Impaired pulmonary VO_{2 k}inetics in cystic fibrosis depend on exercise intensity

Purpose - To investigate the effects of mild-to-moderate cystic fibrosis (CF) on the pulmonary oxygen uptake (VO₂) kinetics of 7 pediatric patients (13.5 +/- 2.8 y) versus 7 healthy matched controls (CON; 13.6 +/- 2.4 y). We hypothesized that CF would slow the VO₂ kinetic response at the onset of moderate (MOD) and very heavy (VH) intensity cycling.

Methods - Changes in breath-by-breath VO₂, near-infrared spectroscopy-derived muscle deoxygenation ([HHb]) at the m. vastus lateralis and thoracic bioelectrical impedance-derived heart rate, stroke volume index (SVI) and cardiac index (CI) were measured during repeat transitions to MOD (90% of the gas exchange threshold) and VH (Δ60%) intensity cycling exercise.

Results - During MOD, the phase II VO₂ [tau] (p=0.84; effect size (ES) = 0.11) and overall mean response time (MRT) (p=0.52; ES=0.11) were not significantly slower in CF versus CON. However, during VH exercise, the phase II VO₂ [tau] (p=0.02, ES=1.28) and MRT (p=0.01, ES=1.40) were significantly slower in CF. Cardiac function, central O₂ delivery (SVI and CI) and muscle [HHb] kinetics were unaltered in CF. However, the arterial-venous O₂ content difference (C_(a-)VO₂) was reduced during VH at 30 s (p=0.03, ES=0.37), with a trend for reduced levels at 0 s (p=0.07, ES=0.25), 60 s (p=0.05, ES=0.28) and 120 s (p=0.07, ES=0.25) in CF. Furthermore, ΔC_(a-)VO₂ significantly correlated with the VH phase II VO₂ [tau] (r= -0.85; p=0.02) and MRT (r = -0.79; p=0.03) in CF only.

Conclusion - Impairments in muscle oxidative metabolism during constant work rate exercise are intensity-dependent in young people with mild-to-moderate CF. Specifically, VO₂ kinetics are slowed during VH but not MOD cycling and appear to be mechanistically linked to impaired muscle O₂ extraction and utilization.