Effect of respiratory rate on airway deadspace ventilation during exercise in cystic fibrosis

Abstract

Gas exchange during exercise in patients with cystic fibrosis (CF) is characterised by an elevated physiological deadspace to tidal volume ratio. While this has been attributed to alveolar ventilation–perfusion mismatch, there are other potential causes of the high proportion of wasted ventilation, including factors relating to the volume and the ventilation of the airway deadspace. CF ( n=6, F=1, FEV 1 26–63% pred) and control ( n=6, F=2) subjects completed steady-state exercise on a cycle ergometer. Gas exchange was measured breath-by-breath and the volume of the airway deadspace ( V D aw ) determined using the equal areas method. Exercise data were interpolated to a CO 2 output of 0.7 l/min. V D aw was similar in the two groups both at rest and during exercise. However, the airway deadspace ventilation ( V ̇ D aw ) (median (inter-quartile range)), patients, 6.8 (5.1–7.1) l/min; controls, 4.9 (3.5–5.6) l/min, P<0.05) was significantly greater in the CF group due to a greater respiratory frequency. These results indicate that in CF patients, abnormally increased V ̇ D aw is an important contributor to the total (physiological) deadspace ventilation. Exercise performance in CF might be enhanced by efforts directed at facilitating an increase in exercise tidal volume and therefore the adoption of a more efficient pattern of breathing.