Body Fluids and Electrolytes

Body Fluids and Electrolytes

• Body water makes up roughly 60% of body weight.
• Intracellular fluid (ICF) constitutes about 40% of total body water and is found within cells.
• Extracellular fluid (ECF) accounts for about 20% of total body water and is located outside cells.
• Interstitial fluid (IF) makes up about 15% of total body water, surrounding cells
• Plasma makes up roughly 5% of total body water, it is the fluid component of blood.
• Cell and capillary membranes divide the body's fluid compartments.
• Plasma/ECF contains cations, including sodium (Na+), potassium (K+), magnesium (Mg++), and calcium (Ca++), and anions, including chloride (Cl-), phosphate (Phos), bicarbonate (HCO3-), and proteins (Prot).
• Sodium is the predominant cation, with chloride and bicarbonate as major anions in ECF.
• Osmoreceptors detect changes in plasma osmolality.
• Hormones like ADH, aldosterone, ANGII, and ANP regulate fluid and electrolyte balance.
• Water balance relies on dietary absorption and urinary excretion.
• ECF changes affect the intracellular environment.

Acid-Base Status

• Normal blood pH is about 7.4; lower values indicate acidity, and higher values indicate alkalinity.
• Buffering systems maintain pH homeostasis.
• Carbon dioxide (CO2) plays a vital role in respiratory regulation.
• Increased CO2 leads to decreased pH, resulting in acidosis.
• Decreased CO2 leads to increased pH, resulting in alkalosis.
• The equilibrium between CO2, water, H+, and HCO3- mediates the relationship between CO2 and pH.
• The kidneys regulate bicarbonate (HCO3-) levels.
• Increased HCO3- leads to increased pH, resulting in alkalosis.
• Decreased HCO3- leads to decreased pH, resulting in acidosis.
• pH = 6.1 + log [HCO3-] / (0.03 x PCO2) illustrates the relationship between pH, bicarbonate, and the partial pressure of carbon dioxide.

Analyzing Acid-Base Status

• Key parameters to assess:
  • Blood pH measured at 37 degrees C (temperature corrected).
  • pCO2 (partial pressure of carbon dioxide) measured at 37 degrees C (temperature corrected).
  • Standard bicarbonate (SB), which is measured bicarbonate concentration standardized to a pCO2 of 40 mmHg and normal body temperature.
  • Base excess (BE), which is the amount of acid or alkali needed to return the blood to normal pH, representing all bases over the normal.
  • Anion gap
  • Strong Ion Difference (SID)

Anion Gap

• The difference between the concentrations of commonly measured cations (Na+, K+) and anions (Cl-, HCO3-) in plasma.
• Under normal conditions, the concentrations of all anions and cations in plasma must be equal, resulting in no anion gap in theory (Law of Electroneutrality).
• Calculated as Anion gap = [Na+] - ([HCO3-] + [Cl-]).
• The normal value is 12 (± 4) mEq/L.
• An anion gap > 16 suggests increased unmeasured anions like sulfates, phosphates, proteinates, and organic acids.
• Useful in evaluating metabolic acidosis when a blood gas analyzer is unavailable.

Strong Ion Difference (SID)

• Considers ions that completely dissociate in solution (strong ions).
• Strong cations in plasma are Na+, K+, Ca++, and Mg++.
• Strong anions are Cl−, lactate, β-hydroxybutyrate, acetoacetate, and SO42−.
• Calculated as SID = [Na+ + K+ + Ca++ + Mg++] – [Cl- + lactate].
• Increased SID suggests metabolic alkalosis.
• Decreased SID suggests metabolic acidosis.

Fluid Therapy

• Corrects fluid, electrolyte, and acid-base imbalances by increasing circulating volume, cardiac output, RBF, and GFR.
• Other purposes are parenteral nourishment, stimulation of organ function, and administration of certain drugs.
• The two phases are:
  • Correction of imbalance: Rapid replacement to address acute deficits (e.g., 4-6 hours); for acute cases like hypotension and shock: 80-90 mL/kg/hour in dogs, 50-55 mL/kg/hour in cats (monitor lungs and nostrils).
  • Maintenance therapy: To meet ongoing physiological needs (e.g., 2-4 days); maintenance needs: 40-65 ml/kg/24 h (mature animals), 130 ml/kg/24 h (immature animals).
• Institute based on assessment of dehydration.
• Clinical signs of dehydration and estimated percentage loss:
  • Loss of skin elasticity: 5%
  • Oral mucous membranes becoming tacky: 6-7%
  • Prolonged capillary refill time: 6-8%
  • Skin tenting that persists: 8-10%
  • Eyes sunken back into orbits: 10%
  • Cool extremities, early shock: 10-12%
• Dehydration estimation based on eye recession (neonatal calves): Degree of eye recession (mm) x 1.6 = % dehydration; intravenous fluids are recommended when dehydration is ≥ 8% (eye recession ≥ 4 mm).
• Replacement volume (liters) = body weight (kg) x % dehydration.

Routes of Fluid Administration

• Intravenous (IV) is preferred for severely ill patients, maintenance, and surgical procedures.
  • It allows for large volumes and hypertonic fluids (monitor osmolality and rate).
  • Maintain asepsis (risk of infection, thrombosis, phlebitis, embolism).
  • Vein access may be difficult in very small or severely ill patients.
• Subcutaneous (SC) is convenient for mild to moderate deficits in small animals.
  • Use isotonic solutions only.
  • Maximum volumes: 10 mL/kg or 50-200 mL per site in small animals (humerus, femur).
• Oral (PO) is easy and safe.
  • Contraindicated in cases of diarrhea or vomiting.
  • Variations include nasogastric and intraruminal administration.
• Intraperitoneal (IP) is good for electrolyte and water absorption.
  • Large volumes can be administered.
  • Use isotonic solutions only.
  • Asepsis is essential due to the risk of peritonitis.
• Intraosseous (IO), also known as intramedullary, is useful in very young or very small animals.
  • It provides rapid access to blood capillaries in bone marrow, leading to rapid absorption and dispersion.
  • Administration via catheter into bone marrow (tibial tuberosity, trochanteric fossa of the femur, wing of the ilium, greater tubercle of the humerus).
  • Periosteum analgesia (1% lidocaine) is needed.
  • Asepsis is crucial to prevent osteomyelitis.
• Per-rectum is useful for very young animals (good absorption of water, K+, Na+, Cl-).

Rate of Flow

• Recall Poiseuille’s law: Flow ∝ (radius of catheter)^4; therefore, the diameter of the catheter is very important for rapid fluid flow.