Objective To observe oxygen uptake efficiency plateau (OUEP,i.e.highest (V)O2/(V)E) and carbon dioxide output efficiency (lowest (V)E/(V)CO2) parameter changes during exercise in normal subjects.Methods Five healthy volunteers performed the symptom limited maximal cardiopulmonary exercise test (CPET) at Harbor-UCLA Medical Center.(V)O2/(V) E and (V) E/(V) CO2 were determined by both arterial and central venous catheters.After blood gas analysis of arterial and venous sampling at the last 30 seconds of every exercise stage and every minute of incremental loading,the continuous parameter changes of hemodynamics,pulmonary ventilation were monitored and oxygen uptake ventilatory efficiency ((V)O2/(V)E and (V) E/(V)CO2) was calculated.Results During CPET,as the loading gradually increased,cardiac output,heart rate,mixed venous oxygen saturation,arteriovenous oxygen difference,minute ventilation,minute alveolar ventilation,tidal volume,alveolar ventilation and pulmonary ventilation perfusion ratio increased near-linearly (P ＜0.05-0.01,vs.resting); arterial oxygen concentration maintained at a high level without significant change (P ＞ 0.05) ; stroke volume,respiratory rate,arterial partial pressure of carbon dioxide,arterial blood hydrogen ion concentration and dead space ventilation ratio significantly changed none-linearly (compare resting state P ＜ 0.05-0.01).OUE during exercise increased from 30.9 ± 3.3 at resting state to the highest plateau 46.0 ± 4.7 (P ＜ 0.05 vs.resting state),then,declined gradually after anaerobic threshold (P ＜ 0.05 -0.01,vs.OUEP) and reached 36.6 ±4.4 at peak exercise.The (V)E/(V)CO2 during exercise decreased from the resting state (39.2 ± 6.5) to the minimum value (24.2 ± 2.4) after AT for a few minutes (P ＞ 0.05 vs.earlier stage),then gradually increased after the ventilatory compensation point (P ＜ 0.05 vs.earlier stage) and reached to 25.9 ± 2.7 at peak exercise.Conclusions Cardiac and lung function as well as metabolism change during CPET is synchronous.In the absence of pulmonary limit,appearing before and after anaerobic threshold,OUEP and lowest (V) E/(V) CO2 could be used as reliable parameters representing the circulatory function.