The Impact of CRRT Modality on Filter Life
Condition: AKI
Study type: Interventional

Summary: 
      The investigators plan to start patients who need CRRT on either CVVH or CVVHD by block
      randomization, and then to measure filter life.
    

Detailed description: 
      Continuous renal replacement therapy (CRRT) is a dialysis modality used in critically ill
      intensive care unit (ICU) patients because it is associated with less hemodynamic instability
      than traditional hemodialysis. CRRT can be conducted using one of two different modalities,
      which are continuous venovenous hemofiltration (CVVH) and continuous venovenous hemodialysis
      (CVVHD). CVVH used conductive clearance, whereas CVVHD uses diffusive clearance (CVVHD).

      In CVVH, plasma water is removed across the filter using transmembrane pressure which forces
      fluid into the effluent space. Solutes are removed via "solvent drag", which is the process
      by which solutes follow fluid flow. The rate of fluid removal is high (usually around 2-3
      L/hr), and so hemodynamic collapse would quickly occur in the fluid were not replaced. The
      patient is therefore given "replacement fluid" which can be given pre-filter or post-filter,
      and usually nearly matches the fluid removal rate. For instance, if 2 L are removed per hour,
      2 L are given back in the form of replacement fluid, if volume neutrality is desired. If
      fluid removal is desired, 1.9 L (e.g) may be given back, resulting in the net loss of 0.1 L
      of fluid per hour. The composition of the replacement fluid determines the serum
      concentration. As an example, if 2 L of fluid with a potassium concentration of 6.0 mmol/L (a
      high amount, 4.0 is normal) is removed and replaced with 2 L of replacement fluid with a
      potassium concentration of 2 mmol/L, then 8 mmol of potassium will be "removed" per hour, and
      the potassium concentration of the serum will fall.

      In CVVHD, dialysate flows in the effluent space, creating a gradient which leads to movement
      of solutes either into or out of the blood, depending on the gradient direction. For
      instance, if serum has a potassium concentration of 6 mmol/L, and the dialysate has a
      concentration of 2 mmol/L, potassium will move from the blood into the dialysate, until the
      dialysate concentration is also equal to 6 mmol/L. At that point, net potassium movement will
      be 0, as the concentration will be equal. As with CVVH, with a dialysate flow rate of 2 L/hr
      will result in the removal of 8 mmol of potassium per hour. While these technical details
      concern only the specialist, the important take away is that both of these modalities achieve
      equivalent clearance, via different mechanisms. Notably, there are no differences in clinical
      outcomes with use of one or the other modality, and these modalities are currently thought to
      have complete equipoise. The decision is generally based on provider comfort and preference.
      There is no clinical scenario where one modality would be preferable to the other, based on
      current evidence.

      A major issue when using CRRT is filter clotting, which occurs on average every 30 hours.
      Clotting renders the circuit unusable, and requires that the machine be reset, which is a
      process that takes 1-2 hours on average, and potentially longer depending on the availability
      of the dialysis nurses who change the circuits. This results in less delivery of dialysis
      than what is prescribed, and clotting is often also associated with the loss of the blood
      that is clotted in the circuit. This could be up to 150 ccs, which is a potentially
      significant amount in critically ill patients. As noted within the figures, post-filter CVVH
      is associated with hemoconcentration, and is thought to be associated with more clotting than
      CVVHD, where no hemoconcentration occurs. The situation is more complicated in pre-filter
      CVVH, however. As in Figure 3, the fluid is initially diluted, and then the fluid is removed
      over the course of flow through the filter. Some Nephrologists believe that this initial
      dilution of the blood results in decreased risk of clotting. However, other nephrologists
      believe that due to differential flow rates in the blood and replacement fluid that
      hemoconcentration still occurs within the filter, albeit to a much lesser degree than in
      post-filter CVVH (this argument is difficult to explain conceptually to a lay audience, but
      an attempt is made within Figure 3). It is therefore unclear currently whether pre-filter
      CVVH and CVVHD have equal or unequal rates of clotting. Because centers tend to use
      exclusively one modality or the other (or a combination, which is beyond the scope of this
      summary), retrospective analyses are often not possible due to lack of a comparison group.

      Figure 3. In pre-filter CVVH, replacement fluid is administered prior to the blood passing
      through the filter. Nephrologists argue about the effect this has on the rates of
      hemoconcentration. Some would argue that after the initial dilution, the blood only returns
      to its initial concentration by the end of the filter, and thus over the course of the
      filter, there is actually hemodilution that decreases likelihood of clotting. Other
      nephrologists argue like so: With a blood flow rate of 12 L/hr (standard), and a fluid flow
      rate of 2 L/hr, then 14 L/hr enter the fluid, and 2 L/hr are removed, which is 14% (2/14) of
      the fluid that entered. If the fluid flow rate is increased to 4 L/hr, then 16 L /hr enter
      the filter, and 4L/hr are removed, which is 25% (4/16). This would therefore theoretically
      result in higher degrees of clotting. The question of pre-filter CVVH and its impact on
      clotting relative to CVVHD is therefore unanswered, and is the reason for the proposed
      study."

      In short, the investigators plan to start patients who need CRRT on either CVVH or CVVHD by
      block randomization, and then to measure how often the filters on the machine need to be
      replaced.

      The investigators plan to exclude minors, prisoners, and patients undergoing a certain
      procedure called "extracorporeal membrane oxygenation" (ECMO). ECMO is designed to pump blood
      and provide oxygen to the patient when the heart and lungs are not working correctly (ECMO
      attempts to do the work of the heart and lungs in instances where the patient's organs have
      failed). CRRT can be added to an ECMO circuit if renal failure is also present, but this is a
      specialized instance with unique risk factors for clotting, and is not representative of the
      general CRRT population.
    

Intervention name(s): CVVH vs CVVHD
Intervention type(s): Other
Intervention description(s): Patients are initiated on CRRT in either the CVVH or CVVHD modality
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: None (Open Label)
Primary purpose: Supportive Care