Hemodialysis FAQs

What is dialysis? What is hemodialysis?

Strictly speaking, dialysis is the diffusion of water and dissolved molecules called solutes across a semi-permeable membrane. In medicine, this is done with the intent of removing waste products from an animal’s or person’s body when their own kidneys fail. In hemodialysis, a machine pumps blood from the patient through an artificial kidney, which removes accumulated wastes (urea, creatinine, phosphate and many others) that the patient’s own kidneys normally would filter from the blood and excrete from the body. The artificial kidney contains many small, hollow fibers (picture a bundle of tiny soda straws) through which the blood passes. The fibers are made of a semi-permeable, synthetic material, which allows passage of water and some small molecules. The fiber bundle is encased in a cylindrical shell through which the dialysate solution flows. Dialysate is a balanced electrolyte, sugar, salt, and water solution that bathes the blood-filled fibers. Wastes move from the blood (the area of high concentration) through the membranes into the dialysate (the area of low concentration), and the used dialysate is discarded. Solute diffusion in hemodialysis is similar to that in the process of brewing tea. Think of the dry tea as the blood with waste products, the tea bag as the dialysis membrane, and the hot water as the dialysate.

In addition to removing solute wastes, the kidneys also maintain normal electrolyte, acid-base, and water balance. Dialysis can be used to correct imbalances in any of these parameters by prescribing more or less of certain substances in the dialysate. If a pet is over-hydrated, water can be removed from the blood by a process called ultrafiltration. During ultrafiltration, pressure across the dialysis membranes draws specifically programmed volumes of water from the blood at a precisely controlled rate. The extra fluid is then discarded with the contaminated dialysate. Conversely, fluids can also be easily administered to a dehydrated pet during a dialysis session. Monitoring equipment helps to ascertain that fluid is administered or removed at a safe rate for the pet.

When is dialysis used in veterinary medicine?

Most human dialysis patients have chronic (long-term, irreversible) renal (kidney) failure; they rely on dialysis treatments 2-3 times per week for their entire lives, unless they receive kidney transplants. In contrast, most veterinary dialysis patients have an acute (recent onset) renal injury, which means that something happened to cause sudden failure of previously adequately-functioning kidneys. Such an injury is often partially or fully reversible. It is important to understand that dialysis does not treat or repair the kidneys themselves. Instead, it serves as a bridging measure by re-establishing and maintaining metabolic balance, in order to give damaged kidneys the opportunity to repair themselves and to recover function. Without the extra time dialysis provides, most patients with severe acute renal failure will die before their kidneys can recover enough function to sustain life.

Most canine dialysis patients have damaged kidneys from antifreeze poisoning, kidney infections (like leptospirosis or pyelonephritis), grape/raisin toxicity, or complications of other systemic disease. Cats, like dogs, may suffer from antifreeze poisoning or kidney infection. Lilies, especially Easter lilies, are potent kidney toxins for cats, also. Ureteral obstruction (blockage of the tubes connecting the kidneys to the bladder) is the most common reason for a cat to need dialysis. Dialysis can stabilize severely ill cats with ureteral obstructions to give the obstruction a chance to resolve and/or to stabilize the cats for surgical correction of the blockage(s). Dialysis treatments two or three times weekly can also maintain pets with chronic renal failure, when medical management alone can no longer provide a good quality of life.

What happens to my pet during a dialysis session?

At the beginning of a session, your pet is greeted (most pets like coming to dialysis) and given a thorough physical examination, including measurement of weight, temperature, heart rate, and blood pressure. Dogs lie on a soft pad, and cats lie on a fleece pad inside half of a pet carrier (cats feel more secure if they are a bit hidden). All pets wear a special harness (like a nylon walking harness that attaches to a leash), which is clipped to leads on three sides of the padded treatment table. This restraint system keeps your pet safely on the table, but lets her or him move and change positions at will. A dialysis team member then unwraps, closely examines, and meticulously cleans the dialysis catheter. These catheters are the lifelines of our patients. We take a small sample of blood from the catheter to measure red cell volume, clotting time, kidney values, and any other parameters the doctor requests for that day. Because your pet has a dialysis catheter in place, use of a needle is not necessary to take blood. In fact, for most patients, the only unpleasant part of a dialysis session is having a temperature taken. 

When the dialysis catheter, dialysis prescription, and patient are all ready, the dialysis machine’s blood lines are attached to the patient’s two catheter lines, and the dialysis treatment begins. Initially, the machine draws blood from your pet through the lines to fill the extracorporeal (outside the body) circuit, and fluids flow from the circuit back to your pet to help make up for the blood being withdrawn. In a cat, about a quarter cup of blood (60 ml or 2 ounces) is outside the body at any given time. In a dog, filling the extracorporeal circuit requires between a quarter and a full cup (60-240 ml or 2-8 ounces) of blood (the circuit size is varied to fit the pet’s size). After this initial equilibration period, blood simultaneously and cyclically leaves and returns to a large blood vessel near the heart, so your pet should not feel any shifts of volume through the course of a dialysis treatment. After acclimating during one or two treatments, most pets spend their time on dialysis comfortably napping.

During a dialysis session, blood pressure, heart rate, clotting time, and machine parameters are recorded at least every 30 minutes. Temperature, ECG and other parameters can be measured if needed. Patient attitude and well-being, blood volume and blood oxygenation are monitored continuously. Pets may eat or drink during dialysis if they choose to. At the end of the session, the blood in the extracorporeal circuit is returned to your pet in a process called rinseback. The catheter access lines are carefully cleaned again, locked with heparin to prevent clot formation, sealed, and bandaged with a neck wrap. After we re-assess physical parameters and feel comfortable that your pet is stable following treatment, she or he may leave the dialysis unit. Most pets feel great after dialysis and many pets are outpatients, living at home and coming to the clinic on dialysis days for 6-8 hours.

Why do treatments take so long?

A standard hemodialysis session lasts about 4 hours in a cat and 4-6 hours in a dog; stable patients are treated 2-3 times per week. This process is extremely efficient, but a single dialysis session must accomplish in 3-5 hours 2-3 times a week what kidneys do 24 hours a day, every day. In addition to the actual dialysis time, patient examination and catheter preparation require 60 minutes or more, prior to and following treatment. Longer treatments are more physiologic, and, in some cases, treatment time may need to be extended to 8 hours or more for the health and safety of your pet. Each session’s prescription (which includes length of treatment), is created at the time your pet’s status is evaluated, and is designed to optimize his or her benefit from a given treatment. Dialysis prescriptions can, and do, change significantly from session to session.

Who are the members of the dialysis team?

The doctors at UCVMC-SD who manage dialysis patients are board certified in veterinary internal medicine and have elected to focus their professional activities on nephrology/urology and hemodialysis. The veterinary technicians who treat and care for dialysis patients are specifically trained to handle the dialysis catheters and machines, and to detect subtle changes in patients that might herald problems. They are skilled in critical care, and their technical expertise is indispensable for safe and successful patient management.

If you ever have questions or concerns regarding your pet’s treatment, disease, or general well-being, never hesitate to ask a dialysis team member. If your pet has a medical emergency related to any aspect of dialysis, a dialysis team member is available to you.