From the Research
A vital capacity less than 20 mL/kg is a critical threshold indicating significant respiratory impairment in myasthenia gravis, prompting consideration for mechanical ventilation or intensive care monitoring, as suggested by the study published in 2005 1. This threshold represents a reduction to about 30-40% of predicted normal values and suggests severe respiratory muscle weakness. Patients approaching this threshold may experience symptoms such as dyspnea, inability to complete sentences, use of accessory muscles, and paradoxical abdominal breathing.
Key Points to Consider:
- Serial measurements of vital capacity are more valuable than single readings, as a declining trend may indicate impending respiratory failure even before reaching the absolute threshold.
- Vital capacity should be measured regularly in myasthenic patients experiencing exacerbations, particularly those with bulbar symptoms or generalized weakness.
- The measurement reflects the maximum volume of air that can be exhaled after maximum inhalation and directly correlates with respiratory muscle strength, which is compromised in myasthenia gravis due to acetylcholine receptor antibodies blocking neuromuscular transmission at the diaphragm and intercostal muscles.
- A decline of 30% or more in maximal inspiratory pressure, as noted in the study 1, predicts a group at higher risk of requiring mechanical or noninvasive ventilation.
- Hypercapnia is frequent and more common in patients who require mechanical ventilation, highlighting the importance of monitoring blood gas values in addition to pulmonary function tests.