Parenteral fluid therapy

4 Parenteral fluid therapy


Parenteral fluid therapy is the most common therapeutic intervention performed in veterinary emergency practice. A thorough understanding of the indications for the use of parenteral fluids, the types of therapeutic fluid available, and the most appropriate protocol for their administration is mandatory both to maximize the benefit and minimize the potential harm associated with this therapy.


Hypovolaemia and dehydration are the most common indications for the use of fluid therapy and it is essential to understand their differences with respect to pathophysiology and clinical assessment in order to administer appropriate fluid therapy (see Ch. 2). This chapter focuses on the different types of parenteral fluid commonly available in nonreferral emergency practice and their appropriate use in hypovolaemia and dehydration.



Types of Parenteral Fluid



Crystalloids


Crystalloids are electrolyte solutions that can pass freely out of the bloodstream through the capillary membrane. Crystalloid solutions are described as isotonic, hypertonic or hypotonic based on how their tonicity compares to that of plasma. The tonicity is related to the sodium concentration and it is the tonicity that determines how the crystalloid solution is distributed between fluid compartments following administration into the bloodstream.


The two most commonly used crystalloid solutions are buffered lactated Ringer’s solution (Hartmann’s solution, compound sodium lactate) and 0.9% sodium chloride (normal strength or physiological saline). Both these solutions are examples of replacement isotonic crystalloids as their tonicity and electrolyte composition are similar to that of extracellular fluid. Following intravascular administration, these fluids equilibrate relatively quickly with the interstitial space and 75–85% of the administered volume is likely to have left the bloodstream 30–60 min after infusion. This is why large volumes are required to expand the intravascular compartment effectively in hypovolaemia and is also the reason why these solutions are used to replenish extravascular fluid losses in dehydration (see below).


Hypertonic (e.g. 7.2–7.5% sodium chloride (hypertonic saline)) and hypotonic (e.g. 0.45% sodium chloride (half strength saline)) crystalloid solutions are also available but their use is much less commonly indicated. Hypertonic saline administration causes plasma volume expansion mainly by drawing water out of cells into the extracellular space down an osmotic gradient. Most of this fluid remains in the interstitial space but a proportion diffuses into the vasculature. The recommended dose is 4 ml/kg i.v. (dogs 4–7 ml/kg, cats 2–4 ml/kg) over a minimum of 5 minutes and a rapid though short-lived effect is typically seen (within 5 minutes). Hypertonic saline is indicated in volume resuscitation, especially in large or giant breed dogs where rapid administration of large volumes of isotonic crystalloids may be impossible. Administration of hypertonic saline must be followed by the use of a replacement isotonic crystalloid due to the osmotic diuresis and rapid sodium redistribution that occur with this treatment. Hypertonic saline is often administered in combination with a colloid solution to prolong intravascular volume expansion. Hypertonic saline is also indicated in the treatment of raised intracranial pressure, especially with concurrent hypovolaemia, where it causes fluid to move out of the brain parenchyma and into the vasculature (see Ch. 28).


Hypotonic saline is most often used in combination with 0.9% sodium chloride to correct hyponatraemia gradually. It is also occasionally used in dehydrated animals with cardiac disease to provide rehydration while limiting the amount of sodium administered.



Clinical Tip



• A 0.18% sodium chloride (with 4% glucose) solution is commercially available. This solution is markedly hypotonic compared to plasma and in the author’s opinion there is no indication for its routine use. It may occasionally be required to treat animals with severe acute hypernatraemia but this is rare in companion animals. If this solution is used, animals must be monitored very closely for electrolyte abnormalities (hyponatraemia in particular) that may be severe with potentially very serious clinical consequences.

• The author is aware of cases in which this solution has been used as a convenient way of administering intravenous glucose supplementation. However, other proprietary glucose solutions (e.g. 5% glucose in 0.9% sodium chloride solution) are available and glucose solutions can also readily be formulated (see Appendix 1, Drug Formulary). The author would therefore strongly discourage the use of the 0.18% sodium chloride solution as a means of administering glucose supplementation.


Uses of isotonic crystalloid solutions


Isotonic crystalloid solutions are used in two main ways:


• High rate isotonic crystalloid therapy is used to treat hypovolaemia either alone or in combination with other fluid types (colloid, hypertonic saline)

• Low rate isotonic crystalloid therapy is used to treat dehydration.


Synthetic colloids


Colloid solutions consist of large (macro) molecules that do not readily leave the intravascular space (through capillary pores) and are hyperoncotic relative to normal animals. Synthetic colloids therefore draw fluid into and hold fluid in the vasculature, causing plasma volume expansion. Commercially available synthetic colloid preparations are often made up in a 0.9% sodium chloride solution. Nonsynthetic (natural) colloid solutions that are currently used therapeutically include plasma and human serum albumin solutions.


The three types of synthetic colloid solution currently in veterinary use are:


• Hydroxyethyl starches: tetrastarch (e.g. Voluven 130/0.4®, Fresenius Kabi); hetastarch (e.g. Hetastarch 600/0.7®, Baxter); pentastarch (e.g. HAES-Steril 200/0.5®, Fresenius Kabi)

• Gelatins (e.g. Gelofusine®, Braun; Haemaccel®, Intervet)

• Dextrans (e.g. Dextran-70®, Pharmacosmos).

The volume and duration of plasma expansion that follows colloid administration depend in part on the specific colloid used (its colloid osmotic pressure (COP)), as well as the dose given and the species in question.



Indications for synthetic colloid use


Synthetic colloids are usually used in hypovolaemic patients in one of two scenarios:


1 When fluid resuscitation with an isotonic crystalloid solution has proved to be ineffective; this is because colloids are more efficient at intravascular volume expansion.

2 When there are specific reasons that warrant the inclusion of a colloid; this might be the case for example in animals that are hypoproteinaemic (e.g. from gastrointestinal or renal loss). Isotonic crystalloids will remain in the circulation for even less time in these animals as plasma proteins (albumin in particular) are partly responsible for retaining these fluids in the intravascular space.

Synthetic colloids are not generally used alone when employed in the treatment of hypovolaemia and are typically discontinued earlier on than the replacement crystalloid solution. It is much less common for patients to remain on long-term colloid therapy (although on-going hypoproteinaemia or the presence of systemic vasculitis or capillary-leak syndrome for example may warrant this treatment).



Adverse effects of synthetic colloids


The most important adverse effect to consider when using synthetic colloids is their effect on coagulation that is likely to be multifactorial in origin. Allergic reactions are also rarely identified.



Haemoglobin-based oxygen-carrying solutions


Haemoglobin-based oxygen-carrying solutions (HBOC) are not blood replacement solutions. They increase plasma haemoglobin concentration and therefore oxygen-carrying capacity but do not contain other blood constituents. The only HBOC currently available for veterinary clinical use is Oxyglobin® (Biopure Corporation; www.biopure.com). This solution is based on polymerized modified bovine haemoglobin and is administered using standard intravenous fluid administration sets. It is currently only licensed for use in dogs but has been used extensively off-licence in cats with great success.


The main indication for Oxyglobin® is in euvolaemic anaemia where it can act as a substitute for the deficient red blood cells and allow improved tissue oxygenation. Unlike with blood transfusions, there are no cellular antigens in Oxyglobin® so typing and crossmatching do not need to be performed. The product as supplied in foil by the manufacturer also has a long shelf-life of 3 years. Oxyglobin® is used in these cases to support the patient while diagnosis is achieved and treatment is instituted and given time to take effect.



Clinical Tip


Administration of Oxyglobin® typically causes a fall in packed cell volume (PCV) due to intravascular volume expansion. Ideally, therefore, plasma haemoglobin concentration should be used to monitor increase in oxygen-carrying capacity in lieu of packed cell volume. An approximately equivalent PCV can be calculated as follows:



image



However, a haemoglobinometer is required to measure free plasma haemoglobin concentration as in-house machines typically calculate haemoglobin concentration from haematocrit. Regardless of whether a haemoglobinometer is available or not, positive clinical response to treatment is the best guide of effective therapy.


As Oxyglobin® contains large molecules it is also a potent colloid solution and can therefore be used very effectively to provide intravascular volume expansion in animals with hypovolaemia. Despite the unique oxygen-carrying benefits of this modified biological colloid, the much greater cost of Oxyglobin® over other available colloids means that its use in hypovolaemia is typically restricted to animals that have suffered significant blood loss.



Clinical Tip



• The volume expanding properties of Oxyglobin® must be borne in mind when it is used to treat anaemia in patients without hypovolaemia as there is a very real risk of fluid overload if inappropriately high rates are used, especially in cats.

• Typical rates for Oxyglobin® use in euvolaemic anaemic patients are 0.5–1 ml/kg/hr in cats and 1–2 ml/kg/hr in dogs.

• Caution is also warranted in disease states associated with an increased risk of fluid overload such as cardiac disease, pulmonary parenchymal disease and cerebral oedema

As with all colloids, Oxyglobin® will interfere with serum total solids measurement via refractometry and results must be interpreted cautiously. Oxyglobin® will also interfere with colorimetric serum biochemistry analysis although the parameters affected will depend on both the analyser and the methodology. Peripheral blood smear evaluation is not affected.



The Fluid Plan


On the basis of the physical examination, and subsequently other findings, it should be possible to answer the following questions (see Ch. 2):


• Is the animal hypovolaemic and if so, is this mild, moderate or severe?

• Is the animal dehydrated and if so, what is the estimated percentage dehydration?


Hypovolaemia


The basic objective is to restore the effective circulating intravascular volume and thereby restore adequate tissue perfusion. Appropriate fluid therapy is therefore provided until end-points suggestive of acceptable systemic perfusion are achieved. This volume expansion is performed over a short period of time – usually a few minutes to an hour but sometimes longer – and may involve the use of both isotonic crystalloids and colloids including Oxyglobin® (plus whole blood and hypertonic saline if available). Isotonic crystalloids are the first choice in the majority of cases.



How?


Hypovolaemia is treated via the intravenous route using one or more of the shortest but largest bore catheters possible. If a peripheral vein cannot be catheterized and a central venous catheter is not available or inappropriate, the intraosseous route may be used initially.



How much and for how long?



Isotonic crystalloids


See Table 4.1 for guidelines for initial rates of isotonic crystalloid fluid therapy in dogs and cats with uncomplicated hypovolaemia. Initial boluses are usually given over 15–20 minutes. For some bigger dogs the use of a pressure infusor (Figure 4.1) around the crystalloid bag can be invaluable in delivering the fluid within a suitable period of time.



Table 4.1 Guidelines for isotonic crystalloid therapy for hypovolaemia in dogs and cats. Initial boluses are usually given over 15–20 minutes

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Related posts:

  1. Jaundice
  2. Shock and dehydration
  3. Pallor and approach to anaemia
  4. Oxygen supplementation

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Parenteral fluid therapy

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