INTRODUCTION

Intravenous fluid therapy is vital for the management of shock, dehydration, and maintenance in animals that require parenteral fluid therapy (see Chapters 61, 62, and 63, Peripheral Venous Catheterization, Intraosseous Catheterization, and Central Venous Catheterization, respectively, and Chapter 65 and 66, Shock Fluids and Fluid Challenge and Transfusion Medicine, respectively). This chapter focuses primarily on the distribution of total body water, patient assessment, and the delivery of synthetic intravenous fluids to maintain normal water, electrolyte, and acid-base status in critically ill dogs and cats that are hemodynamically stable. Because critically ill animals often have fluid and electrolyte balance derangements, overall recovery often depends on recognition and appropriate treatment of these disorders, in addition to diagnosing and treating the primary disease process.

TOTAL BODY WATER

Living organisms are predominantly composed of water. Total body water content is approximately 60% of body weight in a nonobese adult dog or cat. Total body water is distributed between two main compartments: intracellular fluid (ICF) and extracellular fluid (ECF) (Figure 64-1). Each compartment consists of solutes, primarily electrolytes, dissolved in water. The most important determinant of the size of each body fluid compartment is the quantity of solutes contained in that compartment.^1,2

Figure 64-1 The distribution of total body water (TBW). ECF, Extracellular fluid; ICF, intracellular fluid.

The ICF compartment is the larger of the two and comprises 66% of the total body water and 40% of body weight. It is separated from the ECF compartment by the cell membrane, which is very permeable to water but impermeable to most solutes. Cell membranes contain numerous proteins, including ion channels and active solute pumps. The most important active pump is the sodium-potassium ATPase pump, which extrudes three sodium ions out of the cell in exchange for bringing two potassium ions into the cell. This pump is responsible for generation of the electrochemical gradient across cell membranes, typified by a high intracellular potassium concentration, high extracellular sodium concentration, and a negative resting membrane potential.Therefore the most prevalent cation in the ICF is potassium, with much smaller contributions made by magnesium and sodium. The most prevalent anions in the ICF are phosphate and the polyanionic charges of the intracellular proteins.^1,2

The ECF comprises the remaining 33% of the total body water and 20% of body weight. The ECF is subdivided into the plasma (25% of ECF) and interstitial (75% of ECF) fluid compartments. The interstitial fluid bathes all cells and includes lymph. The primary cation in the ECF is sodium and the most prevalent anions are Cl⁻ and HCO₃⁻. The proteins in plasma and the interstitial space also contribute to the negative charges. The oncotic pressure gradient between the intravascular and interstitial spaces is determined by the ratio of proteins in these two compartments.^1,2

MOVEMENT OF FLUIDS WITHIN THE BODY

Water moves freely within most compartments in the body. Small particles such as electrolytes move freely between the intravascular and interstitial compartment, but cannot enter or leave the cellular compartment without a transport system. Larger molecules (>20,000 daltons) do not easily cross the vascular endothelial membrane and may attract small, charged particles, thus creating the colloid osmotic pressure (COP). There are three main natural colloid particles: albumin, globulins, and fibrinogen. An increase in the pressure of fluid within a compartment that pushes against a membrane is known as hydrostatic pressure.

In health, fluid balance is determined by the balance between forces that favor reabsorption of fluid into the vascular compartment (increased COP or decreased hydrostatic pressure) and those that favor filtration out of the vascular space (decreased COP or increased hydrostatic pressure).¹ Changes in the osmolality between any of the fluid compartments within the body will cause free water movement across the respective membrane.

In disease states, both increased fluid losses and decreased intake may lead to dehydration. The nature of the fluid loss (hypotonic, isotonic, or hypertonic) will determine the subsequent changes in osmolality. This will in turn dictate the relative impact on the ICF and ECF compartments.