Regional citrate anticoagulation for slow continuous ultrafiltration: risk of severe metabolic alkalosis

BACKGROUND. Slow continuous ultrafiltration (SCUF) is a safe and efficient treatment for fluid overload in patients who are hemodynamically unstable, have low urine output, and are not in need of dialysis or hemofiltration for solute clearance. Sustained anticoagulation is required for these long treatments, thus posing clinically challenges for patients having contraindications to systemic anticoagulation with heparin. Regional citrate anticoagulation would be an alternative option; however, we believed that this would be problematic due to citrate kinetics that predicted the development of metabolic alkalosis. METHODS. In that patients’ serum bicarbonate reached 45 mEq/L and arterial pH rose to 7.59 after just 3 days of SCUF, we developed equations to study this phenomenon. We report here the acid–base balance calculations quantifying base accumulation in SCUF compared to continuous venovenous hemofiltration (CVVH). RESULTS. This kinetic approach demonstrates the importance of accounting for the high citrate clearance into CVVH hemofiltrate, which prevents development of the alkalosis seen with the relatively low ultrafiltration rates in SCUF: there was net bicarbonate accumulation of ∼1400 mmol/day with SCUF, compared to 664 to as low as 274 mmol/day during CVVH. The calculations underscore the importance of the relative fluid flow rates as well as the bicarbonate and citrate levels in the various infused solutions. We also discuss how citrate’s acid–base effects are potentially complicated by metabolism via gluconeogenic and ketone body pathways. CONCLUSIONS. These acid–base balance findings emphasize why clinicians must be mindful of the risk of metabolic alkalosis when using continuous renal replacement therapy modalities with low rates of ultrafiltration, which thereby presents a contraindication for using citrate anticoagulation for SCUF.