22/04/2026
Selection of Crystalloid and Colloid Fluids in Veterinary Patients
Key Principles
1. Intravascular volume status
2. Cellular hydration status
3. Direction of fluid shifts
4. Appropriate fluid selection
Important Concept
Crystalloids:
Approximately 20–30% remains intravascular, while 70–80% distributes into the interstitial space within 20–60 minutes
1. Intravascular Volume (Estimated Plasma Volume; ePV)
Calculated as: ePV = (100 − Hct) ÷ Hb
- Normal range: 2.5–4.5 dL/g
- Low ePV → hypovolemia / intravascular dehydration
- High ePV → increased intravascular volume or fluid overload
2. Cellular Hydration (Cellular Hydration Index; CHI)
Using CBC values: CHI = (Hb × 3) − Hct − 3
Normal = 0
Negative CHI → cellular dehydration (water leaves cells)
Positive CHI → cellular swelling (water enters cells)
Using blood gas (Nova Prime Vet): CHI = (Hb × 3) − Hct − 4.5
Normal = 0
Note:
If MCHC > 35 g/dL, use CBC-derived CHI, as blood gas Hb/Hct may be affected by hemolysis or artifacts.
3. Direction of Fluid Movement
3.1 Osmolality
Water moves from low osmolality → high osmolality
Normal range: 280–310 mOsm/kg
Total osmolality:
Osm ≈ 2 × Na + (Glucose ÷ 18) + (BUN ÷ 2.8)
Or: 1.86 × Na + (Glucose ÷ 18) + (BUN ÷ 2.8)
Effective Osmolality (Tonicity) Exclude BUN:
Effective Osm ≈ 2 × Na + (Glucose ÷ 18)
(BUN crosses cell membranes freely → minimal effect on transcellular water shift)
3.2 Sodium (Na)
Water generally follows Na (major extracellular osmole), but Na alone is insufficient
Always interpret with Osm + CHI
Corrected Sodium (hyperglycemia):
Na corrected = Na measured + 1.6 × ((Glucose − 100) ÷ 100)
(Use 2.4 if glucose is extremely high)
Situations where water does NOT follow Na:
- Severe hyperglycemia
- Mannitol
- Hypertonic saline
- Ethanol, ketones, other osmotic agents
- Changes in oncotic pressure (protein)
Na − Cl Difference
Normal:
Dogs: 32–40 mEq/L
Cats: 30–38 mEq/L
Interpretation:
Na − Cl < 30–32
→ Relative hyperchloremia
→ Hyperchloremic metabolic acidosis (normal AG)
Examples:
- Diarrhea
- RTA
- Large-volume 0.9% NaCl
- Ureteral obstruction
- Addison’s disease
- Early CKD
Confirm with:
- Low HCO₃⁻
- Low BE
- Normal anion gap
Na − Cl > 40–42
→ Relative hypochloremia
→ Metabolic alkalosis
Examples:
- Vomiting (loss of HCl)
- Diuretics (e.g., furosemide)
- Pyloric obstruction
- GI sequestration
4. Fluid Selection
4.1 Based on Osmolality
Crystalloids
(e.g., LRS, Acetar, Plasma-Lyte)
Indications:
Low ePV
Need intravascular volume expansion
No need for rapid osmotic correction
Fluids
(e.g., D5W, 0.45% NaCl, D5-½NS)
Indications:
High effective osmolality
Intracellular dehydration
Negative CHI
ePV adequate or after perfusion correction
Caution: D5W becomes free water →
Avoid in:
- Shock
- Severe hypovolemia
- Cerebral edema
- Increased ICP
Fluids
(e.g., 3% NaCl, 7.2% NaCl, Mannitol)
Indications:
- Severe hypovolemia
- Severe shock
- Cerebral edema
- Increased ICP
- Severe symptomatic hyponatremia
Key concept: Hypertonic fluids do NOT replace volume
They shift fluid into the intravascular space temporarily
→ Always follow with isotonic fluids
Important Rule:
Do NOT use hypertonic as first-line in most cases of:
Low ePV + Positive CHI
→ Start with isotonic crystalloid
→ Use hypertonic only if:
- Severe shock
- Cerebral edema
- Increased ICP
Examples
Low ePV + Negative CHI + High Osm
→ True dehydration / hypertonic dehydration
→ Start isotonic → correct free water deficit slowly
Low ePV + Positive CHI →
Intracellular fluid excess, vascular depletion
→ Usually start isotonic
Normal/High ePV + Negative CHI → Cellular dehydration from high effective osm
→ Seen in:
- Hypernatremia
- Severe hyperglycemia
- Mannitol
Salt intoxication
4.2 Based on Na / Cl
Examples:
Low Na + Low Cl + metabolic alkalosis → 0.9% NaCl
Normal Na + High Cl → avoid NSS → use balanced fluids
High Na → avoid high-Na fluids
4.3 Colloids
Large molecules → remain intravascular → increase oncotic pressure
Indications:
- Severe hypovolemia not responsive to crystalloids
- Hypoalbuminemia
- Third spacing / capillary leak
- Ascites/edema with low - intravascular volume
- Shock requiring minimal fluid volume
Types
Natural:
- Plasma
- Albumin
- Whole blood / PRBC + plasma
Synthetic:
- HES
-Gelatin
Warnings
HES:
Associated with ↑ AKI and mortality
Especially in sepsis / critical illness
Avoid in:
- Sepsis
- AKI
- Coagulopathy
Gelatin:
- Risk of anaphylaxis
- Coagulopathy
Additional Note
In sepsis or severe capillary leak, colloids may leak → worsen edema
Additional Precautions
Avoid LRS in:
- Severe liver failure
- Marked hyperlactatemia
- When precise lactate monitoring is required
→ Prefer Plasma-Lyte / Acetar
Avoid NSS in:
- Hyperchloremic metabolic acidosis
-Hypertonic caution:
- Existing hypernatremia
- Severe negative CHI
- Chronic hyponatremia (risk of ODS)
Potassium (K) must always be considered
(K is the main intracellular osmole)
Correction Rates Na:
≤ 0.5–1 mEq/L/hr
≤ 10–12 mEq/L/day
Effective Osm:
≤ 0.5–1 mOsm/kg/hr
≤ 10–20 mOsm/kg/day
High-Risk Levels
Na 170 mmol/L
Summary (Clinical Flow)
Evaluate in order:
Osm → Effective Osm → ePV → CHI → Na / Cl / K → Fluid selection
Key Patterns
- Low ePV + Negative CHI → isotonic
- Low ePV + Positive CHI → usually isotonic first
- Normal ePV + Positive CHI → avoid hypotonic
- High ePV + Positive CHI + edema → fluid overload / heart failure
- Low ePV + Low albumin + edema → consider colloid
- Low ePV + hemorrhage → blood products required
- High ePV + edema → colloid may worsen overload
Important Exception
Edema does NOT always mean high ePV
Low ePV can still occur in:
- Sepsis
- Capillary leak
- ARDS
- Reperfusion injury
- Severe hypoalbuminemia
- Early third spacing
