A porcine model of hyperdynamic endotoxemia: pattern of respiratory, macrocirculatory, and regional blood flow changes

CD1d- and MR1-Restricted T Cells in Sepsis

Dysregulated immune responses to infection, such as those encountered in sepsis, can be catastrophic. Sepsis is typically triggered by an overwhelming systemic response to an infectious agent(s) and is associated with high morbidity and mortality even under optimal critical care. Recent studies have implicated unconventional, innate-like T lymphocytes, including CD1d- and MR1-restricted T cells as effectors and/or regulators of inflammatory responses during sepsis. These cell types are typified by invariant natural killer T (iNKT) cells, variant NKT (vNKT) cells, and mucosa-associated invariant T (MAIT) cells. iNKT and vNKT cells are CD1d-restricted, lipid-reactive cells with remarkable immunoregulatory properties. MAIT cells participate in antimicrobial defense, and are restricted by major histocompatibility complex-related protein 1 (MR1), which displays microbe-derived vitamin B metabolites. Importantly, NKT and MAIT cells are rapid and potent producers of immunomodulatory cytokines. Therefore, they may be considered attractive targets during the early hyperinflammatory phase of sepsis when immediate interventions are urgently needed, and also in later phases when adjuvant immunotherapies could potentially reverse the dangerous state of immunosuppression. We will highlight recent findings that point to the significance or the therapeutic potentials of NKT and MAIT cells in sepsis and will also discuss what lies ahead in research in this area.

Lipopolysaccharides (LPS) modulate the metabolism of deoxynivalenol (DON) in the pig

Pigs might be exposed to lipopolysaccharides (LPS) and deoxynivalenol (DON) at the same time, and both toxins are thought to interactively affect the intestinal barrier, the innate immune system, and the xenobiotics metabolism. Hence, we aimed at examining the single and combined effects of both toxins on nutrient digestibility and DON metabolism. For this purpose, barrows (26 ± 4 kg) were fed restrictedly either a control diet (CON) or a diet contaminated with 3.1 mg DON/kg (DON) for 37 days. At day 37 of the experiment, pigs were infused intravenously for 60 min either with 100 μg DON/kg body weight (BW) (CON-DON), 7.5 μg LPS/kg BW (CON-LPS, DON-LPS) or a combination of both substances (CON-DON + LPS), or physiological saline (CON-CON, DON-CON). Blood samples were collected frequently until 3.25 h before the pigs were sacrificed for bile, liver, and kidney collection. The apparent digestibility of N-free extractives was significantly increased by 1 % when the DON-contaminated diet was fed. The total DON content in blood was significantly higher in endotoxemic pigs (34.8 ng/mL; CON-DON + LPS) when compared to the pigs infused with DON alone (18.8 ng/mL; CON-DON) while bile concentrations were not influenced by LPS. DON residue levels in liver and kidney closely reflected the treatment effects as described for blood. In contrast to DON infusion, the LPS challenge resulted in a significantly lower total DON concentration (13.2 vs. 7.5 ng/mL in groups DON-CON and DON-LPS, respectively) when the pigs were exposed to DON through the diet. The conjugation degree for DON in blood and bile was not influenced by treatments. In conclusion, endotoxemic pigs are characterized by higher DON residue levels in blood, liver, and kidney, probably by a compromised elimination.

Hyperoxic ventilation improves survival in pigs during endotoxaemia at the critical hemoglobin concentration

AIM OF THE STUDY

Recently it has been demonstrated that short term hyperoxic ventilation (HV) can improve glucose metabolism, reduce pulmonary and hepatic apoptosis, and improve gastrointestinal perfusion during acute sepsis. However, it is unknown whether additional O(2) improves survival. Therefore we investigated the effects of increased plasma O(2) on survival during extreme anaemia and concomitant endotoxaemia in order to quantify the efficacy of HV.

METHODS

Endotoxaemia (Salmonella abortus equi-LPS) was induced in 14 anesthetized pigs ventilated with room air (FiO(2)=0.21). Simultaneously, animals were haemodiluted by exchange of whole blood for 6% hydroxyethyl starch (200,000:0.5) until the individual critical hemoglobin concentration (Hb(crit)) was achieved (outermost limit of tissue oxygenation). Subsequently, animals were either ventilated with an FiO(2) of 0.21 (NOX, n=7) or an FiO(2) of 1.0 (HOX, n=7), and observed thereafter for 6 h without further intervention.

RESULTS

HV significantly prolonged survival time at Hb(crit) (NOX, 30 [27/35] min; HOX, 172 [111/235] min, p<0.05). In contrast to the NOX group, HV maintained MAP, and improved DO(2) and tissue oxygenation in the HOX group.

CONCLUSION

The improvement of survival, oxygen transport and tissue oxygenation seems to underline the efficacy of HV during endotoxaemia and concomitant acute anaemia. Further studies are needed to transfer these results into daily clinical practice.

Renal Vascular Resistance in Sepsis

AIMS

To assess changes in renal vascular resistance (RVR) in human and experimental sepsis and to identify determinants of RVR.

METHODS

We performed a systematic interrogation of two electronic reference libraries using specific search terms. Subjects were animals and patients involved in experimental and human studies of sepsis and septic acute renal failure, in which the RVR was assessed. We obtained all human and experimental articles reporting RVR during sepsis. We assessed the role of various factors that might influence the RVR during sepsis using statistical methods.

RESULTS

We found no human studies, in which the renal blood flow (and, therefore, the RVR) was measured with suitably accurate direct methods. Of the 137 animal studies identified, 52 reported a decreased RVR, 16 studies reported no change in RVR, and 69 studies reported an increased RVR. Consciousness of animals, duration of measurement, method of induction of sepsis, and fluid administration had no effect on the RVR. On the other hand, on univariate analysis, size of the animals (p < 0.001), technique of measurement (p = 0.017), recovery after surgery (p = 0.004), and cardiac output (p < 0.001) influenced the RVR. Multivariate analysis, however, showed that only cardiac output (p = 0.028) and size of the animals (p = 0.031) remained independent determinants of the RVR.

CONCLUSIONS

Changes in RVR during sepsis in humans are unknown. In experimental sepsis, several factors not directly related to sepsis per se appear to influence the RVR. A high cardiac output and the use of large animals predict a decreased RVR, while a decreased cardiac output and the use of small animals predict an increased RVR.

Renal blood flow in sepsis

Introduction

To assess changes in renal blood flow (RBF) in human and experimental sepsis, and to identify determinants of RBF.

Method

Using specific search terms we systematically interrogated two electronic reference libraries to identify experimental and human studies of sepsis and septic acute renal failure in which RBF was measured. In the retrieved studies, we assessed the influence of various factors on RBF during sepsis using statistical methods.

Results

We found no human studies in which RBF was measured with suitably accurate direct methods. Where it was measured in humans with sepsis, however, RBF was increased compared with normal. Of the 159 animal studies identified, 99 reported decreased RBF and 60 reported unchanged or increased RBF. The size of animal, technique of measurement, duration of measurement, method of induction of sepsis, and fluid administration had no effect on RBF. In contrast, on univariate analysis, state of consciousness of animals (P = 0.005), recovery after surgery (P < 0.001), haemodynamic pattern (hypodynamic or hyperdynamic state; P < 0.001) and cardiac output (P < 0.001) influenced RBF. However, multivariate analysis showed that only cardiac output remained an independent determinant of RBF (P < 0.001).

Conclusion

The impact of sepsis on RBF in humans is unknown. In experimental sepsis, RBF was reported to be decreased in two-thirds of studies (62 %) and unchanged or increased in one-third (38%). On univariate analysis, several factors not directly related to sepsis appear to influence RBF. However, multivariate analysis suggests that cardiac output has a dominant effect on RBF during sepsis, such that, in the presence of a decreased cardiac output, RBF is typically decreased, whereas in the presence of a preserved or increased cardiac output RBF is typically maintained or increased.

Direct assessment and distribution of regional portal blood flow in the pig by means of fluorescent microspheres

Measurement of regional organ blood flow by means of fluorescent microspheres (FM) is an accepted method. However, determination of regional portal blood flow (RPBF) cannot be performed by microspheres owing to the entrapment of the spheres in the upstream capillary bed of the splanchnic organs. We hypothesized that an adequate experimental setting would enable us to measure RPBF by means of FM and to analyze its distribution within the pig liver. A mixing chamber for the injection of FM was developed, and its capability to distribute FM homogeneously in the blood was evaluated in vitro. The chamber was implanted into the portal vein of six anesthetized pigs (23.5 +/- 2.9 kg body wt). Three consecutive, simultaneous injections of FM of two different colors into the chamber were performed. Reference portal blood samples were collected by means of a Harvard pump. At the end of the experiment, the liver was explanted and fixed in formalin before dissection. FM were isolated from the tissue samples by an automated process, and fluorescence intensity was determined. Comparison of 5,458 single RPBF values, determined by simultaneously injected FM, revealed good agreement (bias 2.5%, precision 12.7%) and high correlation (r = 0.97, r2 = 0,95, slope = 1.04, intercept = 0.05). Median RPBF was 1.07 +/- 0.78 ml x min(-1) x g(-1). Allocation of the blood flow values to the anatomic regions of the liver revealed a significantly higher RPBF (P = 0.01) in the liver tissue located close to the diaphragm compared with the rest of the organ and a significantly lower RPBF (P = 0.01) in the left liver lobe compared with the median and right lobes. The results show that the model presented makes it possible to measure RPBF by means of FM reliably and that RPBF is distributed heterogeneously in the porcine liver.

L-arginine supplementation in hyperdynamic endotoxemic pigs: effect on nitric oxide synthesis by the different organs

OBJECTIVES

Under septic conditions, the protective role of nitric oxide in the organs may become compromised at a time of increased demand as a result of decreased availability of L-arginine. It remains unknown whether supplementation with L-arginine, as a substrate, can modulate organ nitric oxide production.

DESIGN

Controlled study with laboratory animals.

SETTING

University research laboratory.

SUBJECTS

Female crossbred pigs.

INTERVENTION

Pigs were challenged with Escherichia coli endotoxin (intravenously) and received intravenous fluid resuscitation for 24 hrs to reproduce a model of long-lasting hyperdynamic endotoxemia. Pigs were infused with either L-arginine or L-alanine intravenously during endotoxin and via the intragastric route after cessation of endotoxin infusion. The effects of L-arginine supplementation on nitric oxide synthesis and the relationships with arginine metabolism were determined with a stable isotope infusion protocol. Also, organ nitrite plus nitrate fluxes were measured. Implantation of multiple catheters enabled in vivo measurements across the hindquarter muscle, the portal-drained viscera, the liver, and the kidneys.

MEASUREMENTS AND RESULTS

The isotope conversion method showed that L-arginine intervention significantly increased nitric oxide production by the portal-drained viscera, liver, and kidneys, resulting in elevated whole-body nitric oxide synthesis under endotoxemic and postendotoxemic conditions. Organ nitrite plus nitrate fluxes only tended to increase because of high variance among data.

CONCLUSIONS

In this endotoxemia model, supplemental use of L-arginine favored nitric oxide synthesis in various organs.

Myocardial dysfunction in sepsis

1. Sepsis is the leading reversible cause of death in patients requiring modern intensive care services. 2. In this group of patients, death usually results from progressive multiple organ failure, rather than overwhelming primary infection. 3. The pathophysiology of sepsis-induced remote organ dysfunction is incompletely understood, although it is believed to result from a systemic inflammatory process that causes tissue injury in the absence of septic shock. 4. As septic shock is the most common early manifestation of severe sepsis, an understanding of mechanisms of myocardial dysfunction is of clinical relevance. In the present review, we will discuss mechanisms of remote organ failure in sepsis, focusing in particular on the pathogenesis of myocardial dysfunction.