What maneuvers are used to assess respiratory function in intubated patients with suspected myasthenic crisis?

Respiratory Function Assessment in Intubated Patients with Suspected Myasthenic Crisis

The most effective maneuvers to assess respiratory function in intubated patients with suspected myasthenic crisis include measuring negative inspiratory force (NIF) and vital capacity (VC), which should be performed frequently to monitor for improvement or deterioration of neuromuscular function.

Key Assessment Maneuvers

Primary Respiratory Function Tests

• Negative Inspiratory Force (NIF)/Maximum Inspiratory Pressure (PImax)

  • Gold standard for assessing respiratory muscle strength 1
  • Measures the maximum negative pressure generated during inspiration
  • Values consistently less than -20 to -30 cmH2O suggest adequate strength for extubation

• Vital Capacity (VC)

  • Measures the maximum volume of air that can be exhaled after maximum inspiration
  • Values consistently above 10-15 mL/kg suggest adequate strength for extubation
  • Trend is more important than absolute values

Additional Assessment Techniques

• Frequent pulmonary function monitoring 1

  • Should be performed at least daily, more frequently when weaning is being considered
  • Declining values may indicate worsening crisis or inadequate treatment

• Prolonged Spontaneous Breathing Trials (SBT)

  • Extended trials (8+ hours) before extubation significantly improve success rates 2
  • Standard short SBTs may not adequately predict extubation success in myasthenic patients

• Daily neurologic review to correlate respiratory function with overall neuromuscular status 1

Monitoring for Cholinergic vs. Myasthenic Crisis

Distinguishing between myasthenic crisis (undertreatment) and cholinergic crisis (overtreatment) is crucial for management:

• Tensilon (edrophonium) test may be required for differential diagnosis 3

  • Improvement suggests myasthenic crisis (needs more anticholinesterase)
  • Worsening suggests cholinergic crisis (needs less anticholinesterase)

• Clinical signs to monitor:

  • Excessive secretions, lacrimation, miosis, bradycardia suggest cholinergic crisis
  • Progressive weakness without cholinergic signs suggests myasthenic crisis

Weaning Protocol for Myasthenic Crisis Patients

1. Initiate weaning assessment when:

   • Triggering factors have been identified and addressed 2
   • Respiratory parameters show consistent improvement
   • Patient is hemodynamically stable

2. Pre-extubation criteria:

   • Stable or improving NIF and VC measurements over 24-48 hours
   • Successful completion of prolonged SBT (8+ hours) 2
   • Resolution of bulbar symptoms (assessed by gag reflex and secretion management)

3. Post-extubation monitoring:

   • Continue frequent respiratory assessments
   • Be prepared for possible reintubation (14.3% failure rate reported) 2
   • Consider non-invasive ventilation support after extubation

Potential Complications and Pitfalls

• Premature extubation risk:

  • Standard weaning parameters may not apply to myasthenic patients
  • Respiratory function can deteriorate rapidly even after successful initial weaning

• Medication considerations:

  • Avoid medications that can worsen myasthenia: β-blockers, magnesium, fluoroquinolones, aminoglycosides, and macrolides 1
  • Monitor for cholinergic crisis with anticholinesterase therapy 3

• Atelectasis and pneumonia prevention:

  • Aggressive respiratory care (suctioning, chest physiotherapy) reduces complications 4
  • Proper positioning and pulmonary hygiene are essential

Treatment Considerations During Assessment

• Immunomodulatory therapy:

  • Plasmapheresis is preferred over IVIG for myasthenic crisis 5
  • Improvement in respiratory parameters should be expected within days of treatment

• Anticholinesterase management:

  • Pyridostigmine dosing should be carefully titrated based on respiratory function tests
  • IV neostigmine may be used when enteral administration is not possible

Remember that myasthenic crisis has a median duration of approximately 2 weeks under appropriate treatment 6, and frequent respiratory function assessment is essential throughout this period to guide management decisions and determine readiness for extubation.