Comparing individual and population differences in minute ventilation/carbon dioxide production slopes using centile growth curves and log-linear allometry

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 (V'(CO2)) and ventilatory efficiency, the latter assessed using the minute ventilation/carbon dioxide production (V'(E)/V'(CO2)) slope. Recently, reference values (centiles) for V'(E)/V'(CO2) slopes for males and females 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 V'(E)/V'(CO2) slopes, fitted using the generalised additive model for location, scale and shape (GAMLSS), to provide individuals with a more precise estimate of where their V'(E)/ V'(CO2) 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=15008) 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 (V'V-E/'(CO2) slopes) will provide more accurate estimates of where an individual's particular V'(E)/V'(CO2) 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 V'(E)/V'(CO2) slopes.