When should intubation be anticipated in a patient with myasthenia crisis?

Anticipating Intubation in Myasthenic Crisis

Intubate early and liberally in myasthenic crisis when forced vital capacity falls below 20 mL/kg, maximal inspiratory pressure is less than 30 cm H₂O, maximal expiratory pressure is less than 40 cm H₂O, or when PCO₂ exceeds 45 mm Hg—early intubation is the most important step in managing myasthenic crisis and prevents prolonged ventilation and complications. 1, 2, 3

Clinical Parameters for Anticipating Intubation

Respiratory Function Thresholds

The following objective measurements predict the need for mechanical ventilation and should trigger intubation planning:

• Forced vital capacity (FVC) <20 mL/kg is strongly associated with need for mechanical ventilation 1
• Maximal inspiratory pressure (negative inspiratory force) <30 cm H₂O indicates inadequate inspiratory muscle strength 1
• Maximal expiratory pressure <40 cm H₂O predicts inability to clear secretions and correlates with extubation failure 1, 4
• PCO₂ >45 mm Hg is the single strongest predictor of noninvasive ventilation failure and need for intubation 3

Bulbar Dysfunction Indicators

Bulbar symptoms are prominent in myasthenic crisis and strongly predict need for intubation:

• Dysphagia with aspiration risk necessitates airway protection 1, 5
• Poor cough strength leads to sputum impaction, the most common cause of extubation failure (61.5% of cases) 4
• Globus events with rapidly exhausting coughing and swallowing are life-threatening 5

Clinical Examination Red Flags

Serial neurologic examinations should focus on:

• Rapidly progressive weakness of respiratory and bulbar muscles developing over minutes to days 5
• Severe dyspnea with visible respiratory distress 5
• Flaccid tetraparesis with immobility indicating generalized weakness 5

Timing Strategy: Early vs. Late Intubation

The threshold for intubation should be low—early intubation prevents prolonged ventilation, reduces pulmonary complications, and shortens ICU and hospital stays. 2, 3

Evidence Supporting Early Intubation

• Patients intubated early have shorter mean ventilation duration (10.4 days) compared to those intubated after failed noninvasive ventilation 3
• Early intubation reduces atelectasis and pneumonia rates (46% vs. 91% in historical series) 6
• Delayed intubation after development of hypercapnia leads to worse outcomes 3

Role of Noninvasive Ventilation (NIV)

NIV with BiPAP can prevent intubation in select patients but requires careful patient selection:

• Consider NIV trial if: APACHE II score <6, serum bicarbonate <30 mmol/L, and PCO₂ ≤45 mm Hg 4, 3
• Proceed directly to intubation if: PCO₂ >45 mm Hg, severe bulbar dysfunction, or inability to protect airway 3
• NIV success rate: 57% can avoid intubation when appropriately selected 4

Monitoring Protocol

Spirometry and Respiratory Mechanics

Perform serial measurements every 4-6 hours in deteriorating patients:

• FVC measurements to track respiratory muscle function 1
• Maximal inspiratory and expiratory pressures to assess diaphragm strength 1
• Single breath count test: counting to <25 (at 2 numbers/second) suggests inadequate respiratory reserve 1

Blood Gas Monitoring

• Rising PCO₂ or end-tidal CO₂ strongly predicts need for mechanical ventilation 1
• Pulse oximetry alone is unreliable as hypoxia develops late in neuromuscular respiratory failure 1
• Arterial blood gases should be obtained when clinical deterioration occurs 1

Autonomic Function

Monitor for dysautonomia that may complicate intubation:

• Cardiac monitoring for tachycardia, bradycardia, or dysrhythmias 1
• Blood pressure monitoring for lability and orthostatic changes 1

Intubation Technique Considerations

Approach and Equipment

• Orotracheal intubation is preferred over nasotracheal approach 2
• Experienced operator should perform the procedure given high-risk nature 1
• Rapid sequence intubation is appropriate in most cases 1

Post-Intubation Management

• Small bore duodenal feeding tubes decrease aspiration risk and improve patient comfort compared to nasogastric tubes 2
• Recruitment maneuvers (30-40 cm H₂O for 25-30 seconds) improve oxygenation after intubation if hemodynamically stable 1
• Aggressive respiratory therapy including suctioning, chest physiotherapy, and bronchodilator treatments reduces atelectasis and pneumonia 6

Common Pitfalls to Avoid

• Waiting for hypoxemia or hypercapnia: These develop late in neuromuscular failure; intubate based on respiratory mechanics 1
• Over-reliance on pulse oximetry: Gas diffusion remains intact until late stages 1
• Attempting NIV with PCO₂ >45 mm Hg: This predicts NIV failure and delays definitive airway management 3
• Underestimating bulbar dysfunction: Aspiration and inability to clear secretions are major causes of complications 4, 5
• Delaying intubation in rapidly progressive weakness: Median crisis duration is 12-14 days of ventilation; early intubation shortens overall course 5, 3