As well as having a lower osmolality than plasma, hypotonic solutions also have lower concentrations of sodium and chloride ions. These solutions are not suitable for bolus therapy due to the redistribution of water in the body to the intracellular spaces when they are administered [ 2 ].
Examples of hypotonic solutions include 5 % dextrose; in this case, the dextrose is immediately metabolised once it has entered the bloodstream [ 1 ]. Although hypotonic solutions allow water to enter the intracellular space, they are unable to maintain electrolyte balance, so a patient ' s electrolyte levels should be monitored routinely while using a hypotonic infusion [ 2 ].
Hypertonic crystalloid fluids have a greater osmolality than plasma. When administered, they can result in water moving from the interstitial and intracellular spaces into the vascular circulation [ 1 ].
Types of fluid loss
Animals can have different types of fluid loss, and knowledge of the type of loss may help the VS to select the most appropriate fluids for a specific condition. There are three main types of dehydration that can occur( Table 1) [ 6 ].
Table 1. Types of dehydration [ 6 ].
Type
Hypertonic dehydration
Description
Occurs following the loss of fluid that contains a high concentration of solutes. Hypertonic dehydration can be caused by heat stroke, overexertion and anxiety.
Hypertonic solutions, such as 7.2 % saline, should be administered with extreme care. Large volumes of hypertonic solutions can be fatal [ 2 ].
When administering hypertonic fluids, the osmotic pressure draws fluid out of the interstitial space and into the circulation. This rapidly increases the intravascular volume, especially when there is available fluid in the interstitial space. Therefore, hypertonic solutions are not recommended for dehydrated patients [ 1 ].
Hypertonic solutions are often used in cases of circulatory shock.
Colloids
Hypotonic dehydration
Isotonic dehydration
Occurs following the loss of water with a low concentration of solutes. This is common in patients that have heart failure and are not receiving the appropriate treatment.
Occurs when the body water that has been lost has the same solute concentration as the water that remains. Many diseases, along with vomiting, diarrhoea, anorexia and haemorrhage, can cause isotonic dehydration.
Colloids are made up of large molecules that help to increase the osmotic pressure of the blood plasma [ 1 ]. This draws fluid out of the tissues and into the bloodstream. Colloids can help treat patients that are experiencing hypotension or profound shock.
There are two different types of colloids that can be used in practice, depending on the severity of the patient ' s condition: synthetic and natural.
Synthetic colloids are manufactured to a specific formulation. There are three common types: gelatins, dextrans and hydroxyethyl starches. Gelatins are the most common type of colloid used in veterinary practice and are formulated from mammalian collagen [ 1 ].
Natural colloids can be used when components such as platelets or red blood cells are needed and a blood product can be used as a natural colloid. Ideally, the blood should be typed and cross-matched with the patient that requires a blood transfusion [ 1 ]. In the UK, animal blood is available from the Pet Blood Bank charity, which operates 24 hours a day, 7 days a week [ 5 ].
Calculating fluid therapy requirements
Both RVNs and SVNs will be required to carry out fluid therapy calculations on a daily basis under the guidance of the VS. Some practices may have set protocols for fluid therapy for anaesthetised patients, but each patient should be assessed throughout their procedure and the fluid rate adjusted accordingly.
Patients receiving fluid therapy due to injury or disease should be evaluated and placed on a suitable product for that condition.
Maintenance requirements
The maintenance fluid requirement for most animals is estimated at 50 ml / kg / 24 hours. This is to replace average fluid losses due to the body ' s normal processes, such as urination, respiration and sweating [ 1 ].
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