Moderate hypothermia reduces hypotensive, but not hypercapnic vasodilation of pial arterioles in rats

Cerebrovascular reactivity is not associated with therapeutic intensity in adult traumatic brain injury: a CENTER-TBI analysis

Background

Impaired cerebrovascular reactivity in adult traumatic brain injury (TBI) is known to be associated with poor outcome. However, there has yet to be an analysis of the association between the comprehensively assessed intracranial hypertension therapeutic intensity level (TIL) and cerebrovascular reactivity.

Methods

Using the Collaborative European Neuro Trauma Effectiveness Research in TBI (CENTER-TBI) high-resolution intensive care unit (ICU) cohort, we derived pressure reactivity index (PRx) as the moving correlation coefficient between slow-wave in ICP and mean arterial pressure, updated every minute. Mean daily PRx, and daily % time above PRx of 0 were calculated for the first 7 days of injury and ICU stay. This data was linked with the daily TIL-Intermediate scores, including total and individual treatment sub-scores. Daily mean PRx variable values were compared for each TIL treatment score via mean, standard deviation, and the Mann U test (Bonferroni correction for multiple comparisons). General fixed effects and mixed effects models for total TIL versus PRx were created to display the relation between TIL and cerebrovascular reactivity.

Results

A total of 249 patients with 1230 ICU days of high frequency physiology matched with daily TIL, were assessed. Total TIL was unrelated to daily PRx. Most TIL sub-scores failed to display a significant relationship with the PRx variables. Mild hyperventilation (p < 0.0001), mild hypothermia (p = 0.0001), high levels of sedation for ICP control (p = 0.0001), and use vasopressors for CPP management (p < 0.0001) were found to be associated with only a modest decrease in mean daily PRx or % time with PRx above 0.

Conclusions

Cerebrovascular reactivity remains relatively independent of intracranial hypertension therapeutic intensity, suggesting inadequacy of current TBI therapies in modulating impaired autoregulation. These findings support the need for investigation into the molecular mechanisms involved, or individualized physiologic targets (ICP, CPP, or Co2) in order to treat dysautoregulation actively.

Electronic supplementary material

The online version of this article (10.1007/s00701-019-03980-8) contains supplementary material, which is available to authorized users.

Hypothermic protection in an acute hypoxia model in rats: Acid-base and oxidant/antioxidant profiles

AIM OF THE STUDY

Recent works demonstrate the benefits of hypothermia when used to preserve brain, cardiac, hepatic, and intestinal function against hypoxic-ischemic injury. However, it is also known that hypothermia affects systemic parameters and also induces the generation of reactive oxygen species in cells and tissues. Here we studied the acid-base related parameters and the antioxidant-oxidant effects of deep hypothermia induction before an acute hypoxic insult in rats.

METHODS

Acid-base indicators and parameters related to oxidative stress were analyzed in hypothermic rats (21-22 degrees C) breathing room air during 2h (control hypothermia), and hypothermic animals switched to hypoxic air (10% O(2)) during the second hour (hypothermia hypoxia group), and they were compared with corresponding normothermia groups maintained at 37 degrees C (control normothermia and normothermia hypoxia groups).

RESULTS

Mild metabolic acidosis appeared early in arterial blood during hypothermia. After exposure to hypoxia, evidence of tissue injury (plasma transaminases and blood lactate) and oxidative stress (increase in lipid peroxidation, decrease in glutathione levels and in the glutathione reduction potential in liver) was found. In contrast, in the hypothermia hypoxia group, plasmatic parameters remained as the control values, and the hepatic glutathione reduction potential were significantly more negative when compared with the normothermia hypoxia group.

CONCLUSIONS

We propose that acidosis induced by hypothermia contributes to the maintenance of intracellular reduction potential in liver, regarding the GSSG/2GSH couple and may help to increase plasmatic antioxidant pool. Our findings provide new insights into the protective effects of hypothermia in vivo.

Deep hypothermia impact on acid-base parameters and liver antioxidant status in an in vivo rat model

Although clinical hypothermia is used for reducing postischemic damage, injurious effects have also been reported. To determine whether hypoxia and oxidative stress are induced by systemic deep hypothermia, we used an in vivo rat model keeping the arterial Pco2 constant. Animals were divided into 4 groups: sham, 2 h deep hypothermia (21 degrees C), 1 h posthypothermia (rewarmed to 37 degrees C after 2 h deep hypothermia), and 3 h normothermia. Blood gases, portal vein blood flow, arterial pressure, and heart rate were monitored throughout the experiment. Liver enzyme antioxidant activity was also examined. The hemodynamic parameters decreased drastically during hypothermia, but were fully restored after rewarming. No changes in hepatic antioxidant activity (catalase, glutathione peroxidase, and superoxide dismutase) were observed. The redox level in liver (GSH/GSSG ratio) was preserved in hypothermia but decreased when animals were rewarmed. ALT did not increase and no evidence of tissue hypoxia was detected in liver regarding the restricted flow during hypothermia. With the described protocol, deep hypothermia is regarded as an experimental safe model.

Cerebral pressure autoregulation is intact and is not influenced by hypothermia after traumatic brain injury in rats

In head-injured patients and experimental traumatic brain injury (TBI), important cerebrovascular abnormalities include decreases in cerebral blood flow (CBF) and impairment of cerebral pressure autoregulation. We evaluated CBF and pressure autoregulation after fluid percussion injury (FPI) and hypothermia in rats with the hypothesis that hypothermia would ameliorate changes in posttraumatic CBF. Male Sprague-Dawley rats, intubated and mechanically ventilated, were prepared for parasagittal FPI (1.8 atm) and laser Doppler CBF flow (LDF) measurement. The abdominal aorta was cannulated for rapid removal and reinfusion of blood. Baseline autoregulatory testing in all groups consisted of LDF measurements at normothermia and a mean arterial pressure (MAP) of 100 mm Hg, followed by randomly ordered changes of MAP to 80, 60, and 40 mm Hg. Animals were then randomized to one of five groups: normothermic control without FPI; normothermia with FPI; hypothermic control (32 degrees C) without FPI; hypothermia initiated before FPI; and hypothermia initiated immediately after FPI injury. For each group, a complete, randomly ordered autoregulatory sequence was performed at 30 and 60 min after FPI or sham TBI. In a second study, rats were prepared identically, maintained at normothermic temperatures and autoregulation was tested before and after TBI using a set of randomly ordered levels of hypotension or using progressive reductions in MAP (i.e., 80, 60, 40 mm Hg) with the hypothesis that the technical manner and timing of decreasing of the blood pressure would effect CBF after TBI. Due to high acute mortality, the group in which hypothermia was induced before FPI was excluded from the analysis. At baseline, autoregulation was similar in all groups. There was no change in CBF or autoregulation in the normothermic control group at 30 and 60 min. In the other groups at 30 and 60 min, there was a similar, statistically significant decrease in absolute CBF (i.e., a decrease of 27-57% of baseline values), but pressure autoregulation was intact except at the lowest blood pressure tested at 60 min, where there was a slight improvement in the hypothermic group. Thus, in these experiments, absolute CBF decreased with hypothermia and FPI, while neither hypothermia nor FPI significantly altered autoregulation. In the second study, autoregulatory function was not different before TBI when comparing random and sequential blood pressure changes, but, when comparing the groups after TBI at the 60 mm Hg blood pressure level, CBF was significantly lower in the sequential group than in the random order group. This suggests that the mechanism of creating hypotension, whether random or sequential, significantly affects the measurement of CBF and autoregulation after TBI in rats.

Uncomplicated rapid posthypothermic rewarming alters cerebrovascular responsiveness

BACKGROUND AND PURPOSE

Recently, we focused on the cerebrovascular protective effects of moderate hypothermia after traumatic brain injury, noting that the efficacy of posttraumatic hypothermia is related to the rate of posthypothermic rewarming. In the current communication, we revisit the use of hypothermia with varying degrees of rewarming to ascertain whether, in the normal cerebral vasculature, varying rates of rewarming can differentially affect cerebrovascular responsiveness.

METHODS

Pentobarbital-anesthetized rats equipped with a cranial window were randomized to 3 groups. In 1 group, a 1-hour period of hypothermia (32 degrees C) followed by slow rewarming (over 90 minutes) was used. In the remaining 2 groups, either a 1- or 2-hour period of hypothermia was followed by rapid rewarming (within 30 minutes). Vasoreactivity to hypercapnia and acetylcholine was assessed before, during, and after hypothermia. Additionally, the vascular responses to sodium nitroprusside (SNP) and pinacidil, a K(ATP) channel opener, were also examined.

RESULTS

Hypothermia itself generated modest vasodilation and reduced vasoreactivity to all utilized agents. The slow rewarming group showed restoration of normal vascular responsivity. In contrast, hypothermia followed by rapid rewarming was associated with continued impaired responsiveness to acetylcholine and arterial hypercapnia. These abnormalities persisted even with the use of more prolonged (2-hour) hypothermia. Furthermore, posthypothermic rapid rewarming impaired the dilator responses of SNP and pinacidil.

CONCLUSIONS

Posthypothermic rapid rewarming caused cerebral vascular abnormalities, including a diminished response to acetylcholine, hypercapnia, pinacidil, and SNP. Our data with acetylcholine and SNP suggest that rapid rewarming most likely causes abnormality at both the vascular smooth muscle and endothelial levels.

Posttraumatic hypothermia followed by slow rewarming protects the cerebral microcirculation

In the clinical and laboratory setting, multiple reports have suggested the efficacy of hypothermia in blunting the damaging consequences of traumatic brain injury (TBI). With the use of posttraumatic hypothermia, it has been recognized that the time of initiation and duration of hypothermia are important variables in determining the degree of neuroprotection provided. Further, it has been recently recognized that the rate of posttraumatic rewarming is an important variable, with rapid rewarming exacerbating neuronal/axonal damage in contrast to slow rewarming which appears to provide enhanced neuroprotection. Although these findings have been confirmed in the brain parenchyma, no information exists for the cerebral microcirculation on the potential benefits of posttraumatic hypothermia followed by either slow or rapid rewarming. In the current communication we assess these issues in the pial circulation using a well-characterized model of TBI. Rats were prepared for the placement of cranial widows for direct assessment of the pial microcirculation prior to and after the induction of impact acceleration injury followed by moderate hypothermia with either subsequent slow or rapid rewarming strategies. The cranial windows allowed for the measurement of pial vessel diameter to assess ACh-dependent and CO2 reactivity in the chosen paradigms. ACh was applied topically to assess ACh-dependent dilation, while CO2 reactivity was assessed by changing the concentration of the inspired gas. Through this approach, it was found that posttraumatic hypothermia followed by slow rewarming maintained normal arteriolar vascular responses in terms of ACh-dependent dilation and CO2 reactivity. In contrast, arterioles subjected to TBI followed by normothermia or hypothermia and rapid rewarming showed impaired vasoreactivity in terms of their ACh-dependent and CO2 responses. This study provides additional evidence of the benefits of posttraumatic hypothermia followed by slow rewarming, demonstrating for the first time that the previously described neuroprotective effects extend to the cerebral microcirculation.

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.

Cerebral blood flow and vascular physiology

The cerebral circulation is tightly regulated to meet the brain's metabolic demands. Although the mechanism is not fully understood, the major physiologic influences on cerebral blood flow have been well documented. In this chapter the basic vascular anatomy, and physiologic control of the cerebral circulation are reviewed. Clinical implications are emphasized.

Adequate cerebral perfusion pressure during rewarming to prevent ischemic deterioration after therapeutic hypothermia

Ischemic deterioration during rewarming is one of the most notable clinical complications after successful therapeutic cerebral hypothermia, but the mechanism is not completely understood. Hypothermia may cause vasoconstriction and relative ischemia, especially with insufficient cerebral perfusion pressure (CPP). Various parameters were evaluated to determine the critical CPP threshold to avoid ischemia during rewarming. Cat experimental head injury was induced by inflating an epidural rubber balloon, and intracranial pressure was maintained at 30 mmHg. During rewarming after cerebral hypothermia, CPP was maintained at >120 mmHg (n = 16), 90 mmHg (n = 11), 60 mmHg (n = 11), and 40 mmHg (n=4) by controlling the blood pressure. Cerebral blood flow, cerebral metabolic rate for oxygen, arteriovenous difference of oxygen (AVDO2), cerebral venous oxygen saturation (ScvO2), and extracellular glutamate concentrations were monitored by glutamate oxidase electrode. After rewarming, the cerebral metabolic parameters were almost restored to the pre-injury level in animals with CPP of more than 90mmHg. However, in the animals with CPP= 60 mmHg, all parameters significantly deteriorated and indicated misery perfusion; ScvO2 was low (29.5+/-1.1%), AVDO2 was significantly high (9.9+/-0.8 ml 100 g(-1) min(-1)) (one-way analysis of variance, p<0.05), and electron microscopic features showed subcellular ischemic change. Extracellular glutamate significantly increased during the rewarming period only in the CPP= 40 mmHg group. CPP less than 60 mmHg during rewarming causes secondary ischemic insult, which might indicate continuation of cerebral vasoconstriction in hypothermia. CPP higher than 90 mmHg is required to avoid the potential risk of relative ischemia after hypothermia.