OVERVIEW
Respiratory muscle fatigue is the inability of the diastolic activity of the respiratory muscles, due to a variety of causes, to generate the thoracic pressure required to maintain a given volume of alveolar ventilation. The normal muscle itself may show a decrease in muscle strength and/or contractile velocity, but this decrease in capacity can be restored with rest. If it does not recover with rest it is called respiratory muscle weakness. The problem of respiratory muscle fatigue or failure has not been taken seriously for a long time, and timely detection and treatment of respiratory muscle fatigue can correct abnormal respiratory mechanics, improve oxygenation, and shorten the duration of mechanical ventilation. Therefore, in recent years, the evaluation of respiratory muscle function in critically ill patients has become one of the important contents of intensive care.
Etiology
1. Insufficient driving of respiratory center
Central lesions such as anterior horn cell degeneration of spinal cord, Guillain-Barre syndrome, coma, etc. can lead to respiratory muscle dysfunction.
2. Neuromuscular diseases
Phrenic nerve injury, neuromuscular junction conduction disorders (e.g. myasthenia gravis), muscle diseases (e.g. progressive muscular atrophy, etc.) can cause respiratory muscle weakness and fatigue.
3. Changes in the initial length and shape of muscles
In emphysema patients, the thorax is lifted up, the diaphragm is low, the muscles are in a shorter initial length state, and the contraction force is decreased. From the residual air position to the total lung position, the external intercostal muscles and auxiliary inspiratory muscles shorten by no more than 20%, while the diaphragm shortens by up to 40%. Therefore, emphysema has a great impact on diaphragm muscle strength.
4. Increased respiratory muscle load
Thoracic and pulmonary diseases cause increased airway resistance, decreased lung compliance, exercise, fever and other factors increase energy metabolism to increase ventilation, can increase the respiratory muscle load, resulting in respiratory muscle fatigue.
5. Insufficient energy supply
Respiratory muscle fatigue can occur when the energy consumption of the respiratory muscle is greater than its energy supply and/or when lactic acid accumulates and blood pH decreases. Such as cardiac insufficiency, anemia, shock, hypoxemia, due to the respiratory muscle blood flow and energy supply is reduced, respiratory muscle fatigue can occur, oxygen can improve the fatigue symptoms.
6.Mechanical ventilation
Mechanical ventilation can replace or assist the respiratory muscle work, so that the fatigue of the respiratory muscle rest, but prolonged mechanical ventilation can lead to respiratory muscle wasting atrophy, so that the respiratory muscle strength and endurance are reduced, resulting in ventilator dependence.
7. Metabolic factors
Hypoxia, hypercapnia, hypocalcemia, hypomagnesemia, hypophosphatemia, hypothyroidism or hyperthyroidism, prolonged or massive use of corticosteroid, drug-induced myasthenia gravis syndrome, etc. can lead to the decrease of respiratory muscle strength.
8. Role of oxygen free radicals
Under certain pathophysiological states (e.g., hypoxia, toxemia, oxidative stress, etc.), the production of reactive oxygen products increases. Reactive oxygen products: such as hydrogen peroxide, superoxide anion radicals, free hydroxyl radicals, etc. are all contributing factors to diaphragmatic fatigue.
Symptoms.
The most common clinical manifestation is dyspnea, which is often aggravated or alleviated with positional changes.
1. Loss of up-and-down movement of the upper ribs during inspiration, with the rib margins moving inward rather than outward.
2. The abdominal wall moves inward during inspiration.
3. Alternation between chest breathing and abdominal breathing.
4. accelerated respiratory rate, respiratory asynchrony (e.g., periodic abdominal and chest pressure respiration alternating with misalignment and non-parallel abdominal pressure respiration, bimodal rows of respiratory movements in the abdomen), and contradictory respiration in the chest and abdomen.
Examination
X-ray observation of decreased amplitude of diaphragmatic movement.
Decrease in lung function measurements.
1. Laboratory examination
(1) Maximum inspiratory pressure (MIP) is the maximum inspiratory oral pressure that can be produced by maximal inspiratory effort when the airway is blocked in the residual ventilatory position (RV) or functional residual ventilatory position (FRC).The main clinical significance of MIP is: ① evaluate the function of the inspiratory muscles in the case of neuromuscular diseases, and provide references for diagnosis of the disease and judgment of the seriousness of the disease, and it is easy to develop respiratory failure when the MIP is less than 30% of the normal expected value. When MIP＜30% of the normal expected value, it is easy to have respiratory failure; ② Evaluation of respiratory muscle function in patients with pulmonary disease (COPD), thoracic deformity and drug intoxication; ③ Used to predict the withdrawal of the machine, it is generally believed that the possibility of successful withdrawal is large when the MIP＜-30cmH20. However, the false-negative rate is very high when MIP is used to predict withdrawal, mainly because the patient cannot cooperate well during measurement.
(2) Transdiaphragmatic pressure (Pdi) Pdi is the pressure difference between the thoracic and abdominal sides of the diaphragm during diaphragmatic contraction and represents the contractile capacity of the diaphragm. Maximum transdiaphragmatic pressure (Pdimax) refers to the maximum value of Pdi produced when inspirating with maximum effort under the state of functional residual airway obstruction (or residual airway position).Pdimax reflects the pressure produced when the diaphragm makes maximum contraction, and it is a reliable index for evaluating respiratory muscle strength. Pdi and Pdimax are both lower when the diaphragm is fatigued, and diaphragm fatigue is indicated when Pdi cannot be maintained at 40% of the Pdimax level. The method of determining trans-diaphragmatic pressure is more complicated, which needs to be measured by esophageal and gastric balloon respectively, and the difference between intra-esophageal pressure and intra-gastric pressure during the inspiratory phase is Pdi.
(3) Diaphragm tension-time index (TTdi) This index is a good indicator of respiratory muscle endurance. It is more important to evaluate endurance than strength for respiratory muscles. Muscle endurance depends on the energy supply, the composition of muscle fibers, and the amount of work done. The amount of work done depends on the force and duration of the muscle contraction. The strength of the diaphragm varies greatly from one individual to another, and in order to minimize individual differences, the ratio of the average value of Pdi and the ratio of Pdimax produced by diaphragmatic contraction is used to reflect the strength of the contraction, and the ratio of the inspiratory time (Ti) to the total time of the respiratory cycle (Ttot) reflects the duration of diaphragmatic contraction, and the product of the two is TTdi. In the presence of inspiratory resistance load, diaphragm fatigue is not likely to occur when the TTdi value is <0.15, and the time to diaphragm fatigue will be significantly shortened when the TTdi value is >0.15. It should be noted that the determination of TTdi is accomplished with an artificially set resistance, and there may be a large discrepancy with voluntary breathing. Therefore, how to determine the threshold of respiratory muscle fatigue in different disease states needs to be further explored.
2. Other auxiliary examinations
(1) Diaphragm electromyography (EMG) can be used to detect the electrophysiologic activity of the diaphragm, intercostal muscles and abdominal muscles. However, it is difficult to perform EMG routinely during mechanical ventilation in critically ill patients, and there are many interfering factors during the examination, so the reproducibility and accuracy of the results are poor. The percutaneous puncture electrode, which passes a fine needle through the skin to the diaphragm, is more accurate and reliable than the percutaneous electrode, and the EMG consists of different frequencies, and its spectrum is mainly between 20 and 250 Hz, and the change in spectral distribution is an early manifestation of fatigue, preceded by a decrease in muscle strength. A 20% decrease in the H/L ratio from the basal value indicates a significant change in the frequency spectrum. The high-frequency component is caused by the accumulation of metabolically toxic substances in the muscle and the recovery period is short (a few minutes), while the low-frequency component is caused by the structural changes in the muscle and the recovery takes more than 24 hours. The presence of respiratory muscle fatigue can be detected early by dynamic observation of EMG. Clinically, an increase in the low-frequency component during withdrawal of mechanical ventilation suggests that at least 24 to 48 hours are needed to recover the contractile function of fatigued respiratory muscles.
(2) Phrenic nerve electrical stimulation method Phrenic nerve stimulation The contraction of the diaphragm is mainly innervated by the phrenic nerve, and the observation of Pdi or EMG after stimulation of the phrenic nerve with body surface or needle electrodes can reflect the function of the diaphragm. The advantage of this method is that it can objectively evaluate the contractile performance of the diaphragm and the mechanical characteristics of the chest wall, and it is not affected by the degree of voluntary effort or breathing mode. The disadvantages are the local pain of stimulation, the difficulty of accurate positioning of electrodes, especially when the patient is agitated, and the change of body position will affect the accuracy of the measurement.It is also difficult to accurately stimulate the phrenic nerve in patients with COPD and obesity if there is a proliferative hypertrophy of the sternocleidomastoid muscle. Therefore, diaphragmatic stimulation has been limited in its application to critically ill patients, and is mainly used in studies of patients with stabilized conditions. Recently, some people use electromagnetic stimulation of the phrenic nerve to study the function of the diaphragm, and found that magnetic stimulation and direct stimulation of the phrenic nerve can effectively stimulate the diaphragm contraction, which can overcome the shortcomings of the direct stimulation method, and its use in the study of the diaphragm function in critically ill patients has also achieved better results.
3.Related examination
Carbon monoxide, prothrombin time, functional residual air volume, residual air volume, total lung volume, and blood creatine phospholipid kinase test.
Diagnosis
Respiratory muscle fatigue can be diagnosed based on the history of underlying respiratory diseases, manifestations such as dyspnea, accelerated respiratory rate, respiratory asynchrony, X-ray and pulmonary function test results, as well as the results of maximal inspiratory pressure, maximal transverse pressure, diaphragm tension, electromyography and other measurements.
Treatment
Active treatment of the cause of the disease.
Symptomatic treatment. Such as respiratory muscle rest, nutrition, etc.
Rehabilitation: respiratory muscle exercises, etc.
Pharmacologic treatment of respiratory muscle fatigue remains highly controversial. Some experiments have shown that aminophylline can increase the contractility and endurance of the diaphragm, but the accuracy of the methods used to evaluate diaphragm function in the experiments is not high, so the reliability of the conclusions obtained is poor. There are also experiments showing that digoxin, β2 agonists, caffeine, etc. can increase diaphragm strength, but their exact efficacy needs to be further confirmed. In patients with COPD, anabolic hormones (growth hormone testosterone, etc.) have recently been used to increase musculoskeletal muscle strength and improve quality of life, and their exact efficacy and price/benefit ratio need to be further evaluated.
Prognosis
The prognosis of the disease is related to the causative factors of the onset, the duration of the disease, and the age and physical condition of the patient.