Abstract: Objective To discuss whether priming solution with blood can reach a physiologic state after ultrafiltration and investigate the influence of this method on electrolytes, acidbase balance and cardiopulmonary function in patients in perioperative period. Methods Forty patients with congenital heart diseases treated in our hospital from February to June 2009 were enrolled in this study. The weight of these patients was less than 8 kg. They were randomly divided into two groups: the experimental group (n=20, 12 males and 8 females; age: 131.00±103.00 d; weight: 4.14±0.96 kg） and the control group (n=20, 11 males and 9 females; age: 127.00±88.00 d; weight: 4.38±1.05 kg). For patients in the experimental group, the priming solution was filtered with a blood ultrafilter in the cardiopulmonary bypass(CPB) circuit for twenty minutes, while the priming solution circulated in the CPB circuit without filtration for patients in the control group before operation. Data were obtained from the priming solution before and after ultrafiltration. Blood gas analysis was done and indexes like the electrolytes were detected during the operation. Fractional shorting (FS), ejection fraction (EF) and cardiac output (CO) were measured by echocardiography. Pulmonary function change was also observed. Results No death occurred in both groups. Mechanical ventilation time for the experimental group was significantly shorter than that of the control group (2.7±0.3 d vs. 4.1±0.4 d，Plt;0.05). After ultrafiltration for the experimental group, all indexes of the priming solution reached normal values: pH increased (from 6.89±0.22 to 7.40±0.57, P=0.001); base excess increased (from -16.12±0.98 mmol/L to +0.31±2.40 mmol/L, P=0.000); potassium concentration decreased (from 10.33±2.13 mmol/L to 4.27±0.93 mmol/L, P=0.000); interleukin8 (IL-8) decreased (from 78.40±6.10 pg/ml to 64.30±48.10 pg/ml, P=0.036); and bradykinin decreased (from 5 982±1 353 pg/ml to 531.00±35.00 pg/ml, P=0.031). The decrease of FS, EF and CO in the experimental group was smaller than that of the control group. Four hours after surgery, CO in the experimental group was significantly higher than that in the control group (2.77±0.95 L/min vs. 1.66±0.75 L/min, P=0.001). Twentyfour hours after surgery, EF in the experimental group was significantly higher than that in the control group (67.44%±6.89% vs. 61.17%±9.02%, P=0.003). Six hours after surgery, alveolararterial oxygen difference (A-aDO2) and respiratory index (RI) increased significantly (Plt;0.05) in both groups, and then started to drop to normal. Patients in the experimental group recovered gas exchange earlier than patients in the control group. Fortyeight hours after operation, A-aDO2 and RI in the experimental group were significantly lower than those in the control group (Plt;0.05). Conclusion Ultrafiltration of priming solution may confer an advantage in maintaining more physiological conditions, reducing inflammatory mediators, and improving cardiopulmonary function after operation, which is very important in performing cardiac surgery on the newborn and infants with complex congenital heart diseases.