High flow oxygen therapy and the work of breathing assessed by thickening fraction of the diaphragm (TFdi): just a side of the moon in cystic fibrosis patients?

Cystic fibrosis (CF) is a life-limiting disorder affecting approximately 70,000 individuals worldwide. Over the past three decades, the median predicted survival rate of CF patients has critically improved thanks to therapies that better address serious complications of the disease (i.e., treatment of pulmonary infections, suppression of airway inflammation; management of airway secretions; interventions supporting the nutritional status) and to novel approaches that address the underlying cause of CF (i.e., drugs restoring CF transmembrane conductance regulator protein function) (1). However, the role of non-invasive ventilation (NIV) along the course of disease still partially belongs to a “grey” therapeutic area of intervention. Actually, NIV is used as bridge to lung transplantation (2), it is an effective airway clearance technique and is superior to low flow oxygen therapy for the treatment of nocturnal hypercapnic respiratory failure, but NIV impact on pulmonary exacerbations, CF progression and exercise capacity is unclear, still lacking long-term randomized controlled trials (3). Notably, according to the British Thoracic Society Guidelines (4), NIV is not indicated in patients with copious secretions; in clinical practice, however, NIV has become a reliable treatment of acute respiratory failure in CF, avoiding the need of invasive mechanical ventilation and its serious complications (5). Sklar and colleagues have interestingly investigated the effect of high flow nasal-oxygen therapy (HFNT) on the work of breathing (WOB) in hypercapnic CF patients, in post-acute care. To this end, they evaluated the thickening fraction of the diaphragm (TFdi) as surrogate of WOB. The Authors found no statistically significant difference in TFdi values between conventional oxygen therapy (COT), HFNT and NIV. However, TFdi could be only a partial expression of the WOB in CF adults. The patients enrolled by Sklar and coworkers were adult and showed a severe airway obstruction at baseline (median FEV1 0.8 L/s, median Tiffeneau 45%), then it is reasonable to argue that a rearrangement of breathing pattern in response to the increased respiratory load was already established before of the entry in the study. As reported by Bellemare (6) in a study performed on CF young adult patients with moderate impairment of pulmonary function tests, not only the diaphragm but also the inspiratory ribcage muscles give a great contribution to the inspiratory pressure development; this contribution is higher than in non-CF subjects. Indeed, the ribcage expansion in CF patients