“Let’s Talk about Peritoneal Dialysis” Post 5: Why Won’t My Patient Ultrafiltrate?

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Is my peritoneal dialysis (PD) patient’s hypervolemia due to ultrafiltration failure (UF)?  UF is a specific diagnosis with a definition of a failure to achieve 400 mL of ultrafiltration during a 4 hour dwell using 4.25% dextrose.  While UF is often diagnosed in the workup of volume expansion, hypervolemia is not always due to UF. 

Before we explore UF further, first we should expand upon the usual differential diagnosis hypervolemia and measures that can be taken.  Simply, hypervolemia is a state in which net sodium balance is positive. The goal of therapy, then, is to achieve net negative sodium balance (be it a dialysis patient or not).  In the PD patient, the first and foremost therapy for hypervolemia is dietary sodium restriction. Without restriction of intake, it is difficult to achieve net negative balance. To enhance excretion, a few techniques can be deployed – administration of diuretics if residual renal function is present, increase of tonicity of the dialysate, increase of total delivered dialysis (increased volume, more exchanges).  Catheter patency or absence of obstruction should be confirmed via rapid “in and out exchange.” In a rapid in and out exchange, 1-2 liters of dialysate are manually instilled and then immediately drained. If the full volume does not drain or is slow to drain, poor ultrafiltration volume may represent retained dialysate and an obstructed catheter.  Dialysate can also be retained if there is a violation of the peritoneum and dialysate leaks into the abdominal cavity or pleural space (more on this to come in a future post!). 

If all of these measures fail, screening for UF is necessary.  Screening is often done at the same time as a peritoneal equilibrium test (PET), as the PET will help determine the next step. When interpreting the PET, the dialysate/plasma (d/p) creatinine is compared to  the patient’s baseline PET (see post 1). This comparison can help diagnose the type of UF. 

D/p creatinine increased from baseline. Type 1 UFThis is the most common form of UF failure and occurs as chronic changes in the peritoneal membrane result in very rapid transport status.  In rapid transporters, the osmotic gradient between the dialysate and the blood required for ultrafiltration is quickly dissipated. The only proven treatment for this is resting the membrane with temporary (and often permanent) switch to hemodialysis.  Where possible, the transition should be made to home hemodialysis.

D/p creatinine unchanged from baseline.  Type 2 UF: The differential diagnosis includes functional alteration of aquaporins or incomplete drainage of dialysate.  Evaluation of sodium sieving helps differentiate these entities.  If sodium sieving is intact, evaluation of leak or obstruction is to be undertaken.  If sodium sieving is impaired, Type 2 UF failure is diagnosed. This is quite rare and may be a clue to the presence of EPS.

D/p creatinine decreased from baseline.  Type 3 UF.  Also rare, this form of UF Failure presents from loss of effective peritoneal surface area, frequently a consequence of peritoneal adhesions or EPS.

Post by: Ankur Shah, MD (@NephroShah)
Nephrologist, Brown University

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