Choosing a crystalloid solution

Table 2 Electrolyte and Buffer Composition and pH of Common Isotonic and Hypotonic Crystalloids

Sodium chloride solution (0.9%) is an excellent choice for promoting renal calcium and potassium excretion. In addition, the fluid does not contain calcium or potassium, so it is useful in treating animals with hypercalcemia and hyperkalemia. Magnesium ions are required as cofactors for a number of metabolic processes, including potassium retention. Refractory hypokalemia, as that observed in animals with diabetic ketoacidosis, can be partially treated or prevented by using a magnesium-containing fluid such as Normosol-R or Plasmalyte M.

The oncotic pressure is the force that holds fluid within a compartment. Starling's forces, which dictate the amount of fluid retention within the vascular space vs. transvascular fluid flux into the interstitium, depend on many factors, including intravascular and interstitial hydrostatic pressure, interstitial and intravascular oncotic pressure, and capillary pore size. As intravascular volume is replenished with a crystalloid fluid, the intravascular hydrostatic pressure increases and causes a dilutional decrease in oncotic pressure, which can result in the movement of osmotically active substances into the interstitium. A colloidal solution contains negatively charged, large-molecular-weight particles that are osmotically active, drawing sodium around their core structures. Wherever sodium is, water follows. By drawing sodium around the particle, water is held within the vascular space.

Colloids replace intravascular fluid deficits only. So colloids are always administered along with crystalloids to restore both intravascular and interstitial fluid volume. Consider colloids whenever hypoproteinemia, decreased oncotic pressure, and increased capillary pore size exist, as in conditions associated with sepsis or systemic inflammatory response syndrome. Although some of the colloid particles can potentially leak into the interstitial space, the net effect is to maintain fluid within the intravascular space to avoid interstitial edema. Whenever a colloid is administered with a crystalloid, reduce the calculated crystalloid fluid requirements by 25% to 50% to avoid volume overload.

Natural colloids

Natural colloid solutions include whole blood and plasma. Fresh whole blood is indicated when loss of both red blood cells and plasma has occurred. The Rule of Ones states that 1 ml fresh blood infused per 1 lb body weight will increase a patient's packed cell volume (PCV) by 1%, provided no ongoing losses are present. This rule is a simple method to calculate the amount of whole blood to administer to reach a target hematocrit. When working in kilograms, the Rule of Ones can be extrapolated: 2.2 ml/kg whole blood will increase an animal's PCV by 1%, provided the PCV of the transfused blood is 40%.⁹ A more accurate method to determine the volume of whole blood to administer is as follows:

ml donor blood needed = [(Desired PCV – Actual Recipient PCV) ÷ Donor PCV] × ml recipient blood volume

(Note: For recipient blood volume, use 90 ml/kg for dogs and 70 ml/kg for cats.)

Packed red blood cells can be administered if the degree of anemia is sufficient to cause clinical signs (e.g. lethargy, inappetence, tachycardia, tachypnea) but plasma proteins are within the normal range.