A 62 year old woman is in the ICU with septic shock and GI bleeding. She develops acute renal failure and is started on continous veno-venous hemodialysis (CVVHD) with regional citrate anticoagulation. Blood flow rate is 100 cc/min and dialysate flow rate is 2000 ml/hour. The bicarbonate rises to 36 mmol/L, forcing the respiratory therapist to hypoventilate the patient in order to normalize the serum pH.
How would you lower the serum HCO3 concentration? Why does this work?
(HINT: use the words “hemofiltration” and “convection” in your answer)
When patients are treated with CVVHD and citrate regional anticoagulation is used, the citrate is metabolized in the liver to bicarbonate. If bicarbonate consumption and/or removal occurs slowly, serum bicarbonate levels will rise and a metabolic alkalosis will develop.
This can be corrected by adding hemofiltration and using normal saline replacement fluid. By doing this, one removes plasma that is rich in bicarbonate (bicarbonate is removed via convection) and replaces it with normal saline that contains no bicarbonate. Over time, the bicarbonate level will fall.