VENTILATORY DRIVE AND CARBON-DIOXIDE RESPONSE IN VENTILATORY FAILURE DUE TO MYASTHENIA-GRAVIS AND GUILLAIN-BARRE-SYNDROME

Objective: To test the hypothesis that either decreased ventilatory drive or decreased CO2 responsiveness accounts for the hypoventilation observed in patients during acute ventilatory failure from myasthenia gravis or Guillain-Barre syndrome. Design: Prospective, consecutive case series evaluating trials of ventilatory muscle performance, ventilatory drive, and CO2 response in patients during recovery from ventilatory failure until they were weaned from mechanical ventilation. Setting: Neurosciences critical care unit in a university hospital. Patients: Seven intubated, mechanically ventilated patients with myasthenia gravis or Guillain-Barre syndrome. Interventions: Patients repeatedly performed mechanically unsupported, spontaneous breathing trials to the limits of endurance. After spontaneous breathing trials, patients underwent CO2 rebreathing studies. Measurements and Main Results: Seventy-three breathing trials were performed in three patients with Guillain-Barre syndrome and four patients with myasthenia gravis. Patients were unable to sustain spontaneous ventilation in 55 trials averaging 27 +/- 5 mins. In these trials, significant increases occurred in mean end-tidal CO2 (41 +/- 1 to 44 +/- 1 torr [5.6 +/- 0.1 to 6.0 +/- 0.1 kPa]) and respiratory rate (31 +/- 1 to 35 +/- 1 breaths/min, p < .01). Ventilatory drive (as measured by airway occlusion pressure for 100 msecs) increased significantly p < .01 from 3.7 +/- 0.3 to 4.9 +/- 0.3 cm H2O. The response of airway occlusion pressure to CO2 rebreathing after these trials was 0.33 +/- 0.07 cm H2O/sec/mm Hg, while the minute ventilation response to CO2 rebreathing was only 0.30 +/- 0.06 L/min/mm Hg. Conclusions: These results suggest that ventilatory drive increases during acute hypoventilation, and the ventilatory drive response to CO2 remains intact, even when the minute ventilation response to CO2 is poor. Therefore, a decrease in ventilatory drive or CO2 response is unlikely to account for hypoventilation during ventilatory failure in patients with myasthenia gravis or Guillain-Barre syndrome.