Hyponatremia and Hypernatremia

An In-Depth Look: CE NORMAL AND ABNORMAL WATER BALANCE Article #1 s e e R a r a m Normal and Abnormal Water a T y b n o i t a r Balance: Hyponatremia and t s u l l I Hypernatremia* Katherine M. James, DVM, PhD, DACVIM Veterinary Information Network Katharine F. Lunn, BVMS, MS, PhD, MRCVS, DACVIMa Colorado State University ABSTRACT: This article discusses normal and abnormal water balance in small animals.The terms and concepts central to understanding the maintenance of normal salt and water balance and the manifestations of dysregulation are presented, the physiology of body fluid balance is reviewed, and the concepts of sodium content and sodium concentration are explored.The emphasis of this review is that abnormalities of serum sodium concentration are, in fact, abnormalities of water balance.Thus, hyponatremia reflects too much water relative to sodium in the extracellular fluid, whereas hypernatremia reflects a relative deficit of water. everal important terms relating to water water. The extracellular fluid (ECF) compart- balance are defined in the box on page ment is further divided into interstitial water and S590. plasma water. The relative distribution of body water for a typical dog is shown in Figure 1; how- TERMS AND CONCEPTS ever, the percentage of total body water in an Body Fluid Balance individual animal is affected by body fat content.1 Balance refers to the ratio of the amount of a Body fluid balance depends on the interrelation- substance assimilated into the body to the ship between two key components that must be amount that is lost from the body. When this simultaneously kept in balance: salt and water. ratio approximates 1:1 (unity), the body is in bal- Salt balance refers to the maintenance of ance for that substance. Under normal steady- optimal body fluid volume, as shown in Figure state conditions, on a day-to-day basis, the 2. It depends primarily on the sodium ion (Na+) volume of total body fluid stays the same; this content of the ECF because Na+ is the principal body fluid balance is achieved when fluid intake extracellular cation. Although the total solute and losses are equivalent. Total concentrations in the extracellular and intracel- body water comprises two lular compartments are equivalent, their com- main compartments: intracel- positions differ; within cells, potassium and •Take CE tests lular water and extracellular magnesium ions are the principal cations, • See full-text articles *A companion article on polyuria and polydipsia begins on page 612. aDr. Lunn discloses that she has received financial support from ContinuEd Continuing CompendiumVet.com Education Company and Fort Dodge Animal Health. October 2007 589 COMPENDIUM 590 CE Normal and Abnormal Water Balance: Hyponatremia and Hypernatremia Glossary of Important Terms Total Body Water (TBW) Content—The total amount of a substance in a given space. The content of a substance such as Na+ only Intracellular water Extracellular water changes when molecules of that substance are gained Interstitial Plasma or lost from a body space, such as the extracellular 60% water water fluid space. This term is used in contrast to TBW TBW 67% 25% 8% concentration, which reflects the content of a TBW TBW TBW substance contained within a certain volume of water, typically plasma or urine. Concentrations of electrolytes (e.g., Na+) are measured by laboratory investigations. The content of a substance is s e e calculated by multiplying the concentration of the R 25% of a r substance by the volume of the fluid space. 75% of ECF a ECF volume volume m a T Osmolality—The concentration of an osmotic solution, y b Water moves Water moves due to due to n especially when measured in osmoles or milliosmoles o i osmotic forces Starling’s forces t a per 1,000 g of solvent (usually water). Osmolarity is a r t s u l l similar term and is often used interchangeably with I osmolality when the solvent is water. However, Figure 1. The distribution of body fluids by osmolarity refers to the measurement in osmoles compartment. Starling’s forces are hydrostatic and oncotic or milliosmoles of solute per liter of water—a forces that determine fluid movement across capillary measurement of osmoles per volume rather than per membranes as a result of filtration. weight of solvent. Osmosis—The movement of water through a semipermeable membrane (such as a cell membrane) into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane. Solutions that cause water to move by osmosis are termed osmotic solutions. Tonicity—The osmotic pressure of a solution. The tonicity of a solution (e.g., an intravenous fluid solution, plasma, urine) is only relevant in reference to another physiologic system. For example, intravenous fluids are hypertonic, isotonic, or s e e hypotonic relative to plasma. A patient’s plasma R a r is hypertonic, isotonic, or hypotonic relative to a m a T normal plasma. A patient’s urine is hypertonic y b (hypersthenuria), isotonic (isosthenuria), or hypotonic n o i t (hyposthenuria) relative to the patient’s plasma. a r t s u l l I Figure 2. Normal salt balance exists when the intake of replacing Na+. These differences in solute composition Na+ in food is equivalent to losses, principally in feces and between intracellular and extracellular space are main- urine. A dog in normal salt balance has an appropriate number of + tained by active ion transport.2 Na in its total body fluid, including the interstitial space (tan) and + the intravascular space (blue and red vessel representing a Because Na is the principal extracellular cation, and composite of arterial, venous, and capillary blood volumes). because the osmolality of body fluids is regulated within Anions (not shown) are present in equal numbers to maintain a very narrow range,2 the Na+ content of the body deter- electroneutrality.Water molecules are present in appropriate mines the volume of ECF and total body fluid volume. proportion (depicted as equal numbers) to maintain a normal If sodium ions (always with accompanying anions) are ECF osmolality.The cells (depicted by a single representative brown cell) contain sufficient intracellular osmoles to maintain an added to the ECF space, the same proportion of water equal concentration of water in intracellular fluid and ECF, molecules must be added to the ECF space or the resulting in an optimal cell volume.(O = osmole) osmolality will increase beyond the limits compatible COMPENDIUM October 2007 Normal and Abnormal Water Balance: Hyponatremia and Hypernatremia CE 591 2 Renal Na+ retention 3 Diarrhea s s e e 1 Renal loss e e R R a a r r a a m m a a T T y y + b b 1 Na n n ingestion o o Salt i i t 2 Vomiting t a a r r t t s s u u l l l l I I Figure 3. A dog in positive salt balance has too many Figure 4. A dog in negative salt balance has too few sodium ions in its total body fluid. Both the interstitial space sodium ions in its total body fluid. Both the interstitial space and vascular volume are expanded.The dog can drink water, and and vascular volume are reduced.The number of water molecules the number of water molecules has increased proportionally to has decreased proportionally through isotonic fluid loss, and maintain a normal ECF osmolality.Thus the equilibrium between normal ECF osmolality is maintained.With mild negative salt the intracellular and extracellular fluid spaces is unchanged (red balance, the equilibrium between the intracellular and arrows of equal size). Positive salt balance can result from (1) an extracellular fluid spaces is unchanged (red arrows of equal size). increase in salt intake through ingestion or intravenous fluid Negative salt balance commonly results from (1) renal or (2 and administration or (2) renal Na+ retention (blue arrows), such as in 3) gastrointestinal or urinary fluid loss.Very little Na+ is present in patients with heart failure or cirrhosis. vomit; however, without oral replacement of Na+, there will be ongoing obligate renal Na+ loss in a vomiting patient. with a healthy state. Thus, increasing the number of sodium ions in the ECF (the Na+ content) increases the ECF volume. Similarly, if sodium ions are removed from the ECF space, water molecules must leave in proportion, thereby reducing the ECF volume; otherwise, the osmolality of the ECF would decrease below the level allowed in a healthy individual. The dog shown in Figure 3 has increased total body fluid and ECF volumes due to increased Na+ content. s + e e Increased Na content can be the result of increased R a + r dietary salt intake or increased renal Na retention. Con- a m a T versely, the dog shown in Figure 4 has decreased total y b + body fluid and ECF volumes due to decreased Na con- n o i t a r tent. This solute deficit can result from gastrointestinal t s u l l or urinary losses. I Water balance (Figure 5) defines the maintenance of Figure 5. Normal water balance exists when water optimal body fluid tonicity. Tonicity refers to the osmotic intake is equal to insensible (nonmeasurable; primarily pressure of a solution—its ability to cause water to move through the respiratory tract) and sensible (measurable in urine and feces) losses. Intracellular fluid and ECF are across a membrane. Body fluid tonicity is tightly regu- isotonic (have an equal water concentration); therefore, water lated because it determines the volume of the body’s movement is in equilibrium (red arrows of equal size).The volume cells, which in turn affects cell function.2 Although ECF of body cells and the plasma osmolality are appropriate.

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