Slow continuous ultrafiltration SCUF removes excessive fluid from the bloodstream via ultrafiltration. No dialysate or replacement fluid is used. Main indication is fluid overload without uremia. Continuous veno-venous hemofiltration CVVH removes larger volumes of fluid mainly via convection.
Replacement fluid is added. No dialysate is used. CVVH is effective method for removal of large molecules. No replacement fluid is used. CVVHD is effective method for removal of small to medium sized molecules.
With CVVH, a dialysis catheter is placed in one of the main veins of the body. This catheter has two separate lines. Blood flows out of the catheter and into the CVVH machine, which then goes into a filter where waste fluid is taken off.
Fluids and electrolytes i. Finally, the blood is returned back to the patient through the catheter. CVVH is typically used until the patient's kidney function returns or hemodialysis can be initiated.
It is run countercurrent to the direction of the blood flow in order to allow for a greater diffusion gradient across a semi-permeable membrane, thereby increasing the effectiveness of solute removal. The dialysate does not cross the filter barrier into the bloodstream thereby meaning that if 1 litre of dialysate was administered per hour, 1 litre of dialysate will drain into the effluent bag.
Dialysate fluids are usually buffered with bicarbonate, as bicarbonate levels are usually low in renal failure. It does not contain the active metabolites that treatment aims to remove such as creatinine and urea. Most of the electrolytes in the dialysate are equal to normal serum concentration, in order to maintain a normal serum electrolyte level for the patient during treatment.
Potassium is the only electrolyte that may need to be added to certain brands of dialysate solution, depending on the aim of patient therapy. This is all dependent on whether the replacement fluid is to be delivered either before pre-dilution or after post-dilution the filter.
If pre-dilution is required, the majority of the rate programmed will be allocated to the pre blood pump stream. It is important to understand that the replacement stream can be programmed as either pre or post , and this indicates whether the fluid rate allocated to the replacement stream will enter the filter circuit before or after the actual filter.
Therefore, some units will deliver their pre-dilution by programming the majority of the fluid rate via the replacement stream set as pre. If post-dilution is required, the majority of the rate programmed will be allocated to the post-replacement stream. The image above will help with the solidification of these modes and principles for you. Slow continuous ultrafiltration SCUF uses the principle of ultrafiltration to remove excess fluid from the fluid overloaded patient.
For this reason, the fluid removed is generally not replaced. Blood is simply taken from the patient, is pumped through the filter allowing ultrafiltration to take place and is then returned to the patient. Continuous veno-venous haemofiltration CVVH uses the principles of ultrafiltration, hydrostatic pressure and convection to remove both fluid and solutes from the patient. Due to the large loss of fluid that occurs in this mode, the patient will require a replacement fluid to be programmed within the filter.
The filter ensures that the amount programmed as the replacement fluid rate is the amount of fluid that is loss during haemofiltration, to ensure that the patient keeps an even balance. This replacement fluid can be programmed to enter the filter either before pre or after post the filter. Continuous veno-venous haemodialysis CVVHD uses the principle of diffusion to remove solutes from the patient. Where blood flows in one direction through the filter, a dialysate fluid flows on the other side of the filter membrane in the opposite direction of the blood.
There are no replacement fluids administered via the filter in this mode. Continuous veno-venous haemodiafiltration CVVHDF uses the principles of both haemofiltration and haemodialysis, as described above. As this mode enables the ultimate removal and replacement of solutes and fluids within the blood, it is the most common mode chosen for CRRT.
It is recommended that this mode is always selected, even if the clinician would like to run only haemofiltration or only dialysis. However, this mode cannot be achieved in the other modes unless the whole filter is re-started.
Using the image above, have a look at the Prismaflex machine that is commonly used for CRRT to try and put all the pieces together. It can be navigated in the following way:. Access pressures are typically negative as the blood has to be pulled from the patient. Filter pressures and return pressures are typically positive as blood has to be pushed through the filter and to the patient, respectively. The filter pressures will begin to rise as the filer pores clog as more pressure is required to get blood through the filter.
Effluent pressures and Transmembrane pressures TMP can be either positive or negative, depending on whether fluid is being pushed or pulled from the blood compartment to the fluid compartment. The effluent pressure will become more negative as the filter pores clog while the TMP will become more positive as the filter pores clog.
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