Effects of nitric oxide on blood flow distribution and O2 extraction capabilities during endotoxic shock

Microcirculatory alterations in shock states

Functional components of the microcirculation provide oxygen and nutrients and remove waste products from the tissue beds of the body's organs. Shock states overwhelmingly stress functional capacity of the microcirculation, resulting in microcirculatory failure. In septic shock, inflammatory mediators contribute to hemodynamic instability. In nonseptic shock states, the microcirculation is better able to compensate for alterations in vascular resistance, cardiac output, and blood pressure. Therefore, global hemodynamic and oxygen delivery parameters are appropriate for assessing, monitoring, and guiding therapy in hypovolemic and cardiogenic shock but, alone, are inadequate for septic shock.

Arginine and citrulline and the immune response in sepsis

Arginine, a semi-essential amino acid is an important initiator of the immune response. Arginine serves as a precursor in several metabolic pathways in different organs. In the immune response, arginine metabolism and availability is determined by the nitric oxide synthases and the arginase enzymes, which convert arginine into nitric oxide (NO) and ornithine, respectively. Limitations in arginine availability during inflammatory conditions regulate macrophages and T-lymfocyte activation. Furthermore, over the past years more evidence has been gathered which showed that arginine and citrulline deficiencies may underlie the detrimental outcome of inflammatory conditions, such as sepsis and endotoxemia. Not only does the immune response contribute to the arginine deficiency, also the impaired arginine de novo synthesis in the kidney has a key role in the eventual observed arginine deficiency. The complex interplay between the immune response and the arginine-NO metabolism is further underscored by recent data of our group. In this review we give an overview of physiological arginine and citrulline metabolism and we address the experimental and clinical studies in which the arginine-citrulline NO pathway plays an essential role in the immune response, as initiator and therapeutic target.

Nitroglycerin reverts clinical manifestations of poor peripheral perfusion in patients with circulatory shock

INTRODUCTION

Recent clinical studies have shown a relationship between abnormalities in peripheral perfusion and unfavorable outcome in patients with circulatory shock. Nitroglycerin is effective in restoring alterations in microcirculatory blood flow. The aim of this study was to investigate whether nitroglycerin could correct the parameters of abnormal peripheral circulation in resuscitated circulatory shock patients.

METHODS

This interventional study recruited patients who had circulatory shock and who persisted with abnormal peripheral perfusion despite normalization of global hemodynamic parameters. Nitroglycerin started at 2 mg/hour and doubled stepwise (4, 8, and 16 mg/hour) each 15 minutes until an improvement in peripheral perfusion was observed. Peripheral circulation parameters included capillary refill time (CRT), skin-temperature gradient (Tskin-diff), perfusion index (PI), and tissue oxygen saturation (StO2) during a reactive hyperemia test (RincStO2). Measurements were performed before, at the maximum dose, and after cessation of nitroglycerin infusion. Data were analyzed by using linear model for repeated measurements and are presented as mean (standard error).

RESULTS

Of the 15 patients included, four patients (27%) responded with an initial nitroglycerin dose of 2 mg/hour. In all patients, nitroglycerin infusion resulted in significant changes in CRT, Tskin-diff, and PI toward normal at the maximum dose of nitroglycerin: from 9.4 (0.6) seconds to 4.8 (0.3) seconds (P < 0.05), from 3.3 °C (0.7 °C) to 0.7 °C (0.6 °C) (P < 0.05), and from [log] -0.5% (0.2%) to 0.7% (0.1%) (P < 0.05), respectively. Similar changes in StO2 and RincStO2 were observed: from 75% (3.4%) to 84% (2.7%) (P < 0.05) and 1.9%/second (0.08%/second) to 2.8%/second (0.05%/second) (P < 0.05), respectively. The magnitude of changes in StO2 was more pronounced for StO2 of less than 75%: 11% versus 4%, respectively (P < 0.05).

CONCLUSIONS

Dose-dependent infusion of nitroglycerin reverted abnormal peripheral perfusion and poor tissue oxygenation in patients following circulatory shock resuscitation. Individual requirements of nitroglycerin dose to improve peripheral circulation vary between patients. A simple and fast physical examination of peripheral circulation at the bedside can be used to titrate nitroglycerin infusion.

Plasma nitrate/nitrite concentrations in dogs with naturally developing sepsis and non-infectious forms of the systemic inflammatory response syndrome

The aim of this prospective observational study was to evaluate the differences in plasma nitrate/nitrite concentrations between dogs with sepsis and those with non-infectious forms of the systemic inflammatory response syndrome (SIRS). Eighteen dogs with sepsis, 20 dogs with SIRS and 29 healthy control dogs were enrolled. Blood samples were obtained from the dogs within 12 hours of admission to the University of Missouri Veterinary Medical Teaching Hospital (MU VMTH) Intensive Care Unit (ICU) in lithium heparin blood tubes. Plasma nitrate/nitrite concentrations were measured using the Greiss reaction. Plasma nitrate/nitrite concentrations at presentation, clinical parameters, organ dysfunction and in-hospital mortality were compared between groups. Plasma total nitrate/nitrite was significantly greater in the sepsis group compared with the control group (P=0.005) and SIRS group (P=0.037). There was no statistical difference in plasma nitrate/nitrite concentration between the SIRS and control groups (P=0.489). The sensitivity was 66.7 per cent (95 per cent CI, 41 to 87 per cent) and the specificity was 75.5 per cent (95 per cent CI, 61 to 87 per cent) for differentiating dogs with sepsis from dogs without sepsis.

Prevention of acute kidney injury and protection of renal function in the intensive care unit. Expert opinion of the Working Group for Nephrology, ESICM

BACKGROUND

Acute renal failure on the intensive care unit is associated with significant mortality and morbidity.

OBJECTIVES

To determine recommendations for the prevention of acute kidney injury (AKI), focusing on the role of potential preventative maneuvers including volume expansion, diuretics, use of inotropes, vasopressors/vasodilators, hormonal interventions, nutrition, and extracorporeal techniques.

METHOD

A systematic search of the literature was performed for studies using these potential protective agents in adult patients at risk for acute renal failure/kidney injury between 1966 and 2009. The following clinical conditions were considered: major surgery, critical illness, sepsis, shock, and use of potentially nephrotoxic drugs and radiocontrast media. Where possible the following endpoints were extracted: creatinine clearance, glomerular filtration rate, increase in serum creatinine, urine output, and markers of tubular injury. Clinical endpoints included the need for renal replacement therapy, length of stay, and mortality. Studies are graded according to the international Grades of Recommendation, Assessment, Development, and Evaluation (GRADE) group system.

CONCLUSIONS AND RECOMMENDATIONS

Several measures are recommended, though none carries grade 1A. We recommend prompt resuscitation of the circulation with special attention to providing adequate hydration whilst avoiding high-molecular-weight hydroxy-ethyl starch (HES) preparations, maintaining adequate blood pressure using vasopressors in vasodilatory shock. We suggest specific vasodilators [corrected] under strict hemodynamic control, sodium bicarbonate for emergency procedures administering contrast media, and periprocedural hemofiltration in severe chronic renal insufficiency undergoing coronary intervention.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s00134-009-1678-y) contains supplementary material, which is available to authorized users.

Invasive amebiasis: a microcirculatory disorder?

The two current models of invasive amebiasis both hold that direct contact of toxic molecules and amebas with tissue produces the necrotic areas characteristic of this disorder. Whereas one model characterizes these toxic molecules as amebic products (e.g., lectins, amebapores, cysteine proteinases and other proteolytic enzymes), the other describes them as products of the inflammatory response (e.g., cytokines, nitric oxide, reactive oxygen intermediates and cytotoxic granules). Both these models can account for necrotic areas with many amebas present and with acute inflammation, but not those with few or no amebas present or with scarce inflammation. A new model poses that an inadequate immune response leads to a continuous and prolonged activation of endothelial cells (ECs) by amebas, amebic molecules and cytokines, which triggers the mechanisms leading to necrosis. Other toxic molecules later contribute to EC activation: nitric oxide, reactive oxygen intermediates, the activated complement and proteases. Hyperactivated endothelial cells continuously express adhesion molecules (e.g., ICAM-1 and E-selectin), pro-coagulant molecules (e.g., tissue factor, von Willebrand factor, and the plasminogen activator inhibitor), resulting in ever greater inflammation and thrombosis, which eventually reduces or blocks blood flow in some vessels and starves certain tissue areas of an adequate oxygen and nutrient supply. When necrotic areas first develop, they are surrounded by inflammatory cells due to the acute inflammation at this stage. However, these cells are starved of oxygen and essential nutrients by the same microcirculatory dysfunction. The increasing concentration of nitric oxide during amebiasis eventually has an anti-inflammatory and vasodilating effect, creating a new mechanism for the microcirculatory dysfunction. This local microcirculatory dysfunction can explain necrotic areas in the presence of many, few, or no amebas, with abundant or scarce inflammation.

Nitric oxide donors in sepsis: a systematic review of clinical and in vivo preclinical data

An abundant literature in the field of sepsis focuses on the role of NO. Inhibiting NO synthesis corrects certain hemodynamic parameters of septic shock but failed to improve outcome in patients. Conversely, administration of NO donors lowers blood pressure but restores microcirculatory flow in patients with sepsis. We undertook a systematic review of the literature to comprehensively summarize the results of studies exploring the effects of systemic NO donors in sepsis. We included both clinical and preclinical data. We described the details surrounding NO donor administration, and the results obtained in each study were regrouped in broad categories. In the case of animal research, we limited our data collection to in vivo protocols and described the sepsis model. Finally, we critically appraised all the studies included in the review. Overall, the reviewed publications aimed for physiopathological description rather than clinical relevance and did not meet the required criteria for extrapolation to clinical practice. With this reserve, NO donors usually improved the outcomes measured (e.g., mortality, pulmonary hypertension, tissue/organ perfusion, etc.) but also lowered blood pressure. We conclude that our findings warrant further animal experimentation designed to maximize clinical relevance.

Resuscitating the microcirculation in sepsis: the central role of nitric oxide, emerging concepts for novel therapies, and challenges for clinical trials

Microcirculatory dysfunction is a critical element of the pathogenesis of severe sepsis and septic shock. In this Bench-to-Bedside review, we present: 1) the central role of the microcirculation in the pathophysiology of sepsis; 2) new translational research techniques of in vivo video microscopy for assessment of microcirculatory flow in human subjects; 3) clinical investigations that reported associations between microcirculatory dysfunction and outcome in septic patients; 4) the potential role of novel agents to "rescue" the microcirculation in sepsis; 5) current challenges facing this emerging field of clinical investigation; and 6) a framework for the design of future clinical trials aimed to determine the impact of novel agents on microcirculatory flow and organ failure in patients with sepsis. We specifically focus this review on the central role and vital importance of the nitric oxide (NO) molecule in maintaining microcirculatory homeostasis and patency, especially when the microcirculation sustains an insult (as with sepsis). We also present the scientific rationale for clinical trials of exogenous NO administration to treat microcirculatory dysfunction and augment microcirculatory blood flow in early sepsis therapy.

The NO donor SIN-1 improves intestinal-arterial P(CO(2)) gap in experimental endotoxemia: an animal study

BACKGROUND

Dysfunction of the microcirculation is a prominent feature of sepsis and endotoxemia. Recently, it has been shown that microcirculatory alterations are completely reversed by local or systemic application of vasodilators in severely septic patients. Therefore, we investigated the influence of vasodilator therapy on microcirculatory dysfunction of the ileum during endotoxic shock in a prospective, controlled animal study.

METHODS

After baseline measurements, shock was induced in 12 domestic pigs by lipopolysaccharide via the mesenteric vein until the mean arterial pressure fell below 60 mmHg. After 30 min in shock, six animals were resuscitated with either fluid alone (control) or fluid and 2 microg/kg/min of the vasodilator 3-morpholino-sydnonimine (SIN-1). The systemic and regional hemodynamics and oxygenation parameters, tonometric ileal P(CO(2)) and microvascular oxygen pressures (muP(O(2))) (by oxygen-dependent Pd-porphyrin phosphorescence) were measured simultaneously.

RESULTS

The ileal-arterial P(CO(2)) gap increased during shock and the ileal mucosal and serosal muP(O(2)) decreased concurrently. SIN-1 in addition to fluid resuscitation significantly improved the ileal-arterial P(CO(2)), whereas fluid alone failed to decrease the P(CO(2)) gap. The SIN-1-induced improvement in the P(CO(2)) gap was accompanied by an increase in serosal muP(O(2)) above shock levels. Mucosal muP(O(2)) was resuscitated to baseline levels in both groups.

CONCLUSION

The application of the vasodilator SIN-1 in addition to fluid resuscitation improves the ileal-arterial P(CO(2)) gap and mucosal muP(O(2)), together with a moderate increase in serosal muP(O(2)), after endotoxic shock. This finding is consistent with the concept that vasodilators may correct pathologic flow distribution within the intestinal wall.

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.

In Vivo Characterization of the Novel Imidazopyridine BYK191023 [2-[2-(4-Methoxy-pyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine], a Potent and Highly Selective Inhibitor of Inducible Nitric-Oxide Synthase

Excessive release of nitric oxide from inducible nitric-oxide synthase (iNOS) has been postulated to contribute to pathology in a number of inflammatory diseases. We recently identified imidazopyridine derivatives as a novel class of potent nitricoxide synthase inhibitors with high selectivity for the inducible isoform. In the present study, we tested the in vivo potency of BYK191023 [2-[2-(4-methoxy-pyridin-2-yl)-ethyl]-3H-imidazo-[4,5-b]pyridine], a selected member of this inhibitor class, in three different rat models of lipopolysaccharide-induced systemic inflammation. Delayed administration of BYK191023 dose-dependently suppressed the lipopolysaccharide-induced increase in plasma nitrate/nitrite (NO(x)) levels with an ED(50) of 14.9 micromol/kg/h. In a model of systemic hypotension following high-dose lipopolysaccharide challenge, curative administration of BYK191023 at a dose that inhibited 83% of the NO(x) increase completely prevented the gradual decrease in mean arterial blood pressure observed in vehicle-treated control animals. The vasopressor effect was specific for endotoxemic animals since BYK191023 did not affect blood pressure in saline-challenged controls. In addition, in a model of lipopolysaccharide-induced vascular hyporesponsiveness, BYK191023 infusion partially restored normal blood pressure responses to norepinephrine and sodium nitroprusside via an l-arginine competitive mechanism. Taken together, BYK191023 is a member of a novel class of highly isoform-selective iNOS inhibitors with promising in vivo activity suitable for mechanistic studies on the role of selective iNOS inhibition as well as clinical development.

Release and hemodynamic influence of nitro-glycerine-derived nitric oxide in endotoxemic rats

BACKGROUND AND AIM

Nitric oxide released from nitro-glycerine (NG) has been considered to improve the microcirculation. Septic conditions are, however, associated with excessive formation of nitric oxide (NO), which is formed from l-arginine by the inducible NO synthase (iNOS) activity. Since the characteristics and influence of NG-derived NO in sepsis remains unclear, the major aims of the present study were to quantify the release and to determine the effects of NO formed from NG on systemic blood pressure under endotoxemic conditions.

MATERIAL AND METHODS

Four hours following endotoxin challenge (10 mg/kg intraperitoneally), rats received an infusion of NG (0.5 or 5.0 micromol/kg/h) over 45 min. We determined the changes in blood pressure and the NO concentrations generated in brain, heart, intestine, kidney, liver, and lung by means of NO trapping and EPR technique.

RESULTS

NG infusion in control rats and endotoxin challenge decreased systemic blood pressure to the same extent. However, in rats subjected to endotoxin challenge NG infusion did not affect the blood pressure. The endotoxin-induced increase in tissue NO concentrations were found to be 15-folds higher than tissue levels of NO following NG infusion.

CONCLUSION

Our results suggest that under endotoxic shock conditions in rats NG may not additionally affect the systemic blood pressure. This may relate to the excessive tissue NO levels induced by endotoxin that are not further increased by NG.

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.

Sepsis: an arginine deficiency state?

OBJECTIVE

Sepsis is a major health problem considering its significant morbidity and mortality rate. The amino acid L-arginine has recently received substantial attention in relation to human sepsis. However, knowledge of arginine metabolism during sepsis is limited. Therefore, we reviewed the current knowledge about arginine metabolism in sepsis.

DATA SOURCE

This review summarizes the literature on arginine metabolism both in general and in relation to sepsis. Moreover, arginine-related therapies are reviewed and discussed, which includes therapies of both nitric oxide (NO) and arginine administration and therapies directed toward inhibition of NO.

DATA

In sepsis, protein breakdown is increased, which is a key process to maintain arginine delivery, because both endogenous de novo production from citrulline and food intake are reduced. Arginine catabolism, on the other hand, is markedly increased by enhanced use of arginine in the arginase and NO pathways. As a result, lowered plasma arginine levels are usually found. Clinical symptoms of sepsis that are related to changes in arginine metabolism are mainly related to hemodynamic alterations and diminished microcirculation. NO administration and arginine supplementation as a monotherapy demonstrated beneficial effects, whereas nonselective NO synthase inhibition seemed not to be beneficial, and selective NO synthase 2 inhibition was not beneficial overall.

CONCLUSIONS

Because sepsis has all the characteristics of an arginine-deficiency state, we hypothesise that arginine supplementation is a logical option in the treatment of sepsis. This is supported by substantial experimental and clinical data on NO donors and NO inhibitors. However, further evidence is required to prove our hypothesis.

Differential effects of a selective inhibitor of soluble guanylyl cyclase on global and regional hemodynamics during canine endotoxic shock

Activation of soluble guanylyl cyclase (sGC) might occur early during septic shock and play a role in the regulation of vascular tone and the redistribution of blood flow. The aim of this study was to assess the effects of sGC inhibition with oxadiazoloquinoxalinone (ODQ) on global and regional hemodynamic parameters in a clinically relevant model of septic shock. Fifteen anesthetized adult mongrel dogs were equipped with femoral and pulmonary artery catheters and ultrasonic flow probes around the mesenteric, femoral and renal arteries. The animals were randomized to receive Escherichia coli endotoxin (2 mg/kg, i.v.) alone, endotoxin followed by ODQ (1 mg/kg i.v.), or ODQ alone. Endotoxin administration was followed by decreases in mean arterial pressure, cardiac index, mesenteric, renal and femoral blood flows (MBF, RBF and FBF), and increases in systemic and pulmonary vascular resistances. Fluid resuscitation restored cardiac index, systemic vascular resistance, pulmonary vascular resistance, MBF, RBF and FBF to pre-endotoxin levels. In the presence of endotoxin, ODQ administration increased MBF and prevented the restoration of FBF. Hence, selective inhibition of sGC may increase splanchnic blood flow in septic shock.

Bench-to-bedside review: sepsis is a disease of the microcirculation

Microcirculatory perfusion is disturbed in sepsis. Recent research has shown that maintaining systemic blood pressure is associated with inadequate perfusion of the microcirculation in sepsis. Microcirculatory perfusion is regulated by an intricate interplay of many neuroendocrine and paracrine pathways, which makes blood flow though this microvascular network a heterogeneous process. Owing to an increased microcirculatory resistance, a maldistribution of blood flow occurs with a decreased systemic vascular resistance due to shunting phenomena. Therapy in shock is aimed at the optimization of cardiac function, arterial hemoglobin saturation and tissue perfusion. This will mean the correction of hypovolemia and the restoration of an evenly distributed microcirculatory flow and adequate oxygen transport. A practical clinical score for the definition of shock is proposed and a novel technique for bedside visualization of the capillary network is discussed, including its possible implications for the treatment of septic shock patients with vasodilators to open the microcirculation.

Hemodynamic effects of glibenclamide during endotoxemia: contrasting findings in vitro versus in vivo

The final common pathway involved in the cardiovascular alterations of septic shock is incompletely defined. The opening of KATP channels is associated with vasorelaxation and alterations in cardiac contractility. This event may be triggered during septic shock by increased nitric oxide (NO) production, by a decreased intracellular content of ATP, or by a change in the transmembrane electrical potential. In the present study, we assessed the effects of glibenclamide, an agent that blocks the opening of KATP channels in vitro, on the contractile response of rat aortic rings to norepinephrine, and in vivo in anesthetized dogs, with or without exposure to Escherichia coli endotoxin. In vitro, glibenclamide decreased the contractile response to norepinephrine in the presence of endotoxin, provided that the endothelium was intact. In vivo, administration of 0.15 mg/kg increased systemic vascular resistance (SVR) in the absence of endotoxin only, and increased myocardial performance. A higher dose of 1 mg/kg increased SVR and decreased myocardial performance, both during endotoxic shock and in control conditions. Renal and mesenteric blood flows decreased, but the respective fractional flows were unchanged. Oxygen delivery decreased in both experimental conditions, but oxygen consumption decreased only in control conditions. The in vitro observations suggest that the opening of KATP channels is involved in the regulation of vascular tone during endotoxemia, via an endothelium-dependent mechanism. As different effects of glibenclamide were observed in vivo, the importance of the opening of KATP channels in endotoxic shock may be limited.

Bench-to-bedside review: microvascular dysfunction in sepsis--hemodynamics, oxygen transport, and nitric oxide

The microcirculation is a complex and integrated system that supplies and distributes oxygen throughout the tissues. The red blood cell (RBC) facilitates convective oxygen transport via co-operative binding with hemoglobin. In the microcirculation oxygen diffuses from the RBC into neighboring tissues, where it is consumed by mitochondria. Evidence suggests that the RBC acts as deliverer of oxygen and 'sensor' of local oxygen gradients. Within vascular beds RBCs are distributed actively by arteriolar tone and passively by rheologic factors, including vessel geometry and RBC deformability. Microvascular oxygen transport is determined by microvascular geometry, hemodynamics, and RBC hemoglobin oxygen saturation. Sepsis causes abnormal microvascular oxygen transport as significant numbers of capillaries stop flowing and the microcirculation fails to compensate for decreased functional capillary density. The resulting maldistribution of RBC flow results in a mismatch of oxygen delivery with oxygen demand that affects both critical oxygen delivery and oxygen extraction ratio. Nitric oxide (NO) maintains microvascular homeostasis by regulating arteriolar tone, RBC deformability, leukocyte and platelet adhesion to endothelial cells, and blood volume. NO also regulates mitochondrial respiration. During sepsis, NO over-production mediates systemic hypotension and microvascular reactivity, and is seemingly protective of microvascular blood flow.

Cardiac response to nitric oxide synthase inhibition using aminoguanidine in a rat model of endotoxemia

This study evaluates the effect of aminoguanidine, a preferential inhibitor of inducible nitric oxide synthase (iNOS), on the prevention of cardiac depression in acute endotoxemia. Cardiac performance was evaluated after 4 h of exposure to endotoxin. Rats (n = 5) were selected randomly to receive, by intraperitoneal injection, one of four treatments: saline, LPS (lipopolysaccharide, E. coli, 4 mg/kg, AG (aminoguanidine 100 mg/kg), and LPS + AG at various times. AG and saline treatments were administered 30 min before LPS and at 1 and 3 h after LPS injection. Hearts were perfused using the Langendorff isolated perfusion system and a balloon-tipped catheter was placed into the left ventricle to measure left ventricular developed pressure (LVDP). Myocyte contractile function was assessed with electrical field stimulation and video microscopy. Tissue was immunostained for the expression of iNOS and for nitrotyrosine, a byproduct of protein nitration by peroxynitrite. Perfused hearts from LPS-treated rats exhibited a 57% decrease (P < 0.05) in LVDP compared to saline-treated animals. No improvement in ventricular function was observed with the administration of AG. Similarly, cardiac myocytes prepared from LPS-treated animals demonstrated a significant (P < 0.05) reduction in percent and velocity of shortening and this effect was unaltered with the same dose of AG. AG administration significantly reduced serum nitrite/nitrate levels (P < 0.05) in endotoxemic rats to control levels. Localized expression of iNOS in the myocardium was lessened with AG treatment and was not associated with peroxynitrite formation in this model of endotoxemia. The results indicate that AG given in vivo before and after endotoxin (at a concentration sufficient to decrease NO production) did not reduce cardiac depression. We conclude that selective inhibition of iNOS and the reduction of NO production do not prevent cardiac dysfunction at an early stage in an acute model of endotoxemia.

Is nitric oxide overproduction the target of choice for the management of septic shock?

Sepsis is a heterogeneous class of syndromes caused by a systemic inflammatory response to infection. Septic shock, a severe form of sepsis, is associated with the development of progressive damage in multiple organs, and is a leading cause of patient mortality in intensive care units. Despite important advances in understanding its pathophysiology, therapy remains largely symptomatic and supportive. A decade ago, the overproduction of nitric oxide (NO) had been discovered as a potentially important event in this condition. As a result, great hopes arose that the pharmacological inhibition of NO synthesis could be developed into an efficient, mechanism-based therapeutic approach. Since then, an extraordinary effort by the scientific community has brought a deeper insight regarding the feasibility of this goal. Here we present in summary form the present state of knowledge of the biological chemistry and physiology of NO. We then proceed to a systematic review of experimental and clinical data, indicating an up-regulation of NO production in septic shock; information on the role of NO in septic shock, as provided by experiments in transgenic mice that lack the ability to up-regulate NO production; effects of pharmacological inhibitors of NO production in various experimental models of septic shock; and relevant clinical experience. The accrued evidence suggests that the contribution of NO to the pathophysiology of septic shock is highly heterogeneous and, therefore, difficult to target therapeutically without appropriate monitoring tools, which do not exist at present.

Time course of inducible nitric oxide synthase activity following endotoxin administration in dogs

An increased production of nitric oxide (NO) via the inducible isoform of NO synthase (iNOS) has been incriminated in the pathogenesis of septic shock. Since the time course of iNOS activity is not known during endotoxic shock in dogs, we measured iNOS activity, estimated by the rate of conversion of (14)C-arginine to (14)C-citrulline in the absence of calcium, in the heart, lung, liver, kidney, and gut at 1, 2, 3, 4, and 6 h after a bolus of Escherichia coli endotoxin (2 mg/kg, iv), in the dog. This model, including generous fluid administration, is associated with typical features of human septic shock, including low systemic vascular resistance, altered myocardial function and limited oxygen extraction capability. An increase in iNOS activity was observed at 4 h in the liver (0.24 vs 0.04 mU/mg/min) and at 6 h in the heart (0.26 vs 0.09 mU/mg/min). These findings may contribute to a better delineation of the involvement of NO in endotoxic shock, and to the evaluation of the therapeutic effects of NO inhibitors.

Diaspirin cross-linked hemoglobin improves oxygen extraction capabilities in endotoxic shock

We studied the effects of diaspirin cross-linked hemoglobin (DCLHb), a cell-free hemoglobin derived from human erythrocytes, on blood flow distribution and tissue oxygen extraction capabilities in endotoxic shock. Eighteen pentobarbital sodium-anesthetized, mechanically ventilated dogs received 2 mg/kg of E. coli endotoxin, followed by saline resuscitation to restore cardiac filling pressures to baseline levels. The animals were randomly divided into three groups: six served as control, six received DCLHb at a dose of 500 mg/kg (group 1) and six DCLHb at a dose of 1,000 mg/kg (group 2). Cardiac tamponade was then induced by saline injection in the pericardial sac to progressively reduce cardiac index and thereby allow study of tissue oxygen extraction capabilities. DCLHb had a dose-dependent vasopressor effect but did not significantly alter cardiac index or regional blood flow. During cardiac tamponade, critical oxygen delivery was 12.8 +/- 0.7 ml. kg(-1). min(-1) in the control group, but 8.6 +/- 0.9 and 8.2 +/- 0.7 ml. kg(-1). min(-1) in groups 1 and 2, respectively (both P < 0.05 vs. control group). The critical oxygen extraction ratio was 39.1 +/- 3.1% in the control group but 58.7 +/- 12.8% and 60.2 +/- 9.0% in groups 1 and 2, respectively. We conclude that DCLHb can improve whole body oxygen extraction capabilities during endotoxic shock in dogs.

Effects of nitric oxide in septic shock

Nitric oxide (NO) is believed to play a key role in the pathogenesis of septic shock, although many aspects of NO's involvement remain poorly defined. Recent years have seen advances in our understanding of the production and effects of NO, but much of the work has been done in animal models and may not be directly relevant to the clinical situation. Differences between species and models can account for many of the apparently conflicting results obtained. Nevertheless, NO-directed strategies have been developed and tested clinically. However, NO can have both beneficial and detrimental effects on many organ systems in sepsis and attempts to nonselectively block all its actions may therefore not yield positive results on outcome. Further exploration and precision of the role of NO and development of techniques to assess the NO balance in individual patients is necessary before further progress can be made in this field.

Norepinephrine and N(G)-monomethyl-L-arginine in hyperdynamic septic shock in pigs: effects on intestinal oxygen exchange and energy balance

OBJECTIVES

To compare the effects of norepinephrine (NOR) and the nonselective nitric oxide synthase inhibitor, N(G)-monomethyl-L-arginine (L-NMMA), on intestinal blood flow, oxygen exchange, and energy metabolism over 24 hrs of hyperdynamic, normotensive porcine endotoxic shock.

DESIGN

Prospective, randomized, experimental study with repeated measures.

SETTING

Investigational animal laboratory.

SUBJECTS

Twenty-seven pigs were divided into three groups: seven animals received no vasopressor therapy (ETX) during endotoxic shock; ten animals were treated with NOR; and ten animals were treated with L-NMMA.

INTERVENTIONS

Pigs were anesthetized, mechanically ventilated, and instrumented. Eight hours later, endotoxic shock was initiated by an infusion of Escherichia coli lipopolysaccharide. Animals were resuscitated by hetastarch directed to maintain the intrathoracic blood volume and a mean arterial pressure (MAP) of >60 mm Hg. Twelve hours after the start of the endotoxin infusion, NOR or L-NMMA was administered for 12 hrs in the treatment groups to maintain a MAP at preshock levels.

MEASUREMENTS AND MAIN RESULTS

ETX caused a continuous fall in MAP, despite a sustained increase in the cardiac output achieved by fluid resuscitation. NOR maintained MAP at preshock levels because of a further rise in cardiac output, whereas hemodynamic stabilization during L-NMMA resulted from systemic vasoconstriction. NOR increased portal venous blood flow concomitant with decreased intestinal oxygen extraction, whereas L-NMMA influenced neither portal venous blood flow nor intestinal oxygen extraction. Mean capillary hemoglobin oxygen saturation of the ileal mucosa as well as the frequency distributions reflecting microcirculatory oxygen availability remained unchanged as well. Nevertheless, portal venous pH similarly decreased and portal venous lactate/pyruvate ratios increased in all three groups. The arterial-ileal mucosal PCO2 gap progressively increased in the ETX and L-NMMA groups, whereas NOR blunted this response.

CONCLUSIONS

Neither treatment could reverse the ETX-induced derangements of cellular energy metabolism as reflected by the increased portal venous lactate/pyruvate ratios. The NOR-induced attenuation of ileal mucosal acidosis was possibly caused by a different pattern of blood flow redistribution compared with L-NMMA.

Minor involvement of nitric oxide during chronic endotoxemia in anesthetized pigs

To study the modifications of hepatic blood flow and hepatic function over time during endotoxemia, 10 pigs received a continuous intravenous infusion of endotoxin (Endo, 160 ng. kg(-1). h(-1)) over 18 h and 7 control (Ctrl) animals received a saline infusion. The involvement of nitric oxide (NO) in this endotoxic model was assessed by measuring plasma concentrations of NO(-)(2), NO(-)(3), and cGMP, by testing vascular reactivity to ACh, and by evaluating inducible NO synthase (NOS 2) expression in hepatic biopsies. Endotoxin induced hypotensive and normokinetic shock in association with few modifications of hepatic blood flow, and hepatic injury was observed in both groups. Endotoxin did not increase plasma concentrations of NO(-)(2), NO(-)(3), and cGMP. The ACh-dependent decrease of mean arterial pressure was reduced in Endo pigs, whereas a minor difference was observed between Ctrl and Endo pigs for ACh-dependent modification of hepatic perfusion. Hepatic NOS 2 mRNA was not detected in Ctrl pigs. In Endo pigs, NOS 2 protein expression was detected only in tissues surrounding the portal vein and the inferior vena cava, whereas NOS 2 mRNA was expressed in all hepatic biopsies. Thus, although endotoxemia induces NOS 2 expression in the liver, our findings show that NO involvement is lower in pigs than in rodents during endotoxemia.

Norepinephrine and nomega-monomethyl-L-arginine in porcine septic shock: effects on hepatic O2 exchange and energy balance

We compared the effects of norepinephrine (NOR; n = 11) and the nonselective nitric oxide synthase inhibitor Nomega-monomethyl-L-arginine (L-NMMA; n = 11) on hepatic blood flow (Q liv), O2 exchange, and energy metabolism over 24 h of hyperdynamic, normotensive porcine endotoxic shock. Endotoxin (ETX; n = 8) caused a continuous fall in mean arterial pressure (MAP) despite a sustained 50% increase in cardiac output (Q) achieved by adequate fluid resuscitation. NOR maintained MAP at preshock levels owing to a further rise in Q, while the comparable hemodynamic stabilization during L-NMMA infusion resulted from systemic vasoconstriction, increasing the systemic vascular resistance (SVR) about 30% from shock level after 6 h of treatment concomitant with a reduction in Q to preshock values. Whereas NOR also increased Q liv and, hence, hepatic O2 delivery (hDO2), but did not affect hepatic O2 uptake (hVO2), L-NMMA influenced neither Q liv nor hDO2 and hVO2. Mean capillary hemoglobin O2 saturation (HbScO2) on the liver surface as well as HbScO2 frequency distributions, which mirror microcirculatory O2 availability, remained unchanged as well. Neither treatment influenced the ETX-induced derangements of cellular energy metabolism reflected by the progressive decrease in hepatic lactate uptake rate and increased hepatic venous lactate/pyruvate ratios. ETX nearly doubled the endogenous glucose production (EGP) rate, which was further increased with NOR, whereas L-NMMA nearly restored EGP to preshock levels. Nevertheless, despite the different mechanisms in maintaining blood pressure neither treatment influenced ETX-induced liver dysfunction.