Troubleshooting CVVH

Even though most studies have not shown any mortality benefit to CVVH when compared to intermittent hemodialysis, that doesn't mean that we don't use CVVH all the time. At our centers (Mass General Hospital and Brigham & Women's Hospital) CVVH is really the preferred modality for patients in the ICU with any hint of hemodynamic instability. Here are the major complications unique to CVVH to be aware of (taken from a recent David Steele lecture I just heard):

1. Citrate Toxicity. For patients in whom a citrate-based replacement solution is used, citrate toxicity may develop in patients with liver disease who lack the ability to convert citrate to bicarbonate. As a result, citrate accumulates and results in a worsening anion gap metabolic acidosis. The increasing citrate levels bind ionized calcium (reducing the iCa level), and the acidosis causes a dissociation of calcium and citrate in vitro, resulting in a rising total Ca level. Patients with citrate toxicity must be converted to a bicarbonate-based replacement solution ASAP.

2. Refractory Acidosis. Occasionally patients who are very hemodynamically stable will remain acidotic even while on CVVH. One solution to this dilemma is to increase the replacement solution rate. For example, at a standard RS rate of 1600 cc/hr, this is delivering a HCO3 concentration of 64meq/hour; increasing the RS rate to 2400 cc/hr will therefore increase the HCO3 delivery rate to 96meq/hour. Another strategy is to add an isotonic NaHCO3 drip: at 250 cc/hour, this results in an additional 37.5 meq/hour, and the additional volume can be handled by ultrafiltration.

3. Electrolytes: Phosphorus and Potassium often need repletion due to the continual nature of CVVH.

4. Recurrent System Clotting: The bane of the Nephrology Fellow is recurrent system clotting, as it may lead to repeated late-night phone calls and unplanned dialysis catheter placements. There are several maneuvers one can perform to limit clotting, though none has a 100% success rate. These include changing to a citrate-based replacement solution, using systemic anticoagulation (e.g., heparin or Argatroban) if it is not contraindicated, increasing the blood flow rate, increasing the replacement solution rate, or performing a heparin prime & dump before beginning. If all else fails, a new dialysis catheter (preferably something with a wide lumen, such as a Niagra catheter) may be warranted.