Respiratory muscle fatigue

Short term evaluation of respiratory effort by premature infants supported with bubble nasal continuous airway pressure using Seattle-PAP and a standard bubble device

Background

Almost one million prematurely born infants die annually from respiratory insufficiency, predominantly in countries with limited access to respiratory support for neonates. The primary hypothesis tested in the present study was that a modified device for bubble nasal continuous positive airway pressure (Bn-CPAP) would provide lower work of spontaneous breathing, estimated by esophageal pressure-rate products.

Methods

Infants born <32 weeks gestation and stable on Bn-CPAP with FiO₂ <0.30 were studied within 72 h following delivery. Esophageal pressures during spontaneous breathing were measured during 2 h on standard Bn-CPAP, then 2 h with Bn-CPAP using a modified bubble device presently termed Seattle-PAP, which produces a different pattern of pressure fluctuations and which provided greater respiratory support in preclinical studies, then 2 h on standard Bn-CPAP.

Results

All 40 infants enrolled completed the study and follow-up through 36 wks post menstrual age or hospital discharge, whichever came first. No infants were on supplemental oxygen at completion of follow-up. No infants developed pneumothoraces or nasal trauma, and no adverse events attributed to the study were observed. Pressure-rate products on the two devices were not different, but effort of breathing, assessed by areas under esophageal pressure-time curves, was lower with Seattle-PAP than with standard Bn-CPAP.

Conclusion

Use of Seattle-PAP to implement Bn-CPAP lowers the effort of breathing exerted even by relatively healthy spontaneously breathing premature neonates. Whether the lower effort of breathing observed with Seattle-PAP translates to improvements in neonatal mortality or morbidity will need to be determined by studies in appropriate patient populations.

[Respiratory pump failure. Clinical symptoms, diagnostics and therapy]

The total anatomical and functional apparatus which allows normal ventilation of the lungs is known as the respiratory pump. An insufficiency of this system, which can be caused by a multitude of reasons, primarily affects the inspiratory musculature and especially the diaphragm. One of the essential clinical characteristics is rapid shallow breathing. Exhaustion of the repiratory musculature due to acute respiratory insufficiency is normally clinically registered but can also be functionally determined in particular by the maximum static inspiratory closed mouth pressure. A further option is invasive measurement of the transdiaphragmal pressure, which however is not suitable as a routine procedure. Mechanical ventilation is used as treatment of respiratory pump insufficiency independent of the cause. This is initially a non-invasive procedure but if unsuccessful intubation and invasive ventilation are indicated. The technical developments in the field of extracorporeal gas exchange systems are very promising. However, in view of the insufficient data, ventilation procedures using masks and tubes still remain the first choice methods.

Respiratory muscle function and activation in chronic obstructive pulmonary disease

Inspiratory muscles are uniquely adapted for endurance, but their function is compromised in chronic obstructive pulmonary disease (COPD) due to increased loads, reduced mechanical advantage, and increased ventilatory requirements. The hyperinflation of COPD reduces the flow and pressure-generating capacity of the diaphragm. This is compensated by a threefold increase in neural drive, adaptations of the chest wall and diaphragm shape to accommodate the increased volume, and adaptations of muscle fibers to preserve strength and increase endurance. Paradoxical indrawing of the lower costal margin during inspiration in severe COPD (Hoover's sign) correlates with high inspiratory drive and severe airflow obstruction rather than contraction of radially oriented diaphragm fibers. The inspiratory muscles remain highly resistant to fatigue in patients with COPD, and the ultimate development of ventilatory failure is associated with insufficient central drive. Sleep is associated with reduced respiratory drive and impairments of lung and chest wall function, which are exaggerated in COPD patients. Profound hypoxemia and hypercapnia can occur in rapid eye movement sleep and contribute to the development of cor pulmonale. Inspiratory muscles adapt to chronic loading with an increased proportion of slow, fatigue-resistant fiber types, increased oxidative capacity, and reduced fiber cross-sectional area, but the capacity of the diaphragm to increase ventilation in exercise is compromised in COPD. In COPD, neural drive to the diaphragm increases to near maximal levels in exercise, but it does not develop peripheral muscle fatigue. The improvement in exercise capacity and dyspnea following lung volume reduction surgery is associated with a substantial reduction in neural drive to the inspiratory muscles.

Modulation of the actomyosin interaction during fatigue of skeletal muscle

Fatigue of skeletal muscle involves many systems beginning with the central nervous system and ending with the contractile machinery. This review concentrates on those factors that directly affect the actomyosin interaction: the build-up of metabolites; myosin phosphorylation; and oxidation of the myofibrillar proteins by free radicals. The decrease in [ATP] and increase in [ADP] appear to play little role in modulating function. The increase in phosphate inhibits tension. The decrease in pH, long thought to be a major factor, is now known to play a more minor role. Myosin phosphorylation potentiates the force achieved in a twitch, and a further role in inhibiting velocity is proposed. Protein oxidation can both potentiate and inhibit the actomyosin interaction. It is concluded that these factors, taken together, do not fully explain the inhibition of the actomyosin interaction observed in living fibers, and thus additional modulators of this interaction remain to be discovered.

Fatigue and respiratory disorders

OBJECTIVES

To analyze the factors at the origin of fatigue in respiratory disorders. To assess fatigue and its functional impact on patients affected from respiratory diseases. To evaluate the results of comprehensive care on fatigue and functional capacity.

MATERIALS AND METHODS

We systematically reviewed the literature in Medline and the Cochrane Library, using the following keywords: fatigue, respiratory disorders, questionnaire, evaluation, assessment, randomized controlled trial, meta-analysis.

RESULTS

Fatigue is a high frequency symptom (90%) and takes an important place, as much as dyspnea, in the genesis of the respiratory induced handicap. Its assessment is varied, according to the studies. It originates from multiple causes, as shown from clinical and experimental studies. The main treatment consists in rehabilitation, using physical exercises. Its efficacy is demonstrated on physical endurance, but is not clear in terms of general fatigue.

CONCLUSION

Although fatigue is very frequent complaint, along with a major disabling condition, the comprehensive assessment of fatigue, in respiratory disorders, including its physical and cognitive components, is not still really codified. Rehabilitation is the main treatment. Its efficiency has been demonstrated on the physical and functional components of fatigue. Its results on perceived fatigue remains to be evaluated.

Acute respiratory failure from abused substances

Acute respiratory failure is a common complication of drug abuse. It is more likely to develop in the setting of chronic lung disease or debility in those with limited respiratory reserve. Drugs may acutely precipitate respiratory failure by compromising respiratory pump function and/or by causing pulmonary pathology. Polysubstance overdoses are common, and clinicians should anticipate complications related to multiple drugs. Impairment of respiratory pump function may develop from central nervous system (CNS) depression (suppression of the medulla oblongata, stroke or seizures) or respiratory muscle fatigue (increased respiratory workload, metabolic acidosis). Drug-related respiratory pathology may result from parenchymal (aspiration-related events, pulmonary edema, hemorrhage, pneumothorax, infectious and non-infectious pneumonitides), airway (bronchospasm and hemorrhage), or pulmonary vascular insults (endovascular infections, hemorrhage, and vasoconstrictive events). Alcohol, cocaine, amphetamines, opiates, and benzodiazepines are the most commonly abused drugs that may induce events leading to acute respiratory failure. While decontamination and aggressive supportive measures are indicated, specific therapies to correct seizures, metabolic acidosis, pneumothorax, infections, bronchospasm, and agitation should be considered. Drug-related respiratory failure when due to CNS depression alone may portend well, but in patients with drug-related significant pulmonary pathology, a protracted course of illness may be anticipated.

Respiratory muscle fatigue

The contribution of respiratory muscle fatigue to the development of ventilatory failure has been the subject of considerable interest and has stimulated much research. Experimental studies in dogs have shown respiratory muscle fatigue to be a cause of ventilatory failure in both cardiogenic and septic shock models. In clinical conditions resulting in acute or chronic hypercapnia, respiratory muscle fatigue is believed to occur; however, the specific role of fatigue has been difficult to prove.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Human respiratory muscles: sensations, reflexes and fatiguability

1. Given the importance of the ventilatory 'pump' muscles, it would not be surprising if they were endowed with both sensory and motor specializations. The present review focuses on some unexpected properties of the respiratory muscle system in human subjects. 2. Although changes in blood gas tension were long held not to influence sensation directly, studies in subjects who are completely paralysed show that increases in arterial CO2 levels elicit strong sensations of respiratory discomfort. 3. Stretch reflexes in human limb muscles contain a monosynaptic spinal excitation and a long-latency excitation. However, inspiratory muscles show an initial inhibition when tested with brief airway occlusions during inspiration. This inhibition does not depend critically on input from pulmonary or upper airway receptors. 4. Human inspiratory muscles (including the diaphragm) have been considered to fatigue during inspiratory resistive loading. However, recent studies using phrenic nerve stimulation to test the force produced by the diaphragm show that carbon dioxide retention (hypoventilation) and voluntary cessation of loading occur before the muscles become overtly fatigued.