Fluid Therapy For Cats

What is fluid therapy?

Fluid therapy is one of the most common medical treatments cats receive. The purposes of fluid therapy are to increase blood volume (dehydration or hypovolemia), change the content of the blood (such as electrolyte, metabolic and acid disorders), change the distribution, increase excretion. The body keeps water levels in check by homeostasis. If there is an excess of water, the kidneys flush it out of the body via the urine, if there is too little water in the body, the kidneys concentrate the urine and the brain sends a signal to drink more water.

The cat receives water by drinking as well as via his food (at differing levels depending on what type of food). Water is lost via urine, sweat, and feces.

Approximately 60% of a cat’s body weight is water (known as total body water/TBW). There are two fluid compartments; Intracellular Fluid (ICF) and Extracellular Fluid (ECF). 2/3rds of total body water is intracellular fluid and the remaining 1/3rd extracellular fluid. Water can pass between the intracellular and extracellular fluid compartments when necessary.

Intracellular fluid

  • Water within the cells. This fluid is high in potassium and magnesium and low in sodium and chloride.

Extracellular fluid

This fluid is further split into two sub-compartments:

  • Intravascular: This compartment is mostly blood, lymph and blood plasma within the blood vessels (arteries, veins, capillaries).
  • Interstitial: The interstitial compartment is the tissue surrounding tissue cells (interstitial fluid).


Electrolytes are also an important part of your cat’s daily requirements. These are minerals in your cat’s blood that allow your cells to generate energy plus perform many other important functions. Electrolytes include:

  • Sodium (Na)
  • Potassium (K)
  • Calcium (Ca)
  • Magnesium (Mg)
  • Chloride (Cl)
  • Bicarbonate (HCO3)

Imbalances may occur either due to increased or decreased electrolytes and are known as hyper (elevated) or hypo (decreased).

  • Calcium: hypercalcemia and hypocalcemia
  • Chloride: hyperchloremia and hypochloremia
  • Magnesium: hypermagnesemia and hypomagnesemia
  • Phosphate: hyperphosphatemia and hypophosphatemia
  • Potassium: hyperkalemia and hypokalemia
  • Sodium: hypernatremia and hyponatremia

Electrolyte imbalances can occur for several reasons, dehydration, kidney disease, pancreatitis, poisoning, vomiting, diarrhea, urinary obstruction (which causes uremic poisoning), burns, nutritional deficiency, vitamin D deficiency, vitamin D toxicity, hyperparathyroidism, hypoparathyroidism.

Osmosis refers to the passage of water solution from outside the cells across the semi-permeable membrane into a region of higher solute concentration. When the solvent molecules are at the same concentration (osmolarity) as that of the cells, the balance of water in and out of the cells is equal (isotonic).

What conditions does fluid therapy treat?

Fluid therapy must be tailored to each individual cat and will be continually re-evaluated. The choice of fluid has many factors which include the underlying disease, the severity of symptoms, size and weight of the cat, the volume required, the rate of administration, where the fluid needs to go (intracellular or extracellular), the cat’s acid-base, electrolyte abnormalities.

The general principals of fluid therapy are to treat:

  • Changes in volume (dehydration, blood loss)
  • Changes in content (electrolyte imbalances)
  • Changes in distribution (pleural effusion)
  • Increase kidney excretion (poisoning)

Dehydration and hypovolemia are the most common indications for fluid therapy. Hypovolemia refers to fluid loss from the intravascular space, which results in perfusion, a decrease in the delivery of the blood to a capillary bed. Blood loss and shock are a common cause of hypovolemia, and fluid therapy can help the remaining red blood cells deliver oxygen to the tissues. Dehydration is a loss of fluid from the extracellular fluid.

Common diseases which may lead to needing fluid therapy include:


  • Kidney failure
  • Vomiting
  • Diarrhea
  • Hyperthermia
  • Inadequate water intake. Often a sick cat will not drink enough fluids


  • Excessive bleeding
  • Burns
  • Shock (which results in extreme dilation of the blood vessels)
  • Sepsis (infection of the blood)


  • Changes in fluid content. Electrolyte, metabolic or acid disorders
  • Promote kidney diuresis to treat poisoning such as uremia (increased fluid will increase excretion, therefore helping to flush the toxin out of the body)
  • During surgery to replace fluids lost due to respiration. Anesthesia can also affect the circulatory system, lowering blood pressure which can lead to a reduction in delivery of blood (known as perfusion) to certain organs such as the kidneys, and fluids can help to provide hemodynamic support
  • Changes in fluid distribution such as pleural or abdominal effusion or edema.

What is in fluids?

Many types of IV fluids may be colloid or crystalloid.

  • Colloid solutions contain salts and larger insoluble molecules such as gelatin. Colloid solutions are restricted to the plasma.
  • Crystalloid solutions may also contain electrolytes. Crystalloid solutions can move around all of the body’s fluid compartments.

There are three categories of fluid:


These fluids are distributed to the intravascular space and have the same osmolarity as the cat’s red blood cells and plasma, meaning that the concentration of dissolved solutes (particles that are dissolved in a solvent, which in this case is water) is the same concentration as it is within the cells. This results in the movement of water into and out of the cells are balanced. Isotonic fluids are given to expand the intracellular space.

Isotonic fluids increase volume.

  • 0.9% (also known as Normal Saline) saline solution is the most commonly used isotonic IV fluid used to treat cats.
  • Ringers solution typically contains potassium, calcium and sodium chloride.
  • Lactated Ringers contain sodium, chloride, potassium, calcium, sodium and lactate.


These fluids have a higher concentration of dissolved solutes, due to this difference between the hypertonic fluid and the red blood cells and plasma, fluid is drawn out of the cells to equalise the concentration of the solutes on either side of the membrane. Hypertonic fluids increase fluid levels in the intravascular compartment, due to water being drawn from the interstitial compartment, the endothelial cells lining the blood vessel walls and red blood cells.

Hypertonic fluids may be used to immediately treat blood loss and hypovolemic shock or cats who cannot receive large volumes of fluids, cats with a sodium deficit and cats with edema.


These fluids have a lower concentration of dissolved solutes, again, due to the different osmolarity between the hypotonic fluid and the red blood cells and plasma, fluid is drawn into the cells to equalise the concentration of the solutes on either side of the membrane.

Hypotonic fluids are used to correct electrolyte imbalances.


When your veterinarian selects a fluid, some questions he must ask include:

  • What is the reason for fluid therapy? Is it to treat dehydration, hypovolemia?
  • What is the extent of dehydration?
  • Is fluid loss ongoing?
  • Is it rehydration or maintaining fluid balance?
  • Are there any electrolyte imbalances?
  • Where is the fluid going to?


When in a veterinary practice, fluids are administered via an intravenous catheter into a vein, usually in the front leg. Other locations of fluid therapy may be:

  • Subcutaneously – Under the skin at the back of the neck
  • Intraperitoneal – Into the peritoneal cavity in the abdomen
  • Intraosseous – Into the bone marrow
  • Oral – Into the mouth


A hospitalised cat will receive fluids via intravenous injection. A small area is shaved, disinfected and a catheter is inserted into the vein. The most common location is the foreleg.


This involves lifting a tent of loose skin at the back of the neck and injecting the fluid into the area. It may be necessary for pet owners to give a cat subcutaneous fluids at home. A lump of fluid may occur after administration, but this will gradually spread throughout the body. If your veterinarian has recommended giving your cat subcutaneous fluids at
home, he or a veterinary nurse will demonstrate how to safely do this. There is also an excellent article on administering fluids here.

The veterinarian will have to calculate how much fluid the cat will need based on the following factors:

  • Level of dehydration
  • How much is still being lost (for example if the cat is still vomiting or has ongoing diarrhea)
  • Maintenance when the cat is stabilised

The body absorbs subcutaneously slower than intravenous fluids, therefore are not suitable for cats with hypovolemia, shock or dehydration.


  • Damage to a vein during injection or during fluid therapy.
  • Air embolism can occur if there is air in the syringe or bag. Air enters the vein and can travel to the brain, heart or lungs, which can be catastrophic.
  • Blood clots can develop during IV therapy.
  • Fluid overload.
  • Electrolyte imbalances.
  • Take extreme care when giving fluids to cats with heart disease.

These side effects are uncommon, and the benefits of fluid therapy by far outweigh the risks.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

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