Coupling of cerebral blood flow and oxygen metabolism in infant pigs during selective brain hypothermia

Cerebral Oxygen Delivery and Consumption in Brain-Injured Patients

Organism survival depends on oxygen delivery and utilization to maintain the balance of energy and toxic oxidants production. This regulation is crucial to the brain, especially after acute injuries. Secondary insults after brain damage may include impaired cerebral metabolism, ischemia, intracranial hypertension and oxygen concentration disturbances such as hypoxia or hyperoxia. Recent data highlight the important role of clinical protocols in improving oxygen delivery and resulting in lower mortality in brain-injured patients. Clinical protocols guide the rules for oxygen supplementation based on physiological processes such as elevation of oxygen supply (by mean arterial pressure (MAP) and intracranial pressure (ICP) modulation, cerebral vasoreactivity, oxygen capacity) and reduction of oxygen demand (by pharmacological sedation and coma or hypothermia). The aim of this review is to discuss oxygen metabolism in the brain under different conditions.

The effect of continuous digital hypothermia on lamellar energy metabolism and perfusion during laminitis development in two experimental models

BACKGROUND

Continuous digital hypothermia (CDH) prevents lamellar failure in the euglycaemic hyperinsulinaemic clamp (EHC) and oligofructose (OF) laminitis models, but the mechanisms remain unclear.

OBJECTIVES

To evaluate the effects of CDH on lamellar energy metabolism and perfusion in healthy horses and during EHC and OF laminitis models.

STUDY DESIGN

In vivo experiment.

METHODS

Archived samples were used from Standardbred geldings that received no treatment (CON) (n = 8) or underwent EHC (n = 8) or OF (n = 6) laminitis models. Both forelimbs were instrumented with a lamellar microdialysis system, and one forelimb was cooled (CDH) with the other maintained at ambient temperature (AMB). Microdialysate was collected every 6 hours and analysed for glucose, lactate and pyruvate concentrations and lactate to pyruvate ratio (L:P). Microdialysis urea clearance was used to estimate lamellar tissue perfusion. Data were analysed using a mixed-effects linear regression model.

RESULTS

Glucose did not change in CDH limbs relative to AMB in CON (P = .3), EHC (P = .3) or OF (P = .6) groups. There was a decrease in lactate (P < .001) and pyruvate (P < .01) in CDH limbs relative to AMB in all groups. L:P decreased in CON CDH relative to CON AMB (P < .001) but was not different in EHC (P = .6) and OF (P = .07) groups. Urea clearance decreased in CDH limbs relative to AMB in CON (P = .002) and EHC (P < .001), but not in OF (P = .4).

MAIN LIMITATIONS

The EHC model may not mimic natural endocrinopathic laminitis.

CONCLUSIONS

CDH caused a marked decrease in lamellar glucose metabolism (CON, EHC and OF) and perfusion (CON and EHC) without affecting lamellar glucose concentration. Although cellular energy failure is not a primary pathophysiological event in EHC and OF laminitis models, CDH may act by limiting energy supply to pathologic cellular processes whilst preserving those critical to lamellar homoeostasis.

Positron Emission Tomography After Ischemic Brain Injury: Current Challenges and Future Developments

Positron emission tomography (PET) is widely used in clinical and animal studies, along with the development of diverse tracers. The biochemical characteristics of PET tracers may help uncover the pathophysiological consequences of cardiac arrest (CA) and ischemic stroke, which include cerebral ischemia and reperfusion, depletion of oxygen and glucose, and neuroinflammation. PubMed was searched for studies of the application of PET for "cardiac arrest," "ischemic stroke," and "targeted temperature management." Available studies were included and classified according to the biochemical properties involved and metabolic processes of PET tracers, and were summarized. The mechanisms of ischemic brain injuries were investigated by PET with various tracers to elucidate the pathological process from the initial decrease of cerebral blood flow (CBF) to the subsequent abnormalities in energy and oxygen metabolism, to the monitoring of inflammation. In general, the trends of cerebral blood flow and oxygen metabolism after ischemic attack are not unidirectional but closely related to the time point of injury and recovery. Glucose metabolism after injury showed significant differences in different brain regions whereas global cerebral metabolic rate of glucose (CMRglc) declined. PET monitoring of neuroinflammation shows comparable efficacy to immunostaining. The technology of PET targeting in brain metabolism and the development of tracers provide new tools to track and evaluate the brain's pathological changes after ischemic brain injury. Despite no existing evidence for an available PET-based prediction method, discoveries of new tracers are expected to provide more possibilities for the whole field.

Perinatal stroke

Perinatal stroke is a heterogeneous syndrome resulting from brain injury of vascular origin that occurs between 20 weeks of gestation and 28 days of postnatal life. The incidence of perinatal stroke is estimated to be between 1:1600 and 1:3000 live births (approximately 2500 children per year in the United States), though its actual incidence is difficult to estimate because it is likely underdiagnosed. Perinatal arterial ischemic stroke (PAIS) accounts for approximately 70% of cases of perinatal stroke. Cerebral sinovenous thrombosis, while less common, also accounts for a large proportion of the morbidity and mortality seen with perinatal stroke. Hemorrhagic stroke leads to disruption of neurologic function due to intracerebral hemorrhage that is nontraumatic in origin. While most cases of PAIS fall into one of these three categories, other patterns of injury should also be considered perinatal stroke. In some cases, the etiology of PAIS is not known but is idiopathic. This chapter will review the classification, risk factors, pathogenesis, clinical presentation, management, and long-term sequelae of perinatal stroke.

Reperfusion Changes After Stroke and Practical Approaches for Neuroprotection

Reperfusion is the first line of care in a growing number of eligible acute ischemic stroke patients. Early reperfusion with thrombolytic drugs and endovascular mechanical devices is associated with improved outcome and lower mortality rates compared with natural history. Reperfusion is not without risk, however, and may result in reperfusion injury, which manifests in hemorrhagic transformation, brain edema, infarct progression, and neurologic worsening. In this article, the functional and structural changes and underlying molecular mechanisms of ischemia and reperfusion are reviewed. The pathways that lead to reperfusion injury and novel neuroprotective strategies with endogenous properties are discussed.

State-Dependent Changes in Brain Glycogen Metabolism

A fundamental understanding of glycogen structure, concentration, polydispersity and turnover is critical to qualify the role of glycogen in the brain. These molecular and metabolic features are under the control of neuronal activity through the interdependent action of neuromodulatory tone, ionic homeostasis and availability of metabolic substrates, all variables that concur to define the state of the system. In this chapter, we briefly describe how glycogen responds to selected behavioral, nutritional, environmental, hormonal, developmental and pathological conditions. We argue that interpreting glycogen metabolism through the lens of brain state is an effective approach to establish the relevance of energetics in connecting molecular and cellular neurophysiology to behavior.

Fetal Cerebrovascular Maturation: Effects of Hypoxia

The human cerebral vasculature originates in the fourth week of gestation and continues to expand and diversify well into the first few years of postnatal life. A key feature of this growth is smooth muscle differentiation, whereby smooth muscle cells within cerebral arteries transform from migratory to proliferative to synthetic and finally to contractile phenotypes. These phenotypic transformations can be reversed by pathophysiological perturbations such as hypoxia, which causes loss of contractile capacity in immature cerebral arteries. In turn, loss of contractility affects all whole-brain cerebrovascular responses, including those involved in flow-metabolism coupling, vasodilatory responses to acute hypoxia and hypercapnia, cerebral autoregulation, and reactivity to activation of perivascular nerves. Future strategies to minimize cerebral injury following hypoxia-ischemic insults in the immature brain might benefit by targeting treatments to preserve and promote contractile differentiation in the fetal cerebrovasculature. This could potentially be achieved through inhibition of receptor tyrosine kinase-mediated growth factors, such as vascular endothelial growth factor and platelet-derived growth factor, which are mobilized by hypoxic and ischemic injury and which facilitate contractile dedifferentiation. Interruption of the effects of other vascular mitogens, such as endothelin and angiotensin-II, and even some miRNA species, also could be beneficial. Future experimental work that addresses these possibilities offers promise to improve current clinical management of neonates who have suffered and survived hypoxic, ischemic, asphyxic, or inflammatory cerebrovascular insults.

Selective brain hypothermia

Selective brain hypothermia is a powerful concept for neuroprotection that has been successfully investigated in a variety of animal models of global and focal ischemia. Its major advantages over systemic hypothermia include rapid induction of cooling, ability to achieve profound target brain temperatures, organ-selective cooling, and temperature control. Clinical systems and devices are available or are currently under development that utilize conductive (surface-cooling pads, closed-loop catheters), convective (transnasal coolant delivery), or mass and energy transport (cold intra-arterial infusion) methods to achieve and maintain selective brain hypothermia. The "ideal" brain-cooling system that is characterized by rapid cooling to profound hypothermia, its ability to maintain selective cooling over several days, and is noninvasive in nature, remains unrealistic. Instead, systems may be identified by their distinct advantages to meet a specific need in the care of a patient. This involves the consideration of the timing of ischemic injury (preischemic, intraischemic, postischemic), extent of ischemic damage (excitotoxicity, inflammation, necrosis, edema), and type and setting of therapeutic intervention (intensive care, interventional therapy, surgery). The successful translation of these systems into clinical practice will depend on smart engineering, safety and efficacy, and usability in current clinical work flow.

Early Transient Mild Hypothermia Attenuates Neurologic Deficits and Brain Damage After Experimental Subarachnoid Hemorrhage in Rats

OBJECTIVE

Metabolic exhaustion in ischemic tissue is the basis for a detrimental cascade of cell damage. In the acute stage of subarachnoid hemorrhage (SAH), a sequence of global and focal ischemia occurs, threatening brain tissue to undergo ischemic damage. This study was conducted to investigate whether early therapy with moderate hypothermia can offer neuroprotection after experimental SAH.

METHODS

Twenty male Sprague-Dawley rats were subjected to SAH and treated by active cooling (34°C) or served as controls by continuous maintenance of normothermia (37.0°C). Mean arterial blood pressure, intracranial pressure, and local cerebral blood flow over both hemispheres were continuously measured. Neurologic assessment was performed 24 hours later. Hippocampal damage was assessed by hematoxylin-eosin and caspase-3 staining.

RESULTS

By a slight increase of mean arterial blood pressure in the cooling phase and a significant reduction of intracranial pressure, hypothermia improved cerebral perfusion pressure in the first 60 minutes after SAH. Accordingly, a trend to increased cerebral blood flow was observed during this period. The rate of injured neurons was significantly reduced in hypothermia-treated animals compared with normothermic controls.

CONCLUSIONS

The results of this series cannot finally answer whether this form of treatment permanently attenuates or only delays ischemic damage. In the latter case, slowing down metabolic exhaustion by hypothermia may still be a valuable treatment during this state of ischemic brain damage and prolong the therapeutic window for possible causal treatments of the acute perfusion deficit. Therefore, it may be useful as a first-tier therapy in suspected SAH.

Assessment of Blood-Brain Barrier Permeability by Dynamic Contrast-Enhanced MRI in Transient Middle Cerebral Artery Occlusion Model after Localized Brain Cooling in Rats

Objective

The purpose of this study was to evaluate the effects of localized brain cooling on blood-brain barrier (BBB) permeability following transient middle cerebral artery occlusion (tMCAO) in rats, by using dynamic contrast-enhanced (DCE)-MRI.

Materials and Methods

Thirty rats were divided into 3 groups of 10 rats each: control group, localized cold-saline (20℃) infusion group, and localized warm-saline (37℃) infusion group. The left middle cerebral artery (MCA) was occluded for 1 hour in anesthetized rats, followed by 3 hours of reperfusion. In the localized saline infusion group, 6 mL of cold or warm saline was infused through the hollow filament for 10 minutes after MCA occlusion. DCE-MRI investigations were performed after 3 hours and 24 hours of reperfusion. Pharmacokinetic parameters of the extended Tofts-Kety model were calculated for each DCE-MRI. In addition, rotarod testing was performed before tMCAO, and on days 1-9 after tMCAO. Myeloperoxidase (MPO) immunohisto-chemistry was performed to identify infiltrating neutrophils associated with the inflammatory response in the rat brain.

Results

Permeability parameters showed no statistical significance between cold and warm saline infusion groups after 3-hour reperfusion 0.09 ± 0.01 min^-1 vs. 0.07 ± 0.02 min^-1, p = 0.661 for K^trans; 0.30 ± 0.05 min^-1 vs. 0.37 ± 0.11 min^-1, p = 0.394 for kep, respectively. Behavioral testing revealed no significant difference among the three groups. However, the percentage of MPO-positive cells in the cold-saline group was significantly lower than those in the control and warm-saline groups (p < 0.05).

Conclusion

Localized brain cooling (20℃) does not confer a benefit to inhibit the increase in BBB permeability that follows transient cerebral ischemia and reperfusion in an animal model, as compared with localized warm-saline (37℃) infusion group.

Endovascular external carotid artery occlusion for brain selective targeting: a cerebrovascular swine model

Background

The choice of an animal model for cerebrovascular research is often determined by the disease subtype to be studied (e.g. ischemic stroke, hemorrhage, trauma), as well as the nature of the intervention to be tested (i.e. medical device or pharmaceutical). Many initial studies are performed in smaller animals, as they are cost-effective and their encephalic vasculature closely models that of humans. Non-human primates are also utilized when confirmation or validation is required on higher levels and to test larger devices. However, working with primates is complex and expensive. Intermediate sized animal models, such as swine and sheep, may represent a valuable compromise. Their cerebrovascular anatomy, however, comes with challenges because of the natural higher external carotid artery perfusion and the existence of a rete mirabile. We describe a modification to the traditional swine cerebrovascular model that significantly enhances selective brain hemispheric perfusion, limiting external carotid perfusion and dilution.

Results

We investigated whether unilateral endovascular coil-embolization of external carotid artery branches in swine would lead to increased brain perfusion, altering cerebral circulation so that it more closely models human cerebral circulation. Equal amounts of approximately 4 °C cold saline were injected in 6 Yorkshire pigs into the ipsilateral common carotid artery before and after embolization. Hemispheric temperature changes from pre- and post-embolization were obtained as a measure of brain perfusion and averaged and compared using non-parametric statistical tests (Wilcoxon signed rank test, Mann–Whitney U Test). Graphs were plotted with absolute changes in hemispheric temperature over time to determine peak temperature drop (PTD) and corresponding time to peak (TTP) following the cold bolus injection. There was a 288 ± 90 % increase in ipsilateral brain cooling after embolization indicating improved selective blood flow to the brain due to this vascular modification.

Conclusion

We have developed an effective, selective vascular brain model in swine that may be useful as a practical and cost-reducing intermediate step for evaluating target dose–responses for central nervous system drugs and brain selective interventions, such as local hypothermia.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-015-1714-7) contains supplementary material, which is available to authorized users.

Brain temperature and its fundamental properties: a review for clinical neuroscientists

Brain temperature, as an independent therapeutic target variable, has received increasingly intense clinical attention. To date, brain hypothermia represents the most potent neuroprotectant in laboratory studies. Although the impact of brain temperature is prevalent in a number of common human diseases including: head trauma, stroke, multiple sclerosis, epilepsy, mood disorders, headaches, and neurodegenerative disorders, it is evident and well recognized that the therapeutic application of induced hypothermia is limited to a few highly selected clinical conditions such as cardiac arrest and hypoxic ischemic neonatal encephalopathy. Efforts to understand the fundamental aspects of brain temperature regulation are therefore critical for the development of safe, effective, and pragmatic clinical treatments for patients with brain injuries. Although centrally-mediated mechanisms to maintain a stable body temperature are relatively well established, very little is clinically known about brain temperature's spatial and temporal distribution, its physiological and pathological fluctuations, and the mechanism underlying brain thermal homeostasis. The human brain, a metabolically "expensive" organ with intense heat production, is sensitive to fluctuations in temperature with regards to its functional activity and energy efficiency. In this review, we discuss several critical aspects concerning the fundamental properties of brain temperature from a clinical perspective.

Regional changes in cerebral blood flow oxygenation can indicate global changes in cerebral blood flow during coronary artery occlusion in juvenile pigs

Near infrared spectroscopy (NIRS) is a widely employed method for assessment of regional cerebral oxygenation (RcStO2). RcStO2 values are expected to vary with changes in the relative amount of oxyhaemoglobin. The present experimental study aimed to assess the response of RcStO2 to controlled alterations of carotid blood flow (CQ). Landrace pigs were anesthetized followed by surgical preparation. Cyclic variations in cardiac output were accomplished by intermittently occluding the main stem of the left coronary artery. A flow measurement probe for assessing CQ was placed around the left carotid artery. One NIRS probe was placed on the left ipsilateral forehead to assess regional cerebral oximetry. Simultaneous registration of CQ and RcStO2 was conducted. There was a strong correlation for variation in CQ and RcStO2 signal values. Based on coherence analysis the fraction of power of the RcStO2 that was coherent with the CQ signal reached 0.84 ± 0.12 (P < 0.05) for frequencies lower than 0.1 Hz. The agreement of the sample-to-sample co-variation, as assessed by the Pearson correlation coefficient, was 0.83 ± 0.08 (P < 0.05). One explanatory component for variations in cerebral oxygenation verified by NIRS should be attributed to variations in the cerebral blood flow.

BRAIN TEMPERATURE CALCULATIONS FOR SWINE USING EXPERIMENTAL MEASUREMENTS OF CEREBRAL BLOOD FLOW

In this study, numerical simulations are performed to analyze the brain temperature reduction in swine during selective head cooling, whole body cooling or while the animals experience ischemia. Brain temperature is calculated using a time dependent thermal model that incorporates available experimental measurements of the rectal temperature, the cerebral blood flow and the cerebral metabolic rate of oxygen consumption.

The calculated temperature distribution is validated against the in vivo temperature measurements recorded during the different experiments. These comparisons help to better understand the relations between brain temperature, blood flow and metabolic activity, which are essential to successfully apply hypothermia in the treatment of brain injury.

The calculations presented here reproduce the temperature behavior observed in all the experiments considered. It is observed that the arterial temperature and the cerebral metabolic rate are important parameters that affect the deep tissue temperature. It is also concluded that the accurate knowledge of parameters such as the skin and bone thermal conductivity are necessary for effective modeling.

Effects of selective head cooling on cerebral blood flow and metabolism in newborn piglets after hypoxia-ischemia

AIM

the effect of selective head cooling on cerebral blood flow (CBF) and cerebral metabolism rate (CMR) was investigated in newborn piglets.

METHODS

seven days old newborn piglets were randomly assigned to one of the following three groups: Selective head cooling in normal piglets (n=4), selective head cooling after HI (n=6) and normal temperature after HI (n=6). CBF was measured with color microspheres. Cerebral oxygenation metabolism rate (CMRO(2)), Cerebral glucose consumption (CMR(Glu)) and Cerebral lactate production (CMR(lac)) were calculated.

RESULT

in normal piglets, CBF, CMRO(2) and CMR(glu) were significantly decreased at both 35°C (P<0.05) and 32°C (P<0.01), while CMR(lac) did not change. Compared to baseline, CBF and CMRO(2) were significantly reduced (P<0.05), while CMR(glu) and CMR(lac) were significantly increased (P<0.01), AVDO(2) was decreased (P<0.05), while AVD(glu) and AVD(lac) were significantly increased (P<0.01 respectively) in HI piglets with normal temperature respectively. Compared to normal temperature after HI, selective head cooling after HI significantly reduced CMR(glu) and CMR(lac), and AVDO(2), AVD(glu), AVD(lac) were improved at 35°C.

CONCLUSION

selective head cooling not only reduced energy consumption, but also improve brain oxygen metabolism in newborn after HI.

Correlation and agreement between the bispectral index vs. state entropy during hypothermic cardio-pulmonary bypass

BACKGROUND

The bispectral index (BIS) and spectral entropy enable monitoring the depth of anaesthesia. Mild hypothermia has been shown to affect the ability of electroencephalography monitors to reflect the anaesthetic drug effect. The purpose of this study was to investigate the effect of hypothermia during a cardio-pulmonary bypass on the correlation and agreement between the BIS and entropy variables compared with normothermic conditions.

METHODS

This prospective clinical study included coronary artery bypass grafting patients (n=25) evaluating correlation and agreement (Bland-Altman analysis) between the BIS and both spectral and response entropy during a hypothermic cardio-pulmonary bypass (31-34 degrees C) compared with nomothermic conditions (34-37.5 degrees C). Anaesthesia was maintained with propofol and sufentanil and adjusted clinically, while the anaesthetist was blinded to the monitors.

RESULTS

The BIS and entropy values decreased during cooling (P<0.05), but the decrease was more pronounced for entropy variables compared with BIS (P<0.05). The correlation coefficients (bias+/-SD; percentage error) between the BIS vs. spectral state entropy and response entropy were r(2)=0.56 (1+/-11; 42%) and r(2)=0.58 (-2+/-11; 43%) under normothermic conditions, and r(2)=0.17 (10+/-12; 77%) and r(2)=0.18 (9+/-11; 68%) under hypothermic conditions, respectively. Bias was significantly increased under hypothermic conditions (P<0.001 vs. normothermia).

CONCLUSION

Acceptable agreement was observed between the BIS and entropy variables under normothermic but not under hypothermic conditions. The BIS and entropy variables may therefore not be interchangeable during a hypothermic cardio-pulmonary bypass.

Selective brain cooling with endovascular intracarotid infusion of cold saline: a pilot feasibility study

BACKGROUND AND PURPOSE

Endovascular brain cooling as a method for rapid and selective induction of hypothermic neuroprotection has not been systematically studied in humans. In this clinical pilot study we investigated the feasibility, safety, and physiologic responses of short-term brain cooling with IC-CSI.

MATERIALS AND METHODS

We studied 18 patients (50 +/- 10 years old, 9 women) undergoing follow-up cerebral angiography after previous treatment of vascular malformations. Isotonic saline (4-17 degrees C) was infused into 1 internal carotid artery at 33 mL/min for 10 minutes. Brain (JVB) and bladder/esophageal temperature measurements (n = 9) were performed. Both MCAs were monitored with transcranial Doppler sonography (n = 13). Arterial and JV blood were sampled to estimate hemodilution and brain oxygen extraction.

RESULTS

JVB temperature dropped approximately 0.84 +/- 0.13 degrees C and systemic temperature by 0.15 +/- 0.08 degrees C from baseline (JVB versus systemic temperature: P = .0006). Systolic MCA-flow velocities decreased from 101 +/- 27 to 73 +/- 18 cm/s on the infused side and from 83 +/- 24 to 78 +/- 21 cm/s on the contralateral side (relative changes, -26 +/- 8% versus -4 +/- 27%; P = .009). Changes in hematocrit (-1.2 +/- 1.1%) and cerebral arteriovenous oxygen difference (0.2 +/- 1.0 mL O(2)/100 mL) were not significant. Doppler data showed no signs of vascular spasm or microemboli. No focal neurologic deficits occurred. Pain was not reported.

CONCLUSIONS

The results of this pilot study suggest that brain cooling can be achieved safely, rapidly, and selectively by means of IC-CSI, opening a new potential avenue for acute neuroprotection. Clinical investigations with control of infusion parameters and measurements of CBF, oxygen consumption, and brain temperature are warranted.

Influences of hypothermia on the cortical blood supply by laser speckle imaging

Induced hypothermia has been broadly applied in neurological intensive care unit (NICU). Meanwhile, accidental hypothermia is also a threatening condition in daily life. It is meaningful to investigate the influences of temperature change on the cerebral blood flow (CBF). In the present study, temporal laser speckle image contrast analysis (tLASCA) was implemented to study the relative CBF change in cerebral artery, vein and capillary level under mild (35 degrees C) and moderate (32 degrees C) hypothermia. Twelve male Sprague-Dawley rats (300 +/-50 g) were anesthetized with sodium pentobarbital and randomly assigned to mild and moderate hypothermia groups (n=9 each). Laser speckle imaging (LSI) trials were acquired from baseline (37 degrees C), hypothermia (35 degrees C or 32 degrees C), and post-rewarming (37 degrees C) phases. In the mild group, mean CBF in different vessels all increased throughout the hypothermic and post-rewarming phases. On the contrary, mean CBF reduced by 10%-20% at 32 degrees C and returned to approximately 95% of the baseline level during the post-rewarming session in the moderate group. Besides, in the moderate group, a CBF rebound in vein was found in the post-rewarming phase. Our results suggested that the CBF changed differently between mild and moderate hypothermia, which may be worth for further study in clinic. And we demonstrated LSI as a promising method to achieve high spatiotemporal resolution CBF change with minimal invasion.

Brain cooling for preterm infants

There is strong evidence that prolonged, moderate cerebral hypothermia initiated within a few hours after severe hypoxia-ischemia and continued until resolution of the acute phase of delayed cell death can reduce neuronal loss and improve behavioral recovery in term infants and adults after cardiac arrest. This review examines the evidence that mild to moderate hypothermia is protective after hypoxia-ischemia in models of preterm brain injury and evaluates the potential risks. Induced hypothermia likely has potential to significantly reduce disability. Cautious, systematic trials are essential before hypothermia can be used in these vulnerable infants.

Brain cooling maintenance with cooling cap following induction with intracarotid cold saline infusion: a quantitative model

Intracarotid cold saline infusion (ICSI) is potentially much faster than whole-body cooling and more effective than cooling caps in inducing therapeutic brain cooling. One drawback of ICSI is hemodilution and volume loading. We hypothesized that cooling caps could enhance brain cooling with ICSI and minimize hemodilution and volume loading. Six-hour-long simulations were performed in a 3D mathematical brain model. The Pennes bioheat equation was used to propagate brain temperature. Convective heat transfer through jugular venous return and the circle of Willis was simulated. Hemodilution and volume loading were modeled using a two-compartment saline infusion model. A feedback method of local brain temperature control was developed where ICSI flow rate was varied based on the rate of temperature change and the deviation of temperature to a target (32 degrees C) within a voxel in the treated region of brain. The simulations confirmed the inability of cooling caps alone to induce hypothermia. In the ICSI and the combination models (ICSI and cap), the control algorithm guided ICSI to quickly achieve and maintain the target temperature. The combination model had lower ICSI flow rates than the ICSI model resulting in a 55% reduction of infusion volume over a 6h period and higher hematocrit values compared to the ICSI model. Moreover, in the combination model, the ICSI flow rate decreased to zero after 4h, and hypothermia was subsequently maintained solely by the cooling cap. This is the first study supporting a role of cooling caps in therapeutic hypothermia in adults.

Hypothermia effects on neurovascular coupling and cerebral metabolic rate of oxygen

Neuronal activation is accompanied by a local increase in cerebral blood flow (CBF) and in cerebral metabolic rate of oxygen (CMRO(2)), caused by neurovascular and neurometabolic coupling. Hypothermia is used as a neuroprotective approach in surgical patients and therapeutically after cardiac arrest or stroke. The effect of hypothermia on neurovascular coupling is of interest for evaluating brain function in these patients, but has not been determined so far. It is not clear whether functional hyperaemia actually operates at subnormal temperatures. In addition, decreasing brain temperature reduces spontaneous CMRO(2) following a known quantitative relationship (Q(10)). Q(10) determination may serve to validate a recently introduced CMRO(2) measurement approach relying on optical measurements of CBF and hemoglobin concentration. We applied this method to investigate hypothermia in a functional study of the somatosensory cortex. Anesthetized Wistar rats underwent surgical implantation of a closed cranial window. Using laser Doppler flowmetry and optical spectroscopy, relative changes in CBF and hemoglobin concentration were measured continuously. At the same time, an electroencephalogram (EEG) was recorded from the measurement site. By the application of ice packs, whole-body hypothermia was induced, followed by rewarming. Spontaneous EEG, CBF and CMRO(2) were measured, interleaved by blocks of electrical forepaw stimulation. The Q(10) obtained from spontaneous CMRO(2) changes of 4.4 (95% confidence interval 3.7-5.1) was close to published values, indicating the reliability of the CMRO(2) measurement. Lowering brain temperature decreased functional changes of CBF and CMRO(2) as well as amplitudes of somatosensory evoked potentials (SEP) to the same degree. In conclusion, neurovascular and neurometabolic coupling is preserved during hypothermia.

Integration of jugular venous return and circle of Willis in a theoretical human model of selective brain cooling

A three-dimensional mathematical model was developed to examine the induction of selective brain cooling (SBC) in the human brain by intracarotid cold (2.8°C) saline infusion (ICSI) at 30 ml/min. The Pennes bioheat equation was used to propagate brain temperature. The effect of cooled jugular venous return was investigated, along with the effect of the circle of Willis (CoW) on the intracerebral temperature distribution. The complete CoW, missing A1 variant (mA1), and fetal P1 variant (fP1) were simulated. ICSI induced moderate hypothermia (defined as 32–34°C) in the internal carotid artery (ICA) territory within 5 min. Incorporation of the complete CoW resulted in a similar level of hypothermia in the ICA territory. In addition, the anterior communicating artery and ipsilateral posterior communicating artery distributed cool blood to the contralateral anterior and ipsilateral posterior territories, respectively, imparting mild hypothermia (35 and 35.5°C respectively). The mA1 and fP1 variants allowed for sufficient cooling of the middle cerebral territory (30–32°C). The simulations suggest that ICSI is feasible and may be the fastest method of inducing hypothermia. Moreover, the effect of convective heat transfer via the complete CoW and its variants underlies the important role of CoW anatomy in intracerebral temperature distributions during SBC.

Heat transfer model of hyporthermic intracarotid infusion of cold saline for stroke therapy

A 3-dimensional hemispheric computational brain model is developed to simulate infusion of cold saline in the carotid arteries in terms of brain cooling for stroke therapy. The model is based on the Pennes bioheat equation, with four tissue layers: white matter, gray matter, skull, and scalp. The stroke lesion is simulated by reducing blood flow to a selected volume of the brain by a factor of one-third, and brain metabolism by 50%. A stroke penumbra was also generated surrounding the core lesion (blood volume reduction 25%, metabolism reduction 20%). The finite difference method was employed to solve the system of partial differential equations. This model demonstrated a reduction in brain temperature, at the stroke lesion, to 32 degrees C in less than 10 minutes.

Temperature gradient between brain tissue and arterial blood mirrors the flow-metabolism relationship in uninjured brain: an experimental study

BACKGROUND

The purpose of the present experimental study was to determine the feasibility and usefulness of brain temperature measurement (T(br)) and the calculated difference between brain temperature and arterial blood temperature (DeltaT(br-a)) in uninjured brain during variations of cerebral perfusion pressure (CPP) and concomitant changes of the regional cerebral blood flow (rCBF).

METHODS

Nine anaesthetized pigs were subjected to controlled CPP decrease to assess the lower cerebral autoregulation threshold. A parenchymal intracranial pressure (ICP) sensor combined with a microthermistor for temperature measurement, a miniaturized Clark-type electrode measuring brain tissue oxygenation (p(ti)O(2)), a small flexible intraparenchymal thermodilution probe for measuring rCBF and cerebral microdialysis were inserted carefully in the frontal white matter.

RESULTS

Analysing the p(ti)O(2) during controlled CPP decrease, we found significant breakpoints of p(ti)O(2) at a CPP of 40 mmHg and 20 mmHg, related to an rCBF of 20 ml/100 g/min and approximately 10 ml/100 g/min. Similarly, the relationship between DeltaT(br-a), and CPP or rCBF revealed a characteristic increase of DeltaT(br-a) in the negative direction up to more than -0.30 degrees C assuming a strong flow dependency.

CONCLUSION

The temperature difference between brain tissue and arterial blood DeltaT(br-a) mainly reflects the cerebral blood flow-brain tissue oxygenation-metabolism relationship as far as the estimation of the individual lower cerebral autoregulation threshold.

A theoretical model of selective cooling using intracarotid cold saline infusion in the human brain

A three-dimensional mathematical model was developed to examine the transient and steady-state temperature distribution in the human brain during selective brain cooling (SBC) by unilateral intracarotid freezing-cold saline infusion. To determine the combined effect of hemodilution and hypothermia from the cold saline infusion, data from studies investigating the effect of these two parameters on cerebral blood flow (CBF) were pooled, and an analytic expression describing the combined effect of the two factors was derived. The Pennes bioheat equation used the thermal properties of the different cranial layers and the effect of cold saline infusion on CBF to propagate the evolution of brain temperature. A healthy brain and a brain with stroke (ischemic core and penumbra) were modeled. CBF and metabolic rate data were reduced to simulate the core and penumbra. Simulations using different saline flow rates were performed. The results suggested that a flow rate of 30 ml/min is sufficient to induce moderate hypothermia within 10 min in the ipsilateral hemisphere. The brain with stroke cooled to lower temperatures than the healthy brain, mainly because the stroke limited the total intracarotid blood flow. Gray matter cooled twice as fast as white matter. The continuously falling hematocrit was the main time-limiting factor, restricting the SBC to a maximum of 3 h. The study demonstrated that SBC by intracarotid saline infusion is feasible in humans and may be the fastest method of hypothermia induction.

Cerebral pathophysiology and clinical neurology of hyperthermia in humans

Deliberate hyperthermia has been used clinically as experimental therapy for neoplastic and infectious diseases. Several case fatalities have occurred with this form of treatment, but most were attributable to systemic complications rather than central nervous system toxicity. Nonetheless, demyelating peripheral neuropathy and neurological symptoms of nausea, delirium, apathy, stupor, and coma have been reported. Temperatures exceeding 40 degrees C cause transient vasoparalysis in humans, resulting in cerebral metabolic uncoupling and loss of pressure-flow autoregulation. These findings may be related to the development of brain edema, intracerebral hemorrhage, and intracranial hypertension observed after prolonged therapeutic hyperthermia. Furthermore, deliberate hyperthermia critically worsens the extent of histopathological damage in animal models of traumatic, ischemic, and hypoxic brain injury. However, it is unknown whether these findings translate to episodes of spontaneous fever in neurologically injured patients. In a clinical setting fever is a strong prognostic marker of a patient's primary degree of neuronal damage, and a causal relation with long-term functional neurological outcome has not been established for most types of brain injury. Furthermore, in the neurosurgical intensive-care unit fever is extremely common whereas antipyretic therapy is only poorly effective. Therefore maintaining strict normothermia may be an impossible goal in many patients. Although there are several physiological arguments for avoiding exogenous hyperthermia in neurologically injured patients, there is no evidence that aggressive attempts at controlling spontaneous fever can improve clinical outcome.

Combined effects of propofol and mild hypothermia on cerebral metabolism and blood flow in rhesus monkey: a positron emission tomography study

PURPOSE

Propofol reduces the cerebral metabolic rate for oxygen (CMRO2), regional CMRO2 (rCMRO2), cerebral blood flow (CBF), and regional CBF (rCBF), but maintains the coupling of cerebral metabolism and blood flow. Under mild to moderate hypothermia, the coupling is maintained, while rCBF is reduced, but no direct measurement of rCMRO2 has yet been reported. This study aimed to evaluate the effects of propofol under normothermic and mild hypothermic temperatures upon rCMRO2, rCBF, and their regional coupling, through direct measurement by positron emission tomography.

METHODS

Rhesus monkeys were anesthetized with 65% nitrous oxide and propofol. Then rCBF and rCMRO2 were measured under four sets of conditions: infusion of a low-propofol dose (12 mg.kg(-1) x h(-1)) at normothermic temperatures (38 degrees C), a high dose (25 mg x kg(-1) x h(-1)) at normothermic temperatures, a low dose under mild hypothermia (35 degrees C), and a high dose under mild hypothermia. The ratio of rCBF/rCMRO(2) was calculated from these data.

RESULTS

Reductions in CMRO2 and rCMRO2 in most regions were associated with two factors: the higher propofol dose and the induction of hypothermia, but there was no interaction between these factors. Concerning blood flow, no significant reduction was observed, except for CBF by the induction of hypothermia. The ratio of rCBF/rCMRO2 was constant in this study setting.

CONCLUSION

During propofol anesthesia, it is possible to reduce cerebral metabolism throughout the entire brain as well as in any brain region by increasing the propofol dose or inducing hypothermia. The concurrent use of these two interventions has an additive effect on metabolism, and can be considered as safe, as their combination does not impair the coupling of cerebral metabolism and blood flow.

Superficial brain is cooler in small piglets: neonatal hypothermia implications

OBJECTIVE

Hypothermia was not neuroprotective in low body weight (BW) infants on subgroup analysis in a recent clinical trial of selective head cooling (SHC) in neonatal encephalopathy (CoolCap Trial).

METHODS

The BW dependence of regional cerebral temperature was investigated in 14 newborn piglets under normothermia (38.5 degrees C), whole-body cooling (WBC; 36.5, 34.5, 32.5, and 30.5 degrees C), or SHC (20, 15, and 10 degrees C).

RESULTS

Normothermia: Lower BW led to lower superficial brain temperature (p < 0.01). Deep to superficial brain and rectal to superficial brain temperature gradients increased with decreasing BW (both p < 0.05). WBC: Lower BW led to lower superficial brain temperature and higher rectal to superficial brain temperature gradient (p < 0.05 and p < 0.01, respectively). SHC: For lower BW, superficial and deep brain temperatures decreased (p < 0.01 and p < 0.05, respectively), whereas rectal to deep, rectal to superficial, and deep to superficial brain temperature gradients increased (p < 0.05, p < 0.01, and p < 0.05, respectively). Compared with SHC alone, superimposition of WBC (34.5 degrees C) reduced all regional temperatures (all p < 0.001); gradients were unaffected.

INTERPRETATION

Brain cooling (under normothermia, WBC, or SHC) was more efficient with lower BW due to greater head surface area-to-volume ratios. In the CoolCap Trial, low BW infants might have been excessively cooled. WBC and SHC may require BW adjustment to accomplish consistent regional temperatures and optimal neuroprotection.

Effects of rapid rewarming on cerebral nitric oxide production and cerebral hemodynamics after hypothermia therapy for kainic acid-induced seizures in immature rabbits

BACKGROUND

The aim of the present study was to investigate whether rapid rewarming after hypothermia therapy during seizures alters the endogenous nitric oxide (NO) production in and around hippocampus, cortical cerebral blood flow (cCBF), and mean arterial blood pressure (MABP) in immature rabbits.

METHODS

The hypothermic rabbits (rectal temperatures, 33 degrees C) were given kainic acid (KA; 12 mg/kg, i.v; at 0 min), followed by cooling (33 degrees C) for 60 min (at 60 min), then either rewarming (RW; 33-37 degrees C) was started (KA[+]RW[+] group, n = 7) or cooling was continued (KA[+]RW[-] group, n = 7) for another 60 min (at the end 120 min). In the KA(-)RW(+) group (n = 5), 0.5 mL normal saline was given (at time 0 min), followed by cooling (33 degrees C) for 60 min (at 60 min), then rewarming to 37 degrees C was started with observation for another 60 min (at the end 120 min). NO production in and around hippocampus was continuously measured by an NO-selective electrode, cCBF by laser Doppler flowmetry, cortical electroencephalogram (EEG), rectal and cerebral temperatures, and MABP during the experiment. Comparisons were made of these parameters between the values at 60 min and 120 min after the KA administrations.

RESULTS

KA administration induced abnormal discharges in both KA(+)RW(+) and KA(+)RW(-) groups at the same degree. The KA(+)RW(+) group had a significant increase in %NO, and significant decreases in %cCBF and MABP after rapid rewarming, compared with before rewarming. In the KA(+)RW(-) group, there were no significant changes in %NO, %cCBF, and MABP between values at 60 and 120 min. These changes after rapid rewarming in the KA(+)RW(+) group were different from those with only elevation in brain temperature from 33 to 37 degrees C without seizures (KA[-]RW[+] group).

CONCLUSIONS

These results suggest that rapid rewarming after hypothermia therapy induces an increase in the NO production in and around hippocampus and the decreases in cCBF and MABP during seizures in immature rabbits.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Secondary injuries in brain trauma: effects of hypothermia

Hypothermia has been shown to be cerebroprotective in traumatized brains. Although a large number of traumatic brain injury (TBI) studies in animals have shown that hypothermia is effective in suppressing a variety of damaging mechanisms, clinical investigations have shown less consistent results. The complexity of damaging mechanisms in human TBI may contribute to these discrepancies. In particular, secondary injuries such as hypotension and hypoxemia may promote poor outcome. However, few experimental TBI studies have employed complex models that included such secondary injuries to clarify the efficacy of hypothermia. This review discusses the effects of hypothermia in various TBI models addressing primary and acute secondary injuries. Included are recently published clinical data using hypothermia as a therapeutic tool for preventing or reducing the detrimental posttraumatic secondary injuries and neurobehavioral deficits. Also discussed are recent successful applications of hypothermia from outside the TBI realm. Based on all available data, some general considerations for the application of hypothermia in TBI patients are given.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Effect of mild hypothermia and hypoxia on blood flow and oxygen consumption of the fetal sheep brain

This study was undertaken to measure the effects of mild hypothermia on cerebral blood flow and metabolism and cardiovascular responses to hypoxia in the fetal sheep. Near-term fetal sheep were chronically instrumented with laser Doppler flowmetry in the parietal cortex for measurement of relative changes in cerebral blood flow, as well as with arterial and sagittal sinus catheters for measurement of oxygen extraction by the brain and a cooling coil around the fetal thorax. Fetuses were studied during cooling alone, cooling with superimposed maternal hypoxia to achieve a fetal arterial Po2 of 1.33 to 1.60 kPa, or hypoxia alone. In response to cooling alone [1.6 degrees +/- 0.1 degrees C (mean +/- SEM) decrease in brain temperature], fetal blood pressure and heart rate both increased significantly whereas cerebral blood flow decreased 14 +/- 4%, commensurate with a 24 +/- 8% decline in cerebral metabolic rate. Administration of moderate hypoxia during cooling resulted in a significant increase in cerebral blood flow, decreased heart rate, and no further increase in blood pressure. In response to hypoxia alone, fetal blood pressure was significantly increased, heart rate was decreased, and cerebral blood flow increased by 24 +/- 8%, whereas cerebral metabolic rate decreased by 38 +/- 13%. Arteriovenous oxygen extraction was unchanged by cooling alone but increased significantly in response to hypoxia administered during cooling. We therefore conclude that oxygen delivery to the fetal sheep brain remains coupled to metabolic rate during hypothermia and that hypothermia does not impair the compensatory cardiovascular responses of the fetus to acute moderate hypoxia.

Induced hypothermia in critical care medicine: a review

BACKGROUND

Clinical trials of induced hypothermia have suggested that this treatment may be beneficial in selected patients with neurologic injury.

OBJECTIVES

To review the topic of induced hypothermia as a treatment of patients with neurologic and other disorders.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

Improved outcome was demonstrated in two prospective, randomized, controlled trials in which induced hypothermia (33 degrees C for 12-24 hrs) was used in patients with anoxic brain injury following resuscitation from prehospital cardiac arrest. In addition, prospective, randomized, controlled trials have been conducted in patients with severe head injury, with variable results. There also have been preliminary clinical studies of induced hypothermia in patients with severe stroke, newborn hypoxic-ischemic encephalopathy, neurologic infection, and hepatic encephalopathy, with promising results. Finally, animal models have suggested that hypothermia that is induced rapidly following traumatic cardiac arrest provides significant neurologic protection and improved survival.

CONCLUSIONS

Induced hypothermia has a role in selected patients in the intensive care unit. Critical care physicians should be familiar with the physiologic effects, current indications, techniques, and complications of induced hyperthermia.

Effects of hypothermia on energy metabolism in Mammalian central nervous system

This review analyzes, in some depth, results of studies on the effect of lowered temperatures on cerebral energy metabolism in animals under normal conditions and in some selected pathologic situations. In sedated and paralyzed mammals, acute uncomplicated 0.5- to 3-h hypothermia decreases the global cerebral metabolic rate for glucose (CMR(glc)) and oxygen (CMRo(2)) but maintains a slightly better energy level, which indicates that ATP breakdown is reduced more than its synthesis. Intracellular alkalinization stimulates glycolysis and independently enhances energy generation. Lowering of temperature during hypoxia-ischemia slows the rate of glucose, phosphocreatine, and ATP breakdown and lactate and inorganic phosphate formation, and improves recovery of energetic parameters during reperfusion. Mild hypothermia of 12 to 24-h duration after normothermic hypoxic-ischemic insults seems to prevent or ameliorate secondary failures in energy parameters. The authors conclude that lowered head temperatures help to protect and maintain normal CNS function by preserving brain ATP supply and level. Hypothermia may thus prove a promising avenue in the treatment of stroke and trauma and, in particular, of perinatal brain injury.

Veno-venous extracorporeal blood shunt cooling to induce mild hypothermia in dog experiments and review of cooling methods

Mild hypothermia (33-36 degrees C) might be beneficial when induced during or after insults to the brain (cardiac arrest, brain trauma, stroke), spinal cord (trauma), heart (acute myocardial infarction), or viscera (hemorrhagic shock). Reaching the target temperature rapidly in patients inside and outside hospitals remains a challenge. This study was to test the feasibility of veno-venous extracorporeal blood cooling for the rapid induction of mild hypothermia in dogs, using a simple pumping-cooling device. Ten custom-bred hunting dogs (21-28 kg) were lightly anesthetized and mechanically ventilated. In five dogs, two catheters were inserted through femoral veins, one peripheral and the other into the inferior vena cava. The catheters were connected via a coiled plastic tube as heat exchanger (15 m long, 3 mm inside diameter, 120 ml priming volume), which was immersed in an ice-water bath. A small roller-pump produced a veno-venous flow of 200 ml/min (about 10% of cardiac output). In five additional dogs (control group), a clinically practiced external cooling method was employed, using alcohol over the skin of the trunk and fanning plus ice-bags. During spontaneous normotension, veno-venous cooling delivered blood into the vena cava at 6.2 degrees C standard deviation (SD 1.4) and decreased tympanic membrane (Tty) temperature from 37.5 to 34.0 degrees C at 5.2 min (SD 0.7), and to 32.0 degrees C at 7.9 min (SD 1.3). Skin surface cooling decreased tympanic temperature from 37.5 to 34.0 degrees C at 19.9 min (SD 3.7), and to 32.0 degrees C at 29.9 (SD 5.1) (P=0.001). Heart rates at Tty 34 and 32 degrees C were significantly lower than at baseline in both groups, but within physiological range, without difference between groups. There were no arrhythmias. We conclude that in large dogs the induction of mild systemic hypothermia with extracorporeal veno-venous blood shunt cooling is simple and four times more rapid than skin surface cooling.

Significant selective head cooling can be maintained long-term after global hypoxia ischemia in newborn piglets

OBJECTIVE

Selective head cooling (SHC) combined with mild body cooling is currently being evaluated as a potentially therapeutic option in the management of neonatal hypoxic-ischemic encephalopathy. It is proposed that SHC enables local hypothermic neuroprotection while minimizing the deleterious side effects of systemic hypothermia. However, there is little evidence that it is possible to cool the brain more than the body for a prolonged period of time. The aim of this study was to examine whether the brain (T(deep brain)) could be cooled to below the rectal temperature (T(rectal)) in our piglet hypoxia ischemia (HI) model for a period of 24 hours, using a head-cooling cap.

METHODS

Eight anesthetized piglets (median age: 15 hours) had subdural and intracerebral basal ganglia temperature probes inserted. After a 45-minute global HI insult (known to produce permanent brain damage), SHC using a cap perfused with cold water (5 degrees C-24 degrees C) combined with overhead body heating to maintain T(rectal) at 34 to 35 degrees C was performed for 24 hours.

RESULTS

The piglets were cooled to a median T(rectal) of 35.0 degrees C (interquartile range [IQR]: 34.7-35.3) for 24 hours. During this time, the median T(deep brain) was 31.4 degrees C (IQR: 30 degrees C-32.2 degrees C), with a median T(rectal) to T(deep brain) gradient of 3.4 degrees C (IQR: 2.7 degrees C-4.8 degrees C). At the end of the cooling period, this gradient was still maintained at a median of 3.3 degrees C (IQR: 2.9 degrees C-3.7 degrees C). The ability to obtain the gradient was not influenced by the size of the piglet (1300-1840 g). Cap cooling lowered scalp temperature (T(scalp)) to a median of 24.9 degrees C (IQR: 22.2 degrees C-29.2 degrees C) and subdural temperature to a median of 28.1 degrees C (IQR: 25.8 degrees C-29.5 degrees C) but did not result in either skin injury or superficial brain hemorrhage. There was no clinically useful correlation between T(scalp) and T(deep brain) or between T(scalp) and T(subdural).

CONCLUSIONS

This study using our piglet HI model shows that it is possible by means of a head-cooling cap to cool the brain more than the body for a 24-hour period while keeping the core temperature mildly hypothermic. However, we were unable to predict temperatures inside the brain using surface temperature probes on the head.

Differences in brain temperature and cerebral blood flow during selective head versus whole-body cooling

OBJECTIVE

To compare brain temperature and cerebral blood flow (CBF) during head and body cooling, with and without systemic hypoxemia.

METHODS

Seventeen newborn swine were studied for either measurement of brain temperature alone (n = 9) or measurement of brain temperature and CBF (n = 8). All animals were ventilated and instrumented, and temperature probes were inserted into the rectum, into the brain at depths of 2 and 1 cm from the cortical surface, and on the dural surface. Blood flow was measured with microspheres. The protocol consisted of a control period, an interval of either head or body cooling, and cooling with 15 minutes of superimposed hypoxia. After a 1-hour recovery period, animals were exposed to the same sequence except that the alternate mode of cooling was evaluated.

RESULTS

Head cooling with a constant rectal temperature resulted in an increase in the temperature gradient across the brain from the warmer central structures to the cooler periphery (brain 2 cm - dura temperature: 1.3 +/- 1.1 degrees C at control to 7.5 +/- 3.5 degrees C during cooling). Hypoxia superimposed on head cooling decreased the temperature gradient by at least 50%. In contrast, body cooling was associated with an unchanged temperature gradient across the brain (brain 2 cm - dura temperature: 1.5 +/- 1.2 degrees C at control to 1.1 +/- 0.9 degrees C during cooling). Hypoxia superimposed on body cooling did not change brain temperature. Both modes of brain cooling resulted in similar reductions of global CBF ( approximately 40%) and O(2) uptake.

CONCLUSION

Brain hypothermia achieved through head or body cooling results in different brain temperature gradients. Alterations in systemic variables (ie, hypoxemia) alters brain temperature differently in these 2 modes of brain cooling. The mode of brain cooling may affect the efficacy of modest hypothermia as a neuroprotective therapy.