Objective: Non-invasive ventilatory therapy has prolonged survival of myopathy patients with hypoventilation. Efficacy of non-invasive ventilation depends on both elastance and resistance of the respiratory system. Although these parameters are important in the prescription of respiratory management, conventional respiratory function test does not show the appropriate answer in patients with severe respiratory muscle weakness. In muscular dystrophy, muscle tends to be shortened due to its fibrosis, when muscle becomes atrophic and weak; fibrosis of respiratory muscle tissues presumably causes high thoracic elastance. We evaluated the total respiratory system elastance and resistance during proportional assist ventilation (PAV) in myopathy patients.
Methods: In PAV with 100% assist, using BiPAP Vision ventilator, airway pressure exceeds 20 cmH2O or tidal volume exceeds 1.5 liter (run-away phenomenon) when the volume assist or the flow assist is higher than the individual elastance or the resistance, respectively. Twenty myopathy patients with ventilatory failure and 7 healthy controls were evaluated, including 7 patients with Duchenne muscular dystrophy (DMD), 2 patients with congenital myopathy (CM), 1 patient with limb-girdle muscular dystrophy (LG), 6 patients with myotonic dystrophy (MyD) and 4 patients with acid maltase deficiency (AMD). Seventeen patients used a nasal mask and 3 patients had a tracheostomy tube. Fifteen patients used a pressure-preset ventilator, and 3 patients used a volume-preset ventilator.
Results: In all patients with DMD, CM and LG, respiratory system elastance was higher than 20 (cmH2O/L) and than in all patients with AMD and MyD except 1 MyD patient. Follow-up measurement after half a or one year showed increase of respiratory system elastance in 2 DMD patients and 1 CM patient, but almost no change in 3 AMD patients. The elastance measured during PAV was consistent with the clinical impression of muscle shortening. One exceptional MyD patient showed extremely high elastance (more than 58 cmH2O/L), which reflected the fixed thoracic spine and increase of abdominal visceral fat. Resistance was normal in all patients except a LG patient with pulmonary aspergillosis and a history of pulmonary tuberculosis who showed 14 (cmH2O/L/s). In a CM patient who developed emphysema, resistance increased from 5 to 12 (cmH2O/L/s) in a year, although forced expiratory volume 1.0% (FEV1.0/FVC) remained normal. Respiratory system resistance measurement was useful to detect a lung disease, because obstructive disorder is underestimated with FEV1.0/FVC when vital capacity is low.
Conclusion: The respiratory system elastance and resistance measured during PAV are useful parameters in evaluation of mechanical features of the lung, thorax and airway. It is recommended to keep both parameters normal in patients who may require ventilatory assist due to progression of respiratory muscle weakness.