(Stroke. 2000;31:1393.)
© 2000 American Heart Association, Inc.
Original Contributions |
Mild and Moderate Hypothermia (
-Stat) Do Not Impair the Coupling Between Local Cerebral Blood Flow and Metabolism in Rats
From the Departments of Anesthesiology and Critical Care Medicine (P.K., T.F., C.L., K.F.W.) and Neurosurgery (A.P.), Faculty of Clinical Medicine Mannheim, and the Department of Physiology I (W.K.), University of Heidelberg, Germany.
Correspondence to Klaus F. Waschke, MD, Department of Anesthesiology and Critical Care Medicine, Faculty of Clinical Medicine Mannheim, University of Heidelberg, Theodor Kutzer Ufer 1-3, D-68167 Mannheim, Germany. E-mail km20{at}rumms.uni-mannheim.de
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Abstract |
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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
PaCO2 (
-stat). Pericranial temperature was
maintained normothermic (37.5°C, n=12) or was reduced to
35°C (n=12) or 32°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°C r=0.96, 32°C r=0.96, P<0.05). The LCBF-to-LCGU ratio increased from 1.5 to 2.5 mL/µmol during anesthesia (P<0.05), remained at 2.4 mL/µmol during mild hypothermia, and decreased during moderate hypothermia (2.1 mL/µmol, 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.
Key Words: autoradiography • autoregulation • cerebral blood flow • cerebral metabolism • hypothermia • rats
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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Hypothermia exerts beneficial effects on neuronal function during focal and global cerebral ischemia.1 2 3 The exact mechanisms responsible for hypothermic neuroprotection remain to be determined but may be attributed to either a reduction in the cerebral metabolic rate (CMR), which could preserve cellular energy stores and aerobic metabolism, or other mechanisms, such as a reduced release of excitatory neurotransmitters or nitric oxide.2 4 5
Moreover, hypothermia directly interferes with many physiological regulatory processes, and little is known about the possible consequences of hypothermia in different organ systems. For example, both cerebral blood flow (CBF) and glucose utilization (CGU) are thermally sensitive. During physiological conditions, CBF is regulated within narrow margins, and a close long-term coupling of local CBF (LCBF) and local CGU (LCGU) was reported.6 The effects of hypothermia on CBF or CGU and the coupling of both have been investigated during various conditions, such as cardiopulmonary bypass or cerebral edema.7 8 9 Hypothermia reduces the CMR for glucose in a graded manner,10 but conflicting results were obtained for CBF. A majority have reported a hypothermia-induced decrease in CBF,11 12 whereas others observed unchanged or even increased CBF,2 13 especially during pH-stat management.14 Furthermore, whether anesthesia and hypothermia interfere with the CBF-CGU coupling remains to be determined. In clinical studies of patients undergoing cardiopulmonary bypass, an unchanged or increased mean CBF associated with a markedly decreased mean CGU ("hyperperfusion") has been interpreted as uncoupling of CBF from metabolism.14 Murkin et al15 even reported independent variations of CBF and metabolism in hypothermic patients who were undergoing cardiac surgery. Because these conclusions were derived from studies that evaluated only global values of CBF and CGU, the question remains of whether uncoupling also exists on a local cerebral level.14 15 Another factor that could interfere with the relationship between CBF and metabolism is anesthesia.16 Because clinical hypothermia is mainly induced during anesthesia, it is difficult to separate the effects of anesthesia from those of hypothermia.
We hypothesized that the investigation of global CBF and CGU values may not accurately reflect the situation that exists on a local cerebral level and that coupling between local LCBF and LCGU is maintained during anesthesia and hypothermia. Therefore, the effects of anesthesia and mild (35°C) or moderate (32°C) hypothermia on the relationship between LCBF and LCGU were investigated in anesthetized rats, and the results were compared with those obtained from conscious normothermic control animals.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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Animals
After approval by the institutional animal care committee (Regierungspräsidium Karlsruhe), the experiments were performed on 48 male Sprague-Dawley rats that weighed 313±18 g (Charles River Deutschland). Animals were kept under temperature-controlled environmental conditions on a 14:10-hour light/dark cycle, fed standard diet (Altromin C 1000), and allowed free access to food and water until the start of the experiments.
Study Groups and Experimental Protocol
Rats were placed in a small box and anesthetized through
inhalation of a gas mixture of isoflurane (1 MAC), oxygen (40%), and
air (remainder). Then, 1 MAC of isoflurane (Forene; Abbott) was
administered with a precalibrated vaporizer (Fortec) through inhalation
via a nose cone. MAC values were corrected for reduced actual body
temperatures as determined by Vitez et al.17 1 MAC of
isoflurane corresponds to 1.2% at 37.5°C, 1.03% at 35°C, and
0.84% at 32°C with a fresh gas flow of 2 L/min. Tracheostomy and
cannulation of right femoral artery and vein were performed with
polyethylene catheters (PE-160 and PE-50; Labokron). Mean
arterial blood pressure and heart rate were continuously
recorded with a quartz pressure transducer (Hewlett-Packard).
Animals were mechanically ventilated (Small Animal Ventilator KTR4;
Hugo Sachs Electronic). Artificial ventilation was performed with
capnometric control of end-tidal PCO2
(Capnometer; Heyer) and continuous pulse oximetry (Onin 8600V Pulse
Oximeter; Onin Medical Inc). Arterial blood gases were
determined with a pH/blood gas analyzer (AVL Gas Check 939;
AVL). Pericranial temperature was measured with ultrafast
microthermocouple probes (diameter 0.3 mm, IT-23, Almemo 2290-3S
Thermometer; Hugo Sachs Electronik) introduced through the masseter
muscle to the outside of the base of the rat skull.
Simultaneously, rectal temperature was measured and body
temperature was kept constant at 37° to 37.5°C with a
temperature-controlled heating pad during the surgical preparation
(Harvard Ltd). Physiological parameters
were assessed and recorded before the determination of CBF and
CGU.
After completion of the surgical preparation, animals were randomly assigned to 1 of the following 4 groups:
(1) Normothermic, conscious control animals (n=12), which
were allowed to recover from anesthesia after
instrumentation of the femoral vessels. Thereafter, the animals were
placed in a rat restrainer and were studied 
60 minutes after
recovery from anesthesia. The temperature-controlled
heating pad was used only during the surgical preparation. After
recovery from anesthesia, animals were able to keep their
body temperature at normothermic levels, and
physiological temperature regulation was not
influenced.
(2) In normothermic, anesthetized animals (n=12), anesthesia was maintained at 1.2% isoflurane (1 MAC). Body temperature was kept constant at 37.5°C with a hollow plastic spiral that surrounded the entire animal body, which was continuously perfused with water of the target temperature. Animals were mechanically ventilated to maintain a PaCO2 of 40 mm Hg, and experiments were started after a 60-minute stabilization period.
(3) The mildly hypothermic, anesthetized animals (n=12) were
first allowed to cool down passively and then were actively cooled
until the target temperature was reached (30 minutes). Active
cooling was performed with the hollow plastic spiral continuously
perfused with water of the target temperature. After the target
temperature of 35°C was reached, pericranial temperature was
maintained constant for 60 minutes until the radioactive tracer was
administered. Blood gases were not corrected for actual body
temperature (-stat management), and ventilatory support was adjusted
to maintain an arterial
PaCO2 of 40 mm Hg.
(4) The moderately hypothermic, anesthetized animals (n=12) were treated as described for the mildly hypothermic group, with the exception that body temperature was decreased to 32°C.
Measurement of LCBF and LCGU
In each group, the 12 rats were randomized to be used either for
the autoradiographic determination of LCBF (n=6) or for the
measurement of LCGU (n=6) according to the methods described by
Sokoloff et al18 and Sakurada et al.19
Previous studies have validated these autoradiographic
methods for a wide range of body and brain temperatures down to a body
temperature of 9°C.10 20 21
For the measurement of LCGU, 125 µCi of
2-[1-14C]deoxy-D-glucose (specific
activity 50 to 56 mCi/mmol; New England Nuclear)/kg BW was injected as
a pulse via the femoral venous catheter over 20 seconds, and timed
arterial blood samples of 80 µL were collected through
the arterial catheter at 15, 30, and 45 seconds and at 1,
2, 3, 5, 7.5, 10, 15, 25, 35, and 45 minutes. The blood samples were
immediately centrifuged and stored on ice until assays were
performed for plasma
2-[1-14C]deoxy-D-glucose and
glucose concentrations. Immediately after the final
arterial blood sample was collected, the animal was
decapitated, and the brain was rapidly removed and frozen in isopentane
chilled to -60°C.
For the measurement of LCBF, 100 µCi of 4-iodo[N-methyl-14C]antipyrine (specific activity 54 mCi/mmol; Amersham-Buchler)/kg BW dissolved in 1 mL of saline was infused continuously at a progressively increasing infusion rate for 1 minute via the femoral venous catheter. The progressively increasing infusion rate, a modification of the method described earlier,19 was chosen to minimize equilibration of rapidly perfused tissues with arterial blood during the period of measurement. During the 1-minute infusion period, 14 to 20 timed blood samples were collected in drops from the free-flowing arterial catheter directly onto filter paper disks (1.3 cm in diameter) that had been prepared in small plastic beakers and weighed. The samples were weighed, and radioactivity was estimated with a liquid scintillation counter (TriCarb 4000 series; Canberra Packard) after extraction of the radioactive compound with ethanol. After the 1-minute infusion and sampling period, the animal was decapitated, and the brain was removed as quickly as possible and frozen in isopentane chilled to -60°C. In both the 2-[1-14C]deoxy-D-glucose and 4-iodo[N-methyl-14C]antipyrine experiments, the frozen brains were coated with chilled embedding medium (Lipshaw), stored at -80°C in plastic bags, cut into 20-µm sections at -20°C in a cryostat, and autoradiographed along with precalibrated [14C]methyl methacrylate standards.
Local tissue concentrations of 14C were determined with the autoradiographs through densitometric analysis. LCGU and LCBF were calculated from the local concentrations of 14C and the time courses of the plasma [14C]deoxyglucose and iodo[14C]antipyrine concentrations, including corrections for the lag and washout in the arterial catheter.18 The washout correction rate constant was 100 min-1, and the brain-blood partition coefficient for iodo[14C]antipyrine was 0.9 in the rats of the present study.22
Autoradiographic images were converted to digitized optical density images with an image processing system (MCID; Imaging Research). For measurements of separate brain structures, an ellipsoid cursor was used, which was adjusted to the size of the individual region. For measurements of mean global CBF or mean global CGU (to be referred to as mean CBF and mean CGU), coronal sections were analyzed as a whole at distances of 200 µm, and the values were summarized to obtain the area-weighted mean values of all measured sections.23
Statistical Analysis
Differences between the experimental groups were evaluated with
ANOVA, and Bonferroni’s correction was used when multiple comparisons
were performed. Data are presented as mean±SD, and
P<0.05 was considered statistically significant. The
overall relationship between LCGU and LCBF in the examined structures
of the brain (Figure
) was assessed with use of the least-squares fit of
the data to y=ax+b, where x
is the mean LCGU in a given region, and y is the mean LCBF
in that same area. Contrasts of slopes of the LCBF-LCGU regression
lines were tested with common t test statistics with
Bonferroni’s correction for multiple comparisons. Because of the
limitations of this kind of analysis, an additional, more
rigorous statistical approach with log-transformed data was applied, in
which the relationship of LCBF and LCGU was examined with a repeated
measures ANOVA according to McCulloch et al24 and
Ford et al.25 For this analysis, a computer
software package (BMDP2v, BMDP Statistical Software Inc) was used that
provided a consideration of interanimal variability and enabled the
detection of heterogeneities in the relationship between LCGU and
LCBF.
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), mildly hypothermic anesthetized
(35°C, 
), and moderately hypothermic anesthetized (32°C,
) animals. For each of the 41 examined brain structures, the mean
LCBF values are plotted against mean LCGU values. The regression lines
were calculated according to
y=ax+b, with
y being LCBF and x being LCGU:
normothermic conscious control animals,
y=1.5x+13, r=0.93;
normothermic anesthetized animals,
y=2.5x+43, r=0.92; mildly
hypothermic anesthetized animals,
y=2.4x+22, r=0.96; and
moderately hypothermic anesthetized animals,
y=2.1x+26, r=0.96
(P<0.05 between the slopes of all groups except for
normothermic anesthetized vs mildly hypothermic
anesthetized rats). |
Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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All animals survived the surgical procedure. Physiological variables of conscious, normothermic anesthetized, and hypothermic anesthetized rats are given in Table 1
. No statistically significant
differences in baseline values were observed between all 4 experimental
groups. Compared with normothermic conscious rats,
isoflurane anesthesia significantly reduced mean
arterial blood pressure, whereas all other variables
remained stable. Mean arterial blood pressure reached
baseline values during mild and moderate hypothermia. Compared with
normothermic anesthetized animals, mildly
hypothermic rats exhibited significantly increased arterial
oxygen tension (PaO2). Moderate
hypothermia was associated with a small, although significant,
reduction in arterial pH, whereas
PaO2 and plasma glucose concentration
were significantly increased compared with normothermic
anesthetized rats (Table 1).
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CBF was measured globally in the entire brain, as well as regionally in
41 different brain structures (Table 2).
Compared with normothermic conscious animals, mean CBF
during isoflurane anesthesia (1 MAC) was 29% higher (94±9
versus 121±18 mL · 100 g-1 ·
min-1; P<0.05). Mild hypothermia
reduced CBF to values in the range of normothermic
conscious control animals (89±9 mL · 100
g-1 · min-1).
During moderate hypothermia, a further significant CBF reduction to
66±6 mL · 100 g-1 ·
min-1 was observed (P<0.05). On a
local cerebral level, compared with normothermic
anesthetized animals, significant decreases in LCBF were
observed in 30 of 41 structures during mild hypothermia and in 36 of 41
structures during moderate hypothermia.
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In addition to blood flow, mean CGU and LCGU were measured in the 4
different groups of rats (Table 3).
During isoflurane anesthesia (1 MAC), mean CGU was reduced
from 57±3 to 32±5 µmol · 100
g-1 · min-1 (44%,
P<0.05) compared with the conscious
normothermic group. No further reduction in mean CGU was
achieved by cooling to 35°C (27±4 µmol · 100
g-1 · min-1).
Moderate hypothermia resulted in a further significant reduction in
mean CGU to 19±2 µmol · 100
g-1 · min-1 (41%
versus normothermic anesthetized and 67% versus
normothermic conscious groups, P<0.05).
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The local changes observed in the 41 brain structures are also shown in
Table 3
. Compared with the normothermic
anesthetized group, LCGU remained unchanged during mild
hypothermia in 31 of 41 brain structures, was significantly reduced by
20% to 50% in 8 structures, and was significantly increased in 2
structures (P<0.05). Moderate hypothermia significantly
reduced LCGU by 20% to 50% in 18 of 41 brain structures and by >50%
in 12 structures (P<0.05). During moderate hypothermia, no
rise in LCGU was observed in any individual structure.
The relationship between local CBF and CGU is plotted in the
Figure. A close coupling between LCBF and LCGU was found in the
conscious and anesthetized normothermic groups, as
well as in both hypothermic groups. This is indicated by the
correlation coefficients between LCBF and LCGU for
normothermic conscious (r=0.93),
normothermic anesthetized (r=0.92), and
hypothermic (35°C r=0.96, 32°C r=0.96) rats.
These correlation coefficients were significantly different from zero
(P<0.05).
The ratio between LCBF and LCGU is reflected by the slope of the
individual LCBF-LCGU regression line (Figure). Isoflurane
anesthesia caused a significant increase in the slope of
the regression line from 1.5 in the normothermic conscious
animals to 2.5 mL/µmol after anesthesia induction
(P<0.05). The slope of the LCBF-LCGU regression line
remained unchanged during mild hypothermia (2.4 mL/µmol) but
decreased significantly during moderate hypothermia (2.1 mL/µmol,
P<0.05).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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The main findings of this study were that (1) during isoflurane anesthesia, mean CBF was increased by one third and mean CGU was reduced by about one half; (2) during mild hypothermia, the anesthesia-induced increase in mean CBF was abolished without further effects on mean CGU; (3) mean CBF and CGU were further reduced during moderate hypothermia; and (4) despite the fact that the percent reductions in LCGU were always larger than the corresponding reductions in LCBF, the coupling between both was preserved on a local cerebral level. These findings indicate that dissociations between global values for CBF and CGU during hypothermia do not necessarily indicate a disturbed LCBF-LCGU coupling on a local cerebral level.
The following issues require consideration for interpretation of our results: influence of anesthesia and hypothermia on the relationship between CBF and metabolism, influence of acid-base management on CBF and CGU during hypothermia, applicability of autoradiographic methods during hypothermia, and study limitations.
Influence of Anesthesia and Hypothermia on the
Relationship Between CBF and Metabolism
Experimental and clinical studies have demonstrated that
anesthesia and hypothermia have contrasting consequences on
mean CBF and CGU. Isoflurane anesthesia (1 MAC) induces a
marked rise in CBF, together with a 40% reduction in glucose
metabolism.16 23 Maekawa et al16
even concluded that CBF and metabolic changes during
isoflurane anesthesia proceed through unrelated regulatory
mechanisms. Similar findings were observed in hypothermia experiments
and in clinical studies that investigated hypothermic
cardiopulmonary bypass. Murkin et al15
investigated the effects of hypothermic cardiopulmonary bypass
on mean CBF and CMRO2. In the pH-stat
management group (blood gases corrected for actual body temperature),
hypothermia resulted in a marked reduction of mean
CMRO2 but unchanged or increased, widely varying
mean CBF, supposedly to indicate flow-metabolism
uncoupling. Similar results were obtained by Stephan et
al.14 Hypothermia at 26°C reduced mean
CMRO2 and cerebral glucose uptake by 58% and
74%, whereas mean CBF was increased by 190%. The authors also
interpreted this finding as uncoupling of flow and
metabolism. Because these studies evaluated only global
values of mean CBF and CMR, the effects of anesthesia and
hypothermia on a local cerebral level could be speculative. In the
present experimental study, anesthesia was the main
factor that contributed to contrasting CBF-CGU changes. Isoflurane
anesthesia at 1 MAC significantly increased mean CBF
(30%) and reduced mean CGU (45%). However, a close LCBF-LCGU
relationship was maintained on a local cerebral level, with only the
LCBF-to-LCGU ratio being reset to a higher level. This resetting also
existed in the mildly and moderately hypothermic anesthetized
rats but with a trend toward normalization with decreasing temperature.
In summary, a marked suppression of mean CGU together with increased,
unchanged, or slightly decreased mean CBF does not necessarily indicate
LCBF-LCGU uncoupling on a local cerebral level.
Influence of Acid-Base Management on CBF and CGU During
Hypothermia
The specific method used for acid-base management during
hypothermia appears to be a main reason for the conflicting results
regarding the CBF-CMR relationship reported in the literature. Two
strategies for hypothermic acid-base management have been suggested:
during -stat management, arterial
CO2 tension is maintained at 40 mm Hg when
measured at 37°C. The dissociation fraction of the imidazole moiety
of histidine is kept constant, and the pH changes parallel the changes
in the neutral pH of water. In pH-stat management, arterial
CO2 tension is maintained at 40 mm Hg when
corrected to the patient’s actual body temperature. Because
temperature correction results in reduced
PaCO2 (due to increased gas
solubility at the lower temperature), the pH-stat strategy requires
controlled hypoventilation or CO2 addition to the
inspired gas, and the net effect is a marked increase in CBF. The
strategy chosen for blood gas management is a major determinant of CBF
and of particular importance for the interpretation of studies on the
relationship between CBF and CGU. Decreased
PaCO2 during -stat management
causes reduced CBF, whereas CBF increases with pH-stat
strategy.26 27 The magnitude of the effects evoked are
closely dependent on the grade of hypothermia. In the majority of
studies, CBF-CGU coupling was preserved during -stat management,
whereas uncoupling was reported during pH-stat
management.14 15
Applicability of Autoradiographic Methods During
Hypothermia
Autoradiographic techniques were used for the
measurement of LCBF and LCGU in the present as well as in prior
hypothermia studies.10 11 These techniques enable the
measurement of mean CBF and CGU and of LCBF and LCGU for every
individual brain structure. Autoradiographic methods have
been widely used in normothermic and hypothermic
conditions, although these techniques are based on assumptions that may
be influenced by hypothermia.18
Autoradiographic LCGU determination is based on the
measurement of the amount of deoxyglucose-6-phosphate in the brain 45
minutes after the injection of radiolabeled DG.
Deoxyglucose-6-phosphate cannot continue the glycolytic pathway, does
not diffuse out of the cell, and therefore is effectively trapped in
the tissue. Autoradiographic CGU determination assumes a
nearly complete phosphorylation of radiolabeled
[14C]deoxyglucose after this 45-minute period.
The enzymatic process of phosphorylation may be slowed
down by hypothermia, potentially resulting in a significant
overestimation of CGU. However, Nakashima et al28
convincingly demonstrated that the rate of
phosphorylation is not significantly influenced by
hypothermia (25°C) and that nearly all of the labeled 2-deoxyglucose
in brain is phosphorylated after the 45-minute waiting
period. The authors concluded that no adaptations in the method of
Sokoloff et al18 are necessary to
autoradiographically assess LCGU in hypothermic rats.
Concerning the measurement of LCBF, the autoradiographic
determination is based on the fast diffusion of
iodo[14C]antipyrine into the brain tissue; this
diffusion is nearly uninfluenced by moderate changes in body
temperature. The iodo[14C]antipyrine method
proved to be reliable during hypothermic
conditions11 28 29 and has even been used for the
determination of CBF in hibernating squirrels with a core temperature
of 9°C.20
Study Limitations
One limitation of the present study is that the effects of
hypothermia cannot be separated from those of isoflurane
anesthesia. Only anesthetized animals were used for
the hypothermia studies due to animal welfare. Furthermore, active
cooling of conscious animals would result in shivering and stress,
probably inducing additional changes in LCBF and LCGU. However,
anesthesia is also used in the clinical setting when
hypothermia is induced for the prevention of brain damage. It can only
be speculated about the effects of hypothermia on LCBF and LCGU in
conscious animals not affected by any anesthetic drugs.
A second limitation of the study might be that we did not directly
measure brain temperature.30 Direct measurement of brain
temperature with implanted probes was avoided because of the possible
interference with the autoradiographic CBF and CGU
measurements. To avoid erroneous measurements, ultrafast
microthermocouple probes were introduced through the masseter muscle to
the outside of the rat skull. Minamisawa et al31 showed
that directly measured brain temperature closely correlates with
subcutaneous skull temperature and that this relationship was lost only
during global forebrain ischemia. Similar results were recently
reported by Brambrink et al.32 We avoided most factors
that increase the likelihood of temperature gradients between skull and
brain temperature, such as global ischemia, opening of the
calvarium, and selective head cooling or warming, and used relatively
long temperature equilibration periods (
60 minutes). Potential
measurement errors induced by temperature gradients between the brain
and skull temperature, for example, should be minimized with this
approach.2
In the present study, moderate degrees of hyperglycemia (225
mg/dL) and hyperoxia (
240 mm Hg) were observed in the
anesthetized, hypothermic animals. Both factors might have
influenced the study results, because both hyperglycemia and hyperoxia
may interfere with CBF and CGU measurements. LCGU assessment with the
[14C]deoxyglucose method is based on a kinetic
analysis of the physical and chemical behaviors of glucose and
[14C]deoxyglucose in the brain. The underlying
equation for LCGU determination requires the knowledge of different
constants, which have previously been derived under normoglycemic
conditions.10 Meanwhile, several authors demonstrated that
hyperglycemia (550 mg/dL) does induce changes in those
constants33 but instead results in only minor alterations
of the derived LCGU values (maximum error <10% at plasma glucose
concentrations observed in the present
study).34 35
Furthermore, the elevated arterial oxygen tension observed
during hypothermia may interfere with CBF measurements. Experimental
and human studies demonstrated that hyperoxia may slightly reduce CBF,
especially under hyperbaric conditions.36 37 However,
PaO2 values observed during
hyperbaric oxygenation are markedly higher than those
in the present investigation
(PaO2 200 mm Hg). James et
al38 observed that increasing
PaO2 above the normal range (up to
450 mm Hg) does not significantly influence gray and white matter
blood flow. In the present study, CBF was significantly (25%)
lower in the moderately compared with the mildly hypothermic rats,
although no significant differences in
PaO2 were observed. We are convinced
that the observed CBF changes were caused by variations in body
temperature and energetic requirements and not by cofactors such as
hyperoxia or methodological errors.
An additional limitation of the present study might be the statistical approach chosen for data analysis. Assessment of the relationship between local CBF and CGU with linear regression analysis and the derived correlation coefficients is open to criticism. A fundamental assumption of this analytical approach is that observations on different regions of the brain are statistically independent. Because observations are repeated on multiple regions of all animals and only the average values for each region are used, the real uncertainty about the relationship between glucose utilization and CBF might be greater than suggested by the regression analysis.24 To exclude erroneous data interpretation with inappropriate statistical analysis, a second approach proposed by McCulloch et al24 and Ford et al25 was used, in which a repeated measures ANOVA was calculated with the use of log-transformed data.
In conclusion, isoflurane anesthesia increased mean CBF by one third and reduced mean CGU by about one half, which has previously been interpreted as uncoupling of CBF and metabolism. Mild hypothermia offset the anesthesia-induced increase in mean CBF without affecting mean CGU, whereas moderate hypothermia reduced both mean CBF and CGU. Although the changes in mean CBF and mean CGU contrasted and the percent reductions in LCGU were always larger than the corresponding reductions in LCBF, a close relationship was maintained between LCBF and LCGU on a local cerebral level under all conditions tested. Therefore, markedly reduced global values for mean CGU at increased, unchanged, or slightly reduced mean CBF do not necessarily indicate a disturbed LCBF-LCGU relationship on a local cerebral level.
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Acknowledgments |
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This work was supported by a grant from the Faculty of Clinical Medicine Mannheim, Germany (Forschungsfond 8-1998/1999).
Received December 9, 1999; accepted February 28, 2000.
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Editorial Comment
Center for Clinical and Molecular Neurobiology, Departments of Neurology and Neuroscience University of Minnesota, Minneapolis, Minnesota
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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Hypothermia has emerged as a promising therapeutic strategy for ischemic and traumatic brain injury (see Tommasino and PicozziR1 for a review). However, the effects of hypothermia on cerebrovascular function have not been extensively studied. In the present article, Krafft and colleagues investigate the effects of 2 levels of hypothermia on cerebral blood flow (CBF) and glucose utilization (CGU) in rats anesthetized with isoflurane. At variance with previous studies in which whole-brain CBF and metabolism were measured, CBF and CGU were measured with quantitative autoradiographic techniques. With this approach, the investigators were able to study the relationship between CBF and CGU in each brain region. To minimize confounding cerebrovascular effects of blood gases/pH management during hypothermia, measurements of arterial PCO2 were not corrected for temperature (
-stat management). At normothermia, isoflurane
anesthesia increased CBF and reduced CGU. Although
hypothermia reduced CBF and CGU, the linear relationship between these
variables was preserved across the brain regions studied. The present study provides evidence of the preservation of coupling between CBF and CGU during graded hypothermia in a well-controlled experimental setting. A potential limitation of this study is that the cerebrovascular actions of hypothermia were not studied in the absence of isoflurane anesthesia; therefore, the effects of isoflurane-induced cerebrovasodilation on CBF-CGU coupling cannot be factored out. However, in humans, hypothermia is commonly used in conjunction with heavy sedation or anesthesia.R2 Therefore, the findings provided in the present study can be valuable, although they should be expanded and refined in future investigations.
Received December 9, 1999; accepted February 28, 2000.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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1. Tommasino C, Picozzi P. Mild hypothermia. J Neurosurg Sci.. 1998;42:37–38.
2.
Schwab S, Schwarz S, Spranger M, Keller E, Bertram M,
Hacke W. Moderate hypothermia in the treatment of patients with severe
middle cerebral artery infarction. Stroke.. 1998;29:2461–2466.
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