Treatment of the two conditions differs. The chronic type of respiratory failure may require only nighttime support of ventilation, which can be provided by negative pressure devices such as a cuirass or preferably, by intermittent positive pressure applied by a tight-fitting mask over the nose (bilevel positive airway pressure [BIPAP] or continuous positive airway pressure [CPAP]). These measures may also be used temporarily in acute situations, but in many cases there will be need of a positive-pressure ventilator that provides a constant volume with each breath. This can be effected only through an endotracheal tube.
Typical ventilator settings in cases of acute mechanical respiratory failure, if there is no pneumonia, are for tidal volumes of 6 to 8 mL/kg, depending on the compliance of the lungs and the patient’s comfort, at a ventilator rate between 4 and 12 breaths per minute, adjusted to the degree of respiratory failure. The tidal volume is kept relatively constant so as to prevent atelectasis, and only the rate is changed as the diaphragm becomes weaker or stronger. Decisions regarding the need for these mechanical devices are frequently difficult, particularly as patients with chronic neuromuscular illnesses often become dependent on a ventilator. Further details regarding the management of ventilation in acute neuromuscular weakness are given in the section on Guillain-Barré syndrome in Chap. 43 (see also monograph by Ropper and colleagues).
The presence of oropharyngeal weakness as a result of the underlying neuromuscular disease may leave the patient’s airway unprotected and require endotracheal intubation before mechanical ventilation becomes necessary. It is even difficult to decide when to remove an endotracheal tube in a patient with oropharyngeal weakness. Because the safety of the swallowing mechanism cannot be assessed with the tube in place, one must be prepared to reintubate the patient or to have a surgeon prepared to perform a tracheostomy after extubation, in the event that aspiration occurs.
We frequently encounter patients in whom the earliest feature of neuromuscular disease is subacute respiratory failure; this is manifest as dyspnea and exercise intolerance but without other overt signs of neuromuscular disease. Most such cases prove to be motor neuron disease, but rare instances of myasthenia gravis (especially the type associated with the MUSK autoantibody), acid maltase deficiency, polymyositis, nemaline myopathy, Lambert-Eaton syndrome, or chronic inflammatory demyelinating polyneuropathy may present in this way. The neurologist may be consulted in these cases after other physicians have found no evidence of intrinsic pulmonary disease. The spirometric flow-volume loop in cases of neuromuscular respiratory failure shows low airflow rates with diminished lung volumes that together simulate restrictive lung disease. Among such patients we have also found instances of isolated unilateral or bilateral phrenic nerve paresis that followed abdominal or cardiac surgery or an infectious illness. The least of these is probably a form of brachial neuritis (see Chap. 46 for a discussion of brachial neuritis).