Respiratory muscle strength and control of ventilation in patients with neuromuscular disease

Pathophysiology of Chronic Hypercapnic Respiratory Failure

Chronic hypercapnic respiratory failure occurs in several conditions associated with hypoventilation. The mechanisms underlying the development of chronic hypercapnia include a combination of processes that increase metabolic CO2 production, reduce minute ventilation (V'e), or increase dead space fraction (Vd/Vt). Fundamental to the pathophysiology is a mismatch between increased load and a reduction in the capacity of the respiratory pump to compensate. Though neural respiratory drive may be decreased in a subset of central hypoventilation disorders, it is more commonly increased in attempting to maintain the load-capacity homeostatic balance.

Can postural changes in spirometry in children with Duchenne muscular dystrophy predict sleep hypoventilation?

AIM

To explore the relationship between postural changes in lung function and polysomnography (PSG) in children with Duchenne muscular dystrophy (DMD).

METHODS

In this prospective cross-sectional study, children with DMD performed spirometry in sitting and supine positions. A control group of age and gender matched healthy children also underwent postural lung function testing. PSG was performed within six months of spirometry.

RESULTS

Seventeen children with DMD, aged 12.3 ± 3 years performed sitting spirometry. 14 (84%) performed acceptable spirometry in the supine position. Mean FEV1sit and FVCsit were 77% (SD ± 22) and 74% (SD ± 20.4) respectively, with mean% ΔFVC(sit-sup) 9% (SD ± 11) (range 2% to 20%), and was significantly greater than healthy controls 4% (n = 30, SD ± 3, P < 0.001). PSG data on the 14 DMD children with acceptable supine spirometry showed total AHI 6.9 ± 5.9/hour (0.3 to 29), obstructive AHI 5.2 ± 4.0/hour (0.2 to 10), and REM AHI 14.1 ± -5.3/hour (0.1 to 34.7). ΔFVC(sit-sup) had poor correlation with hypoventilation on polysomnography.

CONCLUSION

Children with DMD and mild restrictive lung disease showed greater postural changes in spirometry than healthy controls but lower supine spirometry was not predictive of sleep hypoventilation.

Exploring inspiratory occlusion metrics to assess respiratory drive in patients under acute intermittent hypoxia

Patients living with Amyotrophic Lateral Sclerosis (ALS) experience respiratory weakness and, eventually, failure due to inspiratory motor neuron degeneration. Routine pulmonary function tests (e.g., maximum inspiratory pressure (MIP)) are used to assess disease progression and ventilatory compromise. However, these tests are poor discriminators between respiratory drive and voluntary respiratory function at rest. To better understand ALS disease progression, we can look into compensatory strategies and how patients consciously react to the occlusion and the effort produced to meet the ventilatory challenge of the occlusion. This ventilatory challenge, especially beyond the P_0.1 (200 ms and 300 ms), provides information regarding the patient's ability to recruit additional respiratory muscles as a compensatory strategy. Utilizing a standard P_0.1 protocol to assess respiratory drive, we extend the occlusion time analysis to 200 ms and 300 ms (Detected Occlusion Response (DOR)) in order to capture compensatory respiratory mechanics. Furthermore, we followed an Acute Intermittent Hypoxia (AIH) protocol known to increase phrenic nerve discharge to evaluate the compensatory strategies. Inspiratory pressure, the rate of change in pressure, and pressure generation normalized to MIP were measured at 100 ms, 200 ms, and 300 ms after an occlusion. Airway occlusions were performed three times during the experiment (i.e., baseline, 30 and 60 minutes post-AIH). Results indicated that while AIH did not elicit change in the P_0.1 or MIP, the DOR increased for ALS patients. These results support the expected therapeutic role of AIH and indicate the potential of the DOR as a metric to detect compensatory changes.

Methods and Applications in Respiratory Physiology: Respiratory Mechanics, Drive and Muscle Function in Neuromuscular and Chest Wall Disorders

Individuals with neuromuscular and chest wall disorders experience respiratory muscle weakness, reduced lung volume and increases in respiratory elastance and resistance which lead to increase in work of breathing, impaired gas exchange and respiratory pump failure. Recently developed methods to assess respiratory muscle weakness, mechanics and movement supplement traditionally employed spirometry and methods to evaluate gas exchange. These include recording postural change in vital capacity, respiratory pressures (mouth and sniff), electromyography and ultrasound evaluation of diaphragmatic thickness and excursions. In this review, we highlight key aspects of the pathophysiology of these conditions as they impact the patient and describe measures to evaluate respiratory dysfunction. We discuss potential areas of physiologic investigation in the evaluation of respiratory aspects of these disorders.

Pathophysiology of Neuromuscular Respiratory Diseases

Gas exchange between the atmosphere and the human body depends on the lungs and the function of the respiratory pump. The respiratory pump consists of the respiratory control center located in the brain, bony rib cage, diaphragm, and intercostal, accessory, and abdominal muscles. A variety of muscles serve to fine-tune adjustments of ventilation to metabolic demands. Appropriate evaluation and interventions can prevent respiratory complications and prolong life in individuals with neuromuscular diseases. This article discusses normal function of the respiratory pump, general pathophysiologic issues, and abnormalities in more common neuromuscular diseases.

Acute Lobar Atelectasis

Lobar atelectasis (or collapse) is an exceedingly common, rather predictable, and potentially pathogenic companion to many forms of acute illness, postoperative care, and chronic debility. Readily diagnosed by using routine chest imaging and bedside ultrasound, the consequences from lobar collapse may be minor or serious, depending on extent, mechanism, patient vulnerability, abruptness of onset, effectiveness of hypoxic vasoconstriction, and compensatory reserves. Measures taken to reduce secretion burden, assure adequate secretion clearance, maintain upright positioning, reverse lung compression, and sustain lung expansion accord with a logical physiologic rationale. Both classification and logical approaches to prophylaxis and treatment of lobar atelectasis derive from a sound mechanistic knowledge of its causation.

Thoracoabdominal asynchrony and paradoxical motion in middle stage amyotrophic lateral sclerosis

AIM

To assess thoracoabdominal asynchrony (TAA) and the presence of paradoxical motion in middle stage amyotrophic lateral sclerosis (ALS) and its relationships with chest wall tidal volume (V_T,CW), breathing pattern and cough peak flow (CPF).

METHODS

Phase angle (θ) between upper (RCp) and lower ribcage (RCa) and abdomen (AB), as well as percentage of inspiratory time for the lower ribcage (IP_RCa) and abdomen (IP_AB) moving in opposite directions were quantified using optoelectronic plethysmography in 12 ALS patients during quiet breathing and coughing. Paradoxical motion of the compartments was based on threshold values of θ and IP, obtained in twelve age and sex matched healthy persons.

RESULTS

During quiet breathing, significantly higher RCa and AB θ (p < .05), IP_RCa (p = 0.001) and IP_AB (p < 0.05) were observed in ALS patients as compared to controls. In ALS patients, correlations between RCa and AB θ with forced vital capacity (FVC) (r=-0.773, p < 0.01), vital capacity (r=-0.663, p < 0.05) and inspiratory capacity (IC) (r=-0.754, p < 0.01), as well as between RCp and RCa θ with FVC (r=-0.608, p < 0.05) and CPF (r=-0.601, p < 0.05) were found. During coughing, correlations between RCp and AB θ with CPF (r=-0.590, p < 0.05), IC (r=-0.748, p < 0.01) and V_T,CW (r=-0.608, p < 0.05), as well as between RCa and AB θ with CPF (r=-0.670, p < 0.05), IC (r=-0.713, p < 0.05) and peak expiratory flow (r=-0.727, p < 0.05) were also observed in ALS patients. ALS patients with paradoxical motion presented lower vital capacity and FVC_%pred (p < 0.05) compared to those without paradoxical motion.

CONCLUSIONS

Middle stage ALS patients exhibit TAA and paradoxical motion during quiet spontaneous breathing and coughing. In addition, diaphragmatic weakness (i.e. decrease in excursion of the RCa and AB compartments) was observed earlier in the lower ribcage rather than the abdominal compartment in this population.

Diaphragm: Pathophysiology and Ultrasound Imaging in Neuromuscular Disorders

Respiratory muscles are classically involved in neuromuscular disorders, leading to a restrictive respiratory pattern. The diaphragm is the main respiratory muscle involved during inspiration. Ultrasound imaging is a noninvasive, radiation-free, accurate and safe technique allowing assessment of diaphragm anatomy and function. The authors review the pathophysiology of diaphragm in neuromuscular disorders, the methodology and indications of diaphragm ultrasound imaging as well as possible pitfalls in the interpretation of results.

Combined effects of leaks, respiratory system properties and upper airway patency on the performance of home ventilators: a bench study

Background

Combined effects of leaks, mechanical property of respiratory system and upper airway (UA) patency on patient-ventilator synchrony (PVA) and the level of clinically “tolerable” leaks are not well established in home ventilators.

Methods

We comparatively assessed on a bench model, the highest leak level tolerated without inducing significant asynchrony (“critical leak”) in three home ventilators (Astral 150, Trilogy 100 and Vivo 60; noted as A150, T100 and V60 respectively) subjected to three simulated diseased respiratory conditions: chronic obstructive pulmonary disease (COPD), obesity hypoventilation (OHS) and neuromuscular disorders (NMD), with both open and closed UA. Also, total leak values in the device reports were compared to the bench-measured values.

Results

With open UA, all ventilators were able to avoid asynchrony up to a 30 L/min leak and even to 55 L/min in some cases. UA closure and respiratory diseases especially OHS influenced PVA. With closed UA, the critical leak of A150 and T100 remained higher than 55 L/min in COPD and OHS, while for V60 decreased to 41 and 33 L/min respectively. In NMD with closed UA, only T100 reached a high critical leak of 69 L/min. Besides, inspiratory trigger sensitivity change was often necessary to avoid PVA.

Conclusions

Home ventilators were able to avoid PVA in high-level leak conditions. However, asynchrony appeared in cases of abnormal mechanical properties of respiratory system or closed UA. In case of closed UA, the EPAP should be adjusted prior to the inspiratory trigger.

Trial registration

Not applicable.

Electronic supplementary material

The online version of this article (10.1186/s12890-017-0487-2) contains supplementary material, which is available to authorized users.

Neuromuscular disorders and chronic ventilation

Morbidity and mortality have decreased in patients with neuromuscular disease due to implementation of therapies to augment cough and improve ventilation. Infants with progressive neuromuscular disease will eventually develop respiratory complications as a result of muscle weakness and their inability to compensate during periods of increased respiratory loads. The finding of nocturnal hypercapnia is often the trigger for initiating non-invasive ventilation and studies have shown that its use not only may improve sleep-disordered breathing, but also that it may have an effect on daytime function, symptoms related to hypercapnia, and partial pressure of CO₂. It is important to understand the respiratory physiology of this population and to understand the benefits and limitations of assisted ventilation.

Abdominal Binding Improves Neuromuscular Efficiency of the Human Diaphragm during Exercise

We tested the hypothesis that elastic binding of the abdomen (AB) would enhance neuromuscular efficiency of the human diaphragm during exercise. Twelve healthy non-obese men aged 24.8 ± 1.7 years (mean ± SE) completed a symptom-limited constant-load cycle endurance exercise test at 85% of their peak incremental power output with diaphragmatic electromyography (EMGdi) and respiratory pressure measurements under two randomly assigned conditions: unbound control (CTRL) and AB sufficient to increase end-expiratory gastric pressure (Pga,ee) by 5-8 cmH₂O at rest. By design, AB increased Pga,ee by 6.6 ± 0.6 cmH₂O at rest. Compared to CTRL, AB significantly increased the transdiaphragmatic pressure swing-to-EMGdi ratio by 85-95% during exercise, reflecting enhanced neuromuscular efficiency of the diaphragm. By contrast, AB had no effect on spirometric parameters at rest, exercise endurance time or an effect on cardiac, metabolic, ventilatory, breathing pattern, dynamic operating lung volume, and perceptual responses during exercise. In conclusion, AB was associated with isolated and acute improvements in neuromuscular efficiency of the diaphragm during exercise in healthy men. The implications of our results are that AB may be an effective means of enhancing neuromuscular efficiency of the diaphragm in clinical populations with diaphragmatic weakness/dysfunction.

Physiology of respiratory disturbances in muscular dystrophies

Muscular dystrophy is a group of inherited myopathies characterised by progressive skeletal muscle wasting, including of the respiratory muscles. Respiratory failure, i.e. when the respiratory system fails in its gas exchange functions, is a common feature in muscular dystrophy, being the main cause of death, and it is a consequence of lung failure, pump failure or a combination of the two. The former is due to recurrent aspiration, the latter to progressive weakness of respiratory muscles and an increase in the load against which they must contract. In fact, both the resistive and elastic components of the work of breathing increase due to airway obstruction and chest wall and lung stiffening, respectively. The respiratory disturbances in muscular dystrophy are restrictive pulmonary function, hypoventilation, altered thoracoabdominal pattern, hypercapnia, dyspnoea, impaired regulation of breathing, inefficient cough and sleep disordered breathing. They can be present at different rates according to the type of muscular dystrophy and its progression, leading to different onset of each symptom, prognosis and degree of respiratory involvement.

KEY POINTS

A common feature of muscular dystrophy is respiratory failure, i.e. the inability of the respiratory system to provide proper oxygenation and carbon dioxide elimination.In the lung, respiratory failure is caused by recurrent aspiration, and leads to hypoxaemia and hypercarbia.Ventilatory failure in muscular dystrophy is caused by increased respiratory load and respiratory muscles weakness.Respiratory load increases in muscular dystrophy because scoliosis makes chest wall compliance decrease, atelectasis and fibrosis make lung compliance decrease, and airway obstruction makes airway resistance increase.The consequences of respiratory pump failure are restrictive pulmonary function, hypoventilation, altered thoracoabdominal pattern, hypercapnia, dyspnoea, impaired regulation of breathing, inefficient cough and sleep disordered breathing.

EDUCATIONAL AIMS

To understand the mechanisms leading to respiratory disturbances in patients with muscular dystrophy.To understand the impact of respiratory disturbances in patients with muscular dystrophy.To provide a brief description of the main forms of muscular dystrophy with their respiratory implications.

Can daytime measures of lung function predict respiratory failure in children with neuromuscular disease?

Neuromuscular disorders in children are a heterogeneous group of conditions with a variable age of presentation and overlapping clinical manifestations, many of which have progressive respiratory morbidity. Respiratory insufficiency occurs as a consequence of an imbalance between demands on the respiratory system and respiratory muscle capacity. Daytime measures of pulmonary function are used routinely in these children to assess respiratory status and monitor the consequences of the progression of muscle weakness. This review describes the current evidence for daytime pulmonary function tests and their ability to predict imminent respiratory morbidity.

The Respiratory System

This chapter addresses upper airway physiology for the pediatric intensivist, focusing on functions that affect ventilation, with an emphasis on laryngeal physiology and control in breathing. Effective control of breathing ensures that the airway is protected, maintains volume homeostasis, and provides ventilation. Upper airway structures are effectors for all of these functions that affect the entire airway. Nasal functions include air conditioning and protective reflexes that can be exaggerated and involve circulatory changes. Oral cavity and pharyngeal patency enable airflow and feeding, but during sleep pharyngeal closure can result in apnea. Coordination of breathing with sucking and nutritive swallowing alters during development, while nonnutritive swallowing at all ages limits aspiration. Laryngeal functions in breathing include protection of the subglottic airway, active maintenance of its absolute volume, and control of tidal flow patterns. These are vital functions for normal lung growth in fetal life and during rapid adaptations to breathing challenges from birth through adulthood. Active central control of breathing focuses on the coordination of laryngeal and diaphragmatic activities, which adapts according to the integration of central and peripheral inputs. For the intensivist, knowledge of upper airway physiology can be applied to improve respiratory support. In a second part the mechanical properties of the respiratory system as a critical component of the chain of events that result in translation of the output of the respiratory rhythm generator to ventilation are described. A comprehensive understanding of respiratory mechanics is essential to the delivery of optimized and individualized mechanical ventilation. The basic elements of respiratory mechanics will be described and developmental changes in the airways, lungs, and chest wall that impact on measurement of respiratory mechanics with advancing postnatal age are reviewed. This will be follwowed by two sections, the first on respiratory mechanics in various neonatal pathologies and the second in pediatric pathologies. The latter can be classified in three categories. First, restrictive diseases may be of pulmonary origin, such as chronic interstitial lung diseases or acute lung injury/acute respiratory distress syndrome, which are usually associated with reduced lung compliance. Restrictive diseases may also be due to chest wall abnormalities such as obesity or scoliosis (idiopathic or secondary to neuromuscular diseases), which are associated with a reduction in chest wall compliance. Second, obstructive diseases are represented by asthma and wheezing disorders, cystic fibrosis, long term sequelae of neonatal lung disease and bronchiolitis obliterans following hematopoietic stem cell transplantation. Obstructive diseases are defined by a reduced FEV1/VC ratio. Third, neuromuscular diseases, mainly represented by DMD and SMA, are associated with a decrease in vital capacity linked to respiratory muscle weakness that is better detected by PImax, PEmax and SNIP measurements.

[Neuromuscular disease: respiratory clinical assessment and follow-up]

Patients with neuromuscular disease are an important group at risk of frequently suffering acute or chronic respiratory failure, which is their main cause of death. They require follow-up by a pediatric respiratory medicine specialist from birth or diagnosis in order to confirm the diagnosis and treat any respiratory complications within a multidisciplinary context. The ventilatory support and the cough assistance have improved the quality of life and long-term survival for many of these patients. In this paper, the authors review the pathophysiology, respiratory function evaluation, sleep disorders, and the most frequent respiratory complications in neuromuscular diseases. The various treatments used, from a respiratory medicine point of view, will be analyzed in a next paper.

Rate of progression and predictive factors for pulmonary outcome in children and adults with Pompe disease

Respiratory insufficiency is a serious threat to patients with Pompe disease, a neuromuscular disorder caused by lysosomal acid alpha-glucosidase deficiency. Innovative therapeutic options which may stabilize pulmonary function have recently become available. We therefore determined proportion and severity of pulmonary involvement in patients with Pompe disease, the rate of progression of pulmonary dysfunction, and predictive factors for poor respiratory outcome. In a single-center, prospective, cohort study, we measured vital capacity (VC) in sitting and supine positions, as well as maximum inspiratory (MIP) and expiratory (MEP) mouth pressures, and end expiratory CO(2) in 17 children and 75 adults with Pompe disease (mean age 42.7 years, range 5-76 years). Seventy-four percent of all patients, including 53% of the children, had some degree of respiratory dysfunction. Thirty-eight percent had obvious diaphragmatic weakness. Males appeared to have more severe pulmonary involvement than females: at a group level, their mean VC was significantly lower than that of females (p<0.001), they used mechanical ventilation more often than females (p=0.042) and the decline over the course of the disease was significantly different between males and females (p=0.003). Apart from male gender, severe skeletal muscle weakness and long disease duration were the most important predictors of poor respiratory status. During follow-up (average 1.6 years, range 0.5-4.2 years), three patients became ventilator dependent. Annually, there were average decreases in VC in upright position of 0.9% points (p=0.09), VC in supine position of 1.2% points (p=0.049), MIP of 3.2% points (p=0.018) and MEP of 3.8% points (p<0.01). We conclude that pulmonary dysfunction in Pompe disease is much more common than generally thought. Males, patients with severe muscle weakness, and those with advanced disease duration seem most at risk.

Thoracic gunshot wounds: alterations to pulmonary function and respiratory muscle strength

BACKGROUND

The impact on respiratory function of gunshot injuries to the chest is unknown. The objective is to assess pulmonary function and respiratory muscle strength (RMS) in patients who have recently sustained an isolated gunshot injury to the chest.

METHODS

After institutional review board approval, patients with isolated gunshot injuries to the chest were prospectively identified. Study patients underwent pulmonary function testing and an assessment of RMS and gas exchange.

RESULTS

Ten male patients sustaining an isolated pulmonary gunshot wound were prospectively enrolled with a mean age of 29 years ± 10 years and mean Injury Severity Score of 15 ± 5. All patients had an associated pneumothorax (n = 1), hemothorax (n = 4), or a combination of both (n = 5). After removal of all thoracostomy tubes and before discharge [7.4 days ± 5.4 days (range, 2-21 days)], patients underwent respiratory function testing. Lung volume subdivisions were reduced by 25% to 60% of predicted and diffusion capacity by 37% with preservation of the normal ratio of diffusion capacity to alveolar volume. In the six subjects able to perform spirometry in seated and supine postures, forced vital capacity decreased by 20% when changing posture (p = 0.046). Arterial blood gas analysis showed significant reduction in the P(AO)₂/FIO₂ ratio (or increase in AaDO₂). Maximal respiratory pressures were severely reduced from predicted values, the maximal inspiratory pressure by 60% and the maximal expiratory pressure by 78%.

CONCLUSIONS

Lung volumes and RMS are decreased moderately to severely in patients who have sustained an isolated pulmonary gunshot wound. Expiratory muscle force generation is more severely affected than inspiratory muscle force. Further investigation of the long-term impact of these injuries on respiratory function is warranted.

Ventilatory parameters and maximal respiratory pressure changes with age in Duchenne muscular dystrophy patients

The aim of this longitudinal study was to precise, in children with Duchenne muscular dystrophy, the respective functional interest of ventilatory parameters (Vital capacity, total lung capacity and forced expiratory volume in one second [FEV(1)]) in comparison to maximal inspiratory pressure (Pimax) during growth. In ten boys the mean age of 9.1 +/- 1 years) to mean age of 16 +/- 1.4 years followed over a period of 7 years, we found that: (1) ventilatory parameters expressed in percentage of predicted value, after a normal ascending phase, start to decrease between 11 and 12 years, (2) Pimax presented only a decreasing phase since the beginning of the study and thus was already at 67% of predicted value at 12 years while ventilatory parameters was still normal, (3) after 12 years the mean slopes of decrease per year of vital capacity and FEV1 were higher (10.7 and 10.4%) than that of Pimax (6.9%), (4) at 15 years mean values of vital capacity and FEV1 (53.3 and 49.5% of predicted values) was simlar to that of Pimax (48.3%). In conclusion, if at early stages of the disease, Pimax is a more reliable index of respiratory impaiment than ventilatory parameters, the follow-up of ventilatory parameters, when they start to decrease, is a better indicator of disease progression and, at advanced stages they provided same information about the functional impact of disease.

Pulmonary complications of neuromuscular disease: a respiratory mechanics perspective

Paediatric neuromuscular disease compromises both the gas exchange and pump functions of the respiratory system. This can have profound implications for both growth and development of the respiratory system, as well as morbidity and mortality. Aspiration lung disease is common, and leads to increasingly restrictive pulmonary physiology over time. Abnormal lung and chest wall mechanics, and weak respiratory muscles, can combine to cause respiratory failure. Improving the balance between the work of breathing (by decreasing the respiratory load) and the respiratory pump (by improving respiratory muscle strength and decreasing respiratory muscle fatigue) can help prevent the onset of respiratory failure. Airway clearance techniques and non-invasive ventilation are two important tools in this effort. Better ways of assessing the respiratory pump, mechanical function, control and fatigue are needed especially in children.

Sleep, sleep disordered breathing, and nocturnal hypoventilation in children with neuromuscular diseases

Sleep disordered breathing (SDB) is now well recognized in children with neuromuscular diseases (NMD) and may lead to significant morbidity and increased mortality. Predisposing factors to SDB in children with NMD include reduced ventilatory responses, reduced activity of respiratory muscles during sleep and poor lung mechanics due to the underlying neuro-muscular disorder. SDB may present long before signs of respiratory failure emerge. When untreated, SDB may contribute to significant cardiovascular morbidities, neuro-cognitive deficits and premature death. One of the problems in detecting SDB in patients with NMD is the lack of correlation between lung function testing and daytime gas exchange. Polysomnography is the preferred method to evaluate for SDB in children with NMD. When the diagnosis of SDB is confirmed, treatment by non-invasive ventilation (NIV) is usually recommended. However, other modalities of mechanical ventilation do exist and may be indicated in combination with or without other supportive measures.

Response to hypercapnic challenge is associated with successful weaning from prolonged mechanical ventilation due to brain stem lesions

OBJECTIVE

We propose that higher airway occlusion pressure (P0.1) responses to hypercapnic challenge (HC) indicate less severe injury. The study aim was to determine whether P0.1 responses to HC were associated with successful weaning after prolonged mechanical ventilation (PMV) in patients with brainstem lesions and to determine a reference value for clinical use.

DESIGN AND SETTING

Forty-two patients with brainstem lesions on PMV were recruited. Breathing parameters and P0.1 were measured before HC. Three-minute HC challenges with increasing CO(2) concentrations were initiated and P0.1, respiratory rate, minute ventilation (V (e)), tidal volume (V (t)) and end tidal CO(2) were measured.

MEASUREMENTS AND RESULTS

Patients were classified into high (group I) and low (group II) response groups on the basis of P0.1 responses to HC. Increases in V (e) and V (t) after HC were significantly greater in group I patients (12.22 +/- 8.22 vs. 3.08 +/- 4.84 L/min, P < 0.001 and 399.11 +/- 278.18 vs. 110.54 +/- 18.275 ml, P < 0.001). P0.1 levels were significantly higher in group I compared to group II before HC (2.69 +/- 1.81 vs. 1.28 +/- 1.04 cmH(2)O, P = 0.003). The increase in P0.1 following HC was significantly greater in group I compared to group II patients (11.05 +/- 4.06 vs. 2.90 +/- 2.53 cmH(2)O, P < 0.001). Weaning success was significantly higher in group I compared to group II patients (72.2% vs. 33.3%, P = 0.02). A P0.1 increase of >6 cmH(2)O following HC was significantly associated with successful weaning.

CONCLUSIONS

Assessing the P.01 response to serial increases in the level of HC may be a safe means to ascertain whether patients with brainstem lesions are ready for ventilator weaning.

Expiratory Flow Maneuvers in Patients with Neuromuscular Diseases

OBJECTIVES

To compare cough peak flows (CPF), peak expiratory flows (PEF), and potentially confounding flows obtained by lip and tongue propulsion (dart flows, DF) for normal subjects and for patients with neuromuscular disease/restrictive pulmonary syndrome and to correlate them with vital capacity and maximum insufflation capacity.

DESIGN

A cross-sectional analytic study of 125 stable patients and 52 normal subjects in which CPF, PEF, and DF were measured by peak flow meter and vital capacity and maximum insufflation capacity by spirometer.

RESULTS

In normal subjects and in patients, the DF significantly exceeded PEF and CPF (P < or = 0.001). For normal subjects, PEF and CPF were not significantly different. For patients with neuromuscular disease/restrictive pulmonary syndrome, the CPF significantly exceeded PEF (P < 0.05). No normal subjects but 14 patients had DF lower than CPF. Thirteen of these 14 had the ability to air stack (maximum insufflation capacity greater than vital capacity), indicating greater compromise of mouth and lip than of glottic muscles. For 14 of 88 patients, maximum insufflation capacity values did not exceed vital capacity, mostly because of inability to close the glottis (inability to air stack). Nonetheless, for 11 of these 14 patients, the DF were within a standard deviation of the whole patient group; thus, bulbar-innervated muscle dysfunction was not uniform. CPF and PEF correlated with vital capacity (r = 0.85 and 0.86, respectively), and with maximum insufflation capacity (r = 0.76 and 0.72, respectively).

CONCLUSIONS

Measurements of CPF, PEF, and DF are useful for assessing bulbar-innervated, inspiratory, and expiratory muscle function. Care must be taken to not confuse them.

Evaluation of maximal respiratory pressures in myasthenia gravis. Prognostic value

We assess the prognosis of mild forms of myasthenia gravis (MG) by maximal respiratory pressures (MRP) and single fiber electromyography (SFEMG). Fifty MG patients (12 form I, 21 form IIa and 17 form IIb) are valued by MRP [maximal expiratory pressure (MEP) and maximal inspiratory pressure (MIP)] and SFEMG, and are followed-up clinically. We have found in form I patients developing form IIa and form IIa worsening to form IIb, MEP and MIP mean relative values significantly lower than the rest. Inversely, IIb form patients improving to IIa form display MIP mean relative values higher than the rest; no difference appears with MEP. A reduction under 50% of fifth-percentile implies clinical deterioration in forms I and IIa, while its surpassing in IIb form suggests a tendency to improvement. No evident differences are found by SFEMG. MRP allow the follow-up of MG patients and could warn us of a clinical prognosis.

Accuracy of noninvasive estimates of respiratory muscle effort during spontaneous breathing in restrictive diseases

Mean inspiratory pressure (Pi), estimated from the occlusion pressure at the mouth and the inspiratory time, is useful as a noninvasive estimate of respiratory muscle effort during spontaneous breathing in normal subjects and patients with chronic obstructive pulmonary disease. The aim of this study was to compare the Pi with respect to mean esophageal pressure (Pes) in patients with restrictive disorders. Eleven healthy volunteers, 12 patients with chest wall disease, 14 patients with usual interstitial pneumonia, and 17 patients with neuromuscular diseases were studied. Pi, Pes, and mean transdiaphragmatic pressure were simultaneously measured. Tension-time indexes of diaphragm (TTdi) and inspiratory muscles (TTmu) were also determined. In neuromuscular patients, significant correlations were found between Pi and Pes, Pi and transdiaphragmatic pressure, and TTmu and TTdi. A moderate agreement between Pi and Pes and between TTmu and TTdi was found. No significant correlation between these parameters was found in the other patient groups. These findings suggest that Pi is a good surrogate for the invasive measurement of respiratory muscle effort during spontaneous breathing in neuromuscular patients.

Pulmonary complications of chronic neuromuscular diseases and their management

Chronic neuromuscular diseases may affect all major respiratory muscles groups including inspiratory, expiratory, and bulbar, and respiratory complications are the major cause of morbidity and mortality. Untreated, many of these diseases lead inexorably to hypercapnic respiratory failure, precipitated in some cases by chronic aspiration and secretion retention or pneumonia, related to impairment of cough and swallowing mechanisms. Many measures are helpful including inhibition of salivation, cough-assist techniques, devices to enhance communication, and physical therapy. In addition, ventilatory assistance is an important part of disease management for patients with advanced neuromuscular disease. Because of its comfort, convenience, and portability advantages, noninvasive positive pressure ventilation (NPPV) has become the modality of first choice for most patients. Patients to receive NPPV should be selected using consensus guidelines, and initiation should be gradual to maximize the chances for success. Attention should be paid to individual preferences for interfaces and early identification of cough impairment that necessitates the use of cough-assist devices. For patients considered unsuitable for noninvasive ventilation, invasive mechanical ventilation should be considered, but only after a frank but compassionate discussion between the patient, family, physician, and other caregivers.

Pulmonary function testing in neuromuscular disease

Pulmonary function testing is useful in the diagnosis and management of patients with neuromuscular disease. It is important, however, to keep in mind that certain tests commonly used to assess these patients, such as MIPs and MEPs, although useful, are fraught with potential error and rigorous attention should be paid to technical details when performing them. In addition, many studies have shown that pulmonary impairment does not always parallel generalized muscle impairment and thorough testing therefore should be done in any patient with neuromuscular disease to assess the level of respiratory compromise accurately. In addition, the clinician should be aware that the pattern of involvement-bulbar versus inspiratory, versus expiratory muscle weakness-may vary markedly among patients, even with the same diagnosis, so testing should be tailored to detect these patterns. Furthermore, serial follow-up examinations should be performed to track the rate of deterioration so that therapeutic interventions can be initiated before respiratory crises occur.

Lung function measurements in young children with spinal muscle atrophy; a cross sectional survey on the effect of position and bracing

BACKGROUND

Spinal muscular atrophy (SMA) affects respiratory muscles, which in addition to progressive scoliosis leads to respiratory impairment. Children with developing scoliosis are usually treated with spinal bracing to delay the progress.

AIMS

To assess the impact of body position and application of spinal bracing on lung function during tidal breathing in children with SMA.

METHODS

Lung function was determined by tidal flow volume loops and passive respiratory mechanics (single breath occlusion technique) in all eight children in southern Norway with SMA type I and II, in both the sitting and supine position. Additional measurements were performed with and without bracing in five children. Muscle strength was assessed by the Brooks scale.

RESULTS

Tidal expiratory volume (V(E)) and compliance of the respiratory system (CRS) tended to be higher in the sitting compared to the supine position, but this was not statistically significant. However, applying bracing in the sitting position significantly reduced V(E). The highest values of CRS and V(E) were found in the sitting position without bracing.

CONCLUSION

Impairment of tidal respiration must be considered when applying spinal bracing in very young children developing scoliosis with SMA.

Response to influenza immunisation during treatment for cancer

AIMS

To assess the annual risk of influenza infection in children with cancer and the immunogenicity of a trivalent split virus influenza vaccine in these children.

METHODS

Eighty four children with cancer were tested for susceptibility to the circulating strains of influenza virus in autumn 1995 and 1996. Non-immunised children were reassessed the following spring for serological evidence of natural infection. Forty two patients received two doses of influenza vaccine. These children were receiving continuing chemotherapy for acute lymphoblastic leukaemia or were within six months of completing chemotherapy.

RESULTS

Among the 84 children tested for influenza virus susceptibility only 8% of patients were fully protected (antibody titres >/= 40) against all three of the prevalent influenza virus strains; 33% were susceptible to all three viruses. Evidence of acquired natural infection was seen in 30% of unimmunised patients. Among immunised susceptible patients, 66% made some protective response to the vaccine and 55% showed protective antibody titres to all three viral strains following vaccination. Older age was associated with increased response to the H1N1 and H3N2 vaccine components, but total white cell count or neutrophil count at immunisation, type of cancer, or length of time on treatment for acute lymphoblastic leukaemia did not affect response.

CONCLUSIONS

Most children with cancer studied were at risk of influenza infection. A significant response to immunisation was seen, supporting annual influenza vaccination for children being treated for cancer.

A standardized method for the evaluation of respiratory muscle endurance in patients with Duchenne muscular dystrophy

The aim of the study was to develop a standardized method using controlled breathing to quantify respiratory muscle endurance in children with Duchenne muscular dystrophy (DMD) and to test its reproducibility. In 10 DMD patients, all between 10 and 14 years (mean age, 11.5 +/- 1.5 years), except for two patients of 20 and 22 years, and 10 healthy children (mean age, 12 +/- 1 years), we measured the maximal time (Tlim) that a threshold load fixed at 35% of the individual maximal inspiratory pressure (Pimax) could be tolerated. We asked the children to maintain their rest breathing pattern until exhaustion using visual feedback and an auditory signal. The mean Tlim in the DMD children was 4.45 +/- 1.45 min and values were reproducible. All healthy children were able to obtain Tlim values greater than 30 min. The respiratory muscles of DMD children are more susceptible to fatigue than those of healthy subjects. This method should be satisfactory for estimating the effect of treatment and for the specific training of respiratory muscles in DMD patients without significant learning disability.

Pursed lips breathing improves ventilation in myotonic muscular dystrophy

OBJECTIVE

To determine the effects of pursed lips breathing on ventilation, chest wall mechanics, and abdominal muscle recruitment in myotonic muscular dystrophy (MMD).

DESIGN

Before-after trial.

SETTING

University hospital pulmonary function laboratory.

PARTICIPANTS

Eleven subjects with MMD and 13 normal controls.

INTERVENTION

Pursed lips breathing.

OUTCOME MEASURES

Electromyographic (EMG) activity of the transversus abdominis, external oblique, internal oblique, and rectus abdominis was recorded with simultaneous measures of gastric pressure, abdominal plethysmography, and oxygen saturation. Self-reported sensations of dyspnea, respiratory effort, and fatigue were recorded at the end of each trial.

RESULTS

Pursed lips breathing and deep breathing led to increased tidal volume, increased minute ventilation, increased oxygen saturation, reduced respiratory rate, and reduced endexpiratory lung volume. Dyspnea, respiratory effort, and fatigue increased slightly with pursed lips breathing. EMG activity of the transversus abdominis and internal oblique muscles increased in MMD only and was associated with an increase in gastric pressure.

CONCLUSIONS

Pursed lips breathing and deep breathing are effective and easily employed strategies that significantly improve tidal volume and oxygen saturation in subjects with MMD. Abdominal muscle recruitment does not explain the ventilatory improvements, but reduced end-expiratory lung volume may increase the elastic recoil of the chest wall. Further clinical studies are needed to ascertain if the ventilatory improvements with pursed lips breathing and deep breathing improve pulmonary outcomes in MMD.

Pulmonary manifestations of neuromuscular disease with special reference to Duchenne muscular dystrophy and spinal muscular atrophy

Deterioration of respiratory function in patients with neuromuscular disorders is primarily responsible for the high mortality associated with these diseases. A review of Duchenne muscular dystrophy and spinal muscular atrophy, the leading neuromuscular disorders affecting children, will be followed by a critical analysis of the various pathophysiological mechanisms underlying respiratory manifestations in these patients. Among such mechanisms, the role of muscular weakness in preservation of lung function, mucociliary clearance, gas exchange at rest and during exercise, and respiratory control during wakefulness and sleep will be examined in detail. In addition, the potential benefits of respiratory muscle training and of early diagnosis and clinical intervention will be delineated. This review underscores the importance of periodic assessment of pulmonary function during wakefulness and sleep in children affected by neuromuscular diseases as an essential component of multidisciplinary care aimed at improving long-term morbidity, survival, and quality of life.

Mechanism of CO(2) retention in patients with neuromuscular disease

BACKGROUND

In many studies of patients with muscle weakness, chronic hypercapnia has appeared to be out of proportion to the severity of muscle disease, indicating that factors other than muscle weakness are involved in CO(2) retention. In patients with COPD, the unbalanced inspiratory muscle loading-to-strength ratio is thought to trigger the signal for the integrated response that leads to rapid and shallow breathing and eventually to chronic hypercapnia. This mechanism, although postulated, has not yet been assessed in patients with muscular dystrophy.

SUBJECTS

Twenty consecutive patients (mean age, 47.6 years; range, 23 to 67 years) were studied: 11 patients with limb-girdle dystrophy, 3 with Duchenne muscular dystrophy, 1 with Charcot-Marie-Tooth syndrome, 1 with Becker muscular dystrophy, 1 with myotonic dystrophy, 1 with facioscapulohumeral dystrophy, and 2 with amyotrophic lateral sclerosis, without any respiratory complaints. Seventeen normal subjects matched for age and sex were studied as a control group.

METHODS

Routine spirometry and arterial blood gases, maximal inspiratory and expiratory muscle pressures (MIP and MEP, respectively), and pleural pressure during maximal sniff test (Pplsn), were measured. Mechanical characteristics of the lung were assessed by evaluating lung resistance (RL) and dynamic elastance (Eldyn). Eldyn was assessed as absolute value and as percent of Pplsn; Eldyn (%Pplsn) indicates the elastic load per unit of inspiratory muscle force. Breathing pattern was assessed in terms of time (inspiratory time [TI]; respiratory frequency [Rf]) and volume (tidal volume [VT]) components of the respiratory cycle.

RESULTS

A rapid shallow breathing pattern, as indicated by a greater Rf/VT ratio and a lower TI, was found in study patients compared to control subjects. Eldyn was greater in study patients, while MIP, MEP, and Pplsn were lower. PaCO(2) inversely related to VT, TI, and Pplsn (p = 0.012, p = 0.019, and p = 0.002, respectively), whereas it was directly related to Rf, Rf/VT, Eldyn, and Eldyn (%Pplsn) (p < 0.004 to p < 0.0001). Also Eldyn (%Pplsn) inversely related to TI, and the latter positively related to VT. In other words, increase in Eldyn (%Pplsn) was associated with decrease in TI, and the latter was associated with lower VT and greater PaCO(2). Mechanical and breathing pattern variables were introduced in a stepwise multiple regression that selected Eldyn (%Pplsn) (p < 0.0001; r(2) = 0.62) as a unique independent predictor of PaCO(2).

CONCLUSIONS

The present study shows that in patients with neuromuscular disease, elastic load and respiratory muscle weakness are responsible for a rapid and shallow breathing pattern leading to chronic CO(2) retention.

The control of breathing in clinical practice

The control of breathing results from a complex interaction involving the respiratory centers, which feed signals to a central control mechanism that, in turn, provides output to the effector muscles. In this review, we describe the individual elements of this system, and what is known about their function in man. We outline clinically relevant aspects of the integration of human ventilatory control system, and describe altered function in response to special circumstances, disorders, and medications. We emphasize the clinical relevance of this topic by employing case presentations of active patients from our practice.

Respiratory muscle training in neuromuscular disease: long-term effects on strength and load perception

PURPOSE

Deterioration of respiratory muscle function in patients with neuromuscular disorders is primarily responsible for the high morbidity and mortality associated with these diseases.

METHODS

The potential benefit of respiratory muscle training (RMT) on preservation of respiratory muscle strength and respiratory load perception (RLP) was examined in 21 children (mean age: 12.2 +/- 1.8 yr [SD], 16 male) with Duchenne's muscular dystrophy or spinal muscular atrophy type III, and in 20 age-, weight-, and sex-matched controls. Subjects were randomly allocated to undergo incremental RMT with resistive inspiratory and expiratory loads for a period of 6 months (trained group, TR) or to perform similar exercises with no load (NT). Maximal static inspiratory (Pi(max)) and expiratory (Pe(max)) pressures and RLP (modified Borg visual analog scale 0-10) were assessed on two separate occasions before beginning of the training protocol, monthly throughout RMT duration, and every 3-6 months upon cessation of RMT for 1 yr.

RESULTS

In controls, no significant changes in maximal static pressures or load perception occurred during RMT or thereafter. Training in neuromuscular disorder (NMD) patients was associated with improvements in Pi(max) (mean delta max: +19.8 +/- 3.8 cmH2O in TR vs +4.2 +/- 3.6 cmH2O in NT; P < 0.02) and in Pe(max) (mean delta max: +27.1 +/- 4.9 cmH2O in TR vs -1.8 +/- 3.4 cmH2O in NT; P < 0.004). Similarly, RLP significantly decreased during the RMT period in TR (mean delta: 1.9 +/- 0.3; P < 0.01) but did not change in NT (-0.2 +/- 0.2). In addition, with cessation of RMT, static pressures returned to pretraining values in TR within approximately 3 months. However, RLP was still improved after 12 months.

CONCLUSIONS

We conclude that in children with NMD, although RMT-induced increases in expiratory muscle strength are rapidly reversible, long-lasting improvements in RLP occur and could be associated with decreased respiratory symptoms.

A technique for global assessment of respiratory muscle performance at different lung volumes

A system for noninvasive assessment of an all-inclusive function of respiratory muscles at different lung volumes is presented. The apparatus was based on the interrupter technique and facilitated simultaneous measurements of mouth pressure and airflow rate during dynamic or quasistatic manoeuvres. In this study, mouth pressure values were continuously acquired during and after interruption of a forced inspiratory or expiratory manoeuvre for as long as the subject could sustain an elevated mouth pressure against the obstructed opening. These measurements provided information on both muscle strength and power. A total of 420 forced maximal inspiratory and expiratory manoeuvres performed by six healthy subjects were monitored at different lung volumes. The pattern of maximal pressure-time curves was consistent for the same subject regardless of lung volume. Similar values of maximal mouth pressure can be generated by healthy subjects by using either a flange-style mouthpiece or facial mask. For both methods mouth pressure shows a significant (p < 0.05) second order dependency on lung volume for both inspiration and expiration. The standard deviation of measurements from a single subject about a second order curve is of the order of 5-15%. The findings of interchangeability between methods of measurement may be useful in allegedly non-compliant patients.

Hypoventilation syndromes

In summary, alveolar hypoventilation can be associated with a diverse group of disorders, collectively referred to as the hypoventilation syndromes. Most have associated hypercapnia and hypoxemia while awake, with a significant worsening in gas exchange during sleep. In some disorders, gas exchange abnormalities are manifested only during periods of sleep. Signs and symptoms suggestive of the underlying disorder leads one to investigate for associated hypoventilation. Proper diagnosis allows the implementation of appropriate therapy, which may both improve gas exchange and associated symptoms, and impact overall survival.

Evaluation and treatment of respiratory failure in neuromuscular disease

Respiratory failure is a relatively uncommon feature of most neuromuscular disease. It accounts for most of the morbidity and mortality associated with these disorders, however. In most cases, aggressive supportive care, specific immunologic treatment, and treatment of associated medical conditions result in improvement in respiratory function and a favorable outcome.

Myasthenia gravis and upper airway obstruction

Respiratory impairment in myasthenia gravis is usually attributed to weakness of the diaphragm and thoracic chest wall muscles, and is rarely attributed to upper airway obstruction. Myasthenia gravis is characterized by weakness of the striated muscles and usually affects those innervated by the bulbar cranial nerves. Weakness of these bulbar and upper airway muscles can lead to upper airway obstruction. To our knowledge, there are only five case reports in the literature associating upper airway obstruction with myasthenia gravis. Therefore, we attempted to further define its occurrence in myasthenia gravis patients by reviewing their flow volume loops. We present a case of upper airway obstruction causing respiratory symptoms in a myasthenia gravis patient. We then surveyed a total of 61 patients with myasthenia gravis who were tested in our pulmonary function laboratory between February 1990 and August 1993. Of these 61 patients, 12 had flow volume loops and 7 of these 12 disclosed a pattern of extrathoracic upper airway obstruction. The FVC was 80% or more in five of seven patients. Our data suggest that upper airway obstruction is much more common in patients with myasthenia gravis than previously recognized. In conclusion, we recommend the performance of flow volume loops in patients with myasthenia gravis to evaluate their respiratory impairment.