Anterior magnetic phrenic nerve stimulation: laboratory and clinical evaluation

Evaluation of computed tomography in the diagnosis of ultrasound-proven diaphragm dysfunction

INTRODUCTION

Computed tomography (CT) is routinely employed on the evaluation of dyspnea, yet limited data exist on its assessment of diaphragmatic muscle. This study aimed to determine the capability of CT in identifying structural changes in the diaphragm among patients with ultrasound-confirmed diaphragmatic dysfunction.

METHODS

Diaphragmatic ultrasounds conducted between 2018 and 2021 at our center in Marseille, France, were retrospectively collected. Diaphragmatic pillars were measured on CT scans at the L1 level and the celiac artery. Additionally, the difference in height between the two diaphragmatic domes in both diaphragmatic dysfunction cases and controls was measured and compared.

RESULTS

A total of 65 patients were included, comprising 24 with diaphragmatic paralysis, 13 with diaphragmatic weakness, and 28 controls. In the case group (paralysis and weakness) with left dysfunctions (n = 24), the CT thickness of the pillars at the level of L1 and the celiac artery was significantly thinner compared with controls (2.0 mm vs. 7.4 mm and 1.8 mm vs. 3.1 mm, p < 0.001 respectively). Significantly different values were observed for paralysis (but not weakness) in the right dysfunction subgroup (n = 15) (2.6 mm vs. 7.4 mm and 2.2 mm vs. 3.8 mm, p < 0.001 respectively, for paralysis vs. controls). Regardless of the side of dysfunction, a significant difference in diaphragmatic height was observed between cases and controls (7.70 cm vs. 1.16 cm and 5.51 cm vs. 1.16 cm, p < 0.001 for right and left dysfunctions, respectively). Threshold values determined through ROC curve analyses for height differences between the two diaphragmatic domes, indicative of paralysis or weakness in the right dysfunctions, were 4.44 cm and 3.51 cm, respectively. Similarly for left dysfunctions, the thresholds were 2.70 cm and 2.48 cm, respectively, demonstrating good performance (aera under the curve of 1.00, 1.00, 0.98, and 0.79, respectively).

CONCLUSION

In cases of left diaphragmatic dysfunction, as well as in paralysis associated with right diaphragmatic dysfunction, CT revealed thinner pillars. Additionally, a notable increase in the difference in diaphragmatic height demonstrated a strong potential to identify diaphragmatic dysfunction, with specific threshold values.

Effects of inspiratory muscle training on exertional breathlessness in patients with unilateral diaphragm dysfunction: a randomised trial

Background

Unilateral diaphragm dysfunction (UDD) is an underdiagnosed cause of dyspnoea. Inspiratory muscle training (IMT) is the only conservative treatment for UDD, but the mechanisms of improvement are unknown. We characterised the effects of IMT on dyspnoea, exercise tolerance and respiratory muscle function in people with UDD.

Methods

15 people with UDD (73% male, 61±8 years) were randomised to 6 months of IMT (50% maximal inspiratory mouth pressure (PI,max), n=10) or sham training (10% PI,max, n=5) (30 breaths twice per day). UDD was confirmed by phrenic nerve stimulation and persisted throughout the training period. Symptoms were assessed by the transitional dyspnoea index (TDI) and exercise tolerance by constant-load cycle tests performed pre- and post-training. Oesophageal (Pes) and gastric (Pga) pressures were measured with a dual-balloon catheter. Electromyography (EMG) and oxygenation (near-infrared spectroscopy) of respiratory muscles were assessed continuously during exercise.

Results

The IMT group (from 45±6 to 62±23% PI,max) and sham group (no progression) completed 92 and 86% of prescribed sessions, respectively. PI,max, TDI scores and cycle endurance time improved significantly more after IMT versus sham (mean between-group differences: 28 (95% CI 13-28) cmH2O, 3.0 (95% CI 0.9-5.1) points and 6.0 (95% CI 0.4-11.5) min, respectively). During exercise at iso-time, Pes, Pga and EMG of the scalene muscles were reduced and the oxygen saturation indices of the scalene and abdominal muscles were higher post- versus pre-training only in the IMT group (all p<0.05).

Conclusion

The effects of IMT on dyspnoea and exercise tolerance in UDD were not mediated by an improvement in isolated diaphragm function, but may reflect improvements in strength, coordination and/or oxygenation of the extra-diaphragmatic respiratory muscles.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Electrophysiological assessment of respiratory function

While the traditional lung function tests are used to assess lung capacity and pulmonary function, they cannot evaluate respiratory driving function and the integrity of the conduction pathway from the central nervous system to the respiratory motor neuron in the spinal cord and to the diaphragm. The inspiratory trigger is sent from the central nervous system through the phrenic nerve and drives the diaphragm to generate inspiratory movement. Therefore, phrenic nerve stimulation and diaphragmatic electromyography are two fundamental methods to assess respiratory function. There are several useful tools to assess respiratory motor system including electrical or magnetic phrenic nerve stimulation, diaphragmatic needle electromyography, and diaphragmatic ultrasound. By these means, physicians can assess current respiratory status in different neurological diseases that affect respiratory muscles, follow-up of the severity of respiratory impairment, help to predict the chance of successfully weaning from ventilatory support, and confirm clinical diagnoses such as diaphragmatic myoclonus. Although some of these tests require special training, applying these neurophysiological assessments in clinical practice is highly recommended.

Heart Failure Results in Inspiratory Muscle Dysfunction Irrespective of Left Ventricular Ejection Fraction

BACKGROUND

Exercise intolerance in heart failure with reduced ejection fraction (HFrEF) or heart failure with preserved ejection fraction (HFpEF) results from both cardiac dysfunction and skeletal muscle weakness. Respiratory muscle dysfunction with restrictive ventilation disorder may be present irrespective of left ventricular ejection fraction and might be mediated by circulating pro-inflammatory cytokines.

OBJECTIVE

To determine lung and respiratory muscle function in patients with HFrEF/HFpEF and to determine its associations with exercise intolerance and markers of systemic inflammation.

METHODS

Adult patients with HFrEF (n = 22, 19 male, 61 ± 14 years) and HFpEF (n = 8, 7 male, 68 ± 8 years) and 19 matched healthy control subjects underwent spirometry, measurement of maximum mouth occlusion pressures, diaphragm ultrasound, and recording of transdiaphragmatic and gastric pressures following magnetic stimulation of the phrenic nerves and the lower thoracic nerve roots. New York Heart Association (NYHA) class and 6-min walking distance (6MWD) were used to quantify exercise intolerance. Levels of circulating interleukin 6 (IL-6) and tumor necrosis factor-α (TNF-α) were measured using ELISAs.

RESULTS

Compared with controls, both patient groups showed lower forced vital capacity (FVC) (p < 0.05), maximum inspiratory pressure (PImax), maximum expiratory pressure (PEmax) (p < 0.05), diaphragm thickening ratio (p = 0.01), and diaphragm strength (twitch transdiaphragmatic pressure in response to supramaximal cervical magnetic phrenic nerve stimulation) (p = 0.01). In patients with HFrEF, NYHA class and 6MWD were both inversely correlated with FVC, PImax, and PEmax. In those with HFpEF, there was an inverse correlation between amino terminal pro B-type natriuretic peptide levels and FVC (r = -0.77, p = 0.04). In all HF patients, IL-6 and TNF-α were statistically related to FVC.

CONCLUSIONS

Irrespective of left ventricular ejection fraction, HF is associated with respiratory muscle dysfunction, which is associated with increased levels of circulating IL-6 and TNF-α.

Respiratory Muscle and Lung Function in Lung Allograft Recipients: Association with Exercise Intolerance

BACKGROUND

In lung transplant recipients (LTRs), restrictive ventilation disorder may be present due to respiratory muscle dysfunction that may reduce exercise capacity. This might be mediated by pro-inflammatory cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6).

OBJECTIVE

We investigated lung respiratory muscle function as well as circulating pro-inflammatory cytokines and exercise capacity in LTRs.

METHODS

Fifteen LTRs (6 female, age 56 ± 14 years, 63 ± 45 months post-transplantation) and 15 healthy controls matched for age, sex, and body mass index underwent spirometry, measurement of mouth occlusion pressures, diaphragm ultrasound, and recording of twitch transdiaphragmatic (twPdi) and gastric pressures (twPgas) following magnetic stimulation of the phrenic nerves and the lower thoracic nerve roots. Exercise capacity was quantified using the 6-min walking distance (6MWD). Plasma IL-6 and TNF-α were measured using enzyme-linked immunosorbent assays.

RESULTS

Compared with controls, patients had lower values for forced vital capacity (FVC; 81 ± 30 vs.109 ± 18% predicted, p = 0.01), maximum expiratory pressure (100 ± 21 vs.127 ± 17 cm H2O, p = 0.04), diaphragm thickening ratio (2.2 ± 0.4 vs. 3.0 ± 1.1, p = 0.01), and twPdi (10.4 ± 3.5 vs. 17.6 ± 6.7 cm H2O, p = 0.01). In LTRs, elevation of TNF-α was related to lung function (13 ± 3 vs. 11 ± 2 pg/mL in patients with FVC ≤80 vs. >80% predicted; p < 0.05), and lung function (forced expiratory volume after 1 s) was closely associated with diaphragm thickening ratio (r = 0.81; p < 0.01) and 6MWD (r = 0.63; p = 0.02).

CONCLUSION

There is marked restrictive ventilation disorder and respiratory muscle weakness in LTRs, especially inspiratory muscle weakness with diaphragm dysfunction. Lung function impairment relates to elevated levels of circulating TNF-α and diaphragm dysfunction and is associated with exercise intolerance.

Electrophysiological Properties of the Human Diaphragm Assessed by Magnetic Phrenic Nerve Stimulation: Normal Values and Theoretical Considerations in Healthy Adults

PURPOSE

This study determined normal values for motor evoked potentials (MEPs) and compound muscle action potentials (CMAPs) of the diaphragm following cortical and cervical magnetic stimulation (COMS and CEMS) of the phrenic nerves in healthy adults.

METHODS

Using surface electrodes, diaphragmatic MEP and CMAP were recorded in 70 subjects (34 ± 13 years, 25 men) following supramaximal cortical magnetic stimulation and CEMS at functional residual capacity and using a standardized inspiratory pressure trigger (-0.5 kPa). All healthy volunteers underwent standard spirometry and measurement of maximum inspiratory and expiratory pressure.

RESULTS

At functional residual capacity, upper limit of normal for MEP latency was 25 ms in men and 23 ms in women (p < 0.05), and upper limit of normal for CMAP latency was 6 ms. In contrast to MEP and CMAP amplitude, corresponding latencies showed little interindividual and intraindividual variability. Use of an inspiratory pressure trigger enhanced reproducibility and amplitude of diaphragm MEP. Diaphragm responses to both cortical and cervical magnetic stimulation were symmetrical and independent of age (in our cohort), with higher values for latency and amplitude in men (each p < 0.05). Diaphragm CMAP amplitude showed weak-moderate correlations with forced vital capacity (r = 0.47; p < 0.01), maximum inspiratory pressure (r = 0.39; p < 0.01), and maximum expiratory pressure (r = 0.32; p < 0.01).

CONCLUSIONS

Combination of cortical magnetic stimulation and CEMS of the phrenic nerves is feasible and allows noninvasive assessment of both central and peripheral conductivity of the diaphragm and the inspiratory pathway.

β-hydroxy-β-methylbutyrate (HMB) prevents sepsis-induced diaphragm dysfunction in mice

Infections induce severe respiratory muscle weakness. Currently there are no treatments for this important clinical problem. We tested the hypothesis that β-hydroxy-β-methylbutyrate (HMB) would prevent sepsis-induced diaphragm weakness. Four groups of adult male mice were studied: controls (saline-injected), sepsis (intraperitoneal lipopolysaccharide), sepsis+HMB (injected intravenously), and HMB. Diaphragm force generation and indices of caspase 3, calpain, 20S proteasomal subunit, and double-stranded RNA-dependent protein kinase (PKR) activation were assessed after 24h. Sepsis elicited large reductions in diaphragm specific force generation at all stimulation frequencies. Endotoxin also activated caspase 3, calpain, the 20S proteasomal subunit and PKR in the diaphragm. HMB blocked sepsis-induced caspase 3, 20S proteasomal and PKR activation, but did not prevent calpain activation. Most importantly, HMB administration significantly attenuated sepsis-induced diaphragm weakness, preserving muscle force generation at all stimulation frequencies (p<0.01). We speculate that HMB may prove to be an important therapy in infected patients, with the potential to increase diaphragm strength, to reduce the duration of mechanical ventilation and to decrease mortality in this patient population.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Clinical review: Respiratory monitoring in the ICU - a consensus of 16

Monitoring plays an important role in the current management of patients with acute respiratory failure but sometimes lacks definition regarding which 'signals' and 'derived variables' should be prioritized as well as specifics related to timing (continuous versus intermittent) and modality (static versus dynamic). Many new techniques of respiratory monitoring have been made available for clinical use recently, but their place is not always well defined. Appropriate use of available monitoring techniques and correct interpretation of the data provided can help improve our understanding of the disease processes involved and the effects of clinical interventions. In this consensus paper, we provide an overview of the important parameters that can and should be monitored in the critically ill patient with respiratory failure and discuss how the data provided can impact on clinical management.

Assessment of upper airway dynamics by anterior magnetic phrenic stimulation in conscious sleep apnea patients

Phrenic nerve magnetic stimulation (PNMS) performed anterolaterally at the base of the neck (BAMPS) and cervical magnetic stimulation are common techniques for assessing upper airway (UA) mechanical properties in conscious humans. We considered that if NMS performed at the sternal level (a-MS) could induce a similar percentage of flow-limited twitches as BAMPS in conscious subjects, gauging UA dynamic properties by PNMS would be simplified. Instantaneous flow, pharyngeal and esophageal pressures, as well as thoraco-abdominal motion were recorded in 10 conscious sleep apnea patients. BAMPS and a-MS were applied at end expiration. The percentage of flow-limited twitches, maximal tolerated intensity, and minimal stimulator output associated with flow-limited twitches were similar between BAMPS and a-MS. Examining the effects of stimulation site, stimulation intensity and site*intensity interaction on the characteristics of flow-limited twitches, the former was responsible for more negative peak esophageal pressure (BAMPS: −11.5 ± 0.9 cmH2O; a-MS: −6.5 ± 1.1 cmH2O; P = 0.002) and UA closing pressure (BAMPS: −7.7 ± 0.5 cmH2O; a-MS: −5.8 ± 0.6 cmH2O; P = 0.02) as well as for lower mean linear upper airway resistance (UAR) (BAMPS 3.5 ± 0.4 cmH2O·l−1·s−1; a-MS 2.2 ± 0.4 cmH2O·l−1·s−1; P = 0.02). a-MS systematically evoked outward/inward thoracic displacement, although this movement pattern was observed in only 50% of patients when they were subjected to BAMPS. Linear UAR of BAMPS-induced flow-limited twitches was lower in the presence of initial outward thoracic movement (2 ± 0.05 cmH2O·l−1·s−1) than with inward motion (4.3 ± 1.5 cmH2O·l−1·s−1; P = 0.03). We conclude that a-MS represents a practical and functional technique to evaluate UA mechanical properties in conscious sleep apnea patients.

Repetitive magnetic stimulation of the phrenic nerves for diaphragm conditioning: a normative study of feasibility and optimal settings

Electrical stimulation can enhance muscle function. We applied repetitive cervical magnetic phrenic stimulation (rCMS) to induce diaphragm contractions in 7 healthy subjects (800 ms trains; transdiaphragmatic pressure (Pdi) measurements; tolerance ratings). Each rCMS train produced a sustained diaphragm contraction. Sixty-five percent of the maximal available output at 15 Hz proved the best compromise between Pdi and discomfort with nonfatiguing contractions. rCMS appears feasible and should be investigated for diaphragm conditioning in appropriate clinical populations.

Identification of prolonged phrenic nerve conduction time in the ICU: magnetic versus electrical stimulation

PURPOSE

Retrospective study of prospectively collected data to assess the reliability of cervical magnetic stimulation (CMS) to detect prolonged phrenic nerve (PN) conduction time at the bedside. Because PN injuries may cause diaphragm dysfunction, their diagnosis is relevant in intensive care units (ICU). This is achieved by studying latency and amplitude of diaphragm response to PN stimulation. Electrical stimulation (ES) is the gold standard, but it is difficult to perform in the ICU. CMS is an easy noninvasive tool to assess PN integrity, but co-activates muscles that could contaminate surface chest electromyographic recordings.

METHODS

In a first set of 56 ICU patients with suspected PN injury, presence and latency of compound motor action potentials elicited by CMS and ES were compared. With ES as the reference method, CMS was evaluated as a test designed to indicate presence or absence of PN injury. In eight additional patients, intramuscular diaphragm recordings were compared with surface diaphragm recordings and with the electromyograms of possible contamination sources.

RESULTS

The sensitivity of CMS to diagnose abnormal PN conduction was 0.91, and specificity was 0.84, whereas positive and negative predictive values were 0.81 and 0.92, respectively. Passing-Bablok regression analysis suggested no differences between the two measures. The correlation between PN latency in response to CMS and ES was significant. The "diaphragm surface" and "needle" latencies were close, and were significantly different from those of possibly contaminating muscles. One hemidiaphragm showed likely signal contamination.

CONCLUSION

CMS provides an easy reliable tool to detect prolonged PN conduction time in the ICU.

Electromagnetic ventilation: first evaluation of a new method for artificial ventilation in humans

Current methods of artificial ventilation cannot prevent diverse problems associated with mechanical ventilation. In contrast to all current forms of mechanical ventilation, electromagnetic stimulation can activate respiratory muscles directly. However, it is not known if and to what extent electromagnetic stimulation can ventilate humans. In 10 volunteers we stimulated the lateral neck using magnetic stimulators. Over 63 s we stimulated nine times with a frequency of 25 HZ for 1.1 s using 600 V, 900 V, and 1,200 V. The minimum stimulation time for each volunteer was 9 min. A Capnomac monitor measured minute ventilation. Electromagnetic stimulation was well tolerated and safe. Bilateral stimulation with 600 V achieved considerable minute ventilation (median +/- SD, 7.2 +/- 3.4 L/min) that increased at higher voltage levels (P < 0.0001). 900 V achieved sufficient minute ventilation in all volunteers (11.5 +/- 5.0 L/min; 1200 V, 14.0 +/- 4.9 L/min). This first evaluation of electromagnetic ventilation demonstrates that it can be used to ventilate humans sufficiently. This method may be developed to a new mode of ventilation.

Sepsis-induced myopathy

Sepsis is a major cause of morbidity and mortality in critically ill patients, and despite advances in management, mortality remains high. In survivors, sepsis increases the risk for the development of persistent acquired weakness syndromes affecting both the respiratory muscles and the limb muscles. This acquired weakness results in prolonged duration of mechanical ventilation, difficulty weaning, functional impairment, exercise limitation, and poor health-related quality of life. Abundant evidence indicates that sepsis induces a myopathy characterized by reductions in muscle force-generating capacity, atrophy (loss of muscle mass), and altered bioenergetics. Sepsis elicits derangements at multiple subcellular sites involved in excitation contraction coupling, such as decreasing membrane excitability, injuring sarcolemmal membranes, altering calcium homeostasis due to effects on the sarcoplasmic reticulum, and disrupting contractile protein interactions. Muscle wasting occurs later and results from increased proteolytic degradation as well as decreased protein synthesis. In addition, sepsis produces marked abnormalities in muscle mitochondrial functional capacity and when severe, these alterations correlate with increased death. The mechanisms leading to sepsis-induced changes in skeletal muscle are linked to excessive localized elaboration of proinflammatory cytokines, marked increases in free-radical generation, and activation of proteolytic pathways that are upstream of the proteasome including caspase and calpain. Emerging data suggest that targeted inhibition of these pathways may alter the evolution and progression of sepsis-induced myopathy and potentially reduce the occurrence of sepsis-mediated acquired weakness syndromes.

Increased load on the respiratory muscles in obstructive sleep apnea

We wished to quantify, in patients with obstructive sleep apnoea (OSA), the activity of the respiratory muscles in relation to upper airway occlusion and patency in sleep. We hypothesized that particular levels of neuromuscular activation are directly associated with upper airway patency. 21 patients with previously diagnosed OSA and 21 healthy control subjects underwent respiratory muscle testing and polysomnography. Neural respiratory drive, as measured by the electromyogram of the diaphragm (EMG(di)) was elevated in the obese OSA patients, awake and supine (13.1(5.6)%max), compared to normal subjects (mean (SD) 8.1(2.3)%max, p<0.01). During unobstructed breathing in sleep (stage N2) normal subjects had an EMG(di) of 7.7(3.9) compared to 22.8(19.2)%max in the OSA group (p<0.001). Prior to airway occlusion, EMG(submandibular) and EMG(di) dropped markedly, and then, following occlusion, increased progressively to their highest levels at airflow onset. Patients with OSA require specific and increased levels of neural respiratory drive to sustain ventilation in sleep.

Weaning Strategies in Problem Patients

Weaning usually accounts for approximately 40–50% of the total duration of mechanical ventilation. Approximately two-thirds of patients can tolerate withdrawal of ventilation without the need for more gradual weaning, but there are a significant number of patients for whom weaning is difficult. Weaning failure is defined as the failure of a spontaneous breathing trial, or the need for re-intubation within 48 hours of extubation. This article reviews the causes of failure to wean, and outlines a practical approach to dealing with the difficult-to-wean patient. The key to successful weaning combines an approach which optimises ventilation at night, adopts a stepwise approach to reducing ventilatory dependence during the day, and uses non-invasive ventilation as a ‘bridge’ out of the ICU. Having a weaning protocol and ensuring it is initiated in a timely manner is likely to be as important as what is in the protocol.

Exercise-induced respiratory muscle fatigue: implications for performance

It is commonly held that the respiratory system has ample capacity relative to the demand for maximal O2and CO2transport in healthy humans exercising near sea level. However, this situation may not apply during heavy-intensity, sustained exercise where exercise may encroach on the capacity of the respiratory system. Nerve stimulation techniques have provided objective evidence that the diaphragm and abdominal muscles are susceptible to fatigue with heavy, sustained exercise. The fatigue appears to be due to elevated levels of respiratory muscle work combined with an increased competition for blood flow with limb locomotor muscles. When respiratory muscles are prefatigued using voluntary respiratory maneuvers, time to exhaustion during subsequent exercise is decreased. Partially unloading the respiratory muscles during heavy exercise using low-density gas mixtures or mechanical ventilation can prevent exercise-induced diaphragm fatigue and increase exercise time to exhaustion. Collectively, these findings suggest that respiratory muscle fatigue may be involved in limiting exercise tolerance or that other factors, including alterations in the sensation of dyspnea or mechanical load, may be important. The major consequence of respiratory muscle fatigue is an increased sympathetic vasoconstrictor outflow to working skeletal muscle through a respiratory muscle metaboreflex, thereby reducing limb blood flow and increasing the severity of exercise-induced locomotor muscle fatigue. An increase in limb locomotor muscle fatigue may play a pivotal role in determining exercise tolerance through a direct effect on muscle force output and a feedback effect on effort perception, causing reduced motor output to the working limb muscles.

The value of multiple tests of respiratory muscle strength

BACKGROUND

Respiratory muscle weakness is an important clinical problem. Tests of varying complexity and invasiveness are available to assess respiratory muscle strength. The relative precision of different tests in the detection of weakness is less clear, as is the value of multiple tests.

METHODS

The respiratory muscle function tests of clinical referrals who had multiple tests assessed in our laboratories over a 6-year period were analysed. Thresholds for weakness for each test were determined from published and in-house laboratory data. The patients were divided into three groups: those who had all relevant measurements of global inspiratory muscle strength (group A, n = 182), those with full assessment of diaphragm strength (group B, n = 264) and those for whom expiratory muscle strength was fully evaluated (group C, n = 60). The diagnostic outcome of each inspiratory, diaphragm and expiratory muscle test, both singly and in combination, was studied and the impact of using more than one test to detect weakness was calculated.

RESULTS

The clinical referrals were primarily for the evaluation of neuromuscular diseases and dyspnoea of unknown cause. A low maximal inspiratory mouth pressure (Pimax) was recorded in 40.1% of referrals in group A, while a low sniff nasal pressure (Sniff Pnasal) was recorded in 41.8% and a low sniff oesophageal pressure (Sniff Poes) in 37.9%. When assessing inspiratory strength with the combination of all three tests, 29.6% of patients had weakness. Using the two non-invasive tests (Pimax and Sniff Pnasal) in combination, a similar result was obtained (low in 32.4%). Combining Sniff Pdi (low in 68.2%) and Twitch Pdi (low in 67.4%) reduced the diagnoses of patients with diaphragm weakness to 55.3% in group B. 38.3% of the patients in group C had expiratory muscle weakness as measured by maximum expiratory pressure (Pemax) compared with 36.7% when weakness was diagnosed by cough gastric pressure (Pgas), and 28.3% when assessed by Twitch T10. Combining all three expiratory muscle tests reduced the number of patients diagnosed as having expiratory muscle weakness to 16.7%.

CONCLUSION

The use of single tests such as Pimax, Pemax and other available individual tests of inspiratory, diaphragm and expiratory muscle strength tends to overdiagnose weakness. Combinations of tests increase diagnostic precision and, in the population studied, they reduced the diagnosis of inspiratory, specific diaphragm and expiratory muscle weakness by 19-56%. Measuring both Pimax and Sniff Pnasal resulted in a relative reduction of 19.2% of patients falsely diagnosed with inspiratory muscle weakness. The addition of Twitch Pdi to Sniff Pdi increased diagnostic precision by a smaller amount (18.9%). Having multiple tests of respiratory muscle function available both increases diagnostic precision and makes assessment possible in a range of clinical circumstances.

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

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

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3 +/- 4.5 cm H2O (Mean +/- SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0 +/- 3.9 cm H2O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p < 0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0 +/- 9.4 L/ min) and 77.5% of the volume (626 +/- 556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.

Does ICU-acquired paresis lengthen weaning from mechanical ventilation?

OBJECTIVE

To determine whether ICU-acquired paresis (ICUAP) is an independent risk factor of prolonged weaning.

DESIGN

Second study of a prospective cohort of 95 patients who were enrolled in an incidence and risk factor study of ICUAP.

SETTING

Three medical and two surgical ICUs in four hospitals.

PATIENTS AND PARTICIPANTS

Ninety-five patients without pre-existing neuromuscular disease recovering from the acute phase of critical illness after > or =7 days of mechanical ventilation.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Duration of weaning from mechanical ventilation was defined as the duration of mechanical ventilation between awakening (day 1) and successful weaning. Muscle strength was evaluated at day 7 after awakening using the Medical Research Council (MRC) score. Patients with an MRC <48 were considered to have ICUAP. Among the 95 patients (mean age 62.0+/-15.3 years, SAPS 2 on admission 48.7+/-17.4) who regained satisfactory awakening after 7 or more days of mechanical ventilation, 67 (70.5%) were hospitalized in a medical ICU and 28 (29.5%) in a surgical ICU. Median duration (25th-75th percentiles) of weaning was longer in patients with ICUAP than in those without ICUAP: 6 days (1-22 days) vs 3 days (1-7 days); p=0.01; log-rank analysis. In multivariate analysis, the two independent predictors of prolonged weaning were ICUAP [hazard ratio (HR): 2.4; 95% confidence interval (CI): 1.4-4.2] and chronic obstructive pulmonary disease (HR: 2.7; 95% CI: 1.6-4.5)

CONCLUSIONS

ICU-acquired paresis is an independent predictor of prolonged weaning. Prevention of ICU-acquired neuromuscular abnormalities in patients recovering from severe acute illness should result in shorter weaning duration.

Effect of voluntary facilitation on the diaphragmatic response to transcranial magnetic stimulation

We assessed recruitment curves of the surface diaphragm motor-evoked potential (MEP) after transcranial magnetic stimulation during relaxation and at three different levels of facilitation (20, 40, and 60% of maximal inspiratory esophageal pressure) in 10 healthy subjects (six young and four elderly). MEP amplitude recruitment curves varied between individuals during relaxation and at each level of facilitation. Amplitude recruitment curves during relaxation were reproducible in individual subjects. Inspiratory maneuvers caused a decrease in motor threshold and latency and an increase in MEP amplitude, positively correlated to the intensity of facilitation. These changes were similar in young and elderly subjects. The best fit for MEP amplitude recruitment curves for each condition was obtained with a Boltzmann model. The performance of repeated submaximal inspiratory maneuvers did not affect the amplitude recruitment curves of the relaxed diaphragm. We conclude that the recruitment curve of the diaphragm with transcranial magnetic stimulation is repeatable and changes consistently with facilitation and will, therefore, be a robust experimental tool for the investigation of supraspinal pathways to the diaphragm.

The pulmonary physician in critical care. 10: difficult weaning

The study of patients being weaned from mechanical ventilation has offered new insights into the physiology of respiratory failure. Assessment of the balance between respiratory muscle strength, work and central drive is essential if difficulty in weaning occurs, and optimisation of these elements may improve the success of weaning. Psychological support of patients and the creation of units specialising in weaning have also resulted in a higher success rate.

Liberation From Mechanical Ventilation Following Heart Surgery

The definition of ventilator dependency following heart surgery has evolved from a requirement for mechanical ventilation for more than 48 hours to less than 24 hours. Minimization of risk factors assessed in the preoperative period and improved surical and anesthetic techniques lead to improved and shortned postoperative courses and decreased hospital lengths of stay. The management of ventilator dependency following heart surgery should be approached from the perspective of pre-intensive care unit, intensive care unit, and post-intensive care unit Issues. A thorough understanding of risk factors for adverse postoperative morbidity and mortality leads to foused intraoperative and postoperative management aimed at improved quality of life following surgery. Minimizing preoprative risks and matching the postoperative state with criteria for the ideal candidate for early extubation improves outcome and minimizes requirements for mechanical ventilation in the postoperative period. In the event of prolonged requirements for mechanical ventilation, correcting impediments to weanng from mechanical ventilation provides the best circumtances to facilitate the process of rehabilitation and liberaion from mechanical ventilation.

Validation of improved recording site to measure phrenic conduction from surface electrodes in humans

Phrenic nerve stimulation, electrical (ES) or from cervical magnetic stimulation (CMS), allows one to assess the diaphragm contractile properties and the conduction time of the phrenic nerve (PNCT) through recording of an electromyographic response, traditionally by using surface electrodes. Because of the coactivation of extradiaphragmatic muscles, signal contamination can jeopardize the determination of surface PNCTs. To address this, we compared PNCTs with ES and CMS from surface and needle diaphragm electrodes in five subjects (10 phrenic nerves). At a modified recording site, lower and more anterior than usual (lowest accessible intercostal space, costochondral junction) with electrodes 2 cm apart, surface and needle PNCTs were similar (CMS: 6.0 +/- 0.25 ms surface vs. 6.2 +/- 0.13 ms needle, not significant). Electrodes recording the activity of the most likely sources of signal contamination, i.e., the serratus anterior and pectoralis major, showed distinct responses from that of the diaphragm, their earlier occurrence strongly arguing against contamination. With ES and CMS, apparently uncontaminated signals could be consistently recorded from surface electrodes.

Depression of diaphragm contractility by nitrous oxide in humans

Nitrous oxide is widely used in anesthesia and critical care medicine. The effect of nitrous oxide on diaphragm contractility in humans is unknown. We evaluated the effect of a 50% nitrous oxide-50% oxygen mixture on diaphragm contractility in healthy adult volunteers. The sniff transdiaphragmatic pressure (Sn Pdi) and the twitch transdiaphragmatic pressure (Tw Pdi) elicited by bilateral supramaximal phrenic nerve stimulation were measured before during and after inhalation of a mixture of 50% nitrous oxide and 50% oxygen. Sn Pdi decreased by 15.4% during nitrous oxide inhalation, with a value of 136 +/- 21 cm H(2)O before nitrous oxide and a value of 115 +/- 27 cm H(2)O during nitrous oxide inhalation (P = 0.03). Similarly, Tw Pdi decreased from 21.2 +/- 1.8 cm H(2)O before nitrous oxide inhalation to 16.9 +/- 4.1 cm H(2)O during nitrous oxide inhalation (P = 0.03). The effect of nitrous oxide was totally abolished 20 min after its discontinuation. Nitrous oxide has a short-acting suppressant effect on the pressure generating capacity of the diaphragm in healthy humans.

IMPLICATIONS

We investigated whether nitrous oxide (a common component of gas anesthesia) reduces diaphragm strength in humans. Diaphragm strength is reduced by nitrous oxide but the effect wears off within 20 min of administration. Caution is advised when using nitrous oxide without anesthesiologist supervision in patients at risk of ventilatory failure

Evaluation of an inspiratory muscle trainer in healthy humans

The Powerbreathe is an inspiratory muscle trainer promoted as improving inspiratory muscle strength (and consequently exercise performance) in athletes and patients with respiratory disease. No published evidence supports its efficacy. We performed a prospective randomized controlled study in which 12 normal subjects received either Powerbreathe training or sham training for a 6-week period. The primary outcome measure was diaphragm strength evaluated as twitch transdiaphragmatic pressure (Tw Pdi) but secondary outcome measures were provided by full respiratory muscle assessment and cardiopulmonary exercise testing. An advantage to training was observed when outcome was assessed by maximal static inspiratory mouth pressure (mean advantage 14.5 cm H2O, 95% CI 2.2-26.9 cm H2O, P=0.025). However. no significant difference was observed between the groups in any other parameter. In particular the deltaTw Pdi was not different between groups (mean 'advantage' 0.7 cmH2O, 95% CI- 7.0+/-5.5 cmH2O, P=0.8). The continued sale and use of the Powerbreathe device is not justified by our data. A sample size calculation showed that 234 subjects would need to be randomized to definitively refute the hypothesis that Powerbreathe improves Tw Pdi and we argue that such a study is required.