Oxygen uptake efficiency slope: A new index of cardiorespiratory functional reserve derived from the relation between oxygen uptake and minute ventilation during incremental exercise

Abstract

Objectives. We investigated the usefulness of a new variable, oxygen uptake efficiency slope (OUES), as a submaximal measure of cardiorespiratory functional reserve. The OUES is derived from the relation between oxygen uptake (V̇o2 [ml/min]) and minute ventilation (V̇e [liters/min]) during incremental exercise and is determined by V̇o2 = a log V̇e + b, where a = OUES, which shows the effectiveness of V̇o2.Background. Maximal oxygen uptake (V̇o2 max) is effort dependent. There is no standard submaximal measurement of cardiorespiratory reserve that provides generally acceptable results.Methods. Exercise tests, following a standard Bruce protocol, were performed on a treadmill by 108 patients with heart disease and 36 normal volunteers. Expired gas was continuously analyzed. The OUES was calculated from data of the first 75%, 90% and 100% of exercise duration. We also determined the following submaximal variables: the ventilatory anaerobic threshold (VAT), the slope of the regression line of the minute ventilation-carbon dioxide production relation (V̇eVco2 slope) and the extrapolated maximal oxygen consumption (EMOC). We analyzed the relation of OUES and other submaximal variables against V̇o2max and examined the effects of submaximal exercise on OUES.Results. The correlation coefficient of the logarithmic curve-fitting model was 0.978 ± 0.016 (mean ± SD). The OUES and V̇o2-max had a significant correlation (r = 0.941, p < 0.001). The correlation between V̇o2max and OUES was stronger than that between V̇o2 max and VAT, the V̇eV̇o2 slope or EMOC. The OUES values for 100% and 90% of exercise were not different from each other (at an alpha value of 0.05 and treatment effect of 170 the power of the test [1 − beta] was 0.90); OUES for 75% of exercise was slightly lower (3.5%).Conclusions. Our results suggest that OUES may provide an objective, effort-independent estimation of cardiorespiratory functional reserve that is related both to pulmonary dead space and to metabolic acidosis.