What is acute lung injury? What is ARDS?

Redefining ARDS: a paradigm shift

Although the defining elements of "acute respiratory distress syndrome" (ARDS) have been known for over a century, the syndrome was first described in 1967. Since then, despite several revisions of its conceptual definition, it remains a matter of debate whether ARDS is a discrete nosological entity. After almost 60 years, it is appropriate to examine how critical care has modeled this fascinating syndrome and affected patient's outcome. Given that the diagnostic criteria of ARDS (e.g., increased pulmonary vascular permeability and diffuse alveolar damage) are difficult to ascertain in clinical practice, we believe that a step forward would be to standardize the assessment of pulmonary and extrapulmonary involvement in ARDS to ensure that each patient can receive the most appropriate and effective treatment. The selection of treatments based on arbitrary ranges of PaO2/FiO2 lacks sufficient sensitivity to individualize patient care.

Relationship of Extravascular Lung Water and Pulmonary Vascular Permeability to Respiratory Mechanics in Patients with COVID-19-Induced ARDS

During acute respiratory distress syndrome (ARDS), the increase in pulmonary vascular permeability and lung water induced by pulmonary inflammation may be related to altered lung compliance. A better understanding of the interactions between respiratory mechanics variables and lung water or capillary permeability would allow a more personalized monitoring and adaptation of therapies for patients with ARDS. Therefore, our main objective was to investigate the relationship between extravascular lung water (EVLW) and/or pulmonary vascular permeability index (PVPI) and respiratory mechanic variables in patients with COVID-19-induced ARDS. This is a retrospective observational study from prospectively collected data in a cohort of 107 critically ill patients with COVID-19-induced ARDS from March 2020 to May 2021. We analyzed relationships between variables using repeated measurements correlations. We found no clinically relevant correlations between EVLW and the respiratory mechanics variables (driving pressure (correlation coefficient [CI 95%]: 0.017 [-0.064; 0.098]), plateau pressure (0.123 [0.043; 0.202]), respiratory system compliance (-0.003 [-0.084; 0.079]) or positive end-expiratory pressure (0.203 [0.126; 0.278])). Similarly, there were no relevant correlations between PVPI and these same respiratory mechanics variables (0.051 [-0.131; 0.035], 0.059 [-0.022; 0.140], 0.072 [-0.090; 0.153] and 0.22 [0.141; 0.293], respectively). In a cohort of patients with COVID-19-induced ARDS, EVLW and PVPI values are independent from respiratory system compliance and driving pressure. Optimal monitoring of these patients should combine both respiratory and TPTD variables.

Platelet-Rich Plasma Treatment for Chronic Respiratory Disease

This study was conducted to test the hypothesis that platelet-rich plasma (PRP) therapy in chronic respiratory disease patients will cause lung regeneration, thereby slowing the progression of the disease. We performed a search to obtain pertinent articles on the following electronic databases: Google Scholar, PubMed, NCBI, Medscape, and clinicaltrials.gov. Keywords used during in search included "Platelet Rich Plasma" AND "Chronic Respiratory Disease" AND/OR "Chronic Obstructive Pulmonary Disease". A total of 15 articles were chosen for this paper, published from 2011 to 2021, and included case series, lab studies, animal studies, cohort studies, and clinical trials. All statistical data were considered significant if the p-value was less than 5%, or 0.05. Our findings confirmed that PRP therapy successfully caused anti-inflammatory effects and acceleration of tissue regeneration, resulting in improved lung function. This, in turn, slowed the progression of the disease and led to an improved quality of life. Not all chronic respiratory disease patients present in the same manner, but the connecting link is the damaged tissue of the lungs, causing issues with the functionality of the lungs. By adjunctively treating patients with PRP, the high concentration of platelets and their secreted growth factors can help induce an acceleration of healing and regeneration of pulmonary tissue. This, in turn, can slow the progression of the disease, which could lower the overall mortality rate in chronic respiratory disease patients. More studies should be conducted on this topic, specifically large, double-blinded, randomized human trials with controls, to further assess the efficacy and beneficial effects of PRP treatment on the lungs.

OUP accepted manuscript

Background

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are clinically severe respiratory disorders, and there are currently no Food and Drug Administration-approved drug therapies. It is of great interest to us that dimethyl fumarate (DMF) has been shown to have anti-inflammatory effects. The aim of this study was to investigate whether DMF could alleviate lipopolysaccharide(LPS)-induced ALI, and to explore its mechanism of action.

Materials and methods

We established a mice model of ALI with intratracheal instillation of LPS and intraperitoneal injection of DMF to treat ALI. The pathological damage and inflammatory response of lung tissues were observed by hematoxylin and eosin (H&E) staining, ELISA assay and western blot. ATP plus LPS was used for the establishment of ALI in vitro model, the therapeutic effects of DMF was explored by ELISA assay, RT-qPCR, western blot, and flow cytometry, and the therapeutic mechanisms of DMF was explored by administration of Brusatol (BT), a nuclear factor erythroid-2-related factor 2 (Nrf2) inhibitor.

Results

We found that intraperitoneal injection of DMF significantly reduced LPS-induced the pulmonary injury, pulmonary edema, and infiltration of inflammatory mediators. In LPS-induced ALI, NLRP3 inflammasome-mediated pyroptosis was markedly activated, followed by cleavage of caspase-1 and GSDMD. DMF inhibited the activation of the NLRP3 inflammasome and pyroptosis in both lung of ALI mice and ATP plus LPS-induced BEAS-2B cells. Mechanistically, DMF enhanced expressions of Nrf2, leading to inactivation of NLRP3 inflammasome and reduced pyroptosis in vivo and in vitro. Conversely, BT reduced the inhibitory effects of DMF on NLRP3 inflammasome and pyroptosis, and consequently blocked the improvement roles of DMF on ALI.

Conclusions

DMF could improve LPS-induced ALI via inhibiting NLRP3 inflammasome and pyroptosis, and that these effects were mediated by triggering Nrf2 expression, suggesting a therapeutic potential of DMF as an anti-inflammatory agent for ALI/ARDS treatment.

Lung Repair and Regeneration in ARDS: Role of PECAM1 and Wnt Signaling

ARDS is an acute inflammatory pulmonary process triggered by severe pulmonary and systemic insults to the alveolar-capillary membrane. This causes increased vascular permeability and the development of interstitial and alveolar protein-rich edema, leading to acute hypoxemic respiratory failure. Supportive treatment includes the use of lung-protective ventilatory strategies that decrease the work of breathing, can improve oxygenation, and minimize ventilator-induced lung injury. Despite substantial advances in supportive measures, there are no specific pharmacologic treatments for ARDS, and the overall hospital mortality rate remains about 40% in most series. The pathophysiology of ARDS involves interactions among multiple mechanisms, including immune cell infiltration, cytokine storm, alveolar-capillary barrier disruption, cell apoptosis, and the development of fibrosis. Here we review some new developments in the molecular basis of lung injury, with a focus on possible novel pharmacologic interventions aimed at improving the outcomes of patients with ARDS. Our focus is on platelet-endothelial cell adhesion molecule-1, which contributes to the maintenance and restoration of vascular integrity following barrier disruption. We also highlight the wingless-related integration site signaling pathway, which appears to be a central mechanism for lung healing as well as for fibrotic development.

Severity and Outcome of ARDS: The Present Place of Extracorporeal Lung Assist (ECLA)

Within the last decade extracorporeal lung assist has been recommended for the treatment of acute respiratory distress syndrome. However, this recommandation was challenged by several recent clinical studies and reviews. The goal of our analysis was therefore to investigate data on outcome and severity of gas exchange disturbance published from patients treated with ECLA. These data were compared to a historical control group consisting of ARDS patients treated conventionally. Computerized (MEDLINE 1967-95) literature search using the keywords ARDS, ECLA, ECMO, ECCO2R and HUMAN was performed. Only clinical studies published as full papers reporting data on both, patients mortality and oxygenation index (PaO2/FiO2) were included. Overall mean mortality reported was 53±22% in 17 studies (419 patients), with no apparent trend towards a higher survival within the last decade with a mean PaO2/FIO2 (14 papers; 61±17 mmHg). However, mean mortality rates of ARDS patients requiring ECLA was 52.3% and 44.9% if patients undergoing ECMO were excluded (3 papers). Therefore the mortality of these patients with severe lung injury was in the range of patients treated conventionally. Patient outcome observed in our analysis is in accordance with the mortality rates from the European ECLA centres published recently (49% in 1993). Therefore, we conclude that the mean mortality rate of patients suffering from severe ARDS treated with ECLA is in the 50% range and does not differ significantly from those of patients treated conventionally, despite significantly poorer pulmonary function.

Dynamics of Regional Lung Inflammation: New Questions and Answers Using PET

The meaning of the term ‘inflammation’ has undergone considerable evolution. It was originally defined around the year 25 A.D. by Aulus Cornelius Celsus [1] and described the body’s acute reaction following a traumatic event, such as a microscopic tear of a ligament or muscle. His original wording: “Notae vero inflammationis sunt quatour: rubor et tumor cum calore et dolore” (true signs of inflammation are four: redness and swelling with heat and pain) still holds. Disturbance of function (functio laesa) is the legendary fifth cardinal sign of inflammation and was added by Galen in the second century A.D. [2]. Recent articles [3] highlight the complicated role that inflammation plays in chronic illnesses, including metabolic, cardiovascular and neurodegenerative diseases. In addition to these difficult-to-treat diseases, more research and research tools are needed to illuminate therapeutic strategies in another difficulty-to-treat inflammatory malady, the acute respiratory distress syndrome (ARDS).

Acute respiratory distress syndrome 40 years later: time to revisit its definition

OBJECTIVE

Acute respiratory distress syndrome is a common disorder associated with significant mortality and morbidity. The aim of this article is to critically evaluate the definition of acute respiratory distress syndrome and examine the impact the definition has on clinical practice and research.

DATA SOURCES

Articles from a MEDLINE search (1950 to August 2007) using the Medical Subject Heading respiratory distress syndrome, adult, diagnosis, limited to the English language and human subjects, their relevant bibliographies, and personal collections, were reviewed.

DATA SYNTHESIS

The definition of acute respiratory distress syndrome is important to researchers, clinicians, and administrators alike. It has evolved significantly over the last 40 years, culminating in the American-European Consensus Conference definition, which was published in 1994. Although the American-European Consensus Conference definition is widely used, it has some important limitations that may impact on the conduct of clinical research, on resource allocation, and ultimately on the bedside management of such patients. These limitations stem partially from the fact that as defined, acute respiratory distress syndrome is a heterogeneous entity and also involve the reliability and validity of the criteria used in the definition. This article critically evaluates the American-European Consensus Conference definition and its limitations. Importantly, it highlights how these limitations may contribute to clinical trials that have failed to detect a potential true treatment effect. Finally, recommendations are made that could be considered in future definition modifications with an emphasis on the significance of accurately identifying the target population in future trials and subsequently in clinical care.

CONCLUSION

How acute respiratory distress syndrome is defined has a significant impact on the results of randomized, controlled trials and epidemiologic studies. Changes to the current American-European Consensus Conference definition are likely to have an important role in advancing the understanding and management of acute respiratory distress syndrome.

Prone ventilation in trauma or surgical patients with acute lung injury and adult respiratory distress syndrome: is it beneficial?

BACKGROUND

To compare the effectiveness of supine versus prone kinetic therapy in mechanically ventilated trauma and surgical patients with acute lung injury (ALI) and adult respiratory distress syndrome (ARDS).

METHODS

A retrospective review of all patients with ALI/ARDS who were placed on either a supine (roto-rest) or prone (roto-prone) oscillating bed was performed. Data obtained included age, revised trauma score (RTS), base deficit, Injury Severity Score (ISS), head Abbreviated Injury Scale score (AIS), chest (AIS), PaO2/FiO2 ratio, FiO2 requirement, central venous pressure (CVP), days on the bed, ventilator days, use of pressors, complications, mortality, and pulmonary-associated mortality. Data are expressed as mean+/-SE with significance attributed to p<0.05.

RESULTS

From March 1, 2004 through May 31, 2006, 4,507 trauma patients were admitted and 221 were identified in the trauma registry as having ALI or ARDS. Of these, 53 met inclusion criteria. Additionally, 8 general surgery patients met inclusion criteria. Of these 61 patients, 44 patients were positioned supine, 13 were placed prone, and 4 patients that were initially placed supine were changed to prone positioning. There was no difference between the groups in age, CVP, ISS, RTS, base deficit, head AIS score, chest AIS score, abdominal AIS score, or probability of survival. The PaO2/FiO2 ratios were not different at study entry (149 vs. 153, p=NS), and both groups showed improvement in PaO2/FiO2 ratios. However, the prone group had better PaO2/FiO2 ratios than the supine group by day 5 (243 vs. 200, p=0.066). The prone group had fewer days on the ventilator (13.6 vs. 24.2, p=0.12), and shorter hospital lengths of stay (22 days vs. 40 days, p=0.08). There were four patients who failed to improve with supine kinetic therapy that were changed to prone kinetic therapy. These patients had significant improvements in PaO2/FiO2 ratio, and significantly lower FiO2 requirements. There were 18 deaths (7 pulmonary related) in the supine group and 1 death in the prone group (p < 0.01 by chi test).

CONCLUSIONS

ALI/ARDS patients who received prone kinetic therapy had greater improvement in PaO2/FiO2 ratio, lower mortality, and less pulmonary-related mortality than did supine positioned patients. The use of a prone-oscillating bed appears advantageous for trauma and surgical patients with ALI/ARDS and a prospective, randomized trial is warranted.

The Role of CT-scan Studies for the Diagnosis and Therapy of Acute Respiratory Distress Syndrome

CT has provided new insights on the pathophysiology of acute respiratory distress syndrome (ARDS), demonstrating that ARDS does not affect the lung parenchyma homogeneously. These findings suggest that lung edema, as assessed by CT scan, should be included in the definition. Lung CT findings may provide a firm rationale for tailoring tidal volume during mechanical ventilation. Ideally, tidal volume should be proportional to the portion of the lung open to ventilation, as assessed by CT scan, rather than to the body weight. CT assessment of lung recruitability seems to be a prerequisite for a rational setting of positive end-expiratory pressure.

Visualizing lung function with positron emission tomography

Positron emission tomography (PET) provides three-dimensional images of the distributions of radionuclides that have been inhaled or injected into the lungs. By using radionuclides with short half-lives, the radiation exposure of the subject can be kept small. By following the evolution of the distributions of radionuclides in gases or compounds that participate in lung function, information about such diverse lung functions as regional ventilation, perfusion, shunt, gas fraction, capillary permeability, inflammation, and gene expression can be inferred. Thus PET has the potential to provide information about the links between cellular function and whole lung function in vivo. In this paper, recent advancements in PET methodology and techniques and information about lung function that have been obtained with these techniques are reviewed.

Conceptos actuales en la fisiopatología, monitorización y resolución del edema pulmonar

Pulmonary edema, both in its lesional as well as hydrostatic version, is a frequent cause of acute respiratory failure. From the pathophysiological point of view, the most important advance is undoubtedly the knowledge that the reabsorption process of pulmonary edema is an active process with energy consumption. This concept has revolutionized this field due to the possibility of finding substances or factors that stimulate or inhibit this reabsorption. Furthermore, in the monitoring field, significant advances have also been experimented due to the possibility of quantifying the edema in a simple and reliable way with transpulmonary thermodilution.

Development of a clinical definition for acute respiratory distress syndrome using the Delphi technique

PURPOSE

The objective of this study is to describe the implementation of formal consensus techniques in the development of a clinical definition for acute respiratory distress syndrome.

MATERIALS AND METHODS

A Delphi consensus process was conducted using e-mail. Sixteen panelists who were both researchers and opinion leaders were systematically recruited. The Delphi technique was performed over 4 rounds on the background of an explicit definition framework. Item generation was performed in round 1, item reduction in rounds 2 and 3, and definition evaluation in round 4. Explicit consensus thresholds were used throughout.

RESULTS

Of the 16 panelists, 11 actually participated in developing a definition that met a priori consensus rules on the third iteration. New incorporations in the Delphi definition include the use of a standardized oxygenation assessment and the documentation of either a predisposing factor or decreased thoracic compliance. The panelists rated the Delphi definition as acceptable to highly acceptable (median score, 6; range, 5-7 on a 7-point Likert scale).

CONCLUSIONS

We conclude that it is feasible to consider using formal consensus in the development of future definitions of acute respiratory distress syndrome. Testing of sensibility, reliability, and validity are needed for this preliminary definition; these test results should be incorporated into future iterations of this definition.

Causes and timing of death in patients with ARDS

BACKGROUND

Since the early 1980s, case fatality of patients with ARDS has decreased, and explanations are unclear.

DESIGN AND METHODS

Using identical definitions of ARDS and organ failure, we analyzed consecutive cohorts of patients meeting syndrome criteria at our institution in 1982 (n = 46), 1990 (n = 112), 1994 (n = 99), and 1998 (n = 205) to determine causes and timing of death.

RESULTS

Overall case fatality has decreased from 68% in 1981-1982 to a low of 29% in 1996, plateauing since the mid-1990s (p = 0.001 for trend). Sepsis syndrome with multiple organ failure remains the most common cause of death (30 to 50%), while respiratory failure causes a small percentage (13 to 19%) of deaths. The distribution of causes of death has not changed over time. There was no change in the timing of death during the study periods: 26 to 44% of deaths occurred early (< 72 h after ARDS onset), and 56 to 74% occurred late (> 72 h after ARDS onset). However, the increased survival over the past 2 decades is entirely accounted for by patients who present with trauma and other risk factors for their ARDS, while survival for those patients whose risk factor is sepsis has not changed. Additionally, withdrawal of life support in these patients is now occurring at our institution significantly more frequently than in the past, and median time until death has decreased in patients who have support withdrawn.

CONCLUSIONS

While these results do not explain the overall case fatality decline in ARDS, they do indicate that sepsis syndrome remains the leading cause of death and suggest that future therapies to improve survival be targeted at reducing the complications of sepsis.

Physiologic, biochemical, and imaging characterization of acute lung injury in mice

RATIONALE

Most models of acute lung injury in mice have yet to be fully characterized.

OBJECTIVES

To directly compare and contrast endotoxin and oleic acid models of acute lung injury in mice in terms of their physiologic, biochemical, histopathologic, and imaging manifestations.

METHODS

Survival studies, lung weights, x-ray computed tomographic scanning, light and electron microscopy, bronchoalveolar lavage, lung uptake of ((18)F)fluorodeoxyglucose, tissue myeloperoxidase, arterial blood gases, mean arterial pressure, and lung tissue prostanoids were measured in separate groups of C57Bl/6 mice (normal animals, endotoxin only [20 microg/g], oleic acid only [0.15 microl/g], or endotoxin + oleic acid).

RESULTS

Endotoxin alone caused only mild pulmonary neutrophilic inflammation with little functional or structural damage to the alveolar architecture. In contrast, oleic acid caused severe alveolar damage with the development of alveolar edema of the increased-permeability type with associated abnormalities in gas exchange. When given together, endotoxin and oleic acid acted synergistically to increase pulmonary edema and to worsen gas exchange and hemodynamics, thereby increasing mortality. This synergism was significantly attenuated by the prior administration of the endotoxin antagonist E5564 (eritoran).

CONCLUSIONS

Under the conditions of these studies, only mice exposed to oleic acid showed both structural and functional characteristics of acute lung injury. Nevertheless, endotoxin had potent synergistic physiologic effects that increased mortality. Overall, these models, which can be translated to genetically altered mice, are amenable to study with state-of-the-art imaging techniques, and with experimental interventions that can probe the underlying mechanisms of injury.

[Management of acute pulmonary failure: diagnostics-ventilation-withdrawal]

Acute pulmonary failure by definition excludes cardiac insufficiency as the pathogenetic mechanism involved in the development of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). The systemic inflammatory reaction underlying acute pulmonary failure has many etiological causes. One of the most important trigger mechanisms is sepsis. In the realm of cardiac intensive care medicine, the systemic inflammatory reaction is observed in conjunction with assist systems, during extracorporeal circulation, or in the course of cardiogenic shock. In the end, even mechanical ventilation itself can elicit an inflammatory reaction and result in pulmonary failure through ventilator-associated lung injury. Knowledge of the mechanisms has led to the concept of protective ventilation, which exerts both prophylactic and therapeutic effects. Protective ventilation is an integral part of a bundle of therapeutic intensive care measures. Both constitute the essence of management of acute pulmonary failure.

The Changing Face of Organ Failure in ARDS *

OBJECTIVE

To study morbidity and mortality in ARDS patients from 1987 to 1999.

DESIGN

Review of a prospectively collected database of ARDS patients.

SETTING

Large, community hospital located in Salt Lake City, UT.

PATIENTS

ARDS patients identified for the years 1987 to 1999. We prospectively identified ARDS patients at LDS Hospital in Salt Lake City, UT, using PaO(2)/fraction of inspired oxygen ratio (P/F) criteria, the presence of bilateral chest radiograph infiltrates, and the absence of left atrial hypertension.

MEASUREMENTS

We assigned a primary risk factor for ARDS and identified the presence of organ failure before and after ARDS. We compared two temporal groups (ie, 1987 to 1990 vs 1994 to 1999) and used two criteria of arterial hypoxemia (P/F: patients from 1994 to 1999, < or = 105 and < or = 173; patients from 1987 to 1990, < or = 0.2) At 1,500 m (the altitude of Salt Lake City), a PaO(2) of < or = 173 corresponds to an alveolar-arterial oxygen pressure difference of < or = 200 at sea level. We used death at hospital discharge as an end point.

MAIN RESULTS

We identified 516 ARDS patients with a P/F of < or = 105 (1987 to 1990, 256 patients; 1994 to 1999, 260 patients). Patients who had ARDS between 1994 and 1999 with a P/F of < or = 105 had a lower mortality rate than patients between 1987 and 1990 with a P/F of < or = 105 (44% vs 54%, respectively; p <.05). There were 288 patients with a P/F range of 106 to 173 during 1994 to 1999. Patients from 1994 to 1999 with a P/F of < or = 173 had a lower mortality rate compared to patients from 1987 to 1990 (35% vs 54%, respectively; p <.01). Patients from 1994 to 1999 (for both P/F groups) had statistically fewer total nonpulmonary organ failures (ie, more patients had zero organ failures or single organ failures) and fewer specific organ failures (ie, sepsis, cardiovascular failures, and CNS failures). There were statistically fewer cases of cardiovascular failure, sepsis, and in both periods (ie, prior to ARDS and after the onset of ARDS) for 1994-to-1999 patients with a P/F of < or = 105 compared to 1987-to-1990 patients with a P/F of < or = 105.

CONCLUSIONS

Mortality from ARDS has decreased and is associated with decreased organ failure prior to and during the course of ARDS.

Cephalad movement of endotracheal tubes caused by prone positioning pediatric patients with acute respiratory distress syndrome

OBJECTIVE

To test the hypothesis that prone positioning of patients with acute respiratory distress syndrome results in significant cephalad movement of their endotracheal tubes (ETT).

DESIGN

A retrospective review of chest radiographs and patient information.

SETTING

Pediatric intensive care unit of a children's hospital.

MEASUREMENTS AND MAIN RESULTS

Patients with acute respiratory distress syndrome had digital chest radiographs performed before and immediately after prone positioning as per our routine practice. Based on measurements of the length of the thoracic trachea and the length of the thoracic segment of the ETT, the movement of the ETT subsequent to prone positioning was calculated. Fifteen pairs of radiographs of 14 consecutive patients were evaluated. There were seven girls and seven boys, with ages ranging from 2 months to 18 yrs. All patients had a cephalad movement of their ETT ranging from 10% to 57% of their thoracic tracheal length (p < .001) associated with prone positioning. The mean amplitude of this movement was 34% +/- 16%, indicating that if the tip of the ETT is not deeper than one third of the thoracic tracheal length before prone positioning, it might slide into the cervical trachea as a result of this procedure.

CONCLUSIONS

Prone positioning results in cephalad movement of ETT within the trachea. The tip of the ETT should be deeper than one third of the total length of the thoracic trachea before prone positioning to prevent it from moving into the cervical trachea. When prone positioning is done with an ETT originally not deeper than one third of the thoracic trachea, obtaining a chest radiograph immediately after prone positioning is important to determine whether the ETT remained safely situated in the trachea.

Physiologic rationale for ventilator setting in acute lung injury/acute respiratory distress syndrome patients

OBJECTIVES

To review the physiologic approach to setting mechanical ventilation in acute lung injury/acute respiratory distress syndrome.

DATA SOURCES

MEDLINE search from 1979 to the present.

DATA SELECTION

Personal selection of some articles we believe relevant for understanding acute lung injury/acute respiratory distress syndrome physiopathology and its physiologic management.

DATA SUMMARY

Knowing the underlying pathology is key to estimating the potential for recruitment. The potential for recruitment is rather low when the consolidation of pulmonary units exceeds collapse, as in diffuse pneumonia. In contrast, when pulmonary unit collapse exceeds consolidation, as in acute lung injury/acute respiratory distress syndrome from extrapulmonary origin, the potential for recruitment may be high. To exploit the potential for recruitment, a transpulmonary pressure greater than the opening pressure must be applied to the lung. To do so, chest wall elastance must be measured or estimated. To avoid collapse after recruitment, a positive end-expiratory pressure greater than the compressive forces operating on the lung and an alveolar ventilation sufficient to prevent absorption atelectasis must be provided. Indeed, avoidance of stretch (low airway plateau pressure) and prevention of cyclic collapse and reopening (adequate positive end-expiratory pressure and alveolar ventilation) are the physiologic cornerstones of mechanical ventilation in acute lung injury/acute respiratory distress syndrome. When considering all the randomized clinical trials reported so far, it is tempting to speculate that transpulmonary pressure and stresses, rather than tidal volume per se, are the key factors that may have an impact on mortality.

CONCLUSIONS

The majority of physiologic, experimental, and clinical trial data converge on one simple concept: treat the lung gently.

Effect of endotoxin on oleic acid lung injury does not depend on priming

Recent studies have demonstrated significant synergistic physiological and biochemical effects between low-dose endotoxin (Etx) administration and oleic acid (OA)-induced canine lung injury. To evaluate whether this interaction depends on Etx priming of some key cell population, we compared the effects of giving low-dose Etx both after as well as before inducing lung injury with OA. In addition to hemodynamic and blood-gas measurements, positron emission tomographic imaging was used to measure edema accumulation and intrapulmonary blood flow distribution. Biochemical measurements of the stable metabolites of prostacyclin and thromboxane were obtained as well as measurements of isoprostanes and reactive sulfhydryls as evidence for possible concomitant oxidant production. We found that the physiological and biochemical effects of low-dose Etx developed 30-45 min after its administration, regardless of whether Etx was administered before or after OA. No increase in either isoprostane or reactive sulfhydryl production after Etx and/or OA was detected. These data suggest that the synergistic effect of low-dose Etx and OA-induced lung injury is not due to a priming effect of Etx.

Adult respiratory distress syndrome among blunt and penetrating trauma patients: demographics, mortality, and resource utilization over 8 years

PURPOSE

The purpose of this study was to compare demographics, resources used, and mortality rates among a subset of trauma patients (blunt versus penetrating) who developed adult respiratory distress syndrome (ARDS) and over two time periods, 8 years apart.

PATIENTS AND METHODS

This retrospective observational study was based on an analysis of the computerized Trauma Registry of the Shock Trauma Center of the University of Maryland Medical Systems. All trauma patients with ARDS admitted to a 48-bed trauma intensive care unit (ICU) at a Level I Trauma Center during two time periods of 3 consecutive years each were considered: from January 1, 1985, to December 31, 1987 (G1), and January 1, 1993, to December 31, 1995 (G2). Data were collected in the two time periods on demographics, severity (Revised Trauma Score), injury-admission delay time, first 24-hour transfusion fluids and blood, septic and organ system failure complications, life-support techniques, ICU mortality, and length of stay (LOS). The independent variables were the age, type of trauma, severity scores, transport time, fluid therapy, infectious and organ system failure complications, and life-support techniques. The dependent variable was ICU mortality.

RESULTS

A total of 1,108 patients satisfied the entry criteria, 486 in period G1 and 552 in period G2; 929 (89.5%) suffered blunt trauma and 109 (10.5%) penetrating trauma. Mean age was lower for the penetrating trauma populations. There were no significant differences in ICU mortality between the blunt and penetrating trauma patients with ARDS. A significant decrease in ICU mortality was observed between the time periods studied among both blunt (29.5% vs. 21.7%, P <.001) and penetrating trauma patients (25.7% vs. 18.9%, P <.001). A similar rate of renal, hematologic, and cardiovascular organ system failure was observed for both blunt and penetrating trauma patients. Resource utilization measured by ICU LOS and time on mechanical ventilation was also similar in the two groups. The multivariate analysis showed that ICU mortality was related to age, RTS-measured severity, and time period (G1 mortality 1.68-fold that in G2). Renal and hematologic failures entered a second analysis, replacing the time period variable (G2).

CONCLUSIONS

ARDS in patients with blunt and penetrating trauma showed similar trends in ICU mortality, complications, and resource utilization. The ARDS mortality rate decreased over the time period studied in both blunt and penetrating trauma.

A 9-year, single-institution, retrospective review of death rate and prognostic factors in adult respiratory distress syndrome

OBJECTIVE

To evaluate, at a single institution, the adult respiratory distress syndrome (ARDS) death rate in critically ill ventilated surgical/trauma patients and to identify the factors predicting death in these patients.

SUMMARY BACKGROUND DATA

The prognostic features affecting mortality at the onset of ARDS have not been clearly defined. Defining rare characteristics would be valuable because it would allow for better stratification of patients in clinical trials and more appropriate utilization of constrained resources in ICU environments.

METHODS

A retrospective analysis of 980 ventilated surgical and trauma intensive care unit patients from January 1990 to December 1998 was performed at Rhode Island Hospital. One hundred eleven adult intensive care unit patients with ARDS were identified using the criteria of Lung Injury Score more than 2.50 and the definition from the American-European Consensus Conference. Slightly more than half were trauma patients, 57% were men, and the median age was 59 years. The overall death rate was 52%. Patients were segregated by admission date to the intensive care unit (before or after January 1, 1995). Severity of illness was measured by the Revised Trauma Score for trauma patients and the Acute Physiology and Chronic Health Evaluation III for surgical patients. The Multiple Organ Dysfunction Score was determined on the day of onset of ARDS for all patients. Other recorded variables were age, sex, intensive care unit length of stay, length and mode of ventilation, presence or absence of tracheostomy, ventilation variables of peak and mean airway pressures, lung injury scores, elective versus emergency surgery, and presence or absence of pneumonia.

RESULTS

There was a significant decrease in the ARDS death rate from the period 1990 to 1994 to the period 1995 to 1998. The major reason for the decline was a reduction in the posttraumatic ARDS death rate. Lung-protective ventilation strategies were used more frequently in the second period than in the first, and the death rate was significantly decreased in trauma patients in the second period when lung-protective ventilation modes were used. Predictors of death at the onset of ARDS were advanced age, Multiple Organ Dysfunction Score of 8 or more, and Lung Injury Score of 2.76 or more.

CONCLUSION

In this single-institution series, the death rate from ARDS declined from 1990 to 1998, primarily in posttraumatic patients, and the decrease is related to the use of lung-protective ventilation strategies. Based on this patient population, the authors developed a statistical model to evaluate important prognostic indicators (advanced age, organ system and pulmonary dysfunction measurements) at the onset of ARDS.

Platelet-activating factor acetylhydrolases in health and disease

The platelet-activating factor (PAF) acetylhydrolases catalyze hydrolysis of the sn-2 ester bond of PAF and related pro-inflammatory phospholipids and thus attenuate their bioactivity. One secreted (plasma) and four intracellular isozymes have been described. The intracellular isozymes are distinguished by differences in primary sequence, tissue localization, subunit composition, and substrate preferences. The most thoroughly characterized intracellular isoform, Ib, is a G-protein-like complex with two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause Miller-Dieker lissencephaly. Isoform II is a single polypeptide that is homologous to the plasma PAF acetylhydrolase and has antioxidant activity in several systems. Plasma PAF acetylhydrolase is also a single polypeptide with a catalytic triad of amino acids that is characteristic of the alpha/beta hydrolases. Deficiency of this enzyme has been associated with a number of pathologies. The most common inactivating mutation, V279F, is found in >30% of randomly surveyed Japanese subjects (4% homozygous, 27% heterozygous). The prevalence of the mutant allele is significantly greater in patients with asthma, stroke, myocardial infarction, brain hemorrhage, and nonfamilial cardiomyopathy. Preclinical studies have demonstrated that recombinant plasma PAF acetylhydrolase can prevent or attenuate pathologic inflammation in a number of animal models. In addition, preliminary clinical results suggest that the recombinant enzyme may have pharmacologic potential in human inflammatory disease as well. These observations underscore the physiological importance of the PAF acetylhydrolases and point toward new approaches for controlling pathologic inflammation.

Acute respiratory distress syndrome: resource use and outcomes in 1985 and 1995, trends in mortality and comorbidities

PURPOSE

The purpose of this study was to compare resource consumption and mortality between (ARDS) patients with adult respiratory distress syndrome treated at our center in 1985 (45 patients) and those treated in 1995.

MATERIALS AND METHODS

This was a retrospective observational study, considering trauma and nontrauma ARDS separately. We recorded severity index scores (APACHE III), infectious complications and multiorgan failure, intensive care unit (ICU) resource consumption (TISS 28), length of stay, time on mechanical ventilation, and ICU mortality.

RESULTS

We found no variation in overall ARDS mortality and no reduction in mortality in the ARDS trauma group (43.5% in 1985 vs. 38.5% in 1995, not significant) but a significant increase in mortality among nontrauma septic ARDS patients (68.2% vs. 82.9%, P < .001), largely attributable to the new comorbidities of human immunodeficiency virus (HIV) infection and hematologic malignancy. TISS-28 showed an overall reduction over this time period (49.7 +/- 6.6 vs. 38.3 +/- 9.7, P < .001), due to fewer monitoring measures, particularly a lower use of pulmonary artery catheter. There were no overall changes in length of stay or days on mechanical ventilation between 1985 and 1995, but these variables did increase among the trauma subgroup.

CONCLUSION

In our setting, mortality remained constant from 1985 to 1995 among ARDS trauma patients but not among nontrauma ARDS patients because of the new case-mix of the latter population, which now includes HIV and other immunodepressed patients.

Effect of partial liquid ventilation on pulmonary vascular permeability and edema after experimental acute lung injury

We evaluated the effects of partial liquid ventilation (PLV) with two different dosages of the perfluorocarbon LiquiVent (perflubron) on pulmonary vascular permeability and edema formation after oleic acid (OA)-induced acute lung injury in dogs. We used imaging with positron emission tomography to measure fractional pulmonary blood flow, lung water concentration (LWC), and the pulmonary transcapillary escape rate (PTCER) of (68)Ga-labeled transferrin at 5 and 21 h after lung injury in five dogs undergoing conventional mechanical ventilation (CMV), five dogs undergoing low-dose PLV (perflubron at 10 ml/kg), and four dogs undergoing high dose PLV (perflubron at 30 ml/kg). A positive end-expiratory pressure of 7.5 cm H(2)O was used in all dogs. After OA (0.08 ml/kg)- induced lung injury, there were no significant differences or trends for PTCER or LWC at any time when the PLV groups were compared with the CMV group. However, lung tissue myeloperoxidase activity was significantly lower in the combined PLV group than in the CMV group (p = 0.016). We conclude that after OA-induced lung injury, the addition of PLV to CMV does not directly attenuate pulmonary vascular leak or lung water accumulation. Rather, the benefits of such treatment may be due to modifications of the inflammatory response.

Short-term effect of inhaled nitric oxide and prone positioning on gas exchange in patients with severe acute respiratory distress syndrome

OBJECTIVE

To compare the short-term effects of inhaled nitric oxide (NO) and prone positioning in improving oxygenation in acute respiratory distress syndrome (ARDS).

METHODS

Charts of consecutive ARDS patients (lung injury score >2) during a 2-yr period, tested for both inhaled NO and prone positioning efficacy were retrospectively reviewed. Variations in the Pao2/Fio2 ratio induced by inhaled NO and prone positioning were evaluated.

MEASUREMENTS AND MAIN RESULTS

Twenty-seven patients (age, 42+/-17 yrs) were included. Simplified Acute Physiology Score II was 45+/-14. Mortality rate in the intensive care unit was 63%. The causes of ARDS were pneumonia (n = 14), extra-lung infection (n = 5), and noninfectious systemic inflammatory response syndrome (n = 8). Lung injury score was 2.7+/-0.3. At baseline, before the initiation of inhaled NO, the Pao2/Fio2 ratio was 97+/-46 torr and before prone positioning, 92+/-26 torr. Variations in the Pao2/Fio2 ratio were lower at start of NO therapy (11+/-4 ppm) than that observed at prone positioning initiation (23+/-31 vs. 62+/-78 torr, p<.05). An increase in variations in the Pao2/Fio2 ratio of >15 torr was associated with prone positioning in 16 patients (59%) and with NO inhalation in 13 patients (48%) (not significant). An increase in variations in the Pao2/Fio2 ratio of >15 torr was associated with both techniques in only six patients (22%). There was no correlation between the response to prone positioning and the response to inhaled NO (r2 = .005; p = .73).

CONCLUSIONS

Prone positioning improves hypoxemia significantly better than does inhaled NO. The response to one technique is not predictive of the response to the other technique.

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.

Interobserver variability in applying a radiographic definition for ARDS

CONTEXT

Acute lung injury (ALI) and ARDS are currently defined by the American-European Consensus Conference (AECC) definition criteria, which contain a radiographic criterion. The accuracy or reliability of this consensus radiographic definition has not been evaluated, and no radiographic definition of ALI-ARDS has been evaluated by a large international group of experts.

OBJECTIVE

To study the interobserver variability in applying the AECC radiographic criterion for ALI-ARDS.

DESIGN

Survey.

PARTICIPANTS

A convenience sample of 21 experts selected from participants attending the 1997 Toronto Mechanical Ventilation Workshop and from members of the National Institutes of Health ARDS Network.

OUTCOME MEASURES

Participants reviewed 28 randomly selected chest radiograph from critically ill, hypoxemic (PaO(2)/fraction of inspired oxygen ratio, < 300) patients and decided whether the radiograph fulfilled the AECC definition for ALI-ARDS.

RESULTS

Interobserver agreement in applying the AECC definition for ALI-ARDS was moderate (kappa = 0.55; 95% confidence interval, 0.52 to 0.57). Thirteen radiographs (43%) showed nearly complete agreement (defined as 20 or 21 readers in agreement). Nine radiographs (32%) had more than or equal to five dissenting readers. The percentage of radiographs interpreted as consistent with ALI-ARDS by individual readers ranged from 36 to 71%. Participants commented that mild infiltrates, pleural effusions, atelectasis, isolated lower lobe involvement, radiographic technique, and overlying monitoring equipment posed the most difficulties.

CONCLUSIONS

The radiographic criterion used in the current AECC definition for ALI-ARDS showed high interobserver variability when applied by expert investigators in the fields of mechanical ventilation and ARDS. This variability may result in differences in ALI-ARDS populations at different clinical research centers and may make it difficult for clinicians to apply the results of clinical trials to their patients. Modifications to the radiographic criterion or annotated reference radiograph may improve the reliability of future definitions for ALI-ARDS.

Effects of mean airway pressure and tidal excursion on lung injury induced by mechanical ventilation in an isolated perfused rabbit lung model

OBJECTIVE

To study the relative contributions of mean airway pressure (mPaw) and tidal excursion (V(T)) to ventilator-induced lung injury under constant perfusion conditions.

DESIGN

Prospective, randomized study.

SETTING

Experimental animal laboratory.

SUBJECTS

Fifteen sets of isolated rabbit lungs.

INTERVENTIONS

Rabbit lungs were perfused (constant flow, 500 mL/min; capillary pressure, 10 mm Hg) and randomized to be ventilated at identical peak transpulmonary pressure (pressure control ventilation [30 cm H2O and frequency of 20/min]) with three different ventilatory patterns that differed from each other by either mPaw or V(T): group A (low mPaw [13.4+/-0.2 cm H2O]/large V(T) [55+/-8 mL], n = 5); group B (high mPaw [21.2+/-0.2 cm H2O]/small V(T) [18+/-1 mL], n = 5); and group C (high mPaw [21.8+/-0.5 cm H2O]/large V(T) [53+/-5 mL], n = 5).

MEASUREMENTS AND MAIN RESULTS

Continuous weight gain (edema formation), change in ultrafiltration coefficient (deltaKf, vascular permeability index), and histology (lung hemorrhage) were examined. In group A, deltaKf (0.08+/-0.08 g/min/cm H2O/100 g) was less than in group B (0.28+/-0.19 g/min/cm H2O/100 g) or group C (0.41+/-0.29 g/min/cm H2O/100 g) (p = .05). Group A experienced significantly less hemorrhage (histologic score, 5.4+/-2.2) than groups B (10.3+/-2.1) and C (11.1+/-3.0) (p < .05). A similar trend was observed for weight gain. In contrast to tidal excursion, mPaw was found to be a significant factor for lung hemorrhage and increased Kf (two-way analysis of variance; p < .05). Weight gain (r2 = .54, p = .04) and lung hemorrhage (r2 = .65, p = .01) correlated with the mean pulmonary artery pressure changes that resulted from the implementation of the ventilatory strategies. The difference between the changes in mPaw and mean pulmonary artery pressure linearly predicted deltaKf (p = .005 and .05, respectively, r2 = 0.73).

CONCLUSIONS

Under these experimental conditions, mPaw contributes more than tidal excursion to lung hemorrhage and permeability alterations induced by mechanical ventilation.

Neuropsychological sequelae and impaired health status in survivors of severe acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a disease of acute respiratory failure manifested by severe hypoxemia with a high mortality rate. Previous outcome studies of ARDS have assessed survival and/or pulmonary function as the primary outcome variables. Cognitive or psychological outcomes following ARDS have not been described, despite the possibility that ARDS patients are at risk for brain injury through hypoxemia or other mechanisms. In the current study 55 consecutive ARDS survivors completed a battery of neuropsychological tests and questionnaires regarding health status, cognitive and psychological outcomes at the time of hospital discharge and 1 yr after onset of ARDS. At hospital discharge, 100% (55 of 55) of survivors exhibited cognitive and affective impairments, as well as problems with health status which affected their quality of life. At 1 yr after ARDS, 17 of 55 (30%) patients still exhibited generalized cognitive decline. Forty-three of 55 (78%) patients had all or at least one of the following: impaired memory, attention, concentration and/or decreased mental processing speed. One year after ARDS a substantial portion of ARDS survivors exhibit impaired health status and cognitive sequelae which may be due to hypoxemia, emboli, inflammation, drug toxicity, and/or other etiologies.

Incidence and mortality after acute respiratory failure and acute respiratory distress syndrome in Sweden, Denmark, and Iceland. The ARF Study Group

To determine the incidence and 90-d mortality of acute respiratory failure (ARF), acute lung injury (ALI), and the acute respiratory distress syndrome (ARDS), we carried out an 8-wk prospective cohort study in Sweden, Denmark, and Iceland. All intensive care unit (ICU) admissions (n = 13,346) >/= 15 yr of age were assessed between October 6th and November 30th, 1997 in 132 of 150 ICUs with resources to treat patients with intubation and mechanical ventilation (I + MV) >/= 24 h. ARF was defined as I + MV >/= 24 h. ALI and ARDS were defined using criteria recommended by the American-European Consensus Conference on ARDS. Calculation to correct the incidence for unidentified subjects from nonparticipating ICUs was made. No correction for in- or out-migration from the study area was possible. The population in the three countries >/= 15 yr of age was 11.74 million. One thousand two hundred thirty-one ARF patients were included, 287 ALI and 221 ARDS patients were identified. The incidences were for ARF 77.6, for ALI 17.9, and for ARDS 13.5 patients per 100,000/yr. Ninety-day mortality was 41.0% for ARF, including ALI and ARDS patients, 42.2% for ALI not fulfilling ARDS criteria, and 41.2% for ARDS.

The evaluation of lung function with PET

In many ways, the lung is an ideal organ for study with positron emission tomography (PET). First, structure-function relations are homogeneous over larger areas than in other organs (reducing problems associated with otherwise relatively poor spatial resolution and partial-volume averaging). Second, many physiologic and metabolic processes can be studied, including pulmonary blood flow, ventilation, vascular permeability, endothelial receptor and enzyme function, among others. A variety of radiotracers have been used to evaluate pulmonary blood flow with PET, including 68Ga- or 11C-albumin microspheres administered intravenously, H2 15O administered by i.v. infusion, and 13N-N2 administered by inhalation. Pulmonary ventilation has been evaluated with both 13N-N2 and 19Ne gas, also administered by inhalation. In general, the relative advantage of one approach over another depends on site-specific cyclotron capacity and experience, and on the nature and timing of concomitant studies with other positron-emitting radiopharmaceuticals. The various blood flow methods have been used primarily in studies of pulmonary gas exchange, in both experimental animals and in humans. Acute lung injury is usually defined by both an increase in extravascular water (pulmonary edema) and an increase in the permeability of the pulmonary endothelium to protein. Both processes can easily be evaluated with PET. Extravascular water is measured by a combination of scans with i.v. H2 15O and C15O. The latter is administered by inhalation to label the blood pool (to calculate intravascular water concentrations). Pulmonary vascular permeability has been evaluated with dynamic sequential imaging after either 68Ga-transferrin or 11C-methylalbumin infusions. The rate of uptake of either tracer into the pulmonary extravascular space is an index of "leakiness" of the pulmonary endothelium, and is quantified as the pulmonary transcapillary escape rate, or PTCER. PTCER appears to be a highly sensitive index of acute lung injury. Two receptor/ enzyme systems that have been evaluated include the beta-adrenergic receptor system (using 11CGP-12177 as the ligand) and angiotensin converting enzyme (using 18F-fluorocaptopril). In each case, the object is to measure Bmax, or the maximum binding-capacity for the ligand in question. Changes in Bmax can be used to infer changes in protein expression of the receptor or enzyme, or can be used to quantify adequacy of therapy with inhibitor drugs. Given the highly active nature of the pulmonary endothelium, it is likely that many other pulmonary receptor or enzyme systems can be studied in a similar fashion.

Mechanical ventilation of patients with acute lung injury

Ventilatory management of patients with acute lung injury (ALI), particularly its most severe subset, acute respiratory distress syndrome (ARDS), is complex. Newer lung protective strategies emphasize measures to enhance alveolar recruitment and avoid alveolar overdistention, thus minimizing the risk of ventilator-induced lung injury (VILI). Key components of such strategies include the use of smaller-than-conventional tidal volumes which maintain peak transpulmonary pressure below the pressure associated with overdistention, and titration of positive end-expiratory pressure to promote maximal alveolar recruitment. Novel techniques, including prone positioning, inverse ratio ventilation, tracheal gas insufflation, and high frequency ventilation, are considerations in severe ARDS. No single approach is best for all patients; adjustment of ventilatory parameters to individual characteristics, such as lung mechanics and gas exchange, is required.