Comparison of the response to the prone position between pulmonary and extrapulmonary acute respiratory distress syndrome

Translational medicine for acute lung injury

Acute lung injury (ALI) is a complex disease with numerous causes. This review begins with a discussion of disease development from direct or indirect pulmonary insults, as well as varied pathogenesis. The heterogeneous nature of ALI is then elaborated upon, including its epidemiology, clinical manifestations, potential biomarkers, and genetic contributions. Although no medication is currently approved for this devastating illness, supportive care and pharmacological intervention for ALI treatment are summarized, followed by an assessment of the pathophysiological gap between human ALI and animal models. Lastly, current research progress on advanced nanomedicines for ALI therapeutics in preclinical and clinical settings is reviewed, demonstrating new opportunities towards developing an effective treatment for ALI.

Prone positioning may increase lung overdistension in COVID-19-induced ARDS

Real-time effects of changing body position and positive end-expiratory pressure (PEEP) on regional lung overdistension and collapse in individual patients remain largely unknown and not timely monitored. The aim of this study was to individualize PEEP in supine and prone body positions seeking to reduce lung collapse and overdistension in mechanically ventilated patients with coronavirus disease (COVID-19)-induced acute respiratory distress syndrome (ARDS). We hypothesized that prone positioning with bedside titrated PEEP would provide attenuation of both overdistension and collapse. In this prospective observational study, patients with COVID-19-induced ARDS under mechanical ventilation were included. We used electrical impedance tomography (EIT) with decremental PEEP titration algorithm (PEEP_{EIT-titration}), which provides information on regional lung overdistension and collapse, along with global respiratory system compliance, to individualize PEEP and body position. PEEP_{EIT-titration} in supine position followed by PEEP_{EIT-titration} in prone position were performed. Immediately before each PEEP_{EIT-titration}, the same lung recruitment maneuver was performed: 2 min of PEEP 24 cmH₂O and driving pressure of 15 cmH₂O. Forty-two PEEP_{EIT-titration} were performed in ten patients (21 pairs supine and prone positions). We have found larger % of overdistension along the PEEP titration in prone than supine position (P = 0.042). A larger % of collapse along the PEEP titration was found in supine than prone position (P = 0.037). A smaller respiratory system compliance was found in prone than supine position (P < 0.0005). In patients with COVID-19-induced ARDS, prone body position, when compared with supine body position, decreased lung collapse at low PEEP levels, but increased lung overdistension at PEEP levels greater than 10 cm H₂O.

Trial registration number: NCT04460859.

The Association Between Etiologies and Mortality in Acute Respiratory Distress Syndrome: A Multicenter Observational Cohort Study

Background: Lung-protective ventilation (LPV) strategies have been beneficial in patients with acute respiratory distress syndrome (ARDS). As a vital part of LPV, positive end-expiratory pressure (PEEP) can enhance oxygenation. However, randomized clinical trials of different PEEP strategies seem to show no advantages in clinical outcomes in patients with ARDS. A potential reason is that diverse etiologies and phenotypes in patients with ARDS may account for different PEEP responses, resulting in variations in mortality. We consider hospital mortality to be associated with a more specific classification of ARDS, such as sepsis induced or not, and pulmonary or extrapulmonary one. Our study aimed to compare clinical outcomes in various patients with ARDS by etiologies using the China Critical Care Sepsis Trial (CCCST) database. This was a retrospective analysis of a prospective cohort of 2,138 patients with ARDS in the CCCST database. According to ARDS induced by sepsis or not and medical history, patients were stratified into different four groups. Differences among groups were assessed in hospital mortality, ventilation-free days, and other clinical features.

Results: A total of 2,138 patients with ARDS were identified in the database, including 647 patients with sepsis-induced pulmonary ARDS (30.3%), 396 patients with sepsis-induced extrapulmonary ARDS (18.5%), 536 patients with non-sepsis pulmonary ARDS (25.1%), and 559 patients with non-sepsis extrapulmonary ARDS (26.1%). The pulmonary ARDS group had higher mortality compared with the extrapulmonary group (45.9 vs. 23.0%, p < 0.01), longer intensive care unit (ICU) and hospital stays (9 vs. 6 days, p < 0.01, 20 vs. 18 days, p = 0.01, respectively), and fewer ventilation-free days (5 vs. 9 days) in the presence of sepsis. However, the mortality in ARDS without sepsis was inverted compared with extrapulmonary ARDS (pulmonary 23.5% vs. extrapulmonary 29.2%, p = 0.04). After adjusting for the Acute Physiology and Chronic Health Evaluation II and sequential organ failure assessment scores and other clinical features, the sepsis-induced pulmonary condition was still a risk factor for death in patients with ARDS (hazard ratio 0.66, 95% CI, 0.54–0.82, p < 0.01) compared with sepsis-induced extrapulmonary ARDS and other subphenotypes.

Conclusions: In the presence of sepsis, hospital mortality in pulmonary ARDS is higher compared with extrapulmonary ARDS; however, mortality is inverted in ARDS without sepsis. Sepsis-induced pulmonary ARDS should attract more attention from ICU physicians and be cautiously treated.

Trial registration: ChiCTR-ECH-13003934. Registered August 3, 2013, http://www.chictr.org.cn.

Association between ARDS Etiology and Risk of Noninvasive Ventilation Failure

RATIONALE

The etiology of acute respiratory distress syndrome (ARDS) may play an important role in the failure of noninvasive ventilation (NIV).

OBJECTIVES

To explore the association between ARDS etiology and risk of noninvasive ventilation failure.

METHODS

A multicenter prospective observational study was performed in 17 ICUs in China from September 2017 to December 2019. Patients with ARDS who used NIV as a first-line therapy were enrolled. The etiology of ARDS was recorded at study entry.

RESULTS

A total of 306 patients were enrolled. Of the patients, 146 were classified as having pulmonary ARDS (ARDSp) and 160 were classified as having extrapulmonary ARDS (ARDSexp). From initiation to 24 h of NIV, the respiratory rate, heart rate, PaO2/FiO2, and PaCO2 improved slower in patients with ARDSp than those with ARDSexp. Patients with ARDSp experienced more NIV failure (55% vs. 28%; p < 0.01) and higher 28-day mortality (47% vs. 14%; p < 0.01). The adjusted odds ratio of NIV failure and 28-day mortality were 5.47 (95%CI: 3.04-9.86) and 10.13 (95%CI: 5.01-20.46), respectively. In addition, we combined the presence of ARDSp, presence of septic shock, age, non-pulmonary SOFA score, respiratory rate at 1-2 h of NIV, and PaO2/FiO2 at 1-2 h of NIV to develop a risk score of NIV failure. With the increase of the risk score, the rate of NIV failure increased. Using 5.5 as cutoff value to predict NIV failure, the sensitivity and specificity was good both in training and validation cohorts.

CONCLUSIONS

Among patients with ARDS who used NIV as a first-line therapy, ARDSp was associated with slower improvement, more NIV failure, and higher 28-day mortality than ARDSexp. The risk score combined presence of ARDSp, presence of septic shock, age, non-pulmonary SOFA score, respiratory rate at 1-2 h of NIV, and PaO2/FiO2 at 1-2 h of NIV has high accuracy to predict NIV failure among ARDS population.

Prognostic Factors to Predict ICU Mortality in Patients with Severe ARDS Who Received Early and Prolonged Prone Positioning Therapy

Early and prolonged prone positioning (PP) therapy improve survival in advanced ARDS; however, the predictors of mortality remain unclear. The study aims to identify predictive factors correlated with mortality and build-up the prognostic score in patients with severe ARDS who received early and prolonged PP therapy. A total of 116 patients were enrolled in this retrospective cohort study. Univariate and multivariate regression models were used to estimate the odds ratio (OR) of mortality. Factors associated with mortality were assessed by Cox regression analysis and presented as the hazard ratio (HR) and 95% CI. In the multivariate regression model, renal replacement therapy (RRT; OR: 4.05, 1.54–10.67), malignant comorbidity (OR: 8.86, 2.22–35.41), and non-influenza-related ARDS (OR: 5.17, 1.16–23.16) were significantly associated with ICU mortality. Age, RRT, non-influenza-related ARDS, malignant comorbidity, and APACHE II score were included in a composite prone score, which demonstrated an area under the curve of 0.816 for predicting mortality risk. In multivariable Cox proportional hazard model, prone score more than 3 points was significantly associated with ICU mortality (HR: 2.13, 1.12–4.07, p = 0.021). We suggest prone score ≥3 points could be a good predictor for mortality in severe ARDS received PP therapy.

The efficacy and tolerance of prone positioning in non-intubation patients with acute hypoxemic respiratory failure and ARDS: a meta-analysis

Background and aims:

The application of prone positioning with acute hypoxemic respiratory failure (AHRF) or acute respiratory distress syndrome (ARDS) in non-intubation patients is increasing gradually, applying prone positioning for more high-flow nasal oxygen therapy (HFNC) and non-invasive ventilation (NIV) patients. This meta-analysis evaluates the efficacy and tolerance of prone positioning combined with non-invasive respiratory support in patients with AHRF or ARDS.

Methods:

We searched randomized controlled trials (RCTs) (prospective or retrospective cohort studies, RCTs and case series) published in PubMed, EMBASE and the Cochrane Central Register of Controlled Trials from 1 January 2000 to 1 July 2020. We included studies that compared prone and supine positioning with non-invasive respiratory support in awake patients with AHRF or ARDS. The meta-analyses used random effects models. The methodological quality of the RCTs was evaluated using the Newcastle–Ottawa quality assessment scale.

Results:

A total of 16 studies fulfilled selection criteria and included 243 patients. The aggregated intubation rate and mortality rate were 33% [95% confidence interval (CI): 0.26–0.42, I² = 25%], 4% (95% CI: 0.01–0.07, I² = 0%), respectively, and the intolerance rate was 7% (95% CI: 0.01–0.12, I² = 5%). Prone positioning increased PaO₂/FiO₂ [mean difference (MD) = 47.89, 95% CI: 28.12–67.66; p < 0.00001, I² = 67%] and SpO₂ (MD = 4.58, 95% CI: 1.35–7.80, p = 0.005, I² = 97%), whereas it reduced respiratory rate (MD = −5.01, 95% CI: −8.49 to −1.52, p = 0.005, I² = 85%). Subgroup analyses demonstrated that the intubation rate of shorter duration prone (⩽5 h/day) and longer duration prone (>5 h/day) were 34% and 21%, respectively; and the mortality rate of shorter duration prone (⩽5 h/day) and longer duration prone (>5 h/day) were 6% and 0%, respectively. PaO₂/FiO₂ and SpO₂ were significantly improved in COVID-19 patients and non-COVID-19 patients.

Conclusion:

Prone positioning could improve the oxygenation and reduce respiratory rate in both COVID-19 patients and non-COVID-19 patients with non-intubated AHRF or ARDS.

The reviews of this paper are available via the supplemental material section.

Duration of prone position sessions: a prospective cohort study

Background

Prone position (PP) is highly recommended in moderate-to-severe ARDS. However, the optimal duration of PP sessions remains unclear. We searched to evaluate the time required to obtain the maximum physiological effect, and to search for parameters related to patient survival in PP.

Methods and results

It was a prospective, monocentric, physiological study. We included in the study all prone-positioned patients in our ICU between June 2016 and January 2018. Pulmonary mechanics, data from volumetric capnography and arterial blood gas were recorded before prone positioning, 2 h after proning, before return to a supine position (SP) and 2 h after return to SP. Dynamic parameters were recorded before proning and every 30 min during the session until 24 h. 103 patients (ARDS 95%) were included performing 231 PP sessions with a mean length of 21.5 ± 5 h per session. They presented a significant increase in pH, static compliance and P_aO₂/F_iO₂ with a significant decrease in P_aCO₂, P_plat, phase 3 slope of the volumetric capnography, P_etCO₂, V_D/V_T-phy and ΔP. The beneficial physiological effects continued after 16 h of PP and at least up to 24 h in some patients. The evolution of the respiratory parameters during the first session and also during the pooled sessions did not find any predictor of response to PP, whether before, during or 2 h after the return in SP.

Conclusions

PP sessions should be prolonged at least 24 h and be extended in the event that the P_aO₂/F_iO₂ ratio at 24 h remains below 150, especially since no criteria can predict which patient will benefit or not from it.

Trial registration The trial has been registered on 28 June 2016 in ClinicalTrials.gov (NCT 02816190) (https://clinicaltrials.gov/ct2/show/NCT02816190?term=propocap&rank=1).

Molecular Dynamics of Lipopolysaccharide-Induced Lung Injury in Rodents

Acute respiratory distress syndrome (ARDS) is a common disease entity in critical care medicine and is still associated with a high mortality. Because of the heterogeneous character of ARDS, animal models are an insturment to study pathology in relatively standardized conditions. Rodent models can bridge the gap from in vitro investigations to large animal and clinical trials by facilitating large sample sizes under physiological conditions at comparatively low costs. One of the most commonly used rodent models of acute lung inflammation and ARDS is administration of lipopolysaccharide (LPS), either into the airways (direct, pulmonary insult) or systemically (indirect, extra-pulmonary insult). This narrative review discusses the dynamics of important pathophysiological pathways contributing to the physiological response to LPS-induced injury. Pathophysiological pathways of LPS-induced lung injury are not only influenced by the type of the primary insult (e.g., pulmonary or extra-pulmonary) and presence of additional stimuli (e.g., mechanical ventilation), but also by time. As such, findings in animal models of LPS-induced lung injury may depend on the time point at which samples are obtained and physiological data are captured. This review summarizes the current evidence and highlights uncertainties on the molecular dynamics of LPS-induced lung injury in rodent models, encouraging researchers to take accurate timing of LPS-induced injury into account when designing experimental trials.

Differences Between Pulmonary and Extrapulmonary Pediatric Acute Respiratory Distress Syndrome: A Multicenter Analysis

OBJECTIVES

Extrapulmonary pediatric acute respiratory distress syndrome and pulmonary pediatric acute respiratory distress syndrome are poorly described in the literature. We aimed to describe and compare the epidemiology, risk factors for mortality, and outcomes in extrapulmonary pediatric acute respiratory distress syndrome and pulmonary pediatric acute respiratory distress syndrome.

DESIGN

This is a secondary analysis of a multicenter, retrospective, cohort study. Data on epidemiology, ventilation, therapies, and outcomes were collected and analyzed. Patients were classified into two mutually exclusive groups (extrapulmonary pediatric acute respiratory distress syndrome and pulmonary pediatric acute respiratory distress syndrome) based on etiologies. Primary outcome was PICU mortality. Cox proportional hazard regression was used to identify risk factors for mortality.

SETTING

Ten multidisciplinary PICUs in Asia.

PATIENTS

Mechanically ventilated children meeting the Pediatric Acute Lung Injury Consensus Conference criteria for pediatric acute respiratory distress syndrome between 2009 and 2015.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Forty-one of 307 patients (13.4%) and 266 of 307 patients (86.6%) were classified into extrapulmonary pediatric acute respiratory distress syndrome and pulmonary pediatric acute respiratory distress syndrome groups, respectively. The most common causes for extrapulmonary pediatric acute respiratory distress syndrome and pulmonary pediatric acute respiratory distress syndrome were sepsis (82.9%) and pneumonia (91.7%), respectively. Children with extrapulmonary pediatric acute respiratory distress syndrome were older, had higher admission severity scores, and had a greater proportion of organ dysfunction compared with pulmonary pediatric acute respiratory distress syndrome group. Patients in the extrapulmonary pediatric acute respiratory distress syndrome group had higher mortality (48.8% vs 24.8%; p = 0.002) and reduced ventilator-free days (median 2.0 d [interquartile range 0.0-18.0 d] vs 19.0 d [0.5-24.0 d]; p = 0.001) compared with the pulmonary pediatric acute respiratory distress syndrome group. After adjusting for site, severity of illness, comorbidities, multiple organ dysfunction, and severity of acute respiratory distress syndrome, extrapulmonary pediatric acute respiratory distress syndrome etiology was not associated with mortality (adjusted hazard ratio, 1.56 [95% CI, 0.90-2.71]).

CONCLUSIONS

Patients with extrapulmonary pediatric acute respiratory distress syndrome were sicker and had poorer clinical outcomes. However, after adjusting for confounders, it was not an independent risk factor for mortality.

Plasma microRNAs levels are different between pulmonary and extrapulmonary ARDS patients: a clinical observational study

BACKGROUND

Mesenchymal stem cells (MSC) obviously alleviate the damage of the structure and function of pulmonary vascular endothelial cells (VEC). The therapeutic effects of MSC are significantly different between pulmonary ARDS (ARDSp) and extrapulmonary ARDS (ARDSexp). MicroRNAs (miRNAs), as important media of MSC regulating VEC, are not studied between ARDSp and ARDSexp. We aimed to explore the plasma levels difference of miRNAs that regulate VEC function and are associated with MSC (MSC-VEC-miRNAs) between ARDSp and ARDSexp patients.

METHODS

MSC-VEC-miRNAs were obtained through reviewing relevant literatures screened in PubMed database. We enrolled 57 ARDS patients within 24 h of admission to the ICU and then collected blood samples, extracted plasma supernatant. Patients' clinical data were collected. Then, plasma expression of MSC-VEC-miRNAs was measured by real-time fluorescence quantitative PCR. Simultaneously, plasma endothelial injury markers VCAM-1, vWF and inflammatory factors TNF-α, IL-10 were detected by ELISA method.

RESULTS

Fourteen miRNAs were picked out after screening. A total of 57 ARDS patients were included in this study, among which 43 cases pertained to ARDSp group and 14 cases pertained to ARDSexp group. Plasma miR-221 and miR-27b levels in ARDSexp group exhibited significantly lower than that in ARDSp group (miR-221, 0.22 [0.12-0.49] vs. 0.57 [0.22-1.57], P = 0.008, miR-27b, 0.34 [0.10-0.46] vs. 0.60 [0.20-1.46], P = 0.025). Plasma vWF concentration in ARDSexp group exhibited significantly lower than that in ARDSp group (0.77 [0.29-1.54] vs. 1.80 [0.95-3.51], P = 0.048). Significant positive correlation was found between miR-221 and vWF in plasma levels (r = 0.688, P = 0.022). Plasma miR-26a and miR-27a levels in non-survival group exhibited significantly lower than that in survival group (miR-26a, 0.17 [0.08-0.20] vs. 0.69 [0.24-2.33] P = 0.018, miR-27a, 0.23 [0.16-0.58] vs. 1.45 [0.38-3.63], P = 0.021) in ARDSp patients.

CONCLUSION

Plasma miR-221, miR-27b and vWF levels in ARDSexp group are significantly lower than that in ARDSp group. Plasma miR-26a and miR-27a levels in non-survival group are significantly lower than that in survival group in ARDSp patients.

Effect of Manual Hyperinflation on Hemodynamics, Gas Exchange, and Respiratory Mechanics in Ventilated Patients

The authors investigated the effect of manual hyperinflation (MHI) with set parameters applied to patients on mechanical ventilation on hemodynamics, respiratory mechanics, and gas exchange. Sixteen critically ill patients post-septic shock, with acute lung injury, were studied. Heart rate, arterial pressure, and mean pulmonary artery pressure were recorded every minute. Pulmonary artery occlusion pressure, cardiac output, arterial blood gases, and dynamic compliance (Cdyn) were recorded pre- and post-MHI. From this, systemic vascular resistance index (SVRI), cardiac index, oxygen delivery, and partial pressure of oxygen: fraction of inspired oxygen (PaO2:FiO2) ratio were calculated. There were significant increases in SVRI ( P < 0.05) post-MHI and diastolic arterial pressure ( P < 0.01) during MHI. Cdyn increased post-MHI ( P < 0.01) and was sustained at 20 minutes post-MHI ( P < 0.01). Subjects with an intrapulmonary cause of lung disease had a significant decrease ( P = 0.02) in PaO2:FiO2, and those with extrapulmonary causes of lung disease had a significant increase ( P < 0.001) in PaO2:FiO2 post-MHI. In critically ill patients, MHI resulted in an improvement in lung mechanics and an improvement in gas exchange in patients with lung disease due to extrapulmonary events and did not result in impairment of the cardiovascular system.

Efficacy of prone position in acute respiratory distress syndrome patients: A pathophysiology-based review

Acute respiratory distress syndrome (ARDS) is a syndrome with heterogeneous underlying pathological processes. It represents a common clinical problem in intensive care unit patients and it is characterized by high mortality. The mainstay of treatment for ARDS is lung protective ventilation with low tidal volumes and positive end-expiratory pressure sufficient for alveolar recruitment. Prone positioning is a supplementary strategy available in managing patients with ARDS. It was first described 40 years ago and it proves to be in alignment with two major ARDS pathophysiological lung models; the "sponge lung" - and the "shape matching" -model. Current evidence strongly supports that prone positioning has beneficial effects on gas exchange, respiratory mechanics, lung protection and hemodynamics as it redistributes transpulmonary pressure, stress and strain throughout the lung and unloads the right ventricle. The factors that individually influence the time course of alveolar recruitment and the improvement in oxygenation during prone positioning have not been well characterized. Although patients' response to prone positioning is quite variable and hard to predict, large randomized trials and recent meta-analyses show that prone position in conjunction with a lung-protective strategy, when performed early and in sufficient duration, may improve survival in patients with ARDS. This pathophysiology-based review and recent clinical evidence strongly support the use of prone positioning in the early management of severe ARDS systematically and not as a rescue maneuver or a last-ditch effort.

Abdominal Circulatory Interactions

The abdominal compartment is separated from the thoracic compartment by the diaphragm. Under normal circumstances, a large portion of the venous return crosses the splanchnic and nonsplanchnic abdominal regions before entering the thorax and the right side of the heart. Mechanical ventilation may affect abdominal venous return independent of its interactions at the thoracic level. Changes in pressure in the intra-abdominal compartment may have important implications for organ function within the thorax, particularly if there is a sustained rise in intra-abdominal pressure. It is important to understand the consequences of abdominal pressure changes on respiratory and circulatory physiology. This article elucidates important abdominal-respiratory-circulatory interactions and their clinical effects.

S2e guideline: positioning and early mobilisation in prophylaxis or therapy of pulmonary disorders : Revision 2015: S2e guideline of the German Society of Anaesthesiology and Intensive Care Medicine (DGAI)

The German Society of Anesthesiology and Intensive Care Medicine (DGAI) commissioneda revision of the S2 guidelines on "positioning therapy for prophylaxis or therapy of pulmonary function disorders" from 2008. Because of the increasing clinical and scientificrelevance the guidelines were extended to include the issue of "early mobilization"and the following main topics are therefore included: use of positioning therapy and earlymobilization for prophylaxis and therapy of pulmonary function disorders, undesired effects and complications of positioning therapy and early mobilization as well as practical aspects of the use of positioning therapy and early mobilization. These guidelines are the result of a systematic literature search and the subsequent critical evaluation of the evidence with scientific methods. The methodological approach for the process of development of the guidelines followed the requirements of evidence-based medicine, as defined as the standard by the Association of the Scientific Medical Societies in Germany. Recently published articles after 2005 were examined with respect to positioning therapy and the recently accepted aspect of early mobilization incorporates all literature published up to June 2014.

Clinical and biological heterogeneity in acute respiratory distress syndrome: direct versus indirect lung injury

The acute respiratory distress syndrome (ARDS) is a heterogeneous group of illnesses affecting the pulmonary parenchyma with acute onset bilateral inflammatory pulmonary infiltrates with associated hypoxemia. ARDS occurs after 2 major types of pulmonary injury: direct lung injury affecting the lung epithelium or indirect lung injury disrupting the vascular endothelium. Greater understanding of the differences between direct and indirect lung injury may refine the classification of patients with ARDS and lead to development of new therapeutics targeted at specific subpopulations of patients with ARDS.

Does prone positioning improve oxygenation and reduce mortality in patients with acute respiratory distress syndrome?

The emergence of computed tomography imaging more than 25 years ago led to characterization of acute respiratory distress syndrome (ARDS) as areas of relatively normal lung parenchyma juxtaposed with areas of dense consolidation and atelectasis. Given that this heterogeneity is often dorsally distributed, investigators questioned whether care for ARDS patients in the prone position would lead to improved mortality outcomes. This clinical review discusses the physiological rationale and clinical evidence supporting prone positioning in treating ARDS, in addition to its complications and contraindications.

Administration of intrapulmonary sodium polyacrylate to induce lung injury for the development of a porcine model of early acute respiratory distress syndrome

Background

The loss of alveolar epithelial and endothelial integrity is a central component in acute respiratory distress syndrome (ARDS); however, experimental models investigating the mechanisms of epithelial injury are lacking. The purpose of the present study was to design and develop an experimental porcine model of ARDS by inducing lung injury with intrapulmonary administration of sodium polyacrylate (SPA).

Methods

The present study was performed at the Centre for Comparative Medicine, University of British Columbia, Vancouver, British Columbia. Human alveolar epithelial cells were cultured with several different concentrations of SPA; a bioluminescence technique was used to assess cell death associated with each concentration. In the anesthetized pig model (female Yorkshire X pigs (n = 14)), lung injury was caused in 11 animals (SPA group) by injecting sequential aliquots (5 mL) of 1% SPA gel in aqueous solution into the distal airway via a rubber catheter through an endotracheal tube. The SPA was dispersed throughout the lungs by manual bag ventilation. Three control animals (CON group) underwent all experimental procedures and measurements with the exception of SPA administration.

Results

The mean (± SD) ATP concentration after incubation of human alveolar epithelial cells with 0.1% SPA (0.92 ± 0.27 μM/well) was approximately 15% of the value found for the background control (6.30 ± 0.37 μM/well; p < 0.001). Elastance of the respiratory system (E_RS) and the lung (E_L) increased in SPA-treated animals after injury (p = 0.003 and p < 0.001, respectively). Chest wall elastance (E_CW) did not change in SPA-treated animals. There were no differences in E_RS,E_L, or E_CW in the CON group when pre- and post-injury values were compared. Analysis of bronchoalveolar lavage fluid showed a significant shift toward neutrophil predominance from before to after injury in SPA-treated animals (p < 0.001) but not in the CON group (p = 0.38). Necropsy revealed marked consolidation and congestion of the dorsal lung lobes in SPA-treated animals, with light-microscopy evidence of bronchiolar and alveolar spaces filled with neutrophilic infiltrate, proteinaceous debris, and fibrin deposition. These findings were absent in animals in the CON group. Electron microscopy of lung tissue from SPA-treated animals revealed injury to the alveolar epithelium and basement membranes, including intra-alveolar neutrophils and fibrin on the alveolar surface and intravascular fibrin (microthrombosis).

Conclusions

In this particular porcine model, the nonimmunogenic polymer SPA caused a rapid exudative lung injury. This model may be useful to study ARDS caused by epithelial injury and inflammation.

Effects of manually-assisted cough combined with postural drainage, saline instillation and airway suctioning in critically-ill patients during high-frequency oscillatory ventilation: a prospective observational single centre trial

BACKGROUND

Chest physiotherapy may aid sputum clearance during conventional ventilation. However, the role of chest physiotherapy during high-frequency oscillatory ventilation (HFOV) is unclear. This study aimed to determine the effects manually-assisted cough (MAC), postural drainage, saline instillation and airway suction during HFOV.

METHODS

This was an observational study of a chest physiotherapy intervention in adult critically ill patients during HFOV. Measures included gas exchange, HFOV and haemodynamic variables 1 h before, immediately before, and 15 min, 1 h, 6 h and 12 h after intervention. Wet weight of airway secretions was also measured. Linear mixed modelling compared pre-intervention gas exchange, HFOV and haemodynamic variables with the four specified time-points after intervention.

RESULTS

Seventeen adults (ten females) with moderate to severe respiratory failure were studied (age, 49 years SD 14; Acute Physiology and Chronic Health Evaluation II score (APACHE II score) 21 SD 6; PaO2/FiO2 of 139 mmHg SD 51). There was a statistically, although not clinically significant reduction in PaO2/FiO2 for up to 1 h after intervention, but no significant changes in oxygenation index, PaCO2, pH, or haemodynamic parameters up to 12 h after intervention. A reduction in delta pressure (ΔPaw) at 15 min (p < 0.05) and 1 h (p < 0.05) post intervention was not correlated with sputum wet weight.

CONCLUSIONS

MAC, postural drainage, saline instillation and airway suctioning during HFOV in critically ill patients was well tolerated with no clinically significant effect on arterial blood gases or haemodynamics. ΔPaw decreased for up to 1 h after intervention, but was not explained by the weight of sputum removed.

Distinct and replicable genetic risk factors for acute respiratory distress syndrome of pulmonary or extrapulmonary origin

BACKGROUND

The role of genetics in the development of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) from direct or indirect lung injury has not been specifically investigated. The aim of this study was to identify genetic variants contributing to ALI/ARDS from pulmonary or extrapulmonary causes.

METHODS

We conducted a multistage genetic association study. We first performed a large-scale genotyping (50K ITMAT-Broad_CARe Chip) in 1717 critically ill Caucasian patients with either pulmonary or extrapulmonary injury, to identify single nucleotide polymorphisms (SNPs) associated with the development of ARDS from direct or indirect insults to the lung. Identified SNPs (p≤0.0005) were validated in two separated populations (Stage II), with trauma (Population I; n=765) and pneumonia/pulmonary sepsis (Population II; n=838), as causes for ALI/ARDS. Genetic variants replicating their association with trauma related-ALI in Stage II were validated in a second trauma-associated ALI population (n=224, Stage III).

RESULTS

In Stage I, non-overlapping SNPs were significantly associated with ARDS from direct/indirect lung injury, respectively. The association between rs1190286 (POPDC3) and reduced risk of ARDS from pulmonary injury was validated in Stage II (p<0.003). SNP rs324420 (FAAH) was consistently associated with increased risk of ARDS from extrapulmonary causes in two independent ALI-trauma populations (p<0.006, Stage II; p<0.05, Stage III). Meta-analysis confirmed these associations.

CONCLUSIONS

Different genetic variants may influence ARDS susceptibility depending on direct versus indirect insults. Functional SNPs in POPDC3 and FAAH genes may be driving the association with direct and indirect ALI/ARDS, respectively.

Changes in lung composition and regional perfusion and tissue distribution in patients with ARDS

BACKGROUND AND OBJECTIVE

ARDS is characterized by bilateral pulmonary infiltrates and refractory hypoxemia attributed to V/Q mismatch. We used dynamic CT to characterize changes in lung composition, regional perfusion and tissue distribution in patients with ARDS in comparison with healthy subjects.

METHODS

The Fick principle was applied to serial attenuation measurements constructed from sequential CT images acquired during the passage of a bolus of iodinated contrast medium in healthy subjects (n=3) and patients with ARDS (n=11). Perfusion was calculated by the Mullani-Gould method and mapped throughout both lungs. Gradients of perfusion and tissue density against vertical height were constructed.

RESULTS

In comparison with normal individuals, the tissue component of lungs from patients with ARDS was significantly increased (P<0.05). Blood fraction was unchanged. There was a discernable gradient in tissue density from non dependent to dependent regions in the patients with ARDS that was significantly different from controls. The proportion of perfusion applied to consolidated areas (i.e. shunt) correlated significantly (P<0.05) with the severity of hypoxaemia.

CONCLUSIONS

In patients with ARDS there are changes in both lung composition and the distribution of tissue and perfusion that may account in part for the physiological changes that define the syndrome.

Clinical review: Intra-abdominal hypertension: does it influence the physiology of prone ventilation?

Prone ventilation (PV) is a ventilatory strategy that frequently improves oxygenation and lung mechanics in critical illness, yet does not consistently improve survival. While the exact physiologic mechanisms related to these benefits remain unproven, one major theoretical mechanism relates to reducing the abdominal encroachment upon the lungs. Concurrent to this experience is increasing recognition of the ubiquitous role of intra-abdominal hypertension (IAH) in critical illness, of the relationship between IAH and intra-abdominal volume or thus the compliance of the abdominal wall, and of the potential difference in the abdominal influences between the extrapulmonary and pulmonary forms of acute respiratory distress syndrome. The present paper reviews reported data concerning intra-abdominal pressure (IAP) in association with the use of PV to explore the potential influence of IAH. While early authors stressed the importance of gravitationally unloading the abdominal cavity to unencumber the lung bases, this admonition has not been consistently acknowledged when PV has been utilized. Basic data required to understand the role of IAP/IAH in the physiology of PV have generally not been collected and/or reported. No randomized controlled trials or meta-analyses considered IAH in design or outcome. While the act of proning itself has a variable reported effect on IAP, abundant clinical and laboratory data confirm that the thoracoabdominal cavities are intimately linked and that IAH is consistently transmitted across the diaphragm--although the transmission ratio is variable and is possibly related to the compliance of the abdominal wall. Any proning-related intervention that secondarily influences IAP/IAH is likely to greatly influence respiratory mechanics and outcomes. Further study of the role of IAP/IAH in the physiology and outcomes of PV in hypoxemic respiratory failure is thus required. Theories relating inter-relations between prone positioning and the abdominal condition are presented to aid in designing these studies.

[Prone position for the treatment of acute respiratory distress syndrome: a review of current literature]

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) have high incidence and mortality rates. Most of the recently introduced treatments have failed to improve the prognosis of patients with ALI or ARDS or to reduce mortality. Several studies have shown improved oxygenation in the prone position during mechanical ventilation in patients with ARDS. However, current evidence strongly suggests that placing ARDS patients in prone position does not improve survival or reduce the duration of mechanical ventilation. Therefore, though in clinical practice this position may improve refractory hypoxemia in patients with ARDS, there is no evidence to support its systematic use.

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.

Pulmonary vs nonpulmonary sepsis and mortality in acute lung injury

BACKGROUND

Acute lung injury (ALI) is a frequent complication of sepsis. It is unclear if a pulmonary vs nonpulmonary source of sepsis affects mortality in patients with sepsis-induced ALI.

METHODS

Two hundred eighty-eight consecutive patients with sepsis-induced ALI from 14 ICUs at four hospitals in Baltimore, MD were prospectively classified as having a pulmonary vs nonpulmonary source of sepsis. Multiple logistic regression was conducted to evaluate the independent association of a pulmonary vs nonpulmonary source of sepsis with inpatient mortality.

RESULTS

In an unadjusted analysis, in-hospital mortality was lower for pulmonary vs nonpulmonary source of sepsis (42% vs 66%, p < 0.0001). Patients with pulmonary sepsis had lower acute physiology and chronic health evaluation (APACHE) II and sequential organ failure assessment (SOFA) scores, shorter ICU stays prior to the development of ALI, and higher lung injury scores. In the adjusted analysis, several factors were predictive of mortality: age (odds ratio [OR], 1.03; 95% confidence interval [CI], 1.01 to 1.06), Charlson comorbidity index (OR, 1.15; 95% CI, 1.02 to 1.30), ICU length of stay prior to ALI diagnosis (OR, 1.19; 95% CI, 1.01 to 1.39), APACHE II score (OR, 1.07; 95% CI, 1.03 to 1.12), lung injury score (OR, 1.64; 95% CI, 1.11 to 2.43), SOFA score (OR, 1.15; 95% CI, 1.06 to 1.26), and cumulative fluid balance in the first 7 days after ALI diagnosis (OR, 1.06; 95% CI, 1.03 to 1.10). A pulmonary vs nonpulmonary source of sepsis was not independently associated with mortality (OR, 0.72; 95% CI, 0.38 to 1.35).

CONCLUSIONS

Although lower mortality was observed for ALI patients with a pulmonary vs nonpulmonary source of sepsis, this finding is likely due to a lower severity of illness in those with pulmonary sepsis. Pulmonary vs nonpulmonary source of sepsis was not independently predictive of mortality for patients with ALI.

Recruitment maneuver in pulmonary and extrapulmonary experimental acute lung injury

OBJECTIVE

The aim of this study is to test the hypothesis that recruitment maneuvers (RMs) might act differently in models of pulmonary (p) and extrapulmonary (exp) acute lung injury (ALI) with similar transpulmonary pressure changes.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University research laboratory.

SUBJECTS

Wistar rats were randomly divided into four groups. In control groups, sterile saline solution was intratracheally (0.1 mL, Cp) or intraperitoneally (1 mL, Cexp) injected, whereas ALI animals received Escherichia coli lipopolysaccharide intratracheally (100 microg, ALIp) or intraperitoneally (1 mg, ALIexp). After 24 hrs, animals were mechanically ventilated (tidal volume, 6 mL/kg; positive end-expiratory pressure, 5 cm H2O) and three RMs (pressure inflations to 40 cm H2O for 40 secs, 1 min apart) applied.

MEASUREMENTS AND MAIN RESULTS

PaO2, lung resistive and viscoelastic pressures, static elastance, lung histology (light and electron microscopy), and type III procollagen messenger RNA expression in pulmonary tissue were measured before RMs and at the end of 1 hr of mechanical ventilation. Mechanical variables, gas exchange, and the fraction of area of alveolar collapse were similar in both ALI groups. After RMs, lung resistive and viscoelastic pressures and static elastance decreased more in ALIexp (255%, 180%, and 118%, respectively) than in ALIp (103%, 59%, and 89%, respectively). The amount of atelectasis decreased more in ALIexp than in ALIp (from 58% to 19% and from 59% to 33%, respectively). RMs augmented type III procollagen messenger RNA expression only in the ALIp group (19%), associated with worsening in alveolar epithelium injury but no capillary endothelium lesion, whereas the ALIexp group showed a minor detachment of the alveolar capillary membrane.

CONCLUSIONS

Given the same transpulmonary pressures, RMs are more effective at opening collapsed alveoli in ALIexp than in ALIp, thus improving lung mechanics and oxygenation with limited damage to alveolar epithelium.

Is the mortality higher in the pulmonary vs the extrapulmonary ARDS? A meta analysis

BACKGROUND AND AIM

ARDS can occur from the following two pathogenetic pathways: a direct pulmonary injury (ARDSp); and an indirect injury (ARDSexp). The predisposing clinical factor can influence the pathogenesis and clinical outcome of ARDS. This metaanalysis was aimed at evaluating whether there is any difference in mortality between the two groups.

METHODS

We searched the MEDLINE, EMBASE, and CINAHL databases for relevant studies published from 1987 to 2007, and included studies that have reported mortality in the two groups of ARDS. We calculated the odds ratio (OR) and 95% confidence interval (CI) to assess mortality in patients with ARDSp vs patients with ARDSexp and pooled the results using three different statistical models.

RESULTS

Our search yielded 34 studies. In all, the studies involved 4,311 patients with 2,330 patients in the ARDSp group and 1,981 patients in the ARDSexp group. The OR of mortality in ARDSp group compared to the ARDSexp group was 1.11 (95% CI, 0.88 to 1.39), as determined by the random-effects model; 1.04 (95% CI, 0.92 to 1.18), as determined by the fixed-effects model; and 1.04 (95% CI, 0.92 to 1.18), as determined by the exact method, indicating that mortality is similar in the two groups. The mortality was no different whether the studies were classified as prospective (OR, 1.15; 95% CI, 0.87 to 1.51) or retrospective (OR, 1.01; 95% CI, 0.61 to 1.69); small (OR, 1.11; 95% CI, 0.77 to 1.60) or large (OR, 1.1; 95% CI, 0.82 to 1.49); or observational (OR, 1.10; 95% CI, 0.82 to 1.49) or interventional (OR, 0.97; 95% CI, 0.79 to 1.19). There was methodological and statistical heterogeneity (I(2), 50.9%; 95% CI, 21.3 to 66.2%; chi(2) statistic, 67.22; p = 0.0004).

CONCLUSIONS

The results of this study suggest that there is no difference in mortality between these two groups. Further studies should focus on specific etiologies within the subgroups rather than focusing on the broader division of ARDSp and ARDSexp.

[The prone position in acute respiratory distress syndrome: a critical systematic review]

OBJECTIVE

To do a critical systematic review regarding effects of prone positioning (PP) in patients with acute respiratory distress syndrome (ARDS).

METHODS

A systematic review (Highwire, Medline, Cochrane Library from 1976 to 2004), using the keywords: prone position, acute respiratory distress syndrome, allowed us to include the human studies on PP in ARDS patients, independantly of their objectives or their type of protocol. To appreciate the studies validity, we scored the quality evidence of the studies in order to grade our conclusions.

RESULTS AND CONCLUSION

The qualitative analysis of the 58 included studies (1,500 patients returned prone, 4,000 episodes of PP) led to the following main conclusions: 1) the PP improves oxygenation in the majority of ARDS patients (level of evidence I); 2) the PP improves the pulmonary haemodynamics without altering the systemic haemodynamics (level of evidence III); 3) the PP enhances the recruitment maneuvers (level of evidence III); 4) because there are no formal predictive criteria for response to the PP, a "trial of PP" or better two PP trials are necessary to look for the responders; 5) the PP should be performed as early as possible in the course of severe ARDS; 6) the optimal duration of PP is 18 to 23 hours daily, and it should be continued until improvement of arterial oxygenation, or loss of the positive effect of PP on arterial oxygenation or evidently patient's death.

Pathology of Acute Lung Injury and Acute Respiratory Distress Syndrome: A Clinical–Pathological Correlation

Clinical criteria cannot identify all patients who develop acute respiratory distress syndrome, which might indicate the need to develop new tools that improve reliability and diagnostic accuracy. Nevertheless, being conscious of the limitations of the American-European consensus conference criteria continues to be useful in clinical daily practice because these patients would benefit from treatments. This and other management strategies have been evaluated in patients who, fulfilling the clinical criteria of acute respiratory distress syndrome, could not have diffuse alveolar damage.

Alveolar recruitment during prone position: time matters

Alveolar recruitment is one of the beneficial effects of prone positioning in patients with ARDS (acute respiratory distress syndrome). However, responses vary among patients and, therefore, we hypothesized that alveolar recruitment is an individual time-dependent process and its measurement might be helpful to 'dose' prone positioning individually. In 13 patients diagnosed with ARDS, EELV (end-expiratory lung volume) was measured in the supine position, immediately after turning to the prone position, at 1, 2, 4 and 8 h in the prone position and after returning to the supine position. Responders were defined based on a 30% increase in oxygenation. EELV increased in responders, whereas it remained constant in non-responders. The time course was different in individual patients. In some responders, a plateau was reached as early as 2-4 h, whereas, in others, 8 h of prone positioning was not sufficient to allow complete recruitment. The increase in lung volume was associated with both an increase in arterial oxygenation and a decrease in venous admixture. Furthermore, responders had significantly lower baseline EELVs than non-responders. In conclusion, alveolar recruitment during prone positioning has been characterized as an individual time-dependent process. Its measurement might be useful to apply prone positioning more individually and might also help to identify responders.

Validez de los criterios diagnósticos del síndrome de distrés respiratorio agudo

Acute respiratory distress syndrome (ARDS) is defined according to the criteria of the 1994 consensus conference. These criteria aim to <<bring clarity and uniformity to the definition of this clinical entity>>. However, the histological criteria that correspond to ARDS are the criteria of diffuse alveolar damage described in 1976 by Katzenstein et al., which are still valid at present. In the last decade, different studies have been published that have tried to correlate the clinical syndrome with the histological findings. These studies have been basically done in experimental animals, but also by the description of the pulmonary biopsy findings and post-mortem study findings. The present article aims to show discrepancy between clinical and histological diagnosis of the acute pulmonary lesion, basically having an effect on the difficulty of the ARDS diagnosis when its origin is pulmonary and the implications of this discrepancy in the clinical practice and research.

Effects of prone position on the oxygenation of patients with acute respiratory distress syndrome

CONTEXT AND OBJECTIVE

Acute respiratory distress syndrome (ARDS) is characterized by arterial hypoxemia, and prone position (PP) is one possible management strategy. The objective here was to evaluate the effects of PP on oxygenation.

DESIGN AND SETTING

Non-randomized, open, prospective, controlled clinical trial, in a surgical intensive care unit at a tertiary university hospital.

METHODS

Forty-one ARDS patients underwent PP for three-hour periods. Arterial partial oxygen pressure (PaO2) was measured immediately before changing to PP, after 30, 60, 120 and 180 minutes in PP and 60 minutes after returning to dorsal recumbent position (DP). The paired-t and Dunnett tests were used.

RESULTS

A notable clinical improvement in oxygenation (> 15%) was detected in 78.0% of patients. This persisted for 60 minutes after returning to DP in 56% and lasted for 12 and 48 hours in 53.6% and 46.3%, respectively. Maximum improvement was seen after 30 minutes in 12.5% of responding patients and after 180 minutes in 40.6%. No statistically significant associations between PP response and age, gender, weight, PEEP level, tidal volume, respiratory rate, PaO2/FiO2 or duration of mechanical ventilation were detected. One accidental extubation and four cases of deterioration through oxygenation were detected. The 48-hour mortality rate was 17%.

CONCLUSIONS

For a significant number of ARDS patients, PP may rapidly enhance arterial oxygenation and its inclusion for management of severe ARDS is justified. However, it is not a cost-free maneuver and caution is needed in deciding on using PP.

Alveolar recruitment in acute lung injury

Alveolar recruitment is one of the primary goals of respiratory care for acute lung injury. It is aimed at improving pulmonary gas exchange and, even more important, at protecting the lungs from ventilator-induced trauma. This review addresses the concept of alveolar recruitment for lung protection in acute lung injury. It provides reasons for why atelectasis and atelectrauma should be avoided; it analyses current and future approaches on how to achieve and preserve alveolar recruitment; and it discusses the possibilities of detecting alveolar recruitment and derecruitment. The latter is of particular clinical relevance because interventions aimed at lung recruitment are often undertaken without simultaneous verification of their effectiveness.

Posição prona

A posição prona é uma manobra utilizada para combater a hipoxemia nos pacientes com síndrome do desconforto respiratório agudo. Apesar de hoje ser considerada um modo eficaz de melhorar a oxigenação, os mecanismos fisiológicos que levam à melhora da função respiratória ainda não estão completamente esclarecidos. O objetivo principal desta revisão é discutir os aspectos fisiológicos e clínicos da posição prona na síndrome do desconforto respiratório agudo.

Pulmonary and extrapulmonary acute respiratory distress syndrome: are they different?

PURPOSE OF REVIEW

Acute respiratory distress syndrome has been considered a morphologic and functional expression of lung injury caused by a variety of insults. Two distinct forms of acute respiratory distress syndrome/acute lung injury are described, because there are differences between pulmonary acute respiratory distress syndrome (direct effects on lung cells) and extrapulmonary acute respiratory distress syndrome (reflecting lung involvement in a more distant systemic inflammatory response). This article will focus on the differences in lung histology and morphology, respiratory mechanics, and response to ventilatory strategies and pharmacologic therapies in pulmonary and extrapulmonary acute respiratory distress syndrome.

RECENT FINDINGS

Many researchers recognize that experimental pulmonary and extrapulmonary acute respiratory distress syndrome are not identical. In addition, clinical studies have described the detection of differences radiographically, functionally, and by analysis of the responses to therapeutic interventions (ventilatory strategies, positive end-expiratory pressure, prone position, drugs). However, there are contradictions among the different studies addressing these issues, which could be attributed to the fact that the distinction between pulmonary and extrapulmonary acute respiratory distress syndrome is not always clear and simple. Furthermore, there may be frequent overlapping in pathogenetic mechanisms and morphologic alterations.

SUMMARY

The understanding of acute respiratory distress syndrome needs to take into account its origin. If each pathogenetic mechanism were to be considered, clinical management would be more precise, and probably the outcome could include real amelioration.

Recruitment maneuvers during prone positioning in patients with acute respiratory distress syndrome

OBJECTIVE

To evaluate the interaction of recruitment maneuvers and prone positioning on gas exchange and venous admixture in patients with early extrapulmonary acute respiratory distress syndrome ventilated with high levels of positive end-expiratory pressure. We hypothesized that a sustained inflation performed after 6 hrs of prone positioning would induce sustained improvement in oxygenation (Pao2/Fio2) and venous admixture.

DESIGN

Prospective, interventional study.

SETTING

Tertiary care, postoperative intensive care unit.

PATIENTS

Fifteen patients with early extrapulmonary acute respiratory distress syndrome.

INTERVENTIONS

After 6 hrs of prone positioning, a sustained inflation was performed with 50 cm H2O maintained for 30 secs. Data were recorded in supine position, after 6 hrs of prone positioning, at 3, 30, and 180 mins following the sustained inflation.

MEASUREMENTS AND MAIN RESULTS

A response to prone positioning was observed in nine of 15 patients leading to an improvement of Pao2/Fio2 (147 +/- 37 torr vs. 225 +/- 77 torr, p = .005) and venous admixture (35.4 +/- 8.3% vs. 28.9 +/- 9.8%, p = .001). Six patients did not respond to prone positioning. Following the sustained inflation, the responders to prone positioning showed a further increase of Pao2/Fio2 and decrease of venous admixture at 3 mins (Pao2/Fio2, 225 +/- 77 torr vs. 368 +/- 90 torr, p = .018; venous admixture, 28.9 +/- 9.8% vs. 18.9 +/- 6.7%, p = .05). In all six nonresponders to prone positioning, an improvement of Pao2/Fio2 and venous admixture occurred at 3 mins following the sustained inflation (128 +/- 18 torr vs. 277 +/- 59 torr, p = .03; venous admixture, 34.2 +/- 6.0% vs. 23.8 +/- 6.3%, p = .05). The beneficial effects of the sustained inflation remained significantly elevated over 3 hrs in responders and nonresponders to prone positioning.

CONCLUSION

In patients with early extrapulmonary acute respiratory distress syndrome, a sustained inflation performed after 6 hrs of prone positioning induced further and sustained improvement of oxygenation and venous admixture in both responders and nonresponders to prone positioning.

Prone positioning in the patient who has acute respiratory distress syndrome: the art and science

Acute respiratory distress syndrome (ARDS) remains a significant contributor to the morbidity and mortality of patients in the ICU. A variety of treatments are used to support the lung of the patient who has ARDS and improve gas exchange during the acute injury phase. It seems, however, that the simple, safe, and noninvasive act of prone positioning of the critically ill patient who has ARDS may improve gas exchange while preventing potential complications of high positive end-expiratory pressure, volutrauma, and oxygen toxicity. This article provides the critical care nurse with the physiologic rationale for use of the prone position, indications and contraindications for use, safe strategies for prone positioning, and care techniques and monitoring methods of the patient who is in the prone position.

Bench-to-bedside review: chest wall elastance in acute lung injury/acute respiratory distress syndrome patients

The importance of chest wall elastance in characterizing acute lung injury/acute respiratory distress syndrome patients and in setting mechanical ventilation is increasingly recognized. Nearly 30% of patients admitted to a general intensive care unit have an abnormal high intra-abdominal pressure (due to ascites, bowel edema, ileus), which leads to an increase in the chest wall elastance. At a given applied airway pressure, the pleural pressure increases according to (in the static condition) the equation: pleural pressure = airway pressure x (chest wall elastance/total respiratory system elastance). Consequently, for a given applied pressure, the increase in pleural pressure implies a decrease in transpulmonary pressure (airway pressure - pleural pressure), which is the distending force of the lung, implies a decrease of the strain and of ventilator-induced lung injury, implies the need to use a higher airway pressure during the recruitment maneuvers to reach a sufficient transpulmonary opening pressure, implies hemodynamic risk due to the reductions in venous return and heart size, and implies a possible increase of lung edema, partially due to the reduced edema clearance. It is always important in the most critically ill patients to assess the intra-abdominal pressure and the chest wall elastance.

Recovery from lung injury in survivors of acute respiratory distress syndrome: difference between pulmonary and extrapulmonary subtypes

OBJECTIVE

To determine whether long-term outcome differs between acute respiratory distress syndrome (ARDS) resulting from pulmonary (ARDSp) and extrapulmonary (ARDSexp) causes.

DESIGN

Observational study.

SETTING

Medical intensive care unit of a university hospital.

PATIENTS

Twenty-nine ARDS patients (16 ARDSp and 13 ARDSexp) who survived over 6 months after diagnosis.

MEASUREMENTS AND RESULTS

The two groups did not differ according to demographic data and severity indices on admission. The duration of ICU stay (median 21 days [interquartile range, 12-43 days] vs 12 [6.5-20] days, p=0.097) tended to be longer and total ventilation time (360 [96-700] h vs 144 [42.5-216] h, p=0.045) were longer in the patients with ARDSp. The ARDSp patients showed more severe abnormalities on thin-section computed tomography (CT), including ground-glass opacity (GGO; 6 [3-16] vs 0 [0-2.5], p=0.002), reticular density (12 [8-14] vs 5 [2-9], p=0.033) and the sum of all four patterns of lesion (20 [11-27] vs 5 [2-12], p=0.006). There were no between-group differences in Spitzer's Quality of Life index and the Chronic Respiratory Questionnaire.

CONCLUSIONS

These results suggest that ARDSp would leave more severe lung sequelae than ARDSexp, but the clinical relevance of their difference is questionable.

SARS in the Intensive Care Unit

Approximately 20% of patients with severe acute respiratory syndrome (SARS) develop respiratory failure that requires admission to an intensive care unit (ICU). Old age, comorbidity, and elevated lactate dehydrogenase on hospital admission are associated with increased risk for ICU admission. ICU admission usually is late and occurs 8 to 10 days after symptom onset. Acute respiratory distress syndrome occurs in almost all admitted patients and most require mechanical ventilation. ICU admission is associated with significant morbidity, particularly an apparent increase in the incidence of barotrauma and nosocomial sepsis. Long-term mortality for patients admitted to the ICU ranges from 30% to 50%. Many procedures in ICUs pose a high risk for transmission of SARS coronavirus to health care workers. Contact and airborne infection isolation precautions, in addition to standard precautions, should be applied when caring for patients with SARS. Ensuring staff safety is important to maintain staff morale and delivery of adequate services.

New aspects of ventilation in acute lung injury

Recent recognition that artificial ventilation may cause damage to the acutely injured lung has caused renewed interest in ventilation techniques that minimise this potential harm. Many ventilation techniques have proved beneficial in small trials of very specific patient groups, but most have subsequently failed to translate into improved patient outcome in larger trials. An exception to this is 'protective ventilation' using reduced tidal volumes (to lower airway pressure) and increased PEEP (to reduce pulmonary collapse). Results of trials of protective ventilation have been encouraging, and the technique should now be adopted more widely. High frequency ventilation, inverse ratio ventilation, prone positioning and inhaled nitric oxide are all techniques that may be considered when, in spite of optimal artificial ventilation, the patient's gas exchange remains dangerously poor. Under these circumstances, the choice of technique is dependent on their availability, local expertise and individual patient needs.

Impact of randomized trial results on acute lung injury ventilator therapy in teaching hospitals

Reducing tidal volumes administered to patients with acute lung injury is the only intervention reported to decrease mortality resulting from this life-threatening condition. Whereas many medical advances are slowly brought into practice, clinicians in teaching hospitals are often assumed to be early adopters of new medical advances. Our objective was to examine trends in the ventilatory prescription for 398 patients with acute lung injury treated in three teaching hospitals from 1994 to 2001. There was no change in tidal volumes until mid to late 1998, when volumes started to slowly decline at the rate of 48.0 (95% confidence interval, 21.0 to 74.4) ml/year. In the 2 years after the results were released from a large trial that demonstrated the superiority of 6 ml/kg tidal volume therapy over 12 ml/kg, clinicians prescribed tidal volumes of 651 +/- 128 ml or 10.1 +/- 1.9 ml/kg. Tidal volumes after intubation were minimally reduced over the subsequent 2 days of mechanical ventilation (mean reduction, 33 ml). Hospital category, male sex, and disease onset before May 1999 were associated with higher volumes whereas lung injury severity was inversely associated. We conclude that clinicians practicing at these teaching hospitals have not rapidly adopted low tidal volume ventilation that may reduce mortality.

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 alveolar recruitment maneuver in early acute respiratory distress syndrome according to antiderecruitment strategy, etiological category of diffuse lung injury, and body position of the patient

OBJECTIVE

To assess how the level of positive end-expiratory pressure (PEEP) (antiderecruitment strategy), etiological category of diffuse lung injury, and body position of the patient modify the effect of the alveolar recruitment maneuver (ARM) in acute respiratory distress syndrome (ARDS).

DESIGN

Prospective clinical trial.

SETTING

Medical intensive care unit at a tertiary hospital.

PATIENTS

Forty-seven patients with early ARDS, including 19 patients from our preliminary study.

INTERVENTION

From baseline ventilation at a tidal volume of 8 mL/kg and PEEP of 10 cm H2O, the ARM (a stepwise increase in the level of PEEP up to 30 cm H2O with a concomitant decrease in the magnitude of tidal volume down to 2 mL/kg) was given with (ARM + PEEP, n = 20) or without (ARM only, n = 19) subsequent increase of PEEP to 15 cm H2O. In eight other patients, PEEP was increased to 15 cm H2O without a preceding ARM (PEEP only).

MEASUREMENTS AND RESULTS

In all three groups, Pao2 was increased by the respective intervention (all p<.05). In the ARM-only group, Pao2 at 15 mins after intervention was lower than Pao2 immediate after intervention (p =.046). In the ARM + PEEP group, no such decrease in Pao2 was observed, and Pao2 at 15, 30, 45, and 60 mins after intervention was higher than in the ARM-only group (all p<.05). Compared with the PEEP-only group, Pao2 of the ARM + PEEP group was higher immediately after intervention and at the later time points (all p <.05). Compared with patients with ARDS associated with direct lung injury (pulmonary ARDS), patients with ARDS associated with indirect lung injury (extrapulmonary ARDS) showed a greater increase in Pao2 (27 +/- 21% vs. 130 +/- 112%; p=.002) and a greater decrease in radiologic scores (1.0 +/- 2.4 vs. 3.4 +/- 1.5; p=.005) after the ARM. The increase in Pao2 induced by the ARM was greater for patients in the supine position than for patients in the prone position (61 +/- 82% vs. 21 +/- 14%; p=.028). Consequently, Pao immediately after the ARM was similar in the two groups of patients in different positions.

CONCLUSIONS

After the ARM, a sufficient level of PEEP is required as an antiderecruitment strategy. Pulmonary ARDS and extrapulmonary ARDS may be different pathophysiologic entities. An effective ARM may obviate the need for the prone position in ARDS at least in terms of oxygenation.

Pulmonary versus extrapulmonary acute respiratory distress syndrome: different diseases or just a useful concept?

The acute respiratory distress syndrome may complicate both pulmonary and extrapulmonary conditions. There is a growing belief that the predisposition to, and clinical course of, the syndrome may be influenced by the extent to which the lung is directly involved in the precipitating pathologic changes. Several studies have highlighted differences in morphology and respiratory physiology between the two subgroups in the early stages of acute respiratory distress syndrome. Further, preliminary reports have suggested that the effects of therapeutic interventions such as alterations in positive end-expiratory pressure, prone ventilation, and the use of inhaled vasoactive agents may differ between pulmonary and extrapulmonary acute respiratory distress syndrome. There are, however, inconsistencies between various studies addressing these issues, which may relate in part to differences in etiologic case mix. There are also practical difficulties in assigning certain cases to one of these two groups. Finally, there are as yet no outcome data to support any modification of clinical management on the basis of this distinction.