Characterization of neuroprotection from excitotoxicity by moderate and profound hypothermia in cultured cortical neurons unmasks a temperature-insensitive component of glutamate neurotoxicity

Although profound hypothermia has been used for decades to protect the human brain from hypoxic or ischemic insults, little is known about the underlying mechanism. We therefore report the first characterization of the effects of moderate (30 degrees C) and profound hypothermia (12 degrees to 20 degrees C) on excitotoxicity in cultured cortical neurons exposed to excitatory amino acids (EAA; glutamate, N-methyl-D-aspartate [NMDA], AMPA, or kainate) at different temperatures (12 degrees to 37 degrees C). Cooling neurons to 30 degrees C and 20 degrees C was neuroprotective, but cooling to 12 degrees C was toxic. The extent of protection depended on the temperature, the EAA receptor agonist employed, and the duration of the EAA challenge. Neurons challenged briefly (5 minutes) with all EAA were protected, as were neurons challenged for 60 minutes with NMDA, AMPA, or kainate. The protective effects of hypothermia (20 degrees and 30 degrees C) persisted after rewarming to 37 degrees C, but rewarming from 12 degrees C was deleterious. Surprisingly, however, prolonged (60 minutes) exposures to glutamate unmasked a temperature-insensitive component of glutamate neurotoxicity that was not seen with the other, synthetic EAA; this component was still mediated via NMDA receptors, not by ionotropic or metabotropic non-NMDA receptors. The temperature-insensitivity of glutamate toxicity was not explained by effects of hypothermia on EAA-evoked [Ca2+]i increases measured using high- and low-affinity Ca2+ indicators, nor by effects on mitochondrial production of reactive oxygen species. This first characterization of excitotoxicity at profoundly hypothermic temperatures reveals a previously unnoticed feature of glutamate neurotoxicity unseen with the other EAA, and also suggests that hypothermia protects the brain at the level of neurons by blocking, rather than slowing, excitotoxicity.     

Inhibition of shivering increases core temperature afterdrop and attenuates rewarming in hypothermic humans

During severe hypothermia, shivering is absent. To simulate severe hypothermia, shivering in eight mildly hypothermic subjects was inhibited with meperidine (1.5 mg/kg). Subjects were cooled twice (meperidine and control trials) in 8 degrees C water to a core temperature of 35.9 +/- 0.5 (SD) degrees C, dried, and then placed in sleeping bags. Meperidine caused a 3.2-fold increase in core temperature afterdrop (1.1 +/- 0.6 vs. 0.4 +/- 0.2 degree C), a 4.3-fold increase in afterdrop duration (89.4 +/- 31.4 vs. 20.9 +/- 5.7 min), and a 37% decrease in rewarming rate (1.2 +/- 0.5 vs. 1.9 +/- 0.9 degrees C/h). Meperidine inhibited overt shivering. Oxygen consumption, minute ventilation, and heart rate decreased after meperidine injection but subsequently returned toward preinjection values after 45 min postimmersion. This was likely due to the increased thermoregulatory drive with the greater afterdrop and the short half-life of meperidine. These results demonstrate the effectiveness of shivering heat production in attenuating the postcooling afterdrop of core temperature and potentiating core rewarming. The meperidine protocol may be valuable for comparing the efficacy of various hypothermia rewarming methods in the absence of shivering.     

Comparison of brain tissue metabolic changes during ischemia at 35 degrees and 18 degrees C

BACKGROUND: We evaluated brain tissue oxygen pressure (PO2), carbon dioxide pressure (PCO2) and pH during ischemia with brain temperature at 35 degrees and 18 degrees C in the same patient. METHODS: Surgery was performed in a 60-year-old woman to clip a large aneurysm in the left internal carotid artery (ICA). A Paratrend 7 probe measuring PO2, PCO2, and pH was inserted into tissue at risk for ischemia during ICA occlusion and brain protection was provided with 9% desflurane. One week later, hypothermic circulatory arrest with brain temperature at 18 degrees C was performed for aneurysm clipping and tissue measurements were obtained during ischemia and rewarming. RESULTS: At 35 degrees C, ICA occlusion for 16 minutes produced tissue hypoxia (PO2 = 0) and acidosis (pH = 6.70). The rate of increase of hydrogen ion (H+) reached 50 nEq.L(-1).min(-1) during ICA occlusion and there was a slow recovery of acidosis at the end of the ischemic period. During hypothermic circulatory arrest, tissue PO2 was sensitive to decreases in blood pressure and decreased rapidly during exsanguination. Although tissue pH decreased to 6.5 with 30 min of no pump flow, the rate of H+ increase during hypothermic arrest was one-third of that seen during ischemia at 35 degrees C. During rewarming from profound hypothermia, two phases of recovery from acidosis were observed, one during CO2 clearance and one after tissue reoxygenation. Recovery of acidosis occurred sooner at 18 degrees C than at 35 degrees C. CONCLUSIONS: These results show that tissue acidosis develops more slowly and recovers more rapidly with hypothermic ischemia. This may be an important mechanism of reduced ischemic injury during hypothermia.     

Mild hypothermia as a protective therapy during intracranial aneurysm surgery: a randomized prospective pilot trial

OBJECTIVE: To conduct a pilot trial of mild intraoperative hypothermia during cerebral aneurysm surgery. METHODS: One hundred fourteen patients undergoing cerebral aneurysm clipping with (n = 52) (World Federation of Neurological Surgeons score < or =III) and without (n = 62) acute aneurysmal subarachnoid hemorrhage (SAH) were randomized to normothermic (target esophageal temperature at clip application of 36.5 degrees C) and hypothermic (target temperature of 33.5 degrees C) groups. Neurological status was prospectively evaluated before surgery, 24 and 72 hours postoperatively (National Institutes of Health Stroke Scale), and 3 to 6 months after surgery (Glasgow Outcome Scale). Secondary outcomes included postoperative critical care requirements, respiratory and cardiovascular complications, duration of hospitalization, and discharge disposition. RESULTS: Seven hypothermic patients (12%) could not be cooled to within 1 degrees C of target temperature; three of the seven were obese. Patients randomized to the hypothermic group more frequently required intubation and rewarming for the first 2 hours after surgery. Although not achieving statistical significance, patients with SAH randomized to the hypothermic group, when compared with patients in the normothermic group, had the following: 1) a lower frequency of neurological deterioration at 24 and 72 hours after surgery (21 versus 37-41%), 2) a greater frequency of discharge to home (75 versus 57%), and 3) a greater incidence of good long-term outcomes (71 versus 57%). For patients without acute SAH, there were no outcome differences between the temperature groups. There was no suggestion that hypothermia was associated with excess morbidity or mortality. CONCLUSION: Mild hypothermia during cerebral aneurysm surgery is feasible in nonobese patients and is well tolerated. Our results indicate that a multicenter trial enrolling 300 to 900 patients with acute aneurysmal SAH will be required to demonstrate a statistically significant benefit with mild intraoperative hypothermia.     

Severe accidental hypothermia successfully treated by warmed peritoneal lavage

A case of severe accidental hypothermia (core temperature 20 degrees C) in an elderly woman successfully treated by warmed lavage of peritoneal cavity is reported. The various available rewarming techniques and some observations about the differential diagnosis between the severe hypothermic patient with cardiac arrest and the hypothermic dead person are briefly reviewed on the basis of current literature.     

Temporal relation of ATP-sensitive potassium-channel activation and contractility before cardioplegia

BACKGROUND: Pharmacologic treatment using potassium-channel openers (PCOs) before cardioplegic arrest has been demonstrated to provide beneficial effects on left ventricular performance with subsequent reperfusion and rewarming. However, the PCO treatment interval necessary to provide protective effects during cardioplegic arrest remains to be defined. The present study was designed to determine the optimum period of PCO treatment that would impart beneficial effects on left ventricular myocyte contractility after simulated cardioplegic arrest. METHODS: Left ventricular porcine myocytes were assigned randomly to three groups: (1) normothermic control = 37 degrees C for 2 hours; (2) cardioplegia = K+ (24 mEq/L) at 4 degrees C for 2 hours followed by reperfusion and rewarming; and (3) PCO and cardioplegia = 1 to 15 minutes of treatment with the PCO aprikalim (100 micromol/L) at 37 degrees C followed by hypothermic (4 degrees C) cardioplegic arrest and subsequent rewarming. Myocyte contractility was measured after rewarming by videomicroscopy. A minimum of 50 myocytes were examined at each treatment and time point. RESULTS: Myocyte velocity of shortening was reduced after cardioplegic arrest and rewarming compared with normothermic controls (63+/-3 microm/s versus 32+/-2 microm/s, respectively; p < 0.05). With 3 minutes of PCO treatment, myocyte velocity of shortening was improved after cardioplegic arrest to values similar to those of normothermic controls (56+/-3 microm/s). Potassium channel opener treatment for less than 3 minutes did not impart a protective effect, and the protective effect was not improved further with more prolonged periods of PCO treatment. CONCLUSIONS: A brief interval of PCO treatment produced beneficial effects on left ventricular myocyte contractile function in a simulated model of cardioplegic arrest and rewarming. These results suggest that a brief period of PCO treatment may provide a strategy for myocardial protection during prolonged cardioplegic arrest in the setting of cardiac operation.     

Interactions between hypothermia and the latency to ischemic depolarization: implications for neuroprotection

BACKGROUND: The authors postulated that hypothermic neuroprotection can be attributed to a delayed onset of ischemic depolarization. METHODS: Halothane-anesthetized rats were prepared for near-complete forebrain ischemia. Direct current (DC) potential microelectrodes were placed in hippocampal CA1. The pericranial temperature was maintained at 31 degrees C, 33 degrees C, 35 degrees C, or 37 degrees C (n = 6 per group). Bilateral carotid occlusion with systemic hypotension was initiated for 10 min. The time to onset of the DC shift was recorded. In a second experiment, rats were assigned to 37 degrees C or 31 degrees C for 10 min of ischemia, or to 31 degrees C for 14 min of ischemia (n = 8 per group). These durations of ischemia were defined to allow 9 min of ischemic depolarization in the 37 degrees C-10 min and 31 degrees C-14 min groups. Neurologic and histologic outcomes were examined 7 days later. RESULTS: Hippocampal CA1 time to depolarization increased with decreasing temperature (P < 0.0001). Time to depolarization was increased by approximately 4 min in the rats maintained at 31 degrees C compared with those at 37 degrees C. Time to repolarization during reperfusion was not affected by temperature. Increasing the duration of ischemia from 10 min to 14 min with the pericranial temperature maintained at 31 degrees C resulted in a duration of depolarization that was equivalent in the 37 degrees C-10 min and 31 degrees C-14 min groups. However, hippocampal CA1 damage was not increased (31 degrees C-10 min = 4 +/- 1% dead neurons; 31 degrees C-14 min = 6 +/- 1% dead neurons, 95% CI, -1% to 3%; 37 degrees C-10 min = 90 +/- 17% dead neurons). CONCLUSIONS: Despite similar durations of DC depolarization, outcome in hypothermic rats was markedly improved compared with normothermic rats. This indicates that hypothermic neuroprotection can be attributed to mechanisms other than the delay in time to onset of ischemic depolarization.     

Terson''s syndrome in a patient with acute promyelocytic leukemia on all-trans retinoic acid treatment

Hypothermia induced by surface cooling has shown to protect vulnerable regions of the brain during an ischemic insult. This study evaluated the neuroprotective efficacy of neurotensin, a potent hypothermic agent, using a 5-min carotid occlusion procedure in the gerbil. In Experiment 1, the dose-response and time course of neurotensin-induced hypothermia were evaluated (n = 5/dose). Central infusion of 10, 20, and 30 micrograms neurotensin were found to significantly decrease core body temperature of conscious gerbils within 30 min of administration. In Experiment 2, gerbils pretreated with 30 micrograms neurotensin were permitted to become hypothermic or were maintained at 37 degrees-38 degrees C (rectal) during ischemic insult. Other gerbils were pretreated with peptide vehicle prior to ischemic insult (at 37 degrees -38 degrees C) or underwent a sham procedure (n = 6/condition). At 24 h after surgery, gerbils were tested for increased locomotor activity in an open-field apparatus. Gerbils pretreated with peptide vehicle or neurotensin and maintained at 37 degrees-38 degrees C during ischemia had significantly higher activity levels compared to the other treated groups. In contrast, gerbils made hypothermic with neurotensin exhibited activity levels similar to sham gerbils. Histological assessment revealed that neurotensin-induced hypothermia protected the CA1 region from ischemic damage.     

Induction of tolerance to hypothermia and hyperthermia by a common mechanism in mammalian cells

Pretreatment by hypothermic (25 degrees C) cycling (PHC) of attached exponential-phase V79 Chinese hamster cells by Method 4 (24 hr at 25 degrees C + 1.5 hr at 37 degrees C + 24 hr at 25 degrees C + trypsin + 3 hr at 37 degrees C) or by Method 3 (48 hr at 25 degrees C + trypsin + 3 hr at 37 degrees C) make mammalian V79 cells significantly more resistant to 43 degrees C hyperthermia. There is no significant difference in the 43 degrees C curves whether Method 3 or 4 is used for pre-exposure. If pre-exposure at 15 or 10 degrees C, the resistance to hyperthermia is significantly reduced. PHC by Method 4 significantly increases survival of cells exposed to 5 degrees C and, to a lesser extent, to 10 degrees C. The increase in hyper- and hypothermic survival after PHC cannot be accounted for by changes in cell cycle distribution. Heat-shock protein synthesis is not induced by PHC; hence, protection does not result from newly synthesized proteins. When cells are made tolerant to hyperthermia by a pretreatment in 2% DMSO for 24 hr at 37 degrees C (Method 8), the cells are not more resistant to subsequent exposures to hypothermia, either at 5 or 10 degrees C. The results imply that there may be two mechanisms of inducing resistance to hyperthermia, only one of which also confers resistance to hypothermia.     

Experimental hypothermia: effects of core cooling and rewarming on hemodynamics, coronary blood flow, and myocardial metabolism in dogs

Conflicting results have been reported as to the extent that cardiovascular function can be reestablished after rewarming from hypothermia. We measured hemodynamic function, myocardial metabolism and tissue water content in dogs core-cooled to 25 degrees C and later rewarmed. At 25 degrees C left ventricular (LV) systolic pressure (LVSP) was 54% +/- 4%, maximum rate of LV pressure rise (LV dP/dtmax) 44% +/- 5%, aortic pressure (AOP) 50% +/- 6%, heart rate (HR) 40% +/- 0%, cardiac output (CO) 37% +/- 5%, myocardial blood flow (MBF) 34% +/- 5%, and myocardial oxygen consumption (MVO2) 8% +/- 1%, compared to precooling. Stroke volume (SV) and LV end-diastolic pressure (LVEDP) were unchanged. As normothermia (37 degrees C) was reestablished, the depression of cardiac function and myocardial metabolism remained the same as that at 25 degrees C: LVSP 71% +/- 6%, LV dP/dtmax 73% +/- 7%, SV 60% +/- 9%, AOP 70% +/- 6%, CO 57% +/- 9%, MBF 53% +/- 8%, and MVO2 44% +/- 8% HR, in contrast, recovered to precooling values. The arterial concentrations of glucose and free fatty acids (FFA) did not change significantly during the experimental period, whereas an increase in lactate of nonmyocardial origin appeared after rewarming. Increased myocardial contents of creatine phosphate and water were found during both hypothermia and rewarming. The present study demonstrates a persistent depression of cardiac function after hypothermia and rewarming in spite of adequate energy stores. Thus, a direct influence on myocardial contractile function by the cooling and rewarming process is suggested.     

Systemic hypothermia after spinal cord compression injury in the rat: does recorded temperature in accessible organs reflect the intramedullary temperature in the spinal cord?

This article addresses one basic issue regarding the use of systemic hypothermia in the acute management of spinal cord injury, namely, how to interpret temperature recordings in accessible organs such as the rectum or esophagus with reference to the spinal cord temperature. Thirty-six rats, divided into six groups, were randomized to laminectomy or to severe spinal cord compression trauma, and were further randomized to either a cooling/rewarming procedure or continuous normothermia (esophageal temperature 38 degrees C) for 90 min. The first procedure comprised normothermia during the surgical procedure, followed by lowering of the esophageal temperature from 38 degrees C to 30 degrees C (the hypothermic level), a 20-min steady-state period at 30 degrees C, rewarming to 38 degrees C, and finally a 20-min steady-state period at 38 degrees C. The esophageal, rectal, and epidural temperatures were recorded in all animals. The intramedullary temperature was also recorded invasively in four of the six groups. We conclude that the esophageal temperature is safe and easy to record and, in our setting, reflects the epidural temperature. The differences registrated may reflect a true deviation of the intramedullary temperature due to initial environmental exposure and secondary injury processes. Our results indicate that the esophageal temperature exceeds the intramedullary temperature during the initial recording and final steady state following rewarming, but not during the most crucial part of the experiment, the hypothermic period. The core temperature measured in the esophagus can therefore be used to evaluate the intramedullary temperature during alterations of the systemic temperature and during hypothermic periods.     

Changes in myocardial ultrastructure induced by cooling as well as rewarming

The aim of the present study was to investigate if hypothermia and rewarming, without accompanying cardiac ischaemia or cardioplegia, causes myocardial damage. Anaesthetized rats were subjected to a cooling procedure (4 h at 15-13 degrees C) where spontaneous cardiac electromechanical activity was maintained, followed by rewarming. Control rats, hypothermic rats and posthypothermic rats were perfusion-fixed, the hearts removed and the ventricles examined using an electron microscope. Based on morphometric methodology volume fractions as well as absolute volumes of cellular and subcellular components of the ventricles were assessed. In hypothermic hearts capillary volume fraction was significantly decreased, which was probably due to a decrease in perfusion pressure. The cytosolic volume increased in both absolute values and as a fraction of the myocyte: from 25 +/- 11 in controls to 43 +/- 8 microliters and from 0.067 +/- 0.023 to 0.102 +/- 0.013, respectively. There was a corresponding relative decrease in the volume fraction of myofilaments from 0.598 +/- 0.030 to 0.548 +/- 0.024. In posthypothermic hearts significant tissue swelling was apparent, dominated by a significant increase in myocyte volume from 372 +/- 66 in controls to 522 +/- 166 microliters. Similar changes were measured in mitochondrial and cytosolic volumes. In conclusion, the myocardial ultrastructure was altered during hypothermia as well as after rewarming. Posthypothermic myocardium showed generalized cellular swelling and areas of cellular necrosis.     

Mild posttraumatic hypothermia reduces mortality after severe controlled cortical impact in rats

The effect of posttraumatic hypothermia (brain temperature controlled at 32 degrees C for 4 h) on mortality after severe controlled cortical impact (CCI) was studied in rats. Four posttraumatic brain temperatures were compared: 37 degrees C (n = 10), 36 degrees C (n = 4), 32 degrees C (n = 10), and uncontrolled (UC; n = 6). Rats were anesthetized and subjected to severe CCI (4.0-m/s velocity, 3.0-mm depth) to the exposed left parietal cortex. At 10 min posttrauma the rats were cooled or maintained at their target brain temperature, using external cooling or warming. Brain temperature in the UC group was recorded but not regulated, and rectal temperature was maintained at 37 +/- 0.5 degrees C. After 4 h, rats were rewarmed over a 1-h period to 37 degrees C, extubated, and observed for 24 h. In the 37 and 36 degree C groups, 24-h mortality was 50% (37 degrees C = 5/10, 36 degrees C = 2/4). In the 32 degree C group, 24-h mortality was 10% (1/10). In the UC group, brain temperature was 35.4 +/- 0.6 degrees C during the 4-h treatment period and 24-h mortality was 0% (0/6). Mortality was higher in groups with brain temperatures > or = 36 degrees C versus those with brain temperatures < 36 degrees C (50 vs. 6%, respectively; p < 0.05). Additionally, electroencephalograms (EEG) were recorded in subsets of each temperature group and the percentage of time that the EEG was suppressed (isoelectric) was determined. Percentage of EEG suppression was greater in the hypothermic (32 degrees C, n = 6; UC, n = 4) groups than in the normothermic (36 degrees C, n = 3; 37 degrees C, n = 6) groups (23.3 +/- 14.3 vs. 1.2 +/- 3.1%, respectively; p < 0.05). Posttraumatic hypothermia suppressed EEG during treatment and reduced mortality after severe CCI. The threshold for this protective effect appears to be a brain temperature < 36 degrees C. Thus, even mild hypothermia may be beneficial after severe brain trauma.     

Cerebral hypoxia during cardiopulmonary bypass: a magnetic resonance imaging study

BACKGROUND: Neurocognitive deficits after open heart operations have been correlated to jugular venous oxygen desaturation on rewarming from hypothermic cardiopulmonary bypass (CPB). Using a porcine model, we looked for evidence of cerebral hypoxia by magnetic resonance imaging during CPB. Brain oxygenation was assessed by T2*-weighted imaging, based on the blood oxygenation level-dependent effect (decreased T2*-weighted signal intensity with increased tissue concentrations of deoxyhemoglobin). METHODS: Pigs were placed on normothermic CPB, then cooled to 28 degrees C for 2 hours of hypothermic CPB, then rewarmed to baseline temperature. T2*-weighted, imaging was undertaken before CPB, during normothermic CPB, at 30-minute intervals during hypothermic CPB, after rewarming, and then 15 minutes after death. Imaging was with a Bruker 7.0 Tesla, 40-cm bore magnetic resonance scanner with actively shielded gradient coils. Regions of interest from the magnetic resonance images were analyzed to identify parenchymal hypoxia and correlated with jugular venous oxygen saturation. Post-hoc fuzzy clustering analysis was used to examine spatially distributed regions of interest whose pixels followed similar time courses. Attention was paid to pixels showing decreased T2* signal intensity over time. RESULTS: T2* signal intensity decreased with rewarming and in five of seven experiments correlated with the decrease in jugular venous oxygen saturation. T2* imaging with fuzzy clustering analysis revealed two diffusely distributed pixel groups during CPB. One large group of pixels (50% +/- 13% of total pixel count) showed increased T2* signal intensity (well-oxygenated tissue) during hypothermia, with decreased intensity on rewarming. Changes in a second group of pixels (34% +/- 8% of total pixel count) showed a progressive decrease in T2* signal intensity, independent of temperature, suggestive of increased brain hypoxia during CPB. CONCLUSIONS: Decreased T2* signal intensity in a diffuse spatial distribution indicates that a large proportion of cerebral parenchyma is hypoxic (evidenced by an increased proportion of tissue deoxyhemoglobin) during CPB in this porcine model. Neuronal damage secondary to parenchymal hypoxia may explain the postoperative neuropsychological dysfunction after cardiac operations.     

Pyruvate reverses fatty-acid-induced depression of ventricular function and calcium overload after hypothermia in guinea pig hearts

OBJECTIVE: High levels of free fatty acids have been shown to impair mechanical recovery and calcium homeostasis of isolated rat hearts following hypothermic perfusion. The objective of the present study was to investigate whether inhibition of fatty acid oxidation through activation of pyruvate dehydrogenase by millimolar concentrations of pyruvate could influence functional recovery and Ca2+ homeostasis after a hypothermic insult. METHODS: Ventricular function and myocardial calcium ([Ca]total) were measured in 3 different groups of Langendorff-perfused guinea pig hearts exposed to 40 min hypothermic (15 degrees C) perfusion, followed by 30 min rewarming at 37 degrees C. The hearts were perfused with either 11.1 mM glucose (G), glucose and 1.2 mM palmitate (GP), or glucose, palmitate and 5 mM pyruvate (GPP) as energy substrates. RESULTS: All groups showed marked elevations in [Ca]total during hypothermia (from 0.6-0.7 mumol.g dry wt-1 to 9.3-12.2 mumol.g dry wt-1 at 40 min hypothermia, P < 0.05), associated with a pronounced increase in left ventricular end-diastolic pressure (LVEDP from 0-2 to 50-60 mmHg). Following rewarming, GP-perfused hearts showed significantly lower recovery of mechanical function compared to both G- and GPP-perfused hearts (% recovery of left ventricular developed pressure: 27 +/- 8 vs. 62 +/- 3 and 62 +/- 8%, respectively, P < 0.05). The reduced mechanical recovery of GP-perfused hearts was associated with elevated [Ca]total. In separate experiments we found that addition of 1.2 mM palmitate reduced glucose oxidation ([14C]glucose) from 1.77 +/- 0.28 mumol.min-1.g dry wt-1 (G-perfused hearts) to 0.15 +/- 0.04 mumol.min-1.g dry wt-1 (GP-perfused hearts, P < 0.05), implying that fatty acids had become the major substrate for oxidative phosphorylation. Fatty acid oxidation was, however, less pronounced after further addition of 5 mM pyruvate. Thus, palmitate oxidation ([3H]palmitate) was more than 40% lower in GPP-perfused than in GP-perfused hearts (0.83 +/- 0.22 vs. 1.41 +/- 0.12 mumol.min-1.g dry wt-1, P < 0.05). CONCLUSIONS: The present results demonstrate impaired ventricular function and calcium homeostasis after hypothermia in guinea pig hearts perfused with fatty acids in addition to glucose, as compared to hearts perfused with glucose alone. Furthermore, we show that these unfavourable effects of fatty acids can be overcome by an exogenous supply of pyruvate.     

Cerebrovascular relaxation responses to endothelium-dependent and -independent vasodilators after normothermic and hypothermic cardiopulmonary bypass in the rabbit

BACKGROUND: Cardiopulmonary bypass causes activation of leukocytes and increased concentrations of proinflammatory mediators, which may result in endothelial dysfunction. Because hypothermia attenuates many inflammatory processes, the authors hypothesized that hypothermic cardiopulmonary bypass would be associated with better endothelial function than normothermic cardiopulmonary bypass. METHODS: Isoflurane-anesthetized New Zealand White rabbits were randomized to undergo 90 min of either normothermic (37 degrees C, n=9) or hypothermic (27 degrees C, n=9) cardiopulmonary bypass with terminal rewarming. A third group served as anesthetized normothermic non-cardiopulmonary bypass surgical controls (n=8). Basilar artery and descending thoracic aorta were isolated from each animal. In vitro vessel relaxation responses to increasing concentrations of acetylcholine (which induces endothelial release of nitric oxide) and nitroprusside (which provides exogenous nitric oxide) were measured in phenylephrine-precontracted vessel rings. RESULTS: There were no differences in vessel relaxation responses between normothermic and hypothermic cardiopulmonary bypass groups in basilar artery or aorta. In contrast, basilar arteries from non-cardiopulmonary bypass controls had increased relaxation responses to both acetylcholine (P=0.004) and nitroprusside (P=0.031) compared with the pooled cardiopulmonary bypass animal data. CONCLUSIONS: The authors observed no differences in endothelial or vascular smooth muscle function between normothermic and hypothermic cardiopulmonary bypass groups. Compared with non-cardiopulmonary bypass controls, cardiopulmonary bypass appeared to decrease basilar artery smooth muscle relaxation in response to endogenous and exogenous nitric oxide.     

Body temperature in mice: a quantitative measure of alcohol tolerance and physical dependence

Mice undergoing withdrawal after chronic ethanol consumption were found to be hypothermic if kept at room temperature. The extent of the hypothermia correlated well with the behavioral withdrawal symptoms and could be used as a quantitative measure of the severity and time course of the withdrawal syndrome. Placing mice in a cold environment (4 degrees C) exacerbated the hypothermia whereas placing animals at 34 degrees C reversed the hypothermia and produced hyperthermia. It was concluded that the temperature set point mechanism and the ability to regulate around this set point was disturbed in animals physically dependent on alcohol. During consumption of the ethanol-containing diets, mice exhibited tolerance to the hypothermic effects of an acutely administered dose od ethanol. Tolerance to the hypothermic effects of ethanol mirrored the development of behavioral tolerance as measured by performance on a tilting plane. Temperature and behavioral tolerance were both shown to extend well beyond the period of the withdrawal syndrome. Ethanol-treated mice were found to be cross-tolerant to the hypothermic effects of barbiturates but not to the hypothermia produced by the monoamine oxidase inhibitor, pargyline.     

Selective convective brain cooling during normothermic cardiopulmonary bypass in dogs

BACKGROUND: Neurologic complications, primarily resulting from ischemic insults, represent the leading cause of morbidity and disability, and the second most common source of death, after cardiac operations. Previous studies have reported that increases (as occur during the rewarming phase of cardiopulmonary bypass [CPB]) or decreases in brain temperature of a mere 0.5 degrees to 2 degrees C can significantly worsen or improve, respectively, postischemic neurologic outcome. The purpose of the present study was to evaluate a novel approach of selectively cooling the brain during hypothermic CPB and subsequent rewarming. METHODS: Sixteen dogs were anesthetized with either intravenous pentobarbital or inhaled halothane (n = 8 per group). Normocapnia (alpha stat technique) and a blood pressure near 75 mm Hg were maintained. Temperatures were monitored by placing thermistors in the esophagus (i.e., core), parietal epidural space, and brain parenchyma at depths of 1 and 2 cm beneath the dura. During CPB, core temperature was actively cycled from 38 degrees C to 28 degrees C, and then returned to 38 degrees C. Forced air pericranial cooling (air temperature of approximately 13 degrees C) was initiated simultaneous with the onset of CPB, and maintained throughout the bypass period. Brain-to-core temperature gradients were calculated by subtracting the core temperature from regional brain temperatures. RESULTS: In halothane-anesthetized dogs, brain temperatures at all monitoring sites were significantly less than core during all phases of CPB, with one exception (2 cm during systemic cooling). Brain cooling was most prominent during and after systemic rewarming. For example, during systemic rewarming, average temperatures in the parietal epidural space, and 1 and 2 cm beneath the dura, were 3.3 degrees +/- 1.3 degrees C (mean +/- standard deviation), 3.2+/-1.4 degrees C, and 1.6 degrees +/-1.0 degrees C, cooler than the core, respectively. Similar trends, but of a greater magnitude, were noted in pentobarbital-anesthetized dogs. For example, during systemic rewarming, corresponding brain temperatures were 6.5 degrees +/-1.7 degrees C, 6.3 degrees +/-1.6 degrees C, and 4.2+/-1.3 degrees C cooler than the core, respectively. CONCLUSIONS: The magnitude of selective brain cooling observed in both study groups typically exceeded the 0.5 degrees to 2.0 degrees C change previously reported to modulate ischemic injury, and was most prominent during the latter phases of CPB. When compared with previous research from our laboratory, application of cold forced air to the cranial surface resulted in brain temperatures that were cooler than those observed during hypothermic CPB without pericranial cooling. On the basis of the assumption that similar beneficial brain temperature changes can be induced in humans, we speculate that selective convective brain cooling may enable clinicians to improve neurologic outcome after hypothermic CPB.     

Cerebral oxygenation during cardiopulmonary bypass in children

OBJECTIVE: Previous work has found cerebral oxygen extraction to decrease during hypothermic cardiopulmonary bypass in children. To elucidate cardiopulmonary bypass factors controlling cerebral oxygen extraction, we examined the effect of perfusate temperature, pump flow rate, and hematocrit value on cerebral hemoglobin-oxygen saturation as measured by near infrared spectroscopy. METHODS: Forty children less than 7 years of age scheduled for cardiac operations with continuous cardiopulmonary bypass were randomly assigned to warm bypass, hypothermic bypass, hypothermic low-flow bypass, or hypothermic low-hematocrit bypass. For warm bypass, arterial perfusate was 37 degrees C, hematocrit value 23%, and pump flow 150 ml/kg per minute. Hypothermic bypass differed from warm bypass only in initial perfusate temperature (22 degrees C); hypothermic low-flow bypass and low-hematocrit bypass differed from hypothermic bypass only in pump flow (75 ml/kg per minute) and hematocrit value (16%), respectively. Cerebral oxygen saturation was recorded before bypass (baseline), during bypass, and for 15 minutes after bypass had been discontinued. RESULTS: In the warm bypass group, cerebral oxygen saturation remained at baseline levels during and after bypass. In the hypothermic bypass group, cerebral oxygen saturation increased 20% +/- 2% during bypass cooling (p < 0.001), returned to baseline during bypass rewarming, and remained at baseline after bypass. In the hypothermic low-flow and hypothermic low-hematocrit bypass groups, cerebral oxygen saturation remained at baseline levels during bypass but increased 6% +/- 2% (p = 0.05) and 10% +/- 2% (p < 0.03), respectively, after bypass was discontinued. CONCLUSIONS: In children, cortical oxygen extraction is maintained during warm cardiopulmonary bypass at full flow and moderate hemodilution. Bypass cooling can decrease cortical oxygen extraction but requires a certain pump flow and hematocrit value to do so. Low-hematocrit hypothermic bypass and low-flow hypothermic bypass can also alter cortical oxygen extraction after discontinuation of cardiopulmonary bypass.     

Recombinant human megakaryocyte growth and development factor attenuates postbypass thrombocytopenia

BACKGROUND: Cardiopulmonary bypass contributes to platelet loss and dysfunction by exposure to shear stresses, foreign surfaces, and hypothermia. This study tested the hypothesis that pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) accelerates recovery of the platelet population after hypothermic extracorporeal circulation (HEC). METHODS: In a blinded study, subcutaneous injections of drug or placebo were given to dogs daily for 3 days preoperatively (day 0, 1, and 2) with no drug on day 3. On day 4, the animal was prepared for arteriovenous HEC. After heparinization, HEC was initiated at 30 to 40 mL x kg(-1) x min(-1). Hypothermic extracorporeal circulation (25 degrees C) was continued for 90 minutes. RESULTS: Preoperative platelet count (x10(3) platelets/microL) did not differ from predrug count in placebo (256+/-27 versus 255+/-20) or PEG-rHuMGDF (271+/-30 versus 291+/-38). During 60 minutes of HEC, the platelet count decreased to approximately 10% of baseline in placebo (29+/-5) and PEG-rHuMGDF (46+/-8), and recovered to approximately 70% of baseline after rewarming (90 minutes of HEC: placebo, 185+/-17, versus PEG-rHuMGDF, 169+/-22). After HEC, platelet count was greater in PEG-rHuMGDF-treated animals (p < 0.05) without altering function (aggregation responses). Within the first 6 hours after HEC, platelet count in PEG-rHuMGDF-treated animals was rising and increased to 260+/-29 (p < 0.01), but was unchanged in placebo animals (186+/-21). Thereafter, platelet count in placebo animals declined to a nadir of 124+/-15 (72 hours after HEC), whereas platelet count in PEG-rHuMGDF animals approximated the preoperative value (>200) at all times. CONCLUSIONS: Appropriately timed presurgical administration of PEG-rHuMGDF counteracts post-HEC relative thrombocytopenia without increasing platelet population and enhancing aggregation preoperatively or during extracorporeal circulation.