Effects of positive end-expiratory pressure on intracranial pressure, cerebral perfusion pressure, and brain oxygenation in acute brain injury: Friend or foe? A scoping review

Background

Patients with acute brain injury (ABI) are a peculiar population because ABI does not only affect the brain but also other organs such as the lungs, as theorized in brain-lung crosstalk models. ABI patients often require mechanical ventilation (MV) to avoid the complications of impaired respiratory function that can follow ABI; MV should be settled with meticulousness owing to its effects on the intracranial compartment, especially regarding positive end-expiratory pressure (PEEP). This scoping review aimed to (1) describe the physiological basis and mechanisms related to the effects of PEEP in ABI; (2) examine how clinical research is conducted on this topic; (3) identify methods for setting PEEP in ABI; and (4) investigate the impact of the application of PEEP in ABI on the outcome.

Methods

The five-stage paradigm devised by Peters et al. and expanded by Arksey and O'Malley, Levac et al., and the Joanna Briggs Institute was used for methodology. We also adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) extension criteria. Inclusion criteria: we compiled all scientific data from peer-reviewed journals and studies that discussed the application of PEEP and its impact on intracranial pressure, cerebral perfusion pressure, and brain oxygenation in adult patients with ABI. Exclusion criteria: studies that only examined a pediatric patient group (those under the age of 18), experiments conducted solely on animals; studies without intracranial pressure and/or cerebral perfusion pressure determinations, and studies with incomplete information. Two authors searched and screened for inclusion in papers published up to July 2023 using the PubMed-indexed online database. Data were presented in narrative and tubular form.

Results

The initial search yielded 330 references on the application of PEEP in ABI, of which 36 met our inclusion criteria. PEEP has recognized beneficial effects on gas exchange, but it produces hemodynamic changes that should be predicted to avoid undesired consequences on cerebral blood flow and intracranial pressure. Moreover, the elastic properties of the lungs influence the transmission of the forces applied by MV over the brain so they should be taken into consideration. Currently, there are no specific tools that can predict the effect of PEEP on the brain, but there is an established need for a comprehensive monitoring approach for these patients, acknowledging the etiology of ABI and the measurable variables to personalize MV.

Conclusion

PEEP can be safely used in patients with ABI to improve gas exchange keeping in mind its potentially harmful effects, which can be predicted with adequate monitoring supported by bedside non-invasive neuromonitoring tools.

Low mitochondrial DNA copy number of resected cecum appendix correlates with high severity of acute appendicitis

BACKGROUND/PURPOSE

The roles of mitochondrial DNA alterations in acute appendicitis (AA) remain unclear. We evaluated the alterations of mtDNA copy number and mtDNA integrity [proportion of mtDNA templates without 8-hydroxyl-2'-deoxyguanosine (8-OHdG)] of the resected cecum appendixes in clinically suspected acute appendicitis (CSAA).

METHODS

A total of 228 CSAA patients, including 50 harbored negative AA (NAA), 155 true AA (TAA) without rupture and 23 TAA with rupture, who underwent appendectomies were enrolled. Tissues of resected cecum appendixes from the paraffin-embedded pathological blocks were subjected to DNA extraction, and their mtDNA copy number and mtDNA integrity were determined by quantitative real-time polymerase chain reaction (Q-PCR).

RESULTS

During the progression of disease severity from NAA to TAA without rupture and further TAA with rupture, increases of white blood cell (WBC) counts (p = 0.001), positive bacterial culture rates in turbid ascites (p = 0.016) and area (p < 0.001)/or volume (p < 0.001) indices of resected cecum appendixes were noted among CSAA patients. On the contrary, decrease of mtDNA copy number (p = 0.003) was observed during disease progression of CSAA patients, especially in female patients (p = 0.007). Furthermore, lower mtDNA copy numbers were correlated with higher WBC counts (p = 0.001) and larger area (p = 0.003) or volume (p < 0.001) indices of the resected cecum appendixes. However, such an alteration was not observed in mtDNA integrity of resected cecum appendixes.

CONCLUSION

We conclude that a low mtDNA copy number of the resected cecum appendix may reflect high severity of acute appendicitis.

Innate immunity of the human newborn: distinct cytokine responses to LPS and other Toll-like receptor agonists

Human newborns are at increased risk of microbial invasion and display diminished responses to many vaccines indicating a functional immaturity of the immune system at birth. Such altered immune reactivity may reflect the demands of in utero existence, including the need to avoid potentially harmful inflammatory immune reactions. Despite normal basal expression of Toll-like receptors and membrane CD14, innate immune responses of neonatal mononuclear cells to lipopolysaccharide are characterized by markedly reduced release of the pro-inflammatory Th1-polarizing cytokines TNF-α and interferon-γ with relative preservation of anti-inflammatory Th2-polarizing cytokines. Differences between newborns and adults with respect to TLR-induced TNF-α release extend to a range of TLR agonists, including bacterial lipopeptides, and are due to differences in soluble factors present in blood plasma. Soluble factors in neonatal blood plasma suppress TLR-induced TNF-α release from monocytes and efforts to identify and characterize these inhibitors are on-going. Such altered immunity to TLR agonists is likely to alter both innate and adaptive immune responses in newborns profoundly. Definition of the mechanisms underlying distinct neonatal immunity promises to identify novel ways to prevent and treat infection in this relatively high-risk population.

NAD+-dependent sirtuin 1 and 6 proteins coordinate a switch from glucose to fatty acid oxidation during the acute inflammatory response

The early initiation phase of acute inflammation is anabolic and primarily requires glycolysis with reduced mitochondrial glucose oxidation for energy, whereas the later adaptation phase is catabolic and primarily requires fatty acid oxidation for energy. We reported previously that switching from the early to the late acute inflammatory response following TLR4 stimulation depends on NAD(+) activation of deacetylase sirtuin 1 (SirT1). Here, we tested whether NAD(+) sensing by sirtuins couples metabolic polarity with the acute inflammatory response. We found in TLR4-stimulated THP-1 promonocytes that SirT1 and SirT 6 support a switch from increased glycolysis to increased fatty acid oxidation as early inflammation converts to late inflammation. Glycolysis enhancement required hypoxia-inducing factor-1α to up-regulate glucose transporter Glut1, phospho-fructose kinase, and pyruvate dehydrogenase kinase 1, which interrupted pyruvate dehydrogenase and reduced mitochondrial glucose oxidation. The shift to late acute inflammation and elevated fatty acid oxidation required peroxisome proliferator-activated receptor γ coactivators PGC-1α and β to increase external membrane CD36 and fatty acid mitochondrial transporter carnitine palmitoyl transferase 1. Metabolic coupling between early and late responses also required NAD(+) production from nicotinamide phosphoryltransferase (Nampt) and activation of SirT6 to reduce glycolysis and SirT1 to increase fatty oxidation. We confirmed similar shifts in metabolic polarity during the late immunosuppressed stage of human sepsis blood leukocytes and murine sepsis splenocytes. We conclude that NAD(+)-dependent bioenergy shifts link metabolism with the early and late stages of acute inflammation.

Challenges for modeling and interpreting the complex biology of severe injury and inflammation

Human injury is associated with inflammatory responses that are modulated by the acute and chronic activity of endogenous factors and exogenous interventions. A characteristic feature of chronic, severe inflammatory states is the diminished signal output variability of many organ systems, including innate immune responsiveness and endogenous neural and endocrine-mediated functions. The attenuation of signal/response variability and integration of feedback capacity may contribute to systemic and tissue-specific deterioration of function. Some well-intentioned therapies directed toward support of systemic and tissue functions may actually promote the loss of system(s) adaptability and contribute to adverse outcomes in severely stressed patients. In vivo and in silico models of stress, injury, and infection have yet to fully define the influences of ongoing stressful stimulae as well as genetic variation and epigenetic factors in the context of an evolving inflammatory state. Experimental and human models incorporating variable, antecedent stress(es) and altered neuroendocrine rhythms might approximate the altered adaptability in immune and organ function responses. Such models may also provide insights into the salient mechanisms of risk and outcome more precisely than do the constrained study conditions of current animal or human models of systemic inflammation.

Propofol increased the interleukin-6 to interleukin-10 ratio more than isoflurane after surgery in long-term alcoholic patients

This study investigated the effect of an anaesthetic regimen on the immune response in 40 long-term alcoholic patients undergoing surgery. Patients were randomly allocated to receive either propofol or isoflurane during surgery. Plasma cytokines interleukin (IL)-6 and IL-10 were measured at defined times and rates of post-operative infections were documented. The IL-6/IL-10 ratio significantly increased with propofol compared with isoflurane on day 1 after surgery and the IL-10 level significantly increased with isoflurane on day 1 after surgery. The overall post-operative infection rate was significantly higher in isoflurane-treated patients. Our findings indicate that propofol anaesthesia might be the more favourable regimen, with the IL-6/IL-10 ratio indicating an attenuation of the immune imbalance after surgery in long-term alcoholic patients. These results support the undertaking of a properly powered clinical trial to determine if propofol anaesthesia can reduce the postoperative infection rate in this special patient population.

Immunologic responses to critical injury and sepsis

Almost 2 million patients are admitted to hospitals in the United States each year for treatment of traumatic injuries, and these patients are at increased risk of late infections and complications of systemic inflammation as a result of injury. Host response to injury involves a general activation of multiple systems in defending the organism from hemorrhagic or infectious death. Clinicians have the capability to support the critically injured through their traumatic insult with surgery and improved critical care, but the inflammatory response generated by such injuries creates new challenges in the management of these patients. It has long been known that local tissue injury induces systemic changes in the traumatized patient that are often maladaptive. This article reviews the effects of injury on the function of immune system cells and highlights some of the clinical sequelae of this deranged inflammatory-immune interaction.

Comparative inflammatory properties of staphylococcal superantigenic enterotoxins SEA and SEG: implications for septic shock

The severity of Staphylococcus aureus sepsis is positively associated with staphylococcal enterotoxin A (SEA) and negatively associated with the enterotoxin gene cluster (egc), which encodes five staphylococcal enterotoxins. We postulated that the variable, clinical severity of S. aureus sepsis might be a result of differences in the inflammatory properties of staphylococcal superantigens. We therefore compared the inflammatory properties of SEA with those of staphylococcal entérotoxin G (SEG), a member of the five egc superantigens. We found that SEA and SEG had similar superantigenic properties, as they induced CD69 expression on T lymphocytes and selective expansion of Vbeta subpopulations. Contrary to SEG, however, SEA induced a strong proinflammatory/Th1 response, including TNF-alpha and MIP-1alpha production. These results suggest that the association of SEA with the severity of S. aureus septic shock, characterized by a deleterious, inflammatory cascade, may be explained partly by the specific proinflammatory properties of this superantigen.

Innate immunity genes influence the severity of acute appendicitis

OBJECTIVE

Using acute appendicitis as a model, we tested the hypothesis that polymorphisms in genes involved in host defense can be associated with the severity of local infection-inflammation in humans.

SUMMARY BACKGROUND DATA

Innate immunity is the body's front-line system for antimicrobial host defense. Local inflammation is a major innate immune mechanism for containing and destroying microbes, but it may also contribute to tissue injury.

METHODS

We studied 134 patients with acute appendicitis treated at an urban hospital. We looked for associations between the severity of appendicitis (uncomplicated vs. perforated or gangrenous), plasma and peritoneal cytokine concentrations, and single nucleotide polymorphisms in genes involved in recognizing bacterial molecules [CD14 (-159 C-->T); TLR4 (896 A-->G)] and in mounting an inflammatory response [IL-6 (-174 G-->C), TNF-alpha (-308 G-->A), IL-1beta (-31 C -->T)].

RESULTS

Ninety-one patients (68%) had uncomplicated appendicitis and 43 (32%) had complicated disease. The SNPs in the CD14, TLR4, IL-1beta, and TNF-alpha genes were not associated with the severity of appendicitis. A strong association was found between C-allele carriage at -174 in the IL-6 gene and decreased risk of complicated disease (adjusted odds ratio = 0.24, 95% CI = 0.07-0.76). Lower plasma and peritoneal fluid IL-6 concentrations in the IL-6 -174 C-carriers than in the GG homozygotes suggest that this polymorphism contributes to decreased IL-6 production in vivo.

CONCLUSIONS

Polymorphism in the IL-6 gene was associated with the severity of appendicitis, even after adjustment for duration of symptoms. The risk for developing appendiceal perforation or gangrene may be determined, in part, by variation in the IL-6 gene.

Lipopolysaccharide (LPS)-binding protein inhibits responses to cell-bound LPS

Lipopolysaccharide (LPS)-binding protein (LBP) is an acute phase reactant that may play a dual role in vivo, both potentiating and decreasing cell responses to bacterial LPS. Whereas low concentrations of LBP potentiate cell stimulation by transferring LPS to CD14, high LBP concentrations inhibit cell responses to LPS. One inhibitory mechanism involves the ability of LBP to neutralize LPS by transferring it to plasma lipoproteins, whereas other inhibitory mechanisms, such as the one described here, do not require exogenous lipoproteins. Here we show that LBP can inhibit monocyte responses to LPS that has already bound to membrane-bound CD14 (mCD14) on the cell surface. LBP caused rapid dissociation of LPS from mCD14 as measured by the ability of LBP to inhibit cross-linking of a radioiodinated, photoactivatable LPS derivative to mCD14. Whereas LBP removed up to 75% of the mCD14-bound LPS in 10 min, this was not accompanied by extensive release of the LPS from the cells. The cross-linking data suggest that much of the LPS that remained bound to the cells was associated with LBP. The ability of LBP to inhibit cell responses could not be explained by its effect on LPS internalization, because LBP did not significantly increase the internalization of the cell-bound LPS. In cell-free LPS cross-linking experiments, LBP inhibited the transfer of LPS from soluble CD14 to soluble MD-2. Our data support the hypothesis that LBP can inhibit cell responses to LPS by inhibiting LPS transfer from mCD14 to the Toll-like receptor 4-MD-2 signaling receptor.