Us veterinary nurses play an enormous role in administering intravenous fluid therapy to our patients, from assessing the patient prior to fluids, placing catheters, calculating fluid requirements, administering treatment and monitoring the patient.

In order to safely administer fluid therapy, it is important that we know where fluid losses originate from, how to calculate fluid requirements to replace different deficits, which fluids are used and when, and the common complications of intravenous fluid therapy in veterinary patients.

Keep reading to learn more about how fluids are usually balanced in the body, the different fluid losses seen, and how these are replaced, and for access to the free download which accompanies this post!

Body Fluid Compartments

Before we begin assessing our patient and formulating a fluid therapy plan, we need to understand how fluid is normally balanced in the body.

Approximately 60% of a patient’s body weight is actually fluid; this is known as ‘total body water’. This can be divided into intracellular and extracellular fluid; Around 40% of a patient’s body weight is intracellular fluid (2/3rds of total body water), and 20% is extracellular fluid (1/3rd of total body water).

Extracellular fluid can be subdivided into interstitial fluid, transcellular fluid and plasma. Interstitial fluid functions to bathe cells and tissues, and comprises around 15% of a patient’s body weight (24% of total body water). Plasma comprises around 4% of a patient’s body weight or 8-10% of total body water; this forms the majority of a patient’s intravascular volume. Transcellular fluids make up around 1% of a patient’s body weight (or 2% of total body water); these include fluids such as bile, cerebrospinal fluid pleural, pericardial and peritoneal fluids and synovial fluid.

The body is a closed system. This means that any losses must come from one or more of these fluid compartments; it is important that we as veterinary nurses consider where these losses have originated from, in order to select an appropriate fluid type, rate and route under the direction of the veterinary surgeon.

Classifying Fluid Deficits

In order to calculate and administer a fluid therapy plan, we must first classify the type and severity of any fluid deficits present. We do this by obtaining a thorough clinical history, performing a thorough clinical examination, and reviewing any relevant diagnostic results (such as PCV, total solids, and biochemistry results)

We can classify fluid losses according to their tonicity (concentration relative to plasma). Fluid losses may be isotonic, hypotonic or hypertonic.

Isotonic fluid losses

This is the loss of fluid with the same concentration as plasma. Isotonic losses occur when water and electrolytes are lost, rather than a primary loss of water alone. Isotonic fluid losses result in extracellular fluid loss. As fluids do not move from other compartments, plasma volume reduces and hypovolaemia results.

Hypotonic fluid losses

This is the loss of fluid with a lower concentration than plasma. Hypotonic losses occur when there is a primary water loss, for example in polyuric patients, adipsic patients, or patients who are not drinking enough to replace fluid losses. Hypotonic losses cause shifting of fluid from the intracellular space, resulting in dehydration.

Hypertonic fluid losses

This is the loss of fluid with a higher concentration than plasma. Hypertonic losses occur in patients with body cavity effusions such as pleural or peritoneal fluid accumulation. When this occurs, intracellular fluid becomes more concentrated than extracellular fluid, causing the extracellular fluid to shift into cells. As this reduces intravascular volume, this causes worsening hypovolaemia.

Dehydration and Hypovolaemia

Another important way we classify fluid deficits is based on the fluid compartments they originate from - i.e. intracellular or extracellular losses.

Hypovolaemia is the loss of extracellular fluid, specifically from the intravascular space. This is a medical emergency as insufficient delivery of blood and oxygen to tissues and organs results.

Dehydration is the loss of intracellular and interstitial fluid; this is corrected over a much longer time period than hypovolaemia.

Calculating Fluid Deficits

In order to calculate a patient’s fluid deficit, we must first determine where these losses have originated from.

Patients with hypovolaemia have signs consistent with poor perfusion. These signs include:

• Tachycardia (in dogs)

• Bradycardia (in cats, and dogs with severe decompensated hypovolaemia)

• Bounding pulses (in dogs)

• Weak pulses (in dogs and cats)

• Hypothermia

• Pale mucous membranes

• Prolonged capillary refill time

• Hypotension (systolic arterial pressure <90-100mmHg)

• Hyperlactataemia (plasma lactate level >2-2.5 mmol/L)

Patients with hypovolaemia or perfusion deficits require rapid fluid rates over short periods of time – this is known as volume resuscitation or a ‘fluid challenge’. Prior to volume resuscitation, dedicated fluid challenge ‘end points’ should be set. These are physical goal parameters such as a reduction in heart rate or increase in blood pressure. A set fluid volume (typically 5-20ml/kg depending on the species or patient) is administered over a 10-30-minute period, and after this has been administered the patient is re-assessed. This volume can then be repeated if needed, based on the patient assessment, until the parameters listed above return to normal.

To calculate and administer a fluid bolus to a hypovolaemic patient:

1. Calculate the total bolus volume required (e.g. 10ml/kg x 4kg = 40ml)

2. Calculate how many boluses fit into a 1-hour period (e.g if you’re giving the bolus over 15 minutes, 60 minutes / 15 minutes = 4)

3. Multiply the bolus volume by 4 to get your ml/hour rate for your infusion pump (e.g. 40ml x 4 = 160ml/hour)

4. Set your infusion pump as follows:

   1. Drip rate: 160ml/hour

   2. Volume to be infused: 40ml

   3. Time: 15 minutes

5. After the bolus has been given, re-examine your patient, see if their vitals have changed, discuss with the veterinary surgeon and repeat the bolus as required under veterinary direction.

Patients with dehydration have alterations to their bodyweight, skin elasticity, mucous membrane tackiness/dryness and eye position. We can assess their percentage dehydration based on the severity of these signs:

% Dehydration

0-4: No examination findings
4-6: Tacky mucous membranes
6-8: Loss of skin turgor, dry mucous membranes
8-10: Loss of skin turgor, dry mucous membranes, retracted globe position/’sunken eyes’
10-12: Persistent skin tent, dry mucous membranes, dull corneas/corneal dryness, retracted globes
>12: Persistent skin tent, dry mucous membranes, retracted globes, dull corneas, evidence of perfusion deficits/hypovolaemia

Once the percentage dehydration has been calculated, we can determine the fluid volume required. This is calculated by multiplying the patient’s body weight by their % dehydration, by 10.

Fluid volume required (ml) = % dehydration x body weight (kg) x 10

In addition to the volume required to correct the patient’s dehydration, we also need to incorporate daily maintenance requirements, and any ongoing fluid losses.

Maintenance requirements are the volumes of fluid required each day to replace normal daily losses (through normal urination, defecation and respiration). A healthy patient would drink this amount per day, but in many unwell patients it needs to be given via intravenous fluid therapy. There are a number of calculations that can be used to determine maintenance requirements; the one used most commonly is:

Maintenance requirements (ml/day) = 50 x bodyweight (kg) 

For example:

A 14kg Cocker Spaniel presents with 6% dehydration. The vet would like this corrected over 24 hours. They ask you to calculate: The total fluid deficit, the patient’s maintenance fluid requirements and the total fluid rate in ml/hour.

Fluid deficit = 14kg x 6% x 10 = 840ml
Maintenance = 50ml/kg/day x 14kg = 700 ml/day
Fluid rate = 840 + 700 = 1540ml/day
1540 ÷ 24 hours = 64ml/hour

To accompany today’s post, I’ve made you a free intravenous fluid therapy rate calculator! Simply fill in the shaded boxes and your fluid rates will automatically calculate for you. To download your copy, enter your email address in the box below and it will be emailed straight to you (check your junk folder!)

So that’s the fundamentals of fluid balance in the body, where we see fluid losses originate from, and how to calculate fluid therapy requirements! Make sure you grab your free IVFT rate calculator, and let me know what your biggest takeaway from this post was in the comments below!

References:

1. Wellman ML, DiBartola, SP and Kohn CW. 2006. Applied physiology of body fluids in dogs and cats. In: DiBartola SP (ed) Fluid, electrolyte and acid-base disorders in small animal practice. 4th ed. Missouri: Elsevier Saunders.

2. Lyons BM and Waddell LS. 2018. Fluid therapy in hospitalized patients, part 1: patient assessment and fluid choices, https://todaysveterinarypractice.com/fluid-therapy-part-1fluid-therapy-hospitalized-patients-patient-assessment-fluid-choices/

3. Hughston L. 2016. Go with the flow: The basics of fluid therapy for small animal veterinary technicians, https://todaysveterinarynurse.com/articles/go-with-the-flow-the-basics-of-fluid-therapy-for-small-animal-veterinary-technicians/.

4. Aldridge P and O’Dwyer L. 2013. Practical Emergency & Critical Care Veterinary Nursing. Chichester: Wiley-Blackwell.

5. Walden LA. 2019. Recognizing and treating shock in cats, https://www.dvm360.com/view/recognizing-and-treating-shock-cats.

6. Davis H, Jensen T, Johnson A, et al. 2013. 2013 AAHA/AAFP fluid therapy guidelines for dogs and cats. Journal of the American Animal Hospital Association 49, pp.149-159.