Effect of Intravenous Fluid Volumes on the Adrenal Glucocorticoid Response After Burn Injury in Swine

Modulation of Burn Hypermetabolism in Preclinical Models

Severe burns elicit a state of physiological stress and increased metabolism to help the body compensate for the changes associated with the traumatic injury. However, this hypermetabolic state is associated with increased insulin resistance, cardiovascular dysfunction, skeletal muscle catabolism, impaired wound healing, and delayed recovery. Several interventions were attempted to modulate burn hypermetabolism, including nutritional support, early excision and grafting, and growth hormone application. However, burn hypermetabolism still imposes significant morbidity and mortality in burn patients. Due to the limitations of in vitro models, animal models are indispensable in burn research. Animal models provide researchers with invaluable tools to test the safety and efficacy of novel treatments or advance our knowledge of previously utilized agents. Several animal studies evaluated novel therapies to modulate burn hypermetabolism in the last few years, including recombinant human growth hormone, erythropoietin, acipimox, apelin, anti-interleukin-6 monoclonal antibody, and ghrelin therapies. Results from these studies are promising and may be effectively translated into human studies. In addition, other studies revisited drugs previously used in clinical practice, such as insulin and metformin, to further investigate their underlying mechanisms as modulators of burn hypermetabolism. This review aims to update burn experts with the novel therapies under investigation in burn hypermetabolism with a focus on applicability and translation. Furthermore, we aim to guide researchers in selecting the correct animal model for their experiments by providing a summary of the methodology and the rationale of the latest studies.

Advantages and Disadvantages of Using Small and Large Animals in Burn Research: Proceedings of the 2021 Research Special Interest Group

Multiple animal species and approaches have been used for modeling different aspects of burn care, with some strategies considered more appropriate or translatable than others. On April 15, 2021, the Research Special Interest Group of the American Burn Association held a virtual session as part of the agenda for the annual meeting. The session was set up as a pro/con debate on the use of small versus large animals for application to four important aspects of burn pathophysiology: burn healing/conversion; scarring; inhalation injury; and sepsis. For each of these topics, 2 experienced investigators (one each for small and large animal models) described the advantages and disadvantages of using these preclinical models. The use of swine as a large animal model was a common theme due to anatomic similarities with human skin. The exception to this was a well-defined ovine model of inhalation injury; both of these species have larger airways which allow for incorporation of clinical tools such as bronchoscopes. However, these models are expensive and demanding from labor and resource standpoints. Various strategies have been implemented to make the more inexpensive rodent models appropriate for answering specific questions of interest in burns. Moreover, modelling burn-sepsis in large animals has proven difficult. It was agreed that the use of both small and large animal models have merit for answering basic questions about the responses to burn injury. Expert opinion and the ensuing lively conversations are summarized herein, which we hope will help inform experimental design of future research.

The Effect of Burn Resuscitation Volumes on the Gut Microbiome in a Swine Model

INTRODUCTION

While recent reports underscore the significance of the gut microbiome (GM) in health and disease, its importance in burn outcomes remains unclear. Moreover, aggressive intravenous (IV) fluid resuscitation of patients may alter intestinal flora. Herein, we describe GM changes following a large burn in swine randomized to different volumes of IV Lactated Ringers' (LR).

METHODS

Anesthetized Yorkshire swine sustained 40% total body surface area full-thickness burns and were randomized to different volumes of IV LR: none (n = 5), 15 mL/kg/d (low; n = 6), or 80 mL/kg/d (high; n = 6). At baseline and days 1 and 2, fecal swabs were collected for 16s rDNA sequencing. Proximal jejunum was collected immediately after euthanasia (day 2) for western blot, histopathology, and cytokine analyses.

RESULTS

Burns produced significant shifts in β-diversity and non-significant reductions in α-diversity that did not recover regardless of treatment group. Burn-induced increases in Proteobacteria and decreases in Firmicutes were attenuated by IV fluids in a dose-dependent manner, and also correlated with α-diversity. IV fluids caused a dose-dependent increase in Bacteroides and prevented a transient increase in the opportunistic pathogen Haemophilus parainfluenzae. While high volumes of IV fluids increased intestinal Hsp70 levels (P = 0.0464), they reduced SGLT1 (P = 0.0213) and caspase3 (P = 0.0139) levels. IV fluids elicited a non-specific cytokine response; however, Bacteroidetes levels correlated with intestinal IL18 levels (P = 0.0166, R = 0.4201).

CONCLUSIONS

We present the first report on the gut microbiome in a porcine burn model, and present data to suggest that IV fluids may influence GM and gut functional proteins following a burn. Overall, burn-induced GM diversity shifts may expose diagnostic and/or therapeutic targets to improve outcomes.

Minimal Effects of Intravenous Administration of Xenogeneic Adipose Derived Stem Cells on Organ Function in a Porcine 40%TBSA Burn Model

Adipose stem cells (ASCs) have shown therapeutic promise for various conditions, including burn injury. While ASCs have immunomodulatory properties, concerns exist over pro-coagulant activity after intravenous (IV) administration. In the present study, we examined IV human ASC delivery in terms of coagulation, organ function, and inflammation in a 40% total body surface area (TBSA) swine burn model. Anesthetized female Yorkshire swine were burned and randomized to receive 15ml/kg Lactated Ringer's containing: no ASCs; a low dose (5x10 5 ASCs/kg), or a high dose (5x10 6 ASCs/kg). For biochemical analysis, blood was collected at baseline (BL), 3, 6, 12, and 24 hours post-burn, while kidney and liver tissue was collected post-euthanasia. A significant, but transient, effect of ASCs was seen on prothrombin times and INR, wherein low doses revealed slight hypercoagulation. Burns increased partial thromboplastin time, fibrinogen, and d-dimer levels, which was unchanged with ASC administration. ASCs tended to exacerbate increases in bilirubin at 3 hours, but this didn't reach statistical significance. A significant effect of ASCs on creatinine and BUN was seen, wherein low doses elevated levels at 24 hours (creatinine, p=0.0012; BUN, p=0.0195). Hepatic and renal TUNEL staining were similar for all groups. A dose-dependent decrease in IL-8 was observed, while low doses significantly increased IL-1RA at 3 (p=0.050), IL-12 at 12 (p=0.021) and IL-6 at 24 hours post-burn (p=0.035). IV administration of xenogeneic ASCs slightly increased coagulation, but effects on burn-induced renal and hepatic dysfunction effects were minimal. Despite some significant immunomodulation, organ dysfunction effects were modest. Collectively, this study provides evidence to be skeptical about xenogeneic ASC administration in regards to burn.

Impact of oral resuscitation on circulating and splenic leukocytes after burns

BACKGROUND

Hemodynamic aberrations after severe burns are treated with aggressive intravenous (IV) fluid resuscitation however, oral resuscitation has been proposed in resource poor scenarios. Previously we have shown that animals receiving oral fluid following burns were able to recover kidney function. However, immune function such as circulating and splenic immune cell populations after oral or intravenous fluid administration was not examined. Herein, we perform a follow up analysis of splenic tissue and plasma from the previous animal study to examine the splenic response following these resuscitation strategies after burn injury.

METHODS

Eighteen anesthetized Yorkshire swine receiving 40%TBSA contact burns were randomized to receive either: (1) no fluids (Fluid Restricted; negative control), (2) 70 mL/kg/d Oral Rehydration Salt solution (Oral), or (3) 2 mL/kg/%TBSA/d of lactated Ringer's solution IV. Blood was drawn for blood cell analysis, and CT scans were performed before and 48 h post-burn, at which point spleens were harvested for histological, Western blot, and RT-PCR analyses.

RESULTS

Splenic artery diameter decreased by -0.97 ± 0.14 mm in fluid-restricted animals, while IV led to an increase of 0.68 ± 0.30 mm. No significant differences were detected in white and red pulp. IV fluids reduced the population of splenic monocytes (CD163; P = 0.001) and neutrophils (MPO protein; P = 0.13), as well as cytokines IL-8 (P = 0.003), IFN-γ (P = 0.11) and TNFα (P = 0.05). Additionally, withholding IV fluids consistently decreased the expression of FoxP3, CCR6, and IL17β in spleen, suggesting a shift in T-cell phenotype with IV resuscitation.

CONCLUSIONS

The route of fluid administration has a minor influence on the changes in circulating and splenic leukocytes post-burn in the acute phase. Further research is needed to help guide resuscitation approaches using immunologic markers of splenic function following burns.