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The earliest application of assisted ventilation in patients with neuromuscular disease was with the iron lung during the poliomyelitis epidemic in the 1940s and 1950s, which enabled patients with severe respiratory insufficiency to recover, become ventilator-free, and go on to lead productive lives.1 In most cases, the device was used for a few weeks to up to 2 years, with eventual recovery of ventilatory function. Many polio patients with disability continue to reside in iron lungs in the community after several decades.2 Others have converted to tracheostomy-assisted positive-pressure ventilation, achieving mobility and the ability to clear airway secretions. With appropriate weaning techniques, some have switched to noninvasive positive-pressure ventilation (NIPPV).3 NIPPV is now preferred to support most patients with chronic neuromuscular disorders and is used increasingly to support patients with acute ventilatory insufficiency, such as with Guillain-Barré syndrome and myasthenia gravis. Table 32-1 lists neuromuscular conditions associated with respiratory impairment and failure.
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The primary muscle of inspiration is the diaphragm, which is innervated by the third through fifth cervical nerves. In addition, the external intercostal muscles provide stability to the rib cage during inspiration. With ascending paralysis, such as with Guillain-Barré syndrome or traumatic quadriplegia above the T10 level (Fig. 32-1),4 there are reductions in vital capacity and other volume subdivisions, as well as rib cage distortion with inspiratory effort. These changes result in regional ventilation–perfusion mismatching, particularly in the dependent portions of the lungs. An early increase in the alveolar-arterial oxygen (O2) difference can be found in some patients with neuromuscular impairment long before hypercapnia develops.5,6 Gas exchange worsens during sleep secondary to alveolar hypoventilation, inhibited intercostal and accessory muscle activity, and dead space ventilation induced by rapid shallow breathing.6–8 Long-term episodic hypoxia in dystrophic mice leads to dysfunction of myofibrillar contractility as there is no change in diaphragmatic collagen content and dry-to-wet ratio.9 Expiratory muscles in muscular dystrophies are more impaired than inspiratory muscles.10 Despite this finding, cough effort can still be preserved if inspiratory volume and respiratory elastic recoil are maintained.11
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