What are the key parameters to monitor in a myasthenia crisis, particularly in relation to respiratory status, such as peak flow?

Monitoring in Myasthenia Crisis

In myasthenia crisis, the critical parameters to monitor are respiratory muscle strength (vital capacity, maximal inspiratory/expiratory pressures), arterial blood gases (particularly PCO₂), and oxygen saturation, NOT peak expiratory flow, as peak flow is primarily used for obstructive airway diseases like asthma and does not adequately assess the neuromuscular respiratory failure that defines myasthenic crisis. 1, 2, 3

Key Respiratory Monitoring Parameters

Vital Capacity and Respiratory Muscle Strength

• Forced vital capacity (FVC) is the primary bedside measurement to assess respiratory muscle weakness and predict need for intubation 4
• Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) directly measure respiratory muscle strength and correlate with ventilation requirements 4
• Lower MEP on arrival is associated with longer ventilation duration (P=0.02), making this a critical prognostic indicator 4
• Serial measurements of these parameters should be obtained to track progression and response to treatment 4

Arterial Blood Gas Monitoring

• PCO₂ levels are the most critical predictor of respiratory failure severity and need for intubation 4
• PCO₂ >45 mm Hg is the only significant predictor of BiPAP failure and need for endotracheal intubation (P=0.04) 4
• Arterial blood gases should be monitored closely, particularly when considering non-invasive ventilation as an alternative to intubation 4
• Hypercapnia indicates impending respiratory failure requiring immediate escalation of ventilatory support 4

Oxygen Saturation

• Pulse oximetry provides continuous monitoring for hypoxemia, though it may not detect early respiratory muscle failure before hypercapnia develops 4
• Oxygen saturation should be maintained, but normal saturation does not exclude the need for ventilatory support if respiratory muscle weakness is progressing 1, 3

Clinical Assessment Parameters

Bulbar Function

• Swallowing ability and airway protection must be assessed continuously, as bulbar weakness leads to aspiration risk even before frank respiratory failure 1, 3
• Globus events with rapidly exhausting coughing and swallowing difficulties are life-threatening red flags requiring immediate ICU admission 1
• The presence of oropharyngeal weakness significantly increases risk of crisis and complications 2, 3

Respiratory Rate and Pattern

• Tachypnea and shallow breathing indicate respiratory muscle fatigue 1
• Inability to count to 20 in one breath or speak in complete sentences suggests severe respiratory compromise 1
• Progressive dyspnea developing over minutes to days requires immediate intervention 1

Why Peak Flow Is NOT Appropriate

Peak expiratory flow (PEF) measures the maximum speed of expiration and is designed to detect obstructive airway diseases like asthma where airway narrowing limits flow 5, 6. In myasthenia gravis:

• The pathophysiology is neuromuscular weakness, not airway obstruction 1, 2, 3
• Respiratory failure results from reduced tidal volumes due to respiratory muscle weakness and upper airway collapse from bulbar weakness, not from airflow limitation 3
• Peak flow measurements do not adequately capture the restrictive pattern and muscle weakness that characterize myasthenic respiratory failure 4

Monitoring Frequency and Thresholds

Pre-Intubation Phase

• Respiratory parameters should be checked every 2-4 hours or more frequently if deteriorating 1
• Early intubation is essential when vital capacity is declining, PCO₂ is rising, or bulbar symptoms worsen with aspiration risk 1, 3
• Do not wait for severe hypoxemia or respiratory arrest, as timely airway protection prevents complications 3

Ventilation Duration Predictors

• Mean ventilation duration is approximately 10.4 days with median 12-14 days 1, 4
• Longer duration is associated with intubation (vs. BiPAP), atelectasis, and lower MEP on arrival 4
• Approximately 20% remain mechanically ventilated after 1 month, usually due to complications 1

Common Pitfalls to Avoid

• Do not rely on peak flow measurements as they are designed for asthma monitoring, not neuromuscular respiratory failure 5, 6
• Do not delay intubation waiting for severe hypoxemia; progressive hypercapnia and declining vital capacity are earlier and more reliable indicators 4
• Do not miss the window for BiPAP trial; if PCO₂ exceeds 45 mm Hg, BiPAP is likely to fail and intubation should be performed promptly 4
• Monitor for aspiration continuously as bulbar weakness can be life-threatening even with adequate gas exchange 1, 3