Fluid Therapy: Veterinary Perioperative Basics

Fluid Therapy Basics

• Fluid therapy aims to maintain or restore effective circulating blood volume.
• Fluids are drugs and can be harmful or beneficial depending on composition, dose, and kinetics.
• Fluid therapy is extremely common in veterinary medicine, and is a standard of care during the perioperative period.
• It is crucial to have access to a vein for drug administration related to fluid therapy.
• Fluid therapy is usually preventative and highly species-dependent.

Perioperative Period Considerations

• The perioperative period spans the time from hospital admission to discharge.
• Imbalances requiring fluid therapy can arise from vasodilation, blood loss, evaporative fluid loss, and pain-induced fluid redistribution during surgery.
• Blood redistribution to vital organs due to fluid loss can lead to metabolic acidosis.
• CVP (central venous pressure) readings will indicate over-hydration, but not necessarily adequate hydration.
• Fluid therapy corrects dehydration and electrolyte imbalances that require 12-24 hours before anesthesia.
• Aim to administer fluids slowly over 6-12 hours, accounting for both maintenance and deficit.
• Be aware that severely diluited plasma protein, RBC and electrolytes may be produced.

Risk of Fluid Therapy

• Fluid overload is a risk associated with fluid therapy.
• Fluid administration requires close monitoring to prevent hypothermia and overhydration, especially in young and small animals.
• Supplementation with a source of calories is recommended
• Auscultation should occur to check for complications
• Overhydration can lead to cerebral, pulmonary, myocardial, and gut edema, as well as renal interstitial edema and impaired lymphatic drainage.
• Large quantities of room temperature fluids can cause hypothermia, hemodilution and coagulation abnormalities.
• Monitor feces for indications of fluid levels, the GUT (gastrointestinal tract) is a better indicator than the kidneys.

Body Fluid Physiology

• Body water distribution varies with age (% of fat) and physical conditioning.
• Maintenance fluid therapy for days is 40-60 mL/kg.
• The blood volume equals the plasma volume plus RBC volume.
• Not all species have the same fluid requirements

Body Fluid Compartments

• Total body water constitutes approximately 60% of body weight.
• Intracellular fluid comprises 40% of body weight, made up with large quantities of K+ ions.
• Extracellular fluid (20% of body weight) is composed of interstitial fluid (15%) and plasma (5%).
• Large quantities of Na+ and Cl- ions are in extracellular fluid compared to other solutes.
• Transcellular fluids make up 2% and are not rapidly exchangeable.
• Fluid inside the vascular compartment equals plasma.
• Maintain fluid inside the vascular space
• Rapid exchange occurs outside the cell
• The ratio of interstitial fluid to plasma is usually 3:1

Circulation and Transvascular Fluid Flux

• Fluid can also eventualy distribute through SQ injections
• Cardiac output is determined by heart rate and stroke volume, influenced by contractility, preload, and afterload.
• Jv= Kf [(Pc-Pif) - (Tc - TIF)]

Starling's Equation

• Fluid is generally not reabsorbed from capillaries except in the gut and kidney, or during acute hypotensive episodes.

Endothelial Glycocalyx

• The endothelial glycocalyx is destroyed by disease and fluid overload, increasing susceptibility to edema.

Interstitium

• The interstitium is a series of fluid-filled spaces made of flexible connective tissue, acting as a reservoir and a 'third space'.

Fluid Disposition

• Fluid from rapid exchange around the normal interstitium, pressure organ
• Slow exchange around the slow exchange organ through weak biomechanics.

Giving Fluids

• Fluids can be given for perfusion, O2 and acid/base balance.

Responding To Anesthesia

• Common problems under anesthesia include break through pain (15%), hypoventilation (14%) and hypotension (10-15%).
• Hypotension serves as a measure of the quality of anesthesia care.
• Anesthetic drugs can cause cardiac contractions and vasodilation, resulting in decreased CO (blood flow) and ABP.
• Anesthetic drugs tend to supress autonomic nervous system reflexes.

Fluid Therapy and Blood Loss

• The fluid distribution of young with small body weights should be supplemented with calories.
• Monitor to avoid over hydration levels.
• Monitor for the signs of shock- tachicardia, extremities.
• Assess for fluid/electrolyte
• The goal is to increase hydration and maintain balance.
• Transfusion triggers include low PCV (aim for a PCV between 40% and 30% as a context) and hemoglobin concentration falls below 7g/dl.

Hypovolemia Management

• Compensatory responses can combat hypovolemia by affecting cardiac stimulation and blood circulation

Hypovolemia

• Decreased atrial pressure
• Hypovalemia starts at 10.15% blood volume levels dependent upon the species
• 50% blood volume loss relates to a 50% mortality
• An HCT of <20% causes blood situations

Fluid Types

• Types of fluids can be crystalloid or colloid, isotonic, hypertonic or hypotonic.
• Isotonic fluids have normal animal osmolality around 300 mOsml/L
• Sodium levels in animals are around ~180 mEq

Fluid Overload Risks

• Watch for dilution, overload and dilution of plasma proteins and RBCs
• Fluid resuscitation comes 12-24 hours too late

Colloids

• Colloids replaced the volume by affecting the glycocalyx
• Larger molecules that act to keep liquids inside tissue, therefore colloids will lead to allergic reactions.
• All colloids must be administered carefully due to advanced cardiac or renal malfunction.

Crystalloid Solutions

• Balanced crystalloid solutions should contain physiologically.

Fluid Balance

• Assess and adjust fluid based on individual volume.

Crystalloids

• Crystalloids are the most popular medical option because they are homogenous. -Crystalloids contain electrolytes.

Factors for Colloids

• Molecular and degradation levels determine if a colloid is correct.

Fluid Distribution

• Total body water is 60% with interstitial being 15%.
• Based upon tissue composition and blood flow
• Most tissues are >60% bone and ≈30% fat.
• Fatty tissue has less water
• Crystalloids must be 3:1 when more anesthesia, since fluids are 1.5:1 when consious.

Dynamic Assessment

• CVP- Central Venous Pressure is important for a general indication.
• Use acute changes in body weight to determine the correct dosage.
• CVP readings can assist with evaluation of fluid overload.

Rate and Fluid

• 5min bolus can be given to ensure adequate hydration and correct fluid balance.
• Use a syringe to give fluids too fast.
• Bolus is important for a slower hydration rating.
• Fluids not exchangeable in
• not an animal in the way that is going out of its way to protuse itself and protect itself.

Considerations to Avoid Fluid Overload

• Decrease patient anesthetic level.
• Ultrasound as a guide.
• Be ware that the distribution of fluids has a time level.