Fluid Therapy | Skills Pool
Fluid Therapy Dehydration assessment, crystalloid vs. colloid selection, resuscitation bolus protocols by species, maintenance calculation, electrolyte correction and potassium supplementation.
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 tentedMucous Membrane Color : Normal pink, pale (early shock/severe dehydration), injected (fever, pain, inflammation)
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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)Severity % Dehydration Clinical Signs Fluid Rate Mild 5% Minimal mucous membrane dryness, slight skin turgor delay Maintenance + deficit over 24h Moderate 7-8% Dry mucous membranes, skin tenting, CRT 2-3 sec Bolus + maintenance Severe 10%+ Very dry mucous membranes, significant skin tenting, CRT >3 sec, lethargy, weak pulses Rapid shock dose
Crystalloid vs. Colloid Selection
Crystalloids (First-Line)
Balanced Solutions (preferred): Lactated Ringer's (LRS), Normosol-R, PlasmaLyte A—electrolyte composition closer to plasma; less hyperchloremiaSaline 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
Acute/severe dehydration (dog with shock): Replace 50% of deficit in first 6 hours (via shock boluses), remainder over 24 hoursModerate dehydration (stable patient): Replace deficit evenly over 24 hoursMild 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
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):
K+ Level Rate Max Concentration >3.0 mEq/L 0.25-0.5 mEq/kg/hr 20 mEq/L 2.0-3.0 mEq/L 0.5-1.0 mEq/kg/hr 40 mEq/L <2.0 mEq/L 1.0-1.5 mEq/kg/hr 60 mEq/L max
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 temperatureShock 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 calculationCrystalloid distribution : 75% shifts to interstitium; third-spacing common in peritonitis, sepsis; colloid consideration in severe casesMaintenance calculation : Variation based on age, metabolism, disease state; formulas are approximationsReferral : Complex cases (multiple electrolyte disturbances, ongoing hemorrhage, sepsis) warrant internist/anesthesiology consultation When to Use
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