Respiratory muscles weakness

The pretreatment of MH-susceptible patients with oral or intravenous dantrolene prior to surgery in order to avoid a crisis is controversial. Most physicians do not recommend prophylactic pretreatment except in patients who have had a previously documented episode. However, if pretreatment is desired, it is recommended that therapy be begun with intravenous dantrolene in a dose of 2 mg/Kg just prior to induction of anesthesia. This prevents the uncertainty of predictive blood values associated with the use of the oral route. The adverse effects of intravenous dantrolene prophylaxis include phlebitis and tissue necrosis. Patients who receive prophylactic treatment with oral dantrolene often complain of incapacitation, gastrointestinal irritation, prolonged drowsiness, and clinically significant respiratory muscle weakness. 

Skeletal muscle dysfunction can cause myalgia, bone pain, weakness, and potentially fatal rhabdomyolysis. Respiratory muscle weakness and diaphragmatic contractile dysfunction can cause acute respiratory failure. 

The most prominent and often limiting feature of dantrolene administration is dose-dependent muscle weakness. Other side effects are drowsiness, dizziness, malaise, fatigue, and diarrhea. Symptomatic hepatitis is reported in 0.5% of patients receiving it and fatal hepatitis in up to 0.2%. Contraindications include respiratory muscle weakness and liver disease. It is suggested that patients on dantrolene therapy be given regular liver function tests. 

Sleeplessness in ALS has numerous causes. Respiratory insufficiency, difficulty repositioning in bed, anxiety and depression can all contribute to poor sleep. Treatment of depression with sedating antidepressants such as mirtazapine, tricyclic antidepressants, or trazadone can help promote sleep. Zolpidem, a non benzodiazepine sleep aid, is effective and carries a low risk of respiratory depression. Other medications that can be helpful include anithistamines, chloral hydrate and selective use of benzodiazepines (Gordon and Mitsumoto, 2006). Non-invasive positive pressure ventilation can help relieve orthopnea in those with respiratory muscle weakness, and special equipment, such as a hospital bed, can reduce nighttime discomfort. 

Proper supportive care and administration of antitoxin are the mainstays of current therapy. Patients who present with respiratory failure will need full ventilatory support however, there will be a subgroup of patients who present early without obvious signs of respiratory muscle paralysis. The negative inspiratory force, pulse oximetry, and gag reflex of these patients should be evaluated serially to determine the degree of respiratory muscle weakness and likelihood of impending respiratory failure. 

Treatment of hypophosphatemia depends on the degree of hypophosphatemia and the presence of symptoms. Patients with moderate hypophosphatemia may require only treatment of the underlying disorder or oral phosphate supplementation. In patients with marked symptoms of hypophosphatemia, particularly if respiratory muscle weakness is present, parenteral administration of phosphate may be indicated. 

Restrictive lung disease can be produced by a number of defects increased elastic recoil (interstitial lung disease), respiratory muscle weakness (myasthenia gravis), mechanical restrictions (pleural effusion), and poor effort. 

Phosphorus is essential in respiratory disease for its role in the synthesis of adenosine triphosphate, inadequate stores of which can lead to respiratory muscle weakness. Critically ill, malnourished patients are at risk for phosphorus depletion. 

A. Acute poisoning. Nausea and vomiting frequently occur within 30-60 minutes of ingestion. Symptoms of serious intoxication include skeletal muscle weakness, tetanic contractions, respiratory muscle weakness, and respiratory arrest. Laboratory findings include hypocalcemia, hypomagnesemia, and hyperkalemia, and increased QT interval and cardiac dysrhythmias may occur. 

D. Intermediate syndrome. Patients may develop proximal muscle weakness over a few days even after resolution of the acute cholinergic crisis. This is often first not as neck weakness, progressing to proximal limb weakness and cranial nerve palsies. Respiratory muscle weakness and respiratory arrest may occur abmptly. The intennediate syndrome is probably caused by prolonged overstimulation of the neuromuscular junction, and may be associated with inadequate oxime therapy. Atropine is not effective. 

Maintain an open ainway and assist ventilation if necessary (see pp 1-7). Pay careful attention to respiratory muscle weakness sudden respiratory arrest may occur. This is often preceded by increasing weakness of neck flexion muscles. If intubation is required, note the potential for interactions between neuromuscular blockers and cholinesterase inhibitors (see p 472). Administer supplemental oxygen. 

Santiago RM, Scharnhorst D, Ratkin G, et al. Respiratory muscle weakness and ventilatory failure in AL amyloidosis with muscular pseudohypertrophy. Am J Med 1987 83(1) 175-178. 

Rochester D. Respiratory muscle weakness, pattern of breathing, and CO2 retention in chronic obstructive pulmonary disease. Am Rev Respir Dis 1991 143 901-903. 

De Troyer A, Borenstein S, Cordier R. Analysis of lung volume restriction in patients with respiratory muscle weakness. Thorax 1980 35 603-610. 

Gibson GJ, Pride NB, Newsom Davis J, et al. Pulmonary mechanics in patients with respiratory muscle weakness. Am Rev Respir Dis 1977 115 389-395. 

Respiratory muscle weakness and respiratory muscle fatigue can decrease the capacity of these muscles to generate and sustain tension. As direct quantification of respiratory muscle tension is clinically impossible, measurements of pressure elicited by respiratory muscle contractions are used to indirectly determine their contribution to ventilator dependence. 

In healthy volunteers, stimulation of the phrenic nerves elicits twitch pressures of 31 to 39 cmH20, whereas in patients with severe COPD, twitch pressures average 19 to 20 cmH20 (53,60). Twitch pressures in patients recovering from an episode of acute respiratory failure are about half of those recorded in ambulatory patients with severe COPD (Fig. 5) (59,61,62). This decrease is in keeping with respiratory muscle weakness in most of these patients. Respiratory muscle weakness in mechanically ventilated patients can result from preexisting conditions or from new-onset conditions (63). 

Figure 6 Relationship between muscle strength and mixed venous partial pressure of CO2 (PVCO2) in patients with respiratory muscle weakness. Respiratory muscle strength is the arithmetic sum of maximum static inspiratory and expiratory mouth pressures (P iax = + TEmax)- The open circles...

Malnutrition is highly prevalent among patients requiring PMV (37) and LTMV (70) and is assoeiated with poor prognosis (71). Malnutrition decreases muscle mass and respiratory muscle strength and enduranee (40). These effects on the respiratory muscles are partially reversible with nutritional support. The process, however, is slow, and in laboratory animals, it can take months of refeeding for muscle mass to return to normal values (72). To date, it remains unclear whether malnutrition by itself ean cause sufficient respiratory muscle weakness to produce ventilator dependence. It is more likely for malnutrition to be a contributing factor and not a sole cause of ventilator dependence. 

New-onset respiratory muscle weakness may result from conditions that are unique to mechanically ventilated patients, such as ventilator-associated respiratory muscle dysfunction, sepsis-associated myopathy, and ICU-acquired paresis (63). New-onset respiratory muscle weakness may also result from conditions that are not unique to critically ill patients, such as acid-base disorders, electrolyte disturbances, decreased oxygen delivery, and medications (63). 

Limitations in the Current Classification of Respiratory Muscle Weakness... 

When studying respiratory muscle weakness, it is necessary to bear in mind our current limited understanding of these disorders. One, the distinction between preexisting conditions and new-onset conditions can be arbitrary. Two, preexisting conditions, such as... 

Other metabolic factors contributing to PMV include hypophosphatemia and hypomagnesemia, both of which have been associated with diminished diaphragmatic function. Hypothyroidism is an uncommon cause of ventilator dependency (27), being associated with respiratory muscle weakness as well as altered ventilatory drive and upper airway obstruction. Hypothyroidism is a potentially treatable cause of failure to wean and it should be considered in patients with prolonged ventilator dependence. 

Abnormal gas exchange, malnutrition, and prolonged use of controlled ventilation, leading to selective diaphragmatic muscle atrophy, add to peripheral and respiratory muscle weakness (38). Many of the above reasons make it likely that patients will benefit from PT that targets respiratory and skeletal muscle function, although clearer evidence of positive outcomes is still required (53). Inspiratory muscle training has been shown to facilitate... 

Only a small proportion of patients fail to wean from mechanical ventilation, but they require a disproportionate amount of resources. Weaning failure has been extensively studied in the clinical literature and several factors are likely to contribute to it. These factors include inadequate ventilatory drive, respiratory muscle weakness, respiratory muscle fatigue, increased work of breathing, or cardiac failure. There is accumulating... 

It seems reasonable to start NIV in symptomatic patients (Table 2) who have signs of respiratory muscle weakness (FVC <80% predicted or SNIP < 40 cmH20) and evidence of nocturnal desaturation or a Paco2 > 49 mmHg (18). Bourke et al. suggested that orthopnea was the most useful criterion for benefit and compliance with NIV (94), and Sivak (96) noted lack of correlation between VC and the success of NIV. In a small prospective study, Jackson et al. (19) suggested that early intervention with NIV, based on nocturnal desaturation (Sao2 < 90% for 1 min), may improve quality of life. 

DMD is an X-linked recessive pattern of inheritance and affects up to 1 in 3300 males (102). Affected patients typically become wheelchair dependent by the age of 10 to -12 years at which time their VC plateaus. With the development of respiratory muscle weakness and skeletal deformity, the VC starts to fall (103). The patients usually remain asymptomatic until the VC is <450 mL (104). Once the forced expiratory volume in one second (FEVi) is <40% predicted and the MIP is worse than 30% predicted, the development of hypercapnia is likely and therefore ABG should be measured (105,106). Obstructive sleep apnea occurs commonly among younger children and hypoventilation in older children (107). 

Patients with copathology contributing to ventilatory failure, e.g., thoracoplasty, respiratory muscle weakness, mmbid obesity... 

Gilmartin JJ, Cooper BG, Griffiths CJ, et al. Breathing during sleep in patients with myotonic dystrophy and non-myotonic respiratory muscle weakness. Q J Med 1991 78 21-31. 

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