Use of Heart Rate Index to Predict Oxygen Uptake - A Validation Study

An equation that uses heart rate index (HRI) defined as HR/HRrest to predict oxygen uptake (VO₂) in METs (e.g., METs = 6 × HRI - 5) has been developed retrospectively from aggregate data of 60 published studies. However, the prediction error of this model as used by an individual has not been established. Therefore, the purpose of this study was to examine the predictive validity of the HRI equation by comparing submaximal and maximal VO₂ predicted by the equation (VO₂-Pred) with that measured by indirect calorimetry (VO₂-Meas). Sixty healthy adults (age 20.5 ± 2.4 yr., body mass 69.4 ± 13.4 kg, height 1.7 ± 0.1 m) underwent a VO₂max test and an experimental trial consisting of a 15-min resting measurement and three successive 10-min treadmill exercise bouts performed at 40%, 60% and 80% of VO₂max. VO₂ and HR were recorded during both the submaximal and maximal exercises and used to obtain VO₂-Pred and VO₂-Meas for each intensity and for VO₂max. Validation was carried out by paired t-test, regression analysis, and Bland-Altman plots. A modest but significant (p < 0.05) correlation was observed between VO₂-Meas and VO₂-Pred at 40% (r = 0.58), 60% (r = 0.53), and 80% of VO₂max (r = 0.56) and at VO₂max (r = 0.50). No differences between VO₂-Pred and VO₂-Meas were found at 40% (5.53 ± 1.21 vs. 5.28 ± 0.98 METs, respectively) of VO₂max, but VO₂-Pred was higher (p < 0.05) than VO₂-Meas at 60% (8.42 ± 1.77 vs. 7.96 ± 1.39 METs, respectively) and 80% (10.79 ± 2.13 vs. 10.29 ± 1.81 METs, respectively) of VO₂max. In contrast, VO₂-Pred was lower (p < 0.05) than VO₂-Meas at VO₂max (12.32 ± 2.30 vs. 13.38 ± 2.24 METs, respectively). Standard errors of the estimate were 0.81, 1.20, 1.54, and 1.97 METs at 40%, 60%, 80% of VO₂max and at VO₂max, respectively. These results suggest that further investigation aimed to establish the accuracy of using HRI to predict VO₂ is warranted.