section epub:type=”chapter” id=”c0070″ role=”doc-chapter”> The goal of individualized fluid management in the perioperative period is to maintain adequate intravascular fluid volume, left-ventricular filling pressure, cardiac output, systemic blood pressure, and oxygen delivery to tissues. The maintenance of appropriate concentrations of body fluid and electrolytes is essential to normal physiologic function of all body systems. This chapter describes basic human physiology in this area and briefly introduces the various types and protocols of fluid management. blood component therapy; body fluids; crystalloid and colloid administration; electrolytes; fluids; perioperative blood and fluid replacement; transfusion reactions; volume assessment and treatment The goal of individualized fluid management in the perioperative period is to maintain adequate intravascular fluid volume, left-ventricular filling pressure, cardiac output, systemic blood pressure, and oxygen delivery to tissues. The maintenance of appropriate concentrations of body fluid and electrolytes is essential to normal physiologic function of all body systems. A description of basic human physiology in this area along with a brief introduction to the various types and protocols of fluid management of the patient is presented in this chapter. Definitions Anions Ions that carry a negative charge and migrate to the anode (terminal) in an electric field. Autologous Originating from the same person such as with an autotransfusion (versus allogenic, from someone other than the patient). Cations Ions that carry a positive charge and migrate to the cathode (terminal) in an electric field. Chvostek Sign An abnormal spasm or twitch of the facial muscles due to hyperexcitability of the nerve; elicited by light tapping over the facial nerve anterior to the ear as it crosses the zygomatic arch. Indicates hypocalcemia. Colloids Artificially made (starches, dextrans, gelatins) or naturally occurring (albumin, red blood cells, or fresh frozen plasma [FFP]) compounds that, because of larger sizes, are retained within a specific fluid compartment for longer periods of time and increase the oncotic pressure of that compartment. Cryoprecipitate A preparation with concentrated clotting factors VIII, XIII, von Willebrand, and fibrinogen needed to restore normal coagulation in hemophilia, von Willebrand disease, and for patients who cannot produce the necessary amount of this important clotting protein on their own. The preparation is collected from fresh human plasma that has been repeatedly frozen and thawed. Crystalloids Balanced electrolyte solutions of mineral salts and additional small water-soluble molecules that are isotonic and can easily move between the intravascular and interstitial compartments. Edema Accumulation of fluid in the interstitial spaces. Hemolysis A rupture of the red cell membrane that causes release of cell contents, cytoplasm, into the surrounding blood plasma. Hemostasis The physiologic process of the arrest of bleeding within a blood vessel by the process of platelet aggregation, the formation of a platelet plug, and the accumulation of fibrin. Hypercalcemia Increased plasma concentration of calcium (>5.6 mEq/L). Hyperkalemia Greater than 5.0–5.5 mEq/L blood concentration of potassium. Hypermagnesemia An increase in the plasma concentration of magnesium (>2.2 mEq/L). Hypernatremia An increase in sodium in the plasma of more than 145 mEq/L. Hypertonic Solutions (Hyperosmotic) Solutions that have more dissolved components (sodium and other electrolytes) and therefore greater osmolality than that of normal blood and cells. Hypocalcemia Reduced plasma concentration of calcium (<4.4 mEq/L). Hypokalemia Less than 3 mEq/L blood concentration of potassium. Hypomagnesemia A decrease in the plasma concentration of magnesium (<1.6 mEq/L). Hyponatremia A decrease of sodium in the plasma of less than 135 mEq/L. Hypotonic Solutions (Hypoosmotic) Solutions that have fewer dissolved components (sodium and other electrolytes) and therefore lower osmolality than that of normal blood and cells. Isotonic Solutions Solutions that have the same osmolality as plasma. Milliequivalent (mEq) Replaced with the International System of Units (SI units) millimole (mmol); mEq/L has been replaced by mmol/L. Osmolality The concentration of a solution, determined by the total number of solute particles in the solution. Tetany A condition characterized by cramps, muscle twitching, sharp flexion of the wrist and ankle joints, and convulsions. Third Space Shifts of fluid and electrolytes from the intravascular space to a nonfunctional extravascular or extracellular space, or a body cavity. Can be seen postoperatively after long extensive surgical procedures. Trousseau Sign A test for latent tetany usually seen as a result of hypocalcemia. Carpopedal spasm is induced through ischemia by inflation of a sphygmomanometer cuff on the upper arm to a pressure that exceeds systolic blood by 20 mm Hg for 3 minutes. The spasm is seen in the wrist, thumb, hand joints, and fingers. Water is the most abundant and essential component of the body. It represents approximately 50% to 60% of adult body weight and 75% to 77% of body weight in infants less than 1 month of age. By approximately 17 years of age, the adult composition is attained, and in a person weighing 154 lb (70 kg), the total body water is approximately 42 L. Because women have higher fat content in their bodies, and because fat is essentially free of water, they have a lower water content than men. Older adults and those with diabetes, hypertension, or obesity also have a lower proportion of water in their bodies. Body water is the substance within which metabolic reactions take place to facilitate the ionization of electrolytes. It acts as a reagent in many chemical reactions; it transports nutrients to cells and removes waste products; and its high specific heat and heat of vaporization make it especially suitable as a temperature regulator. The total amount of body water remains relatively stable; intake usually slightly exceeds bodily needs, and the excess is excreted. Removal or output of water from the body is normally through four types of excretions: through the lungs, gastrointestinal tract, skin, and kidneys. Water intake includes not only the water consumed in beverages but also fluids obtained from the metabolism of solid foods. The water taken in via liquids and food is referred to as exogenous water. Although variance occurs on a day-to-day basis, overall the average adult in a moderate climate with a mixed diet consumes 2500 to 3000 mL of water daily. In the United States, adults drink an approximately 1200 mL of water. Roughly 1000 mL is obtained from beverages and an additional 1500 mL from solid and semisolid foods. The water formed during metabolism of ingested food is called endogenous water. Because metabolism varies with body temperature, the amount of exercise performed, and other factors, the amount of endogenous water available also varies on a daily basis. In a healthy adult who performs a moderate amount of exercise, an average of 300 to 350 mL of endogenous water is available daily. The sensation of thirst is influenced by the thirst center located in the hypothalamus, which is stimulated by either a decrease in extracellular or intravascular (blood) levels of water (hypovolemic thirst) or an increase in serum osmolality, or blood osmotic pressure, caused by elevated extracellular solutes (osmotic thirst). If the fluid volume inside the cells decreases, salivary secretion is reduced, causing a dry mouth and the sensation of thirst.1 In normal circumstances, an individual then drinks and restores the fluid volume (Box 14.1). The surgical patient experiences even greater fluid losses. However, fasting guidelines changed over time due to evidence-based research and the initiation of early recovery after surgery (ERAS) protocols. Research shows that reduced fasting times lower the residual gastric volumes and improve patient outcomes and patient satisfaction. The American Society of Anesthesiologists’ fasting guidelines changed to more liberal recommendations in 1999. (Box 14.2). However, many facilities have yet to change from the age-old fasting recommendations. Therefore, unless a patient is coming to the operating room for a surgical emergency, in many cases adults will be NPO for at least 8 hours. Longer fasting times are generally still enforced in patients at increased risk for aspiration (Box 14.3). Numerous current large-scale studies have changed fluid replacement recommendations for the perioperative period. The goal of surgical fluid management is to avoid hyper- or hypovolemia. Both over- and under- fluid replacement can have deleterious effects on a patient, from fluid overload to kidney damage. Perioperative fluid therapy should be individualized to each patient, with the goal to transition to oral fluid intake from IV as soon as possible.2 The amount of fluid lost through the lungs varies with the humidity, temperature of inspired air, and rate and depth of respiration. As a rule, 300 to 400 mL of water is lost daily. This loss of water via the respiratory tract is termed insensible loss of water, so named because one is not aware of this loss. The water content of inhaled gases decreases as the ambient temperature decreases. Consequently, the insensible water loss from the lungs is higher in cold environments. Therefore, patients with respiratory dysfunction need to consume greater amounts of water to offset the increased insensible water loss when they are in cold environments. In addition, an increase in the respiration rate can increase the water loss to as much as 2000 mL, which is significant in patients with chronic obstructive pulmonary disease (see Chapter 48). The amount of water lost via the feces averages 100 mL per day; however, with vomiting or diarrhea, this loss may be greatly increased. Up to 7000 mL can be lost with diarrhea and 6000 mL with vomiting. The implications to the perianesthesia nurse are great because active vomiting can significantly affect the volume status of the surgical patient. In such cases, fluids should be increased in rate, any ordered antiemetic therapies should be administered, and the anesthesia provider notified. Water lost via the skin can be via insensible or sensible loss. There is constant flow of moisture from deep body layers to the dry surface where the evaporation occurs. Insensible loss depends largely on environmental humidity. Sensible perspiration refers to loss of water with production of sweat. Sweating is a mechanism for regulation of body temperature when the heat produced by metabolic processes is excessive; it is the body’s natural cooling mechanism The amount of sweat varies with exercise and with body temperature. In a moist atmosphere, sweat may be more visible than in a dry atmosphere, but the amount of water lost is the same. Despite its role as a protective mechanism, sweating can become a hazard when body water supplies are low because the body continues to produce sweat to maintain its temperature. In the healthy adult who performs moderate exercise in a comfortable environment, approximately 500 mL of water is lost in both sensible and insensible perspiration. Water loss via the kidneys varies with the supply of body water. The kidneys are able to concentrate the urine, and the specific gravity may approach 1.040 (normal, 1.002 to 1.030). However, if the amount of excess water is great, such as might occur when a large quantity is administered intravenously (IV), the kidneys excrete dilute urine, the specific gravity of which might approach 1.001. In normal conditions, the kidneys excrete approximately 1.5 L of water per day. In patients who are vomiting or have diarrhea, obviously less water is available, and the kidneys respond promptly by curtailing urine output. The two hormones responsible for control of the volume of urine are antidiuretic hormone (ADH) from the posterior pituitary and aldosterone from the adrenal cortex. ADH, by increasing the permeability of the renal distal convoluted tubule and collecting ducts, increases the amount of intravascular water reabsorbed and thus decreases the urine volume. Aldosterone increases sodium and water retention and potassium excretion. Both hormones are secreted in response to lowered blood volume and serve to control output to balance intake. However, a minimal loss of fluids is obligatory; therefore, perioperative monitoring of fluid balance is critical.3,4 Patients undergoing surgery require fluid replacement for that lost through their lungs, gastrointestinal tract, skin, and urine. Consideration for fluid replacement includes their NPO deficit, intraoperative fluids lost through blood loss, third-space fluid shifts, evaporation through incisions, and tissue manipulation. Replacement of fluids is individualized and based on the type of fluid used for replacement (i.e., crystalloids, colloids, blood products), intraoperative events and patient status; the concept of goal-directed fluid therapy (GDT) as the guidelines used. The fluids in the body can be divided into two compartments along with a potential third compartment or space. The two compartments are normally divided relative to the location of the cell membrane: intracellular (inside the cell) and extracellular (outside the cell). The intracellular fluid (ICF) is estimated to be approximately 40% of the body weight, or approximately 28 L of fluid, and represents approximately two-thirds of the total body water. ICF provides a medium for all intracellular activities. The extracellular fluid (ECF) is approximately 20% of the body weight and ranges from 12 to 14 L of fluid. The fluid compartment includes the blood plasma or intravascular fluid, the interstitial fluid (ISF) that bathes the cells, the lymph which transports white cells, the cerebrospinal fluid (CSF), and the transcellular fluids. The transcellular fluids include the synovial fluid, peritoneal fluid, digestive fluids, and fluids of the eye and ear. The lymph, CSF, and transcellular fluids normally constitute approximately 1% of the body mass. Blood constitutes approximately 4% of the body weight, and the ISF constitutes 15.7%.5 There is a potential third compartment that is commonly called the third space. It is a concept defined as a compartment that includes the interstitial spaces swollen by local responses to tissue trauma, inflammation, and hormonal influx from the stress of surgery. This third space can occur even when patients have undergone massive surgical procedures, and the fluid loss, to include insensible loss, is appropriately replaced. This accumulation of fluid in the third-space compartment usually occurs during and immediately after the surgical procedure and is difficult to clinically differentiate from actual blood loss. Clinically, the signs of hypovolemia reflect third-space loss and actual fluid loss. The treatment includes infusion of fluids in the range of 3 to 10 mL/kg/h and is usually adequate along with establishment and treatment of the underlying cause (e.g., active bleeding). The third-space loss usually resolves in several postoperative days, and the nurse on the unit that receives the patient after the postanesthesia care unit (PACU) should be alert for signs of possible fluid overload as the fluid returns to the ECF after surgery. Fluid balance involves not only the total amount of body water but also the maintenance of a relatively constant distribution of that water in the different compartments. Circulation of fluid between compartments depends on the relative hydrostatic and osmotic pressures in each compartment. Hydrostatic pressure is the force that pushes fluid from one compartment to the other. If the hydrostatic pressure in the capillaries (blood pressure) exceeds the pressure in the interstitial space, fluid moves from the capillary into the interstitial space. Osmotic pressure is the “pull” of fluids into the compartment; it is a function of the number of dissolved molecules in the solution and is not influenced by weight or size of the molecule. Electrolytes are the major contributors to the osmotic pressure of the fluids.6 The foremost difference between the two major compartments that make up the ECF is the much higher protein content in the plasma than in the ISF. Because capillary membranes are not selectively permeable to small particles, ions and small molecules can exchange rapidly between the plasma and the ISF. However, proteins remain in the plasma because they are too large to cross the capillary barrier. As a result, the electrolyte composition differs slightly from the plasma and the ISF. The sodium concentration in plasma is slightly greater, whereas the chloride concentration is slightly less than in the ISF and the sum of the diffusible ions. Thus, the osmotic pressure in the plasma is greater than that of ISF. The osmotic pressure caused by plasma colloids is called the colloid osmotic pressure (COP) or oncotic pressure. Protein molecules are responsible for the COP. The proteins that exert a COP help retain the plasma water in the intravascular compartment. Albumin is the major protein in the plasma that contributes to the COP. The ECF is regulated carefully by the kidneys to facilitate the cells being bathed in fluid that contains appropriate concentrations of electrolytes including sodium, potassium, and nutrients. A patient with major abdominal surgery usually excretes large amounts of potassium during the first 48 hours postoperatively and for several days thereafter. The body has significant stores of potassium; therefore, postoperatively, hypokalemia might not be evident for a number of days. Potassium levels are generally monitored closely postoperatively, and replacement is administered IV when needed. It is important to note that plasma potassium measurements do not exactly predict total body potassium because potassium is primarily an intracellular ion. From a clinical chemistry point of view, the international standard unit is the millimole (mmol), commonly called the milliequivalent (mEq). The clinical implication for the perianesthesia nurse is that patients who undergo major surgery should routinely have potassium levels checked and evaluated before surgery, especially if they are receiving any non–potassium-sparing diuretics (see Chapter 13). A delicate balance of pressures keeps fluids passing between compartments (Fig. 14.1). A dynamic equilibrium exists between the plasma and the ISF because proteins are too large to cross the capillary barrier, which creates a COP between the two components. The hydrostatic pressures of the blood and the ISF tend to oppose each other, which is called the effective filtration pressure. Similarly, the COP is the opposition between the blood and the ISF. The final common pathway is that these pressures result in a pulling in opposite directions when in appropriate physiologic equilibrium, which does not allow fluid to accumulate into the interstitial spaces. Edema then results when either of the two pressures is in dysfunction. Blood and lymph capillary show protein and small solutes molecules inside and in interstitial fluid (I F). Hydrostatic pressure and colloid osmotic pressure cause transfer of fluids, indicated by arrows, via capillary membranes. • Pressures at arterial end of tissue capillaries: Blood hydrostatic pressure (B H P): 35 millimeters of mercury, interstitial fluid hydrostatic pressure (I F H P): 2 millimeters of mercury, blood colloid osmotic pressure (B C O P): 24 millimeters of mercury, and interstitial fluid colloid osmotic pressure (I F C O P): 0 millimeter of mercury. Filtration pressure is 33 millimeters mercury. Osmotic pressure is 24 millimeters of mercury, E F P is 9 millimeters of mercury. • Pressures at venous end of tissue capillaries: Blood hydrostatic pressure (B H P): 15 millimeters of mercury, interstitial fluid hydrostatic pressure (I F H P): 1 millimeter of mercury, blood colloid osmotic pressure (B C O P): 25 millimeters of mercury, and interstitial fluid colloid osmotic pressure (I F C O P): 3 millimeters of mercury. Filtration pressure is 14 millimeters mercury. Osmotic pressure is negative 22 millimeters of mercury, E F P is negative 8 millimeters of mercury.
14: Fluids and Electrolytes
Abstract
Keywords
Body fluid balance
Surgical Patient Considerations
Lungs
Gastrointestinal Tract
Skin
Kidneys
Surgical Fluid Loss
Distribution of body fluids
Edema
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