Page 390 - Clinical Small Animal Internal Medicine

358 Section 5 Critical Care Medicine

to dilution. Finally, as the hydrostatic pressure increases normal plasma and a hypertonic solution is greater than
VetBooks.ir in the interstitium, lymphatic drainage is increased. If normal plasma. An example of this can be seen with the
different sodium chloride‐based crystalloids: 0.9%NaCl
increased capillary hydrostatic pressure persists, how-
ever, these safety factors can be overwhelmed and the
NaCl is a hypotonic crystalloid (154 mOsm/L), and 7.2%
meshwork within the interstitium can be disrupted. is an isotonic crystalloid solution (308 mOsm/L), 0.45%
When this occurs, the resistance to further fluid accu- NaCl is a hypertonic crystalloid (2464 mOsm/L).
mulation becomes minimal and manifests clinically as Finally, crystalloids can be described as replacement
pitting edema, chemosis, and, in rare circumstances, fluids or maintenance fluids. The composition of these
pulmonary edema. two categories is quite different and warrants a brief
Fluid dynamics in pulmonary tissue have unique char- explanation. When a crystalloid fluid is administered
acteristics. The pulmonary capillary pressure is usually intravenously, the majority of the volume given will move
lower than in other tissues and the interstitial pressure is out of the vascular space and into the interstitium. When
more negative (some tissues actually have a negative a 1 L bolus of an isotonic crystalloid is given, it can be
interstitial pressure) while the COP is higher than other anticipated that after one hour only 25% of the delivered
tissues. The fact that the pulmonary capillaries are rela- volume remains within the vascular space. The majority
tively permeable to protein contributes to this higher of the delivered volume has moved into the interstitium
COP. In addition, the alveolar epithelial cells actively and for this reason, a replacement fluid is designed to
transport sodium into the interstitial space, promoting approximate the fluid composition of the ECF. By con-
movement of fluid out of the alveolar space and into the trast, a maintenance fluid is designed to meet the ongo-
interstitium. The net result of these characteristics is ing insensible losses of the patient and has a lower
that fluid movement from the alveoli into the interstit- sodium concentration and usually a higher potassium
ium and then to the lymphatics is favored. This tendency concentration. A unique crystalloid fluid that does not
is overcome when the pulmonary capillary pressure fall within the replacement versus maintenance classifi-
exceeds 25 mmHg, at which point fluid will accumulate cation scheme is 5% dextrose in water (D5W) that is
within the alveoli. If the animal also has an increase in composed entirely of dextrose and water. When given,
pulmonary capillary permeability such as occurs in sep- D5W is effectively equivalent to administering pure
sis or systemic inflammatory response syndrome (SIRS), water, as the dextrose is rapidly metabolized to CO 2 and
or if the animal is hypoalbuminemic, pulmonary edema water.
will occur at lower hydrostatic pressures. Specific clinical presentations may respond most
effectively to specific crystalloid choices but in the
majority of instances, a balanced, isotonic crystalloid is
Intravenous Fluid Types appropriate for initial fluid therapy. Some of the clinical
presentations where one fluid may be preferred over
In order to make logical choices with fluid therapy, it is another include hypochloridemic metabolic alkalosis
important to understand the components and behavior and Addisonian crisis (0.9% NaCl) and urethral obstruc-
of the different classes of intravenous fluids available in tion (LRS or Normosol‐R), among others.
clinical practice. Additives such as 50% dextrose, potassium chloride,
magnesium sulfate, and calcium gluconate can be incor-
porated into crystalloid fluids to meet specific patient
Crystalloid Fluids
needs; however, not all fluid additives are compatible
Crystalloid fluids are the most common type of fluids with all isotonic crystalloids. Investigation into compat-
used in veterinary medicine. Crystalloids are defined as ibility is required before fluid supplementation is under-
fluids containing electrolyte and nonelectrolyte solutes taken. Certain crystalloids such as 0.9% NaCl and D5W
that are capable of entering all the body fluid compart- are often used as carriers for a variety of drugs, includ-
ments. They can further be described as “balanced” or ing anesthetics, inotropes, vasopressors, and insulin,
“unbalanced.” A balanced crystalloid has a composition but compatibility should always be verified before
similar to that of the ECF. An example would be lactated administration.
Ringer’s solution (LRS) or Normosol‐R®. An unbalanced Hypertonic saline is available in a variety of concentra-
crystalloid’s composition does not mimic that of the ECF. tions ranging from 3.0% to 7.5% and is a potent expander
An example would be 0.9% NaCl. of the vascular volume. With an osmolarity of
Crystalloids can also be described by their osmolarity. 2567 mOsm/L, 7.5% NaCl will effectively increase the
An isotonic crystalloid has an osmolarity similar to nor- extracellular fluid volume by five times the administered
mal plasma osmolarity (300–310 mOsm/L) while the volume due to the movement of water from the intracel-
osmolarity of a hypotonic solution is less than that of lular and interstitial spaces into the intravascular space