3.2 Fluid Dynamics, Crystalloids, Colloids & Hypertonic Resuscitation

Fluid Dynamics, Crystalloids, Colloids & Hypertonic Resuscitation

VTS Core Concept: Intravenous fluid therapy is a potent pharmacological intervention. Every fluid type possesses specific tonicity, electrolyte composition, buffering capacity, and distribution kinetics. Selecting the incorrect fluid, volume, or infusion rate can precipitate life-threatening cellular edema, hyperchloremic acidemia, acute coagulopathy, or cardiovascular collapse.

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1. Fluid Compartments & The Revised Starling Principle

The 60-40-20 Rule of Body Fluid Distribution

In healthy adult dogs and cats, Total Body Water (TBW) accounts for approximately 60% of total body weight ($600\text{ mL/kg}$).

+--------------------------------------------------------------------------+
|                       TOTAL BODY WATER (60% BW)                          |
+------------------------------------+-------------------------------------+
|   INTRACELLULAR FLUID (ICF)        |     EXTRACELLULAR FLUID (ECF)       |
|         40% of BW                  |            20% of BW                |
|       (2/3 of TBW)                 |          (1/3 of TBW)               |
|                                    +------------------+------------------+
|                                    | Interstitial     | Intravascular    |
|                                    | Fluid (ISF)      | Fluid (IVF)      |
|                                    | 15% of BW        | 5% of BW         |
|                                    | (3/4 of ECF)     | (1/4 of ECF)     |
+------------------------------------+------------------+------------------+

• Intravascular Blood Volume:
  • Canine: $\approx 80-90\text{ mL/kg}$ total blood volume (plasma constitutes $\approx 50-55\text{ mL/kg}$).
  • Feline: $\approx 45-60\text{ mL/kg}$ total blood volume (plasma constitutes $\approx 30-35\text{ mL/kg}$).

The Endothelial Glycocalyx & Revised Starling Model

Historically, the classical Starling principle assumed continuous transcapillary filtration at the arterial end and significant venous reabsorption driven by plasma colloid osmotic pressure (COP). Modern microvascular physiology has revised this model:

1. The Endothelial Glycocalyx Layer (EGL): A complex, $0.5-3.0;\mu\text{m}$ thick, negatively charged meshwork of proteoglycans (syndecans, glypicans), glycosaminoglycans (heparan sulfate, chondroitin sulfate, hyaluronan), and adsorbed plasma proteins (albumin, antithrombin) coating the luminal vascular endothelium.
2. The Non-Reabsorption Phenomenon: In most continuous capillaries, filtration occurs throughout the length of the capillary bed at varying rates; there is virtually no fluid reabsorption into the venous end of the capillary bed. Filtered interstitial fluid is cleared almost exclusively by the lymphatic system.
3. Glycocalyx Degradation (Shedding): Ischemia-reperfusion injury, endotoxemia, sepsis, severe trauma, hypernatremia, and rapid, excessive isotonic crystalloid bolusing (which triggers atrial natriuretic peptide [ANP] release) degrade and shed the glycocalyx. Shedding destroys the semipermeable oncotic barrier, drastically increasing microvascular permeability and precipitating severe extravasation of albumin and water into tissues (edema formation).

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2. Crystalloid Solutions: Balanced vs. Unbalanced

Crystalloids are aqueous solutions of mineral salts and water-soluble molecules with small molecular weights ($< 30\text{ kDa}$) that pass freely across semipermeable capillary membranes.

Isotonic Replacement Crystalloid Comparison

Clinical Distinctions and Safety Considerations

• Distribution Kinetics: Because sodium distributes freely throughout the extracellular fluid compartment ($75%\text{ ISF}$ vs. $25%\text{ IVF}$), only 20% to 25% of an infused isotonic crystalloid bolus remains within the intravascular space 30 to 60 minutes post-infusion. The remaining $75-80%$ redistributes into the interstitial space.
• 0.9% Normal Saline Risks: Normal saline contains a supra-physiological chloride concentration ($154\text{ mEq/L}$ vs. normal plasma $110\text{ mEq/L}$) and an effective Strong Ion Difference ($SID$) of zero. Rapid administration causes hyperchloremic metabolic acidosis and renal vasoconstriction with decreased GFR. Indications are strictly limited to: hypochloremic metabolic alkalosis (upper GI obstruction/vomiting), hypercalcemia (promotes renal calcium excretion), and Addisonian crisis.
• Calcium in LRS: LRS contains $3\text{ mEq/L } Ca^{2+}$. It must never be co-administered in the same IV line with blood products containing citrate anticoagulants (e.g., Packed Red Blood Cells, Fresh Frozen Plasma); ionized calcium binds citrate, reversing anticoagulation and precipitating lethal microthrombi.

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3. Shock Fluid Dosing & The Fractionated Bolus Strategy

Traditional "full shock doses" represent an estimate of the patient's entire circulating blood volume:

• Canine Full Shock Volume: $80-90\text{ mL/kg}$
• Feline Full Shock Volume: $45-60\text{ mL/kg}$

🛑 Critical Modern Standard: The Fractionated Aliquot Approach

Never administer a full calculated shock dose as an open, continuous infusion. Full shock doses lead to severe volume overload, tissue edema, abdominal compartment syndrome, glycocalyx destruction, and dilution of clotting factors and red cell mass.

The Protocol:

1. Administer $1/4\text{ to } 1/3$ of the calculated shock volume over $15-20\text{ minutes}$:
   • Dogs: $15-25\text{ mL/kg}$ isotonic balanced crystalloid IV
   • Cats: $10-15\text{ mL/kg}$ isotonic balanced crystalloid IV
2. Pause the infusion and perform an immediate focused physical reassessment:
   • Mentation, HR, pulse quality, CRT, MM color, blood pressure (MAP $> 65\text{ mmHg}$), blood lactate clearance, and point-of-care lung ultrasound (TFAST/Vet BLUE for B-lines).
3. If endpoints are achieved: Stop shock bolusing and transition to calculated rehydration/maintenance rates.
4. If hypoperfusion persists and no fluid intolerance (pulmonary crackles, B-lines, gallop rhythm) is detected: Administer a second fractionated aliquot ($10-15\text{ mL/kg}$ in dogs, $5-10\text{ mL/kg}$ in cats).

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4. Hypertonic Saline (7.2% - 7.5% NaCl)

Hypertonic Saline (HTS) is an intensely concentrated crystalloid solution ($2400-2560\text{ mOsm/L}$, $\approx 8\times$ physiological osmolarity).

• Dose: $3-5\text{ mL/kg}$ slow IV over $10-15\text{ minutes}$ (dogs and cats).
• Mechanism of Action: Creates a profound osmotic gradient between the intravascular and interstitial spaces. It rapidly draws endogenous water from the swollen interstitial and intracellular compartments into the vascular space, expanding circulating blood volume by $2\text{ to }4\times$ the infused volume (e.g., $100\text{ mL}$ HTS expands intravascular volume by $300-400\text{ mL}$). It also improves cardiac contractility, decreases endothelial cell swelling, and dampens neuroinflammatory cytokine cascades.
• Primary Clinical Indications:
  1. Traumatic Brain Injury (TBI): Rapidly reduces intracranial pressure (ICP) by dehydrating brain parenchyma while simultaneously restoring MAP and cerebral perfusion pressure ($CPP = MAP - ICP$).
  2. Severe Hypovolemic Shock / Massive Hemorrhage: Rapid volume resuscitation when large-volume crystalloids cannot be rapidly infused.
  3. Large-Breed Gastric Dilatation-Volvulus (GDV): Rapid resuscitation prior to emergency surgical decompression.
• Contraindications & Precautions:
  • Dehydration: Must not be used in dehydrated patients; drawing water from an already depleted interstitium induces profound intracellular dehydration.
  • Hypernatremia: Strictly contraindicated if serum $[Na^+] > 160\text{ mEq/L}$.
  • Rapid Bolus Warning: Infusing HTS rapidly ($< 5\text{ minutes}$) triggers a vagally mediated reflex causing severe bradycardia, profound hypotension, and bronchoconstriction.
  • Duration: Intravascular volume expansion lasts only $30-60\text{ minutes}$; it must be followed immediately by isotonic crystalloids.

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5. Colloids: Synthetic vs. Natural

Colloids are high-molecular-weight solutions ($> 30-40\text{ kDa}$) that remain largely confined within the intravascular space, exerting Colloid Osmotic Pressure (COP) (normal plasma $\text{COP} \approx 18-24\text{ mmHg}$).

Synthetic Colloids: Hydroxyethyl Starches (HES)

• Formulations: VetStarch, Voluven (6% HES 130/0.4 in balanced electrolyte solution).
• Dosing: Dogs: $10-20\text{ mL/kg/24 hr}$ (bolus $3-5\text{ mL/kg}$ over $15\text{ min}$); Cats: $5-10\text{ mL/kg/24 hr}$ (bolus $2-3\text{ mL/kg}$ over $20\text{ min}$).
• Critical Toxicities & Risks:
  • Acute Kidney Injury (AKI): Molecules accumulate within renal tubular epithelial cells, causing osmotic nephrosis and acute renal failure.
  • Coagulopathy: HES impairs primary and secondary hemostasis by coating platelets, accelerating the clearance of Factor VIII and von Willebrand Factor (vWF) (acquired type 1 von Willebrand syndrome), and impairing fibrin polymer cross-linking.
  • Reticuloendothelial Tissue Storage: Hydroxyethyl starch polymers cannot be metabolized by tissue enzymes and persist indefinitely within reticuloendothelial macrophages in liver, spleen, and skin (causing severe chronic pruritus).

Natural Colloids

• Fresh Frozen Plasma (FFP) & Frozen Plasma (FP): Administered at $10-20\text{ mL/kg}$; provides albumin, all active clotting factors (FFP), antithrombin, and natural protease inhibitors. Indicated for coagulopathies, rodenticide toxicity, DIC, and severe pancreatitis.
• Canine Serum Albumin (5% or 25%) & Human Serum Albumin (HSA): Provides concentrated oncotic support. 25% HSA carries significant risk of Type I and Type III hypersensitivity reactions (anaphylaxis and fatal immune-complex glomerulonephritis/vasculitis) in dogs; its use is strictly reserved for life-threatening hypoalbuminemia ($[Alb] < 1.5\text{ g/dL}$) unresponsive to other therapies.

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6. Comprehensive Fluid Deficit, Maintenance & Ongoing Losses Calculations

A complete emergency fluid prescription accounts for three distinct components: Deficit + Maintenance + Ongoing Losses.

Step 1: Dehydration Deficit Calculation

$$\text{Fluid Deficit (mL)} = \% \text{ Dehydration (as decimal)} \times \text{Body Weight (kg)} \times 1000\text{ mL/kg}$$

• Clinical Estimation of Dehydration:
  • $< 5%$: Subclinical; no detectable physical exam abnormalities.
  • $5-6%$: Mild loss of skin turgor, slightly dry/tacky mucous membranes.
  • $7-9%$: Moderate loss of skin turgor (prolonged skin tent), tacky to dry MMs, mild enophthalmos (sunken eyes).
  • $10-12%$: Severe loss of skin turgor (persistent skin tent), completely dry/sticky MMs, marked enophthalmos, dull corneas, weak pulses, hypothermia.
  • $\ge 13%$: Extreme dehydration accompanied by decompensated hypovolemic shock, moribund state.
• Replacement Rate: The calculated dehydration deficit is typically replaced over $12\text{ to }24\text{ hours}$ ($50%$ replaced in the first $4-8\text{ hours}$, remainder over the subsequent $16\text{ hours}$).

Step 2: Maintenance Fluid Requirements

• Linear Standard:
  • Dogs: $60\text{ mL/kg/day}$ (approx. $2.5\text{ mL/kg/hr}$)
  • Cats: $40-45\text{ mL/kg/day}$ (approx. $1.7-2.0\text{ mL/kg/hr}$)
• Allometric / Exponential Formula (Gold Standard for Metabolic Accuracy):
  • Dogs: $\text{Maintenance (mL/day)} = 132 \times (\text{BW in kg})^{0.75}$
  • Cats: $\text{Maintenance (mL/day)} = 80 \times (\text{BW in kg})^{0.75}$

Step 3: Ongoing (Anticipated) Losses

• Quantified volume-for-volume every $4-6\text{ hours}$ (vomitus, liquid diarrhea, chest tube/abdominal drain output, or polyuria in AKI/post-obstruction diuresis) using an isotonic replacement crystalloid.