Fluid Therapy

Fluid Therapy

Overview

Systematic fluid assessment, calculation methodology, and species-specific resuscitation protocols. Includes dehydration severity estimation, crystalloid vs. colloid selection, shock dose rates, maintenance formulas, and electrolyte correction guidelines.

When to Use

• User assesses dehydration severity or plans fluid therapy
• User needs shock dose calculation, maintenance rate, or electrolyte supplementation
• Keywords: fluids, crystalloid, colloid, LRS, Normosol, saline, dehydration, shock dose, maintenance rate, bolus, hypovolemia, potassium, electrolytes

Dehydration Assessment

Physical Examination Markers:

• Skin Turgor: Pinch dorsal neck skin; normal returns immediately, mild dehydration (5%) returns slowly (<2 sec), moderate (7-8%) returns over 2+ seconds, severe (10%+) remains tented
• Mucous Membrane Color: Normal pink, pale (early shock/severe dehydration), injected (fever, pain, inflammation)
• Capillary Refill Time (CRT): Normal <2 seconds (dogs/cats); 2-3 seconds = mild dehydration/early shock; >3 seconds = moderate-severe shock
• Eye Globe Position: Normal in orbit, sunken (>5% dehydration), bulging (increased posterior pressure, less common)

Dehydration Severity:

Crystalloid vs. Colloid Selection

Crystalloids (First-Line)

Composition:

• Balanced Solutions (preferred): Lactated Ringer's (LRS), Normosol-R, PlasmaLyte A—electrolyte composition closer to plasma; less hyperchloremia
• Saline Solutions: 0.9% NaCl (normal saline), 3% NaCl (hypertonic)

Advantages: Inexpensive, readily available, redistribution to interstitium allows tissue hydration, less infection risk Disadvantages: Rapid redistribution (only 25% remains intravascular at 1 hour), third-spacing risk, hyperchloremia if large volumes Dosing: Maintenance + deficit replacement + ongoing loss

Colloids (Adjunctive)

Synthetic: Dextran 70/40 (polysaccharide, 6-8 hour duration), hetastarch (starch-based, 12-24 hour duration), Gelatin Natural: Fresh frozen plasma (FFP, contains clotting factors), fresh whole blood (RBCs + plasma)

Indications: Hypoproteinemia (<4.5 g/dL), ongoing protein losses, failed crystalloid resuscitation, bleeding Dosing: Hetastarch 10-20 mL/kg IV over 15-30 min; dextran 5-10 mL/kg; max 40 mL/kg/day total Considerations: Expense, short shelf-life (hetastarch), coagulation effects (dextran, high-dose hetastarch), infection risk lower than colloids

Resuscitation (Hypovolemic Shock)

Modern approach: Incremental boluses, not full shock-dose infusion. Historical "shock rates" (dog 90 mL/kg, cat 60 mL/kg total crystalloid volume) represent the approximate blood volume and are useful as a ceiling, but current RECOVER/critical care guidelines recommend smaller boluses with reassessment between each.

Dogs: 10-20 mL/kg IV bolus over 15-20 minutes, reassess, repeat up to 3-4 times as needed. Total resuscitation volume should not exceed 80-90 mL/kg without reassessing for ongoing losses or considering colloids/blood products. Cats: 5-10 mL/kg IV bolus over 15-20 minutes, reassess. Cats are more sensitive to volume overload; total should not exceed 50-60 mL/kg without reassessment. Monitor for pulmonary edema. Horses: 10-20 mL/kg IV bolus, reassess. Large volumes required due to body size; hypertonic saline (4-5 mL/kg) can be used as a bridge.

Reassessment targets between boluses: Heart rate trending down, CRT improving toward <2 sec, mucous membrane color improving, urine output >1 mL/kg/hr, lactate decreasing.

Example (10 kg dog, hemorrhagic shock):

• Initial bolus: 10 kg x 15 mL/kg = 150 mL LRS IV over 15 min
• Reassess: HR still elevated, CRT still >2 sec → repeat bolus
• Second bolus: 150 mL LRS IV over 15 min
• Reassess: HR normalizing, CRT <2 sec → transition to deficit replacement rate

Maintenance Calculation (Non-Dehydrated Patient)

Formula: 50 mL/kg/day + 50 mL/kg/day for each kg over 20 kg (for dogs)

• 10 kg dog: 500 mL/day = ~20 mL/hr
• 30 kg dog: 500 + (10 × 50) = 1000 mL/day = ~40 mL/hr

Alternative formula (simpler): 1-2 mL/kg/hour maintenance

• 10 kg dog: 10-20 mL/hr
• 30 kg dog: 30-60 mL/hr

Cats: Generally lower requirement; 1-2 mL/kg/hour, or 30-50 mL/day for average adult

Adjustments: Increase for fever (12.5% per degree Celsius above 38.3°C), hyperventilation, drainage losses (wound, fistula, diarrhea), polyuria

Deficit Replacement

Formula: Percent dehydration × body weight = volume to replace

• Example: 8% dehydration in 20 kg dog = 0.08 × 20 kg = 1.6 liters

Replacement timeline:

• Acute/severe dehydration (dog with shock): Replace 50% of deficit in first 6 hours (via shock boluses), remainder over 24 hours
• Moderate dehydration (stable patient): Replace deficit evenly over 24 hours
• Mild dehydration (maintenance only): May not require separate replacement; address ongoing losses

Calculation: Deficit mL/24 hours ÷ 24 hours = additional mL/hr beyond maintenance

Ongoing Loss Replacement

Gastrointestinal Losses (vomiting, diarrhea):

• Estimate volume: small bowel diarrhea >200 mL/day, vomiting varies; weigh bandages/pads
• Electrolyte composition: High chloride, potassium (especially with diarrhea)
• Replacement: Replace estimated loss mL-for-mL; add appropriate electrolytes

Hemorrhage:

• Crystalloid bolus: 3:1 ratio (3 mL crystalloid for each 1 mL blood loss)
• Ongoing: Assess packed cell volume (PCV) trend; transfusion if PCV <15-20%

Insensible Losses (respiration, sweating): ~10-20 mL/kg/day; included in maintenance calculation

Electrolyte Correction

Potassium (K+) Supplementation

Indications: K+ <3.5 mEq/L (hypokalemia), especially with cardiac arrhythmias, weakness, polyuria

Supplementation Rates (IV, in crystalloid):

Max Concentration: 40 mEq/L (peripheral IV), 60 mEq/L (central line) Monitoring: Recheck K+ after 4-6 hours of supplementation; goal 3.5-4.5 mEq/L Caution: Hyperkalemia risk (cardiac arrhythmias); avoid over-rapid infusion

Example (20 kg dog, K+ 2.5 mEq/L):

• Rate: 20 kg × 0.75 mEq/kg/hr = 15 mEq/hr (mid-range)
• Concentration: 40 mEq/L → 15 mEq/hr ÷ 40 mEq/L = 375 mL/hr infusion rate

Sodium (Na+) Correction

Hypernatremia (Na+ >155 mEq/L): Rapid correction risks cerebral edema; correct slowly over 48 hours

• Free water replacement: D5W or 0.45% NaCl
• Formula: (serum Na - 150) × 0.6 × BW (kg) = mEq Na to remove

Hyponatremia (Na+ <125 mEq/L): Symptomatic (<120) requires hypertonic saline (3% NaCl)

• Calculation: (target Na - current Na) × 0.6 × BW = mL of 3% needed
• Infuse 3-5 mL/kg over 15-20 min; recheck; aim for 10 mEq/L increase per 4-6 hours (max 12 mEq/L/24hr to avoid edema)

Acid-Base Considerations

Metabolic Acidosis (common in shock, sepsis):

• Fluid resuscitation (crystalloid) improves perfusion → lactate clearance
• Sodium bicarbonate rarely indicated acutely (empirical dosing risky); recheck ABG after resuscitation
• If needed: mEq bicarb = 0.3 × (desired HCO3 - actual HCO3) × BW (kg); administer slowly IV

Metabolic Alkalosis (contraction alkalosis post-vomiting):

• Chloride-containing fluids (LRS, 0.9% NaCl) preferred
• Address underlying cause (anti-emetics, electrolyte correction)

Monitoring Parameters

Reassessment intervals: q15 min during resuscitation, q30-60 min post-stabilization, q4-6h stable patients

• CRT, mucous membranes, perfusion
• Urine output (goal >1 mL/kg/hr, cats >0.5 mL/kg/hr)
• BUN/creatinine (assess renal perfusion)
• Electrolytes (especially K+ during supplementation)
• PCV (if hemorrhage/transfusion consideration)

Species-Specific Considerations

Dogs: Tolerate larger boluses (10-20 mL/kg increments); lower risk of volume overload than cats; total resuscitation ceiling ~80-90 mL/kg Cats: More sensitive to volume overload; use 5-10 mL/kg boluses; monitor closely for pulmonary edema; total resuscitation ceiling ~50-60 mL/kg Rabbits: Rapid dehydration common; slow crystalloid infusion preferred (interstitial space limited); glucose supplementation often needed Horses: Large volumes; central line access preferred; risk of dependent edema with prolonged standing; monitor carefully

Limitations

• Dehydration assessment: Subjective; CRT/skin turgor affected by age, obesity, skin condition, ambient temperature
• Shock dosing: Empirical dosing; individual variation significant; reassessment critical (ongoing losses, bleeding, fluid sequestration)
• Electrolyte repletion: Risk of overcorrection (especially Na+, K+); serial lab assessment essential, not single calculation
• Crystalloid distribution: 75% shifts to interstitium; third-spacing common in peritonitis, sepsis; colloid consideration in severe cases
• Maintenance calculation: Variation based on age, metabolism, disease state; formulas are approximations
• Referral: Complex cases (multiple electrolyte disturbances, ongoing hemorrhage, sepsis) warrant internist/anesthesiology consultation