Sex Differences in Parameters of Cardiopulmonary Fitness and their Relationship to Clinical Outcomes in Young People with Cystic Fibrosis

Abstract

Background: Females with cystic fibrosis have higher mortality rates than their male counterparts. Pulmonary function defined as forced expiratory volume in 1 s (FEV1) is a predictor of mortality. However, when pulmonary function is accounted for, females still have a higher mortality rate than males. Independent of lung function, peak oxygen uptake (VO2peak) is a significant predictor of mortality, with higher values of VO2peak relating to a lower risk of mortality. In healthy children, VO2peak is significantly different between sexes, however it has yet to be determined whether sex differences in VO2peak occur in CF. Aim: To identify sex differences in exercise capacity in young people with CF, when appropriately scaled for different body size variables and adjusted for key clinical parameters such as lung function and nutritional and maturity status. Methods: 52 young people (29 males and 23 females) aged 8 – 25 y with CF underwent a ramp cycle test to exhaustion. 47 completed an additional supramaximal cycle test to exhaustion at 110 % of ramp test peak power to verify that a true VO2max was obtained. VO2peak was scaled allometrically and using the ratio standard method. ANCOVAs were utilised to remove the influence of clinical parameters. Results: VO2peak was significantly lower in females than males for absolute VO2peak (p < 0.001, 1.41 ± 0.38 L∙min-1 and 2.17 ± 0.82 L∙min-1, respectively), VO2peak/BM*β (p < 0.001, 53.28 ± 10.93 mL·kg*0.82·min-1 and 73.04 ± 19.57 mL·kg*0.82·min-1, respectively), VO2peak/BSA*β (p < 0.001, 814.58 ± 159.05 mL·m2(*1.29)·min-1 and 1108.06 ± 296.24 mL·m2(*1.29)·min-1, respectively), and VO2peak/Stature*β (p<0.001, 464.76 ± 86.69mL·m*2.42·min-1 and 627.77 ± 183.28mL·m*2.42·min-1, respectively). These remained significant with the inclusion of co- variates, peak height velocity (PHV), age, body mass index (BMI) and FEV1. Conclusions: Irrespective of the scaling method and body size variable used or if adjusted for key clinical parameters (age, PHV, BMI and FEV1), young females with CF have a reduced VO2peak compared to males. Future studies should consider sex differences in exercise capacity as a potential contributor to differences in mortality.