Abstract: Objective To investigate the efficacy of the oxygen uptake efficiency slope (OUES), obtained from submaximal exercise, as an index of cardiopulmonary functional reserve in adults. Methods Exercise tests, following a symptom-limited standard Bruce protocol with simultaneous respiratory gas measurement, were performed on a treadmill in 33 healthy adults free of clinically recognized cardiovascular disease and 31 patients with heart disease. The OUES was derived from the relation between oxygen uptake (VO₂) and minute ventilation (VE) during incremental exercise and was determined by VO₂=alog₁₀VE+b, where a=OUES. The OUES was calculated from data of the first 75%, 90% and 100% of exercise duration (as 75%OUES，90%OUES and 100%OUES). Results The 100%OUES，90%OUES and 75%OUES were not significantly different from each other（F=0.212，P=0.809）. The 75%OUES was slightly lower (1.9%) than the 100%OUES. The 100%OUES, the 90%OUES and the 75%OUES correlated with the peak VO₂ （r=0.836,r=0.824 and r=0.777, P<0.001, respectively）. The VAT correlated with the peak VO₂ （r=0.878，P<0.001），and the VE/VCO₂ at VAT correlated with the peak VO₂ （r=-0.584，P<0.001）. The 100%OUES correlated with the 90%OUES and the 75%OUES （r=0.991 and r =0.945, P<0.001, respectively）. Conclusion The OUES is an index of exercise performance and cardiopulmonary functional reserve on healthy adults and patients with cardiac disease. The 75%OUES from submaximal exercise can be an index of cardiopulmonary functional reserve for patients with cardiac disease.

Key words: oxygen uptake efficiency slope, healthy adult, cardiac diseases, cardiopulmonary exercise test