FiftyYears ofResearch inARDS. Spontaneous Breathing during Mechanical Ventilation. Risks, Mechanisms, and Management

Airway Pressure Release Ventilation in Pediatric Acute Respiratory Distress Syndrome. A Randomized Controlled Trial

RATIONALE

Although case series describe benefits of airway pressure release ventilation (APRV), this mode of ventilation has not been evaluated against the conventional low-tidal volume ventilation (LoTV) in children with acute respiratory distress syndrome (ARDS).

OBJECTIVES

To compare the effect of APRV and conventional LoTV on ventilator-free days in children with ARDS.

METHODS

This open-label, parallel-design randomized controlled trial was conducted in a 15-bed ICU. Children aged 1 month to 12 years satisfying the modified Berlin definition were included. We excluded children with air leaks, increased intracranial pressure, poor spontaneous breathing efforts, chronic lung disease, and beyond 24 hours of ARDS diagnosis or 72 hours of ventilation. Children were randomized using unstratified, variable-sized block technique. A priori interim analysis was planned at 50% enrollment. All enrolled children were followed up until 180 days after enrollment or death, whichever was earlier.

MEASUREMENTS AND MAIN RESULTS

The trial was terminated after 50% enrollment (52 children) when analysis revealed higher mortality in the intervention arm. Ventilator-free days were statistically similar in both arms (P = 0.23). The 28-day all-cause mortality was 53.8% in APRV as compared with 26.9% among control subjects (risk ratio, 2.0; 95% confidence interval, 0.97-4.1; Fisher exact P = 0.089). The multivariate-adjusted risk ratio of death for APRV compared with LoTV was 2.02 (95% confidence interval, 0.99-4.12; P = 0.05). Higher mean airway pressures, greater spontaneous breathing, and early improvement in oxygenation were seen in the intervention arm.

CONCLUSIONS

APRV, as a primary ventilation strategy in children with ARDS, was associated with a trend toward higher mortality compared with the conventional LoTV. Limitations should be considered while interpreting these results. Clinical trial registered with www.clinicaltrials.gov (NCT02167698) and Clinical Trials Registry of India (CTRI/2014/06/004677).

High Positive End-Expiratory Pressure Renders Spontaneous Effort Noninjurious

RATIONALE

In acute respiratory distress syndrome (ARDS), atelectatic solid-like lung tissue impairs transmission of negative swings in pleural pressure (Ppl) that result from diaphragmatic contraction. The localization of more negative Ppl proportionally increases dependent lung stretch by drawing gas either from other lung regions (e.g., nondependent lung [pendelluft]) or from the ventilator. Lowering the level of spontaneous effort and/or converting solid-like to fluid-like lung might render spontaneous effort noninjurious.

OBJECTIVES

To determine whether spontaneous effort increases dependent lung injury, and whether such injury would be reduced by recruiting atelectatic solid-like lung with positive end-expiratory pressure (PEEP).

METHODS

Established models of severe ARDS (rabbit, pig) were used. Regional histology (rabbit), inflammation (positron emission tomography; pig), regional inspiratory Ppl (intrabronchial balloon manometry), and stretch (electrical impedance tomography; pig) were measured. Respiratory drive was evaluated in 11 patients with ARDS.

MEASUREMENTS AND MAIN RESULTS

Although injury during muscle paralysis was predominantly in nondependent and middle lung regions at low (vs. high) PEEP, strong inspiratory effort increased injury (indicated by positron emission tomography and histology) in dependent lung. Stronger effort (vs. muscle paralysis) caused local overstretch and greater tidal recruitment in dependent lung, where more negative Ppl was localized and greater stretch was generated. In contrast, high PEEP minimized lung injury by more uniformly distributing negative Ppl, and lowering the magnitude of spontaneous effort (i.e., deflection in esophageal pressure observed in rabbits, pigs, and patients).

CONCLUSIONS

Strong effort increased dependent lung injury, where higher local lung stress and stretch was generated; effort-dependent lung injury was minimized by high PEEP in severe ARDS, which may offset need for paralysis.

Standardized Unloading of Respiratory Muscles during Neurally Adjusted Ventilatory Assist: A Randomized Crossover Pilot Study

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Currently, there is no standardized method to set the support level in neurally adjusted ventilatory assist (NAVA). The primary aim was to explore the feasibility of titrating NAVA to specific diaphragm unloading targets, based on the neuroventilatory efficiency (NVE) index. The secondary outcome was to investigate the effect of reduced diaphragm unloading on distribution of lung ventilation.

METHODS

This is a randomized crossover study between pressure support and NAVA at different diaphragm unloading at a single neurointensive care unit. Ten adult patients who had started weaning from mechanical ventilation completed the study. Two unloading targets were used: 40 and 60%. The NVE index was used to guide the titration of the assist in NAVA. Electrical impedance tomography data, blood-gas samples, and ventilatory parameters were collected.

RESULTS

The median unloading was 43% (interquartile range 32, 60) for 40% unloading target and 60% (interquartile range 47, 69) for 60% unloading target. NAVA with 40% unloading led to more dorsal ventilation (center of ventilation at 55% [51, 56]) compared with pressure support (52% [49, 56]; P = 0.019). No differences were found in oxygenation, CO2, and respiratory parameters. The electrical activity of the diaphragm was higher during NAVA with 40% unloading than in pressure support.

CONCLUSIONS

In this pilot study, NAVA could be titrated to different diaphragm unloading levels based on the NVE index. Less unloading was associated with greater diaphragm activity and improved ventilation of the dependent lung regions.

Airway pressure release ventilation during acute hypoxemic respiratory failure: a systematic review and meta-analysis of randomized controlled trials

Background

Airway pressure release ventilation (APRV) has been considered a tempting mode of ventilation during acute respiratory failure within the concept of open lung ventilation. We performed a systematic review and meta-analysis to verify whether adult patients with hypoxemic respiratory failure have a higher number of ventilator-free days at day 28 when ventilated in APRV compared to conventional ventilation strategy. Secondary outcomes were difference in PaO₂/FiO₂ at day 3, ICU length of stay (LOS), ICU and hospital mortality, mean arterial pressure (MAP), risk of barotrauma and level of sedation. We searched MEDLINE, Scopus and Cochrane Central Register of Controlled Trials database until December 2018.

Results

We considered five RCTs for the analysis enrolling a total of 330 patients. For ventilatory-free day at day 28, the overall mean difference (MD) between APRV and conventional ventilation was 6.04 days (95%CI 2.12, 9.96, p = 0.003; I² = 65%, p = 0.02). Patients treated with APRV had a lower ICU LOS than patients treated with conventional ventilation (MD 3.94 days [95%CI 1.44, 6.45, p = 0.002; I² = 37%, p = 0.19]) and a lower hospital mortality (RD 0.16 [95%CI 0.02, 0.29, p = 0.03; I² = 0, p = 0.5]). PaO₂/FiO₂ at day 3 was not different between the two groups (MD 40.48 mmHg [95%CI − 25.78, 106.73, p = 0.23; I² = 92%, p < 0.001]). MAP was significantly higher during APRV (MD 5 mmHg [95%CI 1.43, 8.58, p = 0.006; I² = 0%, p = 0.92]). Then, there was no difference regarding the onset of pneumothorax under the two ventilation strategies (RR 1.94 [95%CI 0.54, 6.94, p = 0.31; I² = 0%, p = 0.74]). ICU mortality and sedation level were not included into quantitative analysis.

Conclusion

This study showed a higher number of ventilator-free days at 28 day and a lower hospital mortality in acute hypoxemic patients treated with APRV than conventional ventilation, without any negative hemodynamic impact or higher risk of barotrauma. However, these results need to be interpreted with caution because of the low-quality evidence supporting them and the moderate heterogeneity found. Other well-designed RCTs need to be conducted to confirm our findings.

Does volatile sedation with sevoflurane allow spontaneous breathing during prolonged prone positioning in intubated ARDS patients? A retrospective observational feasibility trial

Background

Lung-protective ventilation and prolonged prone positioning (PP) are presented as essential in treating acute respiratory distress syndrome (ARDS). The optimal respirator mode, however, remains controversial. Pressure-supported spontaneous breathing (PS) during ARDS provides several advantages, but is difficult to achieve during PP because of respiratory depression as a side effect of sedative drugs. This study was designed to evaluate the feasibility and safety of PS during PP in ARDS patients sedated with inhaled sevoflurane.

Results

Overall, we have observed 4339 h of prone positioning in 62 patients who had a median of four prone episodes during treatment. Within 3948 h (91%), patients were successfully brought into a pressure-supported spontaneous breathing mode. The median duration of each prone episode was 17 h (IQR 3). Median duration of pressure-supported spontaneous breathing per episode was 16 h (IQR 5). Just one self-extubation occurred during 276 episodes of PP.

Conclusions and implications

Pressure-supported spontaneous breathing during prolonged prone positioning in intubated ARDS patients with or without ECMO can be achieved during volatile sedation with sevoflurane. This finding may provide a basis upon which to question the latest dogma in ARDS treatment. Our concept must be further investigated and compared to controlled ventilation with regard to driving pressure, lung-protective parameters, muscle weakness and mortality before it can be routinely applied.

Outcomes of Patients Presenting with Mild Acute Respiratory Distress Syndrome

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Patients with initial mild acute respiratory distress syndrome are often underrecognized and mistakenly considered to have low disease severity and favorable outcomes. They represent a relatively poorly characterized population that was only classified as having acute respiratory distress syndrome in the most recent definition. Our primary objective was to describe the natural course and the factors associated with worsening and mortality in this population.

Methods

This study analyzed patients from the international prospective Large Observational Study to Understand the Global Impact of Severe Acute Respiratory Failure (LUNG SAFE) who had initial mild acute respiratory distress syndrome in the first day of inclusion. This study defined three groups based on the evolution of severity in the first week: “worsening” if moderate or severe acute respiratory distress syndrome criteria were met, “persisting” if mild acute respiratory distress syndrome criteria were the most severe category, and “improving” if patients did not fulfill acute respiratory distress syndrome criteria any more from day 2.

Results

Among 580 patients with initial mild acute respiratory distress syndrome, 18% (103 of 580) continuously improved, 36% (210 of 580) had persisting mild acute respiratory distress syndrome, and 46% (267 of 580) worsened in the first week after acute respiratory distress syndrome onset. Global in-hospital mortality was 30% (172 of 576; specifically 10% [10 of 101], 30% [63 of 210], and 37% [99 of 265] for patients with improving, persisting, and worsening acute respiratory distress syndrome, respectively), and the median (interquartile range) duration of mechanical ventilation was 7 (4, 14) days (specifically 3 [2, 5], 7 [4, 14], and 11 [6, 18] days for patients with improving, persisting, and worsening acute respiratory distress syndrome, respectively). Admissions for trauma or pneumonia, higher nonpulmonary sequential organ failure assessment score, lower partial pressure of alveolar oxygen/fraction of inspired oxygen, and higher peak inspiratory pressure were independently associated with worsening.

Conclusions

Most patients with initial mild acute respiratory distress syndrome continue to fulfill acute respiratory distress syndrome criteria in the first week, and nearly half worsen in severity. Their mortality is high, particularly in patients with worsening acute respiratory distress syndrome, emphasizing the need for close attention to this patient population.

Driving pressure during proportional assist ventilation: an observational study

Background

During passive mechanical ventilation, the driving pressure of the respiratory system is an important mediator of ventilator-induced lung injury. Monitoring of driving pressure during assisted ventilation, similar to controlled ventilation, could be a tool to identify patients at risk of ventilator-induced lung injury. The aim of this study was to describe driving pressure over time and to identify whether and when high driving pressure occurs in critically ill patients during assisted ventilation.

Methods

Sixty-two patients fulfilling criteria for assisted ventilation were prospectively studied. Patients were included when the treating physician selected proportional assist ventilation (PAV+), a mode that estimates respiratory system compliance. In these patients, continuous recordings of all ventilator parameters were obtained for up to 72 h. Driving pressure was calculated as tidal volume-to-respiratory system compliance ratio. The distribution of driving pressure and tidal volume values over time was examined, and periods of sustained high driving pressure (≥ 15 cmH₂O) and of stable compliance were identified and analyzed.

Results

The analysis included 3200 h of ventilation, consisting of 8.8 million samples. For most (95%) of the time, driving pressure was < 15 cmH₂O and tidal volume < 11 mL/kg (of ideal body weight). In most patients, high driving pressure was observed for short periods of time (median 2.5 min). Prolonged periods of high driving pressure were observed in five patients (8%). During the 661 periods of stable compliance, high driving pressure combined with a tidal volume ≥ 8 mL/kg was observed only in 11 cases (1.6%) pertaining to four patients. High driving pressure occurred almost exclusively when respiratory system compliance was low, and compliance above 30 mL/cmH₂O excluded the presence of high driving pressure with 90% sensitivity and specificity.

Conclusions

In critically ill patients fulfilling criteria for assisted ventilation, and ventilated in PAV+ mode, sustained high driving pressure occurred in a small, yet not negligible number of patients. The presence of sustained high driving pressure was not associated with high tidal volume, but occurred almost exclusively when compliance was below 30 mL/cmH₂O.

Electronic supplementary material

The online version of this article (10.1186/s13613-018-0477-4) contains supplementary material, which is available to authorized users.

Weaning from Mechanical Ventilation in ARDS: Aspects to Think about for Better Understanding, Evaluation, and Management

Acute respiratory distress syndrome (ARDS) is characterized by severe inflammatory response and hypoxemia. The use of mechanical ventilation (MV) for correction of gas exchange can cause worsening of this inflammatory response, called “ventilator-induced lung injury” (VILI). The process of withdrawing mechanical ventilation, referred to as weaning from MV, may cause worsening of lung injury by spontaneous ventilation. Currently, there are few specific studies in patients with ARDS. Herein, we reviewed the main aspects of spontaneous ventilation and also discussed potential methods to predict the failure of weaning in this patient category. We also reviewed new treatments (modes of mechanical ventilation, neuromuscular blocker use, and extracorporeal membrane oxygenation) that could be considered in weaning ARDS patients from MV.

The future of driving pressure: a primary goal for mechanical ventilation?

Background

Management of patients with acute respiratory distress syndrome (ARDS) remains supportive with lung protective mechanical ventilation. In this article, we discuss the physiological concept of driving pressure, current data, ongoing trials, and future directions needed to clarify the role of driving pressure in patients with ARDS.

Body

Driving pressure is the plateau airway pressure minus PEEP. It can also be expressed as the ratio of tidal volume to respiratory system compliance, indicating the decreased functional size of the lung observed in patients with ARDS (i.e., baby lung). Driving pressure as a strong predictor of mortality in patients with ARDS is supported by a post hoc analysis of previous randomized controlled trials and a subsequent meta-analysis. Importantly, the meta-analysis suggested targeting driving pressure below 13–15 cmH2O. Ongoing clinical trials of driving pressure in patients with ARDS focus mainly on physiological rather than clinical outcome but will provide important insights for the design of future clinical trials.

Conclusion

Currently, no definite clinical recommendations on the routine use of driving pressure in patients with ARDS can be made, as the available data are hypothesis-generating. Randomized controlled trials are needed to evaluate the efficacy of a driving pressure-based ventilation strategy.

Respiratory mechanics in patients with acute respiratory distress syndrome

Despite the recognition of its iatrogenic potential, mechanical ventilation remains the mainstay of respiratory support for patients with acute respiratory distress syndrome (ARDS). The low volume ventilation has been recognized as the only method to reduce mortality of ARDS patients and plateau pressure as the lighthouse for delivering safe ventilation. Recent investigations suggest that a ventilation based on lung mechanics (tidal ventilation tailored to the available lung volume able to receive it, i.e., driving pressure) is a successful approach to improve outcome. However, currently available bedside mechanical variables do not consider regional mechanical properties of ARDS affected lungs, which include the role of local stress risers at the boundaries of areas with different aeration. A unifying approach considers lung-related causes and ventilation-related causes of lung injury. These last may be incorporated in the mechanical power (i.e., amount of mechanical energy transferred per unit of time). Ventilation-induced lung injury (which includes the self-inflicted lung injury of a spontaneously breathing patient) can therefore be prevented by the adoption of measures promoting an increase of ventilable lung and its homogeneity and by delivering lower levels of mechanical power. Prone position promotes lung homogeneity without increasing the delivered mechanical power. This review describes the recent developments on respiratory mechanics in ARDS patients, providing both bedside and research insights from the most updated evidence.

Should we titrate mechanical ventilation based on driving pressure?-yes

Recent reports revealed that lower respiratory driving pressure is associated with better ARDS patients survival during invasive mechanical ventilation and less pulmonary complications in surgical patients and at risk ICU patients without ARDS, makes the best understanding of this subject primordial for the future application of mechanical ventilatory support.

Regional distribution of transpulmonary pressure

The pressure across the lung, so-called transpulmonary pressure (P_L), represents the main force acting toward to provide lung movement. During mechanical ventilation, P_L is provided by respiratory system pressurization, using specific ventilator setting settled by the operator, such as: tidal volume (V_T), positive end-expiratory pressure (PEEP), respiratory rate (RR), and inspiratory airway flow. Once P_L is developed throughout the lungs, its distribution is heterogeneous, being explained by the elastic properties of the lungs and pleural pressure gradient. There are different methods of P_L calculation, each one with importance and some limitations. Among the most known, it can be quoted: (I) direct measurement of P_L; (II) elastance derived method at end-inspiration of P_L; (III) transpulmonary driving pressure. Recent studies using pleural sensors in large animal models as also in human cadaver have added new and important information about P_L heterogeneous distribution across the lungs. Due to this heterogeneous distribution, lung damage could happen in specific areas of the lung. In addition, it is widely accepted that high P_L can cause lung damage, however the way it is delivered, whether it's compressible or tensile, may also further damage despite the values of P_L achieved. According to heterogeneous distribution of P_L across the lungs, the interstitium and lymphatic vessels may also interplay to disseminate lung inflammation toward peripheral organs through thoracic lymph tracts. Thus, it is conceivable that juxta-diaphragmatic area associated strong efforts leading to high values of P_L may be a source of dissemination of inflammatory cells, large molecules, and plasma contents able to perpetuate inflammation in distal organs.

Assessing breathing effort in mechanical ventilation: physiology and clinical implications

Recent studies have shown both beneficial and detrimental effects of patient breathing effort in mechanical ventilation. Quantification of breathing effort may allow the clinician to titrate ventilator support to physiological levels of respiratory muscle activity. In this review we will describe the physiological background and methodological issues of the most frequently used methods to quantify breathing effort, including esophageal pressure measurement, the work of breathing, the pressure-time-product, electromyography and ultrasound. We will also discuss the level of breathing effort that may be considered optimal during mechanical ventilation at different stages of critical illness.

Approaches and techniques to avoid development or progression of acute respiratory distress syndrome

PURPOSE OF REVIEW

Despite major improvement in ventilation strategies, hospital mortality and morbidity of the acute respiratory distress syndrome (ARDS) remain high. A lot of therapies have been shown to be ineffective for established ARDS. There is a growing interest in strategies aiming at avoiding development and progression of ARDS.

RECENT FINDINGS

Recent advances in this field have explored identification of patients at high-risk, nonspecific measures to limit the risks of inflammation, infection and fluid overload, prevention strategies of ventilator-induced lung injury and patient self-inflicted lung injury, and pharmacological treatments.

SUMMARY

There is potential for improvement in the management of patients admitted to intensive care unit to reduce ARDS incidence. Apart from nonspecific measures, prevention of ventilator-induced lung injury and patient self-inflicted lung injury are of major importance.

Dissipation of energy during the respiratory cycle: conditional importance of ergotrauma to structural lung damage

PURPOSE OF REVIEW

To describe and put into context recent conceptual advances regarding the relationship of energy load and power to ventilator-induced lung injury (VILI).

RECENT FINDINGS

Investigative emphasis regarding VILI has almost exclusively centered on the static characteristics of the individual tidal cycle - tidal volume, plateau pressure, positive end-expiratory pressure, and driving pressure. Although those static characteristics of the tidal cycle are undeniably important, the 'dynamic' characteristics of ventilation must not be ignored. To inflict the nonrupturing damage we identify as VILI, work must be performed and energy expended by high stress cycles applied at rates that exceed the capacity of endogenous repair. Machine power, the pace at which the work performing energy load is applied by the ventilator, has received increasing scrutiny as a candidate for the proximate and integrative cause of VILI.

SUMMARY

Although the unmodified values of machine-delivered energy or power (which are based on airway pressures and tidal volumes) cannot serve unconditionally as a rigid and quantitative guide to ventilator adjustment for lung protection, bedside consideration of the dynamics of ventilation and potential for ergotrauma represents a clear conceptual advance that complements the static parameters of the individual tidal cycle that with few exceptions have held our scientific attention.

Building on the Shoulders of Giants: Is the use of Early Spontaneous Ventilation in the Setting of Severe Diffuse Acute Respiratory Distress Syndrome Actually Heretical?

Acute respiratory distress syndrome (ARDS) is not a failure of the neurological command of the ventilatory muscles or of the ventilatory muscles; it is an oxygenation defect. As positive pressure ventilation impedes the cardiac function, paralysis under general anaesthesia and controlled mandatory ventilation should be restricted to the interval needed to control the acute cardio-ventilatory distress observed upon admission into the critical care unit (CCU; "salvage therapy" during "shock state"). Current management of early severe diffuse ARDS rests on a prolonged interval of controlled mechanical ventilation with low driving pressure, paralysis (48 h, too often overextended), early proning and positive end-expiratory pressure (PEEP). Therefore, the time interval between arrival to the CCU and switching to spontaneous ventilation (SV) is not focused on normalizing the different factors involved in the pathophysiology of ARDS: fever, low cardiac output, systemic acidosis, peripheral shutdown (local acidosis), supine position, hypocapnia (generated by hyperpnea and tachypnea), sympathetic activation, inflammation and agitation. Then, the extended period of controlled mechanical ventilation with paralysis under general anaesthesia leads to CCU-acquired pathology, including low cardiac output, myoneuropathy, emergence delirium and nosocomial infection. The stabilization of the acute cardio-ventilatory distress should primarily itemize the pathophysiological conditions: fever control, improved micro-circulation and normalized local acidosis, 'upright' position, minimized hypercapnia, sympathetic de-activation (normalized sympathetic activity toward baseline levels resulting in improved micro-circulation with alpha-2 agonists administered immediately following optimized circulation and endotracheal intubation), lowered inflammation and 'cooperative' sedation without respiratory depression evoked by alpha-2 agonists. Normalised metabolic, circulatory and ventilatory demands will allow one to single out the oxygenation defect managed with high PEEP (diffuse recruitable ARDS) under early spontaneous ventilation (airway pressure release ventilation+SV or low-pressure support). Assuming an improved overall status, PaO₂/FiO₂≥150-200 allows for extubation and continuous non-invasive ventilation. Such fast-tracking may avoid most of the CCU-acquired pathologies. Evidence-based demonstration is required.

Mechanical Ventilation in Adults with Acute Respiratory Distress Syndrome. Summary of the Experimental Evidence for the Clinical Practice Guideline

RATIONALE

The American Thoracic Society/European Society for Intensive Care Medicine/Society of Critical Care Medicine guidelines on mechanical ventilation in adult patients with acute respiratory distress syndrome (ARDS) provide treatment recommendations derived from a thorough analysis of the clinical evidence on six clinical interventions. However, each of the recommendations contains areas of uncertainty and controversy, which may affect their appropriate clinical application.

OBJECTIVES

To provide a critical review of the experimental evidence surrounding the pathophysiology of ventilator-induced lung injury and to help clinicians apply the clinical recommendations to individual patients.

METHODS

We conducted a literature search and narrative review.

RESULTS

A large number of experimental studies have been performed with the aim of improving understanding of the pathophysiological effects of mechanical ventilation. These studies have formed the basis for the design of many clinical trials. Translational research has fundamentally advanced understanding of the mechanisms of ventilator-induced lung injury, thus informing the design of interventions that improve survival in patients with ARDS.

CONCLUSIONS

Because daily management of patients with ARDS presents the challenge of competing considerations, clinicians should consider the mechanism of ventilator-induced lung injury, as well as the rationale for interventions designed to mitigate it, when applying evidence-based recommendations at the bedside.

Hypothesis: Fever control, a niche for alpha-2 agonists in the setting of septic shock and severe acute respiratory distress syndrome?

During severe septic shock and/or severe acute respiratory distress syndrome (ARDS) patients present with a limited cardio-ventilatory reserve (low cardiac output and blood pressure, low mixed venous saturation, increased lactate, low PaO2/FiO2 ratio, etc.), especially when elderly patients or co-morbidities are considered. Rescue therapies (low dose steroids, adding vasopressin to noradrenaline, proning, almitrine, NO, extracorporeal membrane oxygenation, etc.) are complex. Fever, above 38.5-39.5°C, increases both the ventilatory (high respiratory drive: large tidal volume, high respiratory rate) and the metabolic (increased O2 consumption) demands, further limiting the cardio-ventilatory reserve. Some data (case reports, uncontrolled trial, small randomized prospective trials) suggest that control of elevated body temperature ("fever control") leading to normothermia (35.5-37°C) will lower both the ventilatory and metabolic demands: fever control should simplify critical care management when limited cardio-ventilatory reserve is at stake. Usually fever control is generated by a combination of general anesthesia ("analgo-sedation", light total intravenous anesthesia), antipyretics and cooling. However general anesthesia suppresses spontaneous ventilation, making the management more complex. At variance, alpha-2 agonists (clonidine, dexmedetomidine) administered immediately following tracheal intubation and controlled mandatory ventilation, with prior optimization of volemia and atrio-ventricular conduction, will reduce metabolic demand and facilitate normothermia. Furthermore, after a rigorous control of systemic acidosis, alpha-2 agonists will allow for accelerated emergence without delirium, early spontaneous ventilation, improved cardiac output and micro-circulation, lowered vasopressor requirements and inflammation. Rigorous prospective randomized trials are needed in subsets of patients with a high fever and spiraling toward refractory septic shock and/or presenting with severe ARDS.

Acute exacerbation of idiopathic pulmonary fibrosis: lessons learned from acute respiratory distress syndrome?

Idiopathic pulmonary fibrosis (IPF) is a fibrotic lung disease characterized by progressive loss of lung function and poor prognosis. The so-called acute exacerbation of IPF (AE-IPF) may lead to severe hypoxemia requiring mechanical ventilation in the intensive care unit (ICU). AE-IPF shares several pathophysiological features with acute respiratory distress syndrome (ARDS), a very severe condition commonly treated in this setting.A review of the literature has been conducted to underline similarities and differences in the management of patients with AE-IPF and ARDS.During AE-IPF, diffuse alveolar damage and massive loss of aeration occurs, similar to what is observed in patients with ARDS. Differently from ARDS, no studies have yet concluded on the optimal ventilatory strategy and management in AE-IPF patients admitted to the ICU. Notwithstanding, a protective ventilation strategy with low tidal volume and low driving pressure could be recommended similarly to ARDS. The beneficial effect of high levels of positive end-expiratory pressure and prone positioning has still to be elucidated in AE-IPF patients, as well as the precise role of other types of respiratory assistance (e.g., extracorporeal membrane oxygenation) or innovative therapies (e.g., polymyxin-B direct hemoperfusion). The use of systemic drugs such as steroids or immunosuppressive agents in AE-IPF is controversial and potentially associated with an increased risk of serious adverse reactions.Common pathophysiological abnormalities and similar clinical needs suggest translating to AE-IPF the lessons learned from the management of ARDS patients. Studies focused on specific therapeutic strategies during AE-IPF are warranted.

Fifty Years of Research in ARDS. Vt Selection in Acute Respiratory Distress Syndrome

Mechanical ventilation (MV) is critical in the management of many patients with acute respiratory distress syndrome (ARDS). However, MV can also cause ventilator-induced lung injury (VILI). The selection of an appropriate Vt is an essential part of a lung-protective MV strategy. Since the publication of a large randomized clinical trial demonstrating the benefit of lower Vts, the use of Vts of 6 ml/kg predicted body weight (based on sex and height) has been recommended in clinical practice guidelines. However, the predicted body weight approach is imperfect in patients with ARDS because the amount of aerated lung varies considerably due to differences in inflammation, consolidation, flooding, and atelectasis. Better approaches to setting Vt may include limits on end-inspiratory transpulmonary pressure, lung strain, and driving pressure. The limits of lowering Vt have not yet been established, and some patients may benefit from Vts that are lower than those in current use. However, lowering Vts may result in respiratory acidosis. Tactics to reduce respiratory acidosis include reductions in ventilation circuit dead space, increases in respiratory rate, higher positive end-expiratory pressures in patients who recruit lung in response to positive end-expiratory pressure, recruitment maneuvers, and prone positioning. Mechanical adjuncts such as extracorporeal carbon dioxide removal may be useful to normalize pH and carbon dioxide levels, but further studies will be necessary to demonstrate benefit with this technology.

Fifty Years of Research in ARDS. Respiratory Mechanics in Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome is a multifactorial lung injury that continues to be associated with high levels of morbidity and mortality. Mechanical ventilation, although lifesaving, is associated with new iatrogenic injury. Current best practice involves the use of small Vt, low plateau and driving pressures, and high levels of positive end-expiratory pressure. Collectively, these interventions are termed "lung-protective ventilation." Recent investigations suggest that individualized measurements of pulmonary mechanical variables rather than population-based ventilation prescriptions may be used to set the ventilator with the potential to improve outcomes beyond those achieved with standard lung protective ventilation. This review outlines the measurement and application of clinically applicable pulmonary mechanical concepts, such as plateau pressures, driving pressure, transpulmonary pressures, stress index, and measurement of strain. In addition, the concept of the "baby lung" and the utility of dynamic in addition to static measures of pulmonary mechanical variables are discussed.

Pharmacological modulation of C-X-C motif chemokine receptor 4 influences development of acute respiratory distress syndrome after lung ischaemia-reperfusion injury

Activation of C-X-C motif chemokine receptor 4 (CXCR4) has been reported to result in lung protective effects in various experimental models. The effects of pharmacological CXCR4 modulation on the development of acute respiratory distress syndrome (ARDS) after lung injury, however, are unknown. Thus, we studied whether blockade and activation of CXCR4 influences development of ARDS in a unilateral lung ischaemia-reperfusion injury rat model. Anaesthetized, mechanically ventilated animals underwent right lung ischaemia (series 1, 30 minutes; series 2, 60 minutes) followed by reperfusion for 300 minutes. In series 1, animals were treated with vehicle or 0.7 μmol/kg of AMD3100 (CXCR4 antagonist) and in series 2 with vehicle, 0.7 or 3.5 μmol/kg ubiquitin (non-cognate CXCR4 agonist) within 5 minutes of reperfusion. AMD3100 significantly reduced PaO₂ /FiO₂ ratios, converted mild ARDS with vehicle treatment into moderate ARDS (PaO₂ /FiO₂ ratio<200) and increased histological lung injury. Ubiquitin dose-dependently increased PaO₂ /FiO₂ ratios, converted moderate-to-severe into mild-to-moderate ARDS and reduced protein content of bronchoalveolar lavage fluid (BALF). Measurements of cytokine levels (TNFα, IL-6, IL-10) in lung homogenates and BALF showed that AMD3100 reduced IL-10 levels in homogenates from post-ischaemic lungs, whereas ubiquitin dose-dependently increased IL-10 levels in BALF from post-ischaemic lungs. Our findings establish a cause-effect relationship for the effects of pharmacological CXCR4 modulation on the development of ARDS after lung ischaemia-reperfusion injury. These data further suggest CXCR4 as a new drug target to reduce the incidence and attenuate the severity of ARDS after lung injury.

Volume-controlled Ventilation Does Not Prevent Injurious Inflation during Spontaneous Effort

RATIONALE

Spontaneous breathing during mechanical ventilation increases transpulmonary pressure and Vt, and worsens lung injury. Intuitively, controlling Vt and transpulmonary pressure might limit injury caused by added spontaneous effort.

OBJECTIVES

To test the hypothesis that, during spontaneous effort in injured lungs, limitation of Vt and transpulmonary pressure by volume-controlled ventilation results in less injurious patterns of inflation.

METHODS

Dynamic computed tomography was used to determine patterns of regional inflation in rabbits with injured lungs during volume-controlled or pressure-controlled ventilation. Transpulmonary pressure was estimated by using esophageal balloon manometry [Pl(es)] with and without spontaneous effort. Local dependent lung stress was estimated as the swing (inspiratory change) in transpulmonary pressure measured by intrapleural manometry in dependent lung and was compared with the swing in Pl(es). Electrical impedance tomography was performed to evaluate the inflation pattern in a larger animal (pig) and in a patient with acute respiratory distress syndrome.

MEASUREMENTS AND MAIN RESULTS

Spontaneous breathing in injured lungs increased Pl(es) during pressure-controlled (but not volume-controlled) ventilation, but the pattern of dependent lung inflation was the same in both modes. In volume-controlled ventilation, spontaneous effort caused greater inflation and tidal recruitment of dorsal regions (greater than twofold) compared with during muscle paralysis, despite the same Vt and Pl(es). This was caused by higher local dependent lung stress (measured by intrapleural manometry). In injured lungs, esophageal manometry underestimated local dependent pleural pressure changes during spontaneous effort.

CONCLUSIONS

Limitation of Vt and Pl(es) by volume-controlled ventilation could not eliminate harm caused by spontaneous breathing unless the level of spontaneous effort was lowered and local dependent lung stress was reduced.

Mechanical Ventilation: State of the Art

Mechanical ventilation is the most used short-term life support technique worldwide and is applied daily for a diverse spectrum of indications, from scheduled surgical procedures to acute organ failure. This state-of-the-art review provides an update on the basic physiology of respiratory mechanics, the working principles, and the main ventilatory settings, as well as the potential complications of mechanical ventilation. Specific ventilatory approaches in particular situations such as acute respiratory distress syndrome and chronic obstructive pulmonary disease are detailed along with protective ventilation in patients with normal lungs. We also highlight recent data on patient-ventilator dyssynchrony, humidified high-flow oxygen through nasal cannula, extracorporeal life support, and the weaning phase. Finally, we discuss the future of mechanical ventilation, addressing avenues for improvement.

The intensive care medicine research agenda for airways, invasive and noninvasive mechanical ventilation

In an important sense, support of the respiratory system has been a defining characteristic of intensive care since its inception. The pace of basic and clinical research in this field has escalated over the past two decades, resulting in palpable improvement at the bedside as measured by both efficacy and outcome. As in all medical research, however, novel ideas built upon observations are continually proposed, tested, and either retained or discarded on the basis of the persuasiveness of the evidence. What follows are concise descriptions of the current standards of management practice in respiratory support, the areas of present-day uncertainty, and our suggested agenda for the near future of research aimed at testing current assumptions, probing uncertainties, and solidifying the foundation on which to base our progress to the next level.

The Diaphragm Acts as a Brake during Expiration to Prevent Lung Collapse

RATIONALE

The diaphragm is the major inspiratory muscle and is assumed to relax during expiration. However, electrical postinspiratory activity has been observed. Whether there is an expiratory diaphragmatic contraction that preserves lung patency has yet to be explored.

OBJECTIVES

We hypothesized the occurrence of an expiratory diaphragmatic contraction directed at stabilizing peripheral airways and preventing or reducing cyclic expiratory lung collapse.

METHODS

Mild acute respiratory distress syndrome was induced in 10 anesthetized, spontaneously breathing pigs. Lung volume was decreased by lowering end-expiratory airway pressure in a stepwise manner. We recorded the diaphragmatic electric activity during expiration, dynamic computed tomographic scans, and respiratory mechanics. In five pigs, the same protocol was repeated during mechanical ventilation after muscle paralysis.

MEASUREMENTS AND MAIN RESULTS

Diaphragmatic electric activity during expiration increased by decreasing end-expiratory lung volume during spontaneous breathing. This enhanced the diaphragm muscle force, to a greater extent with lower lung volume, indicating a diaphragmatic electromechanical coupling during spontaneous expiration. In turn, the resulting diaphragmatic contraction delayed and reduced the expiratory collapse and increased lung aeration compared with mechanical ventilation with muscle paralysis and absence of diaphragmatic activity.

CONCLUSIONS

The diaphragm is an important regulator of expiration. Its expiratory activity seems to preserve lung volume and to protect against lung collapse. The loss of diaphragmatic expiratory contraction during mechanical ventilation and muscle paralysis may be a contributing factor to unsuccessful respiratory support.

Spontaneous breathing: a double-edged sword to handle with care

In acute hypoxemic respiratory failure (AHRF) and acute respiratory distress syndrome (ARDS) patients, spontaneous breathing is associated with multiple physiologic benefits: it prevents muscles atrophy, avoids paralysis, decreases sedation needs and is associated with improved hemodynamics. On the other hand, in the presence of uncontrolled inspiratory effort, severe lung injury and asynchronies, spontaneous ventilation might also worsen lung edema, induce diaphragm dysfunction and lead to muscles exhaustion and prolonged weaning. In the present review article, we present physiologic mechanisms driving spontaneous breathing, with emphasis on how to implement basic and advanced respiratory monitoring to assess lung protection during spontaneous assisted ventilation. Then, key benefits and risks associated with spontaneous ventilation are described. Finally, we propose some clinical means to promote protective spontaneous breathing at the bedside. In summary, early switch to spontaneous assisted breathing of acutely hypoxemic patients is more respectful of physiology and might yield several advantages. Nonetheless, risk of additional lung injury is not completely avoided during spontaneous breathing and careful monitoring of target physiologic variables such as tidal volume (Vt) and driving transpulmonary pressure should be applied routinely. In clinical practice, multiple interventions such as extracorporeal CO₂ removal exist to maintain inspiratory effort, Vt and driving transpulmonary pressure within safe limits but more studies are needed to assess their long-term efficacy.

Effects of pressure support and pressure-controlled ventilation on lung damage in a model of mild extrapulmonary acute lung injury with intra-abdominal hypertension

Intra-abdominal hypertension (IAH) may co-occur with the acute respiratory distress syndrome (ARDS), with significant impact on morbidity and mortality. Lung-protective controlled mechanical ventilation with low tidal volume and positive end-expiratory pressure (PEEP) has been recommended in ARDS. However, mechanical ventilation with spontaneous breathing activity may be beneficial to lung function and reduce lung damage in mild ARDS. We hypothesized that preserving spontaneous breathing activity during pressure support ventilation (PSV) would improve respiratory function and minimize ventilator-induced lung injury (VILI) compared to pressure-controlled ventilation (PCV) in mild extrapulmonary acute lung injury (ALI) with IAH. Thirty Wistar rats (334±55g) received Escherichia coli lipopolysaccharide intraperitoneally (1000μg) to induce mild extrapulmonary ALI. After 24h, animals were anesthetized and randomized to receive PCV or PSV. They were then further randomized into subgroups without or with IAH (15 mmHg) and ventilated with PCV or PSV (PEEP = 5cmH2O, driving pressure adjusted to achieve tidal volume = 6mL/kg) for 1h. Six of the 30 rats were used for molecular biology analysis and were not mechanically ventilated. The main outcome was the effect of PCV versus PSV on mRNA expression of interleukin (IL)-6 in lung tissue. Regardless of whether IAH was present, PSV resulted in lower mean airway pressure (with no differences in peak airway or peak and mean transpulmonary pressures) and less mRNA expression of biomarkers associated with lung inflammation (IL-6) and fibrogenesis (type III procollagen) than PCV. In the presence of IAH, PSV improved oxygenation; decreased alveolar collapse, interstitial edema, and diffuse alveolar damage; and increased expression of surfactant protein B as compared to PCV. In this experimental model of mild extrapulmonary ALI associated with IAH, PSV compared to PCV improved lung function and morphology and reduced type 2 epithelial cell damage.

Mechanical ventilation in the acute respiratory distress syndrome

The management of the acute respiratory distress syndrome (ARDS) patient is fundamental to the field of intensive care medicine, and it presents unique challenges owing to the specialized mechanical ventilation techniques that such patients require. ARDS is a highly lethal disease, and there is compelling evidence that mechanical ventilation itself, if applied in an injurious fashion, can be a contributor to ARDS mortality. Therefore, it is imperative for any clinician central to the care of ARDS patients to understand the fundamental framework that underpins the approach to mechanical ventilation in this special scenario. The current review summarizes the major components of the mechanical ventilation strategy as it applies to ARDS.

Respiratory monitoring in adult intensive care unit

INTRODUCTION

The mortality of patients with respiratory failure has steadily decreased with the advancements in protective ventilation and treatment options. Although respiratory monitoring per se has not been proven to affect the mortality of critically ill patients, it plays a crucial role in patients' care, as it helps to titrate the ventilatory support. Several new monitoring techniques have recently been made available at the bedside. The goals of monitoring comprise alerting physicians to detect the change in the patients' conditions, to improve the understanding of pathophysiology to guide the diagnosis and provide cost-effective clinical management. Areas covered: We performed a review of the recent scientific literature to provide an overview of the different methods used for respiratory monitoring in adult intensive care units, including bedside imaging techniques such as ultrasound and electrical impedance tomography. Expert commentary: Appropriate respiratory monitoring plays an important role in patients with and without respiratory failure as a guiding tool for the optimization of ventilation support, avoiding further complications and decreasing morbidity and mortality. The physician should tailor the monitoring strategy for each individual patient and know how to correctly interpret the data.