Mechanisms of improvement of respiratory failure in patients with restrictive thoracic disease treated with non-invasive ventilation

Background: Nocturnal non-invasive ventilation (NIV) is an effective treatment for hypercapnic respiratory failure in patients with restrictive thoracic disease. We hypothesised that NIV may reverse respiratory failure by increasing the ventilatory response to carbon dioxide, reducing inspiratory muscle fatigue, or enhancing pulmonary mechanics. Methods: Twenty patients with restrictive disease were studied at baseline (D0) and at 5 - 8 days (D5) and 3 months (3M). Results: Mean (SD) daytime arterial carbon dioxide tension (PaCO2) was reduced from 7.1 (0.9) kPa to 6.6 (0.8) kPa at D5 and 6.3 ( 0.9) kPa at 3M ( p = 0.004), with the mean ( SD) hypercapnic ventilatory response increasing from 2.8 (2.3) l/ min/ kPa to 3.6 (2.4) l/ min/ kPa at D5 and 4.3 (3.3) l/ min/ kPa at 3M ( p = 0.044). No increase was observed in measures of inspiratory muscle strength including twitch transdiaphragmatic pressure, nor in lung function or respiratory system compliance. Conclusions: These findings suggest that increased ventilatory response to carbon dioxide is the principal mechanism underlying the long term improvement in gas exchange following NIV in patients with restrictive thoracic disease. Increases in respiratory muscle strength ( sniff oesophageal pressure and sniff nasal pressure) correlated with reductions in the Epworth sleepiness score, possibly indicating an increase in the ability of patients to activate inspiratory muscles rather than an improvement in contractility.