Acute Kidney Injure (AKI) is a syndrome with high incidence and prevalence in Intensive Care Units (ICU). It is estimated that 50% of the in the sector present AKI at some point and 10 to 15% require renal replacement therapy (RRT). Although studies do not show the superiority of continuous methods, the most severely ill patients are directed to this type of RRT. A disadvantage of continuous therapies is the need for anticoagulation. Critically ill patients have a pro-clotting state (inflammation) and several risk factors for bleeding (coagulopathies, postoperative, large vessel puncture).

On the one hand, ineffective anticoagulation compromises the efficiency of the procedure, shortens the life of the extracorporeal system, consumes resources and increases blood loss due to unexpected and early filter clotting. There is no consensus on what would be the optimal blood flow (Qb) in continuous dialysis, especially when regional citrate anticoagulation (RCA) is used. Theoretically, a higher flow rate would prevent stasis in the system and decrease the risk of filter clotting. Studies show conflicting results. Increasing Qb from 150 to 250 mL/min showed that circuit life and the chance of coagulation were similar. On the other hand, blood flow is important for maintaining the filtration fraction (FF), the ratio of ultrafiltrate flow to plasma flow. Ideally, the FF should be kept below 25% to avoid hemoconcentration and coagulation of the filter. Therefore, the higher the convection rate, the higher the blood flow should be to keep the FF in the optimal range. Since the anticoagulation capacity of citrate is dependent on its concentration, around 4 mmol/L of blood, by increasing the blood flow, the citrate infusion is proportionally increased. Theoretically, the higher citrate load offered should be metabolized and, in theory, could cause its overload with the occurrence of metabolic alkalosis and hypernatremia. This situation occurs when its maximum metabolizing capacity is not reached and there is an excess of citrate infusion relative to the buffering requirement. Thus, we intend to evaluate filter useful life, metabolic control, electrolyte profile and acid-base balance in ICU patients undergoing continuous venovenous hemodiafiltration (CVVHDF), regional citrate anticoagulation during blood flow augmentation.

Acute kidney injury (AKI) is a clinical syndrome with a high incidence and prevalence in Intensive care units (ICU). It is estimated that 50% of ICU patients have AKI at some point.

About 10-15% of these individuals require renal replacement therapy (RRT). Although studies have not conclusively shown the superiority of continuous methods, the most severe patients are usually referred for this type of therapy.

The main indications for continuous therapies are hemodynamic instability, cardiogenic shock, severe respiratory insufficiency, risk situations for brain edema, hypercatabolism, need for strict volume control, acute liver disease and major sodium disturbances. One of the main disadvantages of continuous therapies is the necessity of anticoagulation. Critically ill patients have a pro-clotting state (inflammation) and several risk factors for bleeding (coagulopathies, postoperative, large vessel puncture). On the one hand, the lack or ineffective anticoagulation compromises the efficiency of the procedure, shortens the life of the extracorporeal system, consumes resources and increases blood loss due to unexpected and early filter coagulation. On the other hand, excessive use of anticoagulants, especially heparin, is associated with bleeding and increased transfusions.

In this scenario, regional anticoagulation with citrate (RCA) has become the method of choice in the different modalities of continuous dialysis. When compared to heparin, the use of regional citrate anticoagulation is associated with less bleeding and transfusion need and longer life of the extracorporeal system. It also seems to decrease endothelial activation, neutrophil degranulation and activation of the complement system.

The anticoagulate property of citrate is based on its binding to calcium (Ca). Citrate quenches Ca in the extracorporeal system, an essential cofactor in several steps of coagulation. Optimal anticoagulation is achieved when ionic Ca concentration in the extracorporeal circuit is maintained between 0.25 and 0.35 mmol/L. This is usually achieved with a citrate level in the circuit around 4mmol/L of blood. Depending on the modality chosen and other factors, up to 60% of the citrate-Ca complex is eliminated during passage through the filter (molecular weigh of 298 Daltons and partition coefficient of 1.0). The rest is metabolized in the Krebs cycle mainly in the liver, kidneys and skeletal muscles. Each mol of trisodium citrate causes 3 moles of bicarbonate thus correctly, partially or completely, the metabolic acidosis resulting from renal failure. Ca and sodium (Na) are released into the systemic circulation. Trisodium citrate also increases the strong ion difference due to the high sodium concentration in the solution, thus increasing the buffering capacity. In parallel it is necessary the Ca replacement to maintain normal calcemia. The citrate also quenches magnesium, which can lead to a disturbance of this electrolyte.

There is no consensus on what the optimal blood flow (Qb) would be in continuous dialysis, especially when using regional citrate anticoagulation. Theoretically, a higher blood flow would prevent stasis in the system and thus decrease the risk of filter coagulation. Studies show conflicting results. For example, one study evaluated increasing Qb from 150 to 250 mL/min and showed that circuit useful life and the chance of coagulation of the extracorporeal system were similar between the two groups. On the other hand, blood flow is important for maintaining the filtration fraction (FF), the ratio of ultra-filtrated flow to plasma flow (blood flow minus hematocrit). Ideally, the FF should be kept below 25% to avoid hemoconcentration and coagulation of the filter capillary fibers. So the higher the convection rate (ultrafiltration), the higher the blood flow should be to keep the FF in the optimal range.

Since the anticoagulation capacity of citrate is dependent on its concentration, around 4 mmol/L of blood, by increasing blood flow, citrate infusion is proportionally increased. Theoretically, the higher citrate load offered should be metabolized and, in theory, could lead to citrate overload with the occurrence of metabolic alkalosis and hypernatremia. This situation occurs when the maximum capacity of citrate metabolization is not reached and there is an excess of citrate infusion relative to the buffering requirement. The total Ca/systemic ionic Ca ration remains normal, below 2.5. The oversupply of citrate can be easily corrected by decreasing the bicarbonate concentration of the dialysate, increasing the dialysate dose or decreasing the citrate infusion.

Therefore, we intend to evaluate filter useful life, metabolic control, electrolyte profile and acid-base balance in ICU patients with AKI undergoing continuous venovenous hemodiafiltration (CVVHDF), regional anticoagulation with citrate during increased blood flow.

Hypothesis: Increasing blood flow during continuous venovenous hemodiafiltration prevents stasis in the system and thus reduces the risk of filter coagulation. Blood flow is important for maintaining the filtration fraction (FF), the ratio of ultrafiltrate flow to plasma flow (blood flow minus hematocrit). Ideally, the FF should be kept below 25% to avoid hemoconcentration and coagulation of the filter capillary fibers. So the higher convection rate (ultrafiltration), the higher the blood flow should be to keep the FF in the optimal range. Therefore, it is expected that higher blood flow (250 mL/min) will reduce the FF and concomitantly prolong the life of the filter.

The inclusion criteria will be:

• Age greater than 18 years.
• Weight ≥ 50 Kg.
• Agreeing to participate in the study (TCLE duly elucidated and signed by the patient or family member/guardian).
• Admitted to the hospital ICU.
• Acute Kidney Injury in need of RRT and indication (according to the evaluation of the assistant nephrologist) of continuous therapy.

Exclusion criteria will be:

• Age < 18 years.
• Weight < 50 Kg.
• Refusal to participate in the study (absence of informed consent).
• Patient with chronic kidney disease on dialysis