Phrenic nerve stimulation protects against mechanical ventilation-induced diaphragm dysfunction in rats

Intraoperative phrenic nerve stimulation to prevent diaphragm fiber weakness during thoracic surgery

Thoracic surgery rapidly induces weakness in human diaphragm fibers. The dysfunction is thought to arise from combined effects of the surgical procedures and inactivity. This project tested whether brief bouts of intraoperative hemidiaphragm stimulation would mitigate slow and fast fiber loss of force in the human diaphragm. We reasoned that maintenance of diaphragm activity with brief bouts of intraoperative phrenic stimulation would mitigate diaphragm fiber weakness and myofilament protein derangements caused by thoracic surgery. Nineteen adults (9 females, age 59 ± 12 years) with normal inspiratory strength or spirometry consented to participate. Unilateral phrenic twitch stimulation (twitch duration 1.5 ms, frequency 0.5 Hz, current 2x the motor threshold, max 25 mA) was applied for one minute, every 30 minutes during cardiothoracic surgery. Thirty minutes following the last stimulation bout, biopsies were obtained from the hemidiaphragms for single fiber force mechanics and quantitation of myofilament proteins (abundance and phosphorylation) and compared by a linear mixed model and paired t-test, respectively. Subjects underwent 6 ±  2 hemidiaphragm stimulations at 17 ±  6 mA, during 278 ±  68 minutes of surgery. Longer-duration surgeries were associated with a progressive decline in diaphragm fiber force (p < 0.001). In slow-twitch fibers, phrenic stimulation increased absolute force (+25%, p <  0.0001), cross-sectional area (+16%, p < 0.0001) and specific force (+7%, p < 0.0005). Stimulation did not alter contractile function of fast-twitch fibers, calcium-sensitivity in either fiber type, and abundance and phosphorylation of myofilament proteins. In adults without preoperative weakness or lung dysfunction, unilateral phrenic stimulation mitigated diaphragm slow fiber weakness caused by thoracic surgery, but had no effect on myofilament protein abundance or phosphorylation.

Within- and between-day test-retest reliability of responses to rapid bilateral anterolateral magnetic phrenic nerve stimulation in healthy humans (ReStim)

Background

Mechanical ventilation can lead to lung injury and diaphragmatic dysfunction. Rapid bilateral anterolateral magnetic phrenic nerve stimulation (rBAMPS) may attenuate both of the aforementioned issues by inducing diaphragm activation. However, in order for rBAMPS to become part of standard of care, the reliability of inspiratory responses to rBAMPS needs to be established.

Methods

Eighteen healthy participants (9F) underwent five blocks of 1-s rBAMPS at 25 Hz starting at 20% of maximal stimulator output with 10% increments. Three blocks were completed on the same day to test within-day reliability, and two additional blocks were each completed on subsequent days to test between-day reliability. Mean transdiaphragmatic pressure (Pdi,mean), tidal volume (VT), discomfort, pain, and paresthesia were recorded for each rBAMPS. Relative and absolute reliability of both Pdi,mean and VT were quantified by calculating intraclass correlation coefficients (ICC) and standard error of measurements (SEM), respectively. An ordinal regression was used to determine changes of sensory ratings within and between days.

Results

At all stimulator outputs, within-day Pdi,mean displayed "good" reliability (ICC range 0.78-0.89). Between days, Pdi,mean reliability was also "good" (ICC range 0.79-0.87) at stimulator outputs of 20%-50% of maximum, but "moderate" (ICC range 0.56-0.72) at stimulator outputs of 60%-100%. SEM for Pdi,mean within day ranged from 0.9 to 3.4 across tested stimulator outputs and increased on average by 1.4 ± 0.9 between days. The VT reliability was "good" to "excellent" within (ICC range 0.82-0.94) and between (ICC range 0.81-0.96) days at all stimulator outputs. SEM for VT within day ranged from 0.08 to 0.36 and from 0.11 to 0.30 between days and tended to be larger at stimulator outputs greater than 50% of maximum. Subsequent blocks within day were associated with decreased discomfort and pain (P ≤ 0.043), while subsequent days were associated with decreased discomfort and paresthesia (P < 0.001).

Discussion

rBAMPS appears to induce reliable diaphragmatic contractions, while select sensory responses become blunted over repeated stimulations. However, as reliability is slightly lower between days compared to within day, stimulation parameters may need to be adjusted to achieve similar responses on different days.

Research progress on the pathogenesis and treatment of ventilator-induced diaphragm dysfunction

Prolonged controlled mechanical ventilation (CMV) can cause diaphragm fiber atrophy and inspiratory muscle weakness, resulting in diaphragmatic contractile dysfunction, called ventilator-induced diaphragm dysfunction (VIDD). VIDD is associated with higher rates of in-hospital deaths, nosocomial pneumonia, difficulty weaning from ventilators, and increased costs. Currently, appropriate clinical strategies to prevent and treat VIDD are unavailable, necessitating the importance of exploring the mechanisms of VIDD and suitable treatment options to reduce the healthcare burden. Numerous animal studies have demonstrated that ventilator-induced diaphragm dysfunction is associated with oxidative stress, increased protein hydrolysis, disuse atrophy, and calcium ion disorders. Therefore, this article summarizes the molecular pathogenesis and treatment of ventilator-induced diaphragm dysfunction in recent years so that it can be better served clinically and is essential to reduce the duration of mechanical ventilation use, intensive care unit (ICU) length of stay, and the medical burden.

Aldh1a1 and Scl25a30 in diaphragmatic dysfunction

New methods to prevent ventilator-induced diaphragmatic dysfunction (VIDD) are urgently needed, and the cellular basis of VIDD is poorly understood. This study evaluated whether transvenous phrenic nerve stimulation (PNS) could prevent VIDD in rabbits undergoing mechanical ventilation (MV) and explored whether oxidative stress-related genes might be candidate molecular markers for VIDD. Twenty-four adult male New Zealand white rabbits were allocated to control, MV, and PNS groups (n = 8 in each group). Rabbits in the MV and PNS groups underwent MV for 24 h. Intermittent bilateral transvenous PNS was performed in rabbits in the PNS group. Transdiaphragmatic pressure was recorded using balloon catheters. The diameters and cross-sectional areas (CSAs) of types I and II diaphragmatic fibers were measured using immunohistochemistry (IHC) techniques. Genes associated with VIDD were identified by RNA sequencing (RNA-seq), differentially expressed gene (DEG) analysis, and weighted gene co-expression network analysis (WGCNA). Reverse transcription polymerase chain reaction (RT-PCR), Western blotting, and IHC analyses were carried out to verify the transcriptome profile. Pdi_60Hz, Pdi_80Hz, and Pdi_100Hz were significantly higher in the PNS group than in the MV group at 12 and 24 h (P < 0.05 at both time points). The diameters and CSAs of types I (slow-twitch) and II (fast-twitch) fibers were significantly larger in the PNS group than in the MV group (P < 0.05). RNA-seq, RT-PCR, Western blotting, and IHC experiments identified two candidate genes associated with VIDD: Aldh1a1 and Scl25a30. The MV group had significantly higher mRNA and protein expressions of Aldh1a1/ALDH1A1 and significantly lower mRNA and protein expressions of Scl25a30/SCL25A30 than the control or PNS groups (P < 0.05). We have identified two candidate genes involved in the prevention of VIDD by transvenous PNS. These two key genes may provide a theoretical basis for targeted therapy against VIDD.

Inspiratory response and side-effects to rapid bilateral magnetic phrenic nerve stimulation using differently shaped coils: implications for stimulation-assisted mechanical ventilation

BACKGROUND

Rapid magnetic stimulation (RMS) of the phrenic nerves may serve to attenuate diaphragm atrophy during mechanical ventilation. With different coil shapes and stimulation location, inspiratory responses and side-effects may differ. This study aimed to compare the inspiratory and sensory responses of three different RMS-coils either used bilaterally on the neck or on the chest, and to determine if ventilation over 10 min can be achieved without muscle fatigue and coils overheating.

METHODS

Healthy participants underwent bilateral anterior 1-s RMS on the neck (RMS_BAMPS) (N = 14) with three different pairs of magnetic coils (parabolic, D-shape, butterfly) at 15, 20, 25 and 30 Hz stimulator-frequency and 20% stimulator-output with + 10% increments. The D-shape coil with individual optimal stimulation settings was then used to ventilate participants (N = 11) for up to 10 min. Anterior RMS on the chest (RMS_aMS) (N = 8) was conducted on an optional visit. Airflow was assessed via pneumotach and transdiaphragmatic pressure via oesophageal and gastric balloon catheters. Perception of air hunger, pain, discomfort and paresthesia were measured with a numerical scale.

RESULTS

Inspiration was induced via RMS_BAMPS in 86% of participants with all coils and via RMS_aMS in only one participant with the parabolic coil. All coils produced similar inspiratory and sensory responses during RMS_BAMPS with the butterfly coil needing higher stimulator-output, which resulted in significantly larger discomfort ratings at maximal inspiratory responses. Ten of 11 participants achieved 10 min of ventilation without decreases in minute ventilation (15.7 ± 4.6 L/min).

CONCLUSIONS

RMS_BAMPS was more effective than RMS_aMS, and could temporarily ventilate humans seemingly without development of muscular fatigue. Trial registration This study was registered on clinicaltrials.gov (NCT04176744).

Hippocampal epigenetic and insulin-like growth factor alterations in noninvasive versus invasive mechanical ventilation in preterm lambs

BACKGROUND

The brain of chronically ventilated preterm human infants is vulnerable to collateral damage during invasive mechanical ventilation (IMV). Damage is manifest, in part, by learning and memory impairments, which are hippocampal functions. A molecular regulator of hippocampal development is insulin-like growth factor 1 (IGF1). A gentler ventilation strategy is noninvasive respiratory support (NRS). We tested the hypotheses that NRS leads to greater levels of IGF1 messenger RNA (mRNA) variants and distinct epigenetic profile along the IGF1 gene locus in the hippocampus compared to IMV.

METHODS

Preterm lambs were managed by NRS or IMV for 3 or 21 days. Isolated hippocampi were analyzed for IGF1 mRNA levels and splice variants for promoter 1 (P1), P2, and IGF1A and 1B, DNA methylation in P1 region, and histone covalent modifications along the gene locus.

RESULTS

NRS had significantly greater levels of IGF1 P1 (predominant transcript), and 1A and 1B mRNA variants compared to IMV at 3 or 21 days. NRS also led to more DNA methylation and greater occupancy of activating mark H3K4 trimethylation (H3K4me3), repressive mark H3K27me3, and elongation mark H3K36me3 compared to IMV.

CONCLUSIONS

NRS leads to distinct IGF1 mRNA variant levels and epigenetic profile in the hippocampus compared to IMV.

IMPACT

Our study shows that 3 or 21 days of NRS of preterm lambs leads to distinct IGF1 mRNA variant levels and epigenetic profile in the hippocampus compared to IMV. Preterm infant studies suggest that NRS leads to better neurodevelopmental outcomes later in life versus IMV. Also, duration of IMV is directly related to hippocampal damage; however, molecular players remain unknown. NRS, as a gentler mode of respiratory management of preterm neonates, may reduce damage to the immature hippocampus through an epigenetic mechanism.

Magnesium sulfate ameliorates sepsis-induced diaphragm dysfunction in rats via inhibiting HMGB1/TLR4/NF-κB pathway

Diaphragm dysfunction could be induced by sepsis with subsequent ventilatory pump failure that is associated with local infiltration of inflammatory factors in the diaphragm. It has been shown that the administration of anticonvulsant agent, magnesium sulfate (MgSO₄) could decrease systematic inflammatory response. We recently reported that MgSO₄ could inhibit macrophages high mobility group box 1 (HMGB1) secretion that confirms its anti-inflammatory properties. Toll-like receptor 4 (TLR4)/nuclear factor-kappa B (NF-κB) signal pathway appears to be involved in the pathology of septic experimental animal’s inflammatory response and involve in the pathogenic mechanisms of sepsis-induced diaphragm dysfunction. Thus, in this study, we are aiming to explore whether MgSO₄ could ameliorate sepsis-induced diaphragm dysfunction via TLR4/NF-κB pathway in a rodent model with controlled mechanical ventilation (CMV) and subsequent septic challenge.

Assessment of magnetic flux density properties of electromagnetic noninvasive phrenic nerve stimulations for environmental safety in an ICU environment

Diaphragm weakness affects up to 60% of ventilated patients leading to muscle atrophy, reduction of muscle fiber force via muscle fiber injuries and prolonged weaning from mechanical ventilation. Electromagnetic stimulation of the phrenic nerve can induce contractions of the diaphragm and potentially prevent and treat loss of muscular function. Recommended safety distance of electromagnetic coils is 1 m. The aim of this study was to investigate the magnetic flux density in a typical intensive care unit (ICU) setting. Simulation of magnetic flux density generated by a butterfly coil was performed in a Berlin ICU training center with testing of potential disturbance and heating of medical equipment. Approximate safety distances to surrounding medical ICU equipment were additionally measured in an ICU training center in Bern. Magnetic flux density declined exponentially with advancing distance from the stimulation coil. Above a coil distance of 300 mm with stimulation of 100% power the signal could not be distinguished from the surrounding magnetic background noise. Electromagnetic stimulation of the phrenic nerve for diaphragm contraction in an intensive care unit setting seems to be safe and feasible from a technical point of view with a distance above 300 mm to ICU equipment from the stimulation coil.

Case Studies in Physiology: Physiological and clinical effects of temporary diaphragm pacing in two patients with ventilator-induced diaphragm dysfunction

Ventilator-induced diaphragm dysfunction (VIDD) is increasingly recognized as an important side-effect of invasive ventilation in critically ill patients and is associated with poor outcomes. Whether patients with VIDD benefit from temporary diaphragm pacing is uncertain. Intramuscular diaphragmatic electrodes were implanted for temporary stimulation with a pacing device (TransAeris System) in two patients with VIDD. The electrodes were implanted via laparoscopy (first patient) or via bilateral thoracoscopy (second patient). Stimulation parameters were titrated according to tolerance. Diaphragm thickening fraction by ultrasound, maximum inspiratory pressure (Pi_max) and diaphragm electromyography (EMGdi) signal analysis were used to monitor the response to diaphragm pacing. Both patients tolerated diaphragm pacing. In the first patient, improvements in diaphragm excursions were noted once pacing was initiated and diaphragm thickening fraction did not further deteriorate over time. The diaphragm thickening fraction improved in the second patient, and Pi_max as well as EMGdi analysis suggested improved muscle function. This patient could be fully weaned from the ventilator. These case reports present the first experience with temporary diaphragm pacing in critically ill patients with VIDD. Our results should be taken cautiously given the reduced sample size, but provide the proof of concept to put forward the hypothesis that a course of diaphragm pacing may be associated with improved diaphragmatic function. Our findings of the tolerance to the procedure and the beneficial physiological effects are not prove of safety and efficacy, but may set the ground to design and conduct larger studies.NEW & NOTEWORTHY Diaphragmatic electrode implantation and temporary diaphragm pacing have not been previously used in ICU patients with VIDD. Patients were monitored using a multimodal monitoring approach including ultrasound of the diaphragm, measurement of maximum inspiratory pressure and EMG signal analysis. Our results suggest that diaphragm pacing may improve diaphragmatic function, with the potential to prevent and treat VIDD in critically ill patients. Safety and efficacy of this intervention is yet to be proven in larger studies.

Diaphragm function in acute respiratory failure and the potential role of phrenic nerve stimulation

PURPOSE OF REVIEW

The aim of this review was to describe the risk factors for developing diaphragm dysfunction, discuss the monitoring techniques for diaphragm activity and function, and introduce potential strategies to incorporate diaphragm protection into conventional lung-protective mechanical ventilation strategies.

RECENT FINDINGS

It is increasingly apparent that an approach that addresses diaphragm-protective ventilations goals is needed to optimize ventilator management and improve patient outcomes. Ventilator-induced diaphragm dysfunction (VIDD) is common and is associated with increased ICU length of stay, prolonged weaning and increased mortality. Over-assistance, under-assistance and patient-ventilator dyssynchrony may have important downstream clinical consequences related to VIDD. Numerous monitoring techniques are available to assess diaphragm function, including respiratory system pressures, oesophageal manometry, diaphragm ultrasound and electromyography. Novel techniques including phrenic nerve stimulation may facilitate the achievement of lung and diaphragm-protective goals for mechanical ventilation.

SUMMARY

Diaphragm protection is an important consideration in optimizing ventilator management in patients with acute respiratory failure. The delicate balance between lung and diaphragm-protective goals is challenging. Phrenic nerve stimulation may be uniquely situated to achieve and balance these two commonly conflicting goals.

Diaphragm protection: what should we target?

PURPOSE OF REVIEW

Diaphragm weakness can impact survival and increases comorbidities in ventilated patients. Mechanical ventilation is linked to diaphragm dysfunction through several mechanisms of injury, referred to as myotrauma. By monitoring diaphragm activity and titrating ventilator settings, the critical care clinician can have a direct impact on diaphragm injury.

RECENT FINDINGS

Both the absence of diaphragm activity and excessive inspiratory effort can result in diaphragm muscle weakness, and recent evidence demonstrates that a moderate level of diaphragm activity during mechanical ventilation improves ICU outcome. This supports the hypothesis that by avoiding ventilator overassistance and underassistance, the clinician can implement a diaphragm-protective ventilation strategy. Furthermore, eccentric diaphragm contractions and end-expiratory shortening could impact diaphragm strength as well. This review describes these potential targets for diaphragm protective ventilation.

SUMMARY

A ventilator strategy that results in appropriate levels of diaphragm activity has the potential to be diaphragm-protective and improve clinical outcome. Monitoring respiratory effort during mechanical ventilation is becoming increasingly important.

Neuregulin-1β Protects the Rat Diaphragm during Sepsis against Oxidative Stress and Inflammation by Activating the PI3K/Akt Pathway

Sepsis-induced diaphragm dysfunction (SIDD) which is mainly characterized by decrease in diaphragmatic contractility has been identified to cause great harms to patients. Therefore, there is an important and pressing need to find effective treatments for improving SIDD. In addition, acetylcholinesterase (AChE) activity is a vital property of the diaphragm, so we evaluated both diaphragmatic contractility and AChE activity. Though neuregulin-1β (NRG-1β) is known to exert organ-protective effects in some inflammatory diseases, little is known about the potential of NRG-1β therapy in the diaphragm during sepsis. Our study was aimed at exploring the effects of NRG-1β application on diaphragmatic contractility and AChE activity during sepsis. Proinflammatory cytokines, muscle injury biomarkers in serum, contractile force, AChE activity, proinflammatory cytokines, oxidative parameters, histological condition, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining, and expression of phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB/Akt) signaling proteins in the diaphragm were measured and compared between nonseptic and septic groups with or without NRG-1β treatment. In vitro, the effects of NRG-1β on reactive oxygen species (ROS) production in the lipopolysaccharide- (LPS-) stimulated L6 rat muscle skeletal cells with or without the Akt inhibitor MK-2206 were detected. NRG-1β inhibited proinflammatory cytokine release and muscle injury biomarkers soaring in serum and improved the sepsis-induced diaphragm dysfunction and AChE activity decrease significantly during sepsis. Meanwhile, the inflammatory response, oxidative stress, pathological impairment, and cell apoptosis in the diaphragm were mitigated by NRG-1β. And NRG-1β activated the PI3K/Akt signaling in the diaphragm of septic rats. Elevated ROS production in the LPS-stimulated L6 rat skeletal muscle cells was reduced after treatment with NRG-1β, while MK-2206 blocked these effects of NRG-1β. In conclusion, our findings underlined that NRG-1β could reduce circulating levels of proinflammatory cytokines in rats with sepsis, adjust diaphragmatic proinflammatory cytokine level, mitigate diaphragmatic oxidative injury, and lessen diaphragm cell apoptosis, thereby improving diaphragmatic function, and play a role in diaphragmatic protection by activating PI3K/Akt signaling.

Physiopathological mechanisms of diaphragmatic dysfunction associated with mechanical ventilation

Ventilator-induced diaphragm dysfunction (VIDD) is the loss of diaphragmatic muscle strength'related to of mechanical ventilation, noticed during the first day or 48hours after initiating controlled mechanical ventilation. This alteration has been related to disruption on the insulin growth factor/phosphoinositol 3-kinase/kinase B protein pathway (IGF/PI3K/AKT), in addition to an overexpression of FOXO, expression of NF-kB signaling, increase function of muscular ubiquitin ligase and activation of caspasa-3. VIDD has a negative impact on quality of life, duration of mechanical ventilation, and hospitalization stance and cost. More studies are necessary to understated the process and impact of VIDD. This is a narrative review of non-systematic literature, aiming to explain the molecular pathways involved in VIDD.

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.

The Signaling Network Resulting in Ventilator-induced Diaphragm Dysfunction

Mechanical ventilation (MV) is a life-saving measure for those incapable of adequately ventilating or oxygenating without assistance. Unfortunately, even brief periods of MV result in diaphragm weakness (i.e., ventilator-induced diaphragm dysfunction [VIDD]) that may render it difficult to wean the ventilator. Prolonged MV is associated with cascading complications and is a strong risk factor for death. Thus, prevention of VIDD may have a dramatic impact on mortality rates. Here, we summarize the current understanding of the pathogenic events underlying VIDD. Numerous alterations have been proven important in both human and animal MV diaphragm. These include protein degradation via the ubiquitin proteasome system, autophagy, apoptosis, and calpain activity-all causing diaphragm muscle fiber atrophy, altered energy supply via compromised oxidative phosphorylation and upregulation of glycolysis, and also mitochondrial dysfunction and oxidative stress. Mitochondrial oxidative stress in fact appears to be a central factor in each of these events. Recent studies by our group and others indicate that mitochondrial function is modulated by several signaling molecules, including Smad3, signal transducer and activator of transcription 3, and FoxO. MV rapidly activates Smad3 and signal transducer and activator of transcription 3, which upregulate mitochondrial oxidative stress. Additional roles may be played by angiotensin II and leaky ryanodine receptors causing elevated calcium levels. We present, here, a hypothetical scaffold for understanding the molecular pathogenesis of VIDD, which links together these elements. These pathways harbor several drug targets that could soon move toward testing in clinical trials. We hope that this review will shape a short list of the most promising candidates.

Smad3 initiates oxidative stress and proteolysis that underlies diaphragm dysfunction during mechanical ventilation

Prolonged use of mechanical ventilation (MV) leads to atrophy and dysfunction of the major inspiratory muscle, the diaphragm, contributing to ventilator dependence. Numerous studies have shown that proteolysis and oxidative stress are among the major effectors of ventilator-induced diaphragm muscle dysfunction (VIDD), but the upstream initiator(s) of this process remain to be elucidated. We report here that periodic diaphragm contraction via phrenic nerve stimulation (PNS) substantially reduces MV-induced proteolytic activity and oxidative stress in the diaphragm. We show that MV rapidly induces phosphorylation of Smad3, and PNS nearly completely prevents this effect. In cultured cells, overexpressed Smad3 is sufficient to induce oxidative stress and protein degradation, whereas inhibition of Smad3 activity suppresses these events. In rats subjected to MV, inhibition of Smad3 activity by SIS3 suppresses oxidative stress and protein degradation in the diaphragm and prevents the reduction in contractility that is induced by MV. Smad3's effect appears to link to STAT3 activity, which we previously identified as a regulator of VIDD. Inhibition of Smad3 suppresses STAT3 signaling both in vitro and in vivo. Thus, MV-induced diaphragm inactivity initiates catabolic changes via rapid activation of Smad3 signaling. An early intervention with PNS and/or pharmaceutical inhibition of Smad3 may prevent clinical VIDD.

Anatomy and physiology of phrenic afferent neurons

Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that 1) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis; 2) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; 3) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and 4) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.

Decreased intracellular [Ca2+ ] coincides with reduced expression of Dhprα1s, RyR1, and diaphragmatic dysfunction in a rat model of sepsis

INTRODUCTION

Sepsis can cause decreased diaphragmatic contractility. Intracellular calcium as a second messenger is central to diaphragmatic contractility. However, changes in intracellular calcium concentration ([Ca²⁺ ]) and the distribution and co-localization of relevant calcium channels [dihydropyridine receptors, (DHPRα1s) and ryanodine receptors (RyR1)] remain unclear during sepsis. In this study we investigated the effect of changed intracellular [Ca²⁺ ] and expression and distribution of DHPRα1s and RyR1 on diaphragm function during sepsis.

METHODS

We measured diaphragm contractility and isolated diaphragm muscle cells in a rat model of sepsis. The distribution and co-localization of DHPRα1s and RyR1 were determined using immunohistochemistry and immunofluorescence, whereas intracellular [Ca²⁺ ] was measured by confocal microscopy and fluorescence spectrophotometry.

RESULTS

Septic rat diaphragm contractility, expression of DHPRα1s and RyR1, and intracellular [Ca²⁺ ] were significantly decreased in the rat sepsis model compared with controls.

DISCUSSION

Decreased intracellular [Ca²⁺ ] coincides with diaphragmatic contractility and decreased expression of DHPRα1s and RyR1 in sepsis. Muscle Nerve 56: 1128-1136, 2017.

Corrective effect of diaphragm pacing on the decrease in cardiac output induced by positive pressure mechanical ventilation in anesthetized sheep

Positive pressure ventilation (PPV) is a fundamental life support measure, but it decreases cardiac output (CO). Diaphragmatic contractions produce negative intrathoracic and positive abdominal pressures, promoting splanchnic venous return. We hypothesized that: 1) diaphragm pacing alone could produce adequate ventilation without decreasing CO; 2) diaphragm pacing on top of PPV could improve CO. Of 11 anesthetized and mechanically ventilated ewes (39.6±5.9kg), 3 were discarded from analysis because of hemodynamic instability during the experiment, and 8 retained for analysis. Phrenic stimulation electrodes were inserted in the diaphragm (implanted phrenic nerve stimulation, iPS). CO was measured by the thermodilution technique (pulmonary artery catheter). CO during end-expiratory apnea served as reference. Median CO was 9.77 [6.25-11.25] lmin^-1 during end-expiratory apnea, 8.25 [5.06-9.25] lmin^-1 during "PPV" (-15%) (p<0.05), 9.19 [5.60-10.19] lmin^-1 during "PPV-iPS" (NS vs apnea) and 9.37 [6.12-10.48] lmin^-1 during "iPS" (NS vs. apnea). iPS-driven ventilation was comparable to its PPV counterpart (median 92% [74-97], NS). Diaphragm pacing alone can produce adequate ventilation without reducing CO. Superimposed onto PPV, diaphragm pacing can reduce the PPV-induced decrease in CO.

Ventilator-induced diaphragmatic dysfunction: what have we learned?

PURPOSE OF REVIEW

The purpose of the review is to summarize and discuss recent research regarding the role of mechanical ventilation in producing weakness and atrophy of the diaphragm in critically ill patients, an entity termed ventilator-induced diaphragmatic dysfunction (VIDD).

RECENT FINDINGS

Severe weakness of the diaphragm is frequent in mechanically ventilated patients, in whom it contributes to poor outcomes including increased mortality. Significant progress has been made in identifying the molecular mechanisms responsible for VIDD in animal models, and there is accumulating evidence for occurrence of the same cellular processes in the diaphragms of human patients undergoing prolonged mechanical ventilation.

SUMMARY

Recent research is pointing the way to novel pharmacologic therapies as well as nonpharmacologic methods for preventing VIDD. The next major challenge in the field will be to move these findings from the bench to the bedside in critically ill patients.

Myopathic characteristics in septic mechanically ventilated patients

PURPOSE OF REVIEW

Survivors of a critical illness may experience poor physical function and quality of life as a result of reduced skeletal muscle mass and strength during their acute illness. Patients diagnosed with sepsis are particularly at risk, and mechanical ventilation may result in diaphragm dysfunction. Interest in the interaction of these conditions is both growing and important to understand for individualized patient care.

RECENT FINDINGS

This review describes developments in the presentation of both diaphragm and limb myopathy in critical illness, as measured from muscle biopsy and at the bedside with various imaging and strength-testing modalities. The influence of unloading of the diaphragm with mechanical ventilation and peripheral muscles with immobilization in septic patients has been recently questioned. Systemic inflammation appears to primarily accelerate and accentuate dysfunction, which may be remedied by early mobilization and augmented with developing muscle and/or nerve stimulation techniques.

SUMMARY

Many acute muscle changes in septic patients are likely to stem from pre-existing impairments, which should provide context for clinical evaluations of strength. During illness, sarcolemmal injury promotes a cascade of intra-cellular abnormalities. As unique characteristics of ICU-acquired weakness and differential effects on muscle groups are understood, early diagnosis and management should be facilitated.

Early administration of cisatracurium attenuates sepsis-induced diaphragm dysfunction in rats

Sepsis can often induce diaphragm dysfunction, which is associated with localized elaboration of cytokines within the diaphragm. The administration of cisatracurium has been shown to decrease the inflammatory response and to facilitate mechanical ventilation. In this study, we explored whether cisatracurium could attenuate sepsis-induced diaphragm dysfunction in rats. Animals were divided into three groups: (1) the control group: rats underwent a sham surgical procedure with cecal exposure, but the cecum was neither ligated nor punctured; (2) the CLP group: rats underwent cecal ligation and puncture (CLP) and received a continuous infusion of NaCl 0.9 %; and (3) the Cis + CLP group: rats underwent CLP and received a continuous infusion of cisatracurium. After the surgical procedure, all animals underwent controlled mechanical ventilation for 18 h. Plasma concentrations of interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and high-mobility group box 1 (HMGB1) were measured using an enzyme-linked immunosorbent assay. Upon completion of the experimental protocol, diaphragm contractility and HMGB1 protein expression were analyzed. Impaired diaphragm contractile function, including both force-related properties and force-frequency responses, was pronounced after CLP in comparison with that observed in the control rats. Furthermore, CLP elevated serum levels of IL-6, TNF-α, and HMGB1, and induced HMGB1 protein expression in the diaphragm. In contrast, cisatracurium counteracted the sepsis-induced inflammation reaction in the diaphragm and serum and maintained diaphragm function. These data suggest that early infusion of cisatracurium attenuates sepsis-induced diaphragm dysfunction; this may be attributable to its anti-inflammatory action.

Rapid diaphragm atrophy following cervical spinal cord hemisection

A cervical (C2) hemilesion (C2Hx), which disrupts ipsilateral bulbospinal inputs to the phrenic nucleus, was used to study diaphragm plasticity after acute spinal cord injury. We hypothesized that C2Hx would result in rapid atrophy of the ipsilateral hemidiaphragm and increases in mRNA expression of proteolytic biomarkers. Diaphragm tissue was harvested from male Sprague-Dawley rats at 1 or 7 days following C2Hx. Histological analysis demonstrated reduction in cross-sectional area (CSA) of type I and IIa fibers in the ipsilateral hemidiaphragm at 1 but not 7 days. Type IIb/x fibers, however, had reduced CSA at 1 and 7 days. A targeted gene array was used to screen mRNA changes for genes associated with skeletal muscle myopathy and myogenesis; this was followed by qRT-PCR validation. Changes in diaphragm gene expression suggested that profound myoplasticity is initiated immediately following C2Hx including activation of both proteolytic and myogenic pathways. We conclude that an immediate myoplastic response occurs in the diaphragm after C2Hx with atrophy occurring in ipsilateral myofibers within 1 day.

Ventilator-induced diaphragm dysfunction: cause and effect

Mechanical ventilation (MV) is used clinically to maintain gas exchange in patients that require assistance in maintaining adequate alveolar ventilation. Common indications for MV include respiratory failure, heart failure, drug overdose, and surgery. Although MV can be a life-saving intervention for patients suffering from respiratory failure, prolonged MV can promote diaphragmatic atrophy and contractile dysfunction, which is referred to as ventilator-induced diaphragm dysfunction (VIDD). This is significant because VIDD is thought to contribute to problems in weaning patients from the ventilator. Extended time on the ventilator increases health care costs and greatly increases patient morbidity and mortality. Research reveals that only 18–24 h of MV is sufficient to develop VIDD in both laboratory animals and humans. Studies using animal models reveal that MV-induced diaphragmatic atrophy occurs due to increased diaphragmatic protein breakdown and decreased protein synthesis. Recent investigations have identified calpain, caspase-3, autophagy, and the ubiquitin-proteasome system as key proteases that participate in MV-induced diaphragmatic proteolysis. The challenge for the future is to define the MV-induced signaling pathways that promote the loss of diaphragm protein and depress diaphragm contractility. Indeed, forthcoming studies that delineate the signaling mechanisms responsible for VIDD will provide the knowledge necessary for the development of a pharmacological approach that can prevent VIDD and reduce the incidence of weaning problems.