Page 346 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

338 FLUID THERAPY

the administered dextrose and cause increased urinary should attempt to replace losses with a fluid that is similar
losses of electrolytes. in volume and electrolyte composition to that which has
The veterinary practitioner can manage most animals been lost from the body (see Table 14-3). If clinical
requiring fluid therapy with a limited number of crystalloid assessment of the patient suggests a fluid-responsive type
and additive solutions. The most useful crystalloid solutions of shock, the resuscitation phase of fluid therapy should
forroutineuseareabalancedreplacementsolution(e.g.,lac- be instituted. If the patient has abnormally low oncotic
tated Ringer’s solution, Normosol-R, Plasma-Lyte 148), pressure or an underlying disease condition for which a
0.9% saline, and 5% dextrose in water. The solute composi- low-volume resuscitation strategy may prove advanta-
tion of these fluids is compared with that of ECF in geous, synthetic colloids should be considered as the pri-
Figure14-2,andtheelectrolytecompositionofseveralcom- mary fluid choice for resuscitation (see Chapters 23 and
mercially available solutions is summarized in Table 14-6. 27). If neither of these considerations applies, resuscita-
Supplementation of crystalloid solutions with KCl may tion with a balanced crystalloid solution is indicated.
be necessary when body fluid losses include large If there are no clinical signs of hypovolemia, the hydration
amounts of potassium. An empirical scale has been deficit and maintenance needs may be combined and
devised to estimate the amount of potassium to add to administered during the next 24 hours.
parenterally administered fluids (Table 14-7). 17 This pro- Persistent vomiting caused by pyloric obstruction
tocol has not been evaluated experimentally in dogs or would be expected to result in losses of hydrochloric acid,
cats but has been used successfully in clinical veterinary potassium, sodium, and water, potentially producing
patients during the past 30 years. Potassium supplemen- hypokalemia, hypochloremia, and metabolic alkalosis.
tation is discussed in Chapter 5. The initial fluid of choice in this setting is 0.9% NaCl with
Other additive solutions include 50% dextrose, calcium 20 to 30 mEq KCl per liter. Except in the case of vomiting
chloride, calcium gluconate, potassium phosphate, 8.4% ofstomachcontents,lactatedRinger’sisagoodfirstchoice
sodium bicarbonate, and water-soluble B vitamins. Thia- for fluid therapy while awaiting laboratory results. Normal
mine supplementation may be particularly important in saline (0.9% NaCl) is less ideal because it is not a balanced
cats because their requirement for this vitamin may be solution.Itcontainschlorideingreaterconcentrationthan
higher than that of dogs. Phosphate rarely is used as an body fluids (154 mEq/L versus 110 mEq/L in dogs and
additive but often is required in patients with diabetic 120 mEq/L in cats), and as a result of displacement of
ketoacidosis during insulin therapy. 40 Phosphate supple- bicarbonate with chloride in ECF and initiation of natri-
mentation isdiscussedinChapter7.Theoretically,sodium uresis, it has a mild acidifying effect. 32 Examples of fluid
bicarbonate should not be added to solutions containing therapy in specific diseases are listed in Table 14-3.
calcium (e.g., lactated Ringer’s solution, Plasmalyte-R) In one study, five different solutions were administered
because of the risk of forming insoluble calcium carbonate to unanesthetized dogs during a 1-hour period: 0.9%
crystals. Despite this concern, no adverse consequences NaCl, 0.9% NaCl with 5% dextrose, lactated Ringer’s
have been observed when small amounts of sodium bicar- solution, Normosol-R, and Normosol-R with 2% dex-
bonate have been added to lactated Ringer’s solution. 22,23 tran. 32 The approximate composition of these fluids is
Whenadditivesareused,theclinicianmustkeepinmind presented in Table 14-8. The fluids were warmed to body
that the final osmolality of the fluid may be higher than temperature, and no decreases in rectal temperature were
anticipated. The final osmolality may be approximated observed. Laboratory variables were measured after
by adding the number of milliequivalents per liter of elec- 1 hour of infusion. Fluids were administered at 76 mL/
trolyte and millimoles per liter of nonelectrolyte solutes kg/hr except for Normosol-R with dextran, which was
found in the solution. The final osmolality of the solution administered at a rate of 31.5 mL/kg/hr.
also may differ depending on how the solution was Most of the fluids increased heart rate, diastolic arterial
formulated. For example, if 500 mL of lactated Ringer’s pressure, and central venous pressure (CVP), and all of
solution is mixed with 500 mL of 5% dextrose to create a them decreased hematocrit, hemoglobin, and total protein
replacement solution with 2.5% dextrose, the resulting concentrations by 21% to 25%. All solutions except for
solution has an approximate osmolality of 275 mOsm/ Normosol-R and Normosol-R with 2% dextran caused
kg(virtually the same asthat oflactated Ringer’ssolution). an increase in serum chloride concentration, and the saline
Conversely,if50 mLof50%dextroseisaddedto1 Loflac- solutions decreased pH and bicarbonate concentration. All
tated Ringer’s solution, the resulting solution contains solutions except Normosol-R caused a decrease in serum
2.5% dextrose but has an approximate osmolality of 391 potassium concentration. The causes of the decreased
mOsm/kg, which is substantially higher. serum potassium concentrations in these dogs presumably
The choice of fluid to administer is dependent on the includeddilutionandincreaseddistal tubular flowratewith
nature of the disease process and the composition of the enhanced urinary excretion of potassium. The presence
fluid lost. Underlying acid-base and electrolyte of 5% dextrose in two of the solutions resulted in signifi-
disturbances should be taken into consideration when cantly lower serum potassium concentrations, suggesting
choosing the type of fluid to administer. The clinician movement of potassium into cells with glucose.