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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Inspiratory muscles can be exerted to their maximal limits during situations of: 1) high ventilatory demands, such as in exercise; and 2) during cases of high force demands, as in obstructive or restrictive diseases. In either circumstance, the level of sustainable activity (many hours) seems to be about half of the subject's maximal ventilatory capacity (MVC) or their maximal inspiratory pressure (MIP), respectively. The natural history of chronic hypercapnia in chronic obstructive pulmonary disease (COPD) or in neuromuscular disease suggests that spontaneous ventilation is set at a level below that which will trigger muscle fatigue, even if this lower level results in "chronic ventilatory failure". When this type of patient suffers a pathology that further decreases their global respiratory muscle function or increases their load, we have the makings of an unweanable patient; the mechanical ventilator ultimately replaces the lost inspiratory muscle function. Given time for the muscle to recover force and a reduction of the loads should, thus, be the therapeutic focus.
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PMID:The unweanable patient. 771 7

Neuromuscular diseases cause many changes that affect ventilation and ventilatory control. The pattern of ventilation may become abnormal because of muscle disease. Muscle fatigue and discordant breathing can lead to hypoventilation and CO2 retention. Motoneuron destructive and demyelinating disorders inevitably lead to hypoventilation and hypercapnia. Changes in chest wall mechanics can lead to changes in level of ventilation and ventilatory drive. In many neuromuscular disorders, ventilatory response to CO2 is depressed, but this does not imply an abnormal central control mechanism in all instances. Many patients with neuromuscular diseases have a normal ventilatory drive as manifested by a normal P0.1 but have low ventilation because of abnormalities in muscle function and neuromuscular transmission. Central drive is diminished in some patients with neuromuscular disease but not in the majority of cases. Hypoventilation during sleep is a common problem in neuromuscular diseases. Thus, a combination of factors can lead to abnormal patterns of breathing and hypoventilation in these disorders; no single pathophysiologic mechanism can explain all the abnormalities. Clinically, it is important to appreciate the prevalence of ventilatory control disorders and include appropriate evaluations when assessing patients with neuromuscular diseases and offering therapeutic options.
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PMID:Central control of ventilation in neuromuscular disease. 786 78

Weaning patients from mechanical ventilation constitutes a major portion of the workload in an intensive care unit, as over 40% of total ventilator time is consumed by the weaning process. Several pathophysiological mechanisms may be responsible for weaning failure, but the precise role of each is incompletely understood. Patients who fail a weaning trial commonly develop hypercapnia, which appears to be due to decreased tidal volume rather than a primary decrease in respiratory drive. Respiratory muscle performance is impaired as a result of dynamic hyperinflation and paradoxic motion of the rib cage and abdomen. Worsening of pulmonary mechanics will cause further embarrassment of the respiratory muscles. However, the clinical importance of respiratory muscle fatigue remains unclear. Afferent stimuli arising in the lung parenchyma, respiratory muscles, or as a consequence of impaired gas exchange will be transmitted to the respiratory control centers and result in severe dyspnea in patients who fail a weaning trial.
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PMID:Pathophysiology of failure to wean from mechanical ventilation. 799 29

The working ability in the course of hyperventilation can be divided into three stages: the adaptation, stable working ability, the fatigue. Hypoxia decreased the duration and intensity of voluntary hyperventilation. During hypercapnia, the hyperventilation is more intensive and shorter. The data obtained suggest a co-operation of one's own will and chemoreceptor stimuli under hypoxia and hypercapnia.
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PMID:[The dynamic function of the motor apparatus of the respiratory system during maximal voluntary hyperventilation under hypoxia and hypercapnia]. 816 96

To examine respiratory muscle recruitment pattern during inspiratory loading and role of fatigue in limiting endurance, we studied seven normal subjects on 17 +/- 6 days during breathing against progressive inspiratory threshold load. Threshold pressure (Pth) was progressively increased 14 +/- 5 cmH2O every 2 min until voluntary cessation (task failure). Subjects could adopt any breathing pattern. Tidal volume (VT), chest wall motion, end-tidal PCO2, and arterial O2 saturation were measured. At moderate loads [50-75% of maximum Pth (Pthmax)], inspiratory time (TI) decreased and VT/TI and expiratory time increased, increasing time for recovery of muscles between inspirations. At high loads (> 75% Pthmax), VT/TI decreased, which, with progressive decrease in end-expiratory lung volume (EELV) throughout, increased potential for inspiratory force development. Progressive hypoxia and hypercapnia occurred at higher work loads. Immediately after task failure all subjects could recover at high loads and still reachieve initial Pthmax on reimposition of progressive loading. Respiratory pressures were measured in subgroup of three subjects: transdiaphragmatic pressure response to 0.1-ms bilateral supramaximal phrenic nerve stimulation at end expiration initially increased with increasing load/decreasing EELV, consistent with increasing mechanical advantage of diaphragm, but decreased at highest loads, suggesting diaphragm fatigue. Full recovery had not occurred at 30 min after task failure. We demonstrated that progressive threshold loading is associated with systematic changes in breathing pattern that act to optimize muscle strength and increase endurance. Task failure occurred when these compensatory mechanisms were maximal. Inspiratory muscles appeared relatively resistant to fatigue, which was late but persistent.
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PMID:Ventilatory responses to inspiratory threshold loading and role of muscle fatigue in task failure. 817 4

The mechanisms of chronic ventilatory failure in chronic obstructive pulmonary disease are complex. This paper analyses the diverse available information: mechanical factors and gas-exchange, fighter vs. non-fighter, the ventilatory pattern theory and the fatigue threshold theory. Finally we comment on the evidence supporting the new concept that hypercapnia may develop to avoid or prevent fatigue. Indeed, it is very likely that chronic CO2 retention in COPD may develop by mechanical disadvantages of the inspiratory muscles rather than impairment of ventilation-perfusion ratios. This opens a fascinating new research line on the neuromechanical control of breathing. When the respiratory effort is approaching the fatigue level, the respiratory muscles may elicit a negative feedback reflex, the muscle activity is depressed and hypercapnia develops. If this is so, chronic hypercapnia may be an index of imminent fatigue if increases in ventilation or work of breathing are required. Under this condition some degree of central diaphragm fatigue may help to protect the muscle from severe or limiting peripheral fatigue or even muscle injury. Finally, we comment on some therapeutic approaches such as ventilatory stimulants, training, rest and, specially, oxygen administration and the mechanisms involved in the PCO2 increases.
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PMID:[Causes of CO2 retention in patients with chronic obstructive lung disease]. 820 18

1. The physiological basis of inspiratory effort sensation remains uncertain. Previous studies have suggested that pleural pressure, rather than inspiratory muscle fatigue, is the principal determinant of inspiratory effort sensation. However, only a limited range of inspiratory flows and breathing patterns have been examined. We suspected that inspiratory effort sensation was related to the inspiratory muscle tension-time index developed whatever the breathing pattern or load, and that this might explain the additional rise in sensation seen with hypercapnia. 2. To investigate this we measured hypercapnic rebreathing responses in seven normal subjects (six males, age range 21-38 years) with and without an inspiratory resistive load of 10 cm H2O. Pleural and transdiaphragmatic pressures, mouth occlusion pressure and breathing pattern were measured. Diaphragmatic and ribcage tension-time indices were calculated from these data. Inspiratory effort sensation was recorded using a Borg scale at 30s intervals during each rebreathing run. 3. Breathing pattern and inspiratory pressure partitioning were unrelated to changes in inspiratory effort sensation during hypercapnia. Tension-time indices reached pre-fatiguing levels during both free breathing and inspiratory resistive loading. 4. Stepwise multiple regression analysis using pooled mechanical, chemical and breathing pattern variables showed that pleural pressure was more closely related to inspiratory effort sensation than was transdiaphragmatic pressure.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Physiological determinants of inspiratory effort sensation during CO2 rebreathing in normal subjects. 828 53

Since activity of the genioglossus muscle plays a primary role in maintaining upper airway patency during sleep, its strength and endurance characteristics are of potential importance. The purpose of this study was 2-fold. First, to define the strength and endurance characteristics of the normal human genioglossus. Second, we hypothesized that because the genioglossus has a high proportion of fast glycolytic muscle fibers, brief periods of increased activity would make it more susceptible to fatigue. In five normal male subjects strength of the tongue was evaluated by measuring maximal anterior force using a transducer (Fmax). In each subject tongue endurance was then tested at 100%, 80%, and 50% Fmax. To test the effect of a short-term increase in genioglossal activity on its endurance, an inspiratory flow-resistive load with mild hypercapnia was presented to the upper airway for 10 min, after which genioglossal endurance at 80% Fmax was repeated. On a separate day the effect of inspiratory loading plus hypercapnia on thoracic inspiratory muscle endurance was also tested. Our results showed that mean Fmax was 1,267 +/- 125 (SEM) g. Endurance time (Tlim) decreased progressively during 50%, 80% and 100% Fmax trials. Short-term activation of the genioglossus caused a reduction in Tlim at 80% Fmax to 51.4 +/- 4.8% of its value before loading (p < 0.05). Tlim for the inspiratory muscles, however, was unaffected. We conclude that, like other skeletal muscles, genioglossal endurance is reduced as the force of contraction increases. In addition, genioglossal endurance is significantly reduced by short-term activation insufficient to fatigue the thoracic inspiratory muscles.
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PMID:Strength and endurance characteristics of the normal human genioglossus. 831 95

A 68-year-old man with severe dyspnea was admitted as an emergency case. He had no past history of any respiratory or neuromuscular diseases. Immediately after insufflation of oxygen, respiratory arrest occurred. The blood gas analysis showed hypoxemia and severe hypercapnia (PaO2; 32 mmHg, PaCO2; 127 mmHg). We diagnosed as CO2 narcosis, and he was treated with a respirator in the ICU. He showed nonflaccid bilateral diaphragmatic paralysis and muscle atrophy of the upper extremities. As the EMG showed giant spikes of neurogenic pattern, he was diagnosed as ALS. Weaning from the respirator failed because of his respiratory muscle fatigue. He was given rehabilitation during the day time and ventilatory support with the respirator during the night. We conclude that if we meet with an emergency patient with CO2 narcosis without any pulmonary disorder, we have to suspect neuromuscular diseases, e.q. ALS. In some of such cases, mechanical ventilation supports social rehabilitation.
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PMID:[A case of emergency admission for CO2 narcosis in a patient with amyotrophic lateral sclerosis]. 852 59

The most attractive feature of nuclear magnetic resonance spectroscopy (MRS) is the noninvasive and nondestructive measurement of chemical compounds in intact tissues. MRS already has many applications in comparative physiology, usually based on observation of 31P, since levels of phosphorus compounds indicate tissue energy status and are changed during exercise, fatigue, recovery, hypometabolism, anesthesia, hypoxia, hypercapnia, and osmotic and acid stress. Nuclei other than 31P may also be monitored, such as 1H, 13C, 15N, 19F, or 23Na, and applied in biological research. Particularly, 13C-MRS is interesting because it allows the analysis of metabolic pathways in living systems. Applications of MRS in comparative physiology and biochemistry are comprehensively discussed in this review. The main focus is on anaerobic metabolism during hypoxia, ischemia, and exercise. Species as widely different as slime molds, nematodes, frogs, turtles, and ducks have been studied by 31P-MRS. It is not surprising that striking species differences do occur, but many similarities are also observed. Unique is the occurrence of six different phosphagens with different values of Gibbs free energy in polychete worms The presence of a particular phosphagen may be related to the average oxygen tension within the tissues. Phosphagens and their kinases are also discussed in relation to hypercapnia and acid stress. Other topics discussed in this paper are enzyme kinetics, anesthetics, development and growth, parasitism, and the detection of previously unknown compounds.
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PMID:Nuclear magnetic resonance spectroscopy of living systems: applications in comparative physiology. 875 89


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