Intravenous fluid therapy can be used for a variety of reasons, including treatment of shock, maintaining fluid balance in anorectic patients, rehydration, providing electrolytes and glucose, treating intoxications, and for renal support.

Approximately 60% of an adult animal's body weight is water. Of this 60% of water, two thirds of this fluid is in the intracellular space and one third is in the extracellular space. Of this extracellular fluid, ¾ of this is in the interstitial space and ¼ is in the intravascular space. Therefore, the approximate blood volume is 77–78 mL/kg in a dog and 62–66 mL/kg in cats. It is interesting that cats have a lower total blood volume compared to dogs, which is one of the few reasons they are predisposed to fluid overload. Overall, there is net filtration of fluid from the intravascular to the interstitial space which is determined by Starling's principle.

Starling's principle is based on the understanding that transvascular fluid dynamics is dependent on the hydrostatic and oncotic pressures (determined by plasma protein concentration) of the intravascular and interstitial space, as well as capillary permeability, and is described by the equation:

Jv = Kfc[(Pc – Pi) – σ(πc – πi)]
Where,
Jv = transvascular fluid flux
Kfc = filtration coefficient (which is dependent on hydraulic conductance and surface area)
Pc = intravascular hydrostatic pressure
Pi = interstitial hydrostatic pressure
σ = reflection coefficient (which is the fraction of total osmotic pressure exerted by a solute)
πc = plasma oncotic pressure
πi = interstitial oncotic pressure

Fluid Types

Hartmann's, Compound Sodium Lactate (CSL)

• Isotonic crystalloids (same tonicity as blood).
• Na 130 mmol/L, K 4 mmol/L, Cl 109 mmol/L, Ca 4 mmol/L, lactate 26 mmol/L.
• Indications: most common type of fluid which can be used in most situations (shock and maintenance therapy). Because it includes lactate which is metabolised into bicarbonate, it is useful in treating metabolic acidosis.
• Contraindications: Do not give through the same IV line as blood products, as the calcium chelates with the sodium citrate in blood products.
• Although it contains potassium, it can still be used in patients with hyperkalaemia, as the concentration is minimal.
• Shock dose: Dogs 10–20 mL/kg IV; Cats 5–10 mL/kg delivered over 15 minutes IV. Can be repeated up to a total of 4 times.

0.9% NaCl

• Isotonic crystalloid (same tonicity as blood).
• Na 154 mmol/L, Cl 154 mmol/L.
• Indications: Ideal in metabolic alkalosis (although this situation is rare).
• Shock dose: Dogs 10–20 mL/kg IV; Cats 5–10 mL/kg IV deliver over 15 minutes IV. Can be repeated up to a total of 4 times.

0.45% NaCl

• Hypotonic crystalloids (lower tonicity than blood).
• Na 77 mmol/L, Cl 77 mmol/L.
• Indications: Treatment of hypernatraemia.
• Contraindications: Treatment of shock.

Hypertonic Saline (7.5% NaCl)

• Higher tonicity than blood.
• Na 1283 mmol/L, Cl 1283 mmol/L.
• Due to the higher tonicity, free water will be absorbed from the interstitial and intracellular compartments, increasing the intravascular volume.
• Indications: Traumatic brain injury, cerebral oedema, large breed dogs, during cardiopulmonary resuscitation.
• Contraindications: Hypernatraemia; hyponatraemia; dehydration.
• Shock dose: 2–4 mL/kg over 10–15 minutes IV (Do not bolus rapidly).
• Always administer isotonic crystalloids during or after as a bolus or continuous rate infusion.

Artificial Colloids

• Examples: Voluven, volulyte, hetastarch, dextran, gelafusine.
• These are hyperosmolar solutions which increase oncotic pressure in the intravascular space. Unlike hypertonic saline, it doesn't draw fluid in, but instead minimises redistribution of fluid into the interstitial space, hence helps maintain intravascular blood volume.
• Indications: severe hypoproteinaemia; large breed dogs; shock which is non-responsive to isotonic crystalloids.
• Adverse effects/contraindications: acute kidney injury, coagulopathy, increased risk of fluid overload.
• Shock dose: Dogs 5–10 mL/kg; cats 2.5–5 mL/kg delivered over 15 minutes IV. The maximum dose depends on the type of colloids.

The two main ways we use intravenous fluid therapy is in the treatment of shock, in which fluids are administered as a bolus, or as a continuous infusion, in which it is used as maintenance therapy and for rehydration.

Shock

The definition of shock is inadequate oxygen delivery resulting in anaerobic metabolism and mitochondrial dysfunction, eventually resulting in organ dysfunction. There are 4 different types of shock: 1) hypovolaemic; 2) obstructive; 3) cardiogenic; and 4) vasodilatory. The first three types of shock are categorised as "vasoconstrictive shock," where peripheral vasoconstriction occurs in order to maintain perfusion. In vasodilatory shock, vessels remain vasodilated as vasoconstriction is impaired. Table 1 describes common physical examination parameters of vasoconstrictive and vasodilatory shock, which severity worsens from mild, moderate, to decompensated shock.

Table 1. Clinical findings of shock

Hypovolaemic shock is the most common type of shock which occurs due to severe dehydration (>8%), vomiting, diarrhoea, haemorrhage, third space loss of fluids, or polyuria. Treatment of hypovolaemic shock involves fluid resuscitation and correcting the underlying cause.

Obstructive shock is defined by the obstruction of great vessels or the heart. This can occur due to gastric dilation and volvulus, pericardial effusion causing cardiac tamponade, and pneumothorax. Treatment of obstructive shock involves very quickly relieving the obstruction. Gastric dilation and volvulus will also require fluid resuscitation; however, use of fluid in pericardial tamponade may not be required and can be detrimental.

Cardiogenic shock occurs when there is inadequate circulation of blood due to poor left ventricular function. This can occur due to mitral valve disease, myocardial disease, or arrhythmias. This is the only type of shock in which fluid therapy is contraindicated. Cardiogenic shock can be identified by obtaining a good history, auscultating the heart for murmurs or arrhythmias while palpating pulses for pulse deficits, performing an ECG, and/or performing a bedside echocardiogram to assess the left atrium to aortic ratio (LA:Ao) which is usually less than 1.3–1.5 and subjectively assessing the contractility of the heart.

Vasodilatory shock occurs due to systemic inflammatory response syndrome (SIRS), sepsis, anaphylaxis or general anaesthesia. Treatment involves addressing the underlying cause; administering intravenous fluids; +/- vasopressors (noradrenaline or dopamine) if the patient remains hypotensive. During vasodilatory shock, capillaries are more permeable, and patients are often hypoalbuminaemic resulting in increased risk of fluid extravasation and fluid overload.

The aim of treating shock with intravenous fluid therapy is to rapidly fill the intravascular space with intravenous fluids at doses which are described above. Intravenous fluids can be administered by various methods: Pressure bags are ideal in medium- to large-breed dogs, as it can deliver 1 litre of fluid within 8 minutes. If intravenous fluid is delivered too slowly, it can redistribute and will not treat shock. In small- to medium-sized dogs, a fluid pump can be used. In cats or small dogs prone to fluid overload, you must be very careful about bolusing fluids. Safer methods to prevent fluid overload include drawing fluid up in a 50-ml syringe and delivering by hand; using a syringe driver; or using a paediatric burette (which can be used with or without a fluid pump).

Continuous Intravenous Fluid Therapy

Continuous fluid therapy involves 3 components:

• Maintenance: This includes all sensible losses including normal urine output, normal faeces, panting and sweating. Maintenance rate is approximately 2 mL/kg/h.
• Ongoing losses: This includes any insensible losses including vomiting, diarrhoea, polyuria, or 3rd space loss of fluids (cavitary effusions), and wounds. Ongoing losses can be measured (e.g., using a urinary catheter, weighing incontinence sheets) or estimated visually. An approximate rate would be 0.5–1.5 x maintenance rate.
• Replacement: This is the amount of dehydration which needs to be replaced. The volume to be replaced can be calculated by:
  Replacement volume (ml) = % dehydration/100 x body weight (kg) x 1000
  The replacement volume is then divided by how many hours you choose to deliver the replacement volume over, which is standardly 8–24 hours. Patients at risk of fluid overload should have fluid delivered slowly.

Once the rate of each of the 3 components has been calculated, they are added together to determine the total rate.

Example: A 15-year-old 10-kg dog with a history of diarrhoea and vomiting, presents with skin tenting and dry mucus membranes, and not in shock. We estimate this dog to be 8% dehydrated.
Maintenance = 2 mL/kg/h = 20 mL/h
Ongoing losses = estimated at 0.5 x maintenance = 10 mL/h
Replacement = 8/100 x 10 x 1000 = 800 mL/h; delivered over 24 h = 33 mL/h
Total = 20 + 10 + 33 = 63 mL/h

It is always important to monitor your patients for signs of fluid overload. What we describe as fluid overload is interstitial oedema due to increased hydrostatic pressure (e.g., too much fluid therapy), decreased oncotic pressure (e.g., hypoalbuminaemia), or increased capillary permeability (e.g., vasodilatory shock). Patients who are prone to fluid overload include animals with cardiac or respiratory disease, hypoproteinaemia, sepsis, anuric or oliguric renal failure, geriatrics, and cats. Fluid overload is seen as interstitial oedema on the dorsal neck, feet, elbows, hocks; chemosis of the conjunctiva; bilateral serous nasal discharge; and pulmonary oedema. In patients predisposed to fluid overload, careful fluid monitoring is imperative, which can be performed by regular body weight measurements, calculating INS (total volume of fluids administered including oral water and IV infusions) and OUTS (total volume of fluid produced by patients including urine, drains, diarrhoea); and always considering if fluid rates could be reduced.

References

References can be provided at request.