Neuromuscular blockers in early acute respiratory distress syndrome

Use of extracorporeal life support in adults with severe acute respiratory failure

Extracorporeal membrane oxygenation (ECMO) is a recognized and accepted therapeutic option in the treatment of neonatal and pediatric respiratory failure. However, early studies in adults did not demonstrate a survival benefit associated with the utilization of ECMO for severe acute respiratory failure. Despite this historical lack of benefit, use of ECMO in adult patients has seen a recent resurgence. Local successes and a recently published randomized trial have both demonstrated promising results in an adult population with high baseline mortality and limited therapeutic options. This article will review the history of ECMO use for respiratory failure; investigate the driving forces behind the latest surge in interest in ECMO for adults with refractory severe acute respiratory failure; and describe potential applications of ECMO that will likely increase in the near future.

Glucocorticoids in the treatment of acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is characterized by local inflammation and an intense systemic inflammatory reaction. Glucocorticoid administration has been suggested due to their anti-inflammatory properties. However, results from the initial studies of glucocorticoids in ARDS, which evaluated high-dose and short-term treatments, were negative. More recent studies have evaluated the effect of lower doses of glucocorticoids administered over longer periods, but the results thus far have been inconclusive.

[Fiberoptic bronchoscopy in a respiratory intensive care unit]

OBJECTIVE

To describe the indications, diagnostic performance and safety of fiberoptic bronchoscopy (FOB) performed in a respiratory intensive care unit (RICU).

DESIGN

A prospective, observational study was carried out.

SETTING

A 6-bed RICU in a tertiary university hospital.

PATIENTS

Patients admitted to RICU who required FOB.

INTERVENTIONS

None.

MAIN MEASUREMENTS

FOB indications and complications, endoscopic procedures, time required to perform FOB.

RESULTS

Sixty-nine out (23%) of the 297 patients admitted to the RICU underwent a total of 107 FOB. Sixty-eight percent of FOB were performed in patients on mechanical ventilation. FOB was performed for diagnostic and therapeutic purposes in 88 (82%) and 19 cases (18%), respectively. The study of pulmonary infiltrates was the main indication for diagnostic FOB (44 cases; 50%), particularly in immunocompromised patients (24 cases; 27%). In immunocompromised patients the diagnostic performance of FOB was significantly higher than in immunocompetent subjects (48% vs 30%; p<0.01). No major complications were recorded. Only a significant drop in PaO(2)/FiO(2) ratio was observed (182 ± 74 vs 163 ± 79; p<0.005) in patients undergoing bronchoalveolar lavage. Overall mortality in patients in the RICU was 14%. In patients requiring a single FOB procedure, mortality was 25%, versus 45% among those requiring more than one FOB procedure.

CONCLUSIONS

These results show that FOB is used commonly in the RICU. It is a safe and fast procedure that contributes significantly to clinical management. Patients requiring additional FOB during admission to the RICU show high mortality.

High-frequency oscillatory ventilation in ALI/ARDS

In the last 2 decades, our goals for mechanical ventilatory support in patients with acute respiratory distress syndrome (ARDS) or acute lung injury (ALI) have changed dramatically. Several randomized controlled trials have built on a substantial body of preclinical work to demonstrate that the way in which we employ mechanical ventilation has an impact on important patient outcomes. Avoiding ventilator-induced lung injury (VILI) is now a major focus when clinicians are considering which ventilatory strategy to employ in patients with ALI/ARDS. Physicians are searching for methods that may further limit VILI, while still achieving adequate gas exchange.

Mechanical ventilation during acute lung injury: current recommendations and new concepts

Despite a very large body of investigations, no effective pharmacological therapies have been found to cure acute lung injury. Hence, supportive care with mechanical ventilation remains the cornerstone of treatment. However, several experimental and clinical studies showed that mechanical ventilation, especially at high tidal volumes and pressures, can cause or aggravate ALI. Therefore, current clinical recommendations are developed with the aim of avoiding ventilator-induced lung injury (VILI) by limiting tidal volume and distending ventilatory pressure according to the results of the ARDS Network trial, which has been to date the only intervention that has showed success in decreasing mortality in patients with ALI/ARDS. In the past decade, a very large body of investigations has determined significant achievements on the pathophysiological knowledge of VILI. Therefore, new perspectives, which will be reviewed in this article, have been defined in terms of the efficiency and efficacy of recognizing, monitoring and treating VILI, which will eventually lead to further significant improvement of outcome in patients with ARDS.

Extracorporeal membrane oxygenation for ARDS in adults

A 41-year-old woman presents with severe community-acquired pneumococcal pneumonia. Chest radiography reveals diffuse bilateral infiltrates, and hypoxemic respiratory failure develops despite appropriate antibiotic therapy. She is intubated and mechanical ventilation is initiated with a volume- and pressure-limited approach for the acute respiratory distress syndrome (ARDS). Over the ensuing 24 hours, her partial pressure of arterial oxygen (Pao2) decreases to 40 mm Hg, despite ventilatory support with a fraction of inspired oxygen (Fio2) of 1.0 and a positive end-expiratory pressure (PEEP) of 20 cm of water. She is placed in the prone position and a neuromuscular blocking agent is administered, without improvement in her Pao2. An intensive care specialist recommends the initiation of extracorporeal membrane oxygenation (ECMO).

Critical illness polyneuropathy and myopathy: a major cause of muscle weakness and paralysis

Critical illness polyneuropathy (CIP) and myopathy (CIM) are complications of critical illness that present with muscle weakness and failure to wean from the ventilator. In addition to prolonging mechanical ventilation and hospitalisation, CIP and CIM increase hospital mortality in patients who are critically ill and cause chronic disability in survivors of critical illness. Structural changes associated with CIP and CIM include axonal nerve degeneration, muscle myosin loss, and muscle necrosis. Functional changes can cause electrical inexcitability of nerves and muscles with reversible muscle weakness. Microvascular changes and cytopathic hypoxia might disrupt energy supply and use. An acquired sodium channelopathy causing reduced muscle membrane and nerve excitability is a possible unifying mechanism underlying CIP and CIM. The diagnosis of CIP, CIM, or combined CIP and CIM relies on clinical, electrophysiological, and muscle biopsy investigations. Control of hyperglycaemia might reduce the severity of these complications of critical illness, and early rehabilitation in the intensive care unit might improve the functional recovery and independence of patients.

Perioperative pulmonary complications

PURPOSE OF REVIEW

This article reviews current concepts in perioperative pulmonary management.

RECENT FINDINGS

Preoperative risk assessment tools for perioperative pulmonary complications (POPCs) are evolving for both children and adults. Intraoperative management strategies have a demonstrable effect on outcomes. Late POPCs may be preceded by clinical signs.

SUMMARY

POPCs are common and lead to significant resource utilization. Optimal POPC risk mitigation must span all phases of surgical care. Preoperative assessment may identify patients at risk and effectively lower their risk by identifying targeted interventions. Intra-operative strategies impact postoperative outcome. POPCs continue to be a concern for several days postoperatively. We review the current literature on this broad subject with a focus on implementable interventions for the clinician.

Prone positioning in acute respiratory distress syndrome (ARDS): when and how?

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure. It remains one of the most devastating conditions in the intensive care unit. Mechanical ventilation with positive end-expiratory pressure is a cornerstone therapy for ARDS patients. One adjuvant alternative is to place the patient in a prone position. Since it was first described in 1976, prone positioning has been safely employed to improve oxygenation in many patients with ARDS. Prone positioning may also minimize secondary lung injury induced by mechanical ventilation, although this benefit has not been investigated as extensively, despite its potential. In spite of a strong physiological justification, prone positioning is still not widely accepted as an adjunct therapy in ARDS patients and it is only used regularly in only 10% of ICUs. This may be explained in part by the reluctance to change position, risks and unclear effects on relevant outcomes. In this paper, we review all aspects of prone positioning, from the pathophysiology to the clinical studies of patient outcome, and we also discuss the latest controversies surrounding this treatment.

Neurally adjusted ventilatory assist vs. pressure support ventilation in critically ill patients: an observational study

BACKGROUND

During neurally adjusted ventilatory assist (NAVA), the inspiratory support is controlled by the patients' respiratory drive influenced by an operator-controlled gain factor (NAVA level). The purpose of our observational study was to transfer patients from conventional pressure support ventilation (PSV) to NAVA safely. We compared two approaches to set the NAVA level and evaluated the effect of NAVA.

METHODS

We studied mechanically ventilated patients capable of spontaneous breathing. For the change of the ventilator mode, we used a NAVA level calculated to generate a peak inspiratory pressure equal to PSV. We compared this NAVA level with a NAVA level determined by a NAVA level titration. Ventilatory and haemodynamic data were recorded during an observational period of 6 h.

RESULTS

All 20 patients included in the study could be transferred from PSV to NAVA and completed the observation interval. Setting the NAVA level according to prior PSV settings proved to be a feasible approach, but in 75% of our patients, we modified the NAVA level according to the titration results. Gas exchange and ventilatory mechanics during the observation interval remained stable.

CONCLUSIONS

The ventilator mode NAVA seems to be well tolerated in a heterogeneous group of critically ill patients. Pre-setting of the NAVA level during PSV can result in an overestimation of the required ventilator support. An additional titration of the NAVA level ads valuable information although difficult to interpret in some cases.

The challenge of designing a post-critical illness rehabilitation intervention

Post-ICU morbidity is an important issue for patients, families, and the health-care system. Elliott and colleagues outlined the results from their novel report of the very first home-based physiotherapy program to be tested in survivors of critical illness. The authors described an explicit intervention, which included a self-instruction exercise manual, trainer visits, and telephone follow-up, with excellent internal validity and yet no difference in outcome measures at 26-week follow-up. These results are discussed in the context of risk stratification/individual tailoring of post-ICU programs to patient and family needs and suggest that the collection of multiple simultaneous outcome measures across functional, neuropsychological, caregiver, and health-care utilization domains may offer additional insight into the benefits of post-rehabilitation programs.

Sedation and analgesia in the mechanically ventilated patient

Sedation and analgesia are important components of care for the mechanically ventilated patient in the intensive care unit (ICU). An understanding of commonly used medications is essential to formulate a sedation plan for individual patients. The specific physiological changes that a critically ill patient undergoes can have direct effects on the pharmacology of drugs, potentially leading to interpatient differences in response. Objective assessments of pain, sedation, and agitation have been validated for use in the ICU for assessment and titration of medications. An evidence-based strategy for administering these drugs can lead to improvements in short- and long-term outcomes for patients. In this article, we review advances in the field of ICU sedation to provide an up-to-date perspective on management of the mechanically ventilated ICU patient.

Acute lung failure

Lung failure is the most common organ failure seen in the intensive care unit. The pathogenesis of acute respiratory failure (ARF) can be classified as (1) neuromuscular in origin, (2) secondary to acute and chronic obstructive airway diseases, (3) alveolar processes such as cardiogenic and noncardiogenic pulmonary edema and pneumonia, and (4) vascular diseases such as acute or chronic pulmonary embolism. This article reviews the more common causes of ARF from each group, including the pathological mechanisms and the principles of critical care management, focusing on the supportive, specific, and adjunctive therapies for each condition.

Use of computed tomography scanning to guide lung recruitment and adjust positive-end expiratory pressure

PURPOSE OF REVIEW

We discuss the possible role of computed tomography (CT) to guide protective mechanical ventilation in acute lung injury/acute respiratory distress syndrome (ALI/ARDS), especially tidal volume (VT) and positive-end expiratory pressure (PEEP) settings and recruitment manoeuvres.

RECENT FINDINGS

CT should be used as early as possible after the onset of ALI/ARDS and then repeated after 1 week in the absence of clinical improvement. Advantages of CT include: the regional response to recruitment can be determined; it is objective; the morphofunctional correlations obtained are useful for a comprehensive patient evaluation. CT should be performed at different pressure levels to identify potential for recruitment. Initially, one single whole-lung CT scan is performed at end-expiration at PEEP 5-10 cmH2O to evaluate aeration and compute lung weight. Afterwards, two lung CT slices are performed to assess lung recruitability (at PEEP = 5-10 cmH2O; inspiratory plateau pressure of the respiratory system = 45 cmH2O).

SUMMARY

In ALI/ARDS patients, CT reveals discrepancies between bedside chest radiograph and various clinical and physiological parameters, and it is essential to assess lung morphology and recruitability. Specific algorithms, including or not CT, should be used to better identify ALI/ARDS with potential of recruitment and setting of VT and PEEP.

Plasma from septic shock patients induces loss of muscle protein

Introduction

ICU-acquired muscle weakness commonly occurs in patients with septic shock and is associated with poor outcome. Although atrophy is known to be involved, it is unclear whether ligands in plasma from these patients are responsible for initiating degradation of muscle proteins. The aim of the present study was to investigate if plasma from septic shock patients induces skeletal muscle atrophy and to examine the time course of plasma-induced muscle atrophy during ICU stay.

Methods

Plasma was derived from septic shock patients within 24 hours after hospital admission (n = 21) and healthy controls (n = 12). From nine patients with septic shock plasma was additionally derived at two, five and seven days after ICU admission. These plasma samples were added to skeletal myotubes, cultured from murine myoblasts. After incubation for 24 hours, myotubes were harvested and analyzed on myosin content, mRNA expression of E3-ligase and Nuclear Factor Kappa B (NFκB) activity. Plasma samples were analyzed on cytokine concentrations.

Results

Myosin content was approximately 25% lower in myotubes exposed to plasma from septic shock patients than in myotubes exposed to plasma from controls (P < 0.01). Furthermore, patient plasma increased expression of E3-ligases Muscle RING Finger protein-1 (MuRF-1) and Muscle Atrophy F-box protein (MAFbx) (P < 0.01), enhanced NFκB activity (P < 0.05) and elevated levels of ubiquitinated myosin in myotubes. Myosin loss was significantly associated with elevated plasma levels of interleukin (IL)-6 in septic shock patients (P < 0.001). Addition of antiIL-6 to septic shock plasma diminished the loss of myosin in exposed myotubes by approximately 25% (P < 0.05). Patient plasma obtained later during ICU stay did not significantly reduce myosin content compared to controls.

Conclusions

Plasma from patients with septic shock induces loss of myosin and activates key regulators of proteolysis in skeletal myotubes. IL-6 is an important player in sepsis-induced muscle atrophy in this model. The potential to induce atrophy is strongest in plasma obtained during the early phase of human sepsis.

Open lung approach vs acute respiratory distress syndrome network ventilation in experimental acute lung injury

Background

Setting and strategies of mechanical ventilation with positive end-expiratory pressure (PEEP) in acute lung injury (ALI) remains controversial. This study compares the effects between lung-protective mechanical ventilation according to the Acute Respiratory Distress Syndrome Network recommendations (ARDSnet) and the open lung approach (OLA) on pulmonary function and inflammatory response.

Methods

Eighteen juvenile pigs were anaesthetized, mechanically ventilated, and instrumented. ALI was induced by surfactant washout. Animals were randomly assigned to mechanical ventilation according to the ARDSnet protocol or the OLA (n=9 per group). Gas exchange, haemodynamics, pulmonary blood flow (PBF) distribution, and respiratory mechanics were measured at intervals and the lungs were removed after 6 h of mechanical ventilation for further analysis.

Results

PEEP and mean airway pressure were higher in the OLA than in the ARDSnet group [15 cmH₂O, range 14–18 cmH₂O, compared with 12 cmH₂O; 20.5 (sd 2.3) compared with 18 (1.4) cmH₂O by the end of the experiment, respectively], and OLA was associated with improved oxygenation compared with the ARDSnet group after 6 h. OLA showed more alveolar overdistension, especially in gravitationally non-dependent regions, while the ARDSnet group was associated with more intra-alveolar haemorrhage. Inflammatory mediators and markers of lung parenchymal stress did not differ significantly between groups. The PBF shifted from ventral to dorsal during OLA compared with ARDSnet protocol [−0.02 (−0.09 to −0.01) compared with −0.08 (−0.12 to −0.06), dorsal–ventral gradients after 6 h, respectively].

Conclusions

According to the OLA, mechanical ventilation improved oxygenation and redistributed pulmonary perfusion when compared with the ARDSnet protocol, without differences in lung inflammatory response.

Current therapeutic uses, pharmacology, and clinical considerations of neuromuscular blocking agents for critically ill adults

OBJECTIVE

To summarize literature describing use of neuromuscular blocking agents (NMBAs) for common critical care indications and provide a review of NMBA pharmacology, pharmacokinetics, dosing, drug interactions, monitoring, complications, and reversal.

DATA SOURCES

Searches of MEDLINE (1975-May 2011), EMBASE (1980-May 2011), and Cumulative Index to Nursing and Allied Health Literature (1981-May 2011) were conducted to identify observational and interventional studies evaluating the efficacy or safety of NMBAs for management of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), status asthmaticus, elevated intracranial pressure (ICP), and therapeutic hypothermia.

STUDY SELECTION AND DATA EXTRACTION

We excluded case reports, animal- or laboratory-based studies, trials describing NMBA use during rapid sequence intubation or in the operating room, and studies published in languages other than English or French.

DATA SYNTHESIS

Clinical applications of NMBAs in intensive care include, but are not limited to, immobilizing patients for procedural interventions, decreasing oxygen consumption, facilitating mechanical ventilation, reducing intracranial pressure, preventing shivering, and management of tetanus. Recent data on ARDS demonstrated that early application of NMBAs improved adjusted 90-day survival for patients with severe lung injury. These results may lead to increased use of these drugs. While emerging data support the use of cisatracurium in select patients with ALI/ARDS, current literature does not support the use of one NMBA over another for other critical care indications. Cisatracurium may be kinetically preferred for patients with organ dysfunction. Close monitoring with peripheral nerve stimulation is recommended with sustained use of NMBAs to avoid drug accumulation and minimize the risk for adverse drug events. Reversal of paralysis is achieved by discontinuing therapy or, rarely, the use of anticholinesterases.

CONCLUSIONS

NMBAs are high-alert medications used to manage critically ill patients. New data are available regarding the use of these agents for treatment of ALI/ARDS and status asthmaticus, management of elevated ICP, and provision of therapeutic hypothermia after cardiac arrest. To improve outcomes and promote patient safety, intensive care unit team members should have a thorough knowledge of this class of medications.

Diaphragmatic dysfunction in mechanical ventilation

PURPOSE OF REVIEW

It has become clear from experimental data that prolonged mechanical ventilation can induce diaphragm dysfunction, also known as ventilator-induced diaphragm dysfunction. In this article we will discuss most recent understanding on ventilator-induced diaphragm dysfunction and data on diaphragm dysfunction in patients.

RECENT FINDINGS

Over the last year several studies confirmed the existence of diaphragm dysfunction in patients. Known atrophy pathways are activated in patients undergoing prolonged conventional ventilation resulting in muscle proteolysis and a decrease in myofiber content. The loss of diaphragm force is time-dependent, but current data do not distinguish between the role played by other factors involved in diaphragm dysfunction.

SUMMARY

Diaphragm dysfunction occurs in patients, especially when ventilated with controlled modes of ventilation that minimize diaphragm activity. Time on the ventilator seems to be one of the biggest risk factors resulting in difficulties in weaning patients and prolonging time on the ventilator. Future trials should investigate whether improved patient-ventilator synchrony can reduce ventilator-induced diaphragm dysfunction and decrease weaning failure.

The pharmacology of acute lung injury in sepsis

Acute lung injury (ALI) secondary to sepsis is one of the leading causes of death in sepsis. As such, many pharmacologic and nonpharmacologic strategies have been employed to attenuate its course. Very few of these strategies have proven beneficial. In this paper, we discuss the epidemiology and pathophysiology of ALI, commonly employed pharmacologic and nonpharmacologic treatments, and innovative therapeutic modalities that will likely be the focus of future trials.

Human models of acute lung injury

Acute lung injury (ALI) is a syndrome that is characterised by acute inflammation and tissue injury that affects normal gas exchange in the lungs. Hallmarks of ALI include dysfunction of the alveolar-capillary membrane resulting in increased vascular permeability, an influx of inflammatory cells into the lung and a local pro-coagulant state. Patients with ALI present with severe hypoxaemia and radiological evidence of bilateral pulmonary oedema. The syndrome has a mortality rate of approximately 35% and usually requires invasive mechanical ventilation. ALI can follow direct pulmonary insults, such as pneumonia, or occur indirectly as a result of blood-borne insults, commonly severe bacterial sepsis. Although animal models of ALI have been developed, none of them fully recapitulate the human disease. The differences between the human syndrome and the phenotype observed in animal models might, in part, explain why interventions that are successful in models have failed to translate into novel therapies. Improved animal models and the development of human in vivo and ex vivo models are therefore required. In this article, we consider the clinical features of ALI, discuss the limitations of current animal models and highlight how emerging human models of ALI might help to answer outstanding questions about this syndrome.

A practical approach to adult acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a common disease encountered in hospitalized adult patients that, historically, has carried a very high mortality. This article reviews the clinical features and how pathophysiology informs the evidence-based management of ARDS.

Spontaneously regulated vs. controlled ventilation of acute lung injury/acute respiratory distress syndrome

PURPOSE OF REVIEW

To present an updated discussion of those aspects of controlled positive pressure breathing and retained spontaneous regulation of breathing that impact the management of patients whose tissue oxygenation is compromised by acute lung injury.

RECENT FINDINGS

The recent introduction of ventilation techniques geared toward integrating natural breathing rhythms into even the earliest phase of acute respiratory distress syndrome support (e.g., airway pressure release, proportional assist ventilation, and neurally adjusted ventilatory assist), has stimulated a burst of new investigations.

SUMMARY

Optimizing gas exchange, avoiding lung injury, and preserving respiratory muscle strength and endurance are vital therapeutic objectives for managing acute lung injury. Accordingly, comparing the physiology and consequences of breathing patterns that preserve and eliminate breathing effort has been a theme of persisting investigative interest throughout the several decades over which it has been possible to sustain cardiopulmonary life support outside the operating theater.

Functional disability 5 years after acute respiratory distress syndrome

BACKGROUND

There have been few detailed, in-person interviews and examinations to obtain follow-up data on 5-year outcomes among survivors of the acute respiratory distress syndrome (ARDS).

METHODS

We evaluated 109 survivors of ARDS at 3, 6, and 12 months and at 2, 3, 4, and 5 years after discharge from the intensive care unit. At each visit, patients were interviewed and examined; underwent pulmonary-function tests, the 6-minute walk test, resting and exercise oximetry, chest imaging, and a quality-of-life evaluation; and reported their use of health care services.

RESULTS

At 5 years, the median 6-minute walk distance was 436 m (76% of predicted distance) and the Physical Component Score on the Medical Outcomes Study 36-Item Short-Form Health Survey was 41 (mean norm score matched for age and sex, 50). With respect to this score, younger patients had a greater rate of recovery than older patients, but neither group returned to normal predicted levels of physical function at 5 years. Pulmonary function was normal to near-normal. A constellation of other physical and psychological problems developed or persisted in patients and family caregivers for up to 5 years. Patients with more coexisting illnesses incurred greater 5-year costs.

CONCLUSIONS

Exercise limitation, physical and psychological sequelae, decreased physical quality of life, and increased costs and use of health care services are important legacies of severe lung injury.

Acute respiratory distress syndrome: A clinical review

The acute respiratory distress syndrome (ARDS) is a complex disorder of heterogeneous etiologies characterized by a consistent, recognizable pattern of lung injury. Extensive epidemiologic studies and clinical intervention trials have been conducted to address the high mortality of this disorder and have provided significant insight into the complexity of studying new therapies for this condition. The existing clinical investigations in ARDS will be highlighted in this review. The limitations to current definitions, patient selection, and outcome assessment will be considered. While significant attention has been focused on the parenchymal injury that characterizes this disorder and the clinical support of gas exchange function, relatively limited focus has been directed to hemodynamic and pulmonary vascular dysfunction equally prominent in the disease. The limited available clinical information in this area will also be reviewed. The current standards for cardiopulmonary management of the condition will be outlined. Current gaps in our understanding of the clinical condition will be highlighted with the expectation that continued progress will contribute to a decline in disease mortality.

Clinical review: ventilator-induced diaphragmatic dysfunction--human studies confirm animal model findings!

Diaphragmatic function is a major determinant of the ability to successfully wean patients from mechanical ventilation. However, the use of controlled mechanical ventilation in animal models results in a major reduction of diaphragmatic force-generating capacity together with structural injury and atrophy of diaphragm muscle fibers, a condition termed ventilator-induced diaphragmatic dysfunction (VIDD). Increased oxidative stress and exaggerated proteolysis in the diaphragm have been linked to the development of VIDD in animal models, but much less is known about the extent to which these phenomena occur in humans undergoing mechanical ventilation in the ICU. In the present review, we first briefly summarize the large body of evidence demonstrating the existence of VIDD in animal models, and outline the major cellular mechanisms that have been implicated in this process. We then relate these findings to very recently published data in critically ill patients, which have thus far been found to exhibit a remarkable degree of similarity with the animal model data. Hence, the human studies to date have indicated that mechanical ventilation is associated with increased oxidative stress, atrophy, and injury of diaphragmatic muscle fibers along with a rapid loss of diaphragmatic force production. These changes are, to a large extent, directly proportional to the duration of mechanical ventilation. In the context of these human data, we also review the methods that can be used in the clinical setting to diagnose and/or monitor the development of VIDD in critically ill patients. Finally, we discuss the potential for using different mechanical ventilation strategies and pharmacological approaches to prevent and/or to treat VIDD and suggest promising avenues for future research in this area.

Manual muscle strength testing of critically ill patients: feasibility and interobserver agreement

INTRODUCTION

It has been proposed that intensive care unit (ICU)-acquired weakness (ICUAW) should be assessed using the sum of manual muscle strength test scores in 12 muscle groups (the sum score). This approach has been tested in patients with Guillain-Barré syndrome, yet little is known about the feasibility or test characteristics in other critically ill patients. We studied the feasibility and interobserver agreement of this sum score in a mixed cohort of critically ill and injured patients.

METHODS

We enrolled patients requiring more than 3 days of mechanical ventilation. Two observers performed systematic strength assessments of each patient. The primary outcome measure was interobserver agreement of weakness as a binary outcome (ICUAW is sum score less than 48; "no ICUAW" is a sum score greater than or equal to 48) using the Cohen's kappa statistic.

RESULTS

We identified 135 patients who met the inclusion criteria. Most were precluded from study participation by altered mental status or polytrauma. Thirty-four participants were enrolled, and 30 of these individuals completed assessments conducted by both observers. Six met the criteria for ICUAW recorded by at least one observer. The observers agreed on the diagnosis of ICUAW for 93% of participants (Cohen's kappa = 0.76; 95% confidence interval (CI), 0.44 to 1.0). Observer agreement was fair in the ICU (Cohen's kappa = 0.38), and agreement was perfect after ICU discharge (Cohen's kappa = 1.0). Absolute values of sum scores were similar between observers (intraclass correlation coefficient 0.83; 95% CI, 0.67 to 0.91), but they differed between observers by six points or more for 23% of the participants.

CONCLUSIONS

Manual muscle testing (MMT) during critical illness was not possible for most patients because of coma, delirium and/or injury. Among patients who were able to participate in testing, we found that interobserver agreement regarding ICUAW was good, particularly when evaluated after ICU discharge. MMT is insufficient for early detection of ICU-acquired neuromuscular dysfunction in most patients and may be unreliable during critical illness.

Quantifying the roles of tidal volume and PEEP in the pathogenesis of ventilator-induced lung injury

Management of patients with acute lung injury (ALI) rests on achieving a balance between the gas exchanging benefits of mechanical ventilation and the exacerbation of tissue damage in the form of ventilator-induced lung injury (VILI). Optimizing this balance requires an injury cost function relating injury progression to the measurable pressures, flows, and volumes delivered during mechanical ventilation. With this in mind, we mechanically ventilated naive, anesthetized, paralyzed mice for 4 h using either a low or high tidal volume (Vt) with either moderate or zero positive end-expiratory pressure (PEEP). The derecruitability of the lung was assessed every 15 min in terms of the degree of increase in lung elastance occurring over 3 min following a recruitment maneuver. Mice could be safely ventilated for 4 h with either a high Vt or zero PEEP, but when both conditions were applied simultaneously the lung became increasingly unstable, demonstrating worsening injury. We were able to mimic these data using a computational model of dynamic recruitment and derecruitment that simulates the effects of progressively increasing surface tension at the air-liquid interface, suggesting that the VILI in our animal model progressed via a vicious cycle of alveolar leak, degradation of surfactant function, and increasing tissue stress. We thus propose that the task of ventilating the injured lung is usefully understood in terms of the Vt-PEEP plane. Within this plane, non-injurious combinations of Vt and PEEP lie within a "safe region", the boundaries of which shrink as VILI develops.