In patients with mild to moderate-severe cystic fibrosis (CF), reduced breathing reserve as a sole criterion underestimates ventilatory limitation during exercise, according to a study published in Respiratory Medicine.
Assessing the degree of exercise ventilatory limitation (EVL) is traditionally based on measurement of breathing reserve. However, breathing reserve provides limited information on exercise ventilatory response and does not provide any insight on breathing strategy or the degree of expiratory or inspiratory flow and volume constraints during exercise. Thus, by relying exclusively on breathing reserve to diagnose EVL, clinicians may miss important information.
In the current study, researchers evaluated whether ventilatory limitation may be present despite a normal breathing reserve in 20 CF patients with a wide range of pulmonary obstruction severity. After participants completed a symptom-limited incremental exercise test on a cycle ergometer, the researchers found that 6 patients had severe airway obstruction (forced expiratory volume in 1 second [FEV1] <45% predicted) and conventional evidence of ventilatory limitation (low breathing reserve), whereas 14 patients had mild to moderate-severe airway obstruction (FEV1 46-103% predicted) and a normal breathing reserve (12-62 L/min, BR% [17-40]).
However, dynamic respiratory mechanics demonstrated that even CF patients with mild to moderate-severe lung disease had clear evidence of ventilatory limitation during exercise. The investigators also found that inspiratory capacity was decreased by a median of 580 ml during exercise, indicating dynamic hyperinflation. Inspiratory reserve volume at peak exercise was 445 ml, indicating mechanical constraint on the respiratory system. In addition, the exercise tidal flow met or exceeded the expiratory boundary of the maximal flow volume loop over 72% of the expiratory volume (range 40-90%), indicating an expiratory flow limitation.
The authors concluded, “We found evidence of ventilatory limitation at maximal exercise in all CF patients studied.” In CF patients with severe airway obstruction, breathing reserve was reduced, but reduced breathing reserve – the conventional definition of EVL – underestimates ventilatory limitation in patients studied, noted the authors. For these patients, they added, “only assessment of dynamic respiratory mechanics during exercise revealed the ventilatory limitation.”
Reuveny R, Vilozni D, Dagan A, Ashkenazi M, Velner A, Segel MJ. The role of inspiratory capacity and tidal flow in diagnosing exercise ventilatory limitation in cystic fibrosis. Respir Med. Published online December 6, 2021. doi:10.1016/j.rmed.2021.106713