Fluid Therapy

---
name: fluid-therapy
description: Dehydration assessment, crystalloid vs. colloid selection, resuscitation bolus protocols by species, maintenance calculation, electrolyte correction and potassium supplementation.
---

# 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**:
| 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)
**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):
| 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 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