html xmlns=”http://www.w3.org/1999/xhtml” xml:lang=”en” style=”font-size:1.250rem;”> Laura D. Rosenthal DNP, ACNP, FAANP The drugs discussed in this chapter are used to correct disturbances in the volume and ionic composition of body fluids. Standard guidelines do not exist for electrolyte and fluid replacement. Most facilities will have their own protocols regarding recommended replacement. Many of these protocols allow for the replacement of electrolytes by the registered nurse. Three groups of agents are considered for replacement: (1) drugs used to correct disorders of fluid volume and osmolality, (2) drugs used to correct disturbances of hydrogen ion concentration (acid-base status), and (3) drugs used to correct electrolyte imbalances. Good health requires that both the volume and osmolality of extracellular and intracellular fluids remain within a normal range. If a substantial alteration in either the volume or osmolality of these fluids develops, significant harm can result. Maintenance of fluid volume and osmolality is primarily the job of the kidneys, and, even under adverse conditions, renal mechanisms usually succeed in keeping the volume and composition of body fluids within acceptable limits. However, circumstances can arise in which the regulatory capacity of the kidneys is exceeded. When this occurs, disruption of fluid volume, osmolality, or both can result. Abnormal states of hydration can be divided into two major categories: volume contraction and volume expansion. Volume contraction is defined as a decrease in total body water; conversely, volume expansion is defined as an increase in total body water. States of volume contraction and volume expansion have three subclassifications based on alterations in extracellular osmolality. For volume contraction, the subcategories are isotonic contraction, hypertonic contraction, and hypotonic contraction. Volume expansion may also be subclassified as isotonic, hypertonic, or hypotonic. Descriptions and causes of these abnormal states are discussed later. In the clinical setting, changes in osmolality are described in terms of the sodium content of plasma. Sodium is used as the reference for classification because this ion is the principal extracellular solute. In most cases, the total osmolality of plasma is about 2 times the osmolality of sodium. That is, total plasma osmolality usually ranges from 280 to 300 mOsm/kg water. Isotonic contraction is defined as volume contraction in which sodium and water are lost in isotonic proportions. Hence, although there is a decrease in the total volume of extracellular fluid, there is no change in osmolality. Causes of isotonic contraction include vomiting, diarrhea, kidney disease, and misuse of diuretics. Lost volume should be replaced with fluids that are isotonic to plasma. This can be accomplished by infusing isotonic (0.9%) sodium chloride in sterile water, a solution in which both sodium and chloride are present at a concentration of 145 mEq/L. Hypertonic contraction is defined as volume contraction in which loss of water exceeds loss of sodium. Hence there is a reduction in extracellular fluid volume coupled with an increase in osmolality. Because of extracellular hypertonicity, water is drawn out of cells, thereby producing intracellular dehydration and partial compensation for lost extracellular volume. Causes of hypertonic contraction include excessive sweating, osmotic diuresis, and feeding excessively concentrated foods to infants. Hypertonic contraction may also develop secondary to extensive burns or disorders of the central nervous system (CNS) that render the patient unable to experience or report thirst. Volume replacement in hypertonic contraction should be accomplished with hypotonic fluids (e.g., 0.45% sodium chloride) or with fluids that contain no solutes at all. Initial therapy may consist simply of drinking water. Alternatively, 5% dextrose can be infused intravenously. (Because dextrose is rapidly metabolized to carbon dioxide and water, dextrose solutions can be viewed as the osmotic equivalent of water alone.) Volume replenishment should be done in stages. About 50% of the estimated loss should be replaced during the first 24 hours of treatment. The remainder should be replenished over the next 1 to 2 days. Hypotonic contraction is defined as volume contraction in which loss of sodium exceeds loss of water. Hence both the volume and osmolality of extracellular fluid are reduced. Because intracellular osmolality now exceeds extracellular osmolality, extracellular volume becomes diminished further by movement of water into cells. The principal cause of hypotonic contraction is excessive loss of sodium through the kidneys. This may occur because of diuretic therapy, chronic renal insufficiency, or lack of aldosterone (the adrenocortical hormone that promotes renal retention of sodium). If hyponatremia is mild, and if renal function is adequate, hypotonic contraction can be corrected by infusing isotonic sodium chloride solution for injection. When this is done, plasma tonicity will be adjusted by the kidneys. However, if the sodium loss is severe, a hypertonic (e.g., 3%) solution of sodium chloride should be infused. Administration should continue until plasma sodium concentration has been raised to about 130 mEq/L. Patients should be monitored for signs of fluid overload (distention of neck veins, peripheral or pulmonary edema). When hypotonic contraction is due to aldosterone insufficiency, patients should receive hormone replacement therapy along with intravenous (IV) infusion of isotonic sodium chloride. Volume expansion is defined as an increase in the total volume of body fluid. As with volume contraction, volume expansion may be isotonic, hypertonic, or hypotonic. Volume expansion may result from an overdose with therapeutic fluids (e.g., sodium chloride infusion) or may be associated with disease states, such as heart failure, nephrotic syndrome, or cirrhosis of the liver with ascites. The principal drugs employed to correct volume expansion are diuretics and the agents used for heart failure. These drugs are discussed in Chapters 35 and 40, respectively. Maintenance of acid-base balance is a complex process, the full discussion of which is beyond the scope of this text. Hence, discussion here is condensed. Acid-base status is regulated by multiple systems. The most important are (1) the bicarbonate–carbonic acid buffer system, (2) the respiratory system, and (3) the kidneys. The respiratory system influences pH through control of CO2 exhalation. Because CO2 represents volatile carbonic acid, exhalation of CO2 tends to elevate pH (reduce acidity), whereas retention of CO2 (secondary to respiratory slowing) tends to lower pH. The kidneys influence pH by regulating bicarbonate excretion. By retaining bicarbonate, the kidneys can raise pH. Conversely, by increasing bicarbonate excretion, the kidneys can lower pH, and thereby compensate for alkalosis. There are four principal types of acid-base imbalance: (1) respiratory alkalosis, (2) respiratory acidosis, (3) metabolic alkalosis, and (4) metabolic acidosis. Causes and treatments are discussed next. Respiratory alkalosis is produced by hyperventilation. Deep and rapid breathing increases CO2 loss, which in turn lowers the PCO2 (partial pressure of carbon dioxide) of blood and increases pH. Mild hyperventilation may result from a number of causes, including hypoxia, pulmonary disease, and drugs (especially aspirin and other salicylates). Severe hyperventilation can be caused by CNS injury and hysteria. Management of respiratory alkalosis is dictated by the severity of pH elevation. When alkalosis is mild, no specific treatment is indicated. Severe respiratory alkalosis resulting from hysteria can be controlled by having the patient rebreathe his or her CO2-laden expired breath. This can be accomplished by holding a paper bag over the nose and mouth. Although this technique has been used for many years, there remains a lack of evidence regarding its efficacy in respiratory alkalosis. A sedative (e.g., diazepam [Valium]) can help suppress the hysteria.
Agents Affecting the Volume and Ion Content of Body Fluids
Disorders of Fluid Volume and Osmolality
Volume Contraction
Isotonic Contraction
Definition and Causes
Treatment
Hypertonic Contraction
Definition and Causes
Treatment
Hypotonic Contraction
Definition and Causes
Treatment
Volume Expansion
Acid-Base Disturbances
Respiratory Alkalosis
Causes
Treatment
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Agents Affecting the Volume and Ion Content of Body Fluids
Chapter 87
