Comparing individual and population differences in VE/VCO2 slopes using centile growth curves and log-linear allometry

Abstract

Identifying vulnerable groups and/or individuals’ cardiorespiratory fitness (CRF) is an important challenge for clinicians/researchers alike. To quantify CRF accurately, the assessment of several variables is now standard practice including maximal oxygen uptake (VO2) and ventilatory efficiency, the latter assessed using the minute ventilation/carbon dioxide production (VE/VCO2) slope. Recently, reference values (centiles) for VE/VCO2 slopes for men and women aged 20 to 80 have been published, using cardiopulmonary exercise testing (CPX) data (treadmill protocol) from the Fitness Registry and the Importance of Exercise National Database (FRIEND Registry).

In the current observational study we provide centile curves for the FRIEND Registry VE/VCO2 slopes, fitted using the generalised additive model for location, scale and shape (GAMLSS), to provide individuals with a more precise estimate of where their VE/VCO2 slopes fall within the population. We also confirm that by adopting allometric models (incorporating a log-transformation), the resulting ANCOVAs provided more normal and homoscedastic residuals, with superior goodness-of-fit using the Akaike information criterion AIC=14 671 (compared with traditional ANCOVA's AIC=15 008) that confirms allometric models are vastly superior to traditional ANCOVA models.

In conclusion, providing sex-by-age centile curves rather than referring to reference tables for ventilatory efficiency (VE/VCO2 slopes) will provide more accurate estimates of where an individual's particular VE/VCO2 slope falls within the population. Also, by adopting allometric models researchers are more likely to identify real and valid inferences when analysing population/group differences in VE/VCO2 slopes.