Hypotension during endotoxemia in aged humans

The endotoxin hypothesis of Alzheimer's disease

Lipopolysaccharide (LPS) constitutes much of the surface of Gram-negative bacteria, and if LPS enters the human body or brain can induce inflammation and act as an endotoxin. We outline the hypothesis here that LPS may contribute to the pathophysiology of Alzheimer's disease (AD) via peripheral infections or gut dysfunction elevating LPS levels in blood and brain, which promotes: amyloid pathology, tau pathology and microglial activation, contributing to the neurodegeneration of AD. The evidence supporting this hypothesis includes: i) blood and brain levels of LPS are elevated in AD patients, ii) AD risk factors increase LPS levels or response, iii) LPS induces Aβ expression, aggregation, inflammation and neurotoxicity, iv) LPS induces TAU phosphorylation, aggregation and spreading, v) LPS induces microglial priming, activation and neurotoxicity, and vi) blood LPS induces loss of synapses, neurons and memory in AD mouse models, and cognitive dysfunction in humans. However, to test the hypothesis, it is necessary to test whether reducing blood LPS reduces AD risk or progression. If the LPS endotoxin hypothesis is correct, then treatments might include: reducing infections, changing gut microbiome, reducing leaky gut, decreasing blood LPS, or blocking LPS response.

Human lipopolysaccharide models provide mechanistic and therapeutic insights into systemic and pulmonary inflammation

Inflammation is a key feature in the pathogenesis of sepsis and acute respiratory distress syndrome (ARDS). Sepsis and ARDS continue to be associated with high mortality. A key contributory factor is the rudimentary understanding of the early events in pulmonary and systemic inflammation in humans, which are difficult to study in clinical practice, as they precede the patient's presentation to medical services. Lipopolysaccharide (LPS), a constituent of the outer membrane of Gram-negative bacteria, is a trigger of inflammation and the dysregulated host response in sepsis. Human LPS models deliver a small quantity of LPS to healthy volunteers, triggering an inflammatory response and providing a window to study early inflammation in humans. This allows biological/mechanistic insights to be made and new therapeutic strategies to be tested in a controlled, reproducible environment from a defined point in time. We review the use of human LPS models, focussing on the underlying mechanistic insights that have been gained by studying the response to intravenous and pulmonary LPS challenge. We discuss variables that may influence the response to LPS before considering factors that should be considered when designing future human LPS studies.

Sepsis and Other Infectious Disease Emergencies in the Elderly

Waning immunity and declining anatomic and physiologic defenses render the elder vulnerable to a wide range of infectious diseases. Clinical presentations are often atypical and muted, favoring global changes in mental status and function over febrile responses or localizing symptoms. This review encompasses early recognition, evaluation, and appropriate management of these common infections specifically in the context of elders presenting to the emergency department. With enhanced understanding and appreciation of the unique aspects of infections in the elderly, emergency physicians can play an integral part in reducing the morbidity and mortality associated with these often debilitating and life-threatening diseases.

Intravenous Endotoxin Challenge in Healthy Humans: An Experimental Platform to Investigate and Modulate Systemic Inflammation

Activation of inflammatory pathways represents a central mechanism in multiple disease states both acute and chronic. Triggered via either pathogen or tissue damage-associated molecular motifs, common biochemical pathways lead to conserved yet variable physiological and immunological alterations. Dissection and delineation of the determinants and mechanisms underlying phenotypic variance in response is expected to yield novel therapeutic advances.

Intravenous (IV) administration of endotoxin (gram-negative bacterial lipopolysaccharide), a specific Toll-like receptor 4 agonist, represents an in vivo model of systemic inflammation in man. National Institutes for Health Clinical Center Reference Endotoxin (CCRE, Escherichia coli O:113:H10:K negative) is employed to reliably and reproducibly generate vascular, hematological, endocrine, immunological and organ-specific functional effects that parallel, to varying degrees, those seen in the early stages of pathological states. Alteration of dose (0.06 - 4 ng/kg) and time-scale of exposure (bolus vs. infusion) allows replication of either acute or chronic inflammation and a range of severity to be elicited, with higher doses (2 - 4 ng/kg) frequently being used to create a 'sepsis-like' state. Established and novel medicinal compounds may additionally be administered prior to or post endotoxin exposure to appreciate their effect on the inflammatory cascade. Despite limitations in scope and generalizability, human IV endotoxin challenge offers a unique platform to gain mechanistic insights into inducible physiological responses and inflammatory pathways. Rationally employed it may aid translation of this knowledge into therapeutic innovations.

Over-distension of the airways by mechanical ventilation in the elderly: adding insult to injury

Setzer and colleagues demonstrate that older animals are more susceptible to ventilator-induced lung injury than young animals and develop a more pronounced local and systemic cytokine response to high tidal volumes. These data have significant implications for older patients receiving mechanical ventilation if these findings can be translated to human critical care medicine.

Severe sepsis and septic shock in the elderly: An overview

The incidence of severe sepsis and septic shock is increasing in the older population leading to increased admissions to the intensive care units (ICUs). The elderly are predisposed to sepsis due to co-existing co-morbidities, repeated and prolonged hospitalizations, reduced immunity, functional limitations and above all due to the effects of aging itself. A lower threshold and a higher index of suspicion is required to diagnose sepsis in this patient population because the initial clinical picture may be ambiguous, and aging increases the risk of a sudden deterioration in sepsis to severe sepsis and septic shock. Management is largely based on standard international guidelines with a few modifications. Age itself is an independent risk factor for death in patients with severe sepsis, however, many patients respond well to timely and appropriate interventions. The treatment should not be limited or deferred in elderly patients with severe sepsis only on the grounds of physician prejudice, but patient and family preferences should also be taken into account as the outcomes are not dismal. Future investigations in the management of sepsis should not only target good functional recovery but also ensure social independence and quality of life after ICU discharge.

Severe sepsis and septic shock in the elderly: An overview

The incidence of severe sepsis and septic shock is increasing in the older population leading to increased admissions to the intensive care units (ICUs). The elderly are predisposed to sepsis due to co-existing co-morbidities, repeated and prolonged hospitalizations, reduced immunity, functional limitations and above all due to the effects of aging itself. A lower threshold and a higher index of suspicion is required to diagnose sepsis in this patient population because the initial clinical picture may be ambiguous, and aging increases the risk of a sudden deterioration in sepsis to severe sepsis and septic shock. Management is largely based on standard international guidelines with a few modifications. Age itself is an independent risk factor for death in patients with severe sepsis, however, many patients respond well to timely and appropriate interventions. The treatment should not be limited or deferred in elderly patients with severe sepsis only on the grounds of physician prejudice, but patient and family preferences should also be taken into account as the outcomes are not dismal. Future investigations in the management of sepsis should not only target good functional recovery but also ensure social independence and quality of life after ICU discharge.

The stressed host response to infection: the disruptive signals and rhythms of systemic inflammation

The cognate signals from sterile or pathogen-induced sources converge on the same recognition or response pathways. In the surgical patient, a systemic response to infection most often occurs in the context of ongoing inflammatory stress. Such an inflammatory response is modulated initially by the magnitude of injury and by patient-specific (endogenous) factors, such as confounding illness, age, and genetic variation. Over an extended period of stress, treatmentrelated (exogenous) factors add unpredictability to host responses to subsequent challenges, such as acquired infection. The host response is discussed in the context of how existing sterile stressors may modify the response to acquired infection in surgical patients.

The immunopathogenesis of sepsis in elderly patients

Prominent among the numerous events that contribute to the enhanced susceptibility of elderly patients to infection is the decline of immune function that accompanies aging. Elderly patients experience a marked decline in cell-mediated immune function and reduced humoral immune function. Age-dependent defects in T and B cell function are readily demonstrable in elderly patients, yet the essential elements of innate immunity are remarkably well preserved. The cytokine and chemokine signaling networks are altered in elderly patients and tends to favor a type 2 cytokine response over type 1 cytokine responses. The induction of proinflammatory cytokines after septic stimuli is not adequately controlled by anti-inflammatory mechanisms in elderly persons. This immune dysregulation is accompanied by a more pronounced procoagulant state in older patients. These molecular events function in concert to render elderly patients at excess risk for mortality from severe sepsis and septic shock.

Human models of low-grade inflammation: bolus versus continuous infusion of endotoxin

Systemic low-grade inflammation is recognized in an increasing number of chronic diseases. With the aim of establishing an experimental human in vivo model of systemic low-grade inflammation, we measured circulating inflammatory mediators after intravenous administration of Escherichia coli endotoxin (0.3 ng/kg of body weight) either as a bolus injection or as a 4-h continuous intravenous infusion, as well as after saline administration, in 10 healthy male subjects on three separate study days. Only bolus endotoxin caused an increase in heart rate, whereas a slight increase in rectal temperature was observed in both endotoxin groups. Tumor necrosis factor alpha (TNF-alpha), interleukin-6, and neutrophil responses were earlier and more pronounced in the bolus trial compared with the infusion trial results, whereas lymphocytes increased after endotoxin bolus injection as well as infusion without any difference between groups. Finally, endotoxin activated the hypothalamo-pituitary-adrenal axis slightly earlier in the bolus compared to the infusion trial. The continuous endotoxin infusion model may be more representative of human low-grade inflammation than the bolus injection model due to a less dynamic and more sustained increase in circulating levels of inflammatory mediators over time. In conclusion, low-dose endotoxin infusion elicits an inflammatory response, as assessed by a rise in TNF-alpha, and the responses are significantly different according to whether low-dose endotoxin is applied as a bolus injection or as a continuous infusion.