fluid therapy

Fluid Therapy

• Fluid Therapy lecture by Hanna Machin, Dip ACVAA, Dip SIAV, MVetMed, MRCVS, Lecturer in Veterinary Anaesthesia, October 4, 2024.

Learning Objectives

• Describe the normal distribution of fluid within the body.
• Define hypovolaemia and dehydration.
• Describe different types of intravenous fluids available and their relative indications.
• Describe normal fluid requirements and suggest a treatment plan for maintenance.
• Suggest a treatment plan for a hypovolaemic and/or dehydrated patient.
• Identify patients at risk of iatrogenic fluid overload.

Total Body Water

• Adults: 60% of body weight is total body water, 40% is dry matter.
• Neonates: 80% of body weight is total body water, 20% is dry matter.

Fluid Compartments (Adults)

• Total Body Water (TBW): 40% of body mass.
• Intracellular fluid (2/3 of TBW): 40%
• Extracellular fluid (1/3 of TBW): 20%
  • Interstitial fluid (3/4 of ECF): 15%
  • Intravascular fluid (1/4 of ECF): 5%

Total Blood Volume

• Dog/Horse: 8-9% of body mass (80-90 mL/kg).
• Cat/Cattle/Sheep: 6-7% of body mass (60-70 mL/kg).

The Body's Fluid Compartments

• Fluids move between compartments based on tonicity of the fluid, tonicity of the extracellular compartment (Na+), and the size of macromolecules in the fluid.
• Movement occurs across endothelial membranes, which depend on capillary membrane condition, hydrostatic pressure, colloid osmotic pressure, and vascular permeability.

Types of Fluid Loss

• Water & electrolytes (ECF loss): Vomiting, diarrhea, diuresis.
• Protein-rich ECF loss: Transudate, exudate, effusion, severe enteritis, protein-losing enteropathy/nephropathy.
• Pure water: High RR (hyperthermia, pneumonia), water deprivation, excessive water loss.

Fluid Balance

• Gains: Water intake (food & water); metabolic water production (10%).
• Sensible Losses: Urine output.
• Insensible Losses: Faeces, Respiration, Saliva, Cutaneous (sweating), respiratory tract.

Maintenance Requirements

• Dog: a. 60 mL/kg/day; b. 132 x BW (kg)^0.75 ; c. 30 x BW (kg) + 70 = mL/kg/day

• Cat: a. 40 mL/kg/day; b. 80 x BW (kg)^0.75 ; c. 30 x BW (kg) + 70 = mL/kg/day

• Pediatric: Dog: 3 x adult dose; Cat: 2.5 x adult dose.

• Losses = Sensible losses + Insensible losses = ~50 mL/kg/day (Over 24 hrs).

Goal Directed Fluid Therapy

• Restore homeostasis (euvolemia & hydration).
• Correct acid-base & electrolyte imbalances.
• Assess the deficit(s).
• Choose the best fluid type to replace the deficit(s).
• Calculate fluid dose & rate.
• Monitor patient's response & potential complications.
• Reassess treatment plan.
• Adapt plan to patient's needs.
• Goal: Zero balance.

Fluids are Drugs...

• Are IV fluids indicated?
• Correct drug choice.
• Correct dose/volume.
• Administration (rate/bolus).
• Patient considerations (hypotension? causes? contraindications?).
• Monitoring.
• Side effects.

Hypovolemia vs Dehydration

• Hypovolemia: Decrease in fluid volume within the vascular space, leading to tissue perfusion loss, blood and/or fluid & electrolyte loss. Rapid replacement therapy.
• Dehydration: Decrease in fluid volume within the interstitial compartment, affecting all compartments. Causes water & electrolyte imbalance(especially Na+). Requires slower, sustained replacement.

Assessing Intravascular Space Deficits (Hypovolemia)

• History: V+, D+, anorexia, fever, hemorrhage, edema, ascites
• Physical examination: Altered mentation, tachycardia/arrhythmias, changes in CRT & MM (vasoconstriction), weak peripheral pulses, low BP, cold extremities, tachypnoea.
• Laboratory tests: ↓ PCV, TS, ↑ lactate, metabolic acidosis, anaemia, electrolyte abnormalities, ↓ urine output, ↑ urine specific gravity.

Phases of Hypovolemic Shock

• Compensatory
• Early decompensatory
• Late decompensatory

Assessing Dehydration

• Physical Examination Findings by Dehydration percentage.
• Different parameters for assessment include skin turgor, mucous membrane moisture, PCV, total protein, blood urea nitrogen, urine osmolality, and urine specific gravity.

Assessing Intracellular Space

• Sodium concentration
• Free water deficit.
• Na-K-ATPase pumps.
• Water moves freely.
• Dehydration: Hypernatremia and hypertonicity.

Types of Fluids Available

• Crystalloids
• Colloids
• Oxygen-carrying solutions
• Blood products

Crystalloids

• Solutions prepared with crystalline compounds (electrolytes +/– sugar) in water, often with buffers (acetate, lactate, gluconate).
• Classification: tonicity (isotonic, hypotonic, hypertonic) compared to plasma.

Isotonic Crystalloids

• Composition matches extracellular fluid.
• Redistribution of fluid from intravascular to interstitial space over 30-60 minutes.
• Only 25% remains in intravascular space
• Includes: 0.9% NaCl, Hartmann's, Plasma-Lyte 148.

Saline Solution (0.9% NaCl)

• Not balanced.
• Acidifying solution.
• Potential risk for heart or renal disease.

Hartmann's Solution

• Balanced, most used for replacement/resuscitation and peri-operative fluid therapy.
• Contains precursors for bicarbonate (lactate).
• Liver converts lactate into glucose, consuming H+, which is HCO3- sparing effect).
• Careful with blood transfusions (clots).

Hypertonic Crystalloids (7.5% NaCl)

• Rapid intravascular volume expansion (3X volume infused).
• Draws water from ICF & ECF to intravascular space (transient effect).
• Dehydration: Administer with isotonic crystalloids.
• Inotropic effect.
• Vasodilation: Do not administer too fast! Dose: 4–5 mL/kg

Hypotonic Crystalloids

• Water loss treatment.
• Often with dextrose to increase tonicity to avoid RBC damage.
• Water shift from extracellular to intracellular space.
• For hypernatremia treatment only.
• No boluses.

Colloids

• Macro-molecules suspended in crystalloid solutions (proteins, sugars, starches).
• Longer lasting in intravascular space (hours).
• Natural: blood products (plasma, whole blood, albumin).
• Synthetic: Dextrans, hydroxyethyl starches (HESs).

Resuscitation (Hypovolemia)

• Rapid intravascular volume expansion.
• Draws water from interstitial and intracellular spaces (osmotic gradient) into intravascular space.

Fluids Can Be Helpful For...

• Resuscitation (e.g., hypertonic saline, Hartmann's solution, blood products) to correct intravascular volume deficit.
• Replacement (e.g., Hartmann's solution) to replace lost body fluids, electrolytes not compensated by oral intake
• Maintenance (e.g., hypotonic crystalloids) to meet daily basal requirements for water, electrolytes, and glucose.

Treatment Plan for Hypovolemia (Resuscitation)

• Rapid correction of intravascular deficits.
• Buffered isotonic crystalloids (boluses over 15–30 min).
• Only 25% remains in intravascular space after 30–60 min.
• Dog: 15–20 mL/kg; Cat: 5–10 mL/kg.
• Monitor closely, and if hemorrhage, consider blood products.
• +/- hypertonic crystalloids. Dog: 4-6 mL/kg; Cat: 1-4 mL/kg.
• +/- colloids. Dog: 2.5–5 mL/kg; Max amount: Dogs: 20 ml/kg; Cats: 10–15ml/kg.

Treatment Plan For Dehydration

• Balanced isotonic crystalloids (IV or IO).
• PO administration (voluntary oral intake or orogastric tube) if dehydration is not severe.
• Calculate total fluid deficit.
• Administer over 12–24 hours.
• Continuous monitoring.
• If recovery is achieved, continue oral administration if possible.

Treatment Plan if Hypovolemia & Dehydration

• Address hypovolemia first.
• Then rehydration (account for ongoing losses).
• Add maintenance rate.

Treatment Plan for Maintenance

• Hypotonic crystalloids (or balance isotonic crystalloids + K & dextrose).
• Daily fluid requirement for hydrated patients unable to maintain homeostasis through oral ingestion.
• Dog: 60 mL/kg/day (equivalent to 2 mL/kg/hr).
• Cat: 40 mL/kg/day.
• Paediatric patients: 3 x adult dose, 2.5 x adult dose

Fluid Approach in Anaesthesia

• Previous approach: Large volume crystalloids (10 ml/kg/hr).
• Restricted approach: Fluid administration based on patient needs (3ml/kg/hr (cats), 5ml/kg/hr (dogs)), considering normal cardiac & renal function.

Goals of Fluid Therapy During Anaesthesia

• Oxygen delivery & tissue perfusion
• Macro circulation
• Microcirculation
• Maintaining/correcting electrolyte composition & acid-base balance.
• Stabilizing before anaesthesia
• Monitoring.
• Maintaining adequate blood pressure.

Infusion Equipment

• Intravenous cannula
• Intraosseous catheters
• Fluids of choice
• Giving set
• Fluid pump/syringe driver
• Pressure bag

Giving Sets for Free Flow Fluid Administration

• 20 drops/mL (adult set), 60 drops/ml (pediatric set).
• Micro dripper: Delivers 60 drops per mL.
• Gravity assisted delivery.
• Risk of fluid overload and air embolism.

Fluid Rate Calculations Using a Giving Set

• Required information: Giving set type, patient body mass, desired administration rate.
• Calculation steps: 1. ml/kg/hr * body mass (kg) = mL/hr; 2. mL/hr / 60 = mL/min; 3. mL/min / 60 = mL/sec; 4. mL/sec * drops/mL = drops/sec

Infusion Pumps & Syringe Drivers

• VTBI (Volume To Be Infused)-- Total volume of fluid you want to deliver.
• VI (Volume Infused)-- Amount of fluid already in the pump.
• Improve accuracy & consistency.
• Boluses administration.
• Auditory alarms (e.g., air bubbles).

Fluid Therapy Monitoring

• Monitor clinical signs, heart rate, arterial pressure, pulse rate, capillary refill time, mucous membrane colour, core-peripheral temperature gradient, respiratory rate, mental status, urine output, skin turgor, body weight, and signs of edema.
• Monitor laboratory findings (urine specific gravity, hematocrit, total protein, lactate, and electrolytes).

Fluid Therapy Complications

• Fluid overload/intolerance: Interstitial edema & tri-cavitary effusion.
• Electrolyte & acid-base imbalances.
• Dilution coagulopathy.

Fluid Overload Therapy

• If patient has increased body weight, mild to localized peripheral edema, respiratory distress, or dysfunction in the renal, hepatic, or gastrointestinal system.
• Discontinue IV fluids and increase patient mobility.
• Provide oxygen support, administer a diuretic.
• Remove pleural fluid (thoracentesis) or abdominal fluid (abdominocentesis).
• Consult for hemodialysis.
• Consider discontinuing ACE inhibitors and ARBs to improve GFR.

References (partial)

• Authors, year, and names of the 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats.
• Various sources for fluid therapy, dehydration, and hypovolemia in veterinary medicine.

Test your knowledge on the characteristics and implications of hypovolaemia and dehydration. This quiz includes questions on laboratory tests, physical examination findings, and the effects of fluid volume changes. Perfect for medical students and healthcare professionals.