Assessing maximal exercise capacity: peak work or peak oxygen consumption?

BACKGROUND Exercise capacity assessed by cardiopulmonary exercise testing is usually measured by peak oxygen consumption (VO2). However, not uncommonly, patients achieve a relatively higher work load (peak work) compared to their peak VO2. In these situations it is difficult to know which parameter to use in assessing exercise capacity. The purpose of this study was to determine whether there are distinguishing physiological characteristics of patients with discordance between percent-of-predicted peak work versus peak VO2, in order to understand how to use these measurements in interpreting exercise capacity. METHODS We conducted a retrospective study of 172 cardiopulmonary exercise tests performed at our institution between 2003 and 2010. RESULTS The subjects in the higher peak work group demonstrated higher ventilatory efficiency (lower slope of minute ventilation to carbon dioxide production) and lung function (FEV1 and FVC), a greater breathing reserve (higher breathing reserve, lower VE/maximal voluntary ventilation), and achieved a higher maximal heart rate. Subjects in the higher maximum VO2 group were heavier, had lower ventilatory efficiency, and had a reduced breathing reserve. Multivariate logistic regression analysis showed that the predominant independent factors associated with group assignment were body mass index, breathing reserve, and peak heart rate. The subjects with higher percent-of-predicted peak work than peak VO2 had a lower body mass index, a greater breathing reserve, and a higher peak heart rate. CONCLUSIONS The observation that there are distinguishing physiological features between those who have a higher peak work and those who have higher peak VO2 provides insight into the underlying processes determining maximal exercise capacity.