Guillain-Barre Syndrome (GBS) and Myasthenia Gravis (MG) are common causes of acute weakness. About 25% of these patients may develop respiratory failure requiring intubation, so a major concern is determining who requires ICU-level monitoring and whether intubation should be performed. Ideally it would be possible to predict with 100% accuracy which patients would require intubation, allowing pre-emptive elective intubation. In reality such prediction is impossible, so we are often forced to carefully observe patients in the ICU until they declare themselves.
Foreword: Some comments on bedside pulmonary function tests (PFTs)
- MIP = Maximal Inspiratory Pressure. This is the greatest negative pressure the patient can generate, often also referred to as the NIF (Negative Inspiratory Force). It is measured asking patients to inhale as hard as they can with measurement of the negative pressure that they generate using a pressure gauge (image above). This is a measurement of the strength of the inspiratory muscles, primarily the diaphragm.
- MEP = Maximal Expiratory Pressure. This is the opposite of the MIP, specifically the maximal positive pressure the patient can generate. It is measured by asking patients to exhale as hard as they can, and measuring the positive pressure. This is a measurement of expiratory muscle strength, which may correlate clinically with ability to cough and clear secretions.
- FVC = Forced vital capacity. This is the largest volume of gas that a patient can exhale. Patients are asked to take a full breath in and then exhale maximally, with measurement of the exhaled volume. FVC reflects a global measurement of the patient's ventilatory ability, which takes into account inspiratory and expiratory muscle strength as well as pulmonary compliance.
Pearl #1: Do not intubate a patient solely because of poor PFTs
Original data on which the 20-30-40 rule for GBS was based (from Lawn 2011). Note the poor degree of separation between patient groups based on Pimax (a.k.a. MIP) and Pemax (a.k.a. MEP). Based on the vital capacity data above, the specificity of the 30-40-50 rule must be 83% or lower (given that 17% of patients who didn't require ventilation had a vital capacity below 20 ml/kg).
Pearl #2: Don't check the MIP or MEP
Pearl #3: Don't assume respiratory failure is due to respiratory muscle weakness
Pearl #4: Consider early pre-emptive respiratory support with BiPAP or high-flow nasal cannula.
Pearl #5: Try not to chase dysautonomia in GBS. However, be prepared to handle it in the peri-intubation period.
- The only bedside pulmonary function test which is useful is the forced vital capacity (FVC).
- Patients with a FVC < 20 ml/kg are at risk for respiratory failure and should receive ICU-level monitoring.
- Intubation is typically required when the FVC falls below 10-15 ml/kg. However, the decision to intubate is a clinical decision based primarily on ability to protect the airway, work of breathing, vital signs, overall appearance, and trajectory.
- For patients who are dyspneic but don't require intubation, consider trialing BiPAP or high-flow nasal cannula to see if this may improve their comfort and reduce the work of breathing.
- Patients with GBS may have dysautonomia with wide fluctuations in blood pressure. Avoid treating hypertension if possible, as this may exacerbate subsequent episodes of hypotension.
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