Purpose The aim of this observational study was to investigate the influence of continuous positive airway pressure(CPAP)on arterial blood gas and lactate,markers of tissue hypoxia,among obstructive sleep apnea syndrome(OSAS)patients,and determine the risk factor of serum lactate and PH in OSAS patient.Materials and methods One-hundred and nine patients with newly diagnosed OSAS were enrolled in the study.All individuals were treated with CPAP for one night.Venous lactate and arterial blood gas were gathered from all subjects in the morning at the end of polysomnography,and the next morning after CPAP treatment.Results Of the 109 selected subjects,average lactate were 2.23±0.59,and mean PH,Pa02,PaC02 were 7.380±0.23,88.14±17.83,38.70±4.28,respectively.Compared to baseline,lactic acid significantly reduced(2.10±0.50,p=0.03),while PH increased remarkly(7.388±0.27,p<0.05)after CPAP treatment.In addition,age,neck circumference and the polysomnographic parameters,including AHI,ODI,mean Sp02 and TS90%were correlated with lactate(all p<0.05),and significantly associations were found between age,neck circumference,ODI and PH.Finally,after adjusting for confounding factors,TS90%was the major contributing predictor for increased lactate(p<0.05),and age significantly predicated the PH(p<0.05).Conclusions The results indicate that CPAP treatment can reduce serum lactate and increase PH in OSAS patients,and may alleviate acid-base disorders in OSAS.Furthermore,TS90%was a risk factor for elevated lactate,and age was independently associated with PH.