Approach to Fluid Therapy

In a normal animal what are the main mechanisms of fluid intake and loss?

• Intake

  • Eating, drinking

• Loss

  • Urine, feces, evaporation (Skin or lungs)

How might a fluid deficit develop?

1.) Failure of normal intake

2.) Abnormal Losses

• Hemorrhaging

• Vomiting and diarrhea

• Polyuria

• Increased body temperature & panting

• Wound exudation

What is hypovolaemia?

Hypovolemia occurs when you don't have enough fluid (blood) volume circulating in your body.

• Reduction in effective circulating volume

VERY URGENT

What is dehydration?

A reduction in total body water

What occurs when we remove pure water from the ECF?

What occurs when we remove isotonic fluid from the ECF?

Pure Water Removal

• Tonicity Change: The ECF becomes hypertonic relative to the intracellular fluid (ICF).

• Water Movement: Water will move out of the cells into the ECF to balance the osmotic difference, causing cells to shrink (crenate).

• Example: Sweating without replacement, diabetes insipidus

• What’s lost: Mostly water, not electrolytes

• Effect:

  • Increases osmolarity (concentration of solutes in body fluids)

  • Water shifts out of cells to balance extracellular fluid — cells shrink

  • Leads to hypernatremia (high sodium concentration)

Isotonic Fluid Loss

• Tonicity Change: The ECF remains isotonic but with reduced volume.

• Water Movement: There may be no immediate water movement between the ECF and ICF since the tonicity remains the same.

  Has a greater effect on circulating volume.

• Example: Hemorrhage, vomiting, diarrhea, burns

• What’s lost: Water and electrolytes in equal proportion

• Effect:

  • No significant change in osmolarity

  • No water shift between compartments (cells stay the same size)

  • Leads to hypovolemia (low blood volume), but sodium concentration stays normal

What are clinical signs of dehydration?

• Dry mucous membranes

• Loss of skin elasticity = skin tenting

• Sunken eyes

What are the clinical signs of hypovolaemia?

• Depression

• Weak, rapid pulses

• Cold extremities

• Prolonged capillary refill time (CRT)

How can we gauge the level of dehydration based on clinical signs?

<5% - clinically undetectable

5-6% - tacky mucous membranes

6-8% - decreases in skin elasticity (turgor), dry mucous membranes

8 - 10% - retracted sunken eyeballs

10 - 12% - persistent skin tenting, signs of hypovolemic shock, weak rapid pulse, cold extremities, prolonged CRT, depression

>12% - Hypovalemic shock - collapsed & moribund

Are dehydration and hypovolemia mutually exclusive?

• You can have both at once, a severe dehydration might affect the circulating volume

• Hypovolemia may occur if animal loses blood but is not dehydrated also

What are some laboratory tests we can do to look for signs of dehydration/hypovolaemia?

• Increases in…

  • Packed cell volume

  • Total protein (or total solids)

• Urine specific gravity

  • Reduced urine, more concentrated

• Serum lactate

  • Due to reduced perfusion to tissues, not enough O2, shift to anaerobic metabolism, which produces lactate

Why might we give fluids in the perioperative period?

To anticipate fluid deficits

• To supplement normal intake (Maintenence)

  • They are not taking in normal fluids or food

• To counter physiological changes associated with anesthesia

• Replace fluid or blood loss during surgery

• To establish and maintain venous access

What are the two main types of intravenous fluids?

1.) Crystalloids

• Solutions of electrolytes (Sometimes with glucose) in water

• Diffuse rapidly out of vascular compartment and distribute across the ECF

2.) Colloids

• Suspensions of larger molecular weight molecules in water/electrolytes

• Retained in the vascular compartment for longer

  • Both synthetic and natural (Blood Products)

What are the main classifications of crystalloid fluids and subsequently what is each used for?

Classified based on use

• Plasma Volume Expanders

• ECF Replacement Fluids

  • Replace deficits of water and electrolytes

• Alkalinising Fluids

• Acidifying Fluids

  • To acidify or alkalinise the ECF

• Maintenance Fluids

  • To substitute normal drinking for a short period

What are common examples of crystalloid fluids?

• Hartmann’s Solution

  • Balanced

  • Uses lactate as bicarbonate precursor

• 0.9% sodium chloride (Normal Saline)

  • Unbalanced

What are features of replacement fluids?

Used to replace fluid and electrolyte deficits

• Are isotonic, do not induce fluid shift

• Can be balanced or unbalanced

  • Balanced = closer to composition of ECF fluid

What acid-base effects do replacement fluids have?

• Acidifying crystalloids

  • Bicarbonate free

  • High chloride content promotes HCO3- loss in the kidney

• Alkalinising Crystalloids

  • Bicarbonate precursors - metabolized to HCO3-

What are the main features of maintenance fluids?

• Used to maintain normal hydration and euvolaemia

• Mostly isotonic

• Always unbalanced

  • Tend to have low sodium, but higher levels of potassium

  • Many have additions of glucose or dextrose that make it isotonic, we don’t want to add electrolytes

  • (like D5W, 5% dextrose in water) initially stay in the ECF, but the glucose quickly gets metabolized, leaving primarily water to contribute to hydration.

  • Not enough to be a source of energy

• Many practices make their own maintenance fluids

In what circumstances might we add the following concentrated additives?

• Glucose

• Potassium Chloride

• Sodium Bicarbonate

• Glucose

  • Blood Levels

• Potassium Chloride

  • Deficiency levels

• Sodium Bicarbonate

  • Incompatible with Ca due to precipitate formation

What are plasma volume expanders?

• Specifically used in hypovolaemic patients when we need to expand the circulating volume

  • Hypertonic crystalloids

  • Isotonic crystalloids

  • Colloids

What are the features of hypertonic saline?

• 7.2 & 7.5% sodium chloride

• Mechanism

  • Osmotic effect

  • Positive inotrope (Increase HR)

  • Reduced endothelial swelling

• Produces a rapid increase in circulating volume

  • Volume expansion > volume infused

  • Deficit not replaced, so we must follow with isotonic crystalloid

What are the features of artificial colloids?

Describe the purpose, potential adverse effects and availability.

Used less due to negative outcomes

• Potential adverse effects:

  • Acute kidney injury

  • Increased risk of bleeding

  • Anaphylaxis

  • Volume overload a higher risk

• Main purpose

  • To replace blood loss

  • To rapidly expand the circulating volume

  • To support colloid osmotic pressure (oncotic pressure) in hypoproteinaemic patients

• Availability

  • Gelatin-Based in UK

  • Starches (Hydroxyethyl Starches) in Europe and North America

What are the features of gelatin based colloids?

• Molecules are very small, meaning it produces short duration of plasma expansion (2-3 hrs)

• Osmotic pressure and viscosity is similar to plasma

• Most popular colloid in veterinary medicine

• Adverse effects:

  • Reports of histamine release

  • Impair hemostasis

  • Unknown is causes acute kidney injury

What are the features of hydroxyethyl starches?

• Very large synthetic polymers of glucose

• Produce longer duration of plasma volume expansion

• Cause adverse effects:

  • Acute Kidney Injury

  • Impaired hemostasis

Many not available in some countries due to effects.

What are the features of natural colloids?

Blood products

We can use:

1.) Whole blood & hemorrhagic shock

• Fresh or stored

2.) Packed red blood cells & chronic anemia

3.) Plasma e.g. severe hypoproteinaemia, clotting defects

4.) Cryo-precipitate, which concentrates specific clotting factors, von willebrand’s disease

5.) Cryo-supernatant & rodentcide toxicity

6.) Platelets

What are some other, more uncommon blood derivatives?

• Human specific albumin

• Can be used to treat hypoproteinaemia

• Risk of anaphylaxis

• Species specific is preferred but not always available

What is parenteral nutrition?

• Solutions of dextrose, lipid and amino acids

• Infused IV (Central vein)

• Provides a source of energy and protein in critically ill patients

• Given when GI dysfunction precludes enteral feeding

What are the characteristics of each route of fluid administeration?

• Oral

  • Most physiological, economical, safe, composition not as important, calorific needs can be met

• Subcutaneous

  • Easy, can inject at any rate, multiple sites available, slow absorption (Mild dehydration), uncomfortable

• Intravenous

  • Rapid (Suitable for hypovolemia), allows precise control, calorific needs can be met

  • IV access challenging in small patients

• Intraosseous

  • Rapid, specific equipment, strict asepsis

• Intraperitoneal

  • Absorption of large volume, very slow

How do we decide how much fluid to give and how quickly?

Signs of hypovolemia require URGENT treatment

• Aggressive fluid therapy

  • IV fluid bollus or fluid challenge

  • 10-20 ml/kg over 10-15 mins

  • Monitor for positive response and adjust fluid plan

No Signs of hypovolemia

• Come up with fluid therapy plan to replace estimated deficit

• Provide maintenance fluid requirements (M)

• Provide additional fluid to meet on-going losses

• Constant assessment and re-adjustment is needed