Comparative Analysis of Direct and Indirect Methods for the Determination of Maximal Oxygen Uptake in Sedentary Young Adults

Maximal oxygen uptake is a powerful prognostic indicator and a reliable measure of physical conditioning. It can be measured directly by cardiopulmonary exercise testing (CPET) or indirectly by formulas derived from conventional protocols. We compared the VO 2 max obtained by formula using exercise testing with Bruce protocol (BP) with the VO 2 max obtained by CPET on the treadmill. We selected 41 healthy, non-obese, physically inactive young volunteers, aged between 21 and 50 years, residents of Florianópolis, Brazil. Twenty-one women (52%) with mean age of 35.62 ± 8.83 years, and 20 males, with mean age of 32.5 ± 7.18 years participated in the Abstract Background: Maximal oxygen uptake is a powerful prognostic indicator and a reliable measure of physical conditioning. It can be measured directly by cardiopulmonary exercise testing (CPET) or indirectly by formulas derived from conventional protocols. Objective: We compared the VO 2 max obtained by formula using exercise testing with Bruce protocol (BP) with the VO 2 max obtained by CPET on the treadmill. Methods: We selected 41 healthy, non-obese, physically inactive young volunteers, aged between 21 and 50 years, residents of Florianópolis, Brazil. Results: Twenty-one women (52%) with mean age of 35.62 ± 8.83 years, and 20 males, with mean age of 32.5 ± 7.18 years participated in the study. Statistically significant differences were found for VO 2 max between the two methods (BP - 42.31 ± 5.21 ml/kg.min vs. CPET - 30.46 ± 5.50 ml/kg.min., p < 0.0001). The Bruce formula overestimated the result by 34.1% (BP - 45.95 ± 3.94 ml/kg.min vs. CPX - 34.27 ± 4.20 ml/kg.min, p < 0.0001) for men, and by 44.8% (BP - 38.84 ± 3.72 ml/kg.min vs. CPX - 26.83 ± 3.90, p < 0.0001) for women. A moderate correlation was observed between the methods (r = 0.65). When classifying the results according to the table of aerobic capacity of the American Heart Association, the agreement was null (kappa = 0.0034; Pearson chi2 = 0.001). Conclusion: VO 2 estimated by BP is not capable of demonstrating the true aerobic capacity in these individuals, while CPET is an important tool for early detection of diminished functional capacity in sedentary young men and women. (Int J Cardiovasc Sci. 2019;32(4):362-367) study. Statistically significant differences were found for VO 2 max between the two methods (BP - 42.31 ± 5.21 ml/ kg.min vs. CPET - 30.46 ± 5.50 ml/kg.min., p < 0.0001). The Bruce formula overestimated the result by 34.1% (BP - 45.95 ± 3.94 ml/kg.min vs. CPX - 34.27 ± 4.20 ml/ kg.min, p < 0.0001) for men, and by 44.8% (BP - 38.84 ± 3.72 ml/kg.min vs. CPX - 26.83 ± 3.90, p < 0.0001) for women. A moderate correlation was observed between the methods (r = 0.65). When classifying the results according to the table of aerobic capacity of the American Heart Association, the agreement was null (kappa = 0.0034; Pearson chi2 = 0.001). Conclusion: VO 2 estimated by BP is not capable of demonstrating the true aerobic capacity in these individuals, while CPET is an important tool for early detection of diminished functional capacity in sedentary young men and women.


Introduction
Maximal oxygen uptake (VO 2 max) is the product of the arteriovenous oxygen difference and cardiac output. 1,2 It is the most important physiological measurement in defining functional capacity of an individual (aerobic power). 3 VO 2 max varies with body weight, age, physical activity level and presence of cardiorespiratory disease. 1,4,5 The parameter is used to prescribe exercise, evaluate the effects of training and therapeutic interventions, and as a risk stratification tool for the occurrence of cardiovascular disease. 3,6 VO 2 max can be directly measured by analysis of breathing gases during cardiopulmonary exercise testing (CPET) or estimated by the stress test using prediction equations. 2 Physical fitness has been shown in several studies to be an important predictor of all-cause and cardiovascular mortality. [7][8][9][10][11][12] On the other hand, a sedentary lifestyle is an important cardiovascular risk factor, with increasing prevalence in the world population. 13 The Bruce protocol (BP) is the main non-invasive method for cardiovascular assessment performed in asymptomatic individuals. 14 However, functional capacity estimated by formulas during the test may be inaccurate for physically inactive young individuals, leading to a wrong assessment of fitness and minimizing the real cardiovascular risk posed by a low physical fitness, commonly seen in these individuals.
Aiming at evaluating the difference in functional capacity between the direct and indirect method in inactive young individuals, VO 2 max was measured by the CPET and the BP formulas.
All tests were performed by an experienced cardiologist, qualified to perform ergometric test and CPET. Treadmill tests were carried out following the Brazilian Society of Cardiology guidelines on ergometric and cardiopulmonary tests. 3 VO 2 max was considered as the highest VO 2 reached during stress (VO 2 peak). After direct or indirect measurement of VO 2 , participants were classified by cardiorespiratory fitness using the American Heart Association table and grouped into four groups -low, moderate, high and very high. 5 All participants signed the informed consent form and the study protocol was approved by the ethics committee of the institution.
Statistical analysis was performed using the Stata SE 9 and the Microsoft Excel software. The Student's t-test was used to compare means between matched samples. Correlation was analyzed by Pearson correlation. Agreements were analyzed using weighted kappa statistic. A p ≤ 0.05 was set as statistically significant.

Results
Twenty-one (52%) of the 41 individuals included were women. Mean age was 34.1 ± 8.12, varying from 21 to 50 years. Mean body mass index (BMI) was 24.5 Kg/m² ± 3.34, and mean weight and height was 72.8 ± 15.7 kg and 1.73 ± 0.11 m, respectively. Mean VO 2 max was 42.31 ± 5.21 mL/Kg.min for the BP and 30.46 ± 5.50 mL/Kg.min for the CPET with ramp protocol, p < 0.0001. Analysis by sex ( Figure 1) revealed significant difference between VO 2 max estimated by the BP and the CPET (38.85 ± 3.72 mL/Kg.min versus 26.83 ± 3.90 mL/Kg.min, respectively, p < 0.0001) for women, and for men (45.94 ± 3.94 mL/ Kg.min versus BP 34.26 ± 4.21 mL/Kg.min, respectively, p < 0.0001) The BP overestimated VO 2 max by 44.8% for women and by 34.1% for men compared with the CPET.
No difference was found in maximal effort, measured by maximal heart rate (HRmax) between the tests. During the BP and CPET, HR max was 184.8 ± 9.47 vs 183.1 ± 10.03, respectively, for men, and 179.8 ± 11.68 versus 180.8 ± 12.63, respectively, for women, p = NS).
There was a moderate correlation between the two methods (r = 0.65), and the agreement between the tests regarding cardiorespiratory fitness was null ( Figure  2) (Kappa = 0.0034 and chi-square = 0.001). Most participants showed high or very high cardiorespiratory fitness by the BP and moderate or low cardiorespiratory fitness according to the CPET.

Discussion
Cardiorespiratory fitness has been shown to be an important prognostic marker of morbidity and mortality in young, older, healthy individuals with heart diseases. 8,15,16 Most studies have classified individuals according to their performance in ergometric tests. In Brazil and in the USA, the ergometric test on the treadmill has been widely used, and the BP is the main test performed. 14 Although individual risk to stress tests may be stratified by test duration, functional capacity may be overestimated in young adults, even in physically inactive ones. In this regard, when age range is used for risk stratification, conventional stress test is not an accurate test to evaluate functional capacity. Sedentary habits have long-term, cumulative effects, and several studies have shown that it is never too late to decrease the risk by improving physical fitness.
In most computer programs for exercise stress testing in Brazil, VO 2 is estimated using the BP formula, and used to classify subjects according to cardiorespiratory fitness. Our study showed an important overestimation of the indirect assessment of VO 2 by the BP.
In 1973, Bruce et al., 1 studied 295 physically active or inactive adults by exercise testing on the treadmill, and direct analysis of gases, from which derived the formulas currently used. Ong et al., 4 measured VO 2 max by CPET and compared it with that obtained by cycle ergometer test (prediction equations). The formulas overestimated the results by 13.74% for men and 10.55% form women. 4 Fairbarn et al., 17 evaluated 231 non-athletes aged from   19 In our study sample, results obtained from the treadmill test were even higher, maybe because we have used a treadmill ergometer. Similar to our study, Fairbam et al., 17 also reported the biggest differences in VO 2 max among men, although they used an ergometer cycle to assess aerobic capacity. In a nation-wide Brazilian study published in 2011, Peserico et al., 20 assessed aerobic capacity in trained female runners, by measuring VO 2 max both by direct method and indirectly by Foster's formula (1996) using a treadmill ergometer. The authors found that VO 2 was significantly underestimated when estimated by the prediction formula as compared with direct analysis of gases, indicating substantial limitations of the approach in determining functional capacity in these individuals. 20 Most of previous studies reported contrasting results, showing an overestimation of the VO 2 max indirectly estimated by regression and conventional ergometer test, regardless of the protocol and type of ergometer used. 21,22 Also, according to Santos 21 and Rondon et al., 22 the results of VO 2 max obtained by indirect measurement are influenced by cardiorespiratory fitness of the study subjects. In these studies, greater VO 2 max values, estimated by the ACSM formula, were higher in individuals with poor cardiorespiratory fitness than in those with moderate fitness. These findings suggest that both the type of exercise test protocol and the type of prediction formulas may affect VO 2 estimation, by either overestimating or underestimating the true values. Our study was the first to perform treadmill ergometer exercise test in a group composed of both men and women, young and older subjects, physically active and inactive individuals for a comparative analysis of VO 2 max directly measured by CPET with that estimated by formulas.
VO 2 prediction equations derive from studies conducted in North America and Europe. Thus, the results may not be extended to other populations, as pointed by Ong et al., 15 The formula used in the BP 1 was   23 and a direct and unequivocal measurement of VO 2 gives us the chance to evaluate actual cardiorespiratory fitness of each individual by sex and age range.
The fact that the tests were performed on different days, using different protocols (Bruce versus ramp) may have been a limitation of this study. However, the hemodynamic parameters obtained from the tests were not significantly different, allowing us to infer that there was a similar cardiorespiratory and physical performance by the subjects in both tests. In the present study, we found a significant discrepancy in the classification of cardiorespiratory fitness by the AHA and by the VO 2 prediction formulas. The BP is more widely used for VO 2 calculation in clinical practice in Brazil than spiroergometric tests. Therefore, a possible overestimation of VO 2 values by the BP results in an erroneous evaluation of the cardiorespiratory fitness of the individuals. This, in turn, may represent a lack of opportunity to advise young individuals about the negative effects of a sedentary lifestyle and a low cardiorespiratory capacity.

Conclusion
The assessment of aerobic capacity is an important risk stratification tool in young, physically inactive subjects. Prediction formulas of VO 2 max, derived from cycle or treadmill exercise tests using the BP, are not able to reproduce the true cardiorespiratory capacity in this population, overestimating it. The present study draws attention to the need for an accurate measurement of fitness in this group by CPET, aiming at enabling early warning of these individuals about the risks of a sedentary lifestyle.

Author contributions
Conception and design of the research: Rocha Neto AM, Herdy AH. Acquisition of data: Rocha Neto AM, Herdy AH, Souza P. Analysis and interpretation of the data: Rocha Neto AM, Herdy AH, Souza P. Statistical analysis: Rocha Neto AM, Herdy AH, Souza P. Obtaining financing: Rocha Neto AM. Writing of the manuscript: Rocha Neto AM, Souza P. Critical revision of the manuscript for intellectual content: Rocha Neto AM, Souza P.

Potential Conflict of Interest
No potential conflict of interest relevant to this article was reported.

Sources of Funding
There were no external funding sources for this study.

Study Association
This study is not associated with any thesis or dissertation work.

Ethics approval and consent to participate
This article does not contain any studies with human participants or animals performed by any of the authors.