Acute respiratory distress syndrome: current concepts and future directions

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

Extracellular ATP is a danger signal activating P2X7 receptor in a LPS mediated inflammation (ARDS/ALI)

Acute respiratory distress syndrome (ARDS) is a life-threating lung condition resulting from a direct and indirect injury to the lungs [1, 2]. Pathophysiologically it is characterized by an acute alveolar damage, an increased permeability of the microvascular-barrier, leading to protein-rich pulmonary edema and subsequent impairment of arterial oxygenation and respiratory failure [1]. This study examined the role of extracellular ATP in recruiting inflammatory cells to the lung after induction of acute lung injury with lipopolysaccharide (LPS). However, the precise mechanism is poorly understood. Our objective was to investigate the functional role of the P2X7 receptor in the pathogenesis of acute respiratory distress syndrome (ARDS/ acute lung injury (ALI)) in vitro and in vivo. We show that intratracheally applied LPS causes an acute accumulation of ATP in the BALF (bronchoalveolar lavage) and lungs of mice. Prophylactic and therapeutic inhibition of P2X7R signalling by a specific antagonist and knock-out experiments was able to ameliorate the inflammatory response demonstrated by reduced ATP-levels, number of neutrophils and concentration of pro-inflammatory cytokine levels in the BALF. Experiments with chimeric mice showed that P2X7R expression on immune cells was responsible for the observed effect. Consistently, the inflammatory response is diminished only by a cell-type specific knockdown of P2X7 receptor on non-stationary immune cells. Since the results of BALF from patients with acute ARDS or pneumonia simulated the in vivo data after LPS exposure, the P2X7 receptor may be a new therapeutic target for treatment in acute respiratory distress syndrome (ARDS/ALI).

Aerosolized prostacyclins for acute respiratory distress syndrome (ARDS)

BACKGROUND

Acute respiratory distress syndrome (ARDS) is a critical condition that is associated with high mortality and morbidity. Aerosolized prostacyclin has been used to improve oxygenation despite the limited evidence available so far.This review was originally published in 2010 and updated in 2017.

OBJECTIVES

To assess the benefits and harms of aerosolized prostacyclin in adults and children with ARDS.

SEARCH METHODS

In this update, we searched CENTRAL (2017, Issue 4); MEDLINE (OvidSP), Embase (OvidSP), ISI BIOSIS Previews, ISI Web of Science, LILACS, CINAHL (EBSCOhost), and three trials registers. We handsearched the reference lists of the latest reviews, randomized and non-randomized trials, and editorials, and cross-checked them with our search of MEDLINE. We contacted the main authors of included studies to request any missed, unreported or ongoing studies. The search was run from inception to 5 May 2017.

SELECTION CRITERIA

We included all randomized controlled trials (RCTs), irrespective of publication status, date of publication, blinding status, outcomes published or language. We contacted trial investigators and study authors to retrieve relevant and missing data.

DATA COLLECTION AND ANALYSIS

Three authors independently abstracted data and resolved any disagreements by discussion. Our primary outcome measure was all-cause mortality. We planned to perform subgroup and sensitivity analyses to assess the effect of aerosolized prostacyclin in adults and children, and on various clinical and physiological outcomes. We assessed the risk of bias through assessment of methodological trial components and the risk of random error through trial sequential analysis.

MAIN RESULTS

We included two RCTs with 81 participants.One RCT involved 14 critically ill children with ARDS (very low quality of evidence), and one RCT involved 67 critically ill adults (very low quality evidence).Only one RCT (paediatric trial) provided data on mortality and found no difference between intervention and control. However, this trial was eligible for meta-analysis due to a cross-over design.We assessed the benefits and harms of aerosolized prostacyclin. One RCT found no difference in improvement of partial pressure of oxygen in arterial blood/fraction of inspired oxygen (PaO₂/FiO₂) ratio (mean difference (MD) -25.35, 95% confidence interval (CI) -60.48 to 9.78; P = 0.16; 67 participants, very low quality evidence).There were no adverse events such as bleeding or organ dysfunction in any of the included trials. Due to the limited number of RCTs, we were unable to perform the prespecified subgroup and sensitivity analyses or trial sequential analysis.

AUTHORS' CONCLUSIONS

We are unable to tell from our results whether the intervention has an important effect on mortality because the results were too imprecise to rule out a small or no effect. Therefore, no current evidence supports or refutes the routine use of aerosolized prostacyclin for people with ARDS. There is an urgent need for more RCTs.

C1P Attenuates Lipopolysaccharide-Induced Acute Lung Injury by Preventing NF-κB Activation in Neutrophils

Recently, ceramide-1-phosphate (C1P) has been shown to modulate acute inflammatory events. Acute lung injury (Arnalich et al. 2000. Infect. Immun. 68: 1942-1945) is characterized by rapid alveolar injury, lung inflammation, induced cytokine production, neutrophil accumulation, and vascular leakage leading to lung edema. The aim of this study was to investigate the role of C1P during LPS-induced acute lung injury in mice. To evaluate the effect of C1P, we used a prophylactic and therapeutic LPS-induced ALI model in C57BL/6 male mice. Our studies revealed that intrapulmonary application of C1P before (prophylactic) or 24 h after (therapeutic) LPS instillation decreased neutrophil trafficking to the lung, proinflammatory cytokine levels in bronchoalveolar lavage, and alveolar capillary leakage. Mechanistically, C1P inhibited the LPS-triggered NF-κB levels in lung tissue in vivo. In addition, ex vivo experiments revealed that C1P also attenuates LPS-induced NF-κB phosphorylation and IL-8 production in human neutrophils. These results indicate C1P playing a role in dampening LPS-induced acute lung inflammation and suggest that C1P could be a valuable candidate for treatment of ALI.

Comparison of a New Miniaturized Extracorporeal Membrane Oxygenation System With Integrated Rotary Blood Pump to a Standard System in a Porcine Model of Acute Lung Injury

Extracorporeal membrane oxygenation (ECMO) is used for severe acute respiratory distress syndrome. However, available ECMO systems are large and not well designed for fast delivery, emergency implantation, and interhospital transfer. Therefore, a new miniaturized oxygenator with integrated rotary blood pump (ILIAS) was developed and compared with a standard ECMO system in a large animal model. Acute lung injury was induced with repeated pulmonary saline lavage in 14 pigs until PaO2 /FiO2 -ratio was <100 mm Hg with a positive-end-expiratory-pressure of 5 mbar. Pigs were assigned to the following three groups: group 1 (n = 4): control group with conventional ventilation; group 2 (n = 5): standard vv-ECMO; group 3 (n = 5): vv-ILIAS. Gas exchange, hemodynamics, hemolysis, and coagulation activation were examined over a period of 8 h. No device failed during the observation period. PaCO2 decreased from 59.40 ± 4.14 mm Hg to 48.62 ± 4.50 mm Hg after 1 h in the ILIAS group compared with an improvement of PaCO2 from 48.86 ± 7.45 to 40.10 ± 6.02 in the conventional ECMO group (P = not significant [n.s.]). ARDS-induced respiratory acidosis was controlled promptly with a pH of 7.2 ± 0.1 at baseline increasing to 7.4 ± 0.1 in both study groups after 60 min of ECMO support. Mean carbon dioxide transfer was comparable between the conventional ECMO and ILIAS (211.36 ± 78.39 mL/min vs. 219.99 ± 76.72 mL/min, P = n.s.). PaO2 /FiO2 increased from 118.4 ± 15.5 mm Hg to 179.1 ± 72.4 mm Hg in the ILIAS group compared with an improvement of oxygenation from 107.1 ± 24.9 mm Hg to 179.0 ± 45.7 mm Hg in the standard ECMO group (P = n.s.). Mean oxygen transfer was calculated with 136.09 ± 30.25 mL/min for the ILIAS and 129.05 ± 36.28 mL/min for the standard ECMO. Hemodynamic instability or significant activation of the plasmatic coagulation was not observed. However, hemolysis was significantly higher in the ILIAS group compared with the conventional ECMO. As the ILIAS prototype provided excellent gas exchange with hemodynamic stability comparable with a standard ECMO system, we believe this study serves as a proof of concept. Further development and design modifications (optimized rotation speed and surface coating of rotor) are already done and another experiment is projected to reduce hemolysis and platelet consumption for clinical application.

Hypoxia signaling during acute lung injury

Acute lung injury (ALI) is an inflammatory lung disease that manifests itself in patients as acute respiratory distress syndrome and thereby contributes significantly to the morbidity and mortality of patients experiencing critical illness. Even though it may seem counterintuitive, as the lungs are typically well-oxygenated organs, hypoxia signaling pathways have recently been implicated in the resolution of ALI. For example, functional studies suggest that transcriptional responses under the control of the hypoxia-inducible factor (HIF) are critical in optimizing alveolar epithelial carbohydrate metabolism, and thereby dampen lung inflammation during ALI. In the present review we discuss functional roles of oxygenation, hypoxia and HIFs during ALI, mechanisms of how HIFs are stabilized during lung inflammation, and how HIFs can mediate lung protection during ALI.