyethyl starch improves long-term outcome after global cerebral ischemia

OBJECTIVE

The beneficial effect of hypertonic saline solutions in the emergency treatment of shock and traumatic brain injury is well described. The present study determines effects of a single bolus of hypertonic saline on long-term survival, neurologic function, and neuronal survival 10 days after global cerebral ischemia. In addition, we evaluated the therapeutic window for hypertonic saline treatment (early vs. delayed application).

DESIGN

Laboratory experiment.

SETTING

University laboratory.

SUBJECTS

Male Wistar rats weighing 240-330 g.

INTERVENTIONS

Rats were submitted to temporal global cerebral ischemia using temporary bilateral carotid occlusion combined with hypobaric hypotension. Animals received 7.5% saline/6% hydroxyethyl starch (HHS) or vehicle (NaCl 0.9%) at either 1.5 mins (early treatment) or 31.5 mins (delayed treatment) of reperfusion. Regional cerebral blood flow (rCBF) and physiologic variables were measured during insult and early reperfusion. Animal survival and neurologic function were evaluated throughout the 10-day observation period. Quantification of brain injury was performed on day 10.

MEASUREMENTS AND MAIN RESULTS

Early treatment with HHS resulted in a robust restoration of rCBF after ischemia, reduced postischemic mortality by 77% (9% vs. 39% in vehicle-treated controls), ameliorated neurologic performance (Neuro-Deficit-Score 10 days after insult, 96 +/- 0.7 vs. 85 +/- 1.4, mean +/- se), and almost blunted neuronal cell death (hippocampal CA1, 2150 +/- 191 vs. 884 +/- 141 neurons/mm; cortex, 1746 +/- 91 vs. 1060 +/- 112). In contrast, delayed treatment resulted in no sustained effects.

CONCLUSIONS

Timing of HHS treatment is critical after experimental global cerebral ischemia to reduce mortality, improve neurologic function, and neuronal survival. Our results suggest that early application of HHS may be a potential neuroprotective strategy after global cerebral ischemia.

Neuroprotective effects of a postischemic treatment with a bradykinin B2 receptor antagonist in a rat model of temporary focal cerebral ischemia

Bradykinin, an endogenous nonapeptide produced by activation of the kallikrein-kinin system, promotes neuronal tissue damage as well as disturbances in blood-brain barrier function through activation of B2 receptors. In a rat model of focal cerebral ischemia, blockade of B2 receptors before initiation of ischemia with the B2 receptor antagonist, LF 16-0687 Ms, afforded substantial neuroprotection. In order to assess the potential clinical value of this approach, we evaluated the effect of LF 16-0687 Ms given at reperfusion following focal cerebral ischemia on local cerebral blood flow (LCBF), neurological outcome, and infarct size. Sprague-Dawley rats were subjected to MCA occlusion for 90 min by an intraluminal filament. Animals were assigned to one of four treatment arms (n = 7 each): (1) vehicle, (2) LF 16-0687 Ms (1.0 mg/kg/day), (3) LF 16-0687 Ms (3.0 mg/kg/day), or (4) LF 16-0687 Ms (10.0 mg/kg/day) given at reperfusion and repetitively over 2 days. Neurological recovery was examined daily, and infarct volume was assessed histologically on day 7 after ischemia. Physiological parameters and local CBF were not influenced by the treatment. Significant improvement of neurological outcome was observed on postischemic day 3 in animals receiving 1.0 and 3.0 mg/kg/day of LF 16-0687 Ms (P < 0.05). Inhibition of B2 receptors significantly reduced infarct volume in all treated animals predominantly in the cortex. B2 receptor blockade with LF 16-0687 Ms showed neuroprotective effectiveness even when therapy was initiated upon reperfusion, i.e. 90 min after induction of ischemia. Therefore, blockade of B2 receptors seems to be a promising therapeutic approach after focal cerebral ischemia, which deserves further experimental and clinical evaluation.

Capillary flow and diameter changes during reperfusion after global cerebral ischemia studied by intravital video microscopy

The reaction of cerebral capillaries to ischemia is unclear. Based on Hossmann's observation of postischemic "delayed hypoperfusion," we hypothesized that capillary flow is decreased during reperfusion because of increased precapillary flow resistance. To test this hypothesis, we measured cerebral capillary erythrocyte velocity and diameter changes by intravital microscopy in gerbils. A cranial window was prepared over the frontoparietal cortex in 26 gerbils anesthetized with halothane. The animals underwent either a sham operation or fifteen minutes of bilateral carotid artery occlusion causing global cerebral ischemia. Capillary flow velocities were measured by frame-to-frame tracking of fluorescein isothiocyanate labeled erythrocytes in 1800 capillaries after 1-hour reperfusion. Capillary flow velocities were decreased compared to control (0.25 +/- 0.27 mm/s vs. 0.76 +/- 0.45 mm/s; P<0.001). Precapillary arteriole diameters in reperfused animals were reduced to 76.3 +/- 6.9% compared to baseline (P<0.05). Capillary diameters in reperfused animals (2.87 +/- 0.97 microm) were reduced (P<0.001) compared to control (4.08 +/- 1.19 microm). Similar reductions of precapillary (24%) and capillary vessel diameters (30%) and absolute capillary flow heterogeneity indicate that delayed (capillary) hypoperfusion occurs as a consequence of increased precapillary arteriole tone during reperfusion.

Development of a novel fluorescent microsphere technique to combine serial cerebral blood flow measurements with histology in the rat

The aim of the study was to evaluate the microsphere technique for the quantitative assessment of regional cerebral blood flow (rCBF) at different time points in the same animal. Yellow-green and red fluorescent microspheres with a diameter of 15 microm were injected into the rat at two different time points via a cannula inserted into the left ventricle of the heart. The reproducibility of the rCBF measurements in normocapnic conditions (n=7) and the responsiveness of the flow to hypercapnia induced by 7% CO(2) (n=7) was examined. The fluorescent spheres were counted on 100 microm vibratome sections of perfusion-fixed brains and rCBF was calculated. The median total CBF in normocapnic rats was 224 ml/min/100 g for the first microsphere injection and 216 ml/min/100 g for the second one. In the hypercapnic group CBF amounted to 400 ml/min/100 g and after 30 min of normocapnia decreased to 178 ml/min/100 g. No differences between the left and right hemisphere were found and there was no indication that the first injection might have influenced the second one. The described approach allows combining the assessment of rCBF at different time points in physiological or pathological conditions with histological evaluation of related morphological alterations in the same brain region of the same animal.

Comparison of intracranial pressure measured in the cerebral cortex and the cerebellum of the rat

In this study, we evaluated the accuracy of intracranial pressure (ICP) measurement in rats by insertion of a miniature ICP probe in the parenchyma of the cerebellum. A comparison was made between the ICP values measured simultaneously in the parenchyma of the cerebral cortex and the cerebellum. In order to obtain a wide range of ICP, animals were subjected to a severe closed head injury (CHI), a moderate CHI or to a sham operation. ICP values ranged from 0.8 to 43.9 mmHg. After 15 min stabilisation the first measurement was taken and followed by a second measurement 25 min after onset to allow comparison of ICP changes at the two implantation sites. Linear regression analysis showed a highly significant correlation at 15 min: Y = 0.919X + 0.655 (R(2) = 0.977), and at 25 min: Y = 0.931X + 0.698 (R(2) = 0.976). The differences in ICP measurement between cerebellar and cerebral site were not significantly different from zero at both time points. Altman-Bland plots showed that the difference in ICP readings between the two locations could differ maximally by 5.3 mmHg. The largest differences were detected when high ICP values were recorded. We conclude that in rats the ICP measurement in the cerebellum is comparable to the ICP measurement in the cerebral cortex. The cerebellar ICP can be used as a valuable alternative during experimental procedures.

Cerebral blood flow assessment with indocyanine green bolus transit detection by near-infrared spectroscopy in the rat

In this study we evaluated the feasibility of measuring cerebral blood flow in rats by monitoring the transit of an indocyanine green bolus through the brain with multiwavelength near-infrared spectroscopy. Different volumes of a 1 mg/ml indocyanine green solution (5, 15, 25, 50 microl) were injected intravenously in the search for an optimal dose. Clear transit curves were obtained with all doses and a blood flow index could easily be determined. The indocyanine green signal obtained with the bolus of 5 microl rapidly returned to baseline and interfered minimally with the haemoglobin and cytochrome oxidase signals. This dose was used in a second study to evaluate the reproducibility of the signal and the effect of hypercapnia. Two groups of rats received 7 repetitive boli of indocyanine green. In one group, 7% CO(2) was added to the gas mixture before the second, fourth and sixth indocyanine green injection. Hypercapnia consistently caused a significant increase in blood flow index, cerebral haemoglobin concentration and O(2)-saturation. In the control group these variables remained stable in time. We conclude that monitoring of the transit of an indocyanine green bolus with multiwavelength near-infrared spectroscopy can be used to assess cerebral blood flow qualitatively in rats in combination with continuous monitoring of brain oxygenation.

Tolerance-Inducing dose of 3-nitropropionic acid modulates bcl-2 and bax balance in the rat brain: a potential mechanism of chemical preconditioning

Many studies have reported ischemia protection using various preconditioning techniques, including single dose 3-nitropropionic acid (3-NPA), a mitochondrial toxin. However, the cellular signal transduction cascades resulting in ischemic tolerance and the mechanisms involved in neuronal survival in the tolerant state still remain unclear. The current study investigated the mRNA and protein expression of the antiapoptotic bcl-2 and the proapoptotic bax. two antagonistic members of the bcl-2 gene family, in response to a single dose of 3-NPA, to global cerebral ischemia-reperfusion. and to the combination of both 3-NPA-pretreatment and subsequent global cerebral ischemia-reperfusion. Brain homogenates of adult Wistar rats (n = 25) were analyzed for bcl-2 and bax mRNA expression using a new highly sensitive and quantitative polymerase chain reaction (PCR) technique that allows real-time fluorescence measurements of the PCR product (LightCycler; Roche Diagnostics, Mannheim, Germany). Animals for mRNA analysis received 3-NPA (20 mg/kg, intraperitoneal; "chemical preconditioning") or vehicle (normal saline), and were either observed for 24 plus 3 hours or were subjected to 15 minutes of global cerebral ischemia 24 hours after the pretreatment and observed for 3 hours of reperfusion. Immunohistochemistry was applied to serial brain sections of additional rats (n = 68) to determine amount and localization of the respective Bcl-2 and Bax protein expression in various brain areas. One set of animals was injected with 3-NPA and observed for 3, 12, 24, and 96 hours; a second set was exposed to 15 minutes global cerebral ischemia, 3, 12, and 24 hours reperfusion; and a third set was pretreated with 3-NPA or saline 24 hours before the ischemic brain insult and observed for 96 hours of reperfusion. The authors found single dose 3-NPA treatment to be associated with an elevated bcl-2:bax ratio (increased bcl-2 expression, decreased bax expression), both on the transcriptional (mRNA) and the translational (protein) level. The differential influence of 3-NPA was maintained during early recovery from global cerebral ischemia (3 hours), when 3-NPA pretreated animals showed higher bcl-2 and lower bax mRNA levels compared with rats with saline treatment. Respective changes in protein expression were localized predominately in neurons vulnerable to ischemic damage. Compared with baseline, Bcl-2 protein was significantly higher in surviving neurons at 96 hours after the insult, whereas Bax protein remained unchanged. However, at this late time of postischemic recovery (96 hours), the protein expression pattern of surviving neurons was not different between animals with and without 3-NPA pretreatment. To the authors' knowledge, the current study is the first report on the differential expression of pro- and antiapoptotic genes after a single, nonlethal dose of 3-NPA. The current results suggest alterations in the balance between pro- and antiapoptotic proteins as a potential explanation for the reported protection provided by chemical preconditioning using 3-NPA in rats.

Mild and moderate hypothermia (alpha-stat) do not impair the coupling between local cerebral blood flow and metabolism in rats

BACKGROUND AND PURPOSE

The effects of hypothermia on global cerebral blood flow (CBF) and glucose utilization (CGU) have been extensively studied, but less information exists on a local cerebral level. We investigated the effects of normothermic and hypothermic anesthesia on local CBF (LCBF) and local CGU (LCGU).

METHODS

Thirty-six rats were anesthetized with isoflurane (1 MAC) and artificially ventilated to maintain normal PaCO(2) (alpha-stat). Pericranial temperature was maintained normothermic (37.5 degrees C, n=12) or was reduced to 35 degrees C (n=12) or 32 degrees C (n=12). Pericranial temperature was maintained constant for 60 min until LCBF and LCGU were measured with autoradiography. Twelve conscious rats served as normothermic control animals.

RESULTS

Normothermic anesthesia significantly increased mean CBF compared with conscious control animals (29%, P<0.05). Mean CBF was reduced to control values with mild hypothermia and to 30% below control animals with moderate hypothermia (P<0.05). Normothermic anesthesia reduced mean CGU by 44%. No additional effects were observed during mild hypothermia. Moderate hypothermia resulted in a further reduction in mean CGU (41%, P<0.05). Local analysis showed linear relationships between LCBF and LCGU in normothermic conscious (r=0.93), anesthetized (r=0.92), and both hypothermic groups (35 degrees C r=0. 96, 32 degrees C r=0.96, P<0.05). The LCBF-to-LCGU ratio increased from 1.5 to 2.5 mL/micromol during anesthesia (P<0.05), remained at 2.4 mL/micromol during mild hypothermia, and decreased during moderate hypothermia (2.1 mL/micromol, P<0.05).

CONCLUSIONS

Anesthesia and hypothermia induce divergent changes in mean CBF and CGU. However, local analysis demonstrates a well-maintained linear relationship between LCBF and LCGU during normothermic and hypothermic anesthesia.