Its strange, how many people know about the Leventhal criteria of post operative ventilation in a case of Myasthenia gravis. The disease duration should be more than six years. Pyridostigmine requirement more than 750 mg a day.

Vital capacity less than 2.9 L ( average is 4.8 L). Frankly speaking, there's no single criteria which is accurate or perfect but don't forget to look for thymoma and mediastinal tumor.