Mild hypothermia disturbs regional cerebrovascular autoregulation in awake rats

Altered Cerebral Blood Flow and Potential Neuroprotective Effect of Human Relaxin-2 (Serelaxin) During Hypoxia or Severe Hypovolemia in a Sheep Model

Specific neuroprotective strategies to minimize cerebral damage caused by severe hypoxia or hypovolemia are lacking. Based on previous studies showing that relaxin-2/serelaxin increases cortical cerebral blood flow, we postulated that serelaxin might provide a neuroprotective effect. Therefore, we tested serelaxin in two emergency models: hypoxia was induced via inhalation of 5% oxygen and 95% nitrogen for 12 min; thereafter, the animals were reoxygenated. Hypovolemia was induced and maintained for 20 min by removal of 50% of the total blood volume; thereafter, the animals were retransfused. In each damage model, the serelaxin group received an intravenous injection of 30 µg/kg of serelaxin in saline, while control animals received saline only. Blood gases, shock index values, heart frequency, blood pressure, and renal blood flow showed almost no significant differences between control and treatment groups in both settings. However, serelaxin significantly blunted the increase of lactate during hypovolemia. Serelaxin treatment resulted in significantly elevated cortical cerebral blood flow (CBF) in both damage models, compared with the respective control groups. Measurements of the neuroproteins S100B and neuron-specific enolase in cerebrospinal fluid revealed a neuroprotective effect of serelaxin treatment in both hypoxic and hypovolemic animals, whereas in control animals, neuroproteins increased during the experiment. Western blotting showed the expression of relaxin receptors and indicated region-specific differences in relaxin receptor-mediated signaling in cortical and subcortical brain arterioles, respectively. Our findings support the hypothesis that serelaxin is a potential neuroprotectant during hypoxia and hypovolemia. Due to its preferential improvement of cortical CBF, serelaxin might reduce cognitive impairments associated with these emergencies.

Prolonged Recovery From General Anesthesia Possibly Related to Persistent Hypoxemia in a Draft Horse

Horses are susceptible to developing large areas of pulmonary atelectasis during recumbency and anesthesia. The subsequent pulmonary shunt is responsible for significant impairment of oxygenation. Since ventilation perfusion mismatch persists into the post-operative period, hypoxemia remains an important concern in the recovery stall. This case report describes the diagnosis and supportive therapy of persistent hypoxemia in a 914 kg draft horse after isoflurane anesthesia. It highlights how challenging it can be to deal with hypoxemia after disconnection from the anesthesia machine and how life-threatening it can become if refractory to treatment. Furthermore, it stresses the point on the interactions between hypoxemia and other factors, such as residual drug effects and hypothermia, that should also be considered in the case of delayed recovery from general anesthesia.

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.

Redistribution of Cerebral Blood Flow during Severe Hypovolemia and Reperfusion in a Sheep Model: Critical Role of α1-Adrenergic Signaling

Background: Maintenance of brain circulation during shock is sufficient to prevent subcortical injury but the cerebral cortex is not spared. This suggests area-specific regulation of cerebral blood flow (CBF) during hemorrhage. Methods: Cortical and subcortical CBF were continuously measured during blood loss (≤50%) and subsequent reperfusion using laser Doppler flowmetry. Blood gases, mean arterial blood pressure (MABP), heart rate and renal blood flow were also monitored. Urapidil was used for α1A-adrenergic receptor blockade in dosages, which did not modify the MABP-response to blood loss. Western blot and quantitative reverse transcription polymerase chain reactions were used to determine adrenergic receptor expression in brain arterioles. Results: During hypovolemia subcortical CBF was maintained at 81 ± 6% of baseline, whereas cortical CBF decreased to 40 ± 4% (p < 0.001). Reperfusion led to peak CBFs of about 70% above baseline in both brain regions. α1A-Adrenergic blockade massively reduced subcortical CBF during hemorrhage and reperfusion, and prevented hyperperfusion during reperfusion in the cortex. α1A-mRNA expression was significantly higher in the cortex, whereas α1D-mRNA expression was higher in the subcortex (p < 0.001). Conclusions: α1-Adrenergic receptors are critical for perfusion redistribution: activity of the α1A-receptor subtype is a prerequisite for redistribution of CBF, whereas the α1D-receptor subtype may determine the magnitude of redistribution responses.

Laser speckle imaging reveals multiple aspects of cerebral vascular responses to whole body mild hypothermia in rats

In this paper, we present a novel method to study the effect of induced mild hypothermia on cerebral vascular responses. To measure cerebral vascular responses, a minimally invasive imaging method, temporal laser speckle imaging, was developed and adapted for induced-hypothermia rat model. Experiments were carried out in rats under anesthesia. Laser speckle images were acquired at different temperature points, normothermia (37 °Q and mild therapeutic hypothermia (34 °Q. We extracted multiple hemodynamic responses simultaneously from the images, including blood flow, vessel size and deoxy-hemoglobin saturation. A wide-field view of the cerebral vascular response distribution was studied, which showed an inhomogeneous response map across the region of interest. A comparison between responses in arterioles and venules was carried out (blood flow decreased by 58 ± 9 % vs. 27 ± 8 %). The global decrease of blood flow, dilatation in arterioles and decrease of deoxy-hemoglobin saturation in veins at mild hypothermia suggests a beneficial role of circulatory and oxygenation changes in therapeutic hypothermia. The results reported provide a circulatory explanation for the hypothermia therapeutic effects and mechanism.

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.

Role of endothelial nitric oxide synthase in cerebral blood flow changes during kainate seizures: A genetic approach using knockout mice

The role of endothelial nitric oxide (NO) in the cerebrovascular response to partial seizures was investigated in mice deleted for the endothelial NO synthase gene (eNOS-/-) and in their paired wild-type (WT) congeners. Local cerebral blood flow (LCBF, quantitative [14C]iodoantipyrine method) was measured 3-6 h after unilateral kainate (KA) injection in the dorsal hippocampus; controls received saline. In WT mice, KA seizures induced a 22 to 50% LCBF increase restricted to the ipsilateral hippocampus, while significant LCBF decreases (15-33%) were noticed in 22% of the contralateral areas, i.e., the parietal cortex, amygdala and three basal ganglia areas, compared to saline-injected WT mice. In eNOS-/- mice, no LCBF increases were recorded within the epileptic focus and generalized contralateral LCBF decreases (22-46%) were noticed in 2/3 of the brain areas, compared to saline-injected eNOS-/- mice. Thus, endothelial NO is the mediator of the cerebrovascular response within the epileptic focus and participates in the maintenance of LCBF in distant areas.

Regional Heterogeneity of Cerebral Blood Flow Response to Graded Pressure-Controlled Hemorrhage

BACKGROUND

Little is known about the regional distribution of cerebral blood flow (CBF) in nonanesthetized animals during periods of lowered blood pressure. The present investigation addresses the specific reaction patterns of local cerebral blood flow (LCBF) in comparison with mean CBF during graded pressure-controlled hemorrhagic shock in conscious rats.

METHODS

Conscious rats were subjected to graded pressure-controlled hemorrhage (to 85, 70, 55, or 40 mm Hg) by arterial blood withdrawal. After a period of 30 minutes, blood pressure was stabilized by withdrawal or reinfusion of blood. LCBF was determined autoradiographically by the iodo(14C)antipyrine method in 34 brain structures, and mean CBF was calculated and compared with the values of nonhemorrhaged control animals.

RESULTS

Mean CBF remained unchanged except for the group with the lowest blood pressure of 40 mm Hg (decrease in CBF of 28%). Otherwise, LCBF was increased in some brain structures at an unchanged mean CBF. Congruently, at 40 mm Hg, the decrease in mean CBF did not show up in all brain structures, the local pattern of CBF varying between an unchanged and a profoundly decreased CBF. The mean coefficient of variation of CBF was increased with the severity of hemorrhagic shock, which indicates an enhanced heterogeneity of CBF.

CONCLUSION

Because of the substantial heterogeneity in the responses of LCBF to pressure-controlled hemorrhage, autoregulation of CBF during pressure-controlled hemorrhagic shock has to be reconsidered on a regional basis.

Changes in cerebrovascular response during brain hypothermia after traumatic brain injury

OBJECTIVE

Transcranial Doppler sonography (TCD) provides a rapid and noninvasive assessment of cerebral hemodynamics after traumatic brain injury. This study evaluates the change of cerebrovascular response at the rewarming stage of brain hypothermia using TCD.

MATERIAL AND METHODS

Seventeen patients who had suffered from closed brain injury were investigated with daily TCD recordings and the changes in flow velocities were evaluated to determine whether they reflected the temperature during brain hypothermia. All patients who had treated brain hypothermia underwent continuous monitoring of SjO2, mean arterial blood pressure, and intracranial pressure (ICP).

RESULTS

No significant changes in ICP and cerebral perfusion pressure (CPP) were recorded in all cases. Of 17 patients, 5 had a significant increase in SjO2 > 75% or more, retrospectively, with a robust increase in flow velocities of the middle cerebral artery (P < .01) at the rewarming stage. These cases marked a decrease in pulsatility index (P < .05) concomitant with an increase in SjO2 values. The CT scan revealed acute brain swelling in these cases.

CONCLUSION

These data suggest cerebral vasoactivity could be altered at the rewarming stage. Hypothermia runs a risk of hyperemia at the rewarming stage, which induced a decrease in cerebral vasoresistance.

Effect of mild hypothermia on inodilator-induced vasodilation of pial arterioles in cats

BACKGROUND

Mild hypothermia has been proposed as a means of providing cerebral protection after traumatic brain injury. However, hypothermia has been shown to alter not only physiologic but also pharmacologic responses. The purpose of this study was to investigate whether mild hypothermia (3-4 degrees C temperature reduction) could alter cerebral vasodilation induced by inodilators, which are characterized by having an inotropic effect in addition to a vasodilatory effect. Isoproterenol (a beta-adrenergic receptor agonist), colforsin dapropate (an adenylate cyclase stimulant), and amrinone (a phosphodiesterase inhibitor) were chosen as inodilators.

METHODS

The cranial window technique, combined with microscopic video recording, was used. Forty-eight cats were randomly assigned to either a normothermic or a hypothermic group (33 degrees C). Isoproterenol, colforsin dapropate, or amrinone was topically applied in the cranial window and the diameter of pial arterioles was measured.

RESULTS

Topical administration of isoproterenol, colforsin dapropate, and amrinone produced a significant dilation in a dose-dependent manner during normothermia. The vasodilation induced by these inodilators was not affected by mild hypothermia.

CONCLUSION

The vasodilation induced by topical administration of isoproterenol, colforsin dapropate, and amrinone was not affected by mild hypothermia.

Propofol versus isoflurane anesthesia under hypothermic conditions: effects on intracranial pressure and local cerebral blood flow after diffuse traumatic brain injury in the rat

BACKGROUND

The aim of this study was to compare the cerebral protective effects of two known protective anesthetics, isoflurane and propofol, when these were used in combination with moderate hypothermia (33-34 degrees C) after diffuse traumatic brain injury (TBI) in the rat. We assessed cerebral protection by measuring local cerebral blood flow (LCBF), mean arterial blood pressure (MABP), cerebral perfusion pressure (CPP) and intracranial pressure (ICP).

METHODS

Sixteen female Wistar rats weighing 275 to 350 g were anesthetized and subjected to an accelerated-impact weight-drop model of diffuse TBI. Hypothermia (33-34 degrees C) was induced 45 minutes after TBI (baseline), and was maintained for 180 minutes. The isoflurane group (n = 8) received 70% N(2)O in O(2), and isoflurane at 0.9 +/- 0.04%. The propofol group (n = 8) received 70% N(2)O in O(2) and a propofol infusion (12 mg/kg/hr). LCBF was measured by laser Doppler flowmeter. MABP, ICP, and brain and rectal temperatures were measured every 15 minutes from baseline through 180 minutes. Blood gas and hematocrit testing was also done at baseline and every 60 minutes thereafter to assess the animals' physiological state.

RESULTS

In the isoflurane group, MABP and CPP decreased significantly from baseline to 180 minutes (p < 0.05 and p < 0.01, respectively), and MABP was significantly lower than the pressure in the propofol group from 45 minutes through 180 minutes (p < 0.05, p < 0.01). ICP and LCBF remained unchanged in this group. In the propofol group, from baseline to 180 minutes, CPP increased to maximum 120 +/- 8 mmHg at 75 minutes from 98 +/- 5 mmHg (p < 0.05) and ICP fell from 18 +/- 2 mmHg to 7 +/- 1 mmHg (p < 0.01); and the latter was significantly lower than ICP in the isoflurane group (p < 0.05, p < 0.01, p < 0.001). LCBF in this group was significantly higher than LCBF in the isoflurane group in the last 30 minutes of the experiment (p < 0.05). The propofol group showed no change in MABP over the course of the experiment.

CONCLUSION

In the clinical setting, propofol anesthesia may be better for use in combination with hypothermia in cases of traumatic brain injury, as it reduces ICP and increases CPP under these conditions.

Hypothermia attenuates the vasodilatory response of pial arterioles to hemorrhagic hypotension in the cat

We investigated the effect of hypothermia on the vasodilatory response of pial arterioles to hemorrhagic hypotension. The cranial window technique was combined with microscopic video recording in an experiment involving 20 cats anesthetized with pentobarbital. The animals were randomly assigned to either a normothermic or a hypothermic group (32 degrees C). Mean arterial pressure (MAP) was reduced in stepwise increments of 10 mm Hg (from 100 to 50 mm Hg) by blood withdrawal. The diameter of small (50-100 microm) and large (100-200 microm) pial arterioles was measured. In the normothermic group (n = 9), small and large arterioles dilated at a MAP of 60 and 50 mm Hg, and at a MAP of 70, 60, and 50 mm Hg, respectively, compared with baseline values obtained at a MAP of 100 mm Hg. In contrast, in the hypothermic group (n = 11), vasodilation of either small or large arterioles was absent. The percentage diameter of small and large arterioles (percentage of control) was significantly lower at a MAP of 70, 60, and 50 mm Hg in the hypothermic group than the normothermic group. Our in vivo study demonstrates that hypothermia impairs autoregulatory vasodilation of pial arterioles in response to hemorrhagic hypotension.

IMPLICATIONS

Deliberate mild hypothermia has been proposed as a means of providing cerebral protection during neurosurgical procedures. Our results suggest that cerebral blood flow autoregulation in response to hemorrhagic hypotension may be impaired during hypothermic conditions, indicating the importance of maintaining perfusion pressure during hypothermic therapy to prevent cerebral ischemia.

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

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Mechanisms of brain injury during infant cardiac surgery

Neurological injury is a major and often debilitating complication of congenital heart disease and open-heart surgery. Paradoxically, the full impact of this complication has been underscored by the marked decrease in mortality and the rescue of infants with desperate and previously lethal heart conditions. Although recent focus has been on mechanisms of brain injury originating during open-heart surgery, this article also emphasizes the importance of mechanisms initiated or perpetuated during the preoperative and postoperative periods. In addition to the usually implicated mechanism of hypoxia-ischemia, recent genetic advances suggest an important role for genetic deletion syndromes. Inflammatory cascades have been implicated in the end-organ injury seen after cardiopulmonary bypass and might play a role in neurological dysfunction. These mechanisms are reviewed, with an emphasis on recent developments in our understanding of brain injury in this population.