Fluids Used for Replacement

It is usually recommended to ‘replace like with like’. This statement means that whatever fluid has been lost from the body should be replaced with fluid of the same type. For example, a cat that has had a road traffic accident and lost blood should receive blood during its fluid therapy. Similarly, an animal that has lost electrolytes and fluid through vomiting should receive those same electrolytes and fluid during administration of fluid therapy.

Fluid used in intravenous fluid therapy can be divided into three groups: blood, colloids and crystalloids.


1. Blood

Blood is used in cases such as severe anaemia, severe haemorrhage or where there is a clotting problem. Blood must be collected from a donor animal and then infused into a recipient.

Blood donors
The blood donor must be in good health, have previously been screened and known to be negative for infectious diseases, and have a normal packed cell volume.

Donor dogs should weigh a minimum of 25 kg and cats a minimum of 4 kg and should not be obese.

Blood is collected from the donor’s jugular vein into a blood collection bag containing an anticoagulant, often citrate phosphate dextrose (CPD). The patient does not usually require sedation. One unit (400 ml) is collected from dogs and up to 50 ml from a cat.

Ideally blood donors and recipients should be cross-matched to avoid a blood transfusion reaction, but in many veterinary field practices this is not possible due to the time needed for such tests to be undertaken. Cats certainly should be cross-matched or blood typed prior to donation as a transfusion reaction is more likely. Dogs, however, may tolerate one transfusion without side effects.

Blood is administered to the patient via a blood administration set. This has a filter to remove any clots. The patient must be monitored closely for signs of a blood transfusion reaction such as urticaria, muscle tremors, tachycardia or convulsions. If a transfusion reaction is suspected the blood infusion is stopped immediately. A transfusion reaction is most likely to occur within the first hour of the patient receiving blood.


2. Colloids

Colloids are also known as plasma expanders. They remain in the vascular space for longer than crystalloids, as their osmotic potential is so great that colloids draw fluid out of the interstitial and intracellular spaces, into the plasma. Colloids expand and maintain the vascular volume.

Colloids are used either when rapid restoration of the intravascular volume is required or as a second choice when blood is not available. They are likely to be used in situations of shock or when the circulating blood volume needs to be increased to replace recent losses. Examples of situations requiring colloid fluid replacement include:

• Road traffic accidents (shock and possible blood loss).
• Internal haemorrhage.
• External haemorrhage.
• Surgery where blood loss is anticipated.
• Hypovolaemic shock.

Colloids are used in conjunction with standard fluid administration giving sets and equipment.


3. Crystalloids


Crystalloids are a group of water-based fluids, containing a selection of fundamental electrolytes. Whilst administered intravenously and thus entering the intravascular space, crystalloids spread quickly into the extracellular fluid within the body.

Electrolytes
In deciding which crystalloid fluid to use, a veterinary surgeon has to establish which electrolytes have been lost from the patient. The veterinarian will then select a crystalloid fluid that contains the electrolytes that need to be replaced. It is possible to establish which electrolytes have been lost either by measuring blood parameters or by estimating losses from the clinical history.

Electrolytes are the ions dissolved in body fluids. The most important are sodium (Na+), Potassium (K+), Chloride (Cl–) and Bicarbonate (HCO3–). In choosing a crystalloid fluid, the main considerations are:

• Whether sodium or chloride electrolytes have been lost.
• Whether the patient is acidotic or alkalotic.
• Whether potassium is being lost or there is an excess of potassium within the body.
• Whether hypoglycaemia is present.

The fluid containing the electrolytes that have been lost from the body is preferred. A crystalloid fluid containing lactate is metabolized in the body into bicarbonate, which is useful in treating metabolic acidosis. Therefore, the ideal crystalloid fluid for a patient with metabolic or respiratory acidosis is one containing lactate, such as Hartmann’s solution or Darrow’s solution. Conversely, using a crystalloid that contains lactate is not preferred in a patient with alkalosis.