Effect of High volumeHemodiafiltration on Lung Oxygenation

Patients will receive HVHDF treatment for 48 hours. HVHDF will be performed via indwelling central venous catheter. The blood flow will be 180-240 ml/min, and ultrafiltration rate will be 70 ml/kg/h during HVHF. The substitute fluid will be infused with pre-dilution. Heparin will be used for anti-coagulation, whose initial dose will be 15-25 U/kg, and maintenance dose is 5-15 U/kg/h. aPTT time maintained between 60-80 second and will be checked every 12 hours.

Patients will receive the usual care