Enhanced Diaphragm Muscle Function upon Satellite Cell Transplantation in Dystrophic Mice.

The diaphragm muscle is essential for breathing, and its dysfunctions can be fatal. Many disorders affect the diaphragm, including muscular dystrophies. Despite the clinical relevance of targeting the diaphragm, there have been few studies evaluating diaphragm function following a given experimental treatment, with most of these involving anti-inflammatory drugs or gene therapy. Cell-based therapeutic approaches have shown success promoting muscle regeneration in several mouse models of muscular dystrophy, but these have focused mainly on limb muscles. Here we show that transplantation of as few as 5000 satellite cells directly into the diaphragm results in consistent and robust myofiber engraftment in dystrophin- and fukutin-related protein-mutant dystrophic mice. Transplanted cells also seed the stem cell reservoir, as shown by the presence of donor-derived satellite cells. Force measurements showed enhanced diaphragm strength in engrafted muscles. These findings demonstrate the feasibility of cell transplantation to target the diseased diaphragm and improve its contractility.

Postanesthesia Care Unit and Anesthetic Management Outcomes Among Patients Undergoing Noncardiac Surgery: Differences by Race and Ethnicity.

PURPOSE: To investigate the association of patient race and ethnicity with postanesthesia care unit (PACU) outcomes in common, noncardiac surgeries requiring general anesthesia. DESIGN: Single tertiary care academic medical center retrospective matched cohort. METHODS: We matched 1:1 1836 adult patients by race and/or ethnicity undergoing common surgeries. We compared racial and ethnic minority populations (62 American Indian, 250 Asian, 315 Black or African American, 281 Hispanic, and 10 Pacific Islander patients) to 918 non-Hispanic White patients. The primary outcomes were: the use of an appropriate number of postoperative nausea and vomiting (PONV) prophylactics; the incidence of PONV; and the use of a propofol infusion as part of the anesthetic (PROP). Secondary outcomes were: the use of opioid-sparing multimodal analgesia, including the use of regional anesthesia for postoperative pain control; the use of any local anesthetic, including the use of liposomal bupivacaine; the duration until readiness for discharge from the PACU; the time between arrival to PACU and first pain score; and the time between the first PACU pain score of ≥4 and administration of an analgesic. Logistic and linear regression were used for relevant outcomes of interest. FINDINGS: Overall, there were no differences in the appropriate number of PONV prophylactics, nor the incidence of PONV between the two groups. There was, however, a decreased use of PROP (OR = 0.80; 95% CI: 0.69, 0.94; P = .005), PACU length of stay was 9.56 minutes longer (95% CI: 2.62, 16.49; P = .007), and time between arrival to PACU and first pain score was 2.30 minutes longer in patients from racial and ethnic minority populations (95% CI: 0.99, 3.61; P = .001). There were no statistically significant differences in the other secondary outcomes. CONCLUSIONS: The rate of appropriate number of PONV prophylactic medications as well as the incidence of PONV were similar in patients from racial and ethnic minority populations compared to non-Hispanic White patients. However, there was a lower use of PROP in racial and ethnic minority patients. It is important to have a health equity lens to identify differences in management that may contribute to disparities within each phase of perioperative care.

Neuromotor control of spontaneous quiet breathing in awake rats evaluated by assessments of diaphragm EMG stationarity.

The neuromotor control of the diaphragm muscle (DIAm) is dynamic. The activity of the DIAm can be recorded via electromyography (EMG), which represents the temporal summation of motor unit action potentials. Our goal in the present study was to investigate DIAm neuromotor control during quiet spontaneous breathing (eupnea) in awake rats by evaluating DIAm EMG at specific temporal locations defined by motor unit recruitment and derecruitment. We evaluated the nonstationarity of DIAm EMG activity to identify DIAm motor unit recruitment and derecruitment durations. Combined with assessments of root mean square (RMS) and sum of squares (SS) EMG, the durations of these phases provide physiological information about the temporal aspects of motor control. During eupnea in awake rats (n = 10), the duration of motor unit recruitment comprised 61 ± 19 ms of the onset-to-peak duration (214 ± 62 ms) of the DIAm RMS EMG. The peak-to-offset duration of DIAm EMG activity was 453 ± 96 ms, with a terminating period of derecruitment of 161 ± 44 ms. The burst duration was 673 ± 128 ms. Both the RMS EMG amplitude and the SS EMG were higher at the completion of motor unit recruitment than at the start of motor unit derecruitment, suggesting that offset discharge rates were lower than onset discharge rates. Our analyses provide novel insights into the time domain aspects of DIAm neuromotor control and allow indirect estimates of the contribution of recruitment and frequency to RMS EMG amplitude during eupnea in awake rats.NEW & NOTEWORTHY We characterized three phases of neuromotor control-motor unit recruitment, sustained activity, and derecruitment-based on statistical assessments of stationarity of the diaphragm muscle (DIAm) EMG activity in awake rats. Our findings may allow indirect estimates of the contribution of motor unit recruitment and frequency coding toward generating force and provide novel insights about the temporal aspects of DIAm neuromotor control and descending respiratory drive in unanesthetized animals.

Electrophysiological effects of BDNF and TrkB signaling at type-identified diaphragm neuromuscular junctions.

Previous studies show that synaptic quantal release decreases during repetitive stimulation, i.e., synaptic depression. Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). We hypothesized that BDNF mitigates synaptic depression at the neuromuscular junction and that the effect is more pronounced at type IIx and/or IIb fibers compared to type I or IIa fibers given the more rapid reduction in docked synaptic vesicles with repetitive stimulation. Rat phrenic nerve-diaphragm muscle preparations were used to determine the effect of BDNF on synaptic quantal release during repetitive stimulation at 50 Hz. An ∼40% decline in quantal release was observed during each 330-ms duration train of nerve stimulation (intratrain synaptic depression), and this intratrain decline was observed across repetitive trains (20 trains at 1/s repeated every 5 min for 30 min for 6 sets). BDNF treatment significantly enhanced quantal release at all fiber types (P < 0.001). BDNF treatment did not change release probability within a stimulation set but enhanced synaptic vesicle replenishment between sets. In agreement, synaptic vesicle cycling (measured using FM4-64 fluorescence uptake) was increased following BDNF [or neurotrophin-4 (NT-4)] treatment (∼40%; P < 0.05). Conversely, inhibiting BDNF/TrkB signaling with the tyrosine kinase inhibitor K252a and TrkB-IgG (which quenches endogenous BDNF or NT-4) decreased FM4-64 uptake (∼34% across fiber types; P < 0.05). The effects of BDNF were generally similar across all fiber types. We conclude that BDNF/TrkB signaling acutely enhances presynaptic quantal release and thereby may serve to mitigate synaptic depression and maintain neuromuscular transmission during repetitive activation.NEW & NOTEWORTHY Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). Rat phrenic nerve-diaphragm muscle preparations were used to determine the rapid effect of BDNF on synaptic quantal release during repetitive stimulation. BDNF treatment significantly enhanced quantal release at all fiber types. BDNF increased synaptic vesicle cycling (measured using FM4-64 fluorescence uptake); conversely, inhibiting BDNF/TrkB signaling decreased FM4-64 uptake.

Frailty Transitions One Year After Total Joint Arthroplasty: A Cohort Study.

BACKGROUND: Total joint arthroplasty (TJA) is prevalent and offered to patients regardless of frailty status experiencing pain, disability, and functional decline. This study aims to describe changes in levels of frailty 1 year after TJA. METHODS: We identified a retrospective cohort of adult patients undergoing primary TJA between 2005 and 2016 using an institutional total joint registry. Associations between categorized frailty deficit index (FI) and change in FI were analyzed using linear regression models. Mortality, deep periprosthetic joint infection, and reoperation were analyzed using time to event methods. RESULTS: In total, 5341 patients (37.6% non-frail, 39.4% vulnerable, and 23.0% frail) with items necessary to determine FI at 1 year after TJA were included. Preoperatively, 29% of vulnerable patients improved to non-frail 1 year later, compared to only 11% regressing to frail. Four in 10 frail patients improved to vulnerable/non-frail. Improvements in activities of daily living (ADL) were more evident in frail and vulnerable patients, with >30% reduction in the percentage of patients expressing difficulties with walking, climbing stairs, and requiring ADL assistance 1 year after TJA. Increases in frailty 1 year after TJA were associated with significantly increased rates of mortality (hazard ratio [HR] 1.50, 95% confidence interval [CI] 1.24-1.82, P < .001), deep periprosthetic joint infection (HR 3.98, 95% CI 1.85-8.58, P < .001), and reoperation (HR 1.80, 95% CI 1.19-2.72, P = .005). CONCLUSION: Frailty states are dynamic with patient frailty shown to be modifiable 1 year after TJA. Preoperative frailty measurement is an important step toward identifying those that may benefit most from TJA and for postoperative frailty surveillance.

Acute intrathecal BDNF enhances functional recovery after cervical spinal cord injury in rats.

Unilateral C2 hemisection (C2SH) disrupts descending inspiratory-related drive to phrenic motor neurons and thus, silences rhythmic diaphragm muscle (DIAm) activity. There is gradual recovery of rhythmic DIAm EMG activity over time post-C2SH, consistent with neuroplasticity, which is enhanced by chronic (2 wk) intrathecal BDNF treatment. In the present study, we hypothesized that acute (30 min) intrathecal BDNF treatment also enhances recovery of DIAm EMG activity after C2SH. Rats were implanted with bilateral DIAm EMG electrodes to verify the absence of ipsilateral eupneic DIAm EMG activity at the time of C2SH and at 3 days post-C2SH. In those animals displaying no recovery of DIAm EMG activity after 28 days (n = 7), BDNF was administered intrathecally (450 mcg) at C4. DIAm EMG activity was measured continuously both before and for 30 min after BDNF treatment, during eupnea, hypoxia-hypercapnia, and spontaneous sighs. Acute BDNF treatment restored eupneic DIAm EMG activity in all treated animals to an amplitude that was 78% ± 9% of pre-C2SH root mean square (RMS) (P < 0.001). In addition, acute BDNF treatment increased DIAm RMS EMG amplitude during hypoxia-hypercapnia (P = 0.023) but had no effect on RMS EMG amplitude during sighs. These results support an acute modulatory role of BDNF signaling on excitatory synaptic transmission at phrenic motor neurons after cervical spinal cord injury.NEW & NOTEWORTHY Brain-derived neurotrophic factor (BDNF) plays an important role in promoting neuroplasticity following unilateral C2 spinal hemisection (C2SH). BDNF was administered intrathecally in rats displaying lack of ipsilateral inspiratory-related diaphragm (DIAm) EMG activity after C2SH. Acute BDNF treatment (30 min) restored eupneic DIAm EMG activity in all treated animals to 78% ± 9% of pre-C2SH level. In addition, acute BDNF treatment increased DIAm EMG amplitude during hypoxia-hypercapnia but had no effect on EMG amplitude during sighs.

TrkB signaling contributes to transdiaphragmatic pressure generation in aged mice.

Ventilatory deficits are common in old age and may result from neuromuscular dysfunction. Signaling via the tropomyosin-related kinase receptor B (TrkB) regulates neuromuscular transmission and, in young mice, is important for the generation of transdiaphragmatic pressure (Pdi). Loss of TrkB signaling worsened neuromuscular transmission failure and reduced maximal Pdi, and these effects are similar to those observed in old age. Administration of TrkB agonists such as 7,8-dihydroxyflavone (7,8-DHF) improves neuromuscular transmission in young and old mice (18 mo; 75% survival). We hypothesized that TrkB signaling contributes to Pdi generation in old mice, particularly during maximal force behaviors. Old male and female TrkBF616A mice, with a mutation that induces 1NMPP1-mediated TrkB kinase inhibition, were randomly assigned to systemic treatment with vehicle, 7,8-DHF, or 1NMPP1 1 h before experiments. Pdi was measured during eupneic breathing (room air), hypoxia-hypercapnia (10% O2/5% CO2), tracheal occlusion, spontaneous deep breaths ("sighs"), and bilateral phrenic nerve stimulation (Pdimax). There were no differences in the Pdi amplitude across treatments during ventilatory behaviors (eupnea, hypoxia-hypercapnia, occlusion, or sigh). As expected, Pdi increased from eupnea and hypoxia-hypercapnia (∼7 cm H2O) to occlusion and sighs (∼25 cm H2O), with no differences across treatments. Pdimax was ∼50 cm H2O in the vehicle and 7,8-DHF groups and ∼40 cm H2O in the 1NMPP1 group (F8,74 = 2; P = 0.02). Our results indicate that TrkB signaling is necessary for generating maximal forces by the diaphragm muscle in old mice and are consistent with aging effects of TrkB signaling on neuromuscular transmission.NEW & NOTEWORTHY TrkB signaling is necessary for generating maximal forces by the diaphragm muscle. In 19- to 21-mo-old TrkBF616A mice susceptible to 1NMPP1-induced inhibition of TrkB kinase activity, maximal Pdi generated by bilateral phrenic nerve stimulation was ∼20% lower after 1NMPP1 compared with vehicle-treated mice. Treatment with the TrkB agonist 7,8-dihydroxyflavone did not affect Pdi generation when compared with age-matched mice. Inhibition of TrkB kinase activity did not affect the forces generated during lower force behaviors in old age.

How Is Surgical Risk Best Assessed? A Cohort Comparison of Measures in Total Joint Arthroplasty.

BACKGROUND: We designed this study to determine whether a Frailty Deficit Index (FI) confers added risk stratification beyond more traditional methods. The associations of preoperative scores on FI, American Society of Anesthesiologists (ASA) physical status, and Charlson Comorbidity Index (CCI) with complications after total joint arthroplasty (TJA) were compared. METHODS: Using a single institution cohort of adult patients ≥50 years undergoing primary or revision TJA from 2005 to 2016, we assessed how well the FI, CCI, and ASA scores predicted risk of mortality, infection, and reoperation. We performed 7 models for each outcome: FI, ASA, and CCI alone, FI + ASA, FI + CCI, ASA + CCI, and FI + ASA + CCI. Cox proportional hazards regression methods were used to calculate the concordance (C-) statistic, a measure of discrimination. RESULTS: Of 18,397 TJAs included, 98.9% were alive 1 year postoperatively. For mortality, all models had concordance between 0.76 and 0.79, with the FI + ASA + CCI model performing highest (C-statistic 0.79; 95% confidence interval [CI] 0.76-0.82). Unadjusted, FI had the strongest concordance (C-statistic 0.77). In FI + ASA + CCI, each increase in 1 comorbidity (of 32 total comorbidities) in the FI was significantly associated with a 12% increase in the rate of mortality (hazard ratio [HR] 1.12, 95% CI 1.07-1.17, P < .001), 10% increase in infection (HR 1.10, 95% CI 1.06-1.14; P < .001), and 6% increase in reoperation (HR 1.06, 95% CI 1.05-1.08, P < .001). CONCLUSION: Identifying at-risk patients preoperatively is crucial and may result in adjustment of postoperative care. FI was independently associated with risk of adverse outcomes following TJA even after taking into account other predictive measures.

Peripheral Nerve Blockade with Combined Standard and Liposomal Bupivacaine in Major Lower-Extremity Amputation.

BACKGROUND AND OBJECTIVES: Optimizing perioperative analgesia for patients undergoing major lower-extremity amputation remains a considerable challenge. The utility of liposomal bupivacaine as a component of peripheral nerve blockade for lower-extremity amputation is unknown. METHODS: We conducted an observational study comparing three different perioperative analgesic techniques for adults undergoing major lower-extremity amputation under general anesthesia between 2012 and 2017 at an academic medical center: (1) no regional anesthesia, (2) peripheral nerve blockade with standard bupivacaine, and (3) peripheral nerve blockade with a mixture of standard and liposomal bupivacaine. The primary outcome of cumulative opioid oral morphine milligram equivalent utilization in the first 72 hours postoperatively was compared across groups utilizing multivariable linear regression. RESULTS: A total of 631 unique anesthetics were included for 578 unique patients, including 416 (66%) without regional anesthesia, 131 (21%) with peripheral nerve blockade with a mixture of standard and liposomal bupivacaine, and 84 (13%) with peripheral nerve blockade with standard bupivacaine alone. Cumulative morphine equivalents were lower in those receiving peripheral nerve blockade with combined standard and liposomal bupivacaine compared with those not receiving regional anesthesia (multiplicative increase 0.67; 95% CI 0.50 to 0.90; P = 0.007). There were no significant differences in opioid utilization between peripheral nerve blockade groups (P = 0.59). CONCLUSIONS: Peripheral nerve blockade is associated with reduced opioid requirements after lower-extremity amputation compared with general anesthesia alone. However, the incorporation of liposomal bupivacaine is not significantly different to blockade employing only standard bupivacaine.

Disproportionate loss of excitatory inputs to smaller phrenic motor neurons following cervical spinal hemisection.

KEY POINTS: Motor units, comprising a motor neuron and the muscle fibre it innervates, are activated in an orderly fashion to provide varying amounts of force. A unilateral C2 spinal hemisection (C2SH) disrupts predominant excitatory input from medulla, causing cessation of inspiratory-related diaphragm muscle activity, whereas higher force, non-ventilatory diaphragm activity persists. In this study, we show a disproportionately larger loss of excitatory glutamatergic innervation to small phrenic motor neurons (PhMNs) following C2SH, as compared with large PhMNs ipsilateral to injury. Our data suggest that there is a dichotomy in the distribution of inspiratory-related descending excitatory glutamatergic input to small vs. large PhMNs that reflects their differential recruitment. ABSTRACT: Excitatory glutamatergic input mediating inspiratory drive to phrenic motor neurons (PhMNs) emanates primarily from the ipsilateral ventrolateral medulla. Unilateral C2 hemisection (C2SH) disrupts this excitatory input, resulting in cessation of inspiratory-related diaphragm muscle (DIAm) activity. In contrast, after C2SH, higher force, non-ventilatory DIAm activity persists. Inspiratory behaviours require recruitment of only smaller PhMNs, whereas with more forceful expulsive/straining behaviours, larger PhMNs are recruited. Accordingly, we hypothesize that C2SH primarily disrupts glutamatergic synaptic inputs to smaller PhMNs, whereas glutamatergic synaptic inputs to larger PhMNs are preserved. We examined changes in glutamatergic presynaptic input onto retrogradely labelled PhMNs using immunohistochemistry for VGLUT1 and VGLUT2. We found that 7 days after C2SH there was an ∼60% reduction in glutamatergic inputs to smaller PhMNs compared with an ∼35% reduction at larger PhMNs. These results are consistent with a more pronounced impact of C2SH on inspiratory behaviours of the DIAm, and the preservation of higher force behaviours after C2SH. These results indicate that the source of glutamatergic synaptic input to PhMNs varies depending on motor neuron size and reflects different functional control - perhaps separate central pattern generator and premotor circuits. For smaller PhMNs, the central pattern generator for inspiration is located in the pre-Bötzinger complex and premotor neurons in the ventrolateral medulla, sending predominantly ipsilateral projections via the dorsolateral funiculus. C2SH disrupts this glutamatergic input. For larger PhMNs, a large proportion of excitatory inputs appear to exist below the C2 level or from contralateral regions of the brainstem and spinal cord.

Heterogeneous glutamatergic receptor mRNA expression across phrenic motor neurons in rats.

The diaphragm muscle comprises various types of motor units that are recruited in an orderly fashion governed by the intrinsic electrophysiological properties (membrane capacitance as a function of somal surface area) of phrenic motor neurons (PhMNs). Glutamate is the main excitatory neurotransmitter at PhMNs and acts primarily via fast acting AMPA and N-methyl-D-aspartic acid (NMDA) receptors. Differences in receptor expression may also contribute to motor unit recruitment order. We used single cell, multiplex fluorescence in situ hybridization to determine glutamatergic receptor mRNA expression across PhMNs based on their somal surface area. In adult male and female rats (n = 9) PhMNs were retrogradely labeled for analyses (n = 453 neurons). Differences in the total number and density of mRNA transcripts were evident across PhMNs grouped into tertiles according to somal surface area. A ~ 25% higher density of AMPA (Gria2) and NMDA (Grin1) mRNA expression was evident in PhMNs in the lower tertile compared to the upper tertile. These smaller PhMNs likely comprise type S motor units that are recruited first to accomplish lower force, ventilatory behaviors. In contrast, larger PhMNs with lower volume densities of AMPA and NMDA mRNA expression presumably comprise type FInt and FF motor units that are recruited during higher force, expulsive behaviors. Furthermore, there was a significantly higher cytosolic NMDA mRNA expression in small PhMNs suggesting a more important role for NMDA-mediated glutamatergic neurotransmission at smaller PhMNs. These results are consistent with the observed order of motor unit recruitment and suggest a role for glutamatergic receptors in support of this orderly recruitment. Cover Image for this issue: doi: 10.1111/jnc.14747.

Breathing: Motor Control of Diaphragm Muscle.

Breathing occurs without thought but is controlled by a complex neural network with a final output of phrenic motor neurons activating diaphragm muscle fibers (i.e., motor units). This review considers diaphragm motor unit organization and how they are controlled during breathing as well as during expulsive behaviors.

Impaired Autophagy in Motor Neurons: A Final Common Mechanism of Injury and Death.

Autophagy is a cellular digestion process that contributes to cellular homeostasis and adaptation by the elimination of proteins and damaged organelles. Evidence suggests that dysregulation of autophagy plays a role in neurodegenerative diseases, including motor neuron disorders. Herein, we review emerging evidence indicating the roles of autophagy in physiological motor neuron processes and its function in specific compartments. Moreover, we discuss the involvement of autophagy in the pathogenesis of motor neuron diseases, including spinal cord injury and aging, and recent developments that offer promising therapeutic approaches to mitigate effects of dysregulated autophagy in health and disease.

Glutamatergic input varies with phrenic motor neuron size.

Like all skeletal muscles, the diaphragm muscle accomplishes a range of motor behaviors by recruiting different motor unit types in an orderly fashion. Recruitment of phrenic motor neurons (PhMNs) is generally assumed to be based primarily on the intrinsic properties of PhMNs with an equal distribution of descending excitatory inputs to all PhMNs. However, differences in presynaptic excitatory input across PhMNs of varying sizes could also contribute to the orderly recruitment pattern. In the spinal cord of Sprague-Dawley rats, we retrogradely labeled PhMNs using cholera toxin B (CTB) and validated a robust confocal imaging-based technique that utilizes semiautomated processing to identify presynaptic glutamatergic (Glu) terminals within a defined distance around the somal membrane of PhMNs of varying size. Our results revealed an ~10% higher density of Glu terminals at PhMNs in the lower tertile of somal surface area. These smaller PhMNs are likely recruited first to accomplish lower force ventilatory behaviors of the diaphragm as compared with larger PhMNs in the upper tertile that are recruited to accomplish higher force expulsive behaviors. These results suggest that differences in excitatory synaptic input to PhMNs may also contribute to the orderly recruitment of diaphragm motor units.NEW & NOTEWORTHY The distribution of excitatory glutamatergic synaptic input to phrenic motor neurons differs across motor neurons of varying size. These findings support the size principle of motor unit recruitment that underlies graded force generation in a muscle, which is based on intrinsic electrophysiological properties of motor neurons resulting from differences in somal surface area. A higher density of glutamatergic inputs at smaller, more excitable motor neurons substantiates the earlier and more frequent recruitment of these units.

Diaphragm neuromuscular transmission failure in aged rats.

In aging Fischer 344 rats, phrenic motor neuron loss, neuromuscular junction abnormalities, and diaphragm muscle (DIAm) sarcopenia are present by 24 mo of age, with larger fast-twitch fatigue-intermediate (type FInt) and fast-twitch fatigable (type FF) motor units particularly vulnerable. We hypothesize that in old rats, DIAm neuromuscular transmission deficits are specific to type FInt and/or FF units. In phrenic nerve/DIAm preparations from rats at 6 and 24 mo of age, the phrenic nerve was supramaximally stimulated at 10, 40, or 75 Hz. Every 15 s, the DIAm was directly stimulated, and the difference in forces evoked by nerve and muscle stimulation was used to estimate neuromuscular transmission failure. Neuromuscular transmission failure in the DIAm was observed at each stimulation frequency. In the initial stimulus trains, the forces evoked by phrenic nerve stimulation at 40 and 75 Hz were significantly less than those evoked by direct muscle stimulation, and this difference was markedly greater in 24-mo-old rats. During repetitive nerve stimulation, neuromuscular transmission failure at 40 and 75 Hz worsened to a greater extent in 24-mo-old rats compared with younger animals. Because type IIx and/or IIb DIAm fibers (type FInt and/or FF motor units) display greater susceptibility to neuromuscular transmission failure at higher frequencies of stimulation, these data suggest that the age-related loss of larger phrenic motor neurons impacts nerve conduction to muscle at higher frequencies and may contribute to DIAm sarcopenia in old rats. NEW & NOTEWORTHY Diaphragm muscle (DIAm) sarcopenia, phrenic motor neuron loss, and perturbations of neuromuscular junctions (NMJs) are well described in aged rodents and selectively affect FInt and FF motor units. Less attention has been paid to the motor unit-specific aspects of nerve-muscle conduction. In old rats, increased neuromuscular transmission failure occurred at stimulation frequencies where FInt and FF motor units exhibit conduction failures, along with decreased apposition of pre- and postsynaptic domains of DIAm NMJs of these units.

Impact of sarcopenia on diaphragm muscle fatigue.

NEW FINDINGS: What is the central question of this study? Is the residual force generated by the diaphragm muscle after repeated activation reduced with sarcopenia, and is the residual force generated after fatiguing activation sufficient to sustain ventilatory behaviours of diaphragm muscle in young and old rats? What is the main finding and its importance? After diaphragm muscle fatigue, the residual specific force after 120 s of repeated stimulation was unaffected by ageing and was sufficient to accomplish ventilatory behaviours, but not expulsive manoeuvres (e.g. coughing). The inability to perform expulsive behaviours might underlie the increased susceptibility of older individuals to respiratory tract infections. ABSTRACT: Type IIx and/or IIb diaphragm muscle (DIAm) fibres make up more fatigable motor units that are more vulnerable to sarcopenia, i.e. age-associated reductions of specific force and cross-sectional area. In contrast, type I and IIa DIAm fibres form fatigue-resistant motor units that are relatively unchanged with age. The fatigue resistance of the DIAm is assessed by normalizing the residual force generated after a period of repeated supramaximal stimulation (e.g. 120 s) to the initial maximal force. Given that sarcopenia primarily affects more fatigable DIAm motor units, apparent fatigue resistance improves with ageing. However, the central question is whether there is an ageing-related difference in the residual force generated by the DIAm after repeated stimulation and whether this force is sufficient to sustain ventilatory behaviours of DIAm. In 6- and 24-month-old Fischer 344 rats, we assessed the loss of ex vivo DIAm force throughout 120 s of repeated supramaximal stimulation at 10, 40 and 75 Hz. We found that relative fatigue resistance improved in older rats at 40 and 75 Hz stimulation. Across all stimulation frequencies, DIAm residual force was unchanged with age (∼5 N cm-2 ). We conclude that ageing increases the relative contribution of type I and IIa fibres to DIAm force, with decreased contributions of type IIx and/or IIb fibres. The residual force generated by the DIAm after repeated stimulation is sufficient to accomplish ventilatory behaviours, regardless of age.

Frequency-dependent lipid raft uptake at rat diaphragm muscle axon terminals.

INTRODUCTION: In motor neurons, cholera toxin B (CTB) binds to the cell-surface ganglioside GM1 and is internalized and transported via structurally unique components of plasma membranes (lipid rafts). METHODS: Lipid raft uptake by axon terminals adjoining type-identified rat diaphragm muscle fibers was investigated using CTB and confocal imaging. RESULTS: Lipid raft uptake increased significantly at higher frequency stimulation (80 Hz), compared with lower frequency (20 Hz) and unstimulated (0 Hz) conditions. The fraction of axon terminal occupied by CTB was ∼45% at 0- or 20-Hz stimulation, and increased to ∼65% at 80 Hz. Total CTB fluorescence intensity also increased (∼20%) after 80-Hz stimulation compared with 0 Hz. DISCUSSION: Evidence of increased lipid raft uptake at high stimulation frequencies supports an important role for lipid raft signaling at rat diaphragm muscle axon terminals, primarily for motor units physiologically activated at the higher frequencies. Muscle Nerve 59:611-611, 2019.

Anaesthetic care of patients undergoing primary hip and knee arthroplasty: consensus recommendations from the International Consensus on Anaesthesia-Related Outcomes after Surgery group (ICAROS) based on a systematic review and meta-analysis.

BACKGROUND: Evidence-based international expert consensus regarding anaesthetic practice in hip/knee arthroplasty surgery is needed for improved healthcare outcomes. METHODS: The International Consensus on Anaesthesia-Related Outcomes after Surgery group (ICAROS) systematic review, including randomised controlled and observational studies comparing neuraxial to general anaesthesia regarding major complications, including mortality, cardiac, pulmonary, gastrointestinal, renal, genitourinary, thromboembolic, neurological, infectious, and bleeding complications. Medline, PubMed, Embase, and Cochrane Library including Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, NHS Economic Evaluation Database, from 1946 to May 17, 2018 were queried. Meta-analysis and Grading of Recommendations Assessment, Development and Evaluation approach was utilised to assess evidence quality and to develop recommendations. RESULTS: The analysis of 94 studies revealed that neuraxial anaesthesia was associated with lower odds or no difference in virtually all reported complications, except for urinary retention. Excerpt of complications for neuraxial vs general anaesthesia in hip/knee arthroplasty, respectively: mortality odds ratio (OR): 0.67, 95% confidence interval (CI): 0.57-0.80/OR: 0.83, 95% CI: 0.60-1.15; pulmonary OR: 0.65, 95% CI: 0.52-0.80/OR: 0.69, 95% CI: 0.58-0.81; acute renal failure OR: 0.69, 95% CI: 0.59-0.81/OR: 0.73, 95% CI: 0.65-0.82; deep venous thrombosis OR: 0.52, 95% CI: 0.42-0.65/OR: 0.77, 95% CI: 0.64-0.93; infections OR: 0.73, 95% CI: 0.67-0.79/OR: 0.80, 95% CI: 0.76-0.85; and blood transfusion OR: 0.85, 95% CI: 0.82-0.89/OR: 0.84, 95% CI: 0.82-0.87. CONCLUSIONS: Recommendation: primary neuraxial anaesthesia is preferred for knee arthroplasty, given several positive postoperative outcome benefits; evidence level: low, weak recommendation. RECOMMENDATION: neuraxial anaesthesia is recommended for hip arthroplasty given associated outcome benefits; evidence level: moderate-low, strong recommendation. Based on current evidence, the consensus group recommends neuraxial over general anaesthesia for hip/knee arthroplasty. TRIAL REGISTRY NUMBER: PROSPERO CRD42018099935.

Impact of Frailty on Outcomes After Primary and Revision Total Hip Arthroplasty.

BACKGROUND: Frailty and disability from arthritis are closely intertwined and little is known about the impact of frailty on total hip arthroplasty (THA) outcomes. We hypothesized that higher preoperative frailty is associated with more adverse events following THA. METHODS: All patients (≥50 years) undergoing unilateral primary or revision THA at a single institution from 2005 through 2016 were included. We analyzed the association of frailty (measured by a frailty deficit index) with postoperative outcomes in hospital, within 90 days, and within 1 year using multivariable logistic and Cox regression, adjusting for age. RESULTS: Among 8640 patients undergoing THA (6502 primary and 2138 revisions; median age 68 years), 22.7%, 32.9%, and 44.4% were classified as frail, vulnerable, and nonfrail, respectively. Frail patients tended to be female, older, sicker (American Society of Anesthesiologists ≥3), and received general anesthesia more frequently. Relative to nonfrail patients, frail patients had significantly increased odds of wound complications/hematoma (odds ratio 2.01) and reoperation (odds ratio 2.74) while in hospital, and increased risks for mortality (1-year hazards ratio [HR] 5.65), infection (1-year HR 3.63), dislocation (1-year HR 2.10), wound complications/hematoma (1-year HR 2.61), and reoperation (1-year HR 2.22) within 90 days and 1 year. Frailty was also associated with >5.5-fold increased mortality risk 1 year following THA. No significant associations with aseptic loosening, periprosthetic fracture, or heterotopic ossification were observed. CONCLUSION: A higher preoperative frailty index is associated with increased mortality and perioperative complications following primary and revision THA. The proposed frailty deficit index provides clinically important information for healthcare providers to use when counseling patients prior to decision for surgery.

Phrenic motor neuron loss in aged rats.

Sarcopenia is the age-related reduction of muscle mass and specific force. In previous studies, we found that sarcopenia of the diaphragm muscle (DIAm) is evident by 24 mo of age in both rats and mice and is associated with selective atrophy of type IIx and IIb muscle fibers and a decrease in maximum specific force. These fiber type-specific effects of sarcopenia resemble those induced by DIAm denervation, leading us to hypothesize that sarcopenia is due to an age-related loss of phrenic motor neurons (PhMNs). To address this hypothesis, we determined the number of PhMNs in young (6 mo old) and old (24 mo old) Fischer 344 rats. Moreover, we determined age-related changes in the size of PhMNs, since larger PhMNs innervate type IIx and IIb DIAm fibers. The PhMN pool was retrogradely labeled and imaged with confocal microscopy to assess the number of PhMNs and the morphometry of PhMN soma and proximal dendrites. In older animals, there were 22% fewer PhMNs, a 19% decrease in somal surface area, and a 21% decrease in dendritic surface area compared with young Fischer 344 rats. The age-associated loss of PhMNs involved predominantly larger PhMNs. These results are consistent with an age-related denervation of larger, more fatigable DIAm motor units, which are required primarily for high-force airway clearance behaviors. NEW & NOTEWORTHY Diaphragm muscle sarcopenia in rodent models is well described in the literature; however, the relationship between sarcopenia and frank phrenic motor neuron (MN) loss is unexplored in these models. We quantify a 22% loss of phrenic MNs in old (24 mo) compared with young (6 mo) Fischer 344 rats. We also report reductions in phrenic MN somal and proximal dendritic morphology that relate to decreased MN heterogeneity in old compared with young Fischer 344 rats.