The kidney is a tremendous organizer of our small ions. It determines how much calcium to keep and how much to dump. It controls our blood's pH by controlling which acids and bases to keep and which to lose. It controls sodium, potassium, carbon dioxide, water balance, and more. The kidney filters unwanted toxic biochemicals and gets rid of them using a process called excretion. Not only are toxins removed, but they are removed using just the right amount of water so that hydration is maintained. In failure, however, everything goes haywire. The wrong things are dumped, the wrong things are kept, toxins build up and the patient is sick. The state of toxicity that results is called uremia or uremic poisoning.
Almost every animal hospital can provide diuresis, therapy in which extra fluid beyond what the patient can drink is provided, thus giving the kidney a boost to remove toxic waste. This works well in a large number of patients but there comes a time when, even with plenty of fluids, the sick kidney simply cannot get the toxins out. For most patients, this is the end of the line. In fact, however, dialysis may be another choice. It must be understood that dialysis is substantially more expensive than diuresis, and centers that provide dialysis for pets are still few and far between.
Dialysis catheter placed in a dog's jugular vein. Image Courtesy of ACCESS Specialty Animal Hospitals
What Is Dialysis?
Dialysis is a process that can be thought of as cleansing the blood of toxins. This is done using a membrane called a dialyzer membrane and a fluid called dialysate. The blood is separated from the dialysate by the dialyzer membrane. The dialysate fluid is formulated so that the toxins in the blood will be attracted across the dialyzer membrane and into the dialysate fluid.
There are two types of dialysis: peritoneal dialysis and hemodialysis. When most people refer to someone needing dialysis, they mean hemodialysis. The patient is hooked up to the dialysis machine by an intravenous catheter and a pump forces the patient's blood into the machine for cleansing. The detoxified blood is returned to the patient. Peritoneal dialysis is less high-tech and uses the patient's own abdomen as the dialyzer membrane. The belly is filled with dialysate fluid, toxins are attracted into the fluid over several hours and the fluid (with its toxins) is drained from the belly at the end of the procedure.
Image Courtesy of ACCESS Specialty Animal
Hemodialysis (Description and FAQ)
The intravenous catheters used in hemodialysis are very large and very long so as to support the appropriate speed of blood flow through the dialyzer.
The catheters are difficult to place so surgery may be necessary to do so. A long-term IV port under the skin may be needed if treatments are to be ongoing. A single catheter may be in place for months.
The treatment lasts three to five hours, sometimes longer, during which the patient must calmly sit on a table attached to the equipment. Treatments typically are performed three times a week either indefinitely (as in chronic renal failure) or until the kidney has healed (as in acute renal failure).
There are numerous conditions that will benefit from this type of blood cleansing besides kidney failure. Electrolyte imbalances, heart failure, and many poisonings can also be treated with hemodialysis.
Why Is This So Long in Coming to Pets?
Part of the problem has been that dialysis machines are generally designed for human patients. The amount of blood that goes through the human dialysis machine is too large a blood loss for a veterinary patient to withstand so smaller machines had to be built. A veterinary dialysis machine had to be designed for patients as small as a 5 or 6-pound cat. Furthermore, a dialysis center requires specifically trained staff and 24-hour staffing. It was and is difficult to get such facilities financed. The procedure is still expensive and requires a dedicated owner but at least it is now an option.
How Often Is Dialysis Performed on a Patient?
The patient who depends on dialysis to relieve the symptoms of uremia must have dialysis three times a week on average. The pet owner must be able to bring the pet to the dialysis center with this frequency and leave the pet for the several hours needed. Kidney transplant patients, of course, only require dialysis until they are well enough for surgery. Patients with a stone obstructing a ureter (the tube connecting the kidney and bladder) will require dialysis until they are stable for surgery. Patients who have lost kidney function acutely from poisoning, usually antifreeze, or infection (usually leptospirosis) typically require a month of therapy while their kidneys heal. Other patients may require therapy indefinitely.
Does Dialysis Reliably Control Uremic Poisoning?
Not all patients respond to dialysis. How a given patient will do in part depends on what caused the kidney failure in the first place (toxin vs. infection vs. metabolic causes).
Out of 138 dogs who failed to respond to conventional therapy for kidney failure — diuresis — at the University of California, 40% responded to dialysis. Infectious causes (generally leptospirosis) tend to have a better prognosis. As leptospirosis has re-emerged as a common cause of canine acute kidney failure, the overall response rate for dogs receiving dialysis has improved to 50%.
In a study in 2003 of cats requiring hemodialysis, the response rate was 56%; the best prognosis was for those with a ureter obstruction where over 70% survived.
At What Point in Treatment Is Dialysis Recommended?
If dialysis is being considered, it is best not to wait until conventional therapy has completely failed and the pet is on death's door. Let your veterinarian know from the start that this is an option you are interested in so that your veterinarian can consult with the dialysis center on the best time to refer.
What Are the Complications and Disadvantages?
A new world of complications (beyond those of conventionally managed kidney patients) is introduced to the kidney patient on dialysis.
Malnutrition and Nausea
The toxin build-up in kidney failure causes nausea, and appetite loss follows. Toxins further cause ulcers in the stomach and intestine that contribute even more to the loss of appetite. Once the toxins are removed, the intestine heals quickly but appetite loss may persist. Making the nutrition issue worse is the fact that dialysis patients have an increased protein requirement and an increased calorie requirement. It is particularly important for the dialysis patient to get nutrients in one way or another. Feeding tubes or IV feeding may be necessary.
Metabolic Bone Disease
When the sick kidney does not properly get rid of phosphorus, calcium is mobilized in a complicated hormonal reaction. Dialysis patients appear to be at higher risk for broken bones than are conventionally-managed kidney patients.
Carnitine is a nutrient that facilitates the transport of the body's energy sources. Unfortunately, dialysis inherently depletes the patient of carnitine. Carnitine deficiency results in heart disease, low red blood cell count, and muscle weakness. Supplementation is often necessary for dialysis patients if dialysis is going to be regularly performed for periods longer than one month.
Taurine is an amino acid of animal protein origin. As is the case with carnitine, taurine is lost in the process of dialysis. Taurine deficiency can result in heart disease and (in cats) blindness if it is allowed to persist. This amino acid must be supplemented for long-term dialysis patients.
Problems with the Dialysis Catheter
The dialysis catheter may create a blood clot at the catheter tip. This can interfere with the high blood flow rates necessary for proper dialysis. When a clot occurs, a forceful flushing of the catheter may be enough to move it. If not, clot-dissolving medications must be used and this becomes expensive and may lead to inability to clot.
Infection from the Dialysis Catheter
An infected dialysis catheter is bad news. The resulting blood infection can be lethal and, at best, requires months of antibiotic therapy. Be aware of reddening skin or pus at the catheter site. A fever is of great concern so become familiar with taking the pet's temperature.
Continual Renal Replacement Therapy
Continual renal replacement therapy (CRRT) is another form of blood filtration similar to dialysis; in fact, which procedure is the best choice for the human patient is hotly debated in human medicine. While dialysis removes toxins built up over the previous couple of days over a 3 to 5-hour treatment period, CRRT involves continual removal over a 24 to 48-hour period that mimics the patient's natural kidney function more closely. The idea here is to continually remove toxins until they are all gone if that is possible. Sometimes, a couple of treatments are needed. Unlike dialysis, CRRT is not a regularly performed procedure.
The catheters are similar to the dialysis catheters, but because the procedure requires being hooked to the equipment for up to two days, sedation is generally required for this entire period, although this depends on the hospital.
Lower blood flow rates are used in CRRT when compared to hemodialysis, which makes for fewer blood pressure issues. Also, the dialysis complications that are seen over time (carnitine deficiency, high dietary calorie requirement, catheter infection, and most of the other issues listed above) are not problems with CRRT because CRRT is a one-time treatment. Sedation may be necessary for CRRT, depending on the facility.
Blood cleansing therapies such as these are still new to the veterinary field because their use is high maintenance and costly. As pet insurance becomes more popular, expense becomes less of a deterrent, so more facilities are likely to open up. At the present time, dialysis is chiefly used on a temporary basis for acute conditions, but as treatment becomes more available and affordable, more chronic patients are likely to come forward. If dialysis is something you want to consider, let your veterinarian know so that you can be directed to the appropriate specialist. Costs vary and will depend on your pet's health status, the number of treatments necessary, and the facility.
Your veterinarian can refer you to the closest facility, and The American Society of Veterinary Nephrology and Urology (specializing in the kidneys and the larger urinary system) offers a list of facilities in North America. We hope to add more centers as they open. If you are aware of an animal dialysis center not listed here, please contact us so it can be included.