The repeated measurement of vital capacity is a poor predictor of the need for mechanical ventilation in myasthenia gravis.

OBJECTIVE: Testing the hypothesis that, in myasthenia gravis (MG), repeated measurements of vital capacity (VC) and various parameters derived from this measurement [median or lowest value of measured VCs during hospitalization, VC values < 20 ml/kg body weight (BW) or < 13 ml/kg BW, or an index assessing the variability of VC values during the whole ICU stay] could predict the need for intubation and mechanical ventilation (MV), as has been shown in other neuromuscular diseases with respiratory failure.
DESIGN: Retrospective study with medical chart revision of all the patients with MG and respiratory failure admitted to our intensive care unit between 1985 and 1993.
SETTING: Medical intensive care unit (15 beds) of a university hospital. PATIENTS AND METHODS: Five patients suffering from ten episodes of acute respiratory failure due to their decompensated MG. Repeated measurements of arterial blood gases and VC by trained respiratory therapists, at least every 4 h.
RESULTS: There was no difference in any of these parameters between patients eventually requiring MV (four episodes) and those in whom mechanical ventilation was not necessary (six episodes).
CONCLUSIONS: VC repeated measurements is a poor predictor of the need for further MV in MG patients. This can probably be ascribed to the erratic nature of MG, a disease whose course is largely influenced by many parameters (infection, treatment modifications, initiation of corticosteroid therapy, stress, psychological factors, etc.). Early admissions to the ICU of MG patients with respiratory dysfunction is thus recommended.