Can diaphragmatic contractility be assessed by airway twitch pressure in mechanically ventilated patients?

Dyssynchronous diaphragm contractions impair diaphragm function in mechanically ventilated patients

BACKGROUND

Pre-clinical studies suggest that dyssynchronous diaphragm contractions during mechanical ventilation may cause acute diaphragm dysfunction. We aimed to describe the variability in diaphragm contractile loading conditions during mechanical ventilation and to establish whether dyssynchronous diaphragm contractions are associated with the development of impaired diaphragm dysfunction.

METHODS

In patients receiving invasive mechanical ventilation for pneumonia, septic shock, acute respiratory distress syndrome, or acute brain injury, airway flow and pressure and diaphragm electrical activity (Edi) were recorded hourly around the clock for up to 7 days. Dyssynchronous post-inspiratory diaphragm loading was defined based on the duration of neural inspiration after expiratory cycling of the ventilator. Diaphragm function was assessed on a daily basis by neuromuscular coupling (NMC, the ratio of transdiaphragmatic pressure to diaphragm electrical activity).

RESULTS

A total of 4508 hourly recordings were collected in 45 patients. Edi was low or absent (≤ 5 µV) in 51% of study hours (median 71 h per patient, interquartile range 39-101 h). Dyssynchronous post-inspiratory loading was present in 13% of study hours (median 7 h per patient, interquartile range 2-22 h). The probability of dyssynchronous post-inspiratory loading was increased with reverse triggering (odds ratio 15, 95% CI 8-35) and premature cycling (odds ratio 8, 95% CI 6-10). The duration and magnitude of dyssynchronous post-inspiratory loading were associated with a progressive decline in diaphragm NMC (p < 0.01 for interaction with time).

CONCLUSIONS

Dyssynchronous diaphragm contractions may impair diaphragm function during mechanical ventilation.

TRIAL REGISTRATION

MYOTRAUMA, ClinicalTrials.gov NCT03108118. Registered 04 April 2017 (retrospectively registered).

Rapid review of ventilator-induced diaphragm dysfunction

Ventilator-induced diaphragm dysfunction is gaining increased recognition. Evidence of diaphragm weakness can manifest within 12 h to a few days after the initiation of mechanical ventilation. Various noninvasive and invasive methods have been developed to assess diaphragm function. The implementation of diaphragm-protective ventilation strategies is crucial for preventing diaphragm injuries. Furthermore, diaphragm neurostimulation emerges as a promising and novel treatment option. In this rapid review, our objective is to discuss the current understanding of ventilator-induced diaphragm dysfunction, diagnostic approaches, and updates on strategies for prevention and management.

Evaluation of diaphragm functions with diaphragm ultrasound and pulmonary function tests in individuals with Friedreich's ataxia

Background/aim

It is known that the correlation of pulmonary function tests (PFT) with muscle dysfunction is insufficient. Here, we aimed to evaluate the diaphragm functions in individuals with Friedreich's ataxia (FRDA) and to examine its relationship with respiratory parameters and disease severity.

Materials and methods

This prospective study, conducted between November and December 2022, at Erciyes University, included 14 individuals with genetically confirmed FRDA and an age- and gender-matched healthy control group of eight individuals. We examined pulmonary functions with spirometric methods and evaluated diaphragm excursion, and diaphragm thickness-expiratory (Tde) and - end of inspiration (Tdi) with ultrasonography during calm breathing. Thickening fraction (TF) calculated. Also, we examined PaCO2 at rest. The neurological status of individuals was assessed using the Scale for the Assessment and Rating of Ataxia (SARA).

Results

The mean values of FEV1(lt), FEV1(%), FVC (lt), and FVC (%) were higher in the control group (p; <0.001, 0.013, <0.001, and 0.009, respectively). Also, mean Tdi, Tde, excursion and TF were lower in the FRDA group compared to the control group (p = 0.005, 0.294,0.005, and 0.019, respectively). The mean excursion value was 1.13 ± 0.54cm in the FRDA group and 1.71 ± 0.49cm in the control group. There is a strong, negative, and statistically significant correlation between SARA total score with excursion and TF (r = -0.7432, p = 0.002; r = -0.697, p = 0.008). There is no statistically significant relationship between excursion and BMI, standing-to-supine decrease in FVC, FEV1, and PaCO2. Also, the relationship between maximal inspiratory pressure (PImax) and excursion was moderate.

Conclusion

Diaphragm ultrasound may reveal respiratory dysfunction better than PFT. Diaphragm excursion and TF are associated with disease scores in individuals with FDRA. Further studies are needed regarding the detection of alveolar hypoventilation.

Sarcopenia and COVID-19 Outcomes

Coronavirus disease 2019 (COVID-19) spread rapidly and became a severe global public health threat. Older adults have a high risk of COVID-19 and its associated mortality. Sarcopenia has emerged as a predictor of poor outcomes in COVID-19 patients, including lengthy hospital stays, mortality, intensive care unit admission, need for invasive mechanical ventilation, and poor rehabilitation outcomes. Chronic inflammation, immune dysfunction, respiratory muscle dysfunction, and swallowing dysfunction may underlie the association between sarcopenia and the poor outcomes of COVID-19 patients. Interleukin 6 receptor blockers (tocilizumab or sarilumab) are recommended for treating patients with severe COVID-19, and their therapeutic effects on sarcopenia are of great interest. This review aimed to analyze the current reports on the association between sarcopenia and COVID-19 and provide an update on the contribution of sarcopenia to the severity and adverse outcomes of COVID-19 and its underlying mechanisms. We also aimed to explore the different screening tools for sarcopenia concurrent with COVID-19, and advocate for early diagnosis and treatment of sarcopenia. Given that the fight against the COVID-19 pandemic may be long-term, further research into understanding the effects of sarcopenia in patients infected with the Omicron variant is necessary.

Ultrasound and non-ultrasound imaging techniques in the assessment of diaphragmatic dysfunction

Diaphragm muscle dysfunction is increasingly recognized as an important element of several diseases including neuromuscular disease, chronic obstructive pulmonary disease and diaphragm dysfunction in critically ill patients. Functional evaluation of the diaphragm is challenging. Use of volitional maneuvers to test the diaphragm can be limited by patient effort. Non-volitional tests such as those using neuromuscular stimulation are technically complex, since the muscle itself is relatively inaccessible. As such, there is a growing interest in using imaging techniques to characterize diaphragm muscle dysfunction. Selecting the appropriate imaging technique for a given clinical scenario is a critical step in the evaluation of patients suspected of having diaphragm dysfunction. In this review, we aim to present a detailed analysis of evidence for the use of ultrasound and non-ultrasound imaging techniques in the assessment of diaphragm dysfunction. We highlight the utility of the qualitative information gathered by ultrasound imaging as a means to assess integrity, excursion, thickness, and thickening of the diaphragm. In contrast, quantitative ultrasound analysis of the diaphragm is marred by inherent limitations of this technique, and we provide a detailed examination of these limitations. We evaluate non-ultrasound imaging modalities that apply static techniques (chest radiograph, computerized tomography and magnetic resonance imaging), used to assess muscle position, shape and dimension. We also evaluate non-ultrasound imaging modalities that apply dynamic imaging (fluoroscopy and dynamic magnetic resonance imaging) to assess diaphragm motion. Finally, we critically review the application of each of these techniques in the clinical setting when diaphragm dysfunction is suspected.

The Benefit of Non-invasive Ventilation in Motor Neuron Disease

Background:

Motor Neuron Disease (MND) is a progressive neurodegenerative disorder leading to respiratory muscle weakness with dyspnoea, morning headaches, orthopnoea, poor concentration, unrefreshing sleep, fatigue and daytime somnolence. Respiratory failure is the primary cause of death in those with MND.

Methods:

Although guidelines suggest the use of non-invasive ventilation (NIV) in MND, there lacks clear guidance as to when is the optimal time to initiate NIV and which markers of respiratory muscle decline are the best predictors of prognosis. There have been a number of studies that have found a significant survival advantage to the use of NIV in MND. Similarly, in quality-of-life questionnaires, those treated with NIV tend to perform better and maintain a better quality of life for longer. Furthermore, studies also suggest that improved compliance and greater tolerance of NIV confer a survival advantage.

Results and Discussion:

Forced Vital Capacity (FVC) has traditionally been the main pulmonary function test to determine the respiratory function in those with MND; however, FVC may not be entirely reflective of early respiratory muscle dysfunction. Evidence suggests that sniff nasal inspiratory pressure and maximum mouth inspiratory pressure may be better indicators of early respiratory muscle decline. These measures have been shown to be easier to perform later in the disease, in patients with bulbar onset disease, and may indeed be better prognostic indicators.

Conclusion:

Despite ongoing research, there remains a paucity of randomised controlled data in this area. This review aims to summarise the evidence to date on these topics.

Ultrasonographic assessment of parasternal intercostal muscles during mechanical ventilation

Although mechanical ventilation is a lifesaving treatment, abundant evidence indicates that its prolonged use (1 week or more) promotes respiratory muscle weakness due to both contractile dysfunction and atrophy. Along with the diaphragm, the intercostal muscles are one of the most important groups of respiratory muscles. In recent years, muscular ultrasound has become a useful bedside tool for the clinician to identify patients with respiratory muscle dysfunction related to critical illness and/or invasive mechanical ventilation. Images obtained over the course of illness can document changes in muscle dimension and can be used to estimate changes in function. Recent evidence suggests the clinical usefulness of ultrasound imaging in the assessment of intercostal muscle function. In this narrative review, we summarize the current literature on ultrasound imaging of the parasternal intercostal muscles as used to assess the extent of muscle activation and muscle weakness and its potential impact during discontinuation of mechanical ventilation. In addition, we proposed a practical flowchart based on recent evidence and experience of our group that can be applied during the weaning phase. This approach integrates multiple predictive parameters of weaning success with respiratory muscle ultrasound.

Anesthetic management of the diaphragmatic pacemaker placement surgery. Our experience in the Institut Guttmann

BACKGROUND

The diaphragm pacemaker (DP) has proven its utility in replacing mechanical ventilation in patients with chronic spinal cord injury (SCI) and amyotrophic lateral sclerosis (ALS). The DP improves patient quality of life and reduces morbidity and mortality and their associated health care costs. The anesthetic management of these patients and the particularities of the surgical procedure are challenging. The aim of our study is to analyze anesthetic management and intraoperative complications in patients undergoing DP placement in our hospital.

METHODS

We performed a chart review of patients treated between December 2007 and July 2017, recording the patients' preoperative status, anesthetic management and intraoperative complications.

RESULTS

The study included 16 patients (5 pediatric) undergoing DP implantation for chronic SCI (63%), ALS (25%) and other neurologic conditions (12%). Abdominal laparoscopy was performed under general anesthesia, with intravenous (87%) or inhalational (13%) induction and maintenance using total intravenous (50%) or balanced (50%) anesthesia. Rocuronium was administered in one case to permit airway management. Complications included: hypotension (50%), difficulties in mechanical ventilation during laparoscopy (31%), pneumothorax (12.5%) and autonomic dysreflexia (6%).

CONCLUSIONS

DP placement under general anesthesia is a safe intervention in both adult and pediatric patients. Complications derived from both the underlying disease and the surgical technique may appear, and must be rapidly identified and treated to obtain a satisfactory surgical outcome.

Ventilator-induced diaphragm dysfunction: translational mechanisms lead to therapeutical alternatives in the critically ill

Mechanical ventilation [MV] is a life-saving technique delivered to critically ill patients incapable of adequately ventilating and/or oxygenating due to respiratory or other disease processes. This necessarily invasive support however could potentially result in important iatrogenic complications. Even brief periods of MV may result in diaphragm weakness [i.e., ventilator-induced diaphragm dysfunction [VIDD]], which may be associated with difficulty weaning from the ventilator as well as mortality. This suggests that VIDD could potentially have a major impact on clinical practice through worse clinical outcomes and healthcare resource use. Recent translational investigations have identified that VIDD is mainly characterized by alterations resulting in a major decline of diaphragmatic contractile force together with atrophy of diaphragm muscle fibers. However, the signaling mechanisms responsible for VIDD have not been fully established. In this paper, we summarize the current understanding of the pathophysiological pathways underlying VIDD and highlight the diagnostic approach, as well as novel and experimental therapeutic options.

Respiratory muscle testing in amyotrophic lateral sclerosis: a practical approach

In amyotrophic lateral sclerosis (ALS), respiratory muscle weakness leads to respiratory failure and death. Non-invasive positive pressure ventilation (NIPPV) appears to reduce lung function decline, thus improving survival and quality-of-life of patients affected by the disease. Unfortunately, clinical features and timing to start NIPPV are not well defined. Starting from recent findings, we examine established and novel tests of respiratory muscle function that could help clinicians decide whether and when to start NIPPV in ALS. Non-invasive tests estimate the function of inspiratory, expiratory, and bulbar muscles, whereas clinical examination allows to assess the overall neurologic and respiratory symptoms and general conditions. Most of the studies recommend that together with a thorough clinical evaluation of the patient according to current guidelines, vital capacity, maximal static and sniff nasal inspiratory pressures, maximal static expiratory pressures and peak cough expiratory flow, and nocturnal pulse oximetry be measured. A sound understanding of physiology can guide the physician also through the current armamentarium for additional supportive treatments for ALS, such as symptomatic drugs and new treatments to manage sialorrhea and thickened saliva, cough assistance, air stacking, and physiotherapy. In conclusion, careful clinical and functional evaluation of respiratory function and patient's preference are key determinants to decide "when" and "to whom" respiratory treatments can be provided.

Ultrasound evaluation of diaphragm function in mechanically ventilated patients: comparison to phrenic stimulation and prognostic implications

RATIONALE

In intensive care unit (ICU) patients, diaphragm dysfunction is associated with adverse clinical outcomes. Ultrasound measurements of diaphragm thickness, excursion (EXdi) and thickening fraction (TFdi) are putative estimators of diaphragm function, but have never been compared with phrenic nerve stimulation. Our aim was to describe the relationship between these variables and diaphragm function evaluated using the change in endotracheal pressure after phrenic nerve stimulation (Ptr,stim), and to compare their prognostic value.

METHODS

Between November 2014 and June 2015, Ptr,stim and ultrasound variables were measured in mechanically ventilated patients <24 hours after intubation ('initiation of mechanical ventilation (MV)', under assist-control ventilation, ACV) and at the time of switch to pressure support ventilation ('switch to PSV'), and compared using Spearman's correlation and receiver operating characteristic curve analysis. Diaphragm dysfunction was defined as Ptr,stim <11 cm H₂O.

RESULTS

112 patients were included. At initiation of MV, Ptr,stim was not correlated to diaphragm thickness (p=0.28), EXdi (p=0.66) or TFdi (p=0.80). At switch to PSV, TFdi and EXdi were respectively very strongly and moderately correlated to Ptr,stim, (r=0.87, p<0.001 and 0.45, p=0.001), but diaphragm thickness was not (p=0.45). A TFdi <29% could reliably identify diaphragm dysfunction (sensitivity and specificity of 85% and 88%), but diaphragm thickness and EXdi could not. This value was associated with increased duration of ICU stay and MV, and mortality.

CONCLUSIONS

Under ACV, diaphragm thickness, EXdi and TFdi were uncorrelated to Ptr,stim. Under PSV, TFdi was strongly correlated to diaphragm strength and both were predictors of remaining length of MV and ICU and hospital death.

Novel insights in ICU-acquired respiratory muscle dysfunction: implications for clinical care

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency medicine 2017. Other selected articles can be found online at http://ccforum.com/series/annualupdate2017. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Diaphragm Dysfunction: Diagnostic Approaches and Management Strategies

The diaphragm is the main inspiratory muscle, and its dysfunction can lead to significant adverse clinical consequences. The aim of this review is to provide clinicians with an overview of the main causes of uni- and bi-lateral diaphragm dysfunction, explore the clinical and physiological consequences of the disease on lung function, exercise physiology and sleep and review the available diagnostic tools used in the evaluation of diaphragm function. A particular emphasis is placed on the clinical significance of diaphragm weakness in the intensive care unit setting and the use of ultrasound to evaluate diaphragmatic action.

Dysfunction of respiratory muscles in critically ill patients on the intensive care unit

Muscular weakness and muscle wasting may often be observed in critically ill patients on intensive care units (ICUs) and may present as failure to wean from mechanical ventilation. Importantly, mounting data demonstrate that mechanical ventilation itself may induce progressive dysfunction of the main respiratory muscle, i.e. the diaphragm. The respective condition was termed 'ventilator-induced diaphragmatic dysfunction' (VIDD) and should be distinguished from peripheral muscular weakness as observed in 'ICU-acquired weakness (ICU-AW)'. Interestingly, VIDD and ICU-AW may often be observed in critically ill patients with, e.g. severe sepsis or septic shock, and recent data demonstrate that the pathophysiology of these conditions may overlap. VIDD may mainly be characterized on a histopathological level as disuse muscular atrophy, and data demonstrate increased proteolysis and decreased protein synthesis as important underlying pathomechanisms. However, atrophy alone does not explain the observed loss of muscular force. When, e.g. isolated muscle strips are examined and force is normalized for cross-sectional fibre area, the loss is disproportionally larger than would be expected by atrophy alone. Nevertheless, although the exact molecular pathways for the induction of proteolytic systems remain incompletely understood, data now suggest that VIDD may also be triggered by mechanisms including decreased diaphragmatic blood flow or increased oxidative stress. Here we provide a concise review on the available literature on respiratory muscle weakness and VIDD in the critically ill. Potential underlying pathomechanisms will be discussed before the background of current diagnostic options. Furthermore, we will elucidate and speculate on potential novel future therapeutic avenues.

The JAK-STAT pathway is critical in ventilator-induced diaphragm dysfunction

Mechanical ventilation (MV) is one of the lynchpins of modern intensive-care medicine and is life saving in many critically ill patients. Continuous ventilator support, however, results in ventilation-induced diaphragm dysfunction (VIDD) that likely prolongs patients' need for MV and thereby leads to major associated complications and avoidable intensive care unit (ICU) deaths. Oxidative stress is a key pathogenic event in the development of VIDD, but its regulation remains largely undefined. We report here that the JAK-STAT pathway is activated in MV in the human diaphragm, as evidenced by significantly increased phosphorylation of JAK and STAT. Blockage of the JAK-STAT pathway by a JAK inhibitor in a rat MV model prevents diaphragm muscle contractile dysfunction (by ~85%, p < 0.01). We further demonstrate that activated STAT3 compromises mitochondrial function and induces oxidative stress in vivo, and, interestingly, that oxidative stress also activates JAK-STAT. Inhibition of JAK-STAT prevents oxidative stress-induced protein oxidation and polyubiquitination and recovers mitochondrial function in cultured muscle cells. Therefore, in ventilated diaphragm muscle, activation of JAK-STAT is critical in regulating oxidative stress and is thereby central to the downstream pathogenesis of clinical VIDD. These findings establish the molecular basis for the therapeutic promise of JAK-STAT inhibitors in ventilated ICU patients.

Diaphragm dysfunction on admission to the intensive care unit. Prevalence, risk factors, and prognostic impact-a prospective study

RATIONALE

Diaphragmatic insults occurring during intensive care unit (ICU) stays have become the focus of intense research. However, diaphragmatic abnormalities at the initial phase of critical illness remain poorly documented in humans.

OBJECTIVES

To determine the incidence, risk factors, and prognostic impact of diaphragmatic impairment on ICU admission.

METHODS

Prospective, 6-month, observational cohort study in two ICUs. Mechanically ventilated patients were studied within 24 hours after intubation (Day 1) and 48 hours later (Day 3). Seventeen anesthetized intubated control anesthesia patients were also studied. The diaphragm was assessed by twitch tracheal pressure in response to bilateral anterior magnetic phrenic nerve stimulation (Ptr,stim).

MEASUREMENTS AND MAIN RESULTS

Eighty-five consecutive patients aged 62 (54-75) (median [interquartile range]) were evaluated (medical admission, 79%; Simplified Acute Physiology Score II, 54 [44-68]). On Day 1, Ptr,stim was 8.2 (5.9-12.3) cm H2O and 64% of patients had Ptr,stim less than 11 cm H2O. Independent predictors of low Ptr,stim were sepsis (linear regression coefficient, -3.74; standard error, 1.16; P = 0.002) and Simplified Acute Physiology Score II (linear regression coefficient, -0.07; standard error, 1.69; P = 0.03). Compared with nonsurvivors, ICU survivors had higher Ptr,stim (9.7 [6.3-13.8] vs. 7.3 [5.5-9.7] cm H2O; P = 0.004). This was also true for hospital survivors versus nonsurvivors (9.7 [6.3-13.5] vs. 7.8 [5.5-10.1] cm H2O; P = 0.004). Day 1 and Day 3 Ptr,stim were similar.

CONCLUSIONS

A reduced capacity of the diaphragm to produce inspiratory pressure (diaphragm dysfunction) is frequent on ICU admission. It is associated with sepsis and disease severity, suggesting that it may represent another form of organ failure. It is associated with a poor prognosis. Clinical trial registered with www.clinicaltrials.gov (NCT 00786526).

Monitoring of the respiratory muscles in the critically ill

Evidence has accumulated that respiratory muscle dysfunction develops in critically ill patients and contributes to prolonged weaning from mechanical ventilation. Accordingly, it seems highly appropriate to monitor the respiratory muscles in these patients. Today, we are only at the beginning of routinely monitoring respiratory muscle function. Indeed, most clinicians do not evaluate respiratory muscle function in critically ill patients at all. In our opinion, however, practical issues and the absence of sound scientific data for clinical benefit should not discourage clinicians from having a closer look at respiratory muscle function in critically ill patients. This perspective discusses the latest developments in the field of respiratory muscle monitoring and possible implications of monitoring respiratory muscle function in critically ill patients.

Intrinsic apoptosis in mechanically ventilated human diaphragm: linkage to a novel Fos/FoxO1/Stat3-Bim axis

Mechanical ventilation (MV) is a life-saving measure in many critically ill patients. However, prolonged MV results in diaphragm dysfunction that contributes to the frequent difficulty in weaning patients from the ventilator. The molecular mechanisms underlying ventilator-induced diaphragm dysfunction (VIDD) remain poorly understood. We report here that MV induces myonuclear DNA fragmentation (3-fold increase; P<0.01) and selective activation of caspase 9 (P<0.05) and Bcl2-interacting mediator of cell death (Bim; 2- to 7-fold increase; P<0.05) in human diaphragm. MV also statistically significantly down-regulates mitochondrial gene expression and induces oxidative stress. In cultured muscle cells, we show that oxidative stress activates each of the catabolic pathways thought to underlie VIDD: apoptotic (P<0.05), proteasomal (P<0.05), and autophagic (P<0.01). Further, silencing Bim expression blocks (P<0.05) oxidative stress-induced apoptosis. Overlapping the gene expression profiles of MV human diaphragm and H₂O₂-treated muscle cells, we identify Fos, FoxO1, and Stat3 as regulators of Bim expression as well as of expression of the catabolic markers atrogin and LC3. We thus identify a novel Fos/FoxO1/Stat3-Bim intrinsic apoptotic pathway and establish the centrality of oxidative stress in the development of VIDD. This information may help in the design of specific drugs to prevent this condition.

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.

Narrative review: ventilator-induced respiratory muscle weakness

Clinicians have long been aware that substantial lung injury results when mechanical ventilation imposes too much stress on the pulmonary parenchyma. Evidence is accruing that substantial injury may also result when the ventilator imposes too little stress on the respiratory muscles. Through adjustment of ventilator settings and administration of pharmacotherapy, the respiratory muscles may be rendered almost (or completely) inactive. Research in animals has shown that diaphragmatic inactivity produces severe injury and atrophy of muscle fibers. Human data have recently revealed that 18 to 69 hours of complete diaphragmatic inactivity associated with mechanical ventilation decreased the cross-sectional areas of diaphragmatic fibers by half or more. The atrophic injury seems to result from increased oxidative stress leading to activation of protein-degradation pathways. Scientific understanding of ventilator-induced respiratory muscle injury has not reached the stage where meaningful controlled trials can be done, and thus, it is not possible to give concrete recommendations for patient management. In the meantime, clinicians are advised to select ventilator settings that avoid both excessive patient effort and excessive respiratory muscle rest. The contour of the airway pressure waveform on a ventilator screen provides the most practical indication of patient effort, and clinicians are advised to pay close attention to the waveform as they titrate ventilator settings. Research on ventilator-induced respiratory muscle injury is in its infancy and portends to be an exciting area to follow.

Depressive disorders during weaning from prolonged mechanical ventilation

PURPOSE

Patients who require mechanical ventilation are at risk of emotional stress because of total dependence on a machine for breathing. The stress may negatively impact ventilator weaning and survival. The purpose of this study was to determine whether depressive disorders in patients being weaned from prolonged mechanical ventilation are linked to weaning failure and decreased survival.

METHODS

A prospective study of 478 consecutive patients transferred to a long-term acute care hospital for weaning from prolonged ventilation was undertaken. A clinical psychologist conducted a psychiatric interview to assess for the presence of depressive disorders.

RESULTS

Of the 478 patients, 142 had persistent coma or delirium and were unable to be evaluated for depressive disorders. Of the remaining 336 patients, 142 (42%) were diagnosed with depressive disorders. In multivariate analysis, co-morbidity score [odds ratio (OR), 1.23; P = 0.007], functional dependence before the acute illness (OR, 1.70, P = 0.03) and history of psychiatric disorders (OR, 3.04, P = 0.0001) were independent predictors of depressive disorders. The rate of weaning failure was higher in patients with depressive disorders than in those without such disorders (61 vs. 33%, P = 0.0001), as was mortality (24 vs. 10%, P = 0.0008). The presence of depressive disorders was independently associated with mortality (OR, 4.3; P = 0.0002); age (OR, 1.06; P = 0.001) and co-morbidity score (OR, 1.24; P = 0.02) also predicted mortality.

CONCLUSION

Depressive disorders were diagnosed in 42% of patients who were being weaned from prolonged ventilation. Patients with depressive disorders were more likely to experience weaning failure and death.

The pickwickian syndrome-obesity hypoventilation syndrome

Obesity-hypoventilation syndrome (OHS), also historically described as the Pickwickian syndrome, consists of the triad of obesity, sleep disordered breathing, and chronic hypercapnia during wakefulness in the absence of other known causes of hypercapnia. Its exact prevalence is unknown, but it has been estimated that 10% to 20% of obese patients with obstructive sleep apnea have hypercapnia. OHS often remains undiagnosed until late in the course of the disease. Early recognition is important because these patients have significant morbidity and mortality. Effective treatment can lead to significant improvement in patient outcomes, underscoring the importance of early diagnosis. The authors review the definition and epidemiology of OHS, in addition to the current multifaceted understanding of the pathophysiology, and provide useful clinical approaches to diagnosis and treatment.

Controlled twitch mouth pressure reliably predicts twitch esophageal pressure

The present study hypothesized that twitch mouth pressure (TwPmo) can reliably predict intrathoracic pressure swings reflected by twitch esophageal pressure (TwPes) using a controlled and automated trigger technique. TwPmo, TwPes, and transdiaphragmatic pressure (TwPdi) following bilateral anterior magnetic phrenic nerve stimulation were measured in 21 healthy subjects using an inspiratory pressure trigger (0.5kPa, experiment 1), an expiratory pressure trigger (0.5kPa, experiment 2), an inspiratory flow trigger (40ml/s, experiment 3), and no trigger at relaxed functional residual capacity (experiment 4). TwPmo and TwPes were correlated as follows: r=0.99, p<0.0001 (experiment 1); r=0.67, p=0.001 (experiment 2); r=0.96, p<0.0001 (experiment 3); no correlation (experiment 4). Bland and Altman analysis revealed most narrow limits of agreement for TwPmo and TwPes in experiment 1: bias (range) 0.15kPa (-0.03 to 0.32). TwPmo is an excellent predictor for TwPes when using a fully automated and controlled inspiratory pressure trigger. Thus, measurement of TwPmo could become a standard means assessing inspiratory muscle strength.

Assessment of diaphragmatic function with cervical magnetic stimulation in critically ill patients

The objective of this study was to evaluate a non-volitional measurement to assess diaphragmatic function in intubated and mechanically ventilated patients in a prospective pilot interventional clinical trial. The study was conducted in an 18-bed postoperative intensive care unit based at a university hospital. Patients were prospectively assigned to two groups. Group 1 consisted of eight patients with ventilator weaning failure. Group 2 consisted of eight intubated and ventilated patients who were studied shortly after major surgery and were successfully extubated there-after The twitch pressure response after cervical magnetic stimulation of the phrenic nerves was measured at the endotracheal tube at different PEEP levels. In group 2 the twitch transdiaphragmatic pressure, defined as the difference between twitch gastric and twitch oesophageal pressure was also evaluated. In group 1 the mean twitch pressure at the endotracheal tube on PEEP 0, 5 and 10 cmH2O was 5.2, 4.5 and 2.6 cmH2O: In group 2 this was significantly higher (15.1 cmH2O on PEEP 0 and 12.2 cmH2O on PEEP 5). A good correlation was found between twitch diaphragmatic pressure and twitch pressure at the endotracheal tube (r2 = 0.96) and between twitch oesophageal pressure and twitch pressure at the endotracheal tube (r2 = 0.98). Patients with weaning failure have significantly lower twitch pressure at the endotracheal tube suggesting diaphragmatic dysfunction. Twitch pressure at the endotracheal tube may be a useful parameter to screen for diaphragmatic dysfunction in intubated critically ill patients. Further studies are needed to confirm these preliminary findings.

Twitch airway pressure elicited by magnetic phrenic nerve stimulation in anesthetized healthy children

Children with diaphragm dysfunction may be unable to maintain adequate ventilation. Accurate diagnosis is important, but can only be achieved using an appropriate test and reference range. The aim of this study, therefore, was to measure diaphragm contractility and examine the influence of age and maturation, using magnetic phrenic nerve stimulation in healthy children. Anterolateral magnetic stimulation (MS) of the phrenic nerves was performed using a 43-mm figure-eight coil in 23 children (14 male; mean age, 7.8 years; range, 1.8-15.7) anesthetized for minor surgery with sevoflurane gas. The airway was maintained with a cuffed laryngeal mask airway (LMA) which was briefly occluded during MS. Diaphragm contractility was assessed by measuring the airway pressure (TwPaw) elicited by MS. TwPaw responses were obtained in all subjects, with mean (SD) TwPaw 18.2 (6.8) cm H2O bilateral, 7.3 (3.2) cm H2O left unilateral, and 8.6 (3.1) cm H2O right unilateral. Subgroup analysis was performed in 17 of the children who were prepubertal. Their mean (SD) TwPaw was 17.3 (6.8) cm H2O bilateral, 7.1 (3.7) cm H2O left unilateral, and 8.3 (3.3) right unilateral. The mean (SD) intrapatient coefficients of variation for bilateral and left and right unilateral TwPaw were 8.4% (5.2), 6.7% (3.5), and 11.7% (10.3), respectively. Bilateral and left and right unilateral TwPaw were significantly related to age (P < 0.05). In healthy prepubertal children, diaphragm contractility is primarily influenced by age.

Effect of Lung Volume Reduction Surgery on Diaphragmatic Neuromechanical Coupling At 2 Years

STUDY OBJECTIVES

We previously reported that patients with emphysema show an increase in diaphragmatic neuromechanical coupling at 3 months after lung volume reduction surgery. Diaphragmatic neuromechanical coupling was quantified as the quotient of tidal volume (normalized to total lung capacity) to tidal change in transdiaphragmatic pressure (normalized to maximal transdiaphragmatic pressure). As such, neuromechanical coupling estimates the fraction of diaphragmatic capacity used to generate tidal breathing. The present investigation was conducted to determine whether benefit is maintained at 2 years.

SUBJECTS

Fifteen patients with severe COPD, 8 of whom completed the 2-year study.

METHODS

Lung volumes, exercise capacity (6-min walking distance), diaphragmatic function (maximal transdiaphragmatic pressure and twitch transdiaphragmatic pressure elicited by phrenic nerve stimulation), and diaphragmatic neuromechanical coupling were recorded before surgery, and at 3 months and 2 years after surgery.

RESULTS

Two years after surgery, lung volumes deteriorated to preoperative values, but patients showed persistent improvements in 6-min walking distance (p < 0.05). Three months after surgery, maximal transdiaphragmatic pressure (p < 0.05), twitch transdiaphragmatic pressure (p < 0.01), and diaphragmatic neuromechanical coupling (p < 0.01) had increased over preoperative values. The improvements in neuromechanical coupling resulted from improvements in diaphragmatic strength and, to a lesser extent, from a decrease in transdiaphragmatic pressure required to maintain tidal breathing. The change in respiratory muscle function at 2 years varied among patients: diaphragmatic contractility was > 10% of preoperative value in half of the patients who concluded our study, and neuromechanical coupling was > 10% of preoperative value in three fourths of the patients who concluded our study. Patients who maintained their gains in neuromechanical coupling also maintained their gains in 6-min walking distance.

CONCLUSION

Patients undergoing lung volume reduction surgery can maintain early gains in neuromechanical coupling and exercise capacity 2 years later.