Assessment and interpretation of aerobic exercise (dys)function in paediatric patients with cystic fibrosis
Saynor, Zoe Louise
Thesis or dissertation
University of Exeter
Reason for embargo
Final paper has been accepted but not yet published by Medicine and Science in Sports and Exercise.
The purpose of this thesis was to extend our understanding of the assessment and interpretation of aerobic exercise function of paediatric patients with cystic fibrosis (CF). The first investigation sought to establish (1) the validity of traditional criteria to verify maximal oxygen (V ̇O2max) during a maximal cardiopulmonary exercise test (CPET); and (2) the utility of supramaximal verification (Smax) to confirm V ̇O2max. Traditional criteria significantly underreported V ̇O2max, whilst Smax was shown to provide a valid measurement in this patient group. The reproducibility of this CPET protocol, over the short- (48 h) and medium- (4-6 weeks) term, was then established in study two. V ̇O2max was repeatedly determined with no learning effect over 48 h (typical error (TE): ∆150 mL; ∆9.3%) and 4-6 weeks (TE: ∆160 mL; ∆13.3%). Supplementary maximal and submaximal CPET parameters should be incorporated for a comprehensive evaluation of a patient, however they are characterised by greater variability over time. The influence of mild-to-moderate CF on aerobic exercise function and the matching of muscle O2 delivery-to-O2 utilisation during ramp incremental exercise to exhaustion were then examined in study three. Aerobic function was impaired in CF, indicated by very likely reduced fat-free mass normalised V ̇O2max (mean difference, ±90% CI: -7.9 mL∙kg-1∙min-1, ±6.1), very likely lower V ̇O2 gain (-1.44 mL∙min-1∙W-1, ±1.12) and a likely slower V ̇O2 mean response time (MRT) (11 s, ±13). Arterial oxygen saturation was lower in CF, supporting the notion that centrally mediated O2 delivery may be impaired during ramp incremental exercise. Although a faster rate of fractional O2 extraction would be expected in the face of reduced O2 delivery, this was not observed, suggesting additional impairment in O2 extraction and utilisation at the periphery in CF. The fourth study then demonstrated the clinical utility of CPET to assess the response to 12 weeks treatment with Ivacaftor, using a case-based design. Whilst one patient with relatively mild disease demonstrated no meaningful change in V ̇O2max, the second demonstrated a 30% improvement in V ̇O2max, due to increased O2 delivery and extraction. Furthermore, changes in aerobic function were detected earlier than spirometric indices of pulmonary function. This study demonstrated that CPET represents an important and comprehensive clinical assessment tool and its use as an outcome measure in the functional assessment of patients is encouraged. Study five investigated the V ̇O2 kinetics in this patient group. During moderate intensity cycling, the phase II V ̇O2 time constant (τ) (p = 0.84, effect size (ES) = 0.11) and overall MRT (p = 0.52, ES=0.33) were not slower in CF. However, both were slowed during very heavy intensity cycling (p = 0.02, ES = 1.28 and p = 0.01, ES = 1.40, respectively) in CF. Cardiac output and muscle deoxygenation dynamics were unaltered in CF, however, the arterial-venous O2 content difference (C(a-v ̅)O2) was reduced (p=0.03) during VH and ∆C(a-v ̅)O2 correlated with the phase II τ (r= -0.85; p=0.02) and MRT (r = -0.79; p=0.03) in CF. This study showed that impaired oxidative muscle metabolism in this group is exercise intensity-dependent and mechanistically linked to an intrinsic intramuscular impairment, which limits O2 extraction and utilisation. In conclusion, this thesis has provided guidelines for a valid and reproducible CPET protocol for children and adolescents with mild-to-moderate CF, demonstrated the utility of CPET as clinical outcome measure and furthered our understanding of the factors responsible for impaired aerobic exercise function in this patient group.
Royal Devon and Exeter NHS Foundation Trust
University of Exeter
Williams, Craig Anthony
Barker, Alan Robert
PhD in Sport and Health Sciences