Mechanical advantage of the human parasternal intercostal and triangularis sterni muscles

Parasternal intercostal thickening at hospital admission: a promising indicator for mechanical ventilation risk in subjects with severe COVID-19

We aimed to evaluate the ability of parasternal intercostal thickening fraction (PIC TF) to predict the need for mechanical ventilation, and survival in subjects with severe Coronavirus disease-2019 (COVID-19). This prospective observational study included adult subjects with severe COVID-19. The following data were collected within 12 h of admission: PIC TF, respiratory rate oxygenation index, [Formula: see text] ratio, chest CT, and acute physiology and chronic health evaluation II score. The ability of PIC TF to predict the need for ventilatory support (primary outcome) and a composite of invasive mechanical ventilation and/or 30-days mortality were performed using the area under the receiver operating characteristic (AUC) analysis. Multivariate analysis was done to identify the independent predictors for the outcomes. Fifty subjects were available for the final evaluation. The AUC (95% confidence interval [CI]) for the right and left PIC TF ability to predict the need for ventilator support was 0.94 (0.83-0.99), 0.94 (0.84-0.99), respectively, with a cut off value of > 8.3% and positive predictive value of 90-100%. The AUC for the right and left PIC TF to predict invasive mechanical ventilation and/or 30 days mortality was 0.95 (0.85-0.99) and 0.90 (0.78-0.97), respectively. In the multivariate analysis, only the PIC TF was found to independently predict invasive mechanical ventilation and/or 30-days mortality. In subjects with severe COVID-19, PIC TF of 8.3% can predict the need to ventilatory support with a positive predictive value of 90-100%. PIC TF is an independent risk factor for the need for invasive mechanical ventilation and/or 30-days mortality.

Respiratory muscle ultrasonography evaluation and its clinical application in stroke patients: A review

Background

Respiratory muscle ultrasound is a widely available, highly feasible technique that can be used to study the contribution of the individual respiratory muscles related to respiratory dysfunction. Stroke disrupts multiple functions, and the respiratory function is often significantly decreased in stroke patients.

Method

A search of the MEDLINE, Web of Science, and PubMed databases was conducted. We identified studies measuring respiratory muscles in healthy and patients by ultrasonography. Two reviewers independently extracted and documented data regarding to the criteria. Data were extracted including participant demographics, ultrasonography evaluation protocol, subject population, reference values, etc.

Result

A total of 1954 participants from 39 studies were included. Among them, there were 1,135 participants from 19 studies on diaphragm, 259 participants from 6 studies on extra-diaphragmatic inspiratory muscles, and 560 participants from 14 studies on abdominal expiratory muscles. The ultrasonic evaluation of diaphragm and abdominal expiratory muscle thickness had a relatively typically approach, while, extra-diaphragmatic inspiratory muscles were mainly used in ICU that lack of a consistent paradigm.

Conclusion

Diaphragm and expiratory muscle ultrasound has been widely used in the assessment of respiratory muscle function. On the contrary, there is not enough evidence to assess extra-diaphragmatic inspiratory muscles by ultrasound. In addition, the thickness of the diaphragm on the hemiplegic side was lower than that on the non-hemiplegic side in stroke patients. For internal oblique muscle (IO), rectus abdominis muscle (RA), transversus abdominis muscle (TrA), and external oblique muscle (EO), most studies showed that the thickness on the hemiplegic side was lower than that on the non-hemiplegic side.Clinical Trial Registration: The protocol of this review was registered in the PROSPERO database (CRD42022352901).

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Update on Lean Body Mass Diagnostic Assessment in Critical Illness

Acute critical illnesses can alter vital functions with profound biological, biochemical, metabolic, and functional modifications. Despite etiology, patient's nutritional status is pivotal to guide metabolic support. The assessment of nutritional status remains complex and not completely elucidated. Loss of lean body mass is a clear marker of malnutrition; however, the question of how to investigate it still remains unanswered. Several tools have been implemented to measure lean body mass, including a computed tomography scan, ultrasound, and bioelectrical impedance analysis, although such methods unfortunately require validation. A lack of uniform bedside measurement tools could impact the nutrition outcome. Metabolic assessment, nutritional status, and nutritional risk have a pivotal role in critical care. Therefore, knowledge about the methods used to assess lean body mass in critical illnesses is increasingly required. The aim of the present review is to update the scientific evidence regarding lean body mass diagnostic assessment in critical illness to provide the diagnostic key points for metabolic and nutritional support.

Automated evaluation of respiratory signals to provide insight into respiratory drive

The diaphragm muscle (DIAm) is the primary inspiratory muscle in mammals and is highly active throughout life displaying rhythmic activity. The repetitive activation of the DIAm (and of other muscles driven by central pattern generator activity) presents an opportunity to analyze these physiological data on a per-event basis rather than pooled on a per-subject basis. The present study highlights the development and implementation of a graphical user interface-based algorithm using an analysis of critical points to detect the onsets and offsets of individual respiratory events across a range of motor behaviors, thus facilitating analyses of within-subject variability. The algorithm is designed to be robust regardless of the signal type (e.g., EMG or transdiaphragmatic pressure). Our findings suggest that this approach may be particularly beneficial in reducing animal numbers in certain types of studies, for assessments of perturbation studies where the effects are relatively small but potentially physiologically meaningful, and for analyses of respiratory variability.

Semi-automated Detection of the Timing of Respiratory Muscle Activity: Validation and First Application

Background: Respiratory muscle electromyography (EMG) can identify whether a muscle is activated, its activation amplitude, and timing. Most studies have focused on the activation amplitude, while differences in timing and duration of activity have been less investigated. Detection of the timing of respiratory muscle activity is typically based on the visual inspection of the EMG signal. This method is time-consuming and prone to subjective interpretation.

Aims: Our main objective was to develop and validate a method to assess the respective timing of different respiratory muscle activity in an objective and semi-automated manner.

Method: Seven healthy adults performed an inspiratory threshold loading (ITL) test at 50% of their maximum inspiratory pressure until task failure. Surface EMG recordings of the costal diaphragm/intercostals, scalene, parasternal intercostals, and sternocleidomastoid were obtained during ITL. We developed a semi-automated algorithm to detect the onset (EMG, onset) and offset (EMG, offset) of each muscle’s EMG activity breath-by-breath with millisecond accuracy and compared its performance with manual evaluations from two independent assessors. For each muscle, the Intraclass Coefficient correlation (ICC) of the EMG, onset detection was determined between the two assessors and between the algorithm and each assessor. Additionally, we explored muscle differences in the EMG, onset, and EMG, offset timing, and duration of activity throughout the ITL.

Results: More than 2000 EMG, onset s were analyzed for algorithm validation. ICCs ranged from 0.75–0.90 between assessor 1 and 2, 0.68–0.96 between assessor 1 and the algorithm, and 0.75–0.91 between assessor 2 and the algorithm (p < 0.01 for all). The lowest ICC was shown for the diaphragm/intercostal and the highest for the parasternal intercostal (0.68 and 0.96, respectively). During ITL, diaphragm/intercostal EMG, onset occurred later during the inspiratory cycle and its activity duration was shorter than the scalene, parasternal intercostal, and sternocleidomastoid (p < 0.01). EMG, offset occurred synchronously across all muscles (p ≥ 0.98). EMG, onset, and EMG, offset timing, and activity duration was consistent throughout the ITL for all muscles (p > 0.63).

Conclusion: We developed an algorithm to detect EMG, onset of several respiratory muscles with millisecond accuracy that is time-efficient and validated against manual measures. Compared to the inherent bias of manual measures, the algorithm enhances objectivity and provides a strong standard for determining the respiratory muscle EMG, onset.

Ultrasonographic assessment of parasternal intercostal muscles during mechanical ventilation

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

Acute Effects of Inspiratory Loads and Interfaces on Breathing Pattern and Activity of Respiratory Muscles in Healthy Subjects

Objectives

The aim of this study was to evaluate the acute effects of different inspiratory loads and different interfaces on the breathing pattern and activity of the respiratory muscles.

Methods

Twenty healthy adults were recruited and assigned to two groups (20 and 40% of the Maximal Inspiratory Pressure) by way of randomized crossover allocation. Subjects were evaluated during quiet breathing, breathing against inspiratory load, and recovery. The measurements were repeated using two different interfaces (nasal and oral). Chest wall volumes and respiratory muscle activity were assessed with optoelectronic plethysmography and surface electromyography, respectively.

Results

During the application of inspiratory load, significant changes were observed in the respiratory rate (p < 0.04), inspiratory time (p < 0.02), minute ventilation (p < 0.04), tidal volume (p < 0.01), end-inspiratory volume (p < 0.04), end-expiratory volume (p < 0.03), and in the activity of the scalene, sternocleiomastoid, and parasternal portion of the intercostal muscles (RMS values, p < 0.01) when compared to quiet breathing, regardless of the load level or the interface applied. Inspiratory load application yielded significant differences between using nasal and oral interfaces with an increase in the tidal volume (p < 0.01), end-inspiratory volume (p < 0.01), and electrical activity of the scalene and sternocleiomastoid muscles (p < 0.01) seen with using the nasal interface.

Conclusion

The addition of an inspiratory load has a significant effect on the breathing pattern and respiratory muscle electrical activity, and the effects are greater when the nasal interface is applied.

Expiratory muscle dysfunction in critically ill patients: towards improved understanding

Introduction

This narrative review summarizes current knowledge on the physiology and pathophysiology of expiratory muscle function in ICU patients, as shared by academic professionals from multidisciplinary, multinational backgrounds, who include clinicians, clinical physiologists and basic physiologists.

Results

The expiratory muscles, which include the abdominal wall muscles and some of the rib cage muscles, are an important component of the respiratory muscle pump and are recruited in the presence of high respiratory load or low inspiratory muscle capacity. Recruitment of the expiratory muscles may have beneficial effects, including reduction in end-expiratory lung volume, reduction in transpulmonary pressure and increased inspiratory muscle capacity. However, severe weakness of the expiratory muscles may develop in ICU patients and is associated with worse outcomes, including difficult ventilator weaning and impaired airway clearance. Several techniques are available to assess expiratory muscle function in the critically ill patient, including gastric pressure and ultrasound.

Conclusion

The expiratory muscles are the "neglected component" of the respiratory muscle pump. Expiratory muscles are frequently recruited in critically ill ventilated patients, but a fundamental understanding of expiratory muscle function is still lacking in these patients.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Task-dependent output of human parasternal intercostal motor units across spinal levels

KEY POINTS

During breathing, there is differential activity in the human parasternal intercostal muscles and the activity is tightly coupled to the known mechanical advantages for inspiration of the same regions of muscles. It is not known whether differential activity is preserved for the non-respiratory task of ipsilateral trunk rotation. In the present study, we compared single motor units during resting breathing and axial rotation of the trunk during apnoea. We not only confirmed non-uniform recruitment of motor units across parasternal intercostal muscles in breathing, but also demonstrated that the same motor units show an altered pattern of recruitment in the non-respiratory task of trunk rotation. The output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task and these results help us understand the mechanisms that may govern task-dependent differences in motoneurone output.

ABSTRACT

During inspiration, there is differential activity in the human parasternal intercostal muscles across interspaces. We investigated whether the earlier recruitment of motor units in the rostral interspaces compared to more caudal spaces during inspiration is preserved for the non-respiratory task of ipsilateral trunk rotation. Single motor unit activity (SMU) was recorded from the first, second and fourth parasternal interspaces on the right side in five participants in two tasks: resting breathing and 'isometric' axial rotation of the trunk during apnoea. Recruitment of the same SMUs was compared between tasks (n = 123). During resting breathing, differential activity was indicated by earlier recruitment of SMUs in the first and second interspaces compared to the fourth space in inspiration (P < 0.01). By contrast, during trunk rotation, the same motor units showed an altered pattern of recruitment because SMUs in the first interspace were recruited later and at a higher rotation torque than those in the second and fourth interspaces (P < 0.05). Tested for a subset of SMUs, the reliability of the breathing and rotation tasks, as well as the SMU recruitment measures, was good-excellent [intraclass correlation (2,1): 0.69-0.91]. Thus, the output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task. Given that the differential inspiratory output of parasternal intercostal muscles is linked to their relative mechanical effectiveness for inspiration and also that this output is altered in trunk rotation, we speculate that a mechanism matching neural drive to muscle mechanics underlies the task-dependent differences in output of axial motoneurone pools.

Compensatory effects following unilateral diaphragm paralysis

Injury to nerves innervating respiratory muscles such as the diaphragm muscle results in significant respiratory compromise. Electromyography (EMG) and transdiaphragmatic pressure (Pdi) measurements reflect diaphragm activation and force generation. Immediately after unilateral diaphragm denervation (DNV), ventilatory behaviors can be accomplished without impairment, but Pdi generated during higher force non-ventilatory behaviors is significantly decreased. We hypothesized that 1) the initial reduction in Pdi during higher force behaviors after DNV is ameliorated after 14 days, and 2) changes in Pdi over time after DNV are associated with concordant changes in contralateral diaphragm EMG activity and ventilatory parameters. In adult male rats, the reduced Pdi during occlusion (∼40% immediately after DNV) was ameliorated to ∼20% reduction after 14 days. Contralateral diaphragm EMG activity did not significantly change immediately or 14days after DNV compared to the pre-injury baseline for any motor behavior. Taken together, these results suggest that over time after DNV compensatory changes in inspiratory related muscle activation may partially restore the ability to generate Pdi during higher force behaviors.

Postnatal Hyperplasic Effects of ActRIIB Blockade in a Severely Dystrophic Muscle

The efficacy of two ActRIIB ligand-trapping agents (RAP-031 and RAP-435) in treating muscular dystrophy was examined by determining their morphological effects on the severely dystrophic triangularis sterni (TS) muscle of the mdx mouse, a model for Duchenne muscular dystrophy. These agents trap all endogenous ligands to the ActRIIB receptor and thereby block myostatin signaling in a highly selective manner. Short-term (1 month) and long-term (3 months) in vivo treatment of 1-month-old mdx mice increased myonuclei and fiber cross section (FCS) density but did not alter individual fiber size. Vehicle-treated mdx mice exhibited age-dependent increases in myonuclei and FCS density, and age-dependent reductions in centronucleation that were each enhanced by treatment with RAP-435. Distributions of FCS area (FCSA) in the mdx TS were 90% identical to those from untreated age-matched nondystrophic mice and were unaltered by the substantial fiber hyperplasia observed with age and RAP-435 treatment. These results were inconsistent with injury-induced fiber regeneration which produces altered FCSA distributions characterized by a distinct class of smaller regenerated fibers. Nondystrophic mice exhibited a constant postnatal density of fiber cross sections and myonuclei, and RAP-435 treatment of nondystrophic mice increased TS mean FCSA but had no effects on myonuclei or FCS density. These results demonstrating a continual postnatal proliferation and fusion of satellite cells and a response to myostatin blockade characteristic of developing prenatal muscle suggest that the lack of dystrophin directly results in unrestrained postnatal satellite cell activation that is not necessarily dependent upon prior fiber degeneration. J. Cell. Physiol. 232: 1774-1793, 2017. © 2016 Wiley Periodicals, Inc.

Breathing and Singing: Objective Characterization of Breathing Patterns in Classical Singers

Singing involves distinct respiratory kinematics (i.e. movements of rib cage and abdomen) to quiet breathing because of different demands on the respiratory system. Professional classical singers often advocate for the advantages of an active control of the abdomen on singing performance. This is presumed to prevent shortening of the diaphragm, elevate the rib cage, and thus promote efficient generation of subglottal pressure during phonation. However, few studies have investigated these patterns quantitatively and inter-subject variability has hindered the identification of stereotypical patterns of respiratory kinematics. Here, seven professional classical singers and four untrained individuals were assessed during quiet breathing, and when singing both a standard song and a piece of choice. Several parameters were extracted from respiratory kinematics and airflow, and principal component analysis was used to identify typical patterns of respiratory kinematics. No group differences were observed during quiet breathing. During singing, both groups adapted to rhythmical constraints with decreased time of inspiration and increased peak airflow. In contrast to untrained individuals, classical singers used greater percentage of abdominal contribution to lung volume during singing and greater asynchrony between movements of rib cage and abdomen. Classical singers substantially altered the coordination of rib cage and abdomen during singing from that used for quiet breathing. Despite variations between participants, principal component analysis revealed consistent pre-phonatory inward movements of the abdominal wall during singing. This contrasted with untrained individuals, who demonstrated synchronous respiratory movements during all tasks. The inward abdominal movements observed in classical singers elevates intra-abdominal pressure and may increase the length and the pressure-generating capacity of rib cage expiratory muscles for potential improvements in voice quality.

Neural respiratory drive measured during inspiratory threshold loading and acute hypercapnia in healthy individuals

Understanding the effects of respiratory load on neural respiratory drive and respiratory pattern are key to understanding the regulation of load compensation in respiratory disease. The aim of the study was to examine and compare the recruitment pattern of the diaphragm and parasternal intercostal muscles when the respiratory system was loaded using two methods. Twelve subjects performed incremental inspiratory threshold loading up to 50% of their maximal inspiratory pressure, and 10 subjects underwent incremental, steady-state hypercapnia to a maximal inspired CO2 of 5%. The diaphragmatic electromyogram (EMGdi) was measured using a multipair oesophageal catheter, and the parasternal intercostal muscle EMG (sEMGpara) was recorded from bipolar surface electrodes positioned in the second intercostal space. The EMGdi and sEMGpara were analysed over the last minute of each increment of both protocols, normalized using the peak EMG recorded during maximal respiratory manoeuvres and expressed as EMG%max. The EMGdi%max and sEMGpara%max increased in parallel during the two loading methods, although EMGdi%max was consistently greater than sEMGpara%max in both conditions, inspiratory threshold loading [bias (SD) 9 (3)%, 95% limits of agreement 4-15%] and hypercapnia [bias (SD) 6 (3)%, 95% limits of agreement -0.05 to 12%]. Inspiratory threshold loading resulted in more pronounced increases in mean (SD) EMGdi%max [10 (7)-45 (28)%] and sEMGpara%max [5.3 (3.1)-40 (28)%] from baseline compared with EMGdi%max [7 (4)-21 (8)%] and sEMGpara%max [4.7 (2.3)-10 (4)%] during hypercapnia, despite comparable levels of ventilation. These data support the use of sEMGpara%max, as a non-invasive alternative to EMGdi%max recorded with an invasive oesophageal electrode catheter, for the quantification of neural respiratory drive. This technique should make evaluation of respiratory muscle function easier to undertake and therefore more readily acceptable in patients with respiratory disease, in whom transduction of neural respiratory drive to pressure generation can be compromised.

Structural integrity of intramedullary rib fixation using a single bioresorbable screw

BACKGROUND

Operative management of flail chest injury is receiving increasing interest. However, we have noticed in our own practice the difficulty in achieving reliable results with posterior rib fracture fixation. In this article, we analyze and model the physiologic forces acting on posterior rib fractures and assess the suitability of an intramedullary screw fixation technique in this site.

METHODS

Computerized finite element analysis (FEA) was used to model a typical sixth rib and analyze the physiologic forces that act on the rib in vivo. A fracture in the posterior aspect of the rib was incorporated into the model, and an intramedullary screw fixation concept was assessed, using both a bioabsorbable polymer screw and a stainless steel screw.The records of 120 consecutive patients with flail chest were reviewed, and 26 patients were identified as having multiple posterior rib fractures with displacement. These patients formed a clinical correlation group by which to assess the FEA model.

RESULTS

FEA modeling of the posterior rib fracture showed likely posterior displacement in response to physiologic forces. Review of the 26 patients with flail chest and displaced posterior fractures confirmed the direction of displacement. Modeling of an intramedullary screw fixation showed significant stresses in the bone/screw contact areas (stainless steel solution) and the prosthesis itself (bioabsorbable polymer solution)

CONCLUSION

This FEA model demonstrates that physiologic forces cause posterior displacement at posterior rib fracture sites. Fixation solutions to counteract these forces need to overcome significant stresses at both the bone/prosthesis contact regions and within the prosthetic material itself.

LEVEL OF EVIDENCE

Epidemiologic/therapeutic study, level V.

The influence of passive stretch and NF-κB inhibitors on the morphology of dystrophic muscle fibers

The triangularis sterni (TS) is an expiratory muscle that is passively stretched during inspiration. The magnitude of passive stretch depends upon the location of individual fibers within the TS muscle, with fibers located more caudally being stretched ∼ 5% to 10% more than fibers in the cephalad region. In the mdx mouse model for muscular dystrophy, the TS exhibits severe pathological alterations that are ameliorated by treatment with inhibitors of the NF-κB pathway. The purpose of this study was to assess the influence of passive stretch in vivo on fiber morphology in nondystrophic and mdx TS muscles, and the morphological benefits of treating mdx mice with two distinct NF-κB inhibitors, pyrrolidine dithiocarbamate (PDTC), and ursodeoxycholic acid (UDCA). Transmission electron microscopy revealed Z-line streaming, hypercontraction, and disassociation of the plasma membrane from the basal lamina in mdx fibers. In both nondystrophic and mdx TS muscles, fiber density was larger in more caudal regions. In comparison with nondystrophic TS, fibers in the mdx TS exhibited substantial reductions in diameter throughout all regions. In vivo treatment with either PDTC or UDCA tended to increase fiber diameter in the middle and decrease fiber diameter in the caudal TS, while reducing centronucleation in the middle region. These results suggest that passive stretch induces hypercontraction and plasma membrane abnormalities in dystrophic muscle, and that differences in the magnitude of passive stretch may influence fiber morphology and the actions of NF-κB inhibitors on dystrophic morphology.

Interplay Between the Inspiratory and Postural Functions of the Human Parasternal Intercostal Muscles

The parasternal intercostal muscles are obligatory inspiratory muscles. To test the hypothesis that they are also involved in trunk rotation and to assess the effect of any postural role on inspiratory drive to the muscles, intramuscular electromyographic (EMG) recordings were made from the parasternal intercostals on the right side in six healthy subjects during resting breathing in a neutral posture (“neutral breaths”), during an isometric axial rotation effort of the trunk to the right (“ipsilateral rotation”) or left (“contralateral rotation”), and during resting breathing with the trunk rotated. The parasternal intercostals were commonly active during ipsilateral rotation but were consistently silent during contralateral rotation. In addition, with ipsilateral rotation, peak parasternal inspiratory activity was 201 ± 19% (mean ± SE) of the peak inspiratory activity in neutral breaths ( P < 0.001), and activity commenced earlier relative to the onset of inspiratory flow. These changes resulted from an increase in the discharge frequency of motor units (14.3 ± 0.3 vs. 11.0 ± 0.3 Hz; P < 0.001) and the recruitment of new motor units. The majority of units that discharged during ipsilateral rotation were also active in inspiration. However, with contralateral rotation, parasternal inspiratory activity was delayed relative to the onset of inspiratory flow, and peak activity was reduced to 72 ± 4% of that in neutral breaths ( P < 0.001). This decrease resulted from a decrease in the inspiratory discharge frequency of units (10.5 ± 0.2 vs. 12.0 ± 0.2 Hz; P < 0.001) and the derecruitment of units. These observations confirm that in addition to an inspiratory function, the parasternal intercostal muscles have a postural function. Furthermore the postural and inspiratory drives depolarize the same motoneurons, and the postural contraction of the muscles alters their output during inspiration in a direction-dependent manner.

Increases in nuclear p65 activation in dystrophic skeletal muscle are secondary to increases in the cellular expression of p65 and are not solely produced by increases in IkappaB-alpha kinase activity

Dystrophin-deficient muscle exhibits substantial increases in nuclear NF-kappaB activation. To examine potential mechanisms for this enhanced activation, the present study employs conventional Western blot techniques to provide the first determination of the relative expression of NF-kappaB signaling molecules in adult nondystrophic and dystrophic (mdx) skeletal muscle. The results indicate that dystrophic muscle is characterized by increases in the whole cell expression of IkappaB-alpha, p65, p50, RelB, p100, p52, IKK, and TRAF-3. The proportion of phosphorylated IkappaB-alpha, p65, NIK, and IKKbeta, and the level of cytosolic IkappaB-alpha, were also increased in the mdx diaphragm. Proteasomal inhibition using MG-132 increased the proportion of phosphorylated IkappaB-alpha in nondystrophic diaphragm, but did not significantly increase this proportion in the mdx diaphragm. This result suggests that phosphorylated IkappaB-alpha accumulates in dystrophic cytosol because the rate of IkappaB-alpha degradation is lower than the effective rate of IkappaB-alpha synthesis and phosphorylation. Dystrophic increases in SUMO-1 (small ubiquitin modifier-1) protein and in Akt activation were also observed. The results indicate that increases in nuclear p65 activation in dystrophic muscle are not produced solely by increases in the activity of IkappaB-alpha kinase (IKK), but are due primarily to increases in the expression of p65 and other NF-kappaB signaling components.

Neuromechanical matching of drive in the scalene muscle of the anesthetized rabbit

The scalene is a primary respiratory muscle in humans; however, in dogs, EMG activity recorded from this muscle during inspiration was reported to derive from underlying muscles. In the present studies, origin of the activity in the medial scalene was tested in rabbits, and its distribution was compared with the muscle mechanical advantage. We assessed in anesthetized rabbits the presence of EMG activity in the scalene, sternomastoid, and parasternal intercostal muscles during quiet breathing and under resistive loading, before and after denervation of the scalene and after its additional insulation. At rest, activity was always recorded in the parasternal muscle and in the scalene bundle inserting on the third rib (medial scalene). The majority of this activity disappeared after denervation. In the bundle inserting on the fifth rib (lateral scalene), the activity was inconsistent, and a high percentage of this activity persisted after denervation but disappeared after insulation from underlying muscle layers. The sternomastoid was always silent. The fractional change in muscle length during passive inflation was then measured. The mean shortening obtained for medial and lateral scalene and parasternal intercostal was 8.0 +/- 0.7%, 5.5 +/- 0.5%, and 9.6 +/- 0.1%, respectively, of the length at functional residual capacity. Sternomastoid muscle length did not change significantly with lung inflation. We conclude that, similar to that shown in humans, respiratory activity arises from scalene muscles in rabbits. This activity is however not uniformly distributed, and a neuromechanical matching of drive is observed, so that the most effective part is also the most active.

Drive to the human respiratory muscles

The motor control of the respiratory muscles differs in some ways from that of the limb muscles. Effectively, the respiratory muscles are controlled by at least two descending pathways: from the medulla during normal quiet breathing and from the motor cortex during behavioural or voluntary breathing. Neurophysiological studies of single motor unit activity in human subjects during normal and voluntary breathing indicate that the neural drive is not uniform to all muscles. The distribution of neural drive depends on a principle of neuromechanical matching. Those motoneurones that innervate intercostal muscles with greater mechanical advantage are active earlier in the breath and to a greater extent. Inspiratory drive is also distributed differently across different inspiratory muscles, possibly also according to their mechanical effectiveness in developing airway negative pressure. Genioglossus, a muscle of the upper airway, receives various types of neural drive (inspiratory, expiratory and tonic) distributed differentially across the hypoglossal motoneurone pool. The integration of the different inputs results in the overall activity in the muscle to keep the upper airway patent throughout respiration. Integration of respiratory and non-respiratory postural drive can be demonstrated in respiratory muscles, and respiratory drive can even be observed in limb muscles under certain circumstances. Recordings of motor unit activity from the human diaphragm during voluntary respiratory tasks have shown that depending on the task there can be large changes in recruitment threshold and recruitment order of motor units. This suggests that descending drive across the phrenic motoneurone pool is not necessarily consistent. Understanding the integration and distribution of drive to respiratory muscles in automatic breathing and voluntary tasks may have implications for limb motor control.

The output from human inspiratory motoneurone pools

Survival requires adequate pulmonary ventilation which, in turn, depends on adequate contraction of muscles acting on the chest wall in the presence of a patent upper airway. Bulbospinal outputs projecting directly and indirectly to 'obligatory' respiratory motoneurone pools generate the required muscle contractions. Recent studies of the phasic inspiratory output of populations of single motor units to five muscles acting on the chest wall (including the diaphragm) reveal that the time of onset, the progressive recruitment, and the amount of motoneuronal drive (expressed as firing frequency) differ among the muscles. Tonic firing with an inspiratory modulation of firing rate is common in low intercostal spaces of the parasternal and external intercostal muscles but rare in the diaphragm. A new time and frequency plot has been developed to depict the behaviour of the motoneurone populations. The magnitude of inspiratory firing of motor unit populations is linearly correlated to the mechanical advantage of the intercostal muscle region at which the motor unit activity is recorded. This represents a 'neuromechanical' principle by which the CNS controls motoneuronal output according to mechanical advantage, presumably in addition to the Henneman's size principle of motoneurone recruitment. Studies of the genioglossus, an obligatory upper airway muscle that helps maintain airway patency, reveal that it receives simultaneous inspiratory, expiratory and tonic drives even during quiet breathing. There is much to be learned about the neural drive to pools of human inspiratory and expiratory muscles, not only during respiratory tasks but also in automatic and volitional tasks, and in diseases that alter the required drive.

Sternomastoid, rib cage, and expiratory muscle activity during weaning failure

We hypothesized that patients who fail weaning from mechanical ventilation recruit their inspiratory rib cage muscles sooner than they recruit their expiratory muscles, and that rib cage muscle recruitment is accompanied by recruitment of sternomastoid muscles. Accordingly, we measured sternomastoid electrical activity and changes in esophageal (ΔPes) and gastric pressure (ΔPga) in 11 weaning-failure and 8 weaning-success patients. At the start of trial, failure patients exhibited a higher ΔPga-to-ΔPes ratio than did success patients ( P = 0.05), whereas expiratory rise in Pga was equivalent in the two groups. Between the start and end of the trial, failure patients developed additional increases in ΔPga-to-ΔPes ratio ( P < 0.0014) and the expiratory rise in Pga also increased ( P < 0.004). At the start of trial, sternomastoid activity was present in 8 of 11 failure patients contrasted with 1 of 8 success patients. Over the course of the trial, sternomastoid activity increased by 53.0 ± 9.3% in the failure patients ( P = 0.0005), whereas it did not change in the success patients. Failure patients recruited their respiratory muscles in a sequential manner. The sequence began with activity of diaphragm and greater-than-normal activity of inspiratory rib cage muscles; recruitment of sternomastoids and rib cage muscles approached near maximum within 4 min of trial commencement; expiratory muscles were recruited slowest of all. In conclusion, not only is activity of the inspiratory rib cage muscles increased during a failed weaning trial, but respiratory centers also recruit sternomastoid and expiratory muscles. Extradiaphragmatic muscle recruitment may be a mechanism for offsetting the effects of increased load on a weak diaphragm.

Respiratory muscle activity and respiratory obstruction after abdominal surgery

BACKGROUND

Respiratory movements in patients after abdominal surgery are frequently abnormal, with associated disturbances in the pattern of inspiratory pressure generation. The reasons for these abnormalities are not clear and have been attributed to impaired action of the diaphragm. However, an alternative is that partial airway obstruction could trigger reflex activation of the inspiratory ribcage muscles, which would cause a similar pattern of inspiratory pressure change. Direct measurement of electrical activity can indicate if reflex activation of inspiratory muscles occurs when partial airway obstruction is present.

METHODS

In an open study, we implanted electrodes to measure the EMG of scalene, intercostal and external oblique abdominal muscles in patients after lower abdominal surgery. Analgesia was with morphine i.v. by patient control. We used nasal cannulae to measure nasal airflow and compared EMG activity when airway obstruction was present with activity when breathing was not obstructed.

RESULTS

The pattern of activity of the different muscles was distinct. Intercostal activity reached a maximum during inspiration, before the scalene muscles, whereas scalene activity increased in phase with increasing lung volume. Abdominal muscle activity commenced when expiratory flow had ceased and continued until the next inspiration. In all three muscle groups, partial airway obstruction did not alter muscle activity.

CONCLUSIONS

Partial airway obstruction does not activate inspiratory ribcage muscles, in patients receiving morphine for postoperative analgesia after lower abdominal surgery. Changes in respiratory pressures and abnormalities of chest wall movement described in previous studies cannot be attributed to reflex responses and probably result from increased airway resistance and abdominal muscle action.

Acute effects of thixotropy conditioning of inspiratory muscles on end-expiratory chest wall and lung volumes in normal humans

Thixotropy conditioning of inspiratory muscles consisting of maximal inspiratory effort performed at an inflated lung volume is followed by an increase in end-expiratory position of the rib cage in normal human subjects. When performed at a deflated lung volume, conditioning is followed by a reduction in end-expiratory position. The present study was performed to determine whether changes in end-expiratory chest wall and lung volumes occur after thixotropy conditioning. We first examined the acute effects of conditioning on chest wall volume during subsequent five-breath cycles using respiratory inductive plethysmography (n = 8). End-expiratory chest wall volume increased after conditioning at an inflated lung volume (P < 0.05), which was attained mainly by rib cage movements. Conditioning at a deflated lung volume was followed by reductions in end-expiratory chest wall volume, which was explained by rib cage and abdominal volume changes (P < 0.05). End-expiratory esophageal pressure decreased and increased after conditioning at inflated and deflated lung volumes, respectively (n = 3). These changes in end-expiratory volumes and esophageal pressure were greatest for the first breath after conditioning. We also found that an increase in spirometrically determined inspiratory capacity (n = 13) was maintained for 3 min after conditioning at a deflated lung volume, and a decrease for 1 min after conditioning at an inflated lung volume. Helium-dilution end-expiratory lung volume increased and decreased after conditioning at inflated and deflated lung volumes, respectively (both P < 0.05; n = 11). These results suggest that thixotropy conditioning changes end-expiratory volume of the chest wall and lung in normal human subjects.

Spatial distribution of inspiratory drive to the parasternal intercostal muscles in humans

The human parasternal intercostal muscles are obligatory inspiratory muscles with a diminishing mechanical advantage from cranial to caudal interspaces. This study determined whether inspiratory neural drive to these muscles is graded, and whether this distribution matches regional differences in inspiratory mechanical advantage. To determine the neural drive, intramuscular EMG was recorded from the first to the fifth parasternal intercostals during resting breathing in six subjects. All interspaces showed phasic inspiratory activity but the onset of activity relative to inspiratory flow in the fourth and fifth spaces was delayed compared with that in cranial interspaces. Activity in the first, second and third interspaces commenced, on average, within the first 10% of inspiratory time, and sometimes preceded inspiratory airflow. In contrast, activity in the fourth and fifth interspaces began after an average 33% of inspiratory time. The peak inspiratory discharge frequency of motor units in the first interspace averaged 13.4 +/- 1.0 Hz (mean +/- s.e.m.) and was significantly greater than in all other interspaces, in particular in the fifth space (8.0 +/- 1.0 Hz). Phasic inspiratory activity was sometimes superimposed on tonic activity. In the first interspace, only 3% of units had tonic firing, but this proportion increased to 34% in the fifth space. In five subjects, recordings were also made from the medial and lateral extent of the second parasternal intercostal. Both portions showed phasic inspiratory activity which began within the first 6% of inspiratory time. Motor units from the lateral and medial portions fired at the same peak discharge rate (10.4 +/- 0.7 versus 10.7 +/- 0.6 Hz). These observations indicate that the distribution of neural drive to the parasternal intercostals in humans has a rostrocaudal gradient, but that the drive is uniform along the mediolateral extent of the second interspace. The distribution of inspiratory neural drive to the parasternal intercostals parallels the spatial distribution of inspiratory mechanical advantage, while tonic activity was higher where mechanical advantage was lower.

Chronic treatment with agents that stabilize cytosolic IkappaB-alpha enhances survival and improves resting membrane potential in MDX muscle fibers subjected to chronic passive stretch

The potential pathogenic role of increased NFkappaB signaling in passively stretched dystrophic skeletal muscle was examined by treating adult mdx mice with an agent that stabilized cytosolic IkappaB-alpha (pyrollidine dithiocarbamate, PDTC)and examining the effects of this treatment on the chronically stretched mdx triangularis sterni (TS) muscle. Daily PDTC treatment significantly increased the number of surviving striated TS fibers regardless of age. TS fibers from untreated mdx mice had significantly lower resting potentials (RPs) than nondystrophic mice. Treatment with GdCl3 to block resting Ca2+ influx had no effect on RP in either nondystrophic or mdx preparations. Daily treatment with PDTC significantly improved the RP regardless of age. These results are consistent with the hypothesis that passive stretch activates an NFkappaB-mediated pathogenic mechanism in dystrophic muscle and suggest that agents which stabilize cytosolic IkappaB-alpha levels may be useful for treating Duchenne and related muscular dystrophies.

Respiratory action of the intercostal muscles

The mechanical advantages of the external and internal intercostals depend partly on the orientation of the muscle but mostly on interspace number and the position of the muscle within each interspace. Thus the external intercostals in the dorsal portion of the rostral interspaces have a large inspiratory mechanical advantage, but this advantage decreases ventrally and caudally such that in the ventral portion of the caudal interspaces, it is reversed into an expiratory mechanical advantage. The internal interosseous intercostals in the caudal interspaces also have a large expiratory mechanical advantage, but this advantage decreases cranially and, for the upper interspaces, ventrally as well. The intercartilaginous portion of the internal intercostals (the so-called parasternal intercostals), therefore, has an inspiratory mechanical advantage, whereas the triangularis sterni has a large expiratory mechanical advantage. These rostrocaudal gradients result from the nonuniform coupling between rib displacement and lung expansion, and the dorsoventral gradients result from the three-dimensional configuration of the rib cage. Such topographic differences in mechanical advantage imply that the functions of the muscles during breathing are largely determined by the topographic distributions of neural drive. The distributions of inspiratory and expiratory activity among the muscles are strikingly similar to the distributions of inspiratory and expiratory mechanical advantages, respectively. As a result, the external intercostals and the parasternal intercostals have an inspiratory function during breathing, whereas the internal interosseous intercostals and the triangularis sterni have an expiratory function.

Extrajunctional resting Ca2+ influx is not increased in a severely dystrophic expiratory muscle (triangularis sterni) of the mdx mouse

Freshly isolated adult mdx and nondystrophic (C57B110SnJ) muscle fibers were used to examine the potential role of resting Ca2+ influx in the pathogenesis of Duchenne and related dystrophies. Microfluorimetric determinations of resting divalent cation influx were obtained from undissociated intact muscle fibers in the triangularis sterni (TS), a thin expiratory muscle. Morphological evidence indicated severe dystrophic alterations in the mdx TS at 5 months, and a pronounced loss of fibers with connective tissue infiltration in older animals. To examine resting Ca2+ influx, fibers were loaded with FURA PE3 and the rate of quenching of intracellular signal following the extracellular addition of Mn2+ was determined from extrajunctional regions. There was no significant difference in quench rate between nondystrophic and mdx TS fibers. These results indicate that severe dystrophic pathology in the absence of dystrophin is not due to generalized increases in resting Ca2+ influx.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Respiratory effects of the scalene and sternomastoid muscles in humans

Previous studies have shown that in normal humans the change in airway opening pressure (DeltaPao) produced by all the parasternal and external intercostal muscles during a maximal contraction is approximately -18 cmH(2)O. This value is substantially less negative than DeltaPao values recorded during maximal static inspiratory efforts in subjects with complete diaphragmatic paralysis. In the present study, therefore, the respiratory effects of the two prominent inspiratory muscles of the neck, the sternomastoids and the scalenes, were evaluated by application of the Maxwell reciprocity theorem. Seven healthy subjects were placed in a computed tomographic scanner to determine the fractional changes in muscle length during inflation from functional residual capacity to total lung capacity and the masses of the muscles. Inflation induced greater shortening of the scalenes than the sternomastoids in every subject. The inspiratory mechanical advantage of the scalenes thus averaged (mean +/- SE) 3.4 +/- 0.4%/l, whereas that of the sternomastoids was 2.0 +/- 0.3%/l (P < 0.001). However, sternomastoid muscle mass was much larger than scalene muscle mass. As a result, DeltaPao generated by a maximal contraction of either muscle would be 3-4 cmH(2)O, which is about the same as DeltaPao generated by the parasternal intercostals in all interspaces.

Relationship between neural drive and mechanical effect in the respiratory system

The actions of the canine external and internal interosseous intercostal muscles on the lung were assessed by applying the Maxwell reciprocity theorem. The external intercostals in the dorsal part of the cranial interspaces were found to have a large inspiratory effect. However, this effect decreases continuously in the caudal and the ventral direction, such that the muscles in the ventral part of the caudal interspaces have an expiratory effect. The internal intercostals also show marked gradients, such that the muscles in the dorsal part of the caudal interspaces have a large expiratory effect and those in the ventral part of the most cranial interspaces have a small inspiratory effect. During breathing, however, inspiratory activity is found only in the external intercostals with an inspiratory effect, and expiratory activity is confined to the internal intercostals with an expiratory effect. The spatial distribution of inspiratory activity among the canine external intercostals closely mirrors the distribution of inspiratory effect, and the distribution of expiratory activity among the internal intercostals closely mirrors the distribution of expiratory effect. Therefore, the external intercostals have a clear-cut inspiratory action on the lung during breathing, whereas the internal intercostals have a definite expiratory action. The distribution of neural drive among these muscles appears to be equally well matched to the distribution of respiratory effect in humans.

Distribution of inspiratory drive to the external intercostal muscles in humans

The external intercostal muscles in humans show marked regional differences in respiratory effect, and this implies that their action on the lung during breathing is primarily determined by the spatial distribution of neural drive among them. To assess this distribution, monopolar electrodes were implanted under ultrasound guidance in different muscle areas in six healthy individuals and electromyographic recordings were made during resting breathing. The muscles in the dorsal portion of the third and fifth interspace showed phasic inspiratory activity with each breath in every subject. However, the muscle in the ventral portion of the third interspace showed inspiratory activity in only three subjects, and the muscle in the dorsal portion of the seventh interspace was almost invariably silent. Also, activity in the ventral portion of the third interspace, when present, and activity in the dorsal portion of the fifth interspace were delayed relative to the onset of activity in the dorsal portion of the third interspace. In addition, the discharge frequency of the motor units identified in the dorsal portion of the third interspace averaged (mean +/- S.E.M.) 11.9 +/- 0.3 Hz and was significantly greater than the discharge frequency of the motor units in both the ventral portion of the third interspace (6.0 +/- 0.5 Hz) and the dorsal portion of the fifth interspace (6.7 +/- 0.4 Hz). The muscle in the dorsal portion of the third interspace started firing simultaneously with the parasternal intercostal in the same interspace, and the discharge frequency of its motor units was even significantly greater (11.4 +/- 0.3 vs. 8.9 +/- 0.2 Hz). These observations indicate that the distribution of neural inspiratory drive to the external intercostals in humans takes place along dorsoventral and rostrocaudal gradients and mirrors the spatial distribution of inspiratory mechanical advantage.

Respiratory effects of the external and internal intercostal muscles in humans

The current conventional view of intercostal muscle actions is based on the theory of Hamberger (1749) and maintains that as a result of the orientation of the muscle fibres, the external intercostals have an inspiratory action on the lung and the internal interosseous intercostals have an expiratory action. Recent studies in dogs, however, have shown that this notion is only approximate. In the present studies, the respiratory actions of the human external and internal intercostal muscles were evaluated by applying the Maxwell reciprocity theorem. Thus the orientation of the muscle fibres relative to the ribs and the masses of the muscles were first assessed in cadavers. Five healthy individuals were then placed in a computed tomographic scanner to determine the geometry of the ribs and their precise transformation during passive inflation to total lung capacity. The fractional changes in length of lines with the orientation of the muscle fibres were then computed to obtain the mechanical advantages of the muscles. These values were finally multiplied by muscle mass and maximum active stress (3.0 kg cm-2) to evaluate the potential effects of the muscles on the lung. The external intercostal in the dorsal half of the second interspace was found to have a large inspiratory effect. However, this effect decreases rapidly in the caudal direction, in particular in the ventral portion of the ribcage. As a result, it is reversed into an expiratory effect in the ventral half of the sixth and eighth interspaces. The internal intercostals in the ventral half of the sixth and eighth interspaces have a large expiratory effect, but this effect decreases dorsally and cranially. The total pressure generated by all the external intercostals during a maximum contraction would be -15 cmH2O, and that generated by all the internal interosseous intercostals would be +40 cmH2O. These pressure changes are substantially greater than those induced by the parasternal intercostal and triangularis sterni muscles, respectively.

Respiratory muscle function and drive in chronic obstructive pulmonary disease

Respiratory, and particularly inspiratory, muscle function is altered in COPD. Many of these alterations are secondary to a mechanical disadvantage related to hyperinflation. Other factors, including corticosteroid therapy and nutritional depletion, are also deleterious to muscle function. In addition, the load imposed on the respiratory muscles is increased in COPD. Combined with the altered respiratory muscle function, this increase induces important changes in respiratory muscle drive and recruitment. Moreover, the imbalance between respiratory muscle function and load is an important determinant of dyspnea and hypercapnia. Because much of the lung and airway derangements are irreversible in COPD, the respiratory muscles appear to be an attractive target for therapeutic interventions.