The article by Arena et al1 addresses a practical but important issue in the exercise science arena, the assessment of functional capacity, particularly the maximum oxygen uptake (VO_2max). When is it necessary to measure VO₂? When is it appropriate to estimate VO₂ using a variety of submaximal assessment tools and prediction equations?

Arena et al1 discuss the importance of using an appropriately selected testing protocol (the acceleration of exercise intensity) for specific patient populations, in this case, those with heart failure. Another concern for predicting VO_2max is the accuracy of generalized population-based equations derived during submaximal steady-state exercise.

PREDICTING VO2 FROM EXERCISE INTENSITY

Thus, equations derived for the prediction of VO₂ are both equation specific and population specific, addressing two important concerns of exercise professionals when predicting VO₂ and selecting appropriate assessment methods and prediction equations.2

Protocol Specificity

The accuracy of the selected protocol (acceleration rate of exercise intensity) is population specific. In other words, a selected protocol may result in more accurate predictions of VO₂ in one subject group than in another. A more aggressive protocol (more rapid acceleration of exercise intensity) may work well with healthy subjects who have a normal cardiovascular dynamic response to increasing exercise intensity and subsequently normal VO₂ kinetics. On the other hand, those with compromised cardiovascular dynamics and slower VO₂ kinetics, such as patients with heart failure, will tend to be overestimated by these more aggressive and, in many cases, commonly used protocols.

Equation Specificity

Along with protocol selection, prediction equations are outcome specific. Selected equations must be derived from measured parameters consistent with those predicted. For example, the generalized American College of Sports Medicine (ACSM) equations for walking (as well as the other ACSM equations) were developed to estimate the steady-state VO₂ expected at selected exercise intensities. The intent of the ACSM equations was to estimate steady-state exercise conditions, not non-steady-state conditions such as peak exercise values. As expected, by equation design, the ACSM equations will overestimate non-steady-state VO₂, and should not be used to predict VO_2peak.

Population Specificity

Another issue is population specificity. Equations derived using healthy populations may not be accurate for clinical populations, particularly those with complications that may affect oxygen uptake or transport (cardiac or pulmonary), exercise efficiency (orthopedic), or other complications that may alter normal or expected VO₂ dynamics (metabolic). Also, equations derived using a very select population, such as a gender- or age-specific group, may not be accurate across a generalized population.

Estimate Variability

Even if each of the preceding concerns is accounted for, all prediction equations are fraught with wide variability or error for the predicted VO₂, particularly when predicting an individual value. The predicted VO_2max shows systematic error reaching 10%, whereas predictions for individuals show a scatter of +/- 10% of the mean score.3 This error is large enough that it may preclude detection of subtle changes in VO₂ that occur as a result of intervention or may lead to erroneous classification of a patient from a functional capacity or prognostic standpoint.

In summary, it is appropriate and desirable to estimate VO₂ during exercise, particularly when direct measurement is impossible or impractical. In these instances, the exercise professional must ensure that selected prediction equations are applicable to the population being tested, specific to the type of data desired (steady-state versus non-steady-state conditions), and specific to the equipment and protocol used. If not considered, the resulting inaccuracies may augment errors in judgment regarding the patient's clinical management and recommendations for exercise, recreational, and occupational activities.4

MEASURING VO2

Difficulties with measuring VO₂ include expense, equipment, trained technologists, and although tolerable, additional patient discomfort (mouthpiece and noseclip). In addition, the staff must be technically competent and sophisticated enough to acquire accurate data and to interpret accurately and report test outcomes effectively to the clinical staff. Aside from the concerns about measuring VO₂ during exercise tests, when maximum accuracy is essential, gas exchange analysis provides a more precise, reproducible, and objective measure of exercise tolerance.4 The reasons for measuring VO₂ are discussed in the following sections.4,5

Accurate Values

The measurement of VO₂ should be performed when accurate values are essential for the assessment of exercise capacity, when required by research protocol, or when time-dependent follow-up evaluation of treatment results is needed.

Clinical Evaluation

For patients with concomitant cardiac or pulmonary limitations to exercise, the measurement of VO₂ is essential to differentiate the primary limitation. In addition, VO₂ should be measured for those with metabolic or orthopedic limitations that may alter O₂ delivery or use or exercise efficiency.

Clinical Classification

When VO₂ values are a primary determinant for clinical categorization, particularly those that may determine treatment and outcome, VO₂ should be measured. The range of the predictive accuracy of predictive equations may approach or exceed the size or range of the category, resulting in erroneous classification of patients. An example would be the values of 14 ml/kg/min or less required for the placement of patients with severe left ventricular dysfunction on the heart transplantation list.6,7

Therapeutic Interventions

Knowledge of the VO₂ and ventilatory response to exercise will assist with determining the exact degree of functional impairment and help determine an exercise therapy that can be applied at intensities just below the ventilatory threshold or below the level at which ventilation begins to accelerate with increasing exercise intensity. In addition, the measured response to intervention will be more comprehensive including reference values of ventilatory threshold, ventilatory slope, and VO_2max.8

Psychological Factors

Measurement of VO₂ will eliminate the psychological factors that may change treadmill time without changing VO₂ including, feedback, coaching by laboratory personnel, and variation in the patient's ability to tolerate the physical discomfort of exercising, particularly at higher exercise intensities.

Pragmatic Factors

Measurement of VO₂ will minimize the impact on pragmatic or practical factors such as exercise protocol, exercise mode, habituation with procedures, and handrail support during treadmill walking.9,10

The American Heart Association and the American College of Cardiology list of indications for using ventilatory gas exchange with exercise testing concurs.11

SUMMARY

For the assessment of exercise capacity in large populations when the error of the estimate is not crucial and when the needed equipment is not available, the prediction of VO₂ is appropriate and acceptable. When predicting VO₂, the exercise professional must select equations specific to the exercise equipment and protocol used as well as the population tested.

When maximal accuracy of VO₂ measurement is essential, VO₂ must be measured. Such situations may include patient classifications that may affect treatment, research protocols, and repeated testing over time assessing response to a selected treatment method.

References