Muscle membrane properties in A pig sepsis model: Effect of norepinephrine

A meta-analysis of the association between vasopressor use and intensive care unit-acquired weakness

OBJECTIVE

This study aims to clarify the uncertain association between vasopressor administration and the development of intensive care unit-acquired weakness (ICUAW) in critically ill adult patients.

METHODS

We conducted a comprehensive search of PubMed, Embase, Web of Science, and the Cochrane Central Register of Controlled Trials up to October 10, 2023. Titles and abstracts were independently screened by two authors, who then reviewed full texts and extracted relevant data from the studies that met the inclusion criteria. This review included prospective and retrospective cohort studies that explored the relationship between vasopressor use and ICUAW utilizing univariate or multivariate analysis in adult ICU patients.

RESULTS

A total of 15 studies were included in our review, collectively indicating a statistically significant association between the use of vasopressors and the occurrence of ICUAW (odds ratio [OR], 3.43; 95% confidence intervals [CI], 1.95-6.04), including studies utilizing multivariate analysis (OR, 3.43; 95% CI, 1.76-6.70). Specifically, the use of noradrenaline was significantly associated with ICUAW (OR, 4.42; 95% CI, 1.69-11.56). Subgroup and sensitivity analyses further underscored the significant relationship between vasopressor use and ICUAW, particularly in studies focusing on patients with clinical weakness, varying study designs, different sample sizes, and relatively low risk of bias. However, this association was not observed in studies limited to patients with abnormal electrophysiology.

CONCLUSIONS

Our review underscores a significant link between the use of vasopressors and the development of ICUAW in critically ill adult patients. This finding helps better identify patients at higher risk of ICUAW and suggests considering targeted therapies to mitigate this risk.

Muscle excitability testing

Conventional electrophysiological methods, i.e. nerve conduction studies and electromyography are suitable methods for the diagnosis of neuromuscular disorders, however, they provide limited information about muscle fibre membrane properties and underlying disease mechanisms. Muscle excitability testing is a technique that provides in vivo information about muscle fibre membrane properties such as membrane potential and ion channel function. Since the 1960s, various methodologies have been suggested to examine muscle membrane properties but technical difficulties have limited its use. In 2009, an automated, fast and simple application, the so-called multi-fibre muscle velocity recovery cycles (MVRC) has accelerated the use of muscle excitability testing. Later, frequency ramp and repetitive stimulation protocols have been developed. Though this method has been used mainly in research for revealing disease mechanisms across a broad range of neuromuscular disorders, it may have additional diagnostic uses; value has been shown particularly in muscle channelopathies. This review will provide a description of the state-of-the art of methodological and clinical studies for muscle excitability testing.

Muscle velocity recovery cycles as pharmacodynamic biomarker: Effects of mexiletine in a randomized double-blind placebo-controlled cross-over study

Measuring muscle velocity recovery cycles (MVRCs) is a method to obtain information on muscle cell excitability, independent of neuromuscular transmission. The goal was to validate MVRC as a pharmacodynamic (PD) biomarker for drugs targeting muscle excitability. As proof-of-concept, sensitivity of MVRC to detect effects of mexiletine, a voltage-gated sodium channel (Na_v ) blocker, was assessed. In a randomized, double-blind, two-way crossover study, effects of a single pharmacologically active oral dose of 333 mg mexiletine was compared to placebo in 15 healthy male subjects. MVRC was performed predose, and 3- and 5-h postdose using QTrac. Effects of mexiletine versus placebo were calculated using a mixed effects model with baseline as covariate. Mexiletine had significant effects on MVRC when compared to placebo. Early supernormality after five conditioning stimuli was decreased by mexiletine (estimated difference -2.78% [95% confidence interval: -4.16, -1.40]; p value = 0.0003). Moreover, mexiletine decreased the difference in late supernormality after five versus one conditioning stimuli (5XLSN; ED -1.46% [-2.26, -0.65]; p = 0.001). These results indicate that mexiletine decreases the percentage increase in velocity of the muscle fiber action potential after five conditioning stimuli, at long and short interstimulus intervals, which corresponds to a decrease in muscle membrane excitability. This is in line with the pharmacological activity of mexiletine, which leads to use-dependent Na_V 1.4 blockade affecting muscle membrane potentials. This study shows that effects of mexiletine can be detected using MVRC in healthy subjects, thereby indicating that MVRC can be used as a tool to demonstrate PD effects of drugs targeting muscle excitability in early phase drug development.

Excitability properties of mouse and human skeletal muscle fibres compared by muscle velocity recovery cycles

Mouse models of skeletal muscle channelopathies are not phenocopies of human disease. In some cases (e.g. Myotonia Congenita) the phenotype is much more severe, whilst in others (e.g. Hypokalaemic periodic paralysis) rodent physiology is protective. This suggests a species' difference in muscle excitability properties. In humans these can be measured indirectly by the post-impulse changes in conduction velocity, using Muscle Velocity Recovery Cycles (MVRCs). We performed MVRCs in mice and compared their muscle excitability properties with humans. Mouse Tibialis Anterior MVRCs (n = 70) have only one phase of supernormality (increased conduction velocity), which is smaller in magnitude (p = 9 × 10-21), and shorter in duration (p = 3 × 10-24) than human (n = 26). This abbreviated supernormality is followed by a period of late subnormality (reduced velocity) in mice, which overlaps in time with the late supernormality seen in human MVRCs. The period of late subnormality suggests increased t-tubule Na+/K+-pump activity. The subnormal phase in mice was converted to supernormality by blocking ClC-1 chloride channels, suggesting relatively higher chloride conductance in skeletal muscle. Our findings help explain discrepancies in phenotype between mice and humans with skeletal muscle channelopathies and potentially other neuromuscular disorders. MVRCs are a valuable new tool to compare in vivo muscle membrane properties between species and will allow further dissection of the molecular mechanisms regulating muscle excitability.

Critical Illness Myopathy: Diagnostic Approach and Resulting Therapeutic Implications

Purpose of review

Critical illness myopathy (CIM) is a common neuro-muscular complication of intensive care treatment associated with increased morbidity and mortality. The current guidelines for diagnosis include clinical and electrophysiological criteria as well as a muscle biopsy, and allow diagnosis only at an advanced stage of the disease. To date, there is no treatment for CIM available, apart from symptomatic and rehabilitative interventions. In this review, we discuss different diagnostic approaches and describe new treatment possibilities for CIM.

Recent findings

Of the diagnostic approaches evaluated, a new electrophysiological technique for measuring muscle excitability has the greatest potential to allow earlier diagnosis of CIM than the current guidelines do and thereby may facilitate the conduction of future pathophysiological and therapeutic studies. Although clinical trials are still lacking, in animal models, BGP-15, vamorolone, and ruxolitinib have been shown to have anti-inflammatory effects, to reduce muscle wasting and to improve muscle function and survival.

Summary

In recent years, promising methods for early and confirmatory diagnosis of CIM have been developed, but still need validation. Experimental studies on novel pharmacological interventions show promising results in terms of preventive CIM treatments, but future clinical studies will be needed to study the effectiveness and safety of these drugs.

Ageing contributes to phenotype transition in a mouse model of periodic paralysis

BACKGROUND

Periodic paralysis (PP) is a rare genetic disorder in which ion channel mutation causes episodic paralysis in association with hyper- or hypokalaemia. An unexplained but consistent feature of PP is that a phenotype transition occurs around the age of 40, in which the severity of potassium-induced muscle weakness declines but onset of fixed, progressive weakness is reported. This phenotype transition coincides with the age at which muscle mass and optimal motor function start to decline in healthy individuals. We sought to determine if the phenotype transition in PP is linked to the normal ageing phenotype transition and to explore the mechanisms involved.

METHODS

A mouse model of hyperkalaemic PP was compared with wild-type littermates across a range of ages (13-104 weeks). Only male mice were used as penetrance is incomplete in females. We adapted the muscle velocity recovery cycle technique from humans to examine murine muscle excitability in vivo. We then examined changes in potassium-induced weakness or caffeine contracture force with age using ex vivo muscle tension testing. Muscles were further characterized by either Western blot, histology or energy charge measurement. For normally distributed data, a student's t-test (± Welch correction) or one- or two-way analysis of variance (ANOVA) was performed to determine significance. For data that were not normally distributed, Welch rank test, Mann Whitney U test or Kruskal-Wallis ANOVA was performed. When an ANOVA was significant (P < 0.05), post hoc Tukey testing was used.

RESULTS

Both WT (P = 0.009) and PP (P = 0.007) muscles exhibit increased resistance to potassium-induced weakness with age. Our data suggest that healthy-old muscle develops mechanisms to maintain force despite sarcolemmal depolarization and sodium channel inactivation. In contrast, reduced caffeine contracture force (P = 0.00005), skeletal muscle energy charge (P = 0.004) and structural core pathology (P = 0.005) were specific to Draggen muscle, indicating that they are caused, or at least accelerated by, chronic genetic ion channel dysfunction.

CONCLUSIONS

The phenotype transition with age is replicated in a mouse model of PP. Intrinsic muscle ageing protects against potassium-induced weakness in HyperPP mice. However, it also appears to accelerate impairment of sarcoplasmic reticulum calcium release, mitochondrial impairment and the development of core-like regions, suggesting acquired RyR1 dysfunction as the potential aetiology. This work provides a first description of mechanisms involved in phenotype transition with age in PP. It also demonstrates how studying phenotype transition with age in monogenic disease can yield novel insights into both disease physiology and the ageing process itself.

A comprehensive assessment of multi-system responses to a renal inoculation of uropathogenic E. coli in swine

BACKGROUND

The systemic responses to infection and its progression to sepsis remains poorly understood. Progress in the field has been stifled by the shortcomings of experimental models which include poor replication of the human condition. To address these challenges, we developed and piloted a novel large animal model of severe infection that is capable of generating multi-system clinically relevant data.

METHODS

Male swine (n = 5) were anesthetized, mechanically ventilated, and surgically instrumented for continuous hemodynamic monitoring and serial blood sampling. Animals were inoculated with uropathogenic E. coli by direct injection into the renal parenchyma and were maintained until a priori endpoints were met. The natural history of the infection was studied. Animals were not resuscitated. Multi-system data were collected hourly to 6 hours; all animals were euthanized at predetermined physiologic endpoints.

RESULTS

Core body temperature progressively increased from mean (SD) 37.9(0.8)°C at baseline to 43.0(1.2)°C at experiment termination (p = 0.006). Mean arterial pressure did not begin to decline until 6h post inoculation, dropping from 86(9) mmHg at baseline to 28(5) mmHg (p = 0.005) at termination. Blood glucose progressively declined but lactate levels did not elevate until the last hours of the experiment. There were also temporal changes in whole blood concentrations of a number of metabolites including increases in the catecholamine precursors, tyrosine (p = 0.005) and phenylalanine (p = 0.005). Lung, liver, and kidney function parameters worsened as infection progressed and at study termination there was histopathological evidence of injury in these end-organs.

CONCLUSION

We demonstrate a versatile, multi-system, longitudinal, swine model of infection that could be used to further our understanding of the mechanisms that underlie infection-induced multi-organ dysfunction and failure, optimize resuscitation protocols and test therapeutic interventions. Such a model could improve translation of findings from the bench to the bedside, circumventing a significant obstacle in sepsis research.

Intensive Care Unit-Acquired Weakness: Not just Another Muscle Atrophying Condition

Intensive care unit-acquired weakness (ICUAW) occurs in critically ill patients stemming from the critical illness itself, and results in sustained disability long after the ICU stay. Weakness can be attributed to muscle wasting, impaired contractility, neuropathy, and major pathways associated with muscle protein degradation such as the ubiquitin proteasome system and dysregulated autophagy. Furthermore, it is characterized by the preferential loss of myosin, a distinct feature of the condition. While many risk factors for ICUAW have been identified, effective interventions to offset these changes remain elusive. In addition, our understanding of the mechanisms underlying the long-term, sustained weakness observed in a subset of patients after discharge is minimal. Herein, we discuss the various proposed pathways involved in the pathophysiology of ICUAW, with a focus on the mechanisms underpinning skeletal muscle wasting and impaired contractility, and the animal models used to study them. Furthermore, we will explore the contributions of inflammation, steroid use, and paralysis to the development of ICUAW and how it pertains to those with the corona virus disease of 2019 (COVID-19). We then elaborate on interventions tested as a means to offset these decrements in muscle function that occur as a result of critical illness, and we propose new strategies to explore the molecular mechanisms of ICUAW, including serum-related biomarkers and 3D human skeletal muscle culture models.

Critical Illness Myopathy

Critical illness myopathy (CIM) is a primary myopathy associated with increased mortality and morbidity, which frequently develops in severely ill patients. Several risk factors have been suggested for the development of critical illness myopathy. However, neither the exact etiology nor the underlying mechanisms are known in detail. Although for definite diagnosis muscle biopsy is needed, electrophysiological tests are crucial for the diagnosis of probable critical illness myopathy and differential diagnosis. In this review, conventional electrophysiological tests such as nerve conduction studies, needle electromyography, direct muscle stimulation, and repetitive stimulation for diagnosis of critical illness myopathy are summarized. Moreover, studies using the novel method of recording muscle velocity recovery cycles are addressed.