Impaired pulmonary V˙O2 kinetics in cystic fibrosis depend on exercise intensity

Abstract

PURPOSE: To investigate the effects of mild-to-moderate cystic fibrosis (CF) on the pulmonary oxygen uptake (V˙O2) 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 V˙O2 kinetic response at the onset of moderate (MOD) and very heavy (VH) intensity cycling. METHODS: Changes in breath-by-breath V˙O2, 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 V˙O2 τ (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 V˙O2 τ (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-V¯)O2) 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, [INCREMENT]C(a-V¯)O2 significantly correlated with the VH phase II V˙O2 τ (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, V˙O2 kinetics are slowed during VH but not MOD cycling and appear to be mechanistically linked to impaired muscle O2 extraction and utilization.