Lung recruitment maneuvers in acute respiratory distress syndrome

In the experimental setting, repeated derecruitments of the lungs of ARDS models accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate the injury. In the clinical setting, recruitment manuevers that use a continuous positive airway pressure of 40 cmH2O for 40 secs improve oxygenation in patients with early ARDS who do not have an impairment in the chest wall. High intermittent positive end-expiratory pressure (PEEP), intermitent sighs, or high-pressure controlled ventilation improves short-term oxygenation in ARDS patients. Both conventional and electrical impedance thoracictomography studies at the clinical setting indicate that high PEEP associated with low levels of pressure control ventilation recruit the collapsed portions of the ARDS lungs and that adequate PEEP levels are necessary to keep the ARDS lungs opened allowing a more homogenous ventilation. High PEEP/low tidal volume ventilation was seen to reduce inflammatory mediators in both bronchoalveolar lavage and plasma, compared to low PEEP/high tidal volume ventilation, after 36 hours of mechanical ventilation in ARDS patients. Recruitment maneuvers that used continuous positive airway pressure levels of 35-40 cmH2O for 40 secs, with PEEP set at 2 cmH2O above the lower inflection point of the pressure-volume curve, and tidal volume < 6 mL/kg were associated with a 28-day intensive care unit survival rate of 62%. This contrasted with a survival rate of only 29% with conventional ventilation (defined as the lowest PEEP for acceptable oxygenation without hemodynamic impairment with a tidal volume of 12 mL/kg), without recruitment manuevers (number needed to treat = 3; p < 0.001). In the near future, thoracic computed tomography associated with high-performance monitoring of regional ventilation may be used at the bedside to determine the optimal mechanical ventilation of the ARDS keeping an opened lung with a homogenous ventilation.     

Respiratory system mechanics in acute respiratory distress syndrome

Respiratory mechanics research is important to the advancement of ARDS management. Twenty-eight years ago, research on the effects of PEEP and VT indicated that the lungs of ARDS patients did not behave in a manner consistent with homogenously distributed lung injury. Both Suter and colleagues] and Katz and colleagues reported that oxygenation continued to improve as PEEP increased (suggesting lung recruitment), even though static Crs decreased and dead-space ventilation increased (suggesting concurrent lung overdistension). This research strongly suggested that without VT reduction, the favorable effects of PEEP on lung recruitment are offset by lung overdistension at end-inspiration. The implications of these studies were not fully appreciated at that time, in part because the concept of ventilator-associated lung injury was in its nascent state. Ten years later. Gattinoni and colleagues compared measurements of static pressure-volume curves with FRC and CT scans of the chest in ARDS. They found that although PEEP recruits collapsed (primarily dorsal) lung segments, it simultaneously causes overdistension of non-dependent, inflated lung regions. Furthermore, the specific compliance of the aerated, residually healthy lung tissue is essentially normal. The main implication of these findings is that traditional mechanical ventilation practice was injecting excessive volumes of gas into functionally small lungs. Therefore, the emblematic low static Crs measured in ARDS reflects not only surface tension phenomena and recruitment of collapsed airspaces but also overdistension of the remaining healthy lung. The studies reviewed in this article support the concept that lung injury in ARDS is heterogeneously distributed, with resulting disparate mechanical stresses, and indicate the additional complexity from alterations in chest wall mechanics. Most of these studies, however, were published before lung-protective ventilation. Therefore, further studies are needed to refine the understanding of the mechanical effects of lung-protective ventilation. Although low-VT ventilation is becoming a standard of care for ARDS patients, many issues remain unresolved; among them are the role of PEEP and recruitment maneuvers in either preventing or promoting lung injury and the effects of respiratory rate and graded VT reduction on mechanical stress in the lungs. The authors believe that advances in mechanical ventilation that may further improve patient outcomes are likely to come from more sophisticated monitoring capabilities (ie, the ability to measure P1 or perhaps Cslice) than from the creation of new modes of ventilatory support.     

Is permissive hypercapnia a beneficial strategy for pediatric acute lung injury?

It is clear that mechanical ventilation strategies influence the course of lung disease, and the choice of a ventilation strategy that avoids volutrauma and atelectrauma is firmly based on experimental literature and clinical experience. The application of a lung-protective strategy with reduced tidal volumes, effective lung recruitment, adequate PEEP to minimize alveolar collapse during expiration, and permissive hypercapnia has been shown to be advantageous in adult patients who have ARDS, although it has not been systematically studied in children. A significant body of literature confirms the beneficial effects of hypercapnic acidemia in the setting of acute lung injury. As a corollary, experimental evidence indicates that buffering hypercapnic acidosis abrogates its protective effects. The use of permissive hypercapnia as part of a lung-protective strategy in children should be accepted and perhaps even desired, provided it does not result in significant hemodynamic instability. This acceptance should be tempered with the recognition that a low-stretch, reduced-tidal volume strategy without hypercapnia has also been shown to improve outcomes in adults who have ARDS and that HFOV can generally provide lung-protective ventilation without necessarily inducing hypercapnia. Thus, a synthesis of the available clinical and research data strongly supports a graded approach to managing patients who have acute lung injury requiring intubation. The highest priority should be a mechanical ventilation strategy that limits the tidal volume, with the allowance of hypercapnia to a degree that does not compromise hemodynamic status.     

Mechanical ventilation in ARDS: a state-of-the-art review

Mechanical ventilation is an essential component of the care of patients with ARDS, and a large number of randomized controlled clinical trials have now been conducted evaluating the efficacy and safety of various methods of mechanical ventilation for the treatment of ARDS. Low tidal volume ventilation (相似文献   

Pressure-volume curves in the management of acute respiratory distress syndrome

The interpretation of P-V curves is uncertain for several reasons: the influence of chest wall compliance, differences in regional lung compliance and intrapulmonary gas distribution, lung volume history, lung recruitment beyond the LIP, peripheral airway fluid movement, expiratory-flow limitation, differences between inflation and deflation limb characteristics, and interobserver variability in curve analysis. In addition, many studies of acute lung injury have constructed P-V curves following disconnection from the ventilator. The inevitable lung volume changes that occur may alter the elastic and viscoelastic behavior so that the resulting P-V curve characteristics may not accurately reflect conditions during mechanical ventilation. More extensive research seems to be required before P-V curves are used as a routine guide for mechanical ventilation therapy in ARDS. Furthermore, this article suggests that titrating PEEP or VT according to the inflation-limb P-V curve should be done with caution, because the mechanical significance of this information is open to question. Current research suggests the possibility that PEEP could be targeted according to the slope of deflation-limb compliance, because this measure may more accurately reflect global alveolar closing pressures. This type of analysis can be done only by transferring data into software programs that can perform sophisticated curve fitting, and such programs are not readily available to most clinicians. From a practical standpoint, there is no compelling clinical evidence that adjusting mechanical ventilation according to the P-V curve improves mortality or morbidity in ARDS as much or more than can be achieved simply by decreasing the VT and Pplat.     

Influence of tidal volume on alveolar recruitment. Respective role of PEEP and a recruitment maneuver

Both reduction in tidal volume (VT) and alveolar recruitment may be important to limit ventilator-associated lung injury during mechanical ventilation of patients with the acute respiratory distress syndrome (ARDS). The aim of this study was to assess the risk of alveolar derecruitment associated with VT reduction from 10 to 6 ml/kg. Whether this VT-related derecruitment could be reversed, either by a recruitment maneuver or by an increase in positive end-expiratory pressure (PEEP) level, was also investigated. Fifteen patients with ARDS were successively ventilated using conventional VT (CVT = 10 +/- 1 ml/kg) and low VT (LVT = 6 +/- 1 ml/ kg); total PEEP (PEEPtot) was individually set at the lower inflection point (Plip) of the pressure-volume curve (PEEPtot = 11 +/- 4 cm H(2)O). Pressure-volume curves were recorded from zero PEEP (ZEEP) and from PEEP, and recruited volume (Vrec) was calculated as the volume difference between the two curves for a given pressure. Despite a similar PEEPtot, Vrec was significantly lower with LVT than with CVT, indicating low VT-induced alveolar derecruitment. Reduction in VT was associated with a reduced Sa(O(2)). In 10 patients, Vrec was also measured before and after a recruitment maneuver (two sustained inflations at 45 cm H(2)O), and after an increase in PEEP (by 4 cm H(2)O). Low VT-induced derecruitment was reversed by a recruitment maneuver and by increasing PEEP. We conclude that a reduction in VT could be responsible for alveolar derecruitment, which may be transiently reversed by a reexpansion maneuver or prevented by a PEEP increase above Plip.     

Akute respiratorische Insuffizienz

Acute respiratory distress syndrome (ARDS) is the clinical manifestation of an acute lung injury caused by a variety of direct and indirect injuries to the lung. The cardinal clinical feature of ARDS, refractory arterial hypoxemia, is the result of protein-rich alveolar edema with impaired surfactant function, due to vascular leakage and dysfunction with consequently impaired matching of ventilation to perfusion. Better understanding of the pathophysiology of ARDS has led to the development of novel therapies, pharmacological strategies, and advances in mechanical ventilation. However, protective ventilation is the only confirmed option in ARDS management improving survival, and few other therapies have translated into improved oxygenation or reduced ventilation time. The development of innovative therapy options, such as extracorporeal membrane oxygenation, have the potential to further improve survival of this devastating disease.     

Recruitment maneuvers in acute lung injury

Maintaining optimal lung recruitment has a marked effect on the outcome of patients who suffer from ARDS. RMs superimposed on mechanical ventilation have the potential to recruit atelectatic lungs in the course of general anesthesia; however, the physiologic benefits are less evident in ARDS patients who are ventilated at low VT values and high PEEP levels. Currently, the following technical aspects warrant further investigation: optimal time (the first hours after intubation or the first days of ARDS), duration (from 15 seconds to 2 minutes), mode (continuous positive end-expiratory pressure or pressure controlled ventilation with high PEEP), and type of patients (pulmonary versus extrapulmonary ARDS). Before the routine implementation of RMs to recruit the lungs fully in ARDS patients, clinicians also need more information on side effects and contraindications. Although RMs are transient, they may be associated with complications such as hypotension, bradycardia, and barotrauma. Moreover, further studies are needed to compare the efficacy of periodic high-pressure RMs that are superimposed on mechanical ventilation with ventilation using high PEEP levels and low VT values without RMs in patients who have early ARDS after initial hemodynamic stabilization.     

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??Abstract??Mechanical ventilation is still one of the main treatment measures of acute respiratory distress syndrome (ARDS).From the traditional high tidal volume ventilation (10 ~ 15 mL/kg) to the current use of lung protective ventilation strategies??low tidal volume ventilation (VT)??positive end-expiratory pressure (PEEP)??airway pressure release ventilation (APRV)??bilevel positive airway pressure(BIPAP)??considerable progress has been made.In addition to the well-known conventional mechanical ventilation modes and methods??there are many non-standard mechanical ventilation modes and methods??such as prone position ventilation??neurally adjusted ventilatory assist (NAVA)??extracorporeal membrane oxygenation (ECMO)??high-frequency ventilation and etc.All these measures produce unique effects on the treatment of ARDS.     

Prone positioning in patients with acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure that is characterized by marked hypoxemia, bilateral infiltrates on chest radiograph, and no clinical evidence of left ventricular failure. Mechanical ventilation with positive end-expiratory pressure (PEEP) is a cornerstone therapy for ARDS patients. Because the fundamental aim of supportive treatment is to improve arterial oxygenation, several alternatives to mechanical ventilation with PEEP have been used. One of these alternative therapies is prone positioning, which has been used safely to improve oxygenation in many patients with ARDS. Despite encouraging results, however, the use of prone positioning is not widely accepted as an adjunct to therapy in hypoxemic patients because, aside from temporarily improving gas exchange, it does not seem to affect the outcome of these patients. This article reviews the rationale for using prone positioning in ARDS patients who require intubation and mechanical ventilation.     

急性呼吸窘迫综合征家兔肺不同区域有效血流灌注变化及肺保护性通气对其的影响

目的 通过观察家兔急性呼吸窘迫综合征(ARDS)模型肺不同区域有效血流灌注变化及肺保护性通气对其的影响,探讨ARDS所致严重低氧血症的发生机制。方法 采用静脉注射油酸的方法建立家兔ARDS模型,应用PIM-Ⅱ激光多普勒血流灌注扫描仪观察不同肺通气模式[(大潮气、小潮气 外源性呼气末正压(PEEP)、大潮气 俯卧位、俯卧位 小潮气 PEEP]下肺不同区域(肺上区、肺下区腹侧和肺下区背侧)局部有效血流灌注及动脉血气指标的变化。结果 家兔静脉注射油酸后,(1)肺不同区域氧合指数明显下降,应用肺保护性通气(小潮气 PEEP,俯卧位 小潮气 PEEP)后氧合指数明显改善;(2)肺不同区域局部有效血流灌注均有不同程度的下降,以肺下区背侧最为明显,肺下区腹侧次之,肺上区变化最小,应用肺保护性通气后,小潮气 PEEP对改善肺下区背侧胸膜下肺局部有效血流灌注的效果不如俯卧位 小潮气 PEEP。结论小潮气 PEEP、俯卧位 小潮气 PEEP均可良好改善肺局部有效血流灌注,其中俯卧位 小潮气 PEEP效果尤为明显;右-左分流导致的肺内分流可能是ARDS发生严重进行性低氧血症的主要原因之一。     

New modalities of mechanical ventilation: high-frequency oscillatory ventilation and airway pressure release ventilation

Management of acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) is largely supportive, with the use of mechanical ventilation being a central feature. Recent advances in the understanding of ALI/ARDS and mechanical ventilation have revealed that lung-protective ventilation strategies may attenuate ventilator-associated lung injury and improve patient morbidity/mortality. High-frequency oscillatory ventilation and airway pressure release ventilation are two novel alternative modes of ventilation that theoretically fulfill the principles of lung protection and may offer an advantage over conventional ventilation for ALI/ARDS.     

Inverse ratio ventilation in ARDS. Rationale and implementation

Conventional ventilatory support of patients with the adult respiratory distress syndrome (ARDS) consists of volume-cycled ventilation with applied positive end-expiratory pressure (PEEP). Unfortunately, recent evidence suggests that this strategy, as currently implemented, may perpetuate lung damage by overinflating and injuring distensible alveolar tissues. An alternative strategy--termed inverse ratio ventilation (IRV)--extends the inspiratory time, and, in concept, maintains or improves gas exchange at lower levels of PEEP and peak distending pressures. There are two methods to administer IRV: (1) volume-cycled ventilation with an end-inspiratory pause, or with a slow or decelerating inspiratory flow rate; or (2) pressure-controlled ventilation applied with a long inspiratory time. There are several real or theoretical problems common to both forms of IRV: excessive gas-trapping; adverse hemodynamic effects; and the need for sedation in most patients. Although there are many anecdotal reports of IRV, there are no controlled studies that compare outcome in ARDS patients treated with IRV as opposed to conventional ventilation. Nonetheless, clinicians are using IRV with increasing frequency. In the absence of well-designed clinical trials, we present interim guidelines for a ventilatory strategy in patients with ARDS based on the literature and our own clinical experience.     

Evidencias de la posición en decúbito prono para el tratamiento del síndrome de distrés respiratorio agudo: una puesta al día

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.     

Best compliance during a decremental, but not incremental, positive end-expiratory pressure trial is related to open-lung positive end-expiratory pressure: a mathematical model of acute respiratory distress syndrome lungs

A mathematical model of the acute respiratory distress syndrome (ARDS) lung, incorporating simulated gravitational superimposed pressure and alveolar opening and closing pressures, was used to study the mean tidal pressure-volume (PV) slope ("effective compliance") during incremental and decremental positive end-expiratory pressure (PEEP) trials with constant tidal volume (VT) "ventilation." During incremental PEEP, the PEEP giving maximum mean tidal PV slope did not coincide with "open lung PEEP" (minimum PEEP preventing end expiratory collapse of 97.5% of alveoli inflated at end-inspiration), and it varied greatly with varying VT and "lung mechanics." Incremental PEEP with a low VT tests recruitment by the peak pressure, not prevention of collapse by PEEP. During decremental PEEP with a low VT, maximum mean tidal PV slope occurred with PEEP 2-3.5 cm H2O below open-lung PEEP, unless closing pressure was high. High VT, high "specific compliance," and high opening pressures caused slightly greater underestimation of open-lung PEEP. Maximum mean tidal PV slope was always higher (e.g., 93.7 versus 16.69 ml/cm H2O), and the variation in PV slope with PEEP was greater, during decremental PEEP. The maximum PV slope during a decremental PEEP trial with a low VT may be a useful method to determine open-lung PEEP in ARDS, and should be studied clinically.     

ARDSnet ventilatory protocol and alveolar hyperinflation: role of positive end-expiratory pressure

RATIONALE: In patients with acute respiratory distress syndrome (ARDS), a focal distribution of loss of aeration in lung computed tomography predicts low potential for alveolar recruitment and susceptibility to alveolar hyperinflation with high levels of positive end-expiratory pressure (PEEP). OBJECTIVES: We tested the hypothesis that, in this cohort of patients, the table-based PEEP setting criteria of the National Heart, Lung, and Blood Institute's ARDS Network (ARDSnet) low tidal volume ventilatory protocol could induce tidal alveolar hyperinflation. METHODS: In 15 patients, physiologic parameters and plasma inflammatory mediators were measured during two ventilatory strategies, applied randomly: the ARDSnet and the stress index strategy. The latter used the same ARDSnet ventilatory pattern except for the PEEP level, which was adjusted based on the stress index, a monitoring tool intended to quantify tidal alveolar hyperinflation and/or recruiting/derecruiting that occurs during constant-flow ventilation, on a breath-by-breath basis. MEASUREMENTS AND MAIN RESULTS: In all patients, the stress index revealed alveolar hyperinflation during application of the ARDSnet strategy, and consequently, PEEP was significantly decreased (P < 0.01) to normalize the stress index value. Static lung elastance (P = 0.01), plasma concentrations of interleukin-6 (P < 0.01), interleukin-8 (P = 0.031), and soluble tumor necrosis factor receptor I (P = 0.013) were significantly lower during the stress index as compared with the ARDSnet strategy-guided ventilation. CONCLUSIONS: Alveolar hyperinflation in patients with focal ARDS ventilated with the ARDSnet protocol is attenuated by a physiologic approach to PEEP setting based on the stress index measurement.     

Pulmonary and extrapulmonary forms of acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is usually viewed as the functional and morphological expression of a similar underlying lung injury caused by a variety of insults. However, the distinction between ARDS due to a direct (ARDSp) versus an indirect (ARDSexp) lung injury is gaining more attention as a means of better comprehending the pathophysiology of ARDS and for modifying ventilatory management. From the few published studies, we can summarize that: (1) the prevalent damage in early stages of a direct insult is intra-alveolar, whereas in indirect injury it is the interstitial edema. It is possible that the two insults may coexist (i.e., one lung with direct injury (as in pneumonia) and the other with indirect injury, through mediator release from the contralateral pneumonia); (2) the radiological pattern, by chest x-ray or computed tomography (CT), is different in ARDSp (characterized by prominent consolidation) and ARDSexp (characterized by prominent ground-glass opacification); (3) in ARDSp lung elastance is more markedly increased than in ARDSexp, where the main abnormality is the increase in chest wall elastance, due to abnormally high intra-abdominal pressure; (4) positive end-expiratory pressure (PEEP), inspiratory recruitment, and prone position are more effective to improve respiratory mechanics, alveolar recruitment, and gas-exchange in ARDSexp. Further studies are warranted to better define if the distinction between ARDS of different origins can improve clinical management and survival.     

Rechtsherzfunktion bei ARDS und maschineller Beatmung

The right ventricle is the stepchild of intensive care medicine. In diseases of the lung mainly when the relationship between ventilation and perfusion is disturbed, assisted respiration with positive end-expiratory pressure (PEEP) is essential to improve oxygenation. The serious damage to the lung parenchyma as seen in adult (acute) respiratory distress syndrome (ARDS) and pneumonia has considerable consequences for cardiac function. Whereas left ventricular function remains almost completely unaffected well into late stages of the disease, the right ventricle is subjected early to stress from the underlying disease and mechanical ventilation. The effects of therapeutic measures aimed at maintaining oxygenation and ventilation partially have negative consequences for right ventricular function and encourage the development of acute cor pulmonale. They can be the cause of right-sided heart failure.     

Positive end-expiratory pressure after a recruitment maneuver prevents both alveolar collapse and recruitment/derecruitment

We tested the hypothesis that collapsed alveoli opened by a recruitment maneuver would be unstable or recollapse without adequate positive end-expiratory pressure (PEEP) after recruitment. Surfactant deactivation was induced in pigs by Tween instillation. An in vivo microscope was placed on a lung area with significant atelectasis and the following parameters measured: (1) the number of alveoli per field and (2) alveolar stability (i.e., the change in alveolar size from peak inspiration to end expiration). We previously demonstrated that unstable alveoli cause lung injury. A recruitment maneuver (peak pressure = 45 cm H2O, PEEP = 35 cm H2O for 1 minute) was applied and alveolar number and stability were measured. Pigs were then separated into two groups with standard ventilation plus (1) 5 PEEP or (2) 10 PEEP and alveolar number and stability were again measured. The recruitment maneuver opened a significant number of alveoli, which were stable during the recruitment maneuver. Although both 5 PEEP and 10 PEEP after recruitment demonstrated improved oxygenation, alveoli ventilated with 10 PEEP were stable, whereas alveoli ventilated with 5 PEEP showed significant instability. This suggests recruitment followed by inadequate PEEP permits unstable alveoli and may result in ventilator-induced lung injury despite improved oxygenation.