Extracorporeal Ultrafiltration

    I.  Theory and Background Information

II.  Indications and Contraindications For Use                 

   III.  Set - up

           

    IV.  Conduct Historic Perspective
        The idea of using hemofiltration as a means of removing fluid from edematous patients dates back to the early 1900's when patients with renal failure were placed on ultraporous membrane devices to alleviate this edema.  It wasn't until the 1970's however, that the implementation of hemofiltration was used for open heart surgery.  The use of hemofiltration to concentrate blood in the extracorporeal circuit was restricted to severely hemodiluted patients after the surgery was performed.  Hemofiltration under these conditions was found to be particularly helpful in producing higher postoperative hemoglobin concentrations.
        By 1979 the application of hemofiltration was extended to use "during" the bypass period although this application was also limited to patients with renal impairment.  This procedure was chosen as an alternative to hemodialysis to manage fluid homeostasis during and after the bypass procedure.
        During the 1980's it was recognized that this procedure could not only be applied to patients in renal failure but to otherwise normal patients undergoing open heart surgery as a means of volume control in those that had been over hydrated.  Continual use of hemofiltration devices not only proved to be an effective and easy method of volume control, but  it has also served as an effective means for blood conservation by preserving platelets and coagulation factors.
        Today, hemofiltration, or hemoconcentration, has found widespread application as a means of volume control and blood preservation in cardiac surgery on adults as well as children.  Special variations of hemofiltration are used with children undergoing open heart surgery however.  Some of these are called ultrafiltration, modified ultrafiltration (MUF) and (ZBUF).
 

Basic Physiologic Principles of Hemofiltration
        The primary purpose of hemofiltration is to selectively separate excess plasma water and low molecular weight solutes and plasma proteins from blood using a semi-permeable membrane.  The driving force for hemofiltration is hydrostatic pressure unlike hemodialysis which uses oncotic forces as the primary driving force for fluid and/or solute and protein removal from the blood.  Hemofiltration is accomplished by applying a negative pressure to the effluent side of the hemofilter to increase the perfusion pressure and facilitate fluid removal across the membrane.  The trans-membrane pressure can be mathematically expressed as  TMP=(Pa+Pv)/2+Ps where TMP stands for the total trans-membrane pressure, Pa is the arterial or inlet blood pressure, Pv is the venous or outlet blood pressure and Ps is the amount of negative pressure applied to the effluent side of the membrane.  All pressures are measured in mmHg.  The range of trans-membrane pressures vary between devices according to manufacturer guidelines but TMPs of 100-500mmHg are usually suggested.
        Some other factors that may effect the rate of fluid removal from the blood include, but are not limited to:  Blood flow through the filter, viscosity of the blood, blood temperature, and serum protein levels may all affect fluid removal.
        The quality or efficiency of a particular device can be determined by what is known as an ultrafiltration coefficient .  Each hemofilter consists of a thin membrane skin that serves as the primary micro filter stretched over a thicker porous substructure.  Some important characteristics in determining the ultrafiltration coefficient is the size of the pores found in the membrane, the total number of pores, the length of the pores (membrane thickness), and the total length of the membrane itself.  Aside from variations in the membrane itself, other factors that may affect the ultrafiltration coefficient are the hemoglobin concentration of the blood and the fluid temperature along with the TMP.  Ultrafiltration rate can be increased by either increasing the perfusion pressure going into the filter, increasing the blood flow through the filter, or by increasing the negative pressure on the effluent side of the filter.  A plateau exists however where regardless of the intervention, where a max fluid transfer will occur.
 

Technical Considerations
        Historically, ultrafiltration devices have either been made as parallel membrane sheets or of hollow fiber construction.  The hollow fiber design due to its less bulky nature, smaller priming volume, and simplicity of use, has become the most practical choice for hemoconcentrators.  The hollow fibers are filled with thousands of hollow polysulfone, polyacrylolite or cellulose acetate fibers with each fiber having an internal diameter of about 200 microns.
        The placement of the ultrafilter in the extracorporeal circuit really depends on the application for that procedure.  Will hemoconcentration be employed after the procedure or will it be employed throughout.  Another concern may be whether a membrane oxygenator or bubbler will be used.  In either case, if the use of ultrafiltration is decided upon prior to the initiation of bypass, the inlet tube leading to the ultrafiltration device may be placed as a branch connection from the arterial line filter.  The outflow from the device is then returned to the cardiotomy reservoir.  Another method may be to use the recirculation line in the circuit as the inlet line to the filter and again, return the outflow to the cardiotomy.  In either case, only a single pump need be employed, that being the patients main arterial pump.  Some disadvantages to this method may be that, as the patients pressures and flows to the patient change, the flow through the ultrafilter will also change.  Another disadvantage is that pump head flow rates may need to be increased to compensate for the shunt created by the ultrafilter.  The effluent in each can be collected into a graduated device for later measurement and disposal.
        Other variations on these basic hookup procedures may certainly be applied. Some of the main considerations when using this device in the bypass circuit is the pressure head driving the flow to the filter, the filter return path to return the blood to the extracorporeal circuit, and the collection of the effluent.
 

Variations of Hemofiltration
     Variations to using a hemofilter are somewhat new the the perfusion field.  Currently there is ongoing research in the techniques of modified ultrafiltration (MUF), dilutional ultrafiltration (DUF) and "zero-balance" ultrafiltration (ZBUF).  Of which the latter two have included the adult population and were off-shoots of MUF from pediatric CV surgery.  MUF consists of a circuit that allows removal of plasma water from the extracorporeal circuit as well as the patient primarily at the end of the bypass procedure.  It is used primarily in pediatric surgery and only rarely applied to the adult scenario.  DUF is relatively new technique, that consists of actually hemodiluting the patient with a calculated crystalloid volume in the warming phase of bypass.  This hemodilution is then reversed in order to enhance the removal of complement activated inflammatory mediators that are expressed in high levels at this time.  ZBUF is similar to DUF in its hemodilution process and inflammatory mediator removal.  It also takes it a step further in that an excess of volume is removed from the patient in order to increase plasma proteins and Hct.

Further information about these variations can be sought by contacting Suzanne Gooselaw, Minntech Corp.