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

A Systematic Review of Neuroprotective Strategies during Hypovolemia and Hemorrhagic Shock

Severe trauma constitutes a major cause of death and disability, especially in younger patients. The cerebral autoregulatory capacity only protects the brain to a certain extent in states of hypovolemia; thereafter, neurological deficits and apoptosis occurs. We therefore set out to investigate neuroprotective strategies during haemorrhagic shock. This review was performed in accordance to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Before the start of the search, a review protocol was entered into the PROSPERO database. A systematic literature search of Pubmed, Web of Science and CENTRAL was performed in August 2017. Results were screened and evaluated by two researchers based on a previously prepared inclusion protocol. Risk of bias was determined by use of SYRCLE’s risk of bias tool. The retrieved results were qualitatively analysed. Of 9093 results, 119 were assessed in full-text form, 16 of them ultimately adhered to the inclusion criteria and were qualitatively analyzed. We identified three subsets of results: (1) hypothermia; (2) fluid therapy and/or vasopressors; and (3) other neuroprotective strategies (piracetam, NHE1-inhibition, aprotinin, human mesenchymal stem cells, remote ischemic preconditioning and sevoflurane). Overall, risk of bias according to SYRCLE’s tool was medium; generally, animal experimental models require more rigorous adherence to the reporting of bias-free study design (randomization, etc.). While the individual study results are promising, the retrieved neuroprotective strategies have to be evaluated within the current scientific context—by doing so, it becomes clear that specific promising neuroprotective strategies during states of haemorrhagic shock remain sparse. This important topic therefore requires more in-depth research.

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.