The body is made up of around 60–70% water.¹ Of this fluid, 1/3 is extracellular and 2/3 is intracellular. Of the extracellular fluid, approximately ¾ is interstitial and ¼ is intravascular. Fluids move between the various compartments depending on various forces.¹ In certain disease states, fluid deficiencies may occur in one or all of these fluid compartments and fluid therapy is used as a treatment in an attempt to rectify fluid balance. Different types of fluids are used depending on the specific disease. Fluids are classified as either crystalloids or colloids.¹ Crystalloids can be broadly classified as replacement fluids or maintenance fluids according to their composition. Colloids are classified as natural colloids or synthetic colloids.

Crystalloids

Isotonic crystalloids are similar to extracellular fluid.^1,2 They are replacement fluids that are used for dehydrated and/or hypovolaemic patients. These fluids can be given rapidly in large volumes. It has been shown that about 80% of the crystalloid volume given moves into the interstitial compartment within 1 hour of administration. Lactated Ringer's solution and 0.9% normal saline are examples of isotonic replacement crystalloids. They contain high levels of sodium and low levels or no potassium.³ Hypertonic replacement crystalloids such as 7.5% hypertonic saline can be used in very large dogs where isotonic fluids cannot be given quickly enough to achieve the desired effect in cases with severe hypovolemia.² These fluids are able to draw fluid from the other fluid compartments into the vascular space to achieve a much quicker volume expansion effect with a smaller volume of infused fluid. Hypertonic fluids should not be used in dehydrated patients.

Maintenance crystalloids are hypertonic within the vasculature and are also not suitable for dehydrated patients as they increase plasma osmolarity, draw fluid from body tissues and worsen cellular dehydration. Examples of maintenance solutions include Electrolyte No.2 and Maintelyte. Maintenance fluids contain less sodium and more potassium than extracellular fluid. They are used in patients that cannot supply their daily needs for water and electrolytes and are used once replacement fluid therapy has corrected dehydration or shock. These patients are patients that are not eating and drinking enough to supply fluids, but they are not dehydrated, hypotensive or experiencing ongoing fluid loss.³ Maintenance solutions are given slowly to a patient throughout the day. The goal of maintenance fluid therapy is to maintain normal sensible fluid loss via the urinary tract and insensible fluid losses via the gastrointestinal tract (GIT), urinary tract, skin and respiratory system.¹

Due to the high sodium and low potassium content, replacement fluids should not be used for long-term maintenance therapy since they predispose the patient to hypernatraemia with secondary diuresis and subsequent dehydration, as well as hypokalaemia. It is acceptable to modify replacement fluids and to use them as maintenance fluids in the short term in the absence of renal failure or cardiac disease.³

Colloids

In the healthy body, albumin is responsible for 80% of oncotic pull. Sodium together with proteins exerts oncotic pull, allowing water to be drawn into the blood vessels. Thus, oncotic pull is attributed to both proteins and sodium.

Colloids are solutions which are mostly isosmolar and will add to the oncotic pull when administered to a patient.^1,2 Colloids are divided into natural and synthetic colloids. Natural colloids² include all the blood products, whereas synthetic colloids² are solutions that contain complex starches or sugars that are osmotically active. They increase plasma oncotic pressure and hold fluid within the intravascular space.²

Colloids are used in patients with shock, perfusion deficits, hypooncotic states and deficiency of blood components.¹ Synthetic colloids are used in shock and SIRS (systemic inflammatory response syndrome). Natural colloids are used in patients with anaemia, hypoalbuminaemia, coagulation problems and thrombocytopaenia.

It is important to remember that the crystalloid dose should be reduced by 50% when colloids are used in conjunction with crystalloids.

Natural Colloids

Whole Blood

Blood is indicated in cases with a rapid decrease in haematocrit or signs of anaemia. The goal of blood transfusion is to maintain haematocrit at around 30% in the dog and 27% in the cat.¹

Packed red cells are used in patients that suffer from pure red cell loss, whereas whole blood is indicated in patients that suffer from blood loss anaemia.² The volume of blood required to replace the deficit is calculated with the following formula:

 Volume = desired Ht-patient Ht/donor Ht x 90 x weight

This volume is administered over 2 to 6 hours.²

It is important to remember that stored blood contains anticoagulant that precipitates calcium. In patients that receive multiple blood transfusions within a short period of time, hypocalcaemia could become a possible complication.²

Plasma

Plasma can be used in cases of decreased oncotic pull caused by severe hypoproteinaemia, as it will provide oncotic activity. When serum albumin levels are below 20 g/l, the colloid of choice is albumin.² Although albumin is present in plasma transfusions, it is important to take note that a plasma transfusion will not significantly elevate blood albumin levels.²

Plasma is administered at 10–20 ml/kg over 1–2 hours. It is stored frozen and needs to be defrosted very slowly in a basin of warm water.

Synthetic Colloids

Oxypolygelatin

Oxypolygelatin consists of bovine gelatin in NaCl.¹ Administration of 100 ml of oxypolygelatin leads to a 200 ml increase in vascular volume. They have a very short half-life of 2–4 hours.

Dextrans

Dextrans are polysaccharides. Dextran 40 and dextran 70 are available.¹ Administration of 500 ml of dextran 40 leads to a 750 ml increase in vascular volume.

Hydroxyethyl starch

Hydroxyethyl starches are highly branched polysaccharides (amylopectin) that are manufactured from waxy maize and sorghum. Hetastarch, pentastarch and tetrastarch are available. Hetastarch contains the largest particles and has a half-life of 25 hours.¹ Hydroxyethyl starches are enzymatically degraded by amylase and will cause an increase in serum amylase measurements. They cause an approximate increase of 140% in plasma volume within 30 minutes of administration.¹ When used as a constant rate infusion (CRI), larger molecules are added constantly, and colloid oncotic pressure (COP) remains more constant.¹

Oxyglobin®

Oxyglobin® is a polymerized bovine haemoglobin that is very useful as a blood substitute in cases of severe anaemia where blood is not available for transfusion.¹

Fluid Volumes

Fluid volumes will vary between patients, even when the same disease process is present. No two patients will respond exactly the same and require exactly the same fluid therapy plan. However, we need to have a starting point from where we can adapt our patient's plan based on parameters of fluid balance which are monitored regularly.³ To standardize the "starting point" scientists have developed certain formulae which are used to calculate the required fluid volume for a patient.³

Fluid needs include replacement therapy needed in hypovolemia and/or dehydration together with maintenance fluid needs as well as compensation for ongoing losses seen in cases with vomiting, diarrhea, polyuria and third space losses:

 Fluid required = replacement + maintenance + ongoing loss

Fluid deficit is used to calculate the replacement volume. The deficit is an estimation of the percentage dehydration of the patient. In most cases, deficits are replaced over 6–12 hours¹ unless they are at risk of fluid overload due to diseases or old age. This estimate is used to calculate replacement:³

 Replacement volume (l) = body weight (kg) x % dehydration

Maintenance volumes can be read off fluid tables or can be calculated:³

 Dogs: Maintenance = 132 x bodyweight(kg)^0.75

 Cats: Maintenance = 80 x bodyweight(kg)^0.75

This can be simplified by using 2–6 ml/kg/h for dogs and 2–3 ml/kg/h for cats as a rule of thumb for maintenance requirements.³

Ongoing fluid losses can be measured; for example, by measuring the volume of urine produced in excess of normal production or by weighing fluid soaked bandages from seeping wounds; or it can be estimated based on estimated volume of fluid losses via wounds or vomiting.

The three values are calculated and added and the volume is administered over a 24-h period.

However, in some cases, fluids need to be administered more rapidly to treat or prevent shock. In these cases, initial crystalloid fluid rates (shock rates) are calculated at 80–90 ml/kg in dogs and 50–55 ml/kg in cats.³ (Traditionally, these fluids were given over an hour.) A bolus of 25% of the calculated volume of fluid is administered rapidly and the patient is evaluated after each 25% bolus.³ If a patient is not showing a favourable response after receiving 50% of the calculated "shock rates," a colloid could be added to the fluid regimen.³

Frequent monitoring is required throughout the day in order to optimise fluid therapy and ensure that any changes in fluid requirements are observed and addressed by adjusting the fluid therapy plan.

References

1.  Devey J. Crystalloid and colloid fluid therapy. In: Ettinger S, Feldman E, eds. Textbook of Veterinary Internal Medicine. 7th ed. St. Louis, MO: Saunders Elsevier; 2010:487–96.

2.  Donohoe C, ed. The technician's role in fluid therapy - from catheters to colloids, part 2. In: North American Veterinary Community Congress; 2007; Orlando, Florida.

3.  Davis H, Jensen T, Johnson A, Knowles P, Meyer R, Rucinsky R, et al. 2013. AAHA/AAFP fluid therapy guidelines for dogs and cats. J Am Anim Hosp Assoc. 2013;49:149–59.