Choosing a crystalloid solution
Carefully consider the pH, buffer content, and total electrolyte composition of each type of crystalloid fluid (Table 2) when choosing a fluid that is appropriate for specific diseases. For example, if an animal is hyponatremic, a low-sodium
or no-sodium fluid such as 0.45% saline solution or 5% dextrose solution in water would exacerbate the hyponatremia. Administering
an isotonic or a hypertonic fluid such as lactated Ringer's solution, Normosol-R, Plasmalyte M, or 0.9% sodium chloride solution
may be most appropriate to replenish sodium. Also consider the animal's acid-base status. For example, if an animal has metabolic
acidosis as well as hyponatremia, then administering 0.9% sodium chloride solution, which does not contain any buffers or
bicarbonate precursors, could worsen the metabolic acidosis. Administering lactated Ringer's solution, which contains lactate,
or Normosol-R, which contains acetate and gluconate, may be more appropriate.
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 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.