Weaning of the Ventilator in Myasthenia Gravis

FIGURE 24.1 Hypersecretion and neuromuscular respiratory failure in myasthenia gravis. Serial X-chest showing marked left atelectasis from a mucus plug (A) and gradual improvement after intubation and bronchoscopy (B, C).

How do you safely get a patient with a recent flare-up of myasthenia gravis off the ventilator and keep him off the ventilator? For one thing, management of myasthenic crisis with neuromuscular respiratory failure remains poorly defined and largely empirical. Quite a few patients will have to be intubated and reintubated. Many do improve rapidly after specific therapy—IVIG, or plasma exchange. A recent analysis concluded that the treatment effects of these two options in myasthenia gravis-and possibly myasthenia crises- are comparable. Delay of treatment however increases mortality and complications.

After neurologic improvement, weaning from the ventilator becomes a priority. Liberating myasthenic patients from the ventilator, however, remains frustrating for most attending physicians. Moreover, pulmonary infections and atelectases can make weaning even more challenging.

Most neurologists will try to find an adequate dose of pyridostigmine that improves muscle strength and oropharyngeal function and could assist in weaning the patient off the ventilator. However, at the same time a dose of pyridostigmine must be found that does not cause abundant secretions. Thick secretions will predispose the patient to sudden mucus plugs, which, as it was in our case, may occlude a large bronchial branch.

There are some prerequisites to consider for physicians attempting to wean the patient from the ventilator. The first priority remains satisfactory treatment of the myasthenic symptoms. This is best accomplished with plasma exchange in the acute phase. In our experience, once the patient is severely affected and intubated, IVIG less frequently will lead to substantial improvement. Multiple bronchoscopies may be necessary to clear the bronchial tree from secretions, and often an infection becomes apparent that requires specific antibiotic treatment. Only after the secretions and infection are under control can the patient be considered for weaning. In some instances, early tracheostomy is necessary to better clear secretions from the airways.

Treatment with immunosuppressive drugs should be started, and usually this includes prednisone 60 to 90 mg and perhaps a pulse-dose of methylprednisolone (intravenous infusion of 1 gram). Long-term treatment with mycophenolate mofetil, for example, will only take effect several weeks after initiation and cannot be relied on in the acute phase of management.

The prediction of success of extubation is difficult to determine clinically, and we have not found a good way to do it. Some of the extubation parameters described in neuromuscular respiratory failure and their predictive values are shown in Table 24.1. Oropharyngeal function is hard to assess in an intubated patient, and neck flexion or shoulder shrug do not predict failure. Often the patient is extubated, seemingly doing well and holding his own, only to deteriorate with increased work of breathing, shallow breathing, and gradual rise in arterial PCO2. In our patient example, this deterioration was also further complicated by acute mucus plugging that resulted in reintubation.

Several tests have been developed that might be useful in assessing the probability of successful extubation. One recent study proposed the use of a so-called white card test. This white card is placed 1 to 2 cm from the end of the endotracheal tube and any moisture present on the card following two to three coughs is considered a positive test. The patients are positioned with the head of the bed at 30 to 45 degrees and coached to cough maximally. It is unclear whether this test is a better predictor than a simple clinical assessment of cough strength. Another possibility is to place the patient on a spontaneous breathing trial and observe respiratory frequency and tidal volume for about an hour. We recommend that, extubation can be followed by bilevel positive airway pressure (BiPAP) noninvasive ventilation, which augments airflow and maintains positive airway pressure in the inhalation and exhalation phases.

TABLE 24.1 Predictors for Successful Extubation Parameters in Myasthenia Gravis

Test	Predictive Value
Secretion volume	Good
T-piece trials (with assessment of rapid shallow breathing)	Good
Normal chest X-ray	Good
Neurologic examination (oropharyngeal function, head-flexion strength)	Uncertain
White card test	Uncertain
Pulmonary function tests	Poor