Fluid therapy is one of the most common treatments given to cats. 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.
Below is complicated, and please skip if you want to, but I am including it because it may help you to understand why different fluids may be given to your cat.
Your cat receives water by drinking water and other fluids and via his food (at differing levels depending on what type of food he eats, raw/canned food is higher in water than dry/kibble). Water is lost via the urine, sweat (cats do sweat a little through their paws, respiration, in the feces.
Approximately 60% of your 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.
Water within the cells. This fluid is high in potassium and magnesium and low in sodium and chloride.
This fluid is further divided 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, this is known as interstitial fluid.
Electrolytes are also an important part of your cat’s daily requirements. These are minerals in your cat’s blood which allow your cells to generate energy plus perform many other important functions. Electrolytes include:
Imbalances may occur either due to increased or decreased electrolytes and are known as hyper (elevated) or hypo (decreased).
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 in the fluid given to your cat are the same concentration (osmolarity) as that of the cells, the balance of water in and out of the cells is is equal. This is known as isotonic (see below for more information).
What conditions do IV fluids treat?
Fluid therapy must be tailored to each individual cat and will be continually re-evaluated. The choice of fluid has many factors including the underlying disease being treated, 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:
Inadequate water intake. Often a sick cat will not drink enough fluids.
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, IV fluids will be given to your cat to replace fluids lost due to respiration. Anesthesia can also have an effect on 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 the fluid distribution such as pleural or abdominal effusion or edema.
What is in the fluids administered?
There are many types of IV fluids which can be administered. They 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.
IV fluids come in three categories:
Isotonic fluids are distributed to the intravascular space. These fluids 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 is balanced. Isotonic fluids are given to expand the intracellular space.
Isotonic fluids are used to 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.
Hypertonic 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 in order to equalise the concentration of the solutes on either side of the membrane. Hypertonic fluids are used to 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 can not receive large volumes of fluids, cats with a sodium deficit and cats with edema.
Hypotonic 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 in order to equalise the concentration of the solutes on either side of the membrane.
Hypotonic fluids are commonly 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 the dehydration? This is calculated in percentages.
Is fluid loss ongoing?
Is it rehydration or maintain fluid balance? Maintenance is used to treat cats who are hydrated but not eating or drinking as well as cats undergoing surgery.
Are there any electrolyte imbalances?
Where is the fluid going to?
How are fluids administered?
When in a veterinary practice, fluids are most commonly 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
If your cat is hospitalised, he 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 up a ‘tent’ of loose skin at the back of the neck and injecting the fluid into the area. Pet owners may be required to give subcutaneous fluids to their cat at home. A lump of fluid may occur after administration, but this will gradually spread throughout the body.
Your veterinarian will have to calculate how much fluids to give your cat based depending the level of dehydration, how much is still being lost (for example if your cat is still vomiting or has ongoing diarrhea) and maintenance where your cat has been stabilised, however, lower amounts of fluids are continued.
Subcutaneous fluids are absorbed slower than intravenous fluids, therefore are not suitable for cats with hypovolemia, shock or severe dehydration.
Are there any risks to fluid therapy?
A vein may be damaged during the injection or while fluid therapy is under way.
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.
Extreme care should be taken when giving fluids to cats with heart disease.
These side effects are uncommon, and the benefits of fluid therapy by far outweigh the risks.
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