Objective To investigate the effect of PEEP on cardiac preload in patients with septic shock undergoing mechanical ventilation.Method In a prospective clinical trial,we enrolled 15 patients with septic shock admitted to medical intensive care unit(MIGU),Peking Union Medical College Hospital(PUMCH)from Sept 2007 to Mar 2008.All patients were mechanically ventilated with volume control mode.Hemodynamics was monitored by the transpulmonary thermodilution technique and pulse contour analysis(PiCCOplus).PEEP was increased from 0 to 20 emH_2O(if tolerated),by 5 cmH_2O.The effect of PEEP on cardiac preload was analyzed by one-way ANOVA and Pearson's correlation analysis.Results Fifteen patients were enrolled,with 5 females and 10 males.Mean age was(67.6±19.5)years old,APACHE Ⅱ score was(22.1±7.5),baseline PaO_2/FiO_2(225.6±89.2)mmHg at PEEP of(8.5±3.6)cmH_2O.The ICU mortauty was 67%.With increasing level of PEEP,CVP increased significantly(P=0.002),while GEDI did not show any significant change.In addition,ΔCI positively correlated with △GEDI(r=0.6),hut not with ΔCVP.Conclusions GEDI,rather than CVP,could provide vaud estimates of cardiac preload at high PEEP.