1. Identify the fluid compartment deficit
2. Select the appropriate fluid type and administration route
3. Calculate the fluid dose and administration rate
4. Monitor patient response and signs of complications

CAUTION Adequate extracellular fluid volume (vascular and interstitial spaces) must be restored before addressing intracellular fluid deficits

1. Assess the intravascular space (perfusion)
Aim: evaluate effective circulating volume (ECV), also referred to as perfusion status

• Patient history: vomiting, diarrhoea, polyuria, haemorrhage, etc
• Perfusion parameters: mentation, heart rate, capillary refill time, mucous membrane colour, extremity temperature, pulse quality
• Monitored parameters: blood pressure, ECG findings
• Laboratory test results: lactate, blood gases, PCV
• Imaging: microcardia, CVC collapsibility index, effusions

2. Assess the interstitial space (hydration)
Aim: evaluate hydration status (interstitial volume)

• Physical examination: skin turgor, mucous membrane moisture, globe position
• Measure PCV, total protein, urea, urine specific gravity

3. Assess the intracellular space (free water status)
Aim: Identify and calculate solute-free water deficit (free water loss affects intracellular fluid volume)

• Laboratory values: serum sodium concentration
• Calculate the free water deficit to determine intracellular hydration needs
• Free Water Deficit (FWD) in litres (L) = ((Patient Na ÷ Desired Na) − 1) × (0.6 × Weight in kg)

The choice of fluid type and administration route is determined by the patient’s age, underlying conditions, fluid and electrolyte balance and other specific factors

• Replacement fluids: isotonic or hypertonic crystalloids replace fluid and electrolyte losses
• Maintenance fluids: hypotonic fluids meet daily needs in patients not eating or drinking

• i/v, i/o: hypovolaemia or severe dehydration
• s/c: mild dehydration or home care, not for euhydrated patients
• Enteral: mild dehydration if oral intake is possible, or via feeding tubes

Three phases of fluid therapy

Resuscitation:

• For hypovolaemia, give buffered isotonic crystalloid (sodium similar to patient) boluses
• Cats: 5–10 ml/kg i/v, i/o over 15 – 30 min
• Dogs: 15–20 ml/kg i/v, i/o over 15 – 30 min
• The boluses can be repeated if the desired hemodynamic and perfusion goals have not been achieved and the patient remains hypovolemic
• Consider hypertonic crystalloids or colloids if rapid volume expansion is needed
• Ultrasound of cardiac chambers and the caudal vena cava can help assess fluid responsiveness during resuscitation

Rehydration:

• Use isotonic crystalloids over 12–24 hr
• Fluid Deficit (L) = Body Weight (kg) × % Dehydration (as a decimal)
• Add ongoing losses
• Mild cases: oral or s/c fluids
• If both dehydration and hypovolaemia: correct perfusion first, then rehydrate slowly

Maintenance:

• Use hypotonic fluids if not eating/drinking
• Cats: 40 ml/kg/day
• Dogs: 60 ml/kg/day
• Paediatrics: cats: 2.5 × adult dose, dogs: 3× adult dose

• Use trained staff and multiparameter monitoring to assess hydration and volume status
• Choose the fluid delivery method based on patient size, volume needed, and available monitoring
• Use infusion pumps, syringe drivers, or pressure bags appropriately
• Weigh high-risk patients 2–3 times daily and check respiratory rate every 1–2 hours
• Track all fluid inputs and outputs, and watch for oedema or respiratory changes
• Use focused ultrasound to check for effusion, B-lines, or vena cava size when available
• In shock cases, vena cava assessment may be more reliable than heart rate or blood pressure
• If 24-hour monitoring isn’t possible, give IV fluids during open hours and SC fluids overnight
• Always use a new fluid bag and line per patient; do not use IV bags for flushes

• Monitor for 10% body weight gain or positive fluid balance
• Fluid overload ranges from hypervolemia to life-threatening oedema
• Once oedema develops, reversal is difficult, prevention is key
• Causes: excess fluids, kidney/heart/liver disease
• Signs: oedema, effusions, weight gain, respiratory signs
• Management: stop fluids, restrict sodium and water, give diuretics, encourage mobility

• Hydrate before surgery; correct 80% of deficits if possible
• Euhydrated, euvolemic patients receiving injectable anaesthetics for short periods generally do not require i/v fluids
• Initial fluid rates in patients with normal cardiac and renal function:
  Dogs 5 ml/kg/hr, cats 3 ml/kg/hr following 2013 guidelines
• Maintain MAP ≥ 60 mmHg

Management of hypotension

• Assess anaesthetic depth, reducing the vaporiser settings if possible
• Evaluate body temperature, actively warm as needed
• Consider the requirement for additional analgesia, sedatives or local anaesthesia to help reduce vaporiser settings
• Administer a crystalloid fluid bolus of 5 ml/kg over 10 min

For refractory hypotension

• If hypotension persists despite adjusting vaporiser settings and providing crystalloid fluids, administer sympathomimetic therapy if needed (positive inotropes or vasopressors) or consider a colloid bolus  (2–5 ml/kg over 10 minutes)
• In patients with severe or ongoing haemorrhage, address with blood products
• CAUTION Once stabilised, return to 3–5 ml/kg/hr crystalloid, monitor for fluid overload

Key points for fluid therapy in unwell patients

• Always consider enteral fluids if tolerated, water can be given on its own or mixed with food, using feeding tubes where needed
• Do not avoid fluids in anaemic patients if they are dehydrated or hypovolaemic, monitor closely and assess if a blood transfusion is also needed
• Correct electrolyte imbalances carefully

Renal disease/azotaemia

• If not hypotensive, correct dehydration gradually
• IRIS stage 3–4 or anuric patients are at risk for fluid overload, monitor closely
• Tailor the fluid type, rate, and volume to urine output and tolerance

Anaemia

• Do not withhold fluids in anaemic, dehydrated or hypovolemic patients
• Fluid therapy can improve tissue perfusion and oxygen delivery, especially in shock
• Assess for transfusion needs, these patients may become transfusion dependent when appropriately resuscitated or rehydrated

Heart disease

• CAUTION Avoid triggering fluid overload and the onset of heart failure
• Whenever possible, fluid intake should be provided generally (water/canned food)
• When fluid therapy is necessary, administer 0.45% NaCl with 2.5% dextrose i/v at half to daily maintenance rates. Adjust the rate based on the patient’s condition and tolerance.
• Use positive inotropes in hypotensive patients with congestive heart failure, rather than fluid boluses

Cardiorenal disorders

• Careful monitoring for fluid overload is critical in patients with concurrent heart and kidney disease
• Managing both systems at once can be challenging, even with careful treatment plans

Hypovolaemia + oedema

• Giving fluids to hypovolaemic patients with oedema can be challenging, always consider the underlying cause
• If oedema is due to low albumin, raising oncotic pressure may help; options include synthetic colloids (used cautiously), canine albumin or plasma products
• Plasma requires large volumes to affect albumin levels and may risk fluid overload
• Canine-specific albumin can be more effective and safer than plasma or synthetic colloids in dogs
• Avoid colloids in vasculitis-related oedema, as they may worsen fluid leakage
• Crystalloids should be used carefully in all oedematous patients

TBI

• The goal is to support cerebral perfusion pressure and mean arterial pressure
• Human guidelines recommend keeping systolic blood pressure between 100 and 110 mm Hg to improve survival
• Prioritise blood products (packed red cells, plasma, platelets) over crystalloids in cases of active bleeding
• Mannitol or hypertonic saline to reduce ICP (There is no clear evidence favouring one over the other)

Hypokalaemia

• Treat with potassium chloride (KCl) added to i/v fluids, based on the patient’s potassium level
• CAUTION Never exceed 0.5 mEq/kg/hr, as faster rates can be fatal
• CAUTION Never give potassium-supplemented fluids as a bolus
• Mix fluids well before giving to ensure potassium is evenly distributed
• Use fluid pumps or syringe drivers for accurate delivery and train staff to avoid accidental over-infusion
• If hypokalaemia persists despite supplementation, consider checking magnesium levels, as low magnesium can interfere with potassium correction

Hyponatraemia

• Acute euvolaemic hyponatraemia with neurological signs should be treated with 2–6 ml/kg of 3–7.5% hypertonic saline over 10–15 minutes
• Symptomatic chronic hyponatraemia is treated similarly at first, but once signs resolve, slower correction is needed
• In chronic or asymptomatic hyponatraemia, choose isotonic crystalloids with a sodium level around 10 mEq/l higher than the patient’s.
  CAUTION Correct the sodium level slowly, no more than 0.5 mEq/l per hour or 10–12 mEq/l per day to prevent osmotic demyelination syndrome
• Hypovolaemic hyponatraemia should be corrected with fluids that closely match the patient’s sodium concentration. If no commercial fluid matches the patient’s sodium, dilute isotonic crystalloids with sterile water to make a custom fluid but use these only as bolus therapy.
• CAUTION Avoid hypotonic fluids in all hyponatraemic patients

Hypernatraemia

• Treat hypernatraemia based on whether it is acute or chronic
• CAUTION Correct chronic cases slowly to prevent fluid shifts and cerebral oedema (≤0.5 mEq/l/hr)
• Acute hypernatraemia can be corrected more quickly using hypotonic i/v fluids
• Calculate the free water deficit and monitor sodium levels closely throughout treatment

Vasodilatory shock

• Vasodilatory shock causes low blood pressure and poor perfusion due to widespread blood vessel dilation
• It can be difficult to tell hypovolaemic and vasodilatory shock apart based on physical exam alone, and since vasodilatory shock may also involve some hypovolaemia, giving a fluid bolus as a diagnostic and therapeutic trial is a reasonable first step
• Poor or limited response to fluid boluses may suggest vasodilation. In these cases, vasopressor therapy should be considered.
• In suspected anaphylaxis, give adrenaline (epinephrine) promptly as a first-line treatment

Hypothermia

• Use warmed fluids (40–42°C)
• The actual impact of warm fluids on raising body temperature is unclear, but they are currently considered preferable to room temperature fluids
• CAUTION Hypothermic cats in shock are at high risk of fluid overload if over-resuscitated. Use small fluid boluses (e.g. 5 ml/kg) alongside active rewarming

Hyperthermia

• In hyperthermic patients, room temperature fluids can help cool the patient while restoring perfusion

Hypoglycaemia

• Give dextrose to patients showing signs of low blood sugar such as lethargy, weakness, ataxia or seizures
• For bolus treatment, use 0.5–1 mL/kg of 50% dextrose diluted 1:2 to 1:4 and give over 2–5 minutes
• Follow the bolus with a continuous infusion of 1.25–5% dextrose until the patient can maintain normal glucose levels
• The strength of dextrose in fluids depends on how low the glucose is and how fast fluids are being given
• CAUTION Use a central line if the dextrose concentration exceeds 5% to reduce the risk of phlebitis

Bottom line – Key Takeaways:

• Use with caution: Synthetic colloids are controversial due to potential risks (e.g. AKI, coagulopathy)
• Not first-line: Avoid if crystalloids or natural colloids are effective
• Limit dose and duration, monitor kidney and coagulation parameters closely
• Use with caution or avoid in patients with pre-existing azotaemia, coagulopathy, anaemia, or thrombocytopenia
• No consensus exists on safe or optimal use in small animals
• Clinicians must carefully weigh the risk vs benefit for individual patients.

Bottom line – Key takeaways:

• Hypoalbuminaemia <20 g/l (<2.0 g/dl) is linked to worse outcomes (e.g. oedema, poor perfusion, delayed healing)
• Species-specific albumin is preferred but often limited by cost and availability
• Plasma can be used, but requires large volumes and is costly with modest benefit
• Human albumin carries higher risk of adverse reactions
• Nutritional support is often safer easier, and more effective for long-term management

Bottom line – Key takeaways:

• Balanced crystalloids (e.g. LRS, Normosol-R) contain low potassium (4–5 mEq/l) and are safe for initial resuscitation in most hyperkalaemic patients
• Not harmful in cases like urethral obstruction or Addisonian crisis
• Balanced isotonic fluids correct acid-base imbalances faster than acidifying fluid such as 0.9% NaCl
• Balanced isotonic crystalloids can help avoid overly rapid increases in sodium concentration in hyponatraemic patients with concurrent hyperkalaemia, such as those experiencing an Addisonian crisis
• Key priorities: treat the underlying cause and manage severe hyperkalaemia (K⁺ >7 mmol/l) to prevent cardiac complications

Bottom line – Key takeaways:

• i/v fluids are only indicated during CPR if the patient is hypovolaemic or in distributive shock
• Avoid fluids in euvolaemic patients, as they may reduce perfusion pressures
• Hypertonic saline may offer neuroprotection (experimental evidence only)
• Avoid synthetic colloids—no proven benefit and potential risks
• Treat underlying causes: haemorrhage, electrolyte or acid-base imbalances, or lipid-soluble toxicoses with appropriate targeted therapies