Acute lung injury: clinical concept and experimental approaches to pathogenesis

ROS Signaling in the Pathogenesis of Acute Lung Injury (ALI) and Acute Respiratory Distress Syndrome (ARDS)

The generation of reactive oxygen species (ROS) plays an important role for the maintenance of cellular processes and functions in the body. However, the excessive generation of oxygen radicals under pathological conditions such as acute lung injury (ALI) and its most severe form acute respiratory distress syndrome (ARDS) leads to increased endothelial permeability. Within this hallmark of ALI and ARDS, vascular microvessels lose their junctional integrity and show increased myosin contractions that promote the migration of polymorphonuclear leukocytes (PMNs) and the transition of solutes and fluids in the alveolar lumen. These processes all have a redox component, and this chapter focuses on the role played by ROS during the development of ALI/ARDS. We discuss the origins of ROS within the cell, cellular defense mechanisms against oxidative damage, the role of ROS in the development of endothelial permeability, and potential therapies targeted at oxidative stress.

Advances in therapy for acute lung injury

Despite advances in the therapy for acute lung injury and adult respiratory distress syndrome, mortality remains high. The iatrogenic risk of ventilator-induced lung injury might contribute to this high mortality because the lungs are hyperinflated. Low tidal volume and inspiratory pressure are surrogates for the stress and strain concept; but lung compliance, transpulmonary pressure, and chest wall elastance might differ in individual patients. In previous published studies, an increasing number of patients were treated successfully with extracorporeal support. Extracorporeal membrane oxygenation and interventional lung assist allow ultraprotective ventilation strategies. However, these assists have different technical aspects and different indications.

Incidence of acute lung injury in the United States

OBJECTIVE

Recent estimates of acute respiratory distress syndrome (ARDS) incidence have varied from 1.3 to 22 per 100,000 person years (105 person.years); the incidence of acute lung injury (ALI) has varied from 17.9 to 34 cases per 105 person.years. Potential reasons for this wide range include differences in the definition of the syndrome, in the populations sampled, and in the assumptions made to estimate incidence. We hypothesized that careful, prospective, protocol-driven case identification that included the milder hypoxemia criterion for ALI would yield incidence numbers greater than previously reported.

DESIGN

Prospective cohort study with extrapolation to the U.S. population.

SETTING

National Heart, Lung, and Blood Institute-sponsored ARDS Network composed of 20 hospitals.

PATIENTS

As part of the National Institutes of Health-sponsored ARDS network, 20 hospitals prospectively identified patients with ALI from 1996 to 1999. Screening logs from this study were used to estimate ALI rates per intensive care unit (ICU) bed per year. We used the registry and data from the American Hospital Association to estimate the incidence of ALI in the United States.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The ALI per ICU bed incidence in the ARDS network registry varied from 0.7 to 5.8 cases of ALI per ICU bed per year with an average of 2.2 cases of ALI per ICU bed per year. We tested the robustness of the incidence estimate by performing a variety of sensitivity analyses. When we used the conservative assumptions that the ARDS network screening logs were complete at each of the participating hospitals and that ALI cases are limited to academic hospitals with > or =20 adult ICU beds, the incidence of ALI in adults in the United States is 22.4 cases per 105 person.years. Under the less conservative assumption that ALI cases occurred only at hospitals with > or =20 ICU beds, regardless of their academic status, the incidence of ALI in the United States is estimated at 64.2 cases per 105 person.years.

CONCLUSIONS

Based on this analysis, which used prospective clinical trial screening data and conservative assumptions about where patients with ALI are cared for, the incidence of ALI in the United States appears to be higher than previously reported.

Extracorporeal membrane oxygenation for severe acute respiratory failure

Extracorporeal membrane oxygenation (ECMO) is a technique for providing life support, in case the natural lungs are failing and are not able to maintain a sufficient oxygenation of the body's organ systems. ECMO technique was an adaptation of conventional cardiopulmonary bypass techniques and introduced into treatment of severe acute respiratory distress syndrome (ARDS) in the 1970s. The initial reports of the use of ECMO in ARDS patients were quite enthusiastic, however, in the following years it became clear that ECMO was only of benefit in newborns with acute respiratory failure. In neonates treated with ECMO, survival rates of 80% could be achieved. In adult patients with ARDS, two large randomized controlled trials (RCTs) published in 1979 and 1994 failed to show an advantage of ECMO over conventional treatment; survival rates were only 10% and 33%, respectively, in the ECMO groups. Since then, ECMO technology as well as conventional treatment of adult ARDS have undergone further improvements. In conventional treatment lung-protective ventilation strategies were introduced and ECMO was made safer by applying heparin-coated equipment, membranes and tubings. Many ECMO centres now use these advanced ECMO technology and report survival rates in excess of 50% in uncontrolled data collections. The question, however, of whether the improved ECMO can really challenge the advanced conventional treatment of adult ARDS is unanswered and will need evaluation by a future RCT.

Distinct tumor necrosis factor-alpha responses in alveolar and peritoneal macrophages are associated with local levels of endotoxin

Tumor necrosis factor alpha (TNF-alpha) responses of alveolar macrophages (AMs) and peritoneal macrophages (PMs) were studied in rats after intravenous injection of lipopolysaccharide (LPS). High levels of plasma TNF-alpha, increased pulmonary myeloperoxidase activity, and leukopenia occurred within 2 h after LPS injection. Alveolar spaces exhibited a strict compartment property, as manifested by only slightly increased LPS and TNF-alpha levels in alveolar lavage fluid and an unchanged capacity of AMs to produce TNF-alpha. By contrast, the peritoneal cavity had greatly increased local LPS and TNF-alpha levels and a diminished PMs TNF-alpha response to LPS. The amount of LPS in the alveolar spaces was less than 0.2% of the level in peritoneal fluid. These results indicate that activation of resident macrophages is dependent on the amounts of local LPS and, in addition, suggest that resident AMs neither participate in the plasma TNF-alpha response nor contribute to neutrophil sequestration in the lung during the early stages of endotoxemia.