Prone positioning for acute respiratory distress syndrome in the surgical intensive care unit: who, when, and how long?

Optimizing Management of Acute Respiratory Distress Syndrome in Critically Ill Surgical Patients: A Systematic Review

INTRODUCTION

This systematic review aims to evaluate the optimal management of acute respiratory distress syndrome (ARDS) in critically ill surgical patients, specifically focusing on positioning, extracorporeal membrane oxygenation (ECMO) use, ventilation, fluid resuscitation, and pharmacological treatments.

METHODS

A systematic review was conducted utilizing four databases including PubMed, Google Scholar, EMBASE, and ProQuest. This study followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines and was registered with The International Prospective Register of Systematic Reviews. Studies published until May 20, 2024, that assessed the management of ARDS in critically ill surgical adult populations were included in our review. The primary outcome of interest was mortality, with secondary outcomes like intensive care unit (ICU) length of stay (LOS), ventilator days, and oxygenation also being considered.

RESULTS

A total of fifteen studies met inclusion criteria; four studies assessed positional interventions, four assessed treatments with ECMO, three assessed mechanical ventilation settings, and four assessed fluid resuscitation and medications. Prone position was found to decrease mortality, ICU LOS, ventilator days, and increased oxygenation (P < 0.001). ECMO utilization decreased the overall mortality rate when compared to patients without ECMO (36.4% versus 43.9%, P < 0.001). Maintaining a tidal volume ≤8 mL/kg body weight and plateau pressure ≤35 cm H2O on mechanical ventilation also decreased patient mortality (P < 0.001). Finally, conservative fluid management decreased ICU LOS, whereas methylprednisolone use demonstrated decreased mortality.

CONCLUSIONS

Prone positioning, ECMO utilization, lung protective ventilation settings, and methylprednisolone reduced mortality among surgical patients with ARDS. In addition, prone positioning and conservative fluid management were associated with decreased ICU LOS, ventilator days, and improved oxygenation status.

Prone Positioning and Molecular Biomarkers in COVID and Non-COVID ARDS: A Narrative Review

Prone positioning (PP) represents a therapeutic intervention with the proven capacity of ameliorating gas exchanges and ventilatory mechanics indicated in acute respiratory distress syndrome (ARDS). When PP is selectively applied to moderate-severe cases of ARDS, it sensitively affects clinical outcomes, including mortality. After the COVID-19 outbreak, clinical application of PP peaked worldwide and was applied in 60% of treated cases, according to large reports. Research on this topic has revealed many physiological underpinnings of PP, focusing on regional ventilation redistribution and the reduction of parenchymal stress and strain. However, there is a lack of evidence on biomarkers behavior in different phases and phenotypes of ARDS. Patients response to PP are, to date, decided on PaO2/FiO2 ratio improvement, whereas scarce data exist on biomarker tracking during PP. The purpose of this review is to explore current evidence on the clinical relevance of biomarkers in the setting of moderate-severe ARDS of different etiologies (i.e., COVID and non-COVID-related ARDS). Moreover, this review focuses on how PP may modulate biomarkers and which biomarkers may have a role in outcome prediction in ARDS patients.

Effects of prone positioning on ARDS outcomes of trauma and surgical patients: a systematic review and meta-analysis

BACKGROUND

Prone position is an option for rescue therapy for acute respiratory distress syndrome. However, there are limited relevant data among trauma and surgical patients, who may be at increased risk for complications following position changes. This study aimed to identify the benefits and risks of proning in this patient subgroup.

METHODS

Follow the PRISMA 2020, MEDLINE and EMBASE database searches were conducted. Additional search of relevant primary literature and review articles was also performed. A random effects model was used to estimate the PF ratio, mortality rate, mechanical ventilator days, and intensive care unit length of stay using Review Manager 5.4.1 software.

RESULTS

Of 1,128 studies, 15 articles were included in this meta-analysis. The prone position significantly improved the PF ratio compared with the supine position (mean difference, 79.26; 95% CI, 53.38 to 105.13). The prone position group had a statistically significant mortality benefit (risk ratio [RR], 0.48; 95% CI, 0.35 to 0.67). Although there was no significant difference in the intensive care unit length of stay, the prone position significantly decreased mechanical ventilator days (-2.59; 95% CI, -4.21 to -0.97). On systematic review, minor complications were frequent, especially facial edema. There were no differences in local wound complications.

CONCLUSIONS

The prone position has comparable complications to the supine position. With its benefits of increased oxygenation and decreased mortality, the prone position can be considered for trauma and surgical patients. A prospective multicenter study is warranted.

Focused Management of Patients With Severe Acute Brain Injury and ARDS

Considering the COVID-19 pandemic where concomitant occurrence of ARDS and severe acute brain injury (sABI) has increasingly coemerged, we synthesize existing data regarding the simultaneous management of both conditions. Our aim is to provide readers with fundamental principles and concepts for the management of sABI and ARDS, and highlight challenges and conflicts encountered while managing concurrent disease. Up to 40% of patients with sABI can develop ARDS. Although there are trials and guidelines to support the mainstays of treatment for ARDS and sABI independently, guidance on concomitant management is limited. Treatment strategies aimed at managing severe ARDS may at times conflict with the management of sABI. In this narrative review, we discuss the physiological basis and risks involved during simultaneous management of ARDS and sABI, summarize evidence for treatment decisions, and demonstrate these principles using hypothetical case scenarios. Use of invasive or noninvasive monitoring to assess brain and lung physiology may facilitate goal-directed treatment strategies with the potential to improve outcome. Understanding the pathophysiology and key treatment concepts for comanagement of these conditions is critical to optimizing care in this high-acuity patient population.

Prone Position Ventilation in Neurologically Ill Patients: A Systematic Review and Proposed Protocol

OBJECTIVES

Prone positioning has been shown to be a beneficial adjunctive supportive measure for patients who develop acute respiratory distress syndrome. Studies have excluded patients with reduced intracranial compliance, whereby patients with concomitant neurologic diagnoses and acute respiratory distress syndrome have no defined treatment algorithm or recommendations for management. In this study, we aim to determine the safety and feasibility of prone positioning in the neurologically ill patients.

DESIGN AND SETTING

A systematic review of the literature, performed in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analyses 2009 guidelines, yielded 10 articles for analysis. Using consensus from these articles, in combination with review of multi-institutional proning protocols for patients with nonneurologic conditions, a proning protocol for patients with intracranial pathology and concomitant acute respiratory distress syndrome was developed.

MEASUREMENTS AND MAIN RESULTS

Among 10 studies included in the final analysis, we found that prone positioning is safe and feasible in the neurologically ill patients with acute respiratory distress syndrome. Increased intracranial pressure and compromised cerebral perfusion pressure may occur with prone positioning. We propose a prone positioning protocol for the neurologically ill patients who require frequent neurologic examinations and intracranial monitoring.

CONCLUSIONS

Although elevations in intracranial pressure and reductions in cerebral perfusion pressure do occur during proning, they may not occur to a degree that would warrant exclusion of prone ventilation as a treatment modality for patients with acute respiratory distress syndrome and concomitant neurologic diagnoses. In cases where intracranial pressure, cerebral perfusion pressure, and brain tissue oxygenation can be monitored, prone position ventilation should be considered a safe and viable therapy.

Prone Position in Mechanically Ventilated COVID-19 Patients: A Multicenter Study

Background: The prone position (PP) is increasingly used in mechanically ventilated coronavirus disease 2019 (COVID-19) acute respiratory distress syndrome (ARDS) patients. However, studies investigating the influence of the PP are currently lacking in these patients. This is the first study to investigate the influence of the PP on the oxygenation and decarboxylation in COVID-19 patients. Methods: A prospective bicentric study design was used, and in mechanically ventilated COVID-19 patients, PP was indicated from a partial pressure of oxygen in arterial blood (P_aO2)/fraction of inspired oxygen (F_IO2) ratio of <200. Patients were left prone for 16 h each. Pressure levels, F_IO2, were adjusted to ensure a P_aO2 greater than 60 mmHg. Blood gas analyses were performed before (baseline 0.5 h), during (1/2/5.5/9.5/13 h), and after being in the PP (1 h), the circulatory/ventilation parameters were continuously monitored, and lung compliance (LC) was roughly calculated. Responders were defined compared to the baseline value (P_aO2/F_IO2 ratio increase of ≥15%; partial pressure of carbon dioxide (P_aCO2) decrease of ≥2%). Results: 13 patients were included and 36 PP sessions were conducted. Overall, P_aO2/F_IO2 increased significantly in the PP (p < 0.001). Most P_aO2/F_IO2 responders (29/36 PP sessions, 77%) were identified 9.5 h after turning prone (14% slow responders), while most P_aCO2 responders (15/36 PP sessions, 42%) were identified 13 h after turning prone. A subgroup of patients (interval intubation to PP ≥3 days) showed less P_aO2/F_IO2 responders (16% vs. 77%). An increase in P_aCO2 and minute ventilation in the PP showed a significant negative correlation (p < 0.001). LC (median before the PP = 38 mL/cm H₂O; two patients with LC >80 mL/cm H₂O) showed a significant positive correlation with the 28 day survival of patients (p = 0.01). Conclusion: The PP significantly improves oxygenation in COVID-19 ARDS patients. The data suggest that they also benefit most from an early PP. A decrease in minute ventilation may result in fewer P_aCO2 responders. LC may be a predictive outcome parameter in COVID-19 patients. Trial registration: Retrospectively registered.

Acute respiratory distress syndrome in traumatic brain injury: how do we manage it?

Traumatic brain injury (TBI) is an important cause of morbidity and mortality worldwide. TBI patients frequently suffer from lung complications and acute respiratory distress syndrome (ARDS), which is associated with poor clinical outcomes. Moreover, the association between TBI and ARDS in trauma patients is well recognized. Mechanical ventilation of patients with a concomitance of acute brain injury and lung injury can present significant challenges. Frequently, guidelines recommending management strategies for patients with traumatic brain injuries come into conflict with what is now considered best ventilator practice. In this review, we will explore the strategies of the best practice in the ventilatory management of patients with ARDS and TBI, concentrating on those areas in which a conflict exists. We will discuss the use of ventilator strategies such as protective ventilation, high positive end expiratory pressure (PEEP), prone position, recruitment maneuvers (RMs), as well as techniques which at present are used for 'rescue' in ARDS (including extracorporeal membrane oxygenation) in patients with TBI. Furthermore, general principles of fluid, haemodynamic and hemoglobin management will be discussed. Currently, there are inadequate data addressing the safety or efficacy of ventilator strategies used in ARDS in adult patients with TBI. At present, choice of ventilator rescue strategies is best decided on a case-by-case basis in conjunction with local expertise.

Volumetric capnography: lessons from the past and current clinical applications

Dead space is an important component of ventilation–perfusion abnormalities. Measurement of dead space has diagnostic, prognostic and therapeutic applications. In the intensive care unit (ICU) dead space measurement can be used to guide therapy for patients with acute respiratory distress syndrome (ARDS); in the emergency department it can guide thrombolytic therapy for pulmonary embolism; in peri-operative patients it can indicate the success of recruitment maneuvers. A newly available technique called volumetric capnography (Vcap) allows measurement of physiological and alveolar dead space on a regular basis at the bedside. We discuss the components of dead space, explain important differences between the Bohr and Enghoff approaches, discuss the clinical significance of arterial to end-tidal CO₂ gradient and finally summarize potential clinical indications for Vcap measurements in the emergency room, operating room and ICU.

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.

A practical approach to the use of prone therapy in acute respiratory distress syndrome

In this article we propose a practical approach to the use of prone therapy for acute respiratory distress syndrome (ARDS). We have attempted to provide information to improve the understanding and implementation of prone therapy based on the literature available and our own experience. We review the basic physiology behind ARDS and the theoretical mechanism by which prone therapy can be of benefit. The findings of the most significant studies regarding prone therapy in ARDS as they pertain to its implementation are summarized. Also provided is a discussion of the nuances of utilizing prone therapy, including potential pitfalls, complications, and contraindications. The specific considerations of prone therapy in open abdomens and traumatic brain injuries are discussed as well. Finally, we supply suggested protocols for the implementation of prone therapy discussing criteria for initiation and cessation of therapy as well as addressing issues such as the use of neuromuscular blockade and nutritional supplementation.

Using the prone position for ventilated patients with respiratory failure: a review

AIMS

this review explored the evidence relating to prone positioning in ventilated patients diagnosed with respiratory failure, including acute lung injury (ALI) or adult respiratory distress syndrome (ARDS).

BACKGROUND

mortality rates for ventilated patients with ALI or ARDS are high, and there is a growing body of evidence suggesting that the position these patients are nursed in may influence clinical outcomes. However, there are no guidelines to inform nursing practice in positioning these patients.

METHOD

Medline, Scopus, Cinahl and the Cochrane Library databases were searched for original research reports or systematic reviews of evidence between 2000 and 2009. Reference lists of retrieved papers were hand searched and included studies were analysed using the Critical Appraisal and Skills Programme framework. A narrative data synthesis considered the strengths and limitations of studies, and findings were collated and interpreted.

RESULTS

Application of the search strategy identified a systematic review, currently underway, which has not yet reported and 14 relevant studies eligible for inclusion in this review. Analysis showed considerable variation in study design, but suggests that PaO(2) /FiO(2) ratio, incidence of VAP and mortality may be positively affected by prone positioning.

CONCLUSIONS

evidence of the clinical benefits associated with prone positioning is inconclusive and provides little guidance for nursing practice. There is a need for further research into the clinical outcomes of prone positioning, and greater understanding of the practicalities of prone positioning critically ill patients is required.

RELEVANCE TO CLINICAL PRACTICE

nurses have a central role to play in the continual assessment and management of this patient group, including the position they are nursed in, not only to ensure the best clinical outcomes but also to provide care and comfort to the patient and their family. It is therefore important that their nursing practice and interventions are informed by the best available evidence.

Deadspace ventilation: a waste of breath!

Problems with ventilatory efficiency results in abnormal CO(2) clearance. Measuring deadspace ventilation should be the most reliable method of monitoring ventilatory efficiency in mechanically ventilated patients. Since its first description by Bohr in the late 19th century to the current use of single-breath test for volumetric CO(2), our understanding of the physiological deadspace has vastly improved. Yet indices of oxygenation seem to be the mainstay when instigating or fine-tuning ventilatory strategies. Deadspace and with it ventilatory efficiency has been largely forgotten. Recently though there has been a resurgence of interest in ventilatory efficiency. Several indices have been described that either predict deadspace or track ventilatory efficiency at the bedside. Thus making it more accessible and easy to monitor and study in large groups of patients, factors which have perhaps resulted in its under-utilisation in critical care. This review revisits the physiological concepts and methods of measuring deadspace. Described are the various clinical applications of deadspace measurements in the critically unwell. The potential reasons that have led to the variable being under-used are also examined. Finally we describe the indices currently available to track ventilatory efficiency at the bedside.

Acute management of acquired brain injury part I: an evidence-based review of non-pharmacological interventions

PRIMARY OBJECTIVE

To review the literature on non-pharmacological interventions used in acute settings to manage elevated intracranial pressure (ICP) and minimize cerebral damage in patients with acquired brain injury (ABI).

MAIN OUTCOMES

A literature search of multiple databases (CINAHL, EMBASE, MEDLINE and PSYCHINFO) and hand-searched articles covering the years 1980-2008 was performed. Peer reviewed articles were assessed for methodological quality using the PEDro scoring system for randomized controlled trials (RCTs) and the Downs and Black tool for RCTs and non-randomized trials. Levels of evidence were assigned and recommendations made.

RESULTS

Five non-invasive interventions for acute ABI management were assessed: adjusting head posture, body rotation (continuous rotational therapy and prone positioning), hyperventilation, hypothermia and hyperbaric oxygen. Two invasive interventions were also reviewed: cerebrospinal fluid (CSF) drainage and decompressive craniectomy (DC).

CONCLUSIONS

There is a paucity of information regarding non-pharmacological acute management of patients with ABI. Strong levels of evidence were found for only four of the seven interventions (decompressive craniectomy, cerebrospinal fluid drainage, hypothermia and hyperbaric oxygen) and only for specific components of their use. Further research into all interventions is warranted.

[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.

Nonventilatory interventions in the acute respiratory distress syndrome

Acute respiratory distress syndrome was first described in 1967. Acute respiratory distress syndrome and acute lung injury are diseases the busy intensivist treats almost daily. The etiologies of acute respiratory distress syndrome are many. A significant distinction is based on whether the insult to the lung was direct, such as in pneumonia, or indirect, such as trauma or sepsis. Strategies for managing patients with acute respiratory distress syndrome/acute lung injury can be subdivided into 2 large groups, those based in manipulation of mechanical ventilation and those based in nonventilatory modalities. This review focuses on the nonventlilatory strategies and includes fluid restriction, exogenous surfactant, inhaled nitric oxide, manipulation of production, or administration of eicosanoids, neuromuscular blocking agents, prone position ventilation, glucocorticoids, extracorporeal membrane oxygenation, and administration of beta-agonists. Most of these therapies either have not been studied in large trials or have failed to show a benefit in terms of long-term patient mortality. Many of these therapies have shown promise in terms of improved oxygenation and may therefore be beneficial as rescue therapy for severely hypoxic patients. Recommendations regarding the use of each of these strategies are made, and an algorithm for implementing these strategies is suggested.

[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.

Prone position in mechanically ventilated patients with reduced intracranial compliance

BACKGROUND

Prone position has been used for several years to treat acute lung insufficiency, but in previous studies patients with unstable intracranial pressure (ICP) are mostly excluded. The aim of this study was to investigate if prone position is a safe and useful treatment in patients with reduced intracranial compliance.

METHODS

A consecutive, prospective pilot study of 11 patients admitted to the neuro intensive care unit (NICU) due to traumatic brain injury or intracerebral haemorrhage. ICP, cerebral perfusion pressure (CPP), heart rate (HR), mean arterial blood pressure (MABP), arterial partial pressure of oxygen (PaO(2)), arterial partial pressure of carbon dioxide (PaCO(2)), arterial oxygen saturation (SaO(2)) and respiratory system compliance were measured before, three times during and two times after the patients were placed in the prone position.

RESULTS

No significant changes were demonstrated in ICP, CPP or MABP. PaO(2) and SaO(2) were significantly increased in the prone position. HR was significantly increased in the prone position and after 10 min in the supine post-prone position and the respiratory system compliance was increased after 1 h in the supine post-prone position.

CONCLUSION

Turning NICU patients from the supine to the prone position did not influence ICP, CPP or MABP, but significantly improved patient PaO(2), SaO(2) and respiratory system compliance.

Effects of thoraco-pelvic supports during prone position in patients with acute lung injury/acute respiratory distress syndrome: a physiological study

INTRODUCTION

This study sought to assess whether the use of thoraco-pelvic supports during prone positioning in patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS) improves, deteriorates or leaves unmodified gas exchange, hemodynamics and respiratory mechanics.

METHODS

We studied 11 patients with ALI/ARDS, sedated and paralyzed, mechanically ventilated in volume control ventilation. Prone positioning with or without thoraco-pelvic supports was applied in a random sequence and maintained for a 1-hour period without changing the ventilation setting. In four healthy subjects the pressures between the body and the contact surface were measured with and without thoraco-pelvic supports. Oxygenation variables (arterial and central venous), physiologic dead space, end-expiratory lung volume (helium dilution technique) and respiratory mechanics (partitioned between lung and chest wall) were measured after 60 minutes in each condition.

RESULTS

With thoraco-pelvic supports, the contact pressures almost doubled in comparison with those measured without supports (19.1 +/- 15.2 versus 10.8 +/- 7.0 cmH2O, p < or = 0.05; means +/- SD). The oxygenation-related variables were not different in the prone position, with or without thoraco-pelvic supports; neither were the CO2-related variables. The lung volumes were similar in the prone position with and without thoraco-pelvic supports. The use of thoraco-pelvic supports, however, did lead to a significant decrease in chest wall compliance from 158.1 +/- 77.8 to 102.5 +/- 38.0 ml/cmH2O and a significantly increased pleural pressure from 4.3 +/- 1.9 to 6.1 +/- 1.8 cmH2O, in comparison with the prone position without supports. Moreover, when thoraco-pelvic supports were added, heart rate increased significantly from 82.1 +/- 17.9 to 86.7 +/- 16.7 beats/minute and stroke volume index decreased significantly from 37.8 +/- 6.8 to 34.9 +/- 5.4 ml/m2. The increase in pleural pressure change was associated with a significant increase in heart rate (p = 0.0003) and decrease in stroke volume index (p = 0.0241).

CONCLUSION

The application of thoraco-pelvic supports decreases chest wall compliance, increases pleural pressure and slightly deteriorates hemodynamics without any advantage in gas exchange. Consequently, we stopped their use in clinical practice.

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.

Regulation of regional lung perfusion by nitric oxide

Improved oxygenation has previously been shown in patients with acute lung injury when ventilated in prone position. We hypothesized that this was due to higher regional production of nitric oxide in dorsocaudal lung regions. We measured nitric oxide synthase mRNA expression and nitric oxide production by citrulline assay in ventral and dorsal lung tissue from patients. In volunteers, regional lung perfusion in prone and supine postures was assessed by single photon emission computed tomography using (99m)Tc macroaggregated albumin before and after inhibition of nitric oxide synthase by N(G)-monomethyl-L-arginine infusion. Nitric oxide synthase mRNA expression and nitric oxide production were significantly higher in dorsal compared with ventral lung regions. In supine posture, lung perfusion was shifted to ventral parts during nitric oxide synthase inhibition, whereas in the prone posture lung perfusion remained unchanged. Our results suggest a role for endogenous nitric oxide in regulation of regional pulmonary perfusion.

Acute respiratory distress syndrome

Several combination therapies have been described throughout this article: in all likelihood, it is combination therapy that will allow improved survival of ARDS patients. As medicine moves into the future, clinical trials evaluating the efficacy of therapies for ARDS will be performed. In human critical care medicine, a large forward step was taken when ALI and ARDS were clearly defined. Unfortunately. as good as the definition is, ALI and ARDS occur secondary to many different underlying pathologic processes,perhaps obscuring the benefits of certain therapies for ARDS based on the underlying condition, for example, trauma versus sepsis. Selection of patients entering any ARDS trial is crucial: not only must those patients meet the strict definition of ARDS but the underlying disease process should be clearly identified. Identification of patients suffering from different disease processes before the onset of ARDS will allow for stratification of outcomes according to the intervention and the underlying pathology--comparing apples to apples and not to oranges. We are in a unique position in veterinary medicine. Although frequently financially limited by our clients, we have the opportunity to achieve several goals. First, we need to clearly define what constitutes ALI and ARDS in veterinary medicine. Do we want to rely on the human definitions? Probably not; however, as a group, we need to determine what we will accept as definitions. For example, we may not be able perform right heart catheterizations on all our patients to meet the wedge pressure requirement of human beings of less than 18 mm Hg. Do we agree that a PAOP of less than 18 mmHg is appropriate for animals, and is it appropriate for all animals? Will we accept another measure, for example, pulmonary artery diameter increases with echocardiographic evidence of acceptable left heart function? What is acceptable left heart function? As veterinarians, what do we consider to be hypoxemia? Is it the same in all species that we work with? What do we define as acute onset? Most human ARDS cases occur while patients are in hospital being treated for other problems, whereas many of our patients present already in respiratory distress. If we are unable to ventilate patients for economic or practical reasons, what do we use as the equivalent of the Pao2/Flo, ratio'? Reliance on the pathologist is not reasonable, because many disease processes can look similar to ARDS under the microscope. If anything, ALI and ARDS are clinical diagnoses. It is time for veterinarians to reach a consensus on the definition for ALI and ARDS in our patients. Only when we have a consensus of definition can rational prospective clinical trials of therapies be designed.

Inhaled nitric oxide for acute hypoxic respiratory failure in children and adults: a meta-analysis

We systematically reviewed randomized controlled trials examining inhaled nitric oxide (INO) for the treatment of acute respiratory distress syndrome or acute lung injury in children and adults. Qualitative assessments of identified trials were made, and metaanalyses were performed according to Cochrane methodology. Five randomized controlled trials (n = 535) met entry criteria. One study demonstrated significant improvement in oxygenation in the first 4 days of treatment, with no difference after this. There was no difference in ventilator-free days between treatment and placebo groups, and no specific dose of INO was more advantageous than any other. INO had no effect on mortality in trials without crossover of treatment failures to open-label INO (relative risk, 0.98; 95% confidence interval, 0.66-1.44). Other clinical indicators of effectiveness, such as duration of hospital and intensive care stay, were inconsistently reported. Lack of data prevented assessment of all outcomes. If further trials assessing INO in acute respiratory distress syndrome or acute lung injury are to proceed, they should be stratified for primary etiology, incorporate other modalities that may affect outcome, and evaluate clinically relevant outcomes before any benefit of INO can be excluded.

A protocolized approach to pulmonary failure and the role of intermittent prone positioning

INTRODUCTION

We present a series of adult patients treated under a protocol for severe lung failure (acute respiratory distress syndrome [ARDS]) that uses positive end-expiratory pressure (PEEP) optimization and intermittent prone positioning (IPP) to reduce shunt, improve oxygen (O(2)) delivery, and reduce FiO(2).

METHODS

Trauma, emergency vascular, and general surgical patients with PaO(2)/FiO)(2) (PF) ratio < 200 were entered into a protocol designed to maintain mixed venous saturation (SVO(2)) > 70% with FiO(2) < 0.50. Therapy involved a sequential algorithmic approach that included pulmonary artery oximetry, "best-PEEP" determination, optimization of cardiac function, limitation of VO(2), transfusion to hematocrit of 35%, frequent bronchoscopy, rational diuresis and, if the FiO(2) was > 0.50, a trial of IPP with every-6-hour rotations. Unstable spine fractures and pelvic external fixators were the only contraindications to IPP. We collected data prospectively and from the charts and trauma registry.

RESULTS

Forty adults were treated by protocol, 29 were injured and 11 had vascular or general surgical primary problems. The patients were 46.3 +/- 3.1 years old (the trauma patients were 42.9 +/- 3.2, and the vascular/general patients were 62 +/- 7.5 years old). Average Injury Severity Score was 25.9 +/- 3.7 and the Murray lung injury score was 2.65 +/- 0.9. IPP was used in all patients including those with recent tracheostomy, open abdomens, laparotomy, thoracotomy, leg external fixators, central nervous system injury, continuous venovenous hemofiltration and extracorporeal membrane oxygenation cannulae, vasopressor therapy, recent chest wall open reduction and internal fixation, and facial fractures. With the initiation of IPP therapy, the PF ratio increased from 132.1 +/- 8.5 to 231.6 +/- 14.2 (p < 0.001), the FiO(2) was decreased from 65.9 +/- 4.0% to 47.0 +/- 1.1% (p < 0.001), and the SVO(2) increased from 75.3 +/- 1.8% to 78.6 +/- 1.6% (p = 0.023). PEEP and static compliance were unchanged. The duration of IPP was 85.6 +/- 14.9 hours (median, 55 hours; range, 12 to 490 hours). Within 48 hours, all patients were on FiO(2) < or = 50. Mortality was 20% (14% for trauma) and none died of ARDS. The only complications of IPP were one case of partial-thickness skin loss from a malpositioned nasogastric tube and a case of transient lingual edema.

CONCLUSION

IPP was independently responsible for an increase in PF ratio and SVO(2). We effectively and safely used IPP in our patients with ARDS, including many with issues generally considered to be contraindications. IPP and best-PEEP therapy enabled us to wean all of our patients' Fio2 to < or = 0.50 within 48 hours of ARDS onset.

Gas exchange in the ventilated patient

Increased knowledge of the pathophysiologic mechanisms of impaired gas exchange during acute respiratory failure during recent years has stimulated many studies that evaluate different treatments to improve oxygenation and outcome. Changes in body position (mainly prone positioning) can significantly improve gas exchange in patients with acute respiratory distress syndrome and acute lung failure, with few complications related to the maneuver; however, no survival advantage has yet been detected. A correlation between aerated lung tissue and oxygenation also confirms the importance of recruitment maneuvers in improving gas exchange. Recent suggestions that recruitment of alveoli proceeds during most of the inspired vital capacity and not only around the lower inflection point of the pressure-volume curve raises the question how to best perform recruitment maneuvers. New data support the hypothesis that maintenance of even small amount of spontaneous breathing during mechanical ventilation (with airway pressure release ventilation or biphasic positive airway pressure) can improve gas exchange, whereas other unconventional ventilatory modes have not yet proved advantageous. Some mechanisms responsible for the high percentage of nonresponse to inhaled nitric oxide have recently been proposed, and combinations of inhaled nitric oxide with other therapies have been tested. Increased knowledge in this area may, in the future, make inhaled nitric oxide more attractive in the treatment of adult respiratory failure as well as in neonatal intensive care.

Effects of prone position ventilation in ARDS. An evidence-based review of the literature

In summary, the many studies done on PPV show that the technique improves oxygenation most of the time. The mechanisms behind this effect are probably numerous and have yet to be elucidated completely. In addition, PPV is a safe procedure that rarely worsens a patient's respiratory status or causes other complications and is thus a welcome additional therapeutic option when treating patients with ARDS. Despite the recent large, randomized, controlled trial showing no improvement in mortality rate or organ dysfunction overall, there is evidence suggesting that PPV may be of most benefit in more severely ill patients. Further studies will be useful.

Prone positioning and inhaled nitric oxide: synergistic therapies for acute respiratory distress syndrome

BACKGROUND

Inhaled nitric oxide (INO) and prone positioning have both been advocated as methods to improve oxygenation in patients with acute respiratory distress syndrome (ARDS). This study was designed to evaluate the relative contributions of INO and prone positioning alone and in combination on gas exchange in trauma patients with ARDS.

METHODS

Sixteen patients meeting the consensus definition of ARDS were studied. Patients received mechanical ventilation in the supine position, mechanical ventilation plus INO at 1 part per million in the supine position, mechanical ventilation in the PP, and mechanical ventilation in the prone positioning plus INO at 1 part per million. A stabilization period of 1 hour was allowed at each condition. After stabilization,hemodynamic and gas exchange variables were measured.

RESULTS

INO and prone positioning both increased PaO2/FIO2 compared with ventilation in the supine position. PaO2/FIO2 increased by 14% during use of INO, and 10 of 16 patients (62%) responded to INO in the supine position. PaO2/FIO2 increased by 33%, and 14 of 16 patients (87.5%) responded to the prone position. The combination of INO and prone positioning resulted in an improvement in PaO2/FIO2 in 15 of 16 patients(94%), with a mean increase in PaO2/FIO2 of 59%. Pulmonary vascular resistance was reduced during use of INO, with a greater reduction in pulmonary vascular resistance seen with INO plus prone positioning (175 +/- 36 dynes x s/cm5 vs. 134 +/- 28 dynes x s/cm5) compared with INO in the supine position (164 +/- 48 dynes x s/cm5 vs.138 +/- 44 dynes x s/cm5). There were no significant hemodynamic effects of INO or prone positioning and no complications were seen during this relative short duration of study.

CONCLUSIONS

INO and prone positioning can contribute to improved oxygenation in patients with ARDS. The two therapies in combination are synergistic and may be important adjuncts to mechanical ventilation in the ARDS patient with refractory hypoxemia.