Continuous Veno-Venous Hemofiltration (CVVH) can eliminate metabolic products effectively and replace renal function partly. It is widely used in intensive care units (ICUs), especially for patients with acute kidney injury/failure in an unstable hemodynamic status. Lactate is a molecule smaller than glucose, which can pass through filtration membrane freely in CVVH. Therefore, the blood lactate concentrations would no longer reflect tissue oxygenation status in patients with unstable hemodynamic status. However, there is no prospective study evaluated the effect of CVVH on lactate removal in critically ill patients. The influence of different dose of CVVH on lactate elimination is not clear in patients with different level of serum lactate. Our study aimed to find out how the dose of CVVH effects on lactate clearance, and then re-estimate the prognostic value of lactate in critically ill patients with CVVH.

Inclusion Criteria:

1. The critical ill patients with acute kidney injury
2. Patients requiring continuous veno-venous hemofiltration
3. APACHE II scores more than 12
4. Arterial lactate is higher than 0.5mmol/L but lower than 15mmol/L (due to the sensitivity and optimal measuring range of the equipment)
5. Continuous veno-venous hemofiltration for at least 12 hrs
6. Hemo-filter age of <24 hrs

Exclusion Criteria:

1. APACHE II scores lower than 12
2. patients' age are more than 75Y or lower than 18Y

This study was carried out in Surgical Intensive Care Unit (SICU) of the first affiliated hospital, Sun Yat-sen University.The project has been approved by Ethics Committee of Sun Yat-sen University. All the subjects have signed written informed consent.