Endotoxin tolerance variation over 24 h during porcine endotoxemia: association with changes in circulation and organ dysfunction

Abstract

Endotoxin tolerance (ET), defined as reduced inflammatory responsiveness to endotoxin challenge following a first encounter with endotoxin, is an extensively studied phenomenon. Although reduced mortality and morbidity in the presence of ET has been demonstrated in animal studies, little is known about the temporal development of ET. Further, in acute respiratory distress syndrome ET correlates to the severity of the disease, suggesting a complicated relation between ET and organ dysfunction. Eighteen pigs were subjected to intensive care and a continuous endotoxin infusion for 24 h with the aim to study the time course of early ET and to relate ET to outcome in organ dysfunction. Three animals served as non-endotoxemic controls. Blood samples for cytokine analyses were taken and physiological variables registered every third hour. Production of TNF-α, IL-6, and IL-10 before and after endotoxin stimulation ex vivo was measured. The difference between cytokine values after and before ex vivo LPS stimulation (Δ-values) was calculated for all time points. ΔTNF-α was employed as the principal marker of ET and lower ΔTNF-α values were interpreted as higher levels of ET. During endotoxin infusion, there was suppression of ex vivo productions of TNF-α and IL-6 but not of IL-10 in comparison with that at 0 h. The ex vivo TNF-α values followed another time concentration curve than those in vivo. ΔTNF-α was at the lowest already at 6 h, followed by an increase during the ensuing hours. ΔTNF-α at 6 h correlated positively to blood pressure and systemic vascular resistance and negatively to cardiac index at 24 h. In this study a temporal variation of ET was demonstrated that did not follow changes in plasma TNF-α concentrations. Maximal ET occurred early in the course and the higher the ET, the more hyperdynamic the circulation 18 h later.

Figure 1. Basal- and ex vivo-stimulated TNF-α time concentration curves during the experiment.

Values are logarithmically transformed and given as mean ± SE. The log values on the y-axis have been transformed to absolute values. The p-value is the result of the ANOVA for repeated measures group by time interaction analysis comparing the two curves between 3 and 24 h (n = 18). The group analysis as well as the time analysis of the ANOVA for repeated measures comparing the two curves between 3 and 24 h (n = 18), both resulted in p-values of <0.001.

Figure 2. Differences in TNF-α after and before ex vivo stimulation, ΔTNF-α, at different time points during the experiment in the animals given endotoxin (n = 18).

Values are interquartile ranges. Statistical significance at different time points was tested on the individual changes from time zero using Wilcoxon's matched pairs test. The bar marked with § represents stimulation without prior exposure to endotoxin at 0 h. Bars marked with * represent endotoxin tolerance with a significant reduction of ΔTNF-α in comparison with that at 0 h. Unmarked bars indicate that criteria for endotoxin tolerance were not fulfilled.

Figure 3. The development during the experiment of leukocyte count, platelet count and hemoglobin.

Values are given as relative to the baseline value in percent. Mean ± SE, (n = 18). Baseline values: Leukocyte count 10.6±1.0 10⁹×L⁻¹, Platelet count 380±29 10⁹×L⁻¹, Hemoglobin 76±2 g×L⁻¹.

Figure 4. The development during the experiment of left ventricular stroke work index (LVSWI), cardiac index (CI), systemic vascular resistance index (SVRI) and mean arterial pressure (MAP).

Values are given as relative to the baseline value in percent. Mean ± SE, (n = 18). Baseline values: LVSWI 48±3 g×m⁻², CI 4.3±0.3 L×min⁻¹×m⁻², SVRI 1757±127 dyne×s×cm⁻⁵, MAP 92±4 mmHg.

Figure 5. The development during the experiment of compliance, PaO₂/FiO₂, oxygen delivery (DO₂), oxygen consumption (VO₂) and base excess (BE).

Values are given as relative to the baseline value in percent. Mean ± SE, (n = 18). Baseline values: Compliance 25±1 mL×cm H₂0⁻¹, PaO₂/FiO₂ 426±16 mmHg, DO₂ 454±36 mL×min⁻¹, VO₂ 134±8 mL×min⁻¹ and BE 7±0.5 mmol×L⁻¹.