Validity of the six-minute oxygen uptake efficiency slope in obese children to determine cardiorespiratory fitness

Poor cardiorespiratory fitness (CRF) is associated with increased all cause morbidity and mortality. CRF assessment in obese youth provides valuable information on present and future health status as well as assessing efficacy of an exercise intervention. Maximal oxygen uptake (VO2max) is often difficult to achieve in a sedentary paediatric population due to a lack of motivation and peripheral fatigue, resulting in submaximal test results. Due to the tight linear relationship throughout exercise, the oxygen uptake efficiency slope (OUES) holds possible utility as a predictor of VO2max.

PURPOSE: To determine whether a submaximal six-minute OUES (6minOUES) protocol accurately predicts maximal oxygen uptake (VO2max) in obese children.

METHODS: Eighty-nine obese children (BMI>95th percentile for age and sex) completed a maximal graded exercise treadmill test with ventilatory expired gas analysis. Participants who satisfied oxygen uptake plateau criteria and achieved a peak respiratory exchange ratio (RER) ≥1.05 were classified as reaching VO2max and were included in this analysis. The OUES was determined as the slope of the line when VO2 (ml/min) was plotted against logVE. Maximal OUES (OUESmax) was calculated from start to end of the test while 6-minute OUES (6minOUES) was calculated from start of the test to the 6-minute time-point. A multivariate linear regression was used to determine the relationship between VO2max, 6minOUES, and OUESmax, as well as to derive a prediction equation for VO2max.

RESULTS: Thirty-two obese children (age 12.4±1.9; Tanner puberty stage 3 (2-4), female 59%) achieved a VO2max of 31.2±5.5 ml/kg/min. Participants attained an OUESmax of 2.82±0.89 and a 6minOUES of 2.77±1.01. VO2max (L/min) was strongly correlated with OUESmax in L/min (r2 = 0.93, p<0.001) and 6minOUES in L/min (r2 = 0.83, p<0.001) independent of Tanner stage and sex. Furthermore, 6minOUES was strongly correlated with OUESmax (r2 = 0.91, p<0.001). The VO2max prediction equation derived from this cohort is as follows: VO2max= 0.651*6minOUES + 0.026*Tanner Stage + 0.565 (r = 0.90, standard error of the estimate = 0.34).

CONCLUSIONS: The 6minOUES treadmill protocol may be used to predict VO2max in obese children when oxygen uptake plateau criteria and peak RER indicate submaximal exercise performance.