Criteria for Non-Invasive Ventilation in Myasthenia Gravis Patients
Myasthenia gravis patients should receive non-invasive ventilation when they demonstrate signs of respiratory distress with a PCO2 > 45 mmHg, forced vital capacity < 15-20 ml/kg, or oxygen saturation < 92% while awake. 1, 2
Clinical Indicators for NIV Initiation
Respiratory Parameters
- Forced vital capacity (FVC) < 15-20 ml/kg or < 1 liter 1
- Maximum inspiratory pressure (MIP) < 30 cm H2O 1
- Maximum expiratory pressure (MEP) < 40 cm H2O 1, 3
- PCO2 > 45 mmHg (hypercapnia) 2
- Oxygen saturation < 92% while awake 1
Clinical Signs of Respiratory Distress
- Breathlessness at rest or during talking 1
- Inability to count to 15 in a single breath 1
- Use of accessory respiratory muscles 1
- Increased respiratory or heart rate 1
- Abnormal arterial blood gas measurements 1
Monitoring Requirements
Initial Assessment
- Arterial blood gas analysis to evaluate PaO2, PaCO2, and pH 1, 4
- Spirometry measurements including FVC, MIP, and MEP 1, 3
- Single breath count test (counting at rate of two numbers per second while exhaling) - count < 25 suggests respiratory muscle dysfunction 1
- Sniff nasal inspiratory pressure testing to evaluate diaphragm strength 1
Ongoing Monitoring
- Continuous pulse oximetry during NIV initiation 5
- Regular blood pressure monitoring, especially in hypotensive patients 5
- Repeat arterial blood gas analysis 1-2 hours after NIV initiation 5
- Regular assessment of bulbar function and cough strength 3
NIV Settings for Myasthenia Gravis
Initial Settings
- Start with BiPAP (Bilevel Positive Airway Pressure) mode 2
- Initial inspiratory positive airway pressure (IPAP): 8-12 cmH2O 1
- Initial expiratory positive airway pressure (EPAP): 3-5 cmH2O 1
- FiO2: Start at 40% and titrate to maintain SpO2 > 92% 5
Setting Adjustments
- Increase IPAP by 2 cmH2O increments if PCO2 remains elevated 1
- Adjust settings based on patient comfort and respiratory parameters 5
- Consider volume-assured pressure support for patients with fluctuating respiratory muscle strength 6
Predictors of NIV Success vs. Failure
Favorable Factors for NIV Success
- PCO2 < 45 mmHg at NIV initiation 2
- Lower APACHE II score (< 6) 3
- Serum bicarbonate < 30 mmol/L (indicating less chronic respiratory acidosis) 3
- Intact bulbar function 1, 3
Risk Factors for NIV Failure
- PCO2 > 45 mmHg at NIV initiation 2
- Severe bulbar dysfunction with risk of aspiration 1, 3
- Excessive secretions 7
- Inability to cooperate due to altered mental status 5
Special Considerations
Preventing Complications
- Select appropriate mask size and type to minimize air leakage and skin breakdown 1
- Initially hold mask in place before securing with straps to improve patient tolerance 1
- Regularly assess for skin breakdown, especially over nasal bridge 1
- Consider alternating between nasal and full-face masks for prolonged use 1
When to Consider Intubation
- Failure to improve after 1-2 hours of optimized NIV 5
- Worsening hypercapnia despite NIV 5
- Inability to clear secretions 7
- Deteriorating level of consciousness 1
- Hemodynamic instability 5
Benefits of NIV in Myasthenia Gravis
- Prevents intubation in approximately 57% of myasthenic crisis cases 2
- Reduces duration of mechanical ventilation when used as initial ventilatory support 2
- Decreases incidence of ventilator-associated pneumonia and atelectasis 7
- Shortens ICU and hospital length of stay 2
- Improves sleep quality and reduces fatigue in patients with chronic respiratory insufficiency 6
Pitfalls and Caveats
- NIV should not be used in patients with severe bulbar dysfunction due to risk of aspiration 1
- Hypercapnia (PCO2 > 45 mmHg) at NIV initiation is associated with higher failure rates 2
- Poor cough strength is a major cause of extubation failure (61.5% of cases) in patients who require intubation 3
- Oxygen therapy alone should not be used to treat hypoventilation without ventilatory assistance 1
- A decision about escalation to intubation if NIV fails should be made and documented before starting NIV 1