Abnormal tissue oxygenation and cardiovascular changes in endotoxemia

Variations of renal tissue oxygenation during abdominal compartment syndrome and sepsis

PURPOSE

This experimental study was designed to evaluate the renal tissue oxygenation under the coexistence of abdominal compartment syndrome and sepsis.

MATERIAL AND METHODS

Fourteen non-breed dogs were divided into two groups: the control group (8) and the study group (6). Sepsis was established with intravenous endotoxin infusion at 100μg/kg for over 30min. Insufflation of CO₂ in the peritoneal cavity was used for the increase in intra-abdominal pressure (IAP). A special catheter placed and fixed in the renal cortex at a depth of 3mm from the renal capsule was used for the measurement of renal tissue oxygenation.

RESULTS

Study parameters were recorded at the starting phase, at IAP of 15mmHg and 30mmHg and after decompression of the abdomen in the control group, and at the same intervals plus the induction of sepsis, prior to increasing abdominal pressure, in the study group. With the elevation of the IAP a reduction of renal tissue oxygenation presents itself, which is more pronounced in the presence of sepsis, especially for IAP over 15mmHg. Like other parameters, after abdominal decompression the renal tissue oxygenation returns to the initial levels, independently of sepsis.

CONCLUSIONS

The afferent arterioles vasoconstriction, which takes place during sepsis, and the intra-renal shunt, which occurs and leads to blood diversion to the medulla from the renal cortex due to the combination of intra-abdominal hypertension (IAH) and sepsis, seem to explain this finding.

Effects of the Infusion of 4% or 20% Human Serum Albumin on the Skeletal Muscle Microcirculation in Endotoxemic Rats

Background

Sepsis-induced microcirculatory alterations contribute to tissue hypoxia and organ dysfunction. In addition to its plasma volume expanding activity, human serum albumin (HSA) has anti-oxidant and anti-inflammatory properties and may have a protective role in the microcirculation during sepsis. The concentration of HSA infused may influence these effects. We compared the microcirculatory effects of the infusion of 4% and 20% HSA in an experimental model of sepsis.

Methods

Adult male Wistar rats were equipped with arterial and venous catheters and received an intravenous infusion of lipopolysaccharide (LPS, serotype O127:B8, 10 mg/kg over 30 minutes) or vehicle (SHAM, n = 6). Two hours later, endotoxemic animals were randomized to receive 10 mL/kg of either 4% HSA (LPS+4%HSA, n = 6), 20% HSA (LPS+20%HSA, n = 6) or 0.9% NaCl (LPS+0.9%NaCl, n = 6). No fluids were given to an additional 6 animals (LPS). Vessel density and perfusion were assessed in the skeletal muscle microcirculation with sidestream dark field videomicroscopy at baseline (t0), 2 hours after LPS injection (t1), after HSA infusion (t2) and 1 hour later (t3). The mean arterial pressure (MAP) and heart rate were recorded. Serum endothelin-1 was measured at t2.

Results

MAP was stable over time in all groups. The microcirculatory parameters were significantly altered in endotoxemic animals at t1. The infusion of both 4% and 20% HSA similarly increased the perfused vessel density and blood flow velocity and decreased the flow heterogeneity to control values. Microvascular perfusion was preserved in the LPS+20%HSA group at t3, whereas alterations reappeared in the LPS+4%HSA group.

Conclusions

In a rat model of normotensive endotoxemia, the infusion of 4% or 20% HSA produced a similar acute improvement in the microvascular perfusion in otherwise unresuscitated animals.

Short-term hyperoxia does not exert immunologic effects during experimental murine and human endotoxemia

Oxygen therapy to maintain tissue oxygenation is one of the cornerstones of critical care. Therefore, hyperoxia is often encountered in critically ill patients. Epidemiologic studies have demonstrated that hyperoxia may affect outcome, although mechanisms are unclear. Immunologic effects might be involved, as hyperoxia was shown to attenuate inflammation and organ damage in preclinical models. However, it remains unclear whether these observations can be ascribed to direct immunosuppressive effects of hyperoxia or to preserved tissue oxygenation. In contrast to these putative anti-inflammatory effects, hyperoxia may elicit an inflammatory response and organ damage in itself, known as oxygen toxicity. Here, we demonstrate that, in the absence of systemic inflammation, short-term hyperoxia (100% O2 for 2.5 hours in mice and 3.5 hours in humans) does not result in increased levels of inflammatory cytokines in both mice and healthy volunteers. Furthermore, we show that, compared with room air, hyperoxia does not affect the systemic inflammatory response elicited by administration of bacterial endotoxin in mice and man. Finally, neutrophil phagocytosis and ROS generation are unaffected by short-term hyperoxia. Our results indicate that hyperoxia does not exert direct anti-inflammatory effects and temper expectations of using it as an immunomodulatory treatment strategy.

Tissue oxygen tension monitoring of organ perfusion: rationale, methodologies, and literature review

Tissue oxygen tension is the partial pressure of oxygen within the interstitial space of an organ bed. As it represents the balance between local oxygen delivery and consumption at any given time, it offers a ready monitoring capability to assess the adequacy of tissue perfusion relative to local demands. This review covers the various methodologies used to measure tissue oxygen tension, describes the underlying physiological and pathophysiological principles, and summarizes human and laboratory data published to date.

Shock - A reappraisal: The holistic approach

Shock as reaction to life-threatening condition needs to be reclassified in a timely and more scientific synopsis. It is not possible or beneficial any longer to avoid a holistic approach in critical illness. Semantics of critical illness has often been unfriendly in the literature and a simplification with the elimination of conceptual pleonasms and misnomers under the exclusive light of physiology and physiopathology would be advantageous. Speaking one language to describe the same phenomenon worldwide is essential for understanding; moreover, it increases focus on characterization and significance of the phenomena.

Clinical pathology of the shock syndromes

The clinical aspects of shock syndromes are described from their inception as compensated physiology to a stage of decompensation. The clinical significance of hypotension, fluid-responsive and non fluid-responsive hypotension, is discussed. Untimely or inadequate treatment leads to persistent subclinical shock despite adjustments of the macrohemodynamic variables, which evolves in a second hit of physiological deterioration if not aggressively managed. Irreversible shock ensues as consequence of direct hit or as result of inadequate or delayed treatment and is characterized by drug-resistant hypotension.

Physiopathology of shock

Shock syndromes are of three types: cardiogenic, hemorrhagic and inflammatory. Hemorrhagic shock has its initial deranged macro-hemodynamic variables in the blood volume and venous return. In cardiogenic shock there is a primary pump failure that has cardiac output/mean arterial pressure as initial deranged variables. In Inflammatory Shock it is the microcirculation that is mainly affected, while the initial deranged macrocirculation variable is the total peripheral resistance hit by systemic inflammatory response.

Sepsis in diabetes: A bad duo

Increasing incidence of morbidity and mortality of diabetic subjects due to infection necessitates the understanding of its patho-biology and further remedial measures for its prevention and treatment. The increased incidence of infection is because of systemic illness that has compromising effects on multiple organs including the nervous, vascular, musculoskeletal, and immunologic systems of the diabetic patients. Many factors contribute to this condition including hyperglycemia, insulin deficiency, ischemia and impaired immunity. Sepsis, as a separate entity, lead to destruction of cytokine network that can be fatal. Compromised defense mechanisms due to sepsis and cytokine dysregulation in diabetic patients make the situation worse. Early identification of local infection by applying advanced molecular tools, appropriate selection of antibiotics, intensive wound management, control of glycemic status and supportive treatment can reduce the rate of morbidity and mortality due to sepsis in patients with diabetes.

Tissue oxygen tension monitoring: will it fill the void?

PURPOSE OF REVIEW

The holy grail of circulatory monitoring is an accurate, continuous and relatively noninvasive means of assessing the adequacy of organ perfusion. This could be then advantageously used to direct therapeutic interventions to prevent both under-treatment and over-treatment and thus improve outcomes. However, in view of the heterogeneous response (adaptive or maladaptive) of different organs to various shock states, any monitor of perfusion adequacy cannot reflect every organ system, but should at least detect early deterioration in a 'canary' organ. Tissue oxygen tension reflects the balance between local oxygen supply and demand, and could thus be a potentially useful monitoring modality. This article examines the different technologies available and reviews the current literature regarding its utility as a monitor.

RECENT FINDINGS

Tissue oxygen tension, measured at a variety of sites in both human and laboratory studies, does appear to be a sensitive indicator of organ perfusion in different shock states. However, responses can vary not only between organs and between different shock states, but also over time. These changes reflect the particular oxygen supply-demand balance present in that tissue bed at that specific time point in the disease process. The response to a dynamic oxygen challenge test provides further information that allows severity to be more readily differentiated.

SUMMARY

Monitoring of tissue oxygen tension may offer a potentially useful tool for clinical management though significant validation needs to be first performed to confirm its promise.

Temporal changes in tissue cardiorespiratory function during faecal peritonitis

PURPOSE

Sepsis affects both macro- and micro-circulatory transport of oxygen to tissues, causing regional hypoxia. However, this relationship is poorly characterized with respect to inter-organ variability, disease severity and the evolution to organ dysfunction. We hypothesized that an early circulatory insult precedes the development of organ dysfunction, and is more severe in predicted non-survivors. Consequently, we assessed temporal changes in myocardial function and regional tissue oxygenation in peripheral and deep organs in a rat model of faecal peritonitis. We also examined the utility of a dynamic oxygen challenge test to assess the microcirculation.

METHODS

Awake, tethered, fluid-resuscitated male Wistar rats were randomized to receive intraperitoneal injection of faecal slurry, or to act as controls. At either 6 or 24 h post insult, rats were anaesthetized and underwent echocardiography, arterial cannulation and placement of tissue oxygen probes in peripheral (muscle, bladder) and deep (liver and renal cortex) organ beds. Measurements were repeated during fluid loading and an oxygen challenge test (administration of high oxygen concentrations).

RESULTS

Early sepsis (6 h) was characterized by a fall in global oxygen delivery with concurrent decreases in muscle, renal cortical and, especially, liver tissue PO2. By contrast, during established sepsis (24 h), myocardial and circulatory function had largely recovered despite increasing clinical unwellness, hyperlactataemia and biochemical evidence of organ failure. O2 challenge revealed an early depression of response that, by 24 h, had improved in all organ beds bar the kidney.

CONCLUSIONS

This long-term septic model exhibited an early decline in tissue oxygenation, the degree of which related to predicted mortality. Clinical and biochemical deterioration, however, progressed despite cardiovascular recovery. Early circulatory dysfunction may thus be an important trigger for downstream processes that result in multi-organ failure. Furthermore, the utility of tissue PO2 monitoring to highlight the local oxygen supply-demand balance, and dynamic O2 challenge testing to assess microcirculatory function merit further investigation.

Endotoxemia does not limit energy supply in exercising rat skeletal muscle

Although depletion in high-energy phosphorylated compounds and mitochondrial impairment have been reported in septic skeletal muscle at rest, their impact on energy metabolism has not been documented during exercise. In this study we aimed to investigate strictly gastrocnemius muscle function non-invasively, using magnetic resonance techniques in endotoxemic rats. Endotoxemia was induced by injecting animals intraperitoneally at t(0) and t(0) + 24 h with Klebsiella pneumoniae lipopolysaccharides (at 3 mg kg(-1)). Investigations were performed at t(0) + 48 h during a transcutaneous electrical stimulation protocol consisting of 5.7 min of repeated isometric contractions at a frequency of 3.3 HZ. Endotoxin treatment produced a depletion in basal phosphocreatine content and a pronounced reduction in oxidative adenosine triphosphate (ATP) synthesis capacity, whereas the resting ATP concentration remained unchanged. During the stimulation period, endotoxemia caused a decrease in force-generating capacity that was fully accounted for by the loss of muscle mass. It further induced an acceleration of glycolytic ATP production and an increased accumulation of adenosine diphosphate (ADP, an important mitochondrial regulator) that allowed a near-normal rate of oxidative ATP synthesis. Finally, endotoxemia did not affect the total rate of ATP production or the ATP cost of contraction throughout the whole stimulation period. These data demonstrate that, in an acute septic phase, metabolic alterations in resting muscle do not impact energy supply in exercising muscle, likely as a result of adaptive mechanisms.

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.

Endotoxemia causes a paradoxical intracellular pH recovery in exercising rat skeletal muscle

In resting skeletal muscle, endotoxemia causes disturbances in energy metabolism that could potentially disturb intracellular pH (pH(i)) during muscular activity. We tested this hypothesis using in situ (31)P-magnetic resonance spectroscopy in contracting rat gastrocnemius muscle. Endotoxemia was induced by injecting rats intraperitoneally at t(0) and t(0) + 24 h with Klebsiella pneumoniae endotoxin (lipopolysaccharides at 3 mg/kg) or saline vehicle. Muscle function was investigated strictly noninvasively at t(0) + 48 h through a transcutaneous electrical stimulation protocol consisting of 5.7 minutes of repeated isometric contraction at 3.3 HZ, and force production was measured with an ergometer. At rest, endotoxin treatment did not affect pH(i) and adenosine triphosphate concentration, but significantly reduced phosphocreatine and glycogen contents. Endotoxemia produced both a reduction of isometric force production and a marked linear recovery (0.08 +/- 0.01 pH unit/min) of pH(i) during the second part of the stimulation period. This recovery was not due to any phenomenon of fiber inactivation linked to development of muscle fatigue, and was not associated with any change in intracellular proton buffering, net proton efflux from the cell, or proton turnovers through creatine kinase reaction and oxidative phosphorylation. This paradoxical pH(i) recovery in exercising rat skeletal muscle under endotoxemia is likely due to slowing of glycolytic flux following the reduction in intramuscular glycogen content. These findings may be useful in the follow-up of septic patients and in the assessment of therapies.

Mitochondrial function in sepsis: acute phase versus multiple organ failure

OBJECTIVE

To describe temporal changes in mitochondrial function during the septic process, including the recovery phase.

DESIGN

Literature review.

SUBJECTS

Clinical studies and laboratory models.

MAIN RESULTS

Biochemical and ultrastructural mitochondrial abnormalities have been recognized in in vivo, ex vivo, and in vitro laboratory models of sepsis for >30 yrs. Short-term models show variable effects on mitochondrial function and structure; this is likely related to differences in model design, including species, organs studied, degree of septic insult, and degree of resuscitation. Longer-term models more consistently reveal mitochondrial dysfunction and damage. There is a rebound increase in oxygen consumption and resting energy expenditure in the recovery phase of sepsis. This could reflect mitochondrial recovery (biogenesis) that may restore the energy supply needed to fuel restorative metabolic processes and enable patient survival.

CONCLUSION

Mitochondrial dysfunction seems to be intrinsically involved in the pathogenesis of multiple organ failure. As a consequence of a progressive decrease in energy availability, metabolism must decrease or the cell will die. The interplay between adenosine 5'-triphosphate supply and demand, dictated by the degree of mitochondrial dysfunction and the level of metabolic shutdown (analogous to a hibernation-type response), seems to be crucial in determining outcome. Further studies are needed to confirm this hypothesis.

Albumin resuscitation improves ventricular contractility and myocardial tissue oxygenation in rat endotoxemia

OBJECTIVE

Fluid resuscitation to improve delivery of oxygen to vital organs is a principal clinical intervention for septic patients. We previously reported that albumin resuscitation in rat endotoxemia improved contractility in isolated cardiomyocytes, but whether this effect occurs in vivo is unknown. We hypothesized that albumin resuscitation would improve decreased ventricular contractility and myocardial tissue oxygenation in vivo.

DESIGN

Randomized, controlled, prospective animal study.

SETTING

University animal laboratory.

SUBJECTS

Male Sprague-Dawley rats (250-350 g).

INTERVENTIONS

Rats were randomized into three groups: control with no lipopolysaccharide (n = 8), lipopolysaccharide (10 mg/kg) without albumin resuscitation (n = 8), and lipopolysaccharide with albumin resuscitation (n = 6). Five hours after lipopolysaccharide injection, animals were resuscitated with 10 mL/kg 5% rat albumin in 0.9% saline. Six hours after 10 mL/kg lipopolysaccharide, a pressure-volume conductance catheter (MIKRO-Tip 2.0-Fr, Millar Instruments, Houston, TX) was inserted into the left ventricle to quantify maximum elastance as an index of contractility. Myocardial tissue Po2 was measured using a fiberoptic oxygen probe.

MEASUREMENTS AND MAIN RESULTS

Maximum elastance decreased after lipopolysaccharide relative to control (47%, from 5.9 +/- 0.8 to 3.1 +/- 0.4 mm Hg/microL, p < .05). Albumin resuscitation prevented the lipopolysaccharide-induced decrease in maximum elastance (7.0 +/- 1.2 mm Hg/microL, p < .05 vs. lipopolysaccharide). Myocardial tissue Po2 was reduced in endotoxemia compared with control (53%, from 10.1 +/- 0.9 to 4.7 +/- 0.6 mm Hg, p < .05), and albumin resuscitation improved the lipopolysaccharide-induced tissue hypoxia toward the control value (9.0 +/- 1.4 mm Hg, p < .05).

CONCLUSIONS

Albumin resuscitation improved decreased ventricular contractility and myocardial oxygenation in endotoxemic rats. This result suggests that albumin resuscitation may improve ventricular dysfunction by improving myocardial hypoxia.

Myocardial hypoxia-inducible HIF-1alpha, VEGF, and GLUT1 gene expression is associated with microvascular and ICAM-1 heterogeneity during endotoxemia

The systemic inflammatory response to infection is the leading cause of mortality in North American intensive-care units. Although much is known about inflammatory mediators, the relationships between microregional inflammation, microvascular heterogeneity, hypoxia, hypoxia-inducible gene expression, and myocardial dysfunction are unknown. Male Sprague-Dawley rats were injected intraperitoneally with LPS to test the hypothesis that sepsis-induced local inflammation and increased microvascular heterogeneity are spatially and temporally associated with hypoxia, hypoxia-inducible gene expression, and decreased left-ventricular contractility. Using a combination of three-dimensional microvascular imaging, tissue Po(2), and pressure-volume conductance measurements, we found that 5 h after LPS, minimum oxygen-diffusion distances increased (P < 0.05), whereas tissue oxygenation and contractility both decreased (P < 0.05) in the left ventricle. Real-time RT-PCR analysis revealed that the hypoxia-inducible genes hypoxia-inducible factor (HIF)-1alpha, VEGF, and glucose transporter (GLUT) 1 were all upregulated (P < 0.05) in the left ventricle. Tissue regions expressing ICAM-1, obtained by using laser-capture microdissection, had increased HIF-1alpha and GLUT1 (P < 0.05) gene expression. VEGF gene expression was more diffuse. In LPS rats, GLUT1 gene expression correlated (P < 0.05) with left-ventricular contractility. In 5-h hypoxic cardiomyocytes, we found strong transient HIF-1alpha, weak VEGF, and greater prolonged GLUT1 gene expression. By comparison, the HIF-1alpha-GLUT1 gene-induction pattern was reversed in the left ventricle of LPS rats. Together, these results show that LPS induces hypoxia in the left ventricle associated with increased microvascular heterogeneity and decreased contractility. HIF-1alpha and GLUT1 gene induction are related to a heterogeneous ICAM-1 expression and may be cardioprotective during the onset of septic injury.

The effect of hypoxemic resuscitation of hemorrhagic shock on hemodynamic stabilization and inflammatory response: a pilot study in a rat experimental model

BACKGROUND

Resuscitation of hemorrhagic shock is associated with tissue injury. The effect of hypoxemia during resuscitation was investigated.

METHODS

Shock was induced by withdrawing blood to mean arterial pressure (MAP) 40 mm Hg and maintained for 60 minutes in 25 Wistar rats. Animals were randomly divided to receive either normoxemic (controls, FiO2 = 21%, n = 14) or hypoxemic (HypRes, FiO2 = 12%, n = 11) resuscitation by re-infusing their shed blood. Outcome was assessed through hemodynamic and inflammatory parameters. Another nine rats served to correlate different FiO2 to the corresponding PaO2.

RESULTS

At 60 minutes of resuscitation HypRes had higher MAP than control animals (p = 0.008). The respective median (range) malondialdehyde and TNF-alpha levels was 1.7 (1-2.1) versus 3.1 (2.4-4.3) micromol/L, (p = 0.02) and 0 versus 5.8 (0-5.8) pg/mL, (p = 0.025). Glutathione, endotoxin, interferon-gamma, and nitric oxide values were similar between groups. FiO2 of 12% induced only a mild hypoxemia (PaO2 approximately 80 mm Hg).

CONCLUSIONS

Even mild hypoxemia during resuscitation of shock leads to effective hemodynamic stabilization.

Effect of decreased O2 supply on skeletal muscle oxygenation and O2 consumption during sepsis: role of heterogeneous capillary spacing and blood flow

One of the main aspects of the initial phase of the septic inflammatory response to a bacterial infection is abnormal microvascular perfusion, including decreased functional capillary density (FCD) and increased blood flow heterogeneity. On the other hand, one of the most important phenomena observed in the later stages of sepsis is an increased dependence of tissue O(2) utilization on the convective O(2) supply. This "pathological supply dependency" is associated with organ failure and poor clinical outcomes. Here, a detailed theoretical model of capillary-to-tissue O(2) transport during sepsis is used to examine the origins of abnormal supply dependency. With use of three-dimensional arrays of capillaries with heterogeneous spacing and blood flow, steady-state O(2) transport is simulated numerically during reductions in the O(2) supply. Increased supply dependency is shown to occur in sepsis for hypoxic (decreased hemoglobin O(2) saturation) and stagnant (decreased blood flow) hypoxia. For stagnant hypoxia, a reduction in FCD with decreasing blood flow is necessary to obtain the observed increase in supply dependency. Our results imply that supply dependency observed under normal conditions does not have its origin at the level of individual capillaries. In sepsis, however, diffusion limitation and shunting of O(2) by individual capillaries occur to a degree that is dependent on the heterogeneity of septic injury and the arrangement of capillary networks. Thus heterogeneous stoppage of individual capillaries is a likely factor in pathological supply dependency.

Microvascular effects following treatment with polyethylene glycol-albumin in lipopolysaccharide-induced endotoxemia

OBJECTIVE

To determine whether resuscitation with polyethylene glycol conjugated bovine serum albumin (2.5% weight/volume) infused at 16 mL/kg/hr (PEG-BSA-16) or at 24 mL/kg/hr (PEG-BSA-24) for 1 hr improves microcirculatory conditions in endotoxemia compared with dextran 70 (6% weight/volume) infused at 24 mL/kg/hr (Dex).

DESIGN

Prospective study.

SETTING

University research laboratory.

SUBJECTS

Male Golden Syrian hamsters.

INTERVENTIONS

Hamsters implemented with a skinfold window chamber were given an intravenous injection of lipopolysaccharide and resuscitated within 10 mins with Dex, PEG-BSA-16, or PEG-BSA-24.

MEASUREMENTS AND MAIN RESULTS

Hamsters were observed during 24 hrs after lipopolysaccharide injection. Systemic variables measured included mean arterial pressure, heart rate, and systemic arterial blood gas. Microvascular function was characterized by measuring vessel diameter; red blood cell velocity; functional capillary density (FCD); P(O2) in arterioles, venules, and tissue; and perivascular nitric oxide concentration 6 hrs after lipopolysaccharide injection. At 6 hrs, animals with no treatment had the lowest FCD (6.7 +/- 5.7% of baseline). PEG-BSA provided significantly improved microvascular conditions as shown by restoration of FCD. Recovery of FCD was related to improved microvascular flow and perivascular and tissue P(O2), normalization of shear rate, and decreased perivascular nitric oxide concentration. These effects were related to improved fluid retention using PEG-BSA-24 as evidenced by the significantly lower hematocrit at 24 hrs after resuscitation. Nitric oxide at 6 hrs after induction of sepsis achieved perivascular millimolar concentrations, which were reduced to normal values by PEG-BSA-24 treatment. At 6 hrs there were significant differences in FCD, tissue P(O2), and perivascular nitric oxide concentration following PEG-BSA treatment by comparison with Dex treatment, although there were no differences in systemic variables between Dex and PEG-BSA groups.

CONCLUSIONS

PEG-BSA produces improved microcirculatory conditions in the treatment of endotoxemia when compared with dextran 70.

Microvascular resuscitation as a therapeutic goal in severe sepsis

Sepsis causes microvascular dysfunction. Increased heterogeneity of capillary blood flow results in local tissue hypoxia, which can cause local tissue inflammation, impaired oxygen extraction, and, ultimately, organ dysfunction. Microvascular dysfunction is clinically relevant because it is a marker for mortality: it improves rapidly in survivors of sepsis but fails to improve in nonsurvivors. This, along with the fact that resuscitation of mean arterial pressure and cardiac output alone fails to improve microvascular function, means that microvascular resuscitation is therefore a therapeutic goal. In animal studies of sepsis, volume resuscitation improves microvascular permeability and tissue oxygenation, and leads to improved organ function, including a reduction in myocardial dysfunction. Microvascular resuscitation strategies include hemodynamic resuscitation using the linked combination of volume resuscitation, judicious vasopressor use, and inotropes and vasodilators. Alternative vasoactive agents, such as vasopressin, may improve microcirculatory function to a greater degree than conventional vasopressors. Successful modulation of inflammation has a positive impact on endothelial function. Finally, targeted treatment of the endothelium, using activated protein C, also improves microvascular function and ultimately increases survival. Thus, attention must be paid to the microcirculation in patients with sepsis, and therapeutic strategies should be employed to resuscitate the microcirculation in order to avoid organ dysfunction and to reduce mortality.

Effect of sepsis on skeletal muscle oxygen consumption and tissue oxygenation: interpreting capillary oxygen transport data using a mathematical model

Inherent in the inflammatory response to sepsis is abnormal microvascular perfusion. Maldistribution of capillary red blood cell (RBC) flow in rat skeletal muscle has been characterized by increased 1) stopped-flow capillaries, 2) capillary oxygen extraction, and 3) ratio of fast-flow to normal-flow capillaries. On the basis of experimental data for functional capillary density (FCD), RBC velocity, and hemoglobin O2 saturation during sepsis, a mathematical model was used to calculate tissue O2 consumption (V̇o2), tissue Po2 (Pt) profiles, and O2 delivery by fast-flow capillaries, which could not be measured experimentally. The model describes coupled capillary and tissue O2 transport using realistic blood and tissue biophysics and three-dimensional arrays of heterogeneously spaced capillaries and was solved numerically using a previously validated scheme. While total blood flow was maintained, capillary flow distribution was varied from 60/30/10% (normal/fast/stopped) in control to 33/33/33% (normal/fast/stopped) in average sepsis (AS) and 25/25/50% (normal/fast/stopped) in extreme sepsis (ES). Simulations found approximately two- and fourfold increases in tissue V̇o2 in AS and ES, respectively. Average (minimum) Pt decreased from 43 ( 40 ) mmHg in control to 34 ( 27 ) and 26 ( 15 ) mmHg in AS and ES, respectively, and clustering fast-flow capillaries (increased flow heterogeneity) reduced minimum Pt to 14.5 mmHg. Thus, although fast capillaries prevented tissue dysoxia, they did not prevent increased hypoxia as the degree of microvascular injury increased. The model predicts that decreased FCD, increased fast flow, and increased V̇o2 in sepsis expose skeletal muscle to significant regions of hypoxia, which could affect local cellular and organ function.

Fluids reverse the early lipopolysaccharide-induced albumin leakage in rodent mesenteric venules

OBJECTIVE

Volume resuscitation is clinically beneficial in patients with sepsis, but few data exist concerning the effects of fluid administration on early events in the inflammatory process. Vascular permeability, leukocyte rolling and leukocyte adhesion in the rodent mesenteric microcirculation were assessed in vivo using intravital microscopy, and the effect of fluid administration on lipopolysaccharide (LPS)-induced changes recorded.

DESIGN

Prospective, repeated measures study.

SETTING

University hospital laboratory.

SUBJECTS

Male Wistar rats in six groups.

INTERVENTIONS

All animals underwent intravital microscopic examination of mesenteric post-capillary venules. LPS or vehicle was applied topically. Animals received either no additional fluids, 0.9% saline (16 ml/kg per h) or 5% human albumin (16 ml/kg per h) commencing 30 min prior to LPS/vehicle administration.

MEASUREMENTS AND MAIN RESULTS

Leukocyte rolling, firm adhesion and blood velocity were observed directly. Vascular permeability was assessed using the flux of fluorescently labelled albumin into the interstitium. LPS significantly increased the median (IQR) number of leukocytes rolling and firmly adherent relative to baseline (at 60 min rolling increased from 12.0 (10.3-13.8) to 40.3 (36.0-47.5) cells/min; adhesion increased from 1 (1-2) to 17 (12-26) cells/100 microm; n=5, p<0.01). Transvascular albumin flux was significantly increased 45 min after LPS application (p<0.01), but not after vehicle. Administration of either 0.9% saline (n=5) or 5% human albumin (n=6), significantly attenuated LPS-induced increases in albumin flux (p<0.05), leukocyte rolling (p<0.01) and adhesion (p<0.01). Fluid administration did not appear to alter shear rates.

CONCLUSIONS

Pre-emptive volume administration with either saline or albumin prevented early LPS-induced microcirculatory changes by an undefined effect that is unrelated to changes in microvascular flow.

Measurement of oxygen consumption in mouse aortic endothelial cells using a microparticulate oximetry probe

The purpose of this study was to determine the rate of oxygen consumption in mouse aortic endothelial cells (MAECs) and to determine the effect of a variety of inhibitors and stimulators of oxygen consumption measured by electron paramagnetic resonance (EPR) spectroscopy utilizing a new particulate oximetry probe. We have previously demonstrated that the octa-n-butoxy derivative of naphthalocyanine neutral radical (LiNc-BuO) enables accurate, precise, and reproducible measurements of pO(2) in cellular suspensions. In the current study, we carried out measurements to provide an accurate determination of pO(2) in small volume with less number of cells (20,000 cells) that has not been possible with other techniques. To establish the reliability of this method, agents such as menadione, lipopolysaccharide (LPS), potassium cyanide, rotenone, and diphenyleneiodonium chloride (DPI) were used to modulate the oxygen consumption rate in the cells. We observed an increase in oxygen consumption by the cells upon treatment with menadione and LPS, whereas treatment with cyanide, rotenone, and DPI inhibited oxygen consumption. This study clearly demonstrated the utilization of EPR spectrometry with LiNc-BuO probe for determination of oxygen concentration in cultured cells.

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.

Effect of a maldistribution of microvascular blood flow on capillary O(2) extraction in sepsis

Inherent in the remote organ injury caused by sepsis is a profound maldistribution of microvascular blood flow. Using a 24-h rat cecal ligation and perforation model of sepsis, we studied O(2) transport in individual capillaries of the extensor digitorum longus (EDL) skeletal muscle. We hypothesized that erythrocyte O(2) saturation (SO(2)) levels within normally flowing capillaries would provide evidence of either a mitochondrial failure (increased SO(2)) or an O(2) transport derangement (decreased SO(2)). Using a spectrophotometric functional imaging system, we found that sepsis caused 1) an increase in stopped flow capillaries (from 10 to 38%, P < 0.05), 2) an increase in the proportion of fast-flow to normal-flow capillaries (P < 0.05), and 3) a decrease in capillary venular-end SO(2) levels from 58.4 +/- 20.0 to 38.5 +/- 21.2%, whereas capillary arteriolar-end SO(2) levels remained unchanged compared with the sham group. Capillary O(2) extraction increased threefold (P < 0.05) and was directly related to the degree of stopped flow in the EDL. Thus impaired O(2) transport in early stage sepsis is likely the result of a microcirculatory dysfunction.

Flavohemoglobin denitrosylase catalyzes the reaction of a nitroxyl equivalent with molecular oxygen

We have previously reported that bacterial flavohemoglobin (HMP) catalyzes both a rapid reaction of heme-bound O(2) with nitric oxide (NO) to form nitrate [HMP-Fe(II)O(2) + NO --> HMP-Fe(III) + NO(3)(-)] and, under anaerobic conditions, a slower reduction of heme-bound NO to an NO(-) equivalent (followed by the formation of N(2)O), thereby protecting against nitrosative stress under both aerobic and anaerobic conditions, and rationalizing our finding that NO is rapidly consumed across a wide range of O(2) concentrations. It has been alternatively suggested that HMP activity is inhibited at low pO(2) because the enzyme is then in the relatively inactive nitrosyl form [k(off)/k(on) for NO (0.000008 microM) k(off)/k(on) for O(2) (0.012 microM) and K(M) for O(2) = 30-100 microM]. To resolve this discrepancy, we have directly measured heme-ligand turnover and NADH consumption under various O(2)/NO concentrations. We find that, at biologically relevant O(2) concentrations, HMP preferentially binds NO (not O(2)), which it then reacts with oxygen to form nitrate (in essence NO(-) + O(2) --> NO(3)(-)). During steady-state turnover, the enzyme can be found in the ferric (FeIII) state. The formation of a heme-bound nitroxyl equivalent and its subsequent oxidation is a novel enzymatic function, and one that dominates the oxygenase activity under biologically relevant conditions. These data unify the mechanism of HMP/NO interaction with those recently described for the nematode Ascaris and mammalian hemoglobins, and more generally suggest that the peroxidase (FeIII)-like properties of globins have evolved for handling of NO.

Tissue oxygenation and perfusion in patients with systemic sepsis

OBJECTIVE

Multiple organ dysfunction is associated with systemic sepsis. To investigate whether this is attributable to peripheral tissue hypoperfusion and/or cellular hypoxia, simultaneous measurements of tissue perfusion and oxygenation were made in patients with severe sepsis and in controls.

DESIGN

Prospective, observational study.

SETTING

Adult intensive care unit, tertiary referral center.

PATIENTS

Volunteers (group C, n = 7), patients undergoing cardiopulmonary bypass (group B, n = 6), and patients with severe sepsis (group S, n = 6).

INTERVENTIONS

Limb ischemia and reperfusion.

MEASUREMENTS AND MAIN RESULTS

Tissue oxygenation and microvascular flow were measured by using microelectrodes inserted into brachoradialis muscle and overlying subcutaneous tissue. Forearm cutaneous red cell flux and regional blood flow were measured simultaneously. Responses to 20 mins of limb ischemia and subsequent reperfusion were observed. Baseline muscle tissue oxygenation was greater in sepsis (1.7 +/- 0.2, 1.5 +/- 0.7, and 4.4 +/- 0.6 kPa for groups C, B, and S, respectively, mean +/- sem, p <.05), although baseline subcutaneous tissue oxygenation did not vary between groups. During ischemia tissue oxygenation, values decreased in muscle (to 1.3 +/- 0.2, 1.0 +/- 0.4, and 1.5 +/- 0.4 kPa for groups C, B, and S, respectively) and subcutaneous tissue (to 2.0 +/- 0.3, 1.7 +/- 0.5, and 2.3 +/- 0.2 kPa for groups C, B, and S, respectively). Decline in tissue oxygen tension was initially more rapid in septic muscle compared with controls (25% decrease, 68 +/- 23 vs. 176 +/- 38 for group S vs. group C, p <.05, and 50% decrease, 126 +/- 34 vs. 398 +/- 72 secs for group S vs. group C, p <.01). However, overall rate of tissue decline was similar (95% decrease, 444 +/- 122 vs. 614 +/- 96 for group S vs. group C, p >.05). After reperfusion, significant differences in muscle tissue oxygenation reappeared between groups (2.0 +/- 0.3, 1.5 +/- 0.7, and 4.0 +/- 0.4 kPa for groups C, B, and S, respectively, p <.05). There were no differences in time to 25%, 50%, or 95% tissue oxygen recovery. Whole limb reperfusion was significantly less in patient groups compared with controls (10.6 +/- 0.9, 4.5 +/- 1.2, and 4.3 +/- 1.6 mL x 100 mL(-1) x min(-1) for groups C, B, and S, respectively, p <.05).

CONCLUSIONS

Significant differences in tissue oxygenation distribution between muscle and subcutaneous tissues occur in patients with severe sepsis. High baseline muscle tissue oxygen levels are accompanied by rapid extraction of oxygen during stagnant ischemia.