Selecting an Appropriate Fluid

Fluids are classified as crystalloids or colloids.

Crystalloids

Crystalloids contain variable amounts of electrolytes, water and dextrose, and are characterized by tonicity and their effect on acid-base status. Crystalloids are used either to replace sodium loss or maintain the status quo. Replacement fluids contain sodium at concentrations similar to normal plasma while maintenance fluids have sodium concentrations similar to normal total body concentration. Approximately one-third of administered isotonic replacement fluid remains in the intravascular space and two-thirds enter the interstitial space.

Cats normally lose potassium through urine, and this loss is augmented during dehydration, aldosterone release and sodium conservation. Replacement fluids should be supplemented with potassium when used long term. Normal saline is the fluid of choice for hypercalcemia and hyperkalemia given it contains no calcium or potassium. Normal saline may exacerbate volume overload, metabolic acidosis, heart disease, and hypertension.

Maintenance fluids are designed to replace daily sodium losses and are appropriate for long-term administration. Dextrose is commonly supplemented to approximate plasma tonicity and prevent hemolysis. These lower sodium fluids do not stay in the vascular space, do not meaningfully expand blood volume, and thus should never be used for volume resuscitation.

Hypertonic saline is used for rapid intravascular volume expansion. Volume expansion is short lived, as the sodium redistributes throughout the extracellular compartment quickly. Do not administer hypertonic saline faster than 1 ml/kg/min to avoid vagally-mediated bradycardia and potential cardiopulmonary arrest.

Colloids

Synthetic colloids contain high molecular weight particles that allow these fluids to increase plasma osmotic pressure (COP). As albumin is the main contributor to COP, certain colloids are advantageous in the treatment of patients with hypoalbuminemia. The hydroxyethylstarches most commonly used in veterinary medicine are hetastarch, pentastarch, and tetrastarch.

Fresh frozen plasma (FFP) is collected and spun within 6 hours and frozen ideally at -70°C for up to one year. This fluid contains stable clotting factors (II, VII, IX, X), labile clotting factors (V, VIII,), von Willebrand's factor, fibrinogen and albumin; it does not contain red blood cells and platelets. Indications for FFP use are replacement of all clotting factors, anticoagulant rodenticide intoxication, von Willebrand's disease, and hemophilia (A & B). Frozen plasma (FP) is collected similarly to FFP but is stored for longer than one year. It contains stable clotting factors, fibrinogen and albumin, but does not contain red blood cells, platelets and labile clotting factors. FP may be used for hemophilia B.

Human serum albumin has been used in critically ill feline patients to help support blood pressure and to aid in the treatment of significant hypoalbuminemia. Vigano et al. showed administration of 5% HSA was safe in a large group of critically ill hypoalbuminemic cats. Matthews et al. showed 25% HSA could be safely administered to critically ill animals with an expected increase in albumin and blood pressure.

Traditional Starling Model and the Endothelial Glycocalyx

According to traditional Starling's forces, hydrostatic pressure pushes water out of capillaries and oncotic pressure pulls fluid into the intravascular compartment. The difference between these pressures decides whether water leaves capillaries or is pulled back into them. We now know the luminal surface of endothelial cells is lined with a glycocalyx of membrane-bound macromolecules comprised of sulfated proteoglycans, hyaluronan, glycoproteins, and plasma proteins. Dr. Starling didn't appreciate the key role of the endothelial glycocalyx (EGC), but the EGC is a vital regulator of vascular permeability. The high reabsorption of interstitial fluid in the venular segments of the microcirculation postulated by Dr. Starling does not happen. Filtration across the vascular barrier is largely independent of the bulk colloid concentration surrounding the vessel.

In regions with high intravascular pressure, the inwardly directed oncotic pressure gradient across the EGC prevents flooding of the interstitial space in conjunction with the high resistance to flow within the narrow strand gaps of the endothelium. Within low-pressure sections free and easy access of plasma constituents toward the parenchymal cells allows a highly effective exchange of nutrients and waste products, but the fluid shift is modest if the endothelial surface layer is intact because of the low hydrostatic and oncotic pressure gradients in these segments.

The principle role of the EGC is to maintain the vascular permeability barrier. Other meaningful roles of the endothelial glycocalyx include shielding vascular walls from direct exposure to blood flow and mediating shear-stress-dependent nitric oxide production endothelium. The EGC also promotes retention of vascular protective enzymes and helps preserve the intravascular concentration of coagulation inhibition factors. The EGC also helps modulate the inflammatory response by preventing leukocyte adhesion and binding of chemokines, cytokines, and growth factors endothelium.

Hypovolemia vs. Dehydration

Hydration status is a measure of interstitial fluid, and is determined by evaluating skin turgor, moisture of the mucous membranes and possibly enophthalmos. Volume status is a measure of tissue perfusion, and is initially evaluated by checking heart rate, capillary refill time, mucous membrane color and blood pressure. Indiscriminate use of the terms dehydration and hypovolemia risks confusion and therapeutic errors. Hypovolemic cats commonly have prolonged capillary refill times, tend to have pale mucous membranes and are often (but not always) hypotensive. While dogs may present with tachycardia, most cats either have normal heart rates or bradycardia. If hypovolemia is severe, one may see obtundation, weak peripheral pulses, and lack of venous distension when the veins occluded.

Treatment of hypovolemia should typically be finished within 1–2 hours of presenting to the hospital. This type of resuscitation routinely requires rapid administration of large volumes of intravenous replacement crystalloids known as "shock boluses" until endpoints of resuscitation are reached.

Route of Administration

Common routes of fluid administration in cats include intravenous (peripheral, central or PICC line), subcutaneous, enteral, intraosseous, and intraperitoneal. Hypovolemic patients should have at least one short large bore peripheral intravenous catheter placed. If venous access is not immediately possible, the intraosseous route may be used until vascular access is achieve. The subcutaneous route is not appropriate for hypovolemic patients as peripheral vasoconstriction severely limits absorption. With mild dehydration, the subcutaneous route may suffice. Dextrose should not be delivered subcutaneously, and potassium delivered via this route may induce patient discomfort. Subcutaneously fluids are commonly administered at 10–20 ml/kg per site. Enteral water supplementation may be used to help prevent villous atrophy, and may be combined with other forms of enteral nutritional support.

Volume & Rate of Administration

When determining the most appropriate fluid volume and rate of administration, one should consider the three (3) major components of fluid administration: resuscitation, replacement, and maintenance.

During fluid resuscitation, intravascular volume is restored with intravenous fluids. Hypovolemic patients require fluid resuscitation, and volume infused depends of the stage of shock. Stabilizing interventions for patients with shock should target endpoints of resuscitation. With hypoproteinemic hypovolemia, administration of a synthetic colloid may be appropriate. Reassess EOR after each bolus.

After addressing hypovolemia, an appropriate fluid therapy plan must address dehydration, daily physiologic requirements, and ongoing losses. Isotonic crystalloids should be used for fluid replacement (correcting dehydration and replenishing ongoing losses). The volume required to correct dehydration is the product of the estimated % dehydration and body weight in kilograms, and should be delivered over ~6–24 hours. After correcting dehydration, a patient's fluid therapy plan should be reevaluated. Ongoing losses may be estimated by weighing diarrhea and vomitus, and frequent weight monitoring is recommended to help gauge a patient's fluid status.

Daily physiologic requirements, often termed maintenance requirements, may be calculated either with [(30 x BW in kilograms) + 70] or (80 x BW 0.75). Both isotonic and hypotonic crystalloids may be appropriate to meet daily physiologic fluid requirements.

Fluid therapy is commonly employed during the peri-anesthetic period. Potential benefits of providing fluids to healthy patients during the peri-anesthetic period include cardiovascular support, countering potential anesthesia-induced adverse reactions, and correction of normal ongoing losses. The American Animal Hospital Association and American Association of Feline Practitioners recently released fluid therapy guidelines for cats, recommending a peri-anesthetic initial starting rate of 3 ml/kg/h.

References

1.  DiBartola S. Applied physiology of body fluids in dogs and cats. In: DiBartola S, ed. Fluid Therapy in Small Animal Practice. 3rd ed. Philadelphia, PA: WB Saunders; 2012:12–13.

2.  Chappell D, Jacob M. Role of the glycocalyx in fluid management: small things matter. Best Pract Res Clin Anaesthesiol. 2014;28(3):227–234.

3.  Vigano F, Perissinotto L, Bosco VR. Administration of 5% human serum albumin in critically ill small animal patients with hypoalbuminemia: 418 dogs and 170 cats (1994–2008). J Vet Emerg Crit Care. 2010;20(2):237–243.

4.  Matthew KA, Barry M. The use of 25% human serum albumin: outcome and efficacy in raising serum albumin and systemic blood pressure in critically ill dogs and cats. J Vet Emerg Crit Care. 15(2);2005:110–118.

5.  Wehausen CE, Kirby R, Rudolf E. Evaluation of the effects of bovine hemoglobin glutamer-200 on systolic arterial blood pressure in hypotensive cats: 44 cases (1997–2008). J Am Vet Med Assoc. 2011;238(7):909–914.