Role of tissue hypoxia as the mechanism of lactic acidosis during E. coli endotoxemia

Effects of Polymyxin B on Clinical Signs, Serum TNF-α, Haptoglobin and Plasma Lactate Concentrations in Experimental Endotoxaemia in Sheep

Introduction

The experiment evaluated the effects of intravenous administration of polymyxin B on experimental endotoxaemia in sheep.

Material and Methods

Twenty clinically healthy fat-tailed sheep were randomly divided into: a group treated with 6,000 U/kg of polymyxin B, a group at 12,000 U/kg, and positive and negative controls. Endotoxaemia was induced by intravenous administration of lipopolysaccharide (LPS) from E. coli serotype O55:B5 at 0.5 μg/kg. polymyxin was infused intravenously along with 2.5 L of isotonic intravenous fluids at 20 mL/kg/h. The positive control group received LPS and 2.5 L of isotonic fluids, the negatives receiving just 2.5 L of isotonic fluids. Clinical signs were evaluated before and at 1.5, 3, 4.5, 6, 24, and 48 h after LPS administration. Blood was also sampled at the denoted hours and serum haptoglobin, tumour necrosis factor-α (TNF-α), and plasma lactate concentrations were assayed.

Results

The serum concentration of TNF-α in the positive control group increased significantly up to 48 h after LPS administration. The concentration of TNF-α was significantly different from those of the polymyxin B and positive control groups from 3 to 48 h; also, the concentrations of haptoglobin at different times in the polymyxin groups were lower than those of the positive control group and were significant at hours 3 to 48 (P < 0.05). Following the LPS administration, haptoglobin and TNF-α concentrations changed without significant difference between the two polymyxin B groups.

Conclusion

Polymyxin B (6,000 U/kg) restrained blood lactate concentrations. Furthermore, it significantly improved the clinical signs in endotoxaemic animals, including rectal temperature and heart and respiratory rates. Polymyxin B may be an antiendotoxic in fat-tailed sheep.

Effects of polymyxin B on clinical signs, serum TNF-α, haptoglobin and plasma lactate concentrations in experimental endotoxaemia in sheep

Introduction

The experiment evaluated the effects of intravenous administration of polymyxin B on experimental endotoxaemia in sheep.

Material and Methods

Twenty clinically healthy fat-tailed sheep were randomly divided into: a group treated with 6,000 U/kg of polymyxin B, a group at 12,000 U/kg, and positive and negative controls. Endotoxaemia was induced by intravenous administration of lipopolysaccharide (LPS) from E. coli serotype O55:B5 at 0.5 μg/kg. polymyxin was infused intravenously along with 2.5 L of isotonic intravenous fluids at 20 mL/kg/h. The positive control group received LPS and 2.5 L of isotonic fluids, the negatives receiving just 2.5 L of isotonic fluids. Clinical signs were evaluated before and at 1.5, 3, 4.5, 6, 24, and 48 h after LPS administration. Blood was also sampled at the denoted hours and serum haptoglobin, tumour necrosis factor-α (TNF-α), and plasma lactate concentrations were assayed.

Results

The serum concentration of TNF-α in the positive control group increased significantly up to 48 h after LPS administration. The concentration of TNF-α was significantly different from those of the polymyxin B and positive control groups from 3 to 48 h; also, the concentrations of haptoglobin at different times in the polymyxin groups were lower than those of the positive control group and were significant at hours 3 to 48 (P < 0.05). Following the LPS administration, haptoglobin and TNF-α concentrations changed without significant difference between the two polymyxin B groups.

Conclusion

Polymyxin B (6,000 U/kg) restrained blood lactate concentrations. Furthermore, it significantly improved the clinical signs in endotoxaemic animals, including rectal temperature and heart and respiratory rates. Polymyxin B may be an antiendotoxic in fat-tailed sheep.

Intubation trachéale et choc septique : état des lieux

La place de l’intubation dans le choc septique reste mal définie sans recommandation dictée par les sociétés savantes. Cette problématique met en balance les effets potentiellement bénéfiques de la ventilation mécanique (amélioration de l’oxygénation, repos musculaire diaphragmatique…) et délétères (risque du geste d’intubation, atrophie diaphragmatique, lésions pulmonaires induites par la ventilation, vasoplégie liée à la sédation…). L’intubation ne se discute pas lors des cas extrêmes de coma profond ou d’insuffisance respiratoire aiguë sévère. Néanmoins, la place de l’intubation dans les situations intermédiaires est actuellement peu étudiée et non codifiée. Cette mise au point a pour but d’apporter au lecteur une synthèse des résultats issus des dernières grandes études sur le choc septique. L’analyse suggère une association probable, non exclusive, entre gravité et taux d’intubation, qui reste à préciser. Bien que ces études n’aient pas étudié cette question, les données générales décrivent les caractéristiques « patient » et les données ventilatoires. Enfin, les résultats d’une étude spécifique sur le sujet et le projet de recherche en cours INTUBATIC (inclusions terminées) sont présentés, ce qui apportera peut-être une réponse à cette question dans les mois à venir.

Metabolic and hematological consequences of dietary deoxynivalenol interacting with systemic Escherichia coli lipopolysaccharide

Previous studies have shown that chronic oral deoxynivalenol (DON) exposure modulated Escherichia coli lipopolysaccharide (LPS)-induced systemic inflammation, whereby the liver was suspected to play an important role. Thus, a total of 41 barrows was fed one of two maize-based diets, either a DON-diet (4.59 mg DON/kg feed, n = 19) or a control diet (CON, n = 22). Pigs were equipped with indwelling catheters for pre- or post-hepatic (portal vs. jugular catheter) infusion of either control (0.9% NaCl) or LPS (7.5 µg/kg BW) for 1h and frequent blood sampling. This design yielded six groups: CON_CONjugular‑CONportal, CON_CONjugular‑LPSportal, CON_LPSjugular‑CONportal, DON_CONjugular‑CONportal, DON_CONjugular‑LPSportal and DON_LPSjugular‑CONportal. Blood samples were analyzed for blood gases, electrolytes, glucose, pH, lactate and red hemogram. The red hemogram and electrolytes were not affected by DON and LPS. DON-feeding solely decreased portal glucose uptake (p < 0.05). LPS-decreased partial oxygen pressure (pO₂) overall (p < 0.05), but reduced pCO₂ only in arterial blood, and DON had no effect on either. Irrespective of catheter localization, LPS decreased pH and base-excess (p < 0.01), but increased lactate and anion-gap (p < 0.01), indicating an emerging lactic acidosis. Lactic acidosis was more pronounced in the group DON_LPSjugular-CONportal than in CON-fed counterparts (p < 0.05). DON-feeding aggravated the porcine acid-base balance in response to a subsequent immunostimulus dependent on its exposure site (pre- or post-hepatic).

Lactic acidosis: recognition, kinetics, and associated prognosis

Lactic acidosis is a common condition encountered by critical care providers. Elevated lactate and decreased lactate clearance are important for prognostication. Not all lactate in the intensive care unit is due to tissue hypoxia or ischemia and other sources should be evaluated. Lactate, in and of itself, is unlikely to be harmful and is a preferred fuel for many cells. Treatment of lactic acidosis continues to be aimed the underlying source.

Skeletal muscle contractile properties and proinflammatory cytokine gene expression in human endotoxaemia

Background

Muscle dysfunction associated with sepsis contributes to morbidity and mortality but the underlying mechanisms are unclear. This study examined whether muscle weakness relates to an intrinsic defect in contraction, or to central mechanisms associated with acute illness, and whether systemic endotoxaemia induces changes in gene expression for proinflammatory cytokines within human muscle in vivo.

Methods

In this experimental study, 12 healthy men received intravenous Escherichia coli lipopolysaccharide (LPS, 4 ng/kg) or saline (control). Voluntary and electrically stimulated quadriceps contraction, and tumour necrosis factor (TNF) α mRNA expression in quadriceps muscle biopsies were studied before and after the infusion.

Results

Endotoxaemia induced transient weakness of voluntary quadriceps contraction, equivalent to a 7·8 (95 per cent confidence interval 2·1 to 13·5) per cent reduction in contractile force at 180 min (P = 0·027) and a 9·0 (5·2 to 12·8) per cent reduction at 300 min (P = 0·008). Electrically stimulated contraction was unaffected. LPS administration resulted in an apparent fibre-specific induction of TNF-α mRNA.

Conclusion

Endotoxaemia results in a reduction in voluntary muscle contractile force without an apparent defect in stimulated muscle contraction. Loss of volition may be a more important factor than intrinsic dysfunction in acute sepsis-associated human muscle weakness.

Lactic acidosis: from sour milk to septic shock

Lactic acidosis is frequently encountered in the intensive care unit. It occurs when there is an imbalance between production and clearance of lactate. Although lactic acidosis is often associated with a high anion gap and is generally defined as a lactate level >5 mmol/L and a serum pH <7.35, the presence of hypoalbuminemia may mask the anion gap and concomitant alkalosis may raise the pH. The causes of lactic acidosis are traditionally divided into impaired tissue oxygenation (Type A) and disorders in which tissue oxygenation is maintained (Type B). Lactate level is often used as a prognostic indicator and may be predictive of a favorable outcome if it normalizes within 48 hours. The routine measurement of serum lactate, however, should not determine therapeutic interventions. Unfortunately, treatment options remain limited and should be aimed at discontinuation of any offending drugs, treatment of the underlying pathology, and maintenance of organ perfusion. The mainstay of therapy of lactic acidosis remains prevention.

Inflammation increases vulnerability to hypoxia in newborn piglets: effect of reoxygenation with 21% and 100% O2

OBJECTIVE

The purpose of this study was to assess whether inflammation increases vulnerability to hypoxia, and influences the effect of 100% O(2) and 21% O 2 reoxygenation on brain.

STUDY DESIGN

Newborn piglets (n = 31) were randomized to 4 interventional groups: pretreatment with saline or endotoxin. After hypoxia they were reoxygenated with 21% or 100% oxygen for 30 minutes, followed by 21% oxygen for all groups. To assess brain injury we measured extracellular brain tissue glucose, glycerol, and lactate/pyruvate by microdialysis, brain tissue oxygen tension, and laser Doppler flow.

RESULTS

Administration of endotoxin reduced the time to reach base excess (BE) -20 mmol/L by median 32 minutes compared with saline ( P < .05). We found no differences in changes in biochemical markers, brain tissue microcirculation, or oxygen tension between piglets in the 4 groups.

CONCLUSION

Endotoxin and hypoxia acted synergistically in inducing metabolic acidosis. In the presence of experimental inflammation, 21% oxygen seems as effective as 100% O(2) in reoxygenating piglets.

Subcutaneous gas tensions closely track ileal mucosal gas tensions in a model of endotoxaemia without anaerobism

OBJECTIVE

Few comparative data exist on the responses of the subcutaneous and splanchnic circulations to evolving endotoxic shock. We therefore compared continuous subcutaneous pO(2) (pO(2sc)) and pCO(2) (pCO(2sc)) with simultaneous continuous gut luminal pCO(2) (pCO(2gi)) in an animal model of endotoxaemia and examined whether changes in gas tensions track tissue energy charge (EC).

DESIGN

Prospective observational study.

SUBJECTS

Fourteen anaesthetized rats, 7 controls and 7 experimental.

INTERVENTIONS

Controls were injected with saline, the experimental group with 20 mg/kg Klebsiella endotoxin. pCO(2sc), pO(2sc), and pCO(2gi) were measured continuously. Plasma lactate concentrations were measured at defined periods during the study. After 2 h ileal segments were snap frozen and assayed for tissue EC.

MEASUREMENTS AND RESULTS

Endotoxaemia resulted in a significant decrease in mean arterial blood pressure (132+/-9 to 71+/-20 mmHg) and pO(2sc) (71+/-23 to 33+/-22 torr) and a significant increase in pCO(2gi) (58+/-10 to 90+/-20 torr) and pCO(2sc) (56+/-6 to 81+/-25 torr). During endotoxaemia pCO(2gi) was directly correlated with pCO(2sc) (R (2)=0.5) and inversely correlated with pO(2sc) (R (2)=0.63). Plasma lactate concentrations were significantly elevated from baseline in the endotoxin limb. The mean EC was not significantly different in the two groups.

CONCLUSIONS

Both subcutaneous tissue gas tensions and intestinal luminal carbon dioxide tensions are rapidly responsive during evolving hypodynamic endotoxic shock. Alterations in tissue gas tensions were not associated with dysoxia.

Septic diaphragmatic dysfunction is prevented by Mn(III)porphyrin therapy and inducible nitric oxide synthase inhibition

OBJECTIVE

Decreased diaphragmatic contractility and organ failure observed during sepsis is mediated by an overproduction of nitric oxide ((.)NO)-derived species, mitochondria being a major target of oxidative and nitrative stress. We tested the potential protective effects of (a) a novel synthetic antioxidant, the manganese(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl) porphyrin (MnTE-2-PyP(5+)) and (b) the inducible (.)NO synthase inhibitor aminoguanidine (AG) on a rat model of sepsis.

SETTING

University research laboratories.

SUBJECTS AND INTERVENTIONS

Sepsis was induced by cecal ligation and perforation in rats.

MEASUREMENTS AND RESULTS

Systemic hemodynamics, pulmonary gas exchange, in vitro diaphragmatic function and mitochondrial respiration were evaluated. Moreover, plasma and mitochondrial oxidative and nitrative stress parameters were investigated. Sepsis determined diaphragmatic dysfunction and a significant decrease in mitochondrial coupling and respiration. Oxidative stress was evidenced by decreased plasma antioxidants and increased lipid oxidation. Tyrosine nitration was increased in the plasma and mitochondria of the septic animals. These alterations were ameliorated or prevented by either MnTE-2-PyP(5+) or AG.

CONCLUSIONS

Our results demonstrate that overproduction of (.)NO and (.)NO-derived reactive species play a critical role in mitochondrial impairment and diaphragmatic function during sepsis. More importantly, AG but mainly the novel metalloporphyrin MnTE-2-PyP(5+) were able to ameliorate diaphragmatic and mitochondrial dysfunction and could contribute to preventing organ failure during severe sepsis.

Low exogenous lactate clearance as an early predictor of mortality in normolactatemic critically ill septic patients

OBJECTIVE

To evaluate the prognostic value of lactate clearance and lactate production in severely ill septic patients with normal or mildly elevated blood lactate concentration. DESIGN Prospective, observational study.

SETTING

Nineteen-bed mixed medicosurgical intensive care unit.

PATIENTS

Fifty-six patients with severe sepsis and blood lactate concentration <3 mmol/L.

MEASUREMENTS AND MAIN RESULTS

Lactate metabolism was evaluated in all patients. Lactate clearance was measured by modeling the change in arterial blood lactate over time induced by an infusion of 1 mmol/kg sodium lactate for 15 mins. Lactate production was calculated as the product of lactate clearance times the blood lactate concentration before the infusion. Outcome was taken to be mortality at 28 days after the beginning of the septic episode. A logistic regression model taking into account different risk factors was constructed. Among the 56 patients, 17 (30.3%) died before the 28th day. Basal blood lactate concentration was not different between survivors and nonsurvivors, whereas lactate clearance and production were higher in survivors (0.86 +/- 0.32 vs. 0.58 +/- 0.18 L/hr/kg, p < .005, and 1.19 +/- 0.63 vs. 0.89 +/- 0.24 mmol/hr/kg, p = .055, respectively). An increase in blood lactate 45 mins after the end of the lactate infusion (Deltalact-T60) > or = 0.6 mmol/L was predictive of 28-day mortality with 53% sensitivity and 90% specificity. Multivariate analysis showed that only three factors were independently and significantly correlated with 28-day mortality: presence of more than two organ failures (odds ratio, 27; p = .04), age >70 yrs (odds ratio, 5.7; p = .032), and Deltalact-T60 > or =0.6 mmol/L (odds ratio, 14.2; p = .042).

CONCLUSION

Low lactate clearance in severely ill septic patients with normal or mildly elevated blood lactate is predictive of poor outcome independently of other known risk factors such as age and number of organ failures.

Blood pH level modulates organ metabolism of lactate in septic shock in dogs

OBJECTIVE

Lactic acidosis is an important complication of septic shock. Alkali treatment such as sodium bicarbonate is often used to treat the low pH level that develops in sepsis. The effect of this treatment on lactate (Lac) clearance is not clear. In the present study, the objective was to examine whether blood pH level alters Lac metabolism in sepsis. Measurements were determined in a canine model of Escherichia coli sepsis after bolus infusion (5 mmol/kg) of either lactic acid (LA) or sodium lactate (NaL). In one preparation, Lac uptake by the splanchnic organs (SP), liver, lung, kidneys (Kid), and soft tissues of the lower extremity (SOL) was primarily determined, whereas in another preparation, Lac uptake by the head and neck region and lung was obtained.

METHODS

The dogs were studied while anesthetized and ventilated. After 4 hours of sepsis, either LA or NaL was given through a catheter positioned in the abdominal aorta in respective sepsis (SepLA, SepNaL) and nonsepsis groups (ConLA, ConNaL) (n approximately equal to 6 in each preparation). Catheters and flow probes were used to measure organ Lac uptake. Measurements were obtained at end infusion and at 15-minute intervals after infusion until 75 minutes after infusion.

RESULTS

Arterial clearance of Lac in the sepsis groups was slower as compared with the nonsepsis groups. In the liver, sepsis inhibited the uptake of LA as compared with the nonseptic group. In SP, both sepsis and pH affected Lac uptake in which an increase in uptake was found only after NaL infusion in the nonseptic group. In the head and neck region, Lac uptake was pH-level dependent and was found after LA infusion in the sepsis and nonsepsis groups. In the lung, Lac was produced after either LA or NaL infusion in all groups. Neither Kid nor SOL contributed to Lac uptake in any of the groups.

CONCLUSION

Lactate clearance was reduced in sepsis. Both effects of pH level and sepsis modulated the organ uptake of Lac in septic shock. Only a small amount of the total Lac infused could be accounted for by the organs measured in the present study. This suggests that additional organs may account for lactate removal in sepsis.

Impaired hepatic extraction and increased splanchnic production contribute to lactic acidosis in canine sepsis

In septic shock, the extent to which lactic acidosis (LA) is a consequence of splanchnic lactate overproduction (SLP) or impaired hepatic lactate extraction (HLE) is not clear. We examined SLP and HLE in E. coli sepsis in dogs. We further determined the effects of vasopressor treatments, which included phenylephrine, dopamine, norepinephrine, and a combination of dobutamine and norepinephrine treatment, on SLP and HLE in respective groups. The animals were studied while anesthetized and ventilated. During sepsis, SLP increased as compared with presepsis (-0.017 versus 0.07 mmol/min, p < 0.05), but this increase could not be explained by reduced splanchnic oxygen delivery (SOD). During sepsis, HLE increased as compared with baseline (0.8 versus 8%, p < 0.05), but was significantly lower than that found during lactic acid loading in nonseptic dogs. None of the vasopressor treatments had a detrimental effect on SLP. These results indicate that LA in sepsis occurs secondary to an increase in splanchnic lactate production that is not related to reduced splanchnic oxygen delivery, as well as to a decrease in hepatic lactate extraction. Effects of different vasoactive agents did not alter either splanchnic lactate production or hepatic lactate extraction in this sepsis model.

Alteration in diaphragmatic function induced by acute necrotizing pancreatitis in a rodent model

Some of the common complications of acute necrotizing pancreatitis also involve pulmonary complications. These manifestations are often associated with a cephalad diaphragmatic shift. We hypothized that diaphragmatic function might be directly compromised by the acute abdominal process. This hypothesis was tested on an acute necrotizing pancreatitis (ANP) rat model. We assessed the diaphragm and peripheral (Extensor Digitorum Longus and Soleus) muscle properties in vitro using strips in control (C) and ANP animals. Contractile parameters included single twitch and a force-frequency curve (10 to 100 Hz), and an endurance capacity index (T50%) was calculated after a repetitive stimulation (30 Hz). Breathing pattern was not different between control and ANP animals, and muscular histologic examination was normal. However, ANP was associated with a marked decrease in diaphragmatic strength for all frequencies of stimulation when compared with C. Endurance capacity was also reduced in ANP animals as assessed by a lower T50% (ANP: 31 +/- 10.5 s; C: 49 +/- 10.3 s; p < 0.05). By contrast, no significant change in peripheral muscle function was observed in both groups. We conclude that ANP causes impairment in diaphragmatic strength and endurance capacity, whereas peripheral muscles are spared. These findings suggest that alterations in the respiratory pump may be involved in the genesis of acute respiratory failure in ANP.

Relationship between blood lactate and early hepatic dysfunction in acute circulatory failure

PURPOSE

The purpose of this study was to assess the influence of early hepatic dysfunction on lactate level in patients with acute circulatory failure in a retrospective study.

MATERIALS AND METHODS

Blood lactate was compared between patients in acute circulatory failure (systolic blood pressure < or = 80 mm Hg despite fluid challenge) with or without early hepatic dysfunction (bilirubin > 60 micromol/L or SGOT > 100 IU/L during the first 48 hours). Univariate and multivariate analysis were performed to assess the effects of early hepatic dysfunction and other clinical and biological data on serum lactate levels in patients with acute circulatory failure.

RESULTS

The study included 92 patients, mean age 64+/-15 years, mean simplified acute physiology score (SAPS) 18.4+/-4.1. Early hepatic dysfunction was identified in 29 patients (32%). Mean initial blood lactate was 5.54+/-4.78 mmol/L. Overall intensive care unit mortality was 67.3%. Although patients with and without hepatic dysfunction showed no significant difference in terms of mean SAPS, mean lowest systolic blood pressure, and mortality, serum lactate was higher in the group with hepatic dysfunction than in the group without hepatic dysfunction (8.24+/-6.49 mmol/L v4.29+/-3.09 mmol/L, P < .001). Factors independently associated with serum lactate were the existence of early hepatic dysfunction (P < .01), a nondistributive type of shock (P < .05), and the mean initial amount of epinephrine (P < .05).

CONCLUSIONS

This study suggests that early hepatic dysfunction plays an important role in serum lactate elevation in acute circulatory failure.

Acidosis and tissue hypoxia in the critically ill: how to measure it and what does it mean

We routinely monitor blood gases to determine the adequacy of ventilation and the presence of acid-base abnormalities. Changes in the blood are easily assessed, but of more importance is the abnormality at tissue level. Defects in acid-base homoeostasis have major effects on protein function, thus affecting tissue and organ performance. We concentrate on the changes seen in critically ill patients with acidosis because they form a large portion of the workload of the average intensive care unit. In addition, such patients have significant morbidity and mortality. The development of acidemia in the critically ill is often attributed to reductions in oxygen utilization, which in the past has generally been regarded as dysregulation of tissue blood supply. Resulting tissue hypoperfusion leads to anaerobic metabolism and lactic acidosis. Carbon dioxide production increases as anaerobically produced hydrogen ions are buffered by extracellular bicarbonate. The effectiveness of tissue perfusion is the target of much research, and in this review we outline factors that affect tissue acid-base status, techniques to measure tissue acid-base status, and explore the relationship between tissue acidosis and hypoxia in the critically ill. However, things are not always as simple as they may first appear.

Gastric intramucosal acidosis in mechanically ventilated patients: role of mucosal blood flow

OBJECTIVE

To investigate whether gastric intramucosal acidosis is associated with a decreased gastric mucosal blood flow in mechanically ventilated patients.

DESIGN

Prospective, clinical investigation.

SETTING

University hospital intensive care unit.

PATIENTS

Seventeen mechanically ventilated patients with stable hemodynamic status.

INTERVENTIONS

Gastric tonometry and endoscopic assessment of mucosal blood flow.

MEASUREMENTS AND MAIN RESULTS

Six patients had gastric intramucosal acidosis (intramucosal pH [pHi] of 7.24 +/- 0.06), whereas the remaining 11 patients had pHi values within the normal range (7.44 +/- 0.01). No differences were found between intramucosal acidotic and nonacidotic patients with respect to their general and hemodynamic characteristics. Patients with intramucosal acidosis had a lower gastric mucosal blood flow, as assessed by laser-Doppler flowmetry, than nonacidotic patients (1.4 +/- 0.1 vs. 2.1 +/- 0.2 volts, respectively; p < .05). Reflectance spectrophotometry disclosed that patients with low gastric pHi had also a significantly (p < .05) lower hemoglobin content index (61 +/- 4 arbitrary units) than patients with normal pHi (81 +/- 3 arbitrary units), whereas oxygen saturation index was similar for both groups.

CONCLUSION

Our results support the hypothesis that gastric mucosal hypoperfusion underlies the development of intramucosal acidosis in mechanically ventilated patients.

Mild hyperlactatemia in stable septic patients is due to impaired lactate clearance rather than overproduction

A prospective study was conducted on 34 stable septic patients to determine whether mild hyperlactatemia is a marker of lactate overproduction or an indicator of lactate underutilization during sepsis. Plasma lactate clearance and lactate production were evaluated by modeling the lactate kinetic induced by an infusion of 1 mmol/kg L-lactate over 15 min. The patients were divided in two groups depending on their blood lactate: < or = 1.5 mmol/L (n = 20, lactate = 1.2+/-0.2 mmol/L) or > or = 2 mmol/L (n = 10, lactate = 2.6+/-0.6 mmol/L). The hyperlactatemic patients had a lower lactate clearance (473+/-102 ml/kg/h) than those with normal blood lactate (1,002+/-284 ml/kg/h, p < 0.001), whereas lactate production in the two groups was similar (1,194+/-230 and 1,181+/-325 micromol/kg/h, p = 0.90). A second analysis including all the patients confirmed that the blood lactate concentration was closely linked to the reciprocal of lactate clearance (r2 = 0.73, p < 0.001) but not to lactate production (r2 = 0.03, p = 0.29). We conclude that a mild hyperlactatemia occurring in a stable septic patient is mainly due to a defect in lactate utilization.

Ischemia/reperfusion injury to the ileum does not account for the ileal VO2-DO2 alterations induced by endotoxin

PURPOSE

Endotoxin (lipopolysaccharide [LPS])-induced systemic organ injury leads to disruption of normal systemic organ metabolic processes, which are manifest clinically by signs of accelerated anaerobic metabolism (e.g., tissue acidosis and hyperlactatemia) and altered VO2-DO2 relationships. The association of increased anaerobic metabolism with VO2-DO2 alterations has led to the notion that ischemia/ reperfusion (I/R) injury may be a prerequisite for the development of VO2-DO2 alterations during endotoxemia. However, in contrast to sepsis, in which oxygen consumption is often increased, oxygen consumption is severely decreased after I/R injury. Based on these observations, we hypothesized that I/R injury would result in systemic organ VO2-DO2 alterations, which are distinct from those that occur in sepsis.

MATERIALS AND METHODS

We used the in situ autoperfused feline ileal preparation to simultaneously examine microvascular permeability, reflected as the ileal lymph to plasma protein concentration ratio (CL/CP), and ileal VO2-DO2 relationships after either intravenous LPS (2.0 mg/kg; n = 5) or I/R injury (n = 5), and in matching controls (n = 5).

RESULTS

As expected, all LPS-treated and I/R-injured animals were found to have extensive ileal histological damage and marked increases in the CL/CP compared with controls (0.315 +/- 0.009 and 0.329 +/- 0.034, respectively, v 0.097 +/- 0.009; P < .001, both comparisons). In addition, the critical DO2 (DO2c) was elevated, and the critical oxygen extraction was decreased in both the I/R and LPS groups relative to controls. However, as initially hypothesized, the VO2 at the critical DO2 was markedly decreased in the I/R group compared with that of the LPS group.

CONCLUSIONS

These data indicate that I/R injury is insufficient to account for the systemic organ VO2-DO2 alterations that occur with LPS injury.

Measurements of oxygenation and perfusion in skeletal muscle using multiple microelectrodes

This paper describes an apparatus to measure tissue oxygenation and perfusion (as measured by the wash-in rate of gaseous hydrogen) simultaneously at multiple points in muscle using needle microelectrodes. The development of suitable electrodes and apparatus is described, as well as the development of the method and its validation. In particular, the potential for tissue damage secondary to electrode insertion, the need for in vivo voltammetric determination of the operating potential and the extent of any electrode-tissue and of electrode-electrode interactions are explored, and are shown to be insufficient in magnitude to affect the technique. Its subsequent use to characterise oxygenation and perfusion in rabbit skeletal muscle at rest is also described. In resting tibialis anterior muscle of the rabbit the mean pO2 was 18 +/- 13.3 mm Hg and the mean perfusion was 4.4 +/- 1.3 ml s-1 100 g-1. There was a heterogeneity in simultaneously-measured values of pO2 and perfusion at different points within muscle, and also a temporal variation at the same site. The spans between the highest and lowest simultaneously-measured values of pO2 in muscle ranged from 14 to 80 mm Hg, and for perfusion, from 1 to 12 mls-1 100 g-1. No significant correlation was evident from histological examination between either pO2 or perfusion and surrounding fibre type or capillary density.

Cytopathic hypoxia in sepsis

Diminished availability of oxygen at the cellular level might account for organ dysfunction in sepsis. Although the classical forms of tissue hypoxia due to hypoxemia, anemia, or inadequate perfusion all might be important under some conditions, it seems increasingly likely that a fourth mechanism, namely cytopathic hypoxia, might play a role as well. The term cytopathic hypoxia is used to denote diminished production of adenosine triphosphate (ATP) despite normal (or even supranormal) PO2 values in the vicinity of mitochondria within cells. At least in theory, cytopathic hypoxia could be a consequence of several different (but mutually compatible) pathogenic mechanisms, including diminished delivery of a key substrate (e.g., pyruvate) into the mitochondrial tricarboxylic acid (TCA) cycle, inhibition of key mitochondrial enzymes involved in either the TCA cycle or the electron transport chain, activation of the enzyme, poly-(ADP)-ribosylpolymerase (PARP), or collapse of the protonic gradient across the inner mitochondrial membrane leading to uncoupling of oxidation (of NADH and FADH) from phosphorylation of ADP to form ATP. Tantalizing, but limited, data support the view that cytopathic hypoxia occurs in both animals and patients with sepsis or endotoxemia.

Reversal of muscle fatigue in intact rabbits by intravenous potassium chloride

PURPOSE

Skeletal muscle fatigue has been associated with potassium efflux from the myocytes, resulting in endogenous increases in blood potassium concentration ([K+]). Conversely, exogenous increases in extracellular [K+] potentiates contraction in isolated muscle preparations. The mechanisms responsible for these contradictory effects of [K+] on skeletal muscle function are unknown. Moreover, little is known about the effect of exogenous increases in [K+] on force generation by intact animals, given potassium's deleterious effect on cardiac function.

METHODS

We compared the response to exogenous increases in blood [K+] in rabbits given an infusion of potassium chloride (KCl) intravenously (IV) (0.2 mol/L; KCl group; n = 7) to a group given 0.9% sodium chloride (NaCl) (control; n = 7). The rabbits underwent low-frequency, isometric twitch stimulation of the left hindlimb (square wave pulses 100 microseconds, 40V, 0.25 Hz) throughout the experiment. Both groups received 0.9% NaCl (25 mL/h) during the first hour of twitch stimulation and experienced similar decreases in hindlimb forces to 70% of initial force. A continuous infusion of KCl or of saline (60 mL/h) was started, and hindlimb stimulation continued for 2 hours.

RESULTS

There were no changes in [K+] in the control group, and twitch forces progressively declined during the next 2 hours (369 +/- 47 g to 279 +/- 34 g, P < .01). Arterial [K+] increased in the KCl group from 2.6 +/- 0.1 to 10.1 +/- 0.5 mmol/L (P < .01), and hindlimb twitch forces almost doubled (418 +/- 49 g to 756 +/- 55 g, P < .01). Force frequency curves showed improved contractility in the KCl group at stimulation frequencies below 30 Hz.

CONCLUSIONS

Exogenous increases in blood [K+] potentiate skeletal muscle contraction in intact animals and reverse low-frequency twitch fatigue. A possible mechanism may be the maintenance of intracellular [K+] by hindering K+ efflux from skeletal muscle cells.

Endotoxemia causes ileal mucosal acidosis in the absence of mucosal hypoxia in a normodynamic porcine model of septic shock

OBJECTIVE

To evaluate the hypothesis that splanchnic ischemia and mucosal hypoxia are responsible for lipopolysaccharide-induced intramucosal acidosis in pigs.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Surgical research laboratory at a large, university-affiliated medical center.

SUBJECTS

Anesthetized, mechanically ventilated swine.

INTERVENTIONS

Pigs were infused with lactated Ringer's solution (12 mL/kg/hr) and, starting at 30 mins, 25-mL boluses of dextran-70 (maximum 15 mL/kg/hr) to maintain cardiac output at 90% to 110% of the baseline value for each pig. Ileal mucosal hydrogen ion concentration was measured tonometrically. A segment of distal ileum was exteriorized, opened, and placed on a platform to permit measurement of mucosal PO2, using an array of Clark-type microelectrodes and a computerized data acquisition and analysis system. Mucosal perfusion was measured using laser-Doppler flowmetry. The control group (n = 4) received no further interventions. Pigs in the lipopolysaccharide group (n = 6) were infused with 150 micrograms/kg of Escherichia coli lipopolysaccharide over 60 mins. To assess the effect of mucosal acidosis on mucosal PO2 in nonendotoxemic animals, intramucosal hydrogen ion concentration, mucosal PO2, and mucosal perfusion were measured in pigs rendered hypercarbic through deliberate hypoventilation (hypercarbia group; n = 4).

MEASUREMENTS AND MAIN RESULTS

Infusion of lipopolysaccharide resulted in a significant increase in intramucosal hydrogen ion concentration. However, in the lipopolysaccharide group, mucosal perfusion did not change significantly and mucosal PO2 increased significantly. In the hypercarbia group, hypercarbia was associated with significant increases in both intramucosal hydrogen ion concentration and mucosal PO2.

CONCLUSIONS

Mucosal hypoxia is not responsible for lipopolysaccharide-induced mucosal acidosis in this normodynamic pig model of septic shock. A rightward shift of the oxyhemoglobin dissociation curve (the Bohr effect) can explain the increase in mucosal oxygenation observed in endotoxemic pigs.

Inhibitory effect of Escherichia coli endotoxin on skeletal muscle contractility

OBJECTIVE

Endotoxemia in rabbits is associated with decreases in oxygen transport, tissue hypoxia, metabolic acidosis, and impaired oxygen extraction. In this study, we tested the hypothesis that endotoxin also inhibits skeletal muscle contractility directly.

DESIGN

Randomized animal study.

SETTING

Accredited animal research facility.

SUBJECTS

New Zealand white rabbits of either sex, weighing 2.55 +/- 0.20 kg.

INTERVENTIONS

We compared two groups of rabbits (n = 10 each) undergoing continuous electrical stimulation of the left hindlimb (maximal isometric twitch contraction at 0.25 Hz). One group (septic) was given an intravenous infusion of Escherichia coli endotoxin. The control group was subjected to decreases in cardiac output by inflating a balloon placed in the right ventricle.

MEASUREMENTS AND MAIN RESULTS

Endotoxin or balloon inflation resulted in comparable decreases in cardiac output (49% and 53%, respectively). Hindlimb oxygen transport decreased to similar values for both groups (4.9 +/- 0.3 and 4.2 +/- 0.5 mL/min/kg, respectively). Systemic oxygen extraction ratio was greater in the control group (0.72 +/- 0.03) than in the septic group (0.55 +/- 0.04; p < .05). There were no differences in hindlimb oxygen extraction ratio. Decreases in hindlimb forces were greater in the septic group (42 +/- 4%) than in the control group (18 +/- 3%, p < .01). Force frequency curves obtained at the beginning and the end of the experiment showed greater fatigue in the septic group.

CONCLUSIONS

The intravenous infusion of Escherichia coli endotoxin produces a direct inhibitory effect on skeletal muscle contractility in rabbits. This phenomenon is independent of decreases in oxygen transport and blood pH. Our data support the notion of a direct cellular effect of endotoxin, or of an associated cytokine, on skeletal muscle contractility. The mechanism responsible for this phenomenon is unknown.

Detection of tissue hypoxia by arteriovenous gradient for PCO2 and pH in anesthetized dogs during progressive hemorrhage

The present study tested the hypothesis that, during acute bleeding, the development of tissue hypoxia might be reflected by an abrupt widening in arteriovenous gradient for PCO2 (AV PCO2) and for pH (AV pH) as accurately as by an increase in blood lactate levels. Twenty-four anesthetized (isoflurane 1.4% end-tidal), paralyzed, and mechanically ventilated dogs submitted to progressive hemorrhage were studied. Oxygen uptake (VO2) was derived from expired gas analysis and oxygen delivery (DO2) was calculated by the product of the thermodilution cardiac index and the arterial O2 content. During the first part of the protocol, VO2 remained stable as the progressive reduction in DO2 was associated with a corresponding increase in O2 extraction (O2ER). Blood lactate increased slightly but not significantly. AV PCO2 and AV pH increased significantly, essentially related to venous respiratory acidosis. The critical value of DO2 below which VO2 decreased was 8.95 +/- 1.60 mL.min-1.kg-1. Below this value, there was a marked increase in blood lactate and an abrupt widening in AV PCO2 and AV pH gradients. The critical value of DO2 obtained from blood lactate, AV PCO2 and AV pH were similar to those obtained from VO2 (8.60 +/- 1.12; 8.73 +/- 1.40; 8.78 +/- 1.37, respectively; P = not significant). A significant correlation was found, during the hemorrhage protocol, between blood lactate and AV PCO2 (r = 0.84; P < 0.001) or AV pH (r = 0.78; P < 0.001). Therefore, AV PCO2 and AV pH represent simple but reliable indicators of tissue hypoxia during hemorrhagic shock.