Background The heme oxygenase/carbon monoxide(HO/CO)system plays an important role in the development of hepatic fibrosis.The level of the HO/CO can be directly obtained by determining the carboxyhemoglobin(COHb)level.The aims of this study were to reveal the significance of COHb in patients with hepatitis B virus-related cirrhosis(HBC)complicated by hepatic encephalopathy(HE),and to further investigate the influence of the HO/CO pathway on the end-stage cirrhosis,hoping to find a reliable indicator to evaluate the course of HBC.Methods According to the diagnostic criteria,63 HBC inpatients with HE were enrolled in group H.Patients regaining awareness with current therapies were categorized into group P-H.Comparisons were made with a control group(group N)consisting of 20 health volunteers.The levels of COHb,partial pressure of oxygen(PaO2)and oxygen saturation(SaO2)were determined by arterial blood gas analysis method.The incidences of hepatorenal syndrome(HRS),upper gastrointestinal bleeding,esophagogastric varices and spontaneous bacterial peritonitis(SBP)in group H were recorded.COHb levels in different groups were compared,and the correlations of COHb levels with HE grades(Ⅰ,Ⅱ,Ⅲ,and Ⅳ),PaO2,SaO2 and hypoxemia were analyzed.Results The COHb level in group P-H((1.672±0.761)％)was significantly higher than that in group N((0.983±0.231)％)(P＜0.01),and the level in group H((2.102±1.021)％)was significantly higher than groups P-H and N(P＜0.01).A positive correlation was observed between the COHb concentration and the grade of HE(rs=0.357,P=0.004).There were no significant differences of COHb levels between HE patients with and without complications such as esophagogastric varices((2.302±1.072)％ vs.(1.802±1.041)％,P＞0.05)or the occurrence of SBP((2.960±0.561)％ vs.(2.030±1.021)％,P＞0.05).Compared with HE patients with HRS,the level of COHb was significantly higher in HE patients without HRS((2.502±1.073)％ vs.(1.981±1.020)％,P=0.029).The COHb level had a negative correlation with PaO2(r=-0.335,P=0.007)while no statistically significant relationship was found with SaO2(r=-0.071,P＞0.05).However,when the above two parameters met the diagnostic criteria of hypoxemia,the COHb concentration increased((2.621±0.880)％ vs.(1.910±0.931)％,P=0.011).Conclusions COHb is a potential candidate to estimate the severity and therapeutic effect of HE.The levels of COHb may be tissue-specific in cirrhotic patients with different complications.