The 28 mEq of lactate anions can help reverse metabolic acidosis because of their potential for conversion to bicarbonate ions by the patient's liver (in the absence of shock). It is more physiologic than isotonic saline because its electrolyte concentration is similar to that of plasma. Lactated Ringer's solution (LRS) is a polyionic, isotonic (273 mOsm/L) solution. 9 percent (normal or physiologic) saline Ringer's, acetated Ringer's, and 2.5% dextrose in 0.45% saline. Commonly used isotonic solutions include lactated Ringer's.
They are excellent solutions, therefore, for providing rehydration and maintenance needs, especially because they can be administered intravenously, intraosseously, subcutaneously, and intraperitoneally. Isotonic fluids contain the same osmolality as the extracellular fluids (approximately 290 to 310 mOsm/L). Fluids can be conveniently classified on the basis of tonicity (Table 2). This information is important when deciding which fluid to use in each particular clinical situation. Parenteral fluid packages provide a list of the solute content and osmolality. A comparison of the various routes of fluid administration is provided in Table 1. 1,2 After adequate blood pressure is restored, the method for fluid delivery is switched to the intravenous route. The intraosseous method for fluid therapy is a safe and efficacious route in the critically ill patient. Actually, because of the bone marrow's direct access to the systemic circulation, it can be considered as a large rigid vein through which most medications can be safely delivered. Intraosseous fluid therapy is a preferred route for animals weighing less than 5 kg when the intravenous approach is impossible. With proper technique, the advantages far outweigh the disadvantages. Intravenous catheters should be changed and rotated to another site every 72 hours in order to avoid most of these iatrogenic complications.
The more common complications include phlebitis, catheter sepsis, fluid overload, and the inadvertent flow of fluid into the surrounding perivascular subcutaneous tissue. Knowledge of these requirements and the complications that can result from this mode of therapy is important for a successful outcome. Intravenous treatment requires the insertion of a cannula into a vein using sterile technique and the subsequent sterile maintenance of the intravenous delivery system. It allows for a controlled delivery rate to meet the patient's changing needs. Intravenous infusion is the preferred means of delivering fluids to severely dehydrated animals and medium to large dogs. The two latter routes are preferred for the critically ill patient because they give direct access to the intravascular space. In general, fluids can be given by the following routes: (1) oral, (2) subcutaneous, (3) intraperitoneal, (4) intravenous, and (5) intraosseous. Fluid administered intravenously is distributed between the intravascular and extravascular spaces in fractions determined by the compartments' protein and sodium contents. Three quarters of the ECF is interstitial fluid, and the remaining one quarter is intravascular fluid. Approximately two thirds of TBW is intracellular fluid (ICF) and one third is extracellular fluid (ECF). Total body water (TBW) accounts for approximately 60% of the body weight in kilograms (where 1 L H 20 weighs 1.0 kg).
The clinician and staff, therefore, should familiarize themselves with the pathophysiology of the diseases they are treating and how these conditions relate to the various types of fluids that are available for general use. The methods for providing fluids often influence the eventual outcome of the case. Except for the urgency of treatment, the same objectives apply in the critically ill animal. Fluid therapy in clinical medicine is used to fulfill the following objectives: (1) to replace dehydration deficits, (2) to maintain normal hydration, (3) to replace essential electrolytes and nutrients, and (4) to serve as a vehicle for the infusions of certain intravenous medications.
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