N(G)-monomethyl-L-arginine improves survival in a pig model of abdominal sepsis

Effects of Inhaled Nitric Oxide on Inflammation and Apoptosis After Cardiopulmonary Bypass

BACKGROUND

Cardiopulmonary bypass (CPB), a procedure often used during cardiac surgery, is associated with an inflammatory process that leads to lung injury. We hypothesized that inhaled nitric oxide (INO), which has anti-inflammatory properties, possesses the ability to modulate lung cell apoptosis and prevent CPB-induced inflammation.

METHODS

Twenty male pigs were randomly classified into four groups: sham, sham plus INO, CPB, and CPB plus INO. INO (20 ppm) was administered for 24 h after anesthesia. CPB was performed 90 min into INO treatment. BAL fluid and blood were collected at time 0 (before CPB), at 4 h after beginning CPB, and 24 h after beginning CPB (T24).

RESULTS

At T(24), BAL interleukin (IL)-8 levels and neutrophil percentages were elevated significantly in the CPB group. At T(24), INO reduced IL-8 concentrations and attenuated the increase of neutrophil percentage in the CPB-plus-INO group. Nitrite-plus-nitrate (NOx) concentrations were decreased significantly in groups without INO. Moreover, animals treated with INO showed higher rates of pulmonary apoptosis compared to their respective control groups except for the sham-plus-INO group, in which they were diminished.

CONCLUSION

These results demonstrate that NOx production is reduced after CPB, and that INO acts as an anti-inflammatory agent by decreasing neutrophil numbers and their major chemoattractant, IL-8. INO also increases cell apoptosis in the lungs during inflammatory conditions, which may explain, in part, how it resolves pulmonary inflammation.

The anti-inflammatory effect of inhaled nitric oxide on pulmonary inflammation in a swine model

Cardiopulmonary bypass (CPB) is associated with an inflammatory process that leads to lung injury. In this study, we hypothesized that inhaled nitric oxide (INO) possesses the ability to modulate CPB-induced inflammation. Fifteen male pigs were randomly divided into 3 groups: Sham, CPB+LPS (CPB and lipopolysaccharide), and CPB+LPS+INO. INO (20 parts per million) was administered for 24 h after anesthesia. CPB was performed for 90 min, and LPS was infused (1 µg/kg) after CPB. Bronchoalveolar lavage (BAL) fluid and blood were collected at T0(before CPB), at 4 h, and at 24 h. At 24 h, BAL interleukin-8 (IL-8) levels were not increased as expected in the CPB+LPS group compared with the Sham group, but they were reduced significantly in the CPB+LPS+INO group. Cell hypo reactivity observed in the groups receiving LPS also seemed to downregulate endothelial nitric oxide synthase NOS protein expression relative to the Sham group. Nitrite and nitrate (NOx) concentrations were decreased significantly in the groups without INO. Moreover, animals treated with INO showed higher rates of pulmonary apoptosis compared with their respective controls. These results demonstrate that NOx production is reduced after CPB and that INO acts on the inflammatory process by diminishing neutrophils and their major chemoattractant, IL-8. INO also increases cell apoptosis in the lungs under inflammatory conditions, which may explain, in part, how it resolves pulmonary inflammation.Key words: CPB, nitric oxide, apoptosis, LPS, IL-8.

THE DEVELOPMENT OF ACUTE RESPIRATORY DISTRESS SYNDROME AFTER GUT ISCHEMIA/REPERFUSION INJURY FOLLOWED BY FECAL PERITONITIS IN PIGS: A CLINICALLY RELEVANT MODEL

Numerous clinical trials using anti-inflammatory agents for patients with acute respiratory distress syndrome (ARDS) have failed despite efficacy in acute animal models. This underscores the necessity of developing a clinically relevant model of ARDS. Initially, we attempted to induce lung injury in pigs by fecal peritonitis only. When this was unsuccessful, we designed a two-hit model of ischemia/reperfusion (I/R) injury followed by fecal peritonitis to create a clinically applicable model of ARDS. The initial study consisted of Yorkshire swine [group 1, fecal clot (FC), n = 4] that were followed clinically after intraperitoneal placement of a fecal (0.5 mL/kg) blood (2 mL/kg) clot. Blood was sampled daily for cultures, a complete blood count, a lactate level, and various cytokine expression determined by enzyme-linked immunosorbent assay (ELISA). Pigs were treated with antibiotics and fluids, placed on a ventilator before sacrifice to obtain hemodynamic and pulmonary parameters, and underwent histologic lung assessment. Additionally, bronchoalveolar lavage fluid was obtained for protein concentration and cytokine levels. Once it was evident that no lung injury had occurred, we designed a more severe model. A second group of Yorkshire swine [group 2, superior mesenteric artery (SMA) + FC, n = 4] underwent SMA occlusion for 30 min (I/R) followed by intraperitoneal placement of a FC as in the initial group. These pigs were monitored more invasively and continuously in an intensive care setting for 48 h and followed, treated, and assessed in a similar fashion to group 1. Group 1 (FC) pigs survived 9 days and showed signs of sepsis (bacteremia with polymicrobial organisms), an inflammatory response in the form of elevated cytokines, yet no physiologic or histologic evidence of lung injury. Group 2 (SMA + FC) pigs demonstrated more severe sepsis, a significantly increased cytokine response compared with animals in the FC group, and physiologic signs of progressive pulmonary injury. Pigs in the SMA + FC group were sacrificed at 48 h after clinical deterioration (significant decline in oxygenation) and demonstrated pathologic evidence of lung injury indicated by increased bronchoalveolar lavage fluid protein, diffuse and thickened alveolar septae, hyaline membrane formation, and pulmonary edema. The addition of a second "hit" (SMA occlusion, I/R) to a FC sepsis model resulted in severe lung injury that developed within a 3-day period. To our knowledge, this is the first large animal experiment that definitively and consistently causes insidious onset ARDS in pigs. By closely paralleling the clinical development of pulmonary injury, this model should prove invaluable in the study of human ARDS.

Cardiovascular effects of the nitric oxide synthase inhibitor NG-methyl-l-arginine hydrochloride (546C88) in patients with septic shock: Results of a randomized, double-blind, placebo-controlled multicenter study (study no. 144-002)*

OBJECTIVE

To assess the hemodynamic effects of the nitric oxide synthase inhibitor 546C88 in patients with septic shock, although this was not a stated aim of the protocol. The predefined primary efficacy objective of the protocol was resolution of shock determined at the end of a 72-hr treatment period.

DESIGN

Multicentered, randomized, placebo-controlled, safety and efficacy study.

SETTING

Forty-eight intensive care units in Europe, North America, and Australia.

PATIENTS

A total of 312 patients with septic shock diagnosed within 24 hr before randomization.

INTERVENTIONS

Patients were randomly allocated to receive either 546C88 or placebo (5% dextrose) by intravenous infusion for up to 72 hrs. Conventional vasoactive therapy was restricted to norepinephrine, dopamine, and dobutamine. Study drug was initiated at 0.1 mL/kg/hr (5 mg/kg/hr 546C88) and titrated according to response up to a maximum rate of 0.4 mL/kg/hr with the objective to maintain mean arterial pressure at 70 mm Hg while attempting to withdraw any concurrent vasopressor(s).

MEASUREMENTS AND MAIN RESULTS

Requirement for vasopressors, systemic and pulmonary hemodynamics, indices of oxygen transport, and plasma concentrations of arginine and nitrate were assessed over time. The median mean arterial pressure for both groups was maintained > or =70 mm Hg. There was an early increase in systemic and pulmonary vascular tone and oxygen extraction, whereas both cardiac index and oxygen delivery decreased for patients in the 546C88 cohort. Although these parameters subsequently returned toward baseline values, the observed differences between the treatment groups, except for pulmonary vascular resistance and oxygen extraction, persisted throughout the treatment period, despite a reduced requirement for vasopressors in the 546C88 cohort. These changes were associated with a reduction in plasma nitrate concentrations, which were elevated in both groups before the start of therapy.

CONCLUSIONS

The nitric oxide synthase inhibitor 546C88 can reduce the elevated plasma nitrate concentrations observed in patients with septic shock. In this study, treatment with 546C88 for up to 72 hrs was associated with an increase in vascular tone and a reduction in both cardiac index and oxygen delivery. The successful maintenance of a target mean arterial blood pressure > or =70 mm Hg was achieved with a reduction in the requirement for, or withdrawal of, conventional inotropic vasoconstrictor agents (i.e., dopamine and norepinephrine). There were no substantive untoward consequences accompanying these hemodynamic effects. An international, randomized, double-blind, placebo-controlled phase III study has since been conducted in patients with septic shock. Recruitment into the study was discontinued due to the emergence of increased mortality in the 546C88-treated group.

Early immunoneutralization of calcitonin precursors attenuates the adverse physiologic response to sepsis in pigs

OBJECTIVE

The 116 amino acid prohormone procalcitonin and some of its component peptides (collectively termed calcitonin precursors) are important markers and mediators of sepsis. In this study, we sought to evaluate the effect of immunoneutralization of calcitonin precursors on metabolic and physiologic variables of sepsis in a porcine model.

DESIGN

A prospective, controlled animal study.

SETTING

A university research laboratory.

SUBJECTS

30-kg Yorkshire pigs.

INTERVENTIONS

Sepsis was induced in 15 pigs by intraperitoneal instillation of a suspension of cecal content (1 g/kg animal body weight) and a toxinogenic Escherichia coli solution (2 x 10(11) colony-forming units). During induction of sepsis, seven pigs received an intravenous infusion of purified rabbit antiserum, reactive to the aminoterminal portion of porcine prohormone procalcitonin. Another eight control pigs received an intravenous infusion of purified nonreactive rabbit antiserum. For all 15 animals, physiologic data (urine output, core temperature, arterial pressure, heart rate, cardiac index, and stroke volume index) and metabolic data (serum blood urea nitrogen and creatinine, arterial lactate, and pH) were collected or recorded hourly until death at 15 hrs.

MEASUREMENTS AND MAIN RESULTS

In this large-animal model of rapidly lethal peritonitis, serum calcitonin precursors were significantly elevated. Amino-prohormone procalcitonin-reactive antiserum administration resulted in a significant improvement or a beneficial trend in a majority of the measured physiologic and metabolic derangements induced by sepsis. Specifically, arterial pressure, cardiac index, stroke volume index, pH, and creatinine were all significantly improved, while urine output and serum lactate had beneficial trends. Treated animals also experienced a statistically significant increase of short-term survival.

CONCLUSIONS

These data from a large-animal model with polymicrobial sepsis demonstrate the salutary effect of early immunoneutralization of calcitonin precursors on physiologic and metabolic variables. Immunologic blockade of calcitonin precursors may offer a novel therapeutic approach to human sepsis.

The hemodynamic effects of diaspirin cross-linked hemoglobin in dopamine-resistant endotoxic shock in swine

As the blood substitute Diaspirin Cross-linked Hemoglobin (DCLHb) has potent vasopressor activity, we assessed its hemodynamic effects in a clinically relevant dopamine-resistant endotoxic shock model in swine. In a randomized and controlled study, E. coli LPS was administered to anesthetized and invasively monitored swine. Group I (n = 3) control pigs were not resuscitated. Groups II (n = 5) and III (n = 6) pigs received dopamine (DA) after MAP decreased 30%, and hetastarch and DCLHb, respectively, after dopamine-resistance occurred. Progressive hemodynamic decline occurred in Group I pigs. DA failed to restore MAP to baseline. However, 0% and 67% of pigs also treated with heta-starch and DCLHb, respectively, achieved temporary restoration of baseline MAP (p = 0.03), prompting a reduction in the dose of DA in 0% of hetastarch vs. 50% of DCLHb treated pigs. Except for increased MPAP and decreased heart in DCLHb treated pigs (p<0.001), hemodynamics and survival were not different (p>0.05). In conclusion, although DCLHb exacerbated pulmonary hypertension and did not improve O2 utilization or survival, because DCLHb restored MAP to baseline and had a dopamine sparing effect, further investigation of DCLHb's hemodynamic effects in adrenergic agent-resistant endotoxemia is warranted.

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.

[Chronic experimental bacteremia in Yucatan micropigs]

The Yucatan micropig has been used to develop an experimental model of chronic bacteremia. This animal exhibits clinical and biological characteristics that are close to those in humans, and the pharmacokinetic behaviours of many classes of drugs in this model are similar to those in man. Six adult female were intravenously inoculated with a mean Escherichia coli inoculum of 5.1 x 10(9) bacteria. During five days of spontaneous evolution, the medical follow-up includes biological, clinical and bacteriological parameters. A systemic inflammatory syndrome, a sepsis, an organ insufficiency and positive blood cultures mimic the human disease. In all animals there is an adynamia, a lack of motor coordination, an anorexia, a tachypnea, a fever, a leuconeutropenia followed by an hyperleucocytosis, an anemia, a thrombopenia, an acute tubulonephritis and an elevated sedimentation rate. In some cases, there is an increase of the C reactive protein, in others, an increase of IL-6 and IL-8. At day five, all animals are alive, and five micropigs have positive blood cultures. This chronic, reproducible model is thus suitable for further antibacterial treatments evaluations.

Effect of L-arginine on endothelial injury and hemostasis in rabbit endotoxin shock

To investigate whether impaired endothelial function was related to alteration of nitric oxide (NO) formation during endotoxic shock, we studied the effects of supplementation of L-arginine (L-Arg), D-arginine (D-Arg), and N(G)-nitro-L-arginine methyl ester (L-NAME), on endothelial function and structure in a rabbit model. Endotoxic shock was induced by a single lipopolysaccharide bolus (0.5 mg/kg i.v., Escherichia coli endotoxin). Coagulation factors and expression of monocyte tissue factor were determined by functional assays. Endothelium-dependent vascular relaxation was assessed by in vitro vascular reactivity. Immunohistochemical staining (CD31) was performed to assess damaged endothelial cell surface of the abdominal aorta. These parameters were studied 5 days after the onset of endotoxic shock and were compared under three conditions: in absence of treatment, with L-Arg or D-Arg supplementation, or with L-NAME. Both L-Arg and D-Arg significantly improved endothelium-dependent relaxation and endothelial morphological injury. L-NAME did not alter endothelial histological injury induced by lipopolysaccharide. These data indicate that arginine supplementation nonspecifically prevents endothelial dysfunction and histological injury in rabbit endotoxic shock. Moreover, L-Arg has no effect on coagulation activation and expression of monocyte tissue factor induced by endotoxic shock.

Comparison between selective and nonselective nitric oxide synthase inhibition and phenylephrine in normal and endotoxic swine

OBJECTIVE

To compare the cardiopulmonary and peripheral circulatory effects of the nonselective nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) to the more selective inducible NOS inhibitor S-methylisothiourea (SMT) and to phenylephrine (PE) in endotoxic and normal swine.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Research laboratory of academic medical center.

SUBJECTS

Nonanesthetized, sedated, mechanically ventilated, minimally invasive swine model.

INTERVENTIONS

Animals received either lipopolysaccharide (LPS, LPS groups) or equivalent volume of saline (normal groups). LPS animals were further randomized into four groups when mean arterial pressure (MAP) had dropped to <60 mm Hg: the LPS/saline group received saline only; the other groups received either L-NAME, SMT, or PE. These were titrated to elevate MAP by 20-25 mm Hg, and animals were followed for another 3 hrs. Pulmonary artery occlusion pressure was maintained at one to two times baseline with the infusion of saline. Normal groups received the same agents 1 hr after baseline measurements, and drugs were titrated to achieve similar increases in MAP. We measured gastric-arterial PCO2 gradient by tonometry as an index of gastric mucosal perfusion. Left ventricular volumes were determined echocardiographically; right ventricular volumes were determined by a pulmonary arterial catheter equipped with a rapid thermistor. Plasma nitrite/nitrate (NOx) concentrations were measured hourly.

MEASUREMENTS AND MAIN RESULTS

In the LPS groups, all agents elevated MAP and systemic vascular resistance similarly. By hr 4, cardiac output had decreased in all groups, but the decrease with L-NAME (35% +/- 16%) occurred earlier (at hr 3) and was larger than the decrease with SMT at hrs 3 and 5 and larger than the decrease with saline at hrs 3 to 5. L-NAME resulted in a larger increase in mean pulmonary arterial pressure (MPAP) when compared with saline (130% +/- 44% vs. 61% +/- 25%; p < .001) and SMT groups (130% vs. 97% +/- 80%; p < .007). Only L-NAME had detrimental effects on right ventricular function as indicated by an increase in right ventricular end-systolic volume (54 +/- 10 to 87 +/-6 mL; p < .05) and right ventricular end-diastolic volume (90 +/-11 to 128 +/- 18 mL; p < .05). SMT decreased both left ventricular end-systolic volume (10.4 +/- 2 to 7.7 +/- 4 mL; p < .05) and left ventricular end-diastolic volume (18.5 +/- 3 to 14.2 +/- 5 mL; p < .05), indicating improved left ventricular function, whereas L-NAME did not affect left ventricular volumes. Both SMT and PE corrected LPS-induced gastric mucosal acidosis, but L-NAME did not. We did not detect changes in plasma NOx concentrations in any of LPS groups. In the normal groups, all agents increased MAP without changes in plasma NOx concentrations. L-NAME caused a larger decrease in cardiac output, but the increase in MPAP was higher with SMT. Both NOS inhibitors led to left ventricular dilation, but PE did not. Only L-NAME caused right ventricular dilation. There were no changes in gastric-arterial PCO2 gradient.

CONCLUSIONS

In LPS animals, we failed to detect changes in plasma NOx concentrations. Furthermore, for similar increases in MAP, SMT improved gastric mucosal acidosis, had less adverse effects on right ventricular function and MPAP, and may have improved left ventricular function. However, apart from its bene-ficial effects on left ventricular function, SMT was not superior to PE. The results from normal animals indicate that both NOS inhibitors have adverse effects on cardiac function beyond those attributed to increased MAP.

Respiratory failure. Conventional and high-tech support

Although significant progress has been made in the treatment of patients with acute lung failure in the critical care setting, the mortality rate from acute lung injury and ARDS is unacceptably high, given the numbers of patients treated for these syndromes each year. The improved understanding of the pathophysiology of respiratory failure from basic science and clinical research is reflected in improved survival rates over the years. Advances in the mechanical ventilator (through microprocessor technology); biosurface technology; liquid ventilation; and, in some cases, returning to so-called "antiquated" practices of patient care (e.g., prone positioning) seem to have had an impact nonetheless. As refinement continues to occur in these areas, morbidity and mortality from lung failure will have a lesser impact on patients as physicians treat the consequences of organ failure in the ICU.

Hemodynamic homeostasis during acute hypoxia in septic and nonseptic piglets: differential role of prostaglandins and nitric oxide

We studied the hemodynamic responses of 29 anesthetized and mechanically ventilated piglets to acute hypoxia [reduction of Pao2 from 130 to 38 mm Hg induced by inhalation of 7% fraction of inspired oxygen (Fio2) for 7.5 min] before and during group B beta-hemolytic streptococci (GBS) sepsis. During hypoxia, nonseptic piglets maintained stable systemic blood pressure [105+/-9 (SD) to 97+/-14 mm Hg] and cardiac output (CO) (667+/-72 to 685+/-113 mL/min). However, during GBS/hypoxia, systemic blood pressure fell from 94+/-17 to 49+/-25 mm Hg, CO fell from 397+/-146 to 223+/-142 mL/min (both p < 0.001 versus pre-GBS), and cardiac arrest often ensued. We tested three hypotheses that might underlie GBS-induced intolerance to systemic hypoxia: 1) GBS-induced reduction of systemic CO/systemic oxygen delivery (QO2) below a critical QO2 beyond which the superimposition of hypoxia becomes intolerable; this mechanism is unlikely as nonseptic piglets with comparable reductions in CO/QO2 (induced by inflation of a left atrial balloon) tolerated hypoxia well; 2) GBS-induced inhibition of nitric oxide (NO) synthesis that is vital to tolerance of hypoxia; this mechanism is unlikely as infusion of the NO substrate L-arginine did not restore tolerance to hypoxia during GBS infusion (as it did after inhibition of NO synthesis during infusion of N-nitro-L-arginine in nonseptic piglets); and 3) GBS-induced production of pathologic prostaglandins that impaired the piglet's capacity to tolerate hypoxia; this mechanism finds support in the observation that inhibition of prostaglandins with the cyclooxygenase inhibitor indomethacin completely restored the ability of septic piglets to tolerate hypoxia. Further evaluation of GBS-induced intolerance to systemic hypoxia may provide insight into the incompletely understood mechanisms by which sepsis induces circulatory collapse in experimental animals and in humans.

Nitric oxide in the pathogenesis of sepsis

In sepsis and septic shock, inflammatory mediators result in the production of increased concentrations of nitric oxide (NO) from the enzymatic breakdown of the amino acid L-arginine. The increased amounts of NO are responsible for changes in vasomotor tone, decreased vasopressor responsiveness, and decreased myocardial function, characteristic of septic insult. Therapeutic strategies designed to reduce the concentration of NO by inhibiting the action of the nitric oxide synthase enzyme, or by scavenging the excess NO, offer the potential to treat directly the vasomotor abnormalities and myocardial depression seen in sepsis and other inflammatory states. This article reviews the biology of NO in sepsis and discusses strategies for neutralization of the increased NO production, in the setting of severe sepsis and septic shock.